Technical Proposal for the SPIRAL 2 instrumentation

Transcription

1 Technical Proposal for the SPIRAL 2 instrumentation TITLE of the Project: Neutrons For Science (NFS) Abstract (Max 15 lines): The deuterons and protons beams delivered by the LINAG of SPIRAL-2 are particularly well suited to produce high energy neutron in the 1 MeV 40 MeV energy range. The Neutrons For Science (NFS) facility will be composed of mainly two rooms: a converter room where neutrons are produced by the interaction of deuteron or proton beam with thick or thin converters and an experimental hall with a well collimated pulsed neutron beam. A white neutron source covering the 1 MeV 40 MeV energy range and quasi mono-energetic neutron beam will be available. This facility is of first importance for academic research and applied physics. Several research areas will be covered by NFS like the study of the fission process, the transmutation of nuclear waste, the design of future fission and fusion reactors, the nuclear medicine or the test and development of new detectors, etc. In addition, cross-section measurements of neutron- and deuteron- induced reactions could be realized by activation technique in a dedicated irradiation station. Spokespersons (maximum 3) with one corresponding spokesperson: Xavier Ledoux, CEA/DIF/DPTA/SPN Bruyères-le-Châtel, Stanislav Simakov, FZK, Karlsruhe Contact person at GANIL: F. Rejmund Members of the Collaboration: X. Ledoux, E. Bauge, G. Belier, T. Ethvignot, J. Taïeb, C. Varignon, CEA/DIF/DPTA/SPN, Bruyères-le-Châtel, France S. Andriamonje, E. Dupont, D. Doré, F. Gunsing, D. Ridikas, CEA/DSM/IRFU/SPhN, Saclay, France V. Blideanu, CEA/DSM/IRFU/Senac, Saclay, France M. Aïche, G. Barreau, S. Czajkowski, B. Jurado, CENBG, Gradignan, France G. Ban, F. R. Lecolley, J. F. Lecolley, J. L. Lecouey, N. Marie, J. C. Steckmeyer, LPC, Caen, France P. Baumann, P. Dessagne, M. Kerveno, G. Rudolf, IPHC, Strasbourg, France P. Bem, NPI, Řež, Czech Republic J. Blomgren, DNR, Uppsala, Sweden U. Fischer, S. P. Simakov, FZK, Karlsruhe, Germany B. Jacquot, F. Rejmund, GANIL, Caen, France M. Avrigeanu, V. Avrigeanu, C. Borcea, F. Negoita, M. Petrascu, NIPNE, Bucharest, Romania S. Oberstedt, A.J.M. Plompen, JRC/IRMM, Geel, Belgium O. Shcherbakov, PNPI, Gatchina, Russia M. Fallot, L. Giot, Subatech, Nantes, France A. G. Smith, I. Tsekhanovich, Department of Physics and Astronomy, University of Manchester, Manchester, UK O. Serot, J.C. Sublet, CEA/DEN, Cadarache, France L. Perrot, IPNO, Orsay, France T. Caillaud, O. Landoas, B. Rossé, I. Thfoin, CEA/DIF/DCRE/ SCEP, Bruyères-le-Châtel, France

2 1 Introduction and Overview (max. 5 pages) Introduction Gamma production cross sections for inelastic scattering and (n,x) reactions Charged particle production Fission Neutrons and charged particles activation reactions Neutron detector characterizations Key experiments Actinides Fission-Fragment Yields (see Appendix 1) (n,x) cross section measurements by in-beam γ-ray spectroscopy (see Appendix 2) Neutrons and charged particles activation reactions (see Appendix 3) Study of the pre-equilibrium process in the (n,xn) reactions (see Appendix 4) Description of the proposed equipment(s) Design specifications The neutron beam: The Irradiation facility: Simulations Neutron spectra: Energy resolution Neutron overlap Neutron flux in time-of-flight hall Neutron flux for activation measurements Thermal simulations for the converters Collimator design Neutron beam dump Design and construction scheme Calibration procedures Trigger, DAQ, Controls Target requirements Beam requirements Implementation and Installation Experimental hall and Annex facilities Detectors-Machine interface Assembly and installation Commissioning (work plan, cost, necessary manpower and other resources) Operation (running cost, necessary manpower and other resources) Safety issues and proposed solutions Organisations and Responsibilities Management Board WBS - work package break down structure Schedule for the signature of Memorandum of Understanding Planning Finances Manpower Options and possible further upgrades (list) Relations with other projects Other issues...22 Appendix 1: Actinides Fission-Fragment Yields...23 Appendix 2: (n,x) cross section measurements by in-beam γ-ray spectroscopy...26 Appendix 3: Neutrons and charged particles activation reactions...30 Appendix 4: Study of the pre-equilibrium process in the (n,xn) reactions...33 Appendix 5: Neutron beam dump...36 Appendix 6: Neutron monitoring...40 Appendix 7: Safety issues and proposed solutions for NFS...43

3 Technical Proposal for the SPIRAL 2 instrumentation 30 May Introduction and Overview (max. 5 pages) 1.1 Introduction Neutron induced reactions play an important role in a wide range of applications including nuclear power reactors, accelerator driven systems (ADS), fusion technology, medical diagnostics and therapy, radio isotopes production, dosimetry and dose effects and radiation damage and upsets in electronic devices and basic science research. The data used in transport codes are embodied in evaluated data libraries, which are based on measurements and reaction models. As a matter of fact, the quality of the evaluated data depends on the accuracy of the measurements. Today there is still a large demand of data in neutron-induced reactions above, say, 14 MeV. For many cases (n,fission), (n,n'γ), (n,xn), (n,lcp) reaction cross sections are unknown or known with poor precision. The neutron energy range between 1 and 40 MeV concerns several applications in particular, nuclear waste transmutation in ADS; innovative nuclear power reactors (fast reactors), so called Generation IV reactors; fusion applications like the International Fusion Material Irradiation Facility (IFMIF) and the International Thermonuclear Experimental Reactor (ITER) and in particular its projected follow up DEMO. In these cases, nuclear engineers need new and good quality data, relevant codes and theoretical models in order to build reliable evaluated data libraries to be used by nuclear industry. The above experimental data require specific facilities like pulsed neutron beams for cross section measurements by time-of-flight or dedicated irradiation stations for activation analysis. The high neutron fluxes with high and variable energy spectra available at NFS are very attractive to perform the measurements of the transmutation-incineration of nuclear waste and minor actinides in particular. The high neutron fluxes would allow the measurements of small reaction cross-sections and/or with very small targets, which might be rare, expensive, and in some cases radioactive. The energy range and conditions offered by SPIRAL-2/NFS time-of-flight facility is complementary to other facilities in Europe, notably GELINA at IRMM/JRC in Geel and the n_tof facility at CERN. For example, the design of IFMIF requires reliable data up to 55 MeV. In this case, the neutron spectrum will be determined by the activation of dosimetry foils, and for most of the suitable reactions the cross-sections have been measured up to now with unacceptable uncertainty or never at all and the evaluated data files are in disagreement. Gamma production for heating calculations and multiple particle emission to evaluate displacements per atoms (dpa) are absolutely needed. A lot of cross section measurements can be performed by activation technique, and the NFS fluxes are particularly well adapted for fast neutron measurements. They may represent the source for a further step in fastneutron physics formerly developed around 14 MeV, next to that done systematically at IRMM/JRC in Geel through extension into the incident energy range MeV. The even larger importance of the MeV energy range is related to the fact that the pre-equilibrium (PE) processes become dominant just at these energies, being then fully responsible for all other reactions at higher energies. On the other hand, above MeV turn out to be important also the multiple PE processes, adding more questionable points to the understanding of the corresponding experimental data. Therefore, the energy range to be available at SPIRAL-2/NFS is the one most important for the progress of nuclear models which still have many unclear elements. One of them, e.g., concerns an effective nuclear potential depth introduced within LANL-Duke University studies for the PE description, which is changing strongly its value just above the neutron incident energy of 30 MeV [C. Kalbach, Phys. Rev. C 62, (2000)]. These studies have shown a hard change of the assumptions concerning this basic quantity, within the period , so that additional analyses are strongly requested. Moreover, the extension to 100 MeV of the neutron energy range foreseen formerly within the actually starting Phase 2 of the n_tof facility at CERN has been postponed indefinitely, while the measurements performed until now at Louvain-la-Neuve above 33 MeV have been not as systematically and extensive as those still under way at IRMM/JRC up to 21 MeV and proving the usefulness of their extension to at least 40 MeV. 3

4 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 Cross-sections of deuteron induced reactions on structural materials are also needed, and a deuteron activation station in the converter cave could ensure such measurements up to 40 MeV. On the other hand, needs of additional data related at the same time to neutrons and deuterons have been pointed out in the case of (n,d) reactions, unfortunately systematically measured by particle spectroscopy only in the end of 70s at LLNL-Livermore and Ohio University. Their analyses are hardly dependent however by the knowledge of the deuteron optical potential, for whose study there have been obtained spare elastic-scattering data only above 8 MeV. Therefore, in order to provide data requested by the formerly mentioned nuclear engineering design objectives, these both deuteron research goals could be considered at SPIRAL-2/NFS time-of-flight facility. It is clear that the need of studying neutrons induced reaction is very strong. Most of the experiments needed in the pre-cited topics can not be realized today because of the lack of laboratory with well adapted characteristics: neutron flux, energy, purity and/or possibility of using actinide targets. The Neutrons For Science (NFS) facility design described in this technical proposal have all these required features. Several studies and key experiments have already been identified and are shortly described in the text below. For some of them the experimental detection set-ups exist and could start just at the beginning of SPIRAL-2. Last but not least, NFS will open GANIL to a new physicist community. 1.2 Gamma production cross sections for inelastic scattering and (n,x) reactions Inelastic neutron scattering (n,n γ) and (n,xn) cross sections on major construction materials, fuel and inert fuel components, moderators and coolants are among the most needed in nuclear technology applications. This includes ADS, GenIV reactors, fusion devices and security installations. The primary importance of these reactions lies in their impact on the energy distribution of the fast neutron spectrum, thereby significantly altering most reaction rates through the so-called indirect effect. Significant uncertainties in estimated k eff values for fast systems result, as a consequence, from the large uncertainties associated with these cross sections. In some cases, even precision estimates of k eff in light water reactors can be affected. Other issues where inelastic scattering and (n,xn) reactions play an important role concern radiation shielding and, as a consequence, damage to containment structures, as well as radiation heating. In the latter case the gamma-production cross sections themselves are of main importance. Naturally, (n,xn) reactions additionally play an important role in neutron multiplication, e.g. for ADS and for the controlled fusion devices. It should be noted that measurements of inelastic and certainly of (n,xn) cross sections in the energy range of importance - threshold ( 5 MeV) up to 20 (30) MeV - are considerably more sparse than total, fission and capture cross section measurements. The origin of this sparseness and the often considerable scattering of the available data lie in the technical difficulties associated with direct measurements on the basis of neutron detection and the availability of neutron sources with suitable conditions. This energy range is particularly interesting because it corresponds to the opening of new reaction channels like (n,3n),(n,4n), (n,5n) Today, for these reactions on actinides, the data bases rely mostly on models. A typical example is the 233 U(n,2n) reaction competing with the fission of 233 U in the thorium based fuel cycle in a fast neutron flux. As illustrated in Figure 1, the three main data bases disagree strongly in this particular case. Due to the activity of the 233 U, this experiment is a challenge and the use of highly segmented 4π gamma detector like AGATA would probably be necessary. 4

5 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 Figure 1: Evaluated 233 U(n,2n) cross-sections from 3 major data library files. Figure 2: (n,2n) cross-section evaluations and existing data for 241 Am. Equally, the (n,2n) reaction cross sections are poorly known for 241 Am (see Figure 2), being one of the most important minor actinides found in the used nuclear fuel and therefore to be transmuted/to be incinerated. Thanks to the high neutron flux available, such measurements could be performed at NFS by activation technique and off line gamma spectroscopy. 1.3 Charged particle production The production of light charged particles in neutron induced reactions is a particularly poorly known process. It is of primary importance in systems with large high energy neutron fluxes as fast reactors, ADS, IFMIF, etc. The production of hydrogen and helium gas is critical for material behavior. In the spallation projects, the life time of the target and of the target window in particular is strongly related to the damages induced by hadrons and high energy neutrons, both contributing to the damage rates at equal proportions. Gas production is of particular importance in this process. The fuel and its containments in nuclear reactors, all the structural material in the fusion technology like ITER need reliable data on H and He production. Equally, in the neutron therapy treatment of cancer about 40 % of the cell damage is due to neutron-induced emission of light ions. As long as physics is concerned, in the MeV range the pre-equilibrium process plays an important role in the emission of composite particles. Some reaction codes can calculate integral cross-sections but the prediction of double differential cross-section is still not accurate. In this context, the charged particle production data at high energy are scarce and the measurement of double differential cross-sections in neutron induced reactions is needed to constrain and validate the physics models. 1.4 Fission Fission plays an essential role in nuclear reactor physics and in the transmutation of actinides. New reliable measurements of the fission cross-section, especially for minor actinides, are still needed. The isotopes of main interest are those of Np, Am and Cm, but contemporary studies include those of U, Th and Pa that are of interest due to the Th/U fuel cycle. In the framework of actinide incineration with accelerator driven systems, for which the neutron spectrum may be locally hard, Nuclear Energy Agency is recommending to extend the databases for nuclear data at least up to 40 MeV. In addition, such cross section measurements, and an accurate study of their energy dependence in particular, would greatly help nuclear model development (fission barrier systematic, level densities, transition states and fission modes, etc.). The intense neutron flux could also allow a direct measurement of cross-sections which at present are only accessible via direct reactions (also known as surrogate reactions). Up to now, surrogate measurements have shown to be rather successful for determining fission cross sections. 5

6 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 This is rather surprising as surrogate measurements are based on the assumption that the angular momentum distributions populated in the direct and in the neutron-induced reactions are equivalent. Neutron-induced measurements at NFS would allow validating the surrogate reaction method, which is the subject of intense debate currently, and thus improving our understanding of fundamental aspects of nuclear reactions. In addition, the NFS facility would allow reaching neutron energy range not achievable by the surrogate technique, typically above 10 MeV. The prompt and delayed fission gammas and neutrons are of special importance too not only for nuclear energy but also for security projects on detection of nuclear materials on borders. The neutron multiplicity distribution and fission neutron characteristics are of particular interest in the design of fission reactors and ADS. To our knowledge, no high energy data exist for minor actinides which are expected to be burned in ADS. The detailed proposal on fission fragment measurements (A and Z distributions) at SPIRAL-2 is one of the key experiments described in the next section. This experiment is interesting not only for reactor applications (decay heat estimates, core poisoning, safety issues, long-lived nuclear waste, etc.) but also for fundamental physics (deformed shell effects in the fragment distribution). All these studies require very thin actinide samples because of their high alpha activity and to allow the fission fragment detection. As the alpha-activity would have to be limited to 1 MBq this implies that measurements are feasible for 230,232 Th, 231 Pa, 233,234,235,236,238 U, 236,237 Np, 239,240,241,242,244 Pu, 243 Am and 245,246,247,248 Cm as well as for the beta emitters as 233 Pa and 241 Pu. The main advantages of Spiral-2 (white spectrum) over the quasi mono-energetic sources, currently employed to study these reactions, are that the systematic measurements can be performed at the same installation in a reasonable time and in a very broad range of excitation energy without losing good neutron energy resolution and a more reliable control of low energy contaminant neutrons. 1.5 Neutrons and charged particles activation reactions Neutron and charged particles induced activation reactions produce radionuclides decaying with emission of γ rays or β - particles during the time varying from milliseconds to million of years. From the basic point of view the measurements of such cross sections are important for the verification of the nuclear reaction and nuclei structure theoretical models, which also describe many other reactions. The practical applications of activation reactions cross sections have a special importance, since the induced radioactivity determines the nuclear safety issues such as γ-dose rate, heat generation, radioelement transportation and waste management. The new SPIRAL-2 facility makes it possible to perform two types of activation measurements: employing the neutrons distributed in the broad energy range ( white spectrum) or mono-energetic neutron sources as well as direct proton and deuteron beams. The white energy neutron fluxes enable the integral validation of the activation cross section and direct observation of the most dominant γ-ray emitters generated in the investigated material. At NFS the neutron energy will extend up to 40 MeV, total flux in the 1 cm diameter activation foil located at 5 cm from the source will reach n/cm 2 /s, exceeding by a few orders of magnitude the intensity available at facilities of FZK in Karlsruhe and Nuclear Physics Institute in Řeź. Monoenergetic neutron source with variable energy enables the measurements of activation cross sections as a function of energy and thus direct validation of the theoretical model calculations and evaluated libraries for a given energies, i.e. leading to excitation functions of the reactions of interest.. The proton and deuteron induced activation reactions have a great interest for the assessment of induced radioactivities in the accelerator components, targets and beam stoppers as well as isotope production for RIB facilities or nuclear medicine. In particular, the IFMIF facility as well as the SPIRAL- 2 facility itself needs such a data for estimation of the potential radiation hazards from the accelerating 6

7 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 cavities and beam transport elements (Al, Fe, Cr, Cu, Nb) and metal and gaseous impurities of the Li loop (Be, C, O, N, Na, K, S, Ca, Fe, Cr, Ni). The cross sections are needed in the energy range from the activation reaction threshold 2-10 MeV up to 40 MeV both for deuterons and protons. 1.6 Neutron detector characterizations The energy range and time characteristics of NFS will be a unique tool for neutron detectors characterizations. Neutrons detectors are used in numerous topics from academic research to applications like nuclear industry or dose measurements. NFS will allow performing characterizations of detectors which are usually used in intense and high energy neutron environment. This is the case, for example, for Inertial Confinement fusion experiments where such nuclear detectors help to diagnostic the implosion qualities (neutron yields, neutron energy spectra, edc ). NFS neutron beams will also help in providing technical characterizations of these detectors such as their homogeneity, linearity and sensitivity due to high intensity beams and in wide energy range. An important aspect is that we will be able to test these diagnostics in their entireties in the realistic environment (high-energy high-intensity fluxes), i.e. close to the final design conditions. 1.7 Key experiments Several key experiments related to the previously exposed topics have already been identified. For three of them, a detailed description was given in the appendixes 2 to 4 of Letter Of Intent. Some of these experiments are ready to be performed, the experimental set-ups exists and similar experiments were already realized and performed in other laboratories. However, these experiments can not be pursued in existing facilities because the energy range of interest and/or the neutron beam intensity are not adequate Actinides Fission-Fragment Yields (see Appendix 1) This experimental programme aims at measuring the properties of fission-fragment yields for various actinides of interest for new fuel cycle or new type of reactors. This programme is also of considerable interest for fundamental nuclear physics, as nuclear deformed shell effects and nuclear matter viscosity govern fission fragment distribution properties. As discussed in appendix 1, a rather simple experimental set-up based on energy and time-of-flight measurement associated to X-rays detectors would allow for identifying in mass, charge and kinetic energy of the fission fragments with a sufficient accuracy. Only the high intensity neutron beam of NFS renders the use of such simple setup possible, as the very thin targets and the limited solid-angle induced by long flight path for time-offlight accuracy is compensated by the high intensity. As an example, compared to the future n_tof spallation neutron source at CERN (placed at 20 m), NFS will provide better statistics by two orders of magnitude in the same measurement time. A second alternative for the charge and mass identification is envisaged in the frame of a two-arm spectrometer, based on time-of-flight, energy-loss, range, and energy measurement for both fissionfragments in coincidence. This spectrometer exists (STEFF), and is under commissioning at Manchester University (see Figure 3). It has the advantage that it is based on gas detectors, and therefore does not present any damage due to neutron irradiation. It is planned to build a dedicated spectrometer, founded by the UK STFC. 7

8 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 Figure 3: View of the STEFF detector (n,x) cross section measurements by in-beam γ-ray spectroscopy (see Appendix 2) Threshold nuclear reaction such as (n,xn), (n,xp), (n, α), (n,xnα) etc have an important contribution in structural material safety issues as mechanical properties and health risks for operators in accelerator driven systems or new reactors (Generation IV), as the neutron spectrum is harder than in conventional reactors. Experiments using gamma spectroscopy to determine the residual nucleus of the reaction have the advantage to measure different exit channel (neutron or charged particle) with one type of detector. This technique was already successfully used at other neutron facility (Gelina, Geel, Belgium and Louvain-la-Neuve) with neutron energy complementary to the energy covered by NFS. NFS facility will open the study of unknown channels such as (n, 3n), for example, on 232 Th and on Pb isotopes. In addition, the monoenergetic beam of NFS present important advantages compared to other facilities, as the high proton intensity allows for low repetition rate, providing a better background subtraction Neutrons and charged particles activation reactions (see Appendix 3) NFS facility will allow for measuring activation cross section in white and monoenergetic spectrum. The high intensity neutron beam will allow for long lived radioactive isotope production, or the activation of rare materials, or exotic activation channels. In addition, the excitation function of the deuteron-induced reactions could be measured up to 40 MeV, the energy range of interest of IFMIF studies Study of the pre-equilibrium process in the (n,xn) reactions (see Appendix 4) The (n,xn) reactions (induced by one neutron and emitting x neutrons) are the predominant nonelastic process reaction for fast neutrons. For example in the 7-20 MeV energy range the (n,2n) reaction is one of the most important nuclear-reaction channels for non-fissile nuclei. The simulation codes are composed of several models (optical model, direct interaction, pre-equilibrium and evaporation) to reproduce the whole reaction mechanism. Among these processes, the pre-equilibrium emission is by far the least known. Some of the existing models are able to reproduce the integrated observables but fails at describing differential measurements. An experimental program aims at measuring measure the energy spectra of neutrons in the (n,xn) reactions in coincidence with the neutron multiplicity could be realized at NFS. The results will give strong constrains to validate and improve the pre-equilibrium models. 8

9 Technical Proposal for the SPIRAL 2 instrumentation 30 May Description of the proposed equipment(s) 2.1 Design specifications NFS is composed of 2 facilities: an irradiation station for proton, deuteron and neutron induced reactions and a very well collimated neutron beam. Two areas can be distinguished into a targetconverter room and a TOF hall. In the first one there is the beam line extension, the converter for the production of neutrons and a set-up to perform measurement by activation technique in neutron, proton and deuteron induced reactions. The second room is a much bigger hall placed at zero degree in respect with the ion beam line and separated from the converter room by a thick concrete shielding wall pierced of a channel to define the neutron beam. This hall is dedicated to experiments with a pulsed neutron beam in the 100 kev 40 MeV range. The requested design specifications are the following: The neutron beam: The NFS neutron beam should allow to perform time-of-flight (TOF) experiments to measure the neutron energy with a resolution better that 5% on the overall energy range (from 100 kev to 40 MeV). High sensitivity detectors like HPGe or neutron liquid scintillator detectors should be used. These requirements involve a flight path between the neutron production and the sample location of at least 20 m and an ion burst time resolution on the converter not worse than 1ns. To avoid the neutron overlap from consecutive bursts the ion beam frequency should be variable from 1 MHz to 1 khz (to be compared to the LINAG primary frequency of 88 MHz). The use of detectors in a compact geometry around the target involves that the neutron beam must be very well defined in space, and a very efficient collimator between the converter room and the TOF hall is absolutely needed. Two types of neutron spectra will be available at NFS, a continuous one produced by the interaction of the deuteron beam on a thick stopping ( 1 cm) converter and quasi-monokinetic ones produced by the 7 Li(p,n) 7 Be reaction on thin target The Irradiation facility: The measurement of cross-section by activation technique is also one of the NFS proposals. The element to be studied is irradiated by the neutrons, protons or deuterons. Then the activity is measured off-line in order to obtain the reaction cross-sections of interest. In the case of proton or deuteron induced reactions the sample is placed under vacuum in a dedicated reaction chamber. The ion beam charge is measured on a Faraday Cup downstream of the sample. In the case of neutron induced reactions, the sample is placed in air a few centimeters downstream the converter. The flux is measured by a neutron monitor. A dedicated pneumatic air system will allow sending rapidly the samples back and forth between the converter room and the TOF hall where γ spectroscopy will be done with a germanium detector. Hence it will be possible to measure short lived isotopes activations, i.e. well below 1 s half-lives. 2.2 Simulations Neutron spectra: The neutron spectra in deuteron and proton induced reaction on thick and thin converters have been measured previously (Figure 4 and Figure 5). The first one shows the deuteron break-up reaction on 1 cm thick converters. The impinging deuteron is fully stopped in the target and a continuous spectrum is generated with an average energy of about 14 MeV at 0 degrees. We can see on Figure 4 that the use of beryllium instead of carbon allows gaining a factor of 2 in the neutrons yield. The second production mode is obtained by the 7 Li(p,n) 7 Be reaction on a thin target (~1 mm). In 9

10 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 this case, quasi-monoenergetic neutrons are produced at 0 degree with energy E n E p 2 MeV (see Figure 5). The energy resolution and the low energy tail depend mainly on the lithium target thickness. Figure 4 : Neutron spectra in deuteron induced reaction on thick converters Figure 5 : Neutron spectra produced by 7 Li(p,n) 7 Be reaction Energy resolution The energy resolution by the TOF technique is given by equation below. E E = t L ( γ + 1) + t L 2 γ Equation 1 2 Where γ is the Lorentz factor, L the fight path, t the time-of-flight, L and t the associated uncertainties. The time resolution ( t) is a combination of the detector time resolution and of the beam burst duration at the converter point. Two beam line extensions have been investigated, with a cavity of β=0.07 to bunch the beam (black curves) and without (red curves) (see Figures 6 to 9). Calculations were performed for proton and deuteron at maximum energy and at 10 MeV for deuterons. The results show that with the buncher the time distribution (FWHM) is in all cases better than 1 ns. But at high energy, namely 40 MeV and 33 MeV for deuterons and protons respectively, the time distribution is never worse than 1.2 ns FWHM even without the buncher. Figure 6 : 40 MeV deuteron beam time distribution on the converter. Figure 7 : 33 MeV proton beam time distribution on the converter. 10

11 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 Figure 8 : 2 MeV proton beam time distribution on the converter. Figure 9 : 10 MeV deuteron beam time distribution on the converter. By taking into account the beam time resolution, the flight path and typical detector time resolutions, the energy resolution as a function of the neutron energy can be calculated with Equation 1. Two distances have been considered. The nearest from the converter is 5 m and it roughly corresponds to the collimator exit and the highest fluxes. The farthest is 20 m and will be used for high resolution measurements. It can be observed (see Figure 10) that with fast detectors like scintillators or silicon junctions the energy resolution at 40 MeV is better than 1 % and even for slow detectors like HPGe ( t 8 ns) the energy resolution remains below than 5%. Energy Resolution (fwhm %) 8% 4% 0% 5m 1,5 ns 20 m 1,5 ns 20 m 8 ns Energy (MeV) Figure 10 : Energy resolution for several flight path and detector responses. For the high energy ion beam the buncher is not obligatory because it does not degrade the energy resolution. However, the buncher should be an optional upgrade of NFS because it will be needed for the lower energies or for some special studies, e.g. using very fast neutron detectors Neutron overlap When using the TOF technique the overlap of neutrons emitted by different ion beam burst has to be taken into account very carefully. The beam frequency required to avoid this neutron wrap around depends on the flight path (see Figure 11). To cover the whole energy range a minimum neutron beam period of 1 µs is needed for flight path of 5 m, and longer period are needed for higher distance. It corresponds to a frequency of 1MHz to be compared to the own LINAG frequency of 88 MHz. A fast beam chopper is absolutely required to select one unique burst over N with 100<N< Note that the ion beam intensity on the converter is reduced by the same factor and the intensity in the converter room will be limited to 50 µa. 11

12 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 n/ns (a. u.) 1E+11 1E+9 1E+7 1 m 5 m 20 m 1E TOF (ns) Figure 11 : Time of flight distribution of neutrons produced in 40 MeV d + Be reaction for 1 m, 5 m and 20 m path lengths Neutron flux in time-of-flight hall By taking into account the neutron yield production, the beam division and the flight path, the neutron flux can be evaluated and compared to other major time-of-flight facilities in the world namely n_tof at CERN, WNR at Los Alamos and Gelina at Geel. A flight path between 5 and 20 m is available at NFS allowing high intensity flux (5 m) and high resolution measurement (20 m). We can see on Figure 12 that between 1 and 35 MeV NFS is very competitive in terms of average flux in comparison with the 3 neutron beam facilities. It has to be stressed that it is mainly due to the high repetition rate (the flux by per single deuteron burst is clearly lower). Moreover, NFS presents some advantages thanks to the neutron production mechanism itself. In spallation sources (case of n-tof and WNR), the high energy neutrons (up to hundreds MeV), can present challenges for collimation and background. Secondly the gamma-flash, which is known to be very penalizing especially because it induces a dead time, will be probably strongly reduced at NFS. Note that high energy gammas are produced by π 0 decay in spallation sources based on high energy proton accelerator (like CERN) and by bremsstrahlung process in photoneutron sources based on electron accelerator (like Gelina). E dφ/de (cm -2.s -1 ) 1E+8 1E+6 1E+4 WNR n-tof NFS 5 m 50 µa NFS 20 m 12 µa Geel-fast Geel-Mod 1E+2 0,001 0,01 0, Energy (MeV) Figure 12 : Neutron flux at NFS compared to 3 existing facilities. 12

13 Technical Proposal for the SPIRAL 2 instrumentation 30 May Neutron flux for activation measurements Neutron induced reaction cross-section measurements by activation technique are also envisaged in the converter room. The samples to be irradiated have to be placed as close as possible to the converter in order to maximize the available neutron flux. No time structure is required in this case. Nevertheless, the maximum ion beam intensity will be limited to 50 µa in order to reduce the radioprotection constraints and to make the target-converter design easier. Both continuous and quasi-monokinetic spectra can be used. For quasi-mono-energetic neutrons the lithium foil is fixed on a beam stopper made of carbon to stop the proton beam with a very low neutron production rate. In Figure 13 the neutron flux available close to the converter (5 cm) for a beam intensity of 50 µa is presented. Note that even at this reduced intensity the available neutron fluxes are much higher than in the existing neutron facilities in Europe, where irradiations by high energy neutrons are performed. 1E+12 E dφ/de (cm -2.s -1 ) 1E+11 1E+10 1E+09 1E MeV p + 7Li 33 MeV d + C 33 MeV d + Be 40 MeV d + Be Energy (MeV) Figure 13 : Available neutron fluxes for activation experiments Thermal simulations for the converters The maximal power deposition on the NFS converter is about 2 kw, corresponding to a 40 MeV deuteron beam of 50 µa. The area of the deuteron beam spot is approximately of 1 cm 2. Preliminary thermal calculations have been performed with a rotating target of 1 cm thick, 30 cm in diameter and a rotating speed of 100 turns/minute. The converter is located inside a light water cooled cupper chamber (see Figure 14). The simulations (see Figure 15) show that the maximum temperatures are of 630 C and 640 C for graphite and beryllium targe ts respectively. These temperatures are much lower than the melting point for these 2 elements. Further simulations have to be performed to optimize the target design. The same result could probably be obtained with a disk of even smaller diameter and by increasing the rotating speed. Due to the relatively low temperature of the converter, the cupper envelope could be cooled by a pressurized gas system, avoiding the water activation problems. The possibility of using a non rotating beryllium target cooled by liquid or gas has to be studied too. In brief, no major technological challenges are expected for this converter target. 13

14 Technical Proposal for the SPIRAL 2 instrumentation 30 May 2008 Figure 14 : Schematic design of the rotating target for thermal calculations. Figure 15 : Temperature (K) of the Beryllium (left) and carbon (right) converters (see the text for details) Collimator design One of the main specifications of the NFS neutron beam will be its quality in terms of the spatial distribution. Actually, detectors which are very sensitive to the neutrons, like High Purity Germanium or liquid scintillator detectors, have to be used very close to the physics target. A neutron beam diameter of only a few centimeters at the TOF hall is absolutely required. The design of the wall and the channel between the converter room and the time-of-flight hall has to be studied very carefully Neutron beam dump The NFS neutron beam dump will be part of (or very close to) the experimental area. Besides obvious shielding consideration, especially at forward angles, the beam dump should generate as low as possible neutron and photon (gamma or X-rays) back-scattered background in the experimental area. The beam characteristics (spectrum, flux, size) affect the beam dump design. The other 14

15 parameters to consider for the beam dump itself are geometry and materials. Such beam dumps exist in different laboratories hosting pulsed neutron beams. The main characteristics of the beam dumps used at three different facilities hosted by LANSCE (Los Alamos, USA), TSL (Uppsala, Sweden), and CERN (Genève) have been reviewed (see appendix 5). Conventional solutions adopted at other facilities could also respond to NFS needs. However, a detailed study mainly based on neutron transport calculations will be achieved in the near future. 2.3 Design and construction scheme Figure 16 depicts the two parts of the NFS facility. In the first one there is a primary proton/deuteron beam line extension and the target-converter for neutron production. An irradiation station for sample irradiation by proton and deuteron for activation measurements is placed upstream of the converter point. When the 7 Li(p,n) reaction is used to produce quasi-monokinetic neutrons, the proton beam crosses the thin converter and a magnet downstream of the converter point deflects the beam to a beam dump. The second one is the hall dedicated to the time-of-flight measurements. It is separated from the converter room by a collimator composed of a thick concrete wall pierced of a channel defining the neutron beam profile (see the Figure 17). The size of the neutron hall downstream of the collimator will be around Length~20 m X Width~6 m. This size would allow using large experimental set- at distances ranging from 5 up to 25 m. This flexibility is very interesting in terms of flux and energy measurement resolution. In the first part of the hall (corresponding to the shorter flight path) a specific compartment is needed in order to use non-sealed actinide targets (for fission fragment measurements). This compartment, being of the reduced volume ( 50 m 3 ), will be connected to the nuclear ventilation system for safety purposes. When using stable targets, the separating wall will be dismounted to not disturb other experiments using the entire ToF hall. Figure 16 : Scheme of the NFS facility with the converter room and the TOF hall. 15

16 Figure 17 : View of the converter room with the beam line extension, the converter, the clearing magnet, the beam dump and the collimator. A station dedicated to measurements by activation technique for neutron, proton and deuteron induced reactions is also envisaged. No time structure is required in this case and the maximum ion beam intensity will be limited to 50 µa in order to reduce the radioprotection constraints and make the target-converter design easier. For quasi-mono-energetic neutrons replacing the deviation magnet by a beam stopper made of carbon seems to be a more adequate solution since in this case the sample could be positioned closer to the Li target. We note that this technique is successfully used by NPI Řeź. In Figure 13 the neutron flux available close to the converter (5 cm) for a beam intensity of 50 µa is presented. Note that even at this reduced intensity the available neutron fluxes are much higher than in the existing neutron facilities in Europe, where irradiations by high energy neutrons are performed. The LINAG offers the possibility of delivering proton and deuteron beams with variable energy up to 33 and 40 MeV respectively. Thus a charged particle irradiation station can be created without major difficulties. To carry out such measurement the following equipment will be needed: A vacuum chamber (cooled and equipped by the beam-charge-monitor cup) at the end of ion tube for accommodation of foils to be irradiated by the beam A charge-to-digital converter for measurement the whole beam charge and time profile. A system for the activated foils extraction and their transportation to measuring station. A calibrated high purity germanium (HPGe) detector based, spectrometry chain. Set of standard γ-ray sources with known activity (uncertainty 1-2%) for eventual calibration of the HPGe detectors 2.4 Calibration procedures Neutron monitors are absolutely required in order to determine the absolute cross sections, to measure both the energy spectrum and the geometrical distribution of the beam. It will also allow to normalize one measurement with respect to another one. Although some experiments will have their own calibration system, a neutron monitoring in place at NFS at any time is absolutely needed. During the commissioning phase, energy spectrum, flux and geometrical characteristics will be determined as a function of the ion energy and the target-converter. Both absolute calibration and monitoring during 16

17 an experiment will be performed. Unfortunately, a perfect detector covering the whole energy range with high efficiency and time resolution does not exist. Several solutions have been investigated and are developed in the appendix 6. No special R&D is required for such purpose but a calibration experiment in a neutron field reference laboratory of some of the detectors (e.g., liquid scintillator) will be needed. 2.5 Trigger, DAQ, Controls Standard equipment for ion beam line is required: beam profiler, vacuum and temperature sensors (already included in the beam line extension definition). 2.6 Target requirements Targets of stable and radioactive isotopes are required at NFS. The use of actinides is of prime interest for several topics. Two types of targets are distinguished: Thick targets (from several grams to several hundreds of grams) are used for (n,xn), (n,n γ) reactions. These relatively massive samples are sealed with ISO label, and therefore present almost no risks of contamination. The remaining limitations are the irradiation risk and the legal detention authorization. Thin targets (several milligrams) are used for the experiments in which charged particles (fission fragment or light charged particles) are detected. The sample is used in a reaction chamber under vacuum and the main risk is due to contamination especially in accidental situations (fire, drop of the target, degradation during irradiation and contamination of the reaction chamber). The list of the actinides, potentially to be used at NFS, is given in Table 1. Thin samples Thick targets Isotope T1/2 (a) M(mg) A(Bq) M(g) A(Bq) 232Th 1,40E ,04E ,22E U 1,59E ,78E ,07E U 7,04E ,00E ,00E Np 2,14E ,30E ,30E U 4,47E ,21E ,73E Pu 2,41E ,15E ,89E Am 4,33E ,34E Am 7,37E ,71E Cm 8,50E ,17E U 2,46E ,15E U 2,34E ,20E Pu 6,56E ,20E Pu 3,75E ,29E Pu 8,00E ,39E Cm 4,76E ,65E Cm 1,56E ,72E Cm 3,48E ,66E Pa 3,28E ,73E Pu 1,44E ,91E Cf 3,51E ,56E Cf 9,00E ,93E+08 Table 1: List of the required actinides: both thin and thick target characteristics are provided. 17

18 2.7 Beam requirements The primary ion beam line extension is required from the accelerator high energy line room to the converter room. The beams of interest are proton and deuteron at energy from 2 MeV to maximum LINAG energy (33 MeV and 40 MeV for protons and deuterons respectively). The RFQ of A/Q = 1/3 is fully adapted for this purpose. Due to pulsed beam requirement and safety issues the intensity of the primary beam will be limited to 50 µa. In order to use the neutron converter and the irradiation station, the focal point should be tunable on 2 m along the beam axis. A beam spot on the converter of 1 cm in diameter at FWHM is required. A burst time resolution no worse than 1 ns at FWHM is request for high energy deuteron and proton beam. We have already mentioned that no buncher is needed for the first experiments at high energies but it is still an optional for an upgrade of NFS. The beam line which is detailed in the red rectangle of Figure 18 allows reaching all the beam line requirements. Figure 18 : NFS High energy beam line (in the red rectangle). A dedicated pulsed beam is required for TOF measurement and fast chopper is absolutely needed in order to select one burst over N with (100 < N < 10000). It seems not possible to switch the beam at the LINAG exit within the 11 ns separating 2 bursts, so the burst selection should be realized at low energy before and/or after the RFQ. 3 Implementation and Installation 3.1 Experimental hall and Annex facilities As mentioned previously the experimental hall has to be placed at zero degree in respect to the ion beam line. The characteristics are given in section 2.3. Together with the two main experimental parts, two rooms are necessary: one for the data acquisition and one for the electronic crates and modules. 3.2 Detectors-Machine interface 3.3 Assembly and installation The access to the experimental hall has to be large enough to receive large detection set-ups like 4π detectors or individual shielding structures for neutron detectors. The weight of some detectors or shielding can reach 2000 kg. 18

19 4 Commissioning (work plan, cost, necessary manpower and other resources) A commissioning period will be needed to test all the components of the facility for safety issues as well as for the physic characteristics. The following points concerning the safety will have to be addressed: - Converters: working temperature, mechanical properties if rotating target is used, off-line tests of the materials and components, performance of the cooling system - Neutron beam dump - Neutron and gamma detectors for radioprotection - The beam properties like time resolution, focalization and intensity measurements The physical properties will have to be measured in the commissioning phase: - Spatial and time characteristics (resolution and chopped frequency) of the ion beams. - Energy spectra and spatial extension of neutron beams in the TOF hall (see section 2.4). 5 Operation (running cost, necessary manpower and other resources) The duration of a typical experiment will be from 1 to 15 days. It seems by now impossible to run in parallel with other LINAG users. The very low neutron flux attenuation in the thin target allows running several experiments at the same time at several distances in the TOF hall. The users should need the same neutron spectra and time structure. The physicists usually come with their own experimental set-up and a mounting period of several days is necessary before the experiments. Each converter manipulation will require technician and radioprotection supports. To reduce the number of intervention the accepted experiments will be planned during periods with identical specifications. The periods could be divided as follows: Period 1: Irradiation experiment Period 2: TOF experiment with lithium converter Period 3: TOF experiment with thick converter Period 4: Use of actinide target Period 5: Use of stable target 6 Safety issues and proposed solutions See document "Risk analysis for AEL". The study on the safety issues performed by the CEA/IRFU/Senac is given in Appendix 7. The external prompt dose rate due to neutron production has been studied on the basis of a preliminary conceptual design of a deuteron-beryllium converter (conservative approach). The results of calculations are described in this document and the safety and radioprotection issues discussed in terms of recommendations. Residual γ dose rates have not been studied yet but should be significant in the converter room mainly right after irradiation. This study was realized with basic hypothesis: A fixed beryllium converter is in the center of the converter room (a rotating target will not modify the neutron dose values). The results show that with 19

20 this configuration the wall thickness should be enlarged up to 2.5 m for the converter cave and up to 1 m for the TOF hall. Note however that the recommended thicknesses can be reduced with the following design optimizations: A local shielding surrounding the converter could minimize the concrete wall thicknesses and the activation of the different equipments located in the converter room. An optimized design of the collimator and of the neutron beam dump (also required for experimental background constrains) will reduce the neutron dose outside of the TOF hall and allow reducing the wall thickness. The simulations were performed to reach a neutron dose less than the public limit (0.5 µsv/h) outside of the hall. A higher value could be acceptable depending on the classification of the neighboring rooms/area, namely public or controlled zone. 7 Organizations and Responsibilities 7.1 Management Board M. Aïche, IN2P3 V. Blideanu, CEA/DSM P. Dessagne, CNRS F.R. Lecolley, IN2P3 X. Ledoux, CEA/DIF F. Negoita, NIPNE F, Rejmund, GANIL S. Simakov, FZK 7.2 WBS - work package break down structure Task Number Task name Description of Task Participating Members (coordinators in bold) 1 Converter Simulations and design FZK NPI CEA 2 Collimator Simulations and design CEA/DIF CEA/DSM 3 Neutron beam dump Simulations and design CEA/DIF NIPNE 4 Monitoring systems Simulations LPC IPHC CEA/DSM 5 Irradiation station Design FZK NPI 6 Deviating magnet Simulations IPNO GANIL 7.3 Schedule for the signature of Memorandum of Understanding The signature of the Memorandum of Understanding is planed during the last 6 months of the SPIRAL-2 PP project. 20

21 8 Planning Equipment End of R&D Start of construction End of construction Beam line extension Already done Same date as LINAG high energy lines Converters 12/ / /2010 Collimator 06/ / /2011 Neutron beam dump 12/ / /2011 Monitoring system 06/ / /2011 Irradiation station 12/ / /2011 Deviating magnet 12/ / /

22 22

23 Appendix 1: Actinides Fission-Fragment Yields (Extract from the Letter Of Intents) B. Jurado, M. Aïche, G. Barreau, S. Czajkowski CEN Bordeaux-Gradignan F. Rejmund GANIL, Caen Our aim is the systematic measurement of the isotopic fission-fragment yields of different actinides such as the ones mentioned in the scientific case section over a neutron-energy (E n ) range from around 1 MeV to 40 MeV. The intense neutron flux produced by the NFS facility of SPIRAL 2 offers a unique possibility for performing these measurements. 1. Motivation: The existing experimental data on fission-fragment yields are rather scarce and consequently there does not yet exist a calculation of these yield distributions of sufficiently accuracy. A recent publication by Madland [ 1 ] shows that even for the three main actinides involved in current nuclearfission reactors, 235,238 U and 239 Pu, these data are sufficiently complete at only two incident neutron energies: thermal and 14 MeV. Concerning the nuclei involved in the 232 Th fuel cycle and the minor actinides (Np, Am and Cm isotopes), the situation is even worse as some of these nuclei are highly radioactive [2]. Fission fragment distributions of minor actinides are essential for the design of nuclear reactors for the incineration of nuclear waste where significant quantities of minor actinides will be introduced into the reactor fuel. In general, the operation of a reactor is strongly affected by the fissionfragment isotopic yields. In fact, accurate fission-fragment isotopic distributions are essential for estimating the radiotoxicity of the used fuel and the decay heat. The decay heat is the heat produced by the decay of radioactive fission products after a nuclear reactor has been shut down. Decay heat is the principal reason of safety concern in Light Water Reactors. It is the source of 60% of radioactive release risk worldwide. Therefore, it is of high importance to precisely know the amount of decay heat in order to assess core and containment cooling strategy during abnormal event. Besides, some fission products like e.g. Xe, Sm, Eu and Gd isotopes can act as neutron poisons in nuclear reactors. Hence, a good knowledge of their production is of great importance for the neutron balance in critical nuclear systems. Finally, the isotopic distributions of the fission products are also of interest for determining the delayed neutrons, which play a vital role in the controllability of reactors. The direct measurement of delayed neutrons being rather difficult, it was demonstrated recently [3] that one of the ways to estimate the number of delayed neutrons in accelerator-driven systems is to reconstruct the neutron spectrum from the experimental isotopic distributions of fission products, their life-times and the energy released in the beta-decay. Fission-fragment isotopic distributions are also of considerable interest for fundamental nuclear physics and in particular for improving our knowledge on the fission process. The fission-fragment yields are strongly influenced by structural effects: spherical and deformed shell gaps as well as pairing interactions are responsible for the most important features of fission-fragment distributions. The evolution of the fission-fragment distributions with excitation energy will allow investigating the change of the potential energy surface of the nucleus, i.e. the excitation-energy dependence of shell effects. The NFS facility offers a unique opportunity for performing a continuous study of the evolution of shell effects from 1 MeV to 40 MeV neutron energy, which is only available for very few nuclei. This issue is of great importance for the production of superheavy elements in low-energy fusionevaporation reactions. Presently, the reliability on predictions is very limited because the parameterisations used for describing the vanishing of shell effects are based on few experimental information. The energy range available at NFS SPIRAL2 corresponds to the crucial range where shell 23

24 effects pass from a full influence to a quasi-complete disappearance. Furthermore, the variation with the excitation energy of the even-odd effects in fission-fragment distributions is directly related to the dependence of nuclear viscosity with excitation energy. Viscosity is a fundamental parameter of nuclear matter which determines the intensity of the coupling between collective and microscopic degrees of freedom. Its intensity and its dependence with the excitation energy are currently the subject of strong debates. Additionally, these measurements are important for defining the fissionfragment beam intensities delivered by SPIRAL 2. As stated bellow, the experimental set-up necessary for measuring fission-fragment yields can be completed in order to measure the fission cross sections of all proposed actinides. Such data are also of great importance for determining the rate of incineration of minor actinides. On the other hand, the evolution of the fission cross sections from 1 MeV to 40 MeV neutron energy gives key information on fundamental quantities as the level densities, fission barrier parameters and transition states. 2. Experimental technique: The measurement we are interested in requires an experimental set-up that enables measuring the kinetic energy, the mass and the charge of both fission fragments. There are different types of detection systems that allow to determine these three quantities. One possibility is to place the target inside a double, position-sensitive ionisation chamber in order to measure the energy of both fission fragments in coincidence. With this information one can determine the total kinetic energy (TKE) and the mass distributions. The determination of the fission-fragment charge represents great technical challenge. In particular, the precise measurement of the charge of the heavy fission fragment is extremely difficult due to its low velocity. The charge can be for example determined by measuring the X-rays emitted by the fission fragments in coincidence with the fission detectors. However, this can only be done provided the neutron beam is well collimated so as to minimize the scattering of neutrons into the X-rays detectors. A better mass resolution can be obtained by measuring, in addition to the energy, the time-of-flight (ToF) of the fission fragments. While the energy measurement gives the mass of the fragments after neutron evaporation, the ToF measurement allows determining the masses before neutron emission. Therefore, an experimental device that combines the measurement of energy and time permits the determination of the average number of emitted neutrons, which is also a very valuable quantity. A possible scenario for this type of measurement would be to place two position sensitive Si detectors at opposite sides of the target to measure the energy and the ToF difference of both fission fragments. By placing one of the two Si detectors at two different positions one can have an absolute ToF measurement from which the masses previous to neutron evaporation can be obtained. The time resolution could be improved by means of a more sophisticated set-up in which one additional detector should be placed close to the target and two other right before the Si detectors. These ToF detectors could be for example micro-channel-plate or secondary-electron detectors which have a very good time resolution and cause very small energy and angular straggling of the fission fragments. To determine the charge of the fission fragments one could surround the setup with X-ray detectors as explained above. Another possibility would be to perform an energy-loss ( E) measurement of the fission fragments. The charge determination is the most critical part of the measurement and important technical developments should be done in order to achieve the best possible charge resolution. The measurements just described need neutron fluxes as intense as the ones expected for SPIRAL 2. This is due to two main experimental limitations: the small thickness of the actinide targets and the reduced detection efficiency. Indeed, as explained above, these measurements require the detection in coincidence of the two fission fragments. Therefore, thin targets are needed so that both fission fragments can exit the sample. Moreover, some of the targets in which we are interested are highly radioactive and only small amounts can be used due to radiation-protection constraints. On the other hand, the detection efficiency of the full experimental set-up is given by the product of the X-ray detection efficiency and the fission-fragment detection efficiency. The X-ray detection efficiency depends mainly on the number X-ray detectors used, which is essentially limited by the available space. In order to have a good time resolution the fission-fragment detectors should be placed at relative big distances with the consequent solid-angle reduction. These restrictions in the quantity of 24

25 matter and solid angle should be compensated by a rather intense neutron flux in order to have reasonable beam-time requirements. Fission cross sections can be determined simultaneously with the fission product distributions provided the neutron flux is known. A possible way to determine the neutron flux with high precision over a wide range of E n (1 MeV to 40 MeV) is to use the neutron-proton scattering cross section as reference reaction. This can be done by adding to the previous set-up a second part consisting of a thin hydrogenised foil and a E-E telescope which serves to detect the protons recoiling from the neutron scattering in the foil. 3. Counting rates: The table below gives the expected fission rates at different neutron energies for NFS and for the future 20 m flight-path neutron beam at n_tof (CERN). We have estimated the neutron flux at 20 m by multiplying the current n_tof flux by a factor 100. This factor roughly takes into account the reduction of the neutrons flight path from 190 to 20 m. For determining the counting rates we have assumed a 1 cm 2 sample of 243 Am of 150 µg/cm 2 thickness, which is representative of the target samples we would use. We have supposed an experimental set-up with a total efficiency of approximately 10 %. The neutron flux per unit of energy has been integrated over an energy interval of 1 MeV. This implies that the systematic determination of the fission yields as a function of energy will be done in steps of 1 MeV. E n /MeV σ f /b Fiss. rate/s -1 NFS 40 MeV d+be at 5 m Fiss. rate/s -1 n_tof CERN spallation at 20 m 1 0,420 1,162E-02 1,017E ,381 8,594E-02 1,910E ,152 3,125E-01 1,042E ,578 3,119E-01 5,347E ,736 5,990E-02 4,414E-04 At energies around 1 MeV the fluxes of the two facilities are comparable. However, at higher neutron energies the flux of NFS is higher than the flux at n_tof. In order to have good statistics we would need at least fission events for a given E n. The previous table shows that for example at E n = 30 MeV this could be achieved in around 20 days beam time at NFS, while at the future n_tof facility at CERN we would need about 2600 days. References: [1] D. G. Madland, Nucl. Phys. A772 (2006) 113 [2] M. Lammer and A. L. Nichols, Proc. Workshop on Fission, Cadarache, France 2005 [3] D. Ridikas et al., Proc. Workshop on Fission, Cadarache, France

26 Appendix 2: (n,x) cross section measurements by in-beam γ-ray spectroscopy (Extract from the Letter Of Intents) Paule BAUMANN, Philippe DESSAGNE, Maëlle KERVENO, Gérard RUDOLF IPHC Strasbourg Arjan PLOMPEN IRMM/JRC Geel In Accelerator Driven Systems (ADS), particle producing reactions such as (n,xn), (n,α), (n,p), (n,xnα), etc, which have a threshold of several MeV, have more importance than in conventional reactors. Indeed in ADS the external neutron flux provided by the accelerator is produced by spallation reactions, which means that its spectrum extends to several hundreds of MeV. On the other hand, the cross sections of these reactions are badly known. This is due in part to the fact that few neutron beams with energies well above 14 MeV were available in the past, but also that no universal method exists. Activation is only possible with mono-energetic beams, and if the final nucleus has a life time in the range of a few seconds to a few weeks. Direct measurement of the neutrons does not allow to disentangle precisely different values of x, nor neutrons coming possibly from fission. While (n,xn) reactions have cross sections reaching hundreds of mbarns, those of charged particle producing reactions are smaller. Thus only some systems could be measured, and data bases are mostly fed by model predictions, especially in the heavy nuclei. These threshold reactions are not only important in ADS, but also in most of the fast reactors of Generation IV. Even, their impact has become evident in the recent years in thermal reactors. Indeed, they produce rare isotopes such as 232 U or interstitial gazes in amounts which become non negligible when the lifespan of the reactor is extended. 1. White beam At Spiral II, a white beam can be produced by the 12 C(d,n) or 9 Be(d,n) reactions. Because the neutron energy can only be determined then by time-of-flight, in-beam gamma spectroscopy is the only method which can be used to determine (n,x) cross sections with such a beam. In collaboration with IRMM Geel, the GRACE (Groupe de Recherches sur l Aval du Cycle Electronucléaire) group of IPHC has used this technique with success at Gelina. This was possible despite the long flight path (200 m), and the small number of crystals (2) because very thick Pb targets ( g of Pb enriched in 206 Pb, 207 Pb or 208 Pb) could be used. But since neutrons from successive beam bunches must not overlap, the frequency of the Spiral II beam must be lowered by a large factor, and the neutron flux will be reduced by the same factor. This factor depends on the type of reaction which is studied, and the possible background produced by reactions other than the one which is studied, and of course on the length of the flight path. This length should not exceed 20 or 30 m at Spiral II. As a result, the characteristics of the beam will be close to those of the Los Alamos WNR beam [1], with nevertheless a flux multiplied by a factor about 3 for Spiral II in the MeV region, where the (n,2n) reactions have maximum cross section. But in both cases, the data acquisition has to be active continuously, so that the background will be recorded also during 100% of the duty cycle, while low frequency beams like Gelina (800 Hz) and n_tof (0.1 Hz) allow a strong reduction of the background by time selection. This means that, like at WNR, active targets and especially actinides will be difficult to study. When comparing WNR and Spiral II, one must take into account that a large set of HPGe detectors, Geanie (former Berkeley HERA), is in operation at Los Alamos. A competitive facility for gamma-spectrometry at SPIRAL2 in the interest of cross 26

27 section measurements should therefore achieve an absolute peak efficiency comparable to or better than that array (1% at 1.33 MeV). The quality of the measurements would furthermore be substantially enhanced if a significant gamma-gamma coincidence probability could be achieved, so that level and decay data can be improved upon. The presence of such a device at Ganil would allow to measuringe cross sections of (n,2n), (n,nα) and (n,np) reactions. Such cross sections are of interest to studies of structural material safety in terms of mechanical properties and health risks for operators and gas production. Sensitivity to the residual nucleus will result in data complementary to double differential measurements of the outgoing particles, e.g. substantial additional information for the optimization of nuclear model calculations, and in particular the more detailed testing and development of preequilibrium and level density models. If more efficient than Geanie, the array would allow routine study of coincidence events, which is of more general interest than only application driven experiments (low spin structure physics). 2. Mono-energetic beam Figure 1: The 56 Fe(n,α) cross sections in several data bases. Only one experimental point appears in Exfor (experimental data base). At NFS, quasi mono-energetic beams could be produced by d+d, d+t and p+ 7 Li reactions. However, because D and T targets are not likely to stand high intensity deuteron beams, the latter reaction seems more promising. The GRACE group has used in-beam gamma spectroscopy at the Cyclone neutron beam of Louvain-la-Neuve [2]. Because it is produced by the 7 Li+p reaction, this beam is not truly monoenergetic. It is actually composed of a mono-energetic component and a broad continuum due to three-body break up, with almost the same intensity (Fig. 2). The effect of this component is shown below (Fig. 3). At about 35 MeV, one expects the 232 Th(n,5n) cross section to be at its maximum. This is confirmed by the kev line in 228 Th which is best seen at this energy. But one observes also the and the lines in 232 Th and 230 Th, fed by the (n,n ) and the (n,3n) reactions, respectively. For these lines, the feeding is due partly to the cross section of these reactions at 35 MeV, but also partly to the slower neutrons. A deconvolution, requiring the precise knowledge of the neutron spectrum at each energy, is the only way to disentangle the two contributions but limits the precision which can be obtained. 27

28 Figure 2: Relative spectral fluence of the neutron beam at LLN. The thick line is obtained with a NE213 scintillator, the thin one with a fission chamber. Figure 3: γ spectra from a 232 Th target bombarded by neutrons at different energies. Each spectrum corresponds to 7 hours statistics. The Spiral II beam will have several advantages compared to that of Cyclone: the intensity of the proton beam is larger (up to 5 ma instead of 10µA). however, using realistic thicknesses of 7 Li and a 5 ma beam would result in a too large energy deposit in the target. Therefore, one must reduce either the intensity or the frequency of the proton beam, or the thickness of the target. Reducing the frequency presents the very interesting advantage that one can use the time of flight to separate the mono-energetic component from the continuum. Reducing the thickness improves the energy resolution of the mono-energetic component. Typically, a 7 Li target with a thickness of 1 mm and a reduction of the frequency by a factor 100 would yield a neutron flux and an energy deposit similar to that at Cyclone, with an beam energy resolution of 28

29 about 1 MeV at 20 MeV, and a complete rejection of the continuum. In this case, the possibly not perfect rejection of a consecutive beam burst could even be acceptable. the proton beam can have almost any energy below 40 MeV, while at Cyclone it must be above 25 MeV. The interesting range starts at about 7 MeV. the proton energy can be varied more easily. A typical experiment would consist in some 10 neutron energies, each point needing less than one day. A very good collimation of the neutron beam, equivalent to the one at Cyclone, is of course mandatory since HPGe detectors would be used. References [1] Rochman et al., Nucl. Instr. Meth. A523 (2004) 102 [2] Schumacher et al., Nucl. Instr. Meth. A421 (1999)

30 Appendix 3: Neutrons and charged particles activation reactions (Extract from the Letter Of Intents) S.P. Simakov, U. Fischer Forschungszentrum Karlsruhe, Institute for Reactor Safety, Postfach 3640, D Karlsruhe, Germany P. Bem Nuclear Physics Institute, Řež, Czech Republic Neutron and charged particles induced activation reactions produce unstable radionuclides decaying with emission of γ rays or β - particles during the time varying from milliseconds to million of years. From the basic point of view the measurements of such cross sections are important for the verification of the nuclear reaction and nuclei structure theoretical models, which are also used to describe many other reactions. The practical applications of activation reactions cross sections have a special importance, since the induced radioactivity determines the nuclear safety issues such as γ- dose rate, heat generation, waste transportation and storage. So far the measurements of the neutron activation cross sections, its theoretical analyses and generation of the evaluated data libraries covered the neutron energies mainly below 14 MeV due to the availability of the conventional neutron sources. Above this energy the experimental data are still scarce, whereas the evaluated data disagree with the measurements and each other sometimes even drastically as shown in Fig. 1. The new SPIRAL-2 facility makes it possible to perform mainly two types of neutron activation measurements: employing the energy broad distributed ( white ) and mono-energetic neutron sources [1]. The white energy neutron fluxes enable the integral validation of the activation cross section and direct observation of the most dominant γ-ray emitters generated in the material under the investigation. At NFS such source will be produced by a 40 MeV deuteron beam impinging on the carbon or beryllium targets. In this case, the neutron energy will extend up to 40 MeV, total flux in the 1 cm diameter activation foil located at 5 cm from the source will reach n/cm 2 /s, several times exceeding the intensity available today at facilities of Forschungszentrum in Karlsruhe and Nuclear Physics Institute in Řeź. Thanks to the intense neutron beam of SPIRAL-2, a) only small activation samples would be required, b) less beam time would be needed to perform irradiation of such samples. Monoenergetic neutron source with variable energy enables the measurements of activation cross sections as a function of energy and thus direct validation of the theoretical model calculations and evaluated libraries. At the NFS such neutron source will employ 7 Li(p,n) reaction. For the proton beam energy varying up to 35 MeV, the energy of neutrons in forward direction will change up to 32 MeV with overall energy resolution approximately 2 MeV. The target has to be a 2-3 mm thick metallic lithium layer followed by the carbon beam stopper and/or proton deflecting magnet. At several dozens MeV such a neutron source has a low energy tail, that means it will be most suitable for the measurement of the threshold reaction cross sections, what is usually the case for activation ones. These white and mono-energetic NFS neutron sources open the possibility for the validation of the neutron activation cross sections, many of them still lack of the measured data. In addition due to the high neutron source intensity NFS is advantageous in terms of: - production of long lived radioactive isotopes which determines the processing and waste of 30

31 irradiated materials (at the existing facilities it is difficult to detect the radioactive inventories with a half life exceeding a hundreds days, because of low counting rate). - activation of the materials or isotopes available in tiny amounts (the mass of the foils used now typically amounts gram). - sequential nuclear reactions, such as 28 Si(n,np) 27 Al(n,2n) finally producing 26 Al with a half life of 720,000 years, which determines the long term activity of SiC, potential structural material for a fusion power plant. The proton and deuteron induced activation reactions have a great interest for the assessment of induced radioactivities in the accelerator components, targets and beam stoppers as well as isotope production for medicine. In particular, the IFMIF facility needs such a data for estimation of the potential radiation hazards from the accelerating cavities and beam transport elements (Al, Fe, Cr, Cu, Nb) and metal and gaseous impurities of the Li loop (Be, C, O, N, Na, K, S, Ca, Fe, Cr, Ni). The cross sections are needed in the energy range from the activation reaction threshold 2-10 MeV up to 40 MeV both for deuterons and protons. Fig. 2 shows that the present status of the measured and evaluated data is unacceptable. The relevant measurement at NFS could be carried out by inserting a thin foil or set of the foils in the accelerator vacuum tube and irradiating them by charged particle beam with variable or maximum available energy of 40 MeV, correspondingly. Of course, safety and radioprotection issues should be addressed carefully in order to evaluate if such a procedure is technically possible. σ, mb p-d 2 O spectrum IEAF W(n,nα+) 182m Hf 10-1 EAF-2005 d-li spectrum Neutron Energy, MeV Figure 1: Status of neutron activation cross-section for 182m Hf production in 186 W: no experimental data and large discrepancies between evaluated data (in colour). In black are shown neutron energy spectra available from different reactions. 31

32 Nb(d,2n) 93m Mo Cross Section, b Ditroy'00 EAF Ditroi'00_Sig EAF20051_Nb93d2nMo92m Deuteron Energy, MeV Figure 2: Status of deuteron activation cross sections for 93m Mo production in 93 Nb sample: no measurements above 10 MeV, evaluated data essentially disagree with experiment. References : [1] U. Fischer, Nuclear data needs for fusion technology and possible contribution by SPIRAL2, 15 th Colloque GANIL, Giens, France (2006), presentation available from 32

33 Appendix 4: Study of the pre-equilibrium process in the (n,xn) reactions. X. Ledoux, G. Bélier, C. Varignon CEA/DIF/DPTA/Service de Physique Nucléaire Bruyères-le-Châtel 1. Motivations Neutron induced reactions are very important for many applications like Accelerator-Driven Systems, fast-neutron reactors or medical applications. Among the inelastic processes, the (n,xn) reactions are predominant for fast neutrons. For example in the 7-20 MeV energy range the (n,2n) reaction is one of the most important nuclear-reaction channels for non-fissile nuclei. For such energies the reaction codes need several models (optical, direct interaction, pre-equilibrium and evaporation) to be able to calculate the cross sections. Among the corresponding processes, the preequilibrium is clearly the least known. Actually some of the existing models reproduce integrated observables but fail in describing differential measurements. Figure 1: Talys [1] simulations of the neutron spectra emitted in 40 MeV 208 Pb(p,xn) (left) and 40 MeV 208 Pb(n,xn) (right) reactions. In order to constrain strongly the pre-equilibrium models we propose to measure the neutrons energy spectra in the (n,xn) reactions in coincidence with the neutron multiplicity. The energies of interest range from 15 to several tenths of Mega electron Volts. This domain corresponds to an increase of pre-equilibrium process effect as illustrated in the figure 1. This energy range is also characterized by the opening of new reaction channels like (n,3n), (n,4n), (n,5n)... The measurement of the double differential cross-section in tagged (n,xn) reactions would be innovative, and would provide data of prime importance for the nuclear data bases and modeling improvements. 33

34 2. Experiment description The detection set-up (Fig. 2) can be divided into two main parts. The first one consists of neutron detectors placed 1 m from the studied sample at 40, 60 and 80 with respect to the beam direction. These neutr on counters are composed of a NE213 liquid scintillator cells (Φ=12.53 cm and L=5 cm) coupled to a Photonis XP4512B phototube. A pulse shape analysis allows neutron-gamma discrimination while the neutron energy is determined by time-of-flight (TOF). The detection threshold is adjusted at 500 kev to ensure good n-γ discrimination. The efficiency is calculated with the O5S code [2]. Figure 2: Scheme and photo of the experimental set-up. The second part of the set-up is the 4π neutron detector CARMEN (Cells Arrangement Relative to the Measurement of Neutrons) [3], similar to BNB [4,5] or ORION [6,7] detectors. This kind of detector has already been used in the past for (n,2n) integrated cross-section measurements [8]. It consists of two independent vertical hemispheres, whose outer and inner radii are 60 and 15 cm, each one filled with almost 0.5 m 3 of gadolinium-loaded scintillating organic liquid (BC521). The interspace between the 2 hemispheres can be adjusted in order to place extra detectors in the vertical plane containing the target. A small space, about 10 cm, between the hemispheres permits doubledifferential cross sections measurements with the external NE213 detectors previously described. The 15 cm in radius area at the centre of the detector defines the reaction chamber. A horizontal cylindrical channel, 5 cm in radius, allows the neutrons to reach the reaction chamber, the beam exit being ensured by a rectangular wide-mouthed channel. Twelve phototubes surrounding each hemisphere collect the light produced in the scintillator. When a neutron enters the scintillating liquid it interacts with a proton whose recoil produces a so-called prompt light signal. The neutron is then slowed down 34

35 by loosing its energy by inelastic and mainly elastic scattering on hydrogen and carbon nuclei. As the neutron is slowed -down to the thermal energy it can be captured by a gadolinium nucleus whose deexcitation produces a delayed light signal. Due to the low gadolinium concentration, 0.5 % by weight, 50 µs following the first interaction are necessary to capture 99% of the total number of captured neutrons in the scintillator. The big advantage for such detector is to scatter in time the counting of neutrons emitted simultaneously in a nuclear reaction. For each event, two 50 µs gate signals (separated by 50 µs) are generated to measure the neutron and the background multiplicities respectively. The high efficiency of CARMEN, 85% for fission-evaporation neutrons, makes it very sensitive to the neutron and gamma ray background. The data acquisition is triggered by the detection of one neutron in one of the NE213 cell while CARMEN detects the remaining (x-1) neutrons of the reaction. A first campaign of measurements was performed at the Tandem of Bruyères-le-Châtel up to 13.3 MeV. As explained in the first section, NFS would permit to extend this study up to MeV. The use of CARMEN requires a neutron beam with specific constrains. The beam has to be very well collimated, the neutron and gamma background should be as low as possible and a pulsed beam is needed. Indeed by now there are no other facilities, which would allow performing this experiment, and NFS is perfectly adapted for this kind of measurements. References [1] A. J. Koning, S. Hilaire and M. Duijvestijn, TALYS A nuclear reaction code. User manual. (2003). [2] R.E. Textor and V.V. Verbinski, Oak Ridge National Laboratory Report, ORNL 4160 UC-34-Physics ORNL 4160 (1968). [3] I. Lantuéjoul, PhD Thesis, University of Caen (2004). [4] U. Jahnke, G. Ingold, D. Hilscher, H. Orf, E.A. Koop, G. Feige and R. Brandt, Lectures Notes in Physics, Springer, Berlin, 178 (1983) 179. [5] U. Jahnke, C.-M. Herbach, D. Hilscher, V. Tishchenko, J. Galin, A. Letourneau, B. Lott, A. Peghaire, F. Goldenbaum and L. Pienkowski, Nucl. Inst. and Meth. A 508 (2003) 295. [6] Y. Périer, B. Lott, Y. El Masri, J. Galin, Th. Keutgen, J.H. Le Faou, M. Morjean, A. Péghaire, B.M. Quednau and I. Tilquin, Nucl. Inst. And Meth. A 413 (1998) 312. [7] B. Lott, F. Cnigniet, J. Galin, F. Goldenbaum, D. Hilscher, A. Liénard, A. Péghaire, Y. Périer and X. Qian, Nucl. Inst. and Meth. A 414 (1998) 117. [8] J. Fréhaut, Nucl. Instr. and Meth. 135, (1976) 511. [9] J.F. Briesmeister (Ed), MCNP - A General Monte-Carlo N-Particle Transport Code, Version 4B, LA M, Los Alamos National Laboratory, Los Alamos, New Mexico (1997)

36 Appendix 5: Neutron beam dump Summary: The NFS neutron beam dump will be part of (or close to) the experimental area. Besides obvious shielding consideration, especially at forward angles, the beam dump should generate as low as possible neutron and photon (gamma or X-rays) background in the experimental area. The beam characteristics (spectrum, flux, size) affect the beam dump design. The other parameters to consider for the beam dump itself are geometry and materials. The main characteristics of the beam dumps used at three different facilities hosted by LANSCE (Los Alamos, USA), TSL (Uppsala, Sweden), and CERN (Genève) have been reviewed. At the NFS facility, two neutron production modes are envisaged. In the first mode, 40 MeV deuterons break up on a carbon converter to produce neutrons with a continuous energy distribution between ~1 MeV and ~40 MeV and an average energy of ~14 MeV. In the second mode, quasi-monoenergetic neutrons would be produced up to ~30 MeV using the 7 Li(p,n) 7 Be reaction. Thus, NFS would combine some of the advantages of WNR (high intensity neutron flux of the order of 10 6 n/cm2/s) and TSL's or n_tof's (neutrons in the MeV to tens of MeV energy range). Therefore, conventional solutions adopted at other facilities could also respond to NFS needs. 1. Introduction The NFS neutron beam dump will be part of (or close to) the experimental area (Fig. 1). Besides obvious shielding consideration, especially at forward angles, the beam dump should generate as low as possible neutron and photon (gamma and X-rays) background in the experimental area. Figure 1: Schematic view of the NFS facility. The neutron beam dump is on the right side. The beam characteristics (spectrum, flux, size) affect the beam dump design. The other parameters to consider for the beam dump itself are geometry and materials. 2. Beam dump at other facilities In the following, we review the characteristics of the beam dumps used at three different facilities hosted by LANSCE (Los Alamos, USA), TSL (Uppsala, Sweden), and CERN (Genève) WNR (LANSCE) The Weapons Neutron Research center (WNR) at LANSCE operates a spallation neutron source driven by 800 MeV protons. This source is not moderated. Most of the neutrons are emitted via 36

37 an evaporation mechanism with energies around 1 MeV. However, high energy neutrons emitted through more direct reactions are also present. The main characteristics of WNR beam dumps are given below [1]. Neutron beam n spectrum n flux beam size ~1 MeV (peak) up to a few hundreds of MeV 10 6 to 10 7 n/cm 2 /s (at the beam dump) up to 13 cm ( ) at the beam stop (usually much smaller) Beam dump location materials geometry shielding at least 5 meters from the experimental area - borated CH 2 (40% boron) - 5 cm Fe or Cu - 5 cm CH 2-5 cm Fe or Cu etc... up to a thickness of 50 cm cylindrical 1 meter of magnetite(feo)-loaded concrete blocks 2.2. Blue hall (TSL) The blue hall at the Theodor Svedberg Laboratory (TSL) hosts a neutron beam facility driven by a cyclotron. Quasi-monoenergetic neutrons are produced using the 7 Li(p,n) 7 Be reaction. The neutron energy peak can be adjusted between 11 MeV and 175 MeV. The main characteristics of the neutron beam dump are given below [2]. Neutron beam n spectrum 11 MeV 175 MeV (quasi-monoenergetic) n flux 10 3 to 10 4 n/cm 2 /s (at the beam dump) beam size from 7 cm to 2 m ( ) at the beam stop (~20 m) Beam dump location materials geometry shielding 3 meters from the experimental area iron ore surrounded by concrete cylindrical trap (to avoid backscattered neutrons) concrete walls 2.3. n_tof (CERN) The n_tof facility hosted by the CERN is a water-moderated spallation neutron source driven by 20 GeV protons. Due to the moderation process, slow neutrons (~ev) are available in addition to fast evaporation neutrons (~MeV). Moreover, intermediate energy neutrons (~GeV) emitted through more direct reactions are also present. The main characteristics of the n_tof beam dump are given below [3]. Neutron beam n spectrum from a few ev up to a few hundreds of MeV n flux 10 3 to 10 4 n/cm 2 /s (at the beam dump) beam size up to 8 cm ( ) at the beam stop (~200 m) 37

38 Beam dump location materials geometry shielding ~10 meters from the experimental area polyethylene (CH 2 ) + cadmium sheets cylindrical trap (50 cm x 50 cm x 50 cm) concrete walls The n_tof beam dump (Fig. 2) includes three BF 3 detectors to monitor the neutron flux intensity and position. Figure 2: Details of the polyethylene beam dump used at n_tof [3] The experimental area is shielded from backscattered neutrons by a concrete wall as can be seen in Fig. 3. Figure 3: View of the n_tof neutron escape line and beam dump. The experimental area is a few meters behind the concrete wall [3] 3. Beam dump at NFS At the NFS facility, two neutron production modes are envisaged. In the first mode, 40 MeV deuterons break up on a carbon converter to produce neutrons with a continuous energy distribution between ~1 MeV and ~40 MeV and an average energy of ~14 MeV. In the second mode, quasimonoenergetic neutrons would be produced up to ~30 MeV using the 7 Li(p,n) 7 Be reaction. Thus, NFS would combine some of the advantages of WNR (high intensity neutron flux of the order of 10 6 n/cm2/s) and TSL's or n_tof's (neutrons in the MeV to tens of MeV energy range). Therefore, conventional solutions adopted at other facilities could also respond to NFS needs. 38

39 References [1] R.C. Haight, private communication, March [2] A. Prokofiev, private communication, March [3] CERN n_tof Facility: Performance Report, n_tof collaboration, January

40 Appendix 6: Neutron monitoring I - ABSOLUTE CALIBRATION OF NEUTRON FLUX Characteristics of the neutron source, i-e the neutron yield as a function of energy and angle, will be determined in a dedicated experiment. The expected neutron energy range is MeV. Time-of-flight measurements as well as activation measurements may allow for an absolute calibration of the neutron flux (see M.J. Saltmarsh et al. NIM 145 (1977) 81). I.1. Time - of - flight measurements (TOF) The deuteron beam is pulsed at 500 khz, leaving 2 µs between bursts. The burst width is a few hundred picoseconds. The flux estimate is 10 6 n.cm -2.s -1 at a distance of 10 m from the converter. The TOF measurement is performed by measuring the time interval between a fast timing signal issued from the neutron detection device and the stop signal derived from the RF signal of the deuteron accelerator. With a very conservative time resolution of 2 ns and a flight path 10 m long, the expected E/E resolutions are: 2.5 % at 40 MeV, 1,2 % at 10 MeV and 0,4 % at 1 MeV. At large angles ( ϑ =90 shorter flight path ( 2 m) leads to the following values : 12,4 % at 40 MeV, 6,2 % at 10 MeV and 2 % at 1 MeV. Due to the overlap of events from successive beam pulses, low energy measurements (E 0,4 MeV) will need a shortening of the flight path : with a flight path of 5m, an energy resolution of 0,25 % should be reached for a neutron energy of 100 kev. Liquid scintillator coupled to a photo multiplier tube may be used to detect neutrons with high efficiency (E 0.5 at 20 MeV) above threshold energy of 1 MeV. Neutrons and gammas are discriminated in the two dimension plot Q T Q L, Q T being the total integrated charge signal and Q L the integration of the tail of the signal. Due to the high detection efficiency, measurements have to be performed at low intensity. Assuming an intensity of 1 nae the neutron flux should be 35 n.cm -2.s -1 and the counting number 3500 s -1 using a DeMoN module (16 cm in diameter and ε = 0,5). A proton recoil telescope can also be used to detect neutrons: the elastic scattering of neutrons with hydrogen nuclei in a converter, such as CH 2, eject protons which are detected in silicon detectors located behind the converter with respect to the neutron beam. In the energy range of 15 MeV and for a converter thickness of 2 mm, the proton detection efficiency is of the order of With a neutron flux of 10 6 n.cm -2.s -1 and a detector of area 5 x 5 cm 2, the counting number is s -1. For an intensity of 1 µae, it will decrease to 900 s -1. Whatever the device retained, measurement with specific shield in front of the detector should be carried out for estimating the background effect of scattered neutrons. ) the I.2. Sample activation measurements When samples are irradiated by neutrons, the reaction rate R, the number of activated radionuclides per unit of time is given by: R = σ Φ where σ is the neutron cross section reaction and Φ the neutron flux. From the measured activities and knowing the excitation function of the reaction, the neutron yields can be accessed. Specific reactions should be considered: 40

41 Reaction Threshold energy (MeV) Half-life Gamma energy (kev) 27 Al(n,α) 24 Na h Ni(n,p) 58 Co 71.3 d Co(n,2n) 58 Co d Nb(n,2n) 92m Nb d In(n,n ) 115m In h II - NEUTRON MONITORING DURING THE NFS EXPERIMENTS The monitoring of the neutron flux, during the experiments carried out at NFS, is mandatory for relative normalization of the measurements. Simple devices, easy to implement, reliable and robust are needed. They will be calibrated in the neutron source characteristics determination phase. II.1. Proton recoil telescope A proton recoil telescope made of a CH 2 converter and three silicon detectors with respective thicknesses of 300, 300 and 1000 µm (G. Ban et al., NIMA 577 (2007) 696) was developed specifically for monitoring 14 MeV neutrons. The detection of triple coincidences between the three silicon detectors allows for an unambiguous detection of the most energetic protons and therefore, of the fast neutrons. Such a device could be adapted for the monitoring of the NFS neutron source. The use of three elements, each of them with a thickness of 1 mm, will allow for the monitoring of different parts of the energy spectrum: - protons stopped in the first element are associated with the "low energy" part of the spectrum (E < 12 MeV) ; - coincident events between the first two silicon detectors correspond to the MeV energy range ; - and triple coincidences give information for the energy domain above 18 MeV. Note that protons with energy larger than 22.7 MeV will cross the three detectors. This is not a problem as the proton energy spectra measurement is not mandatory. With such a device, the low energy part of the neutron energy spectrum will suffer from an energy threshold due to the gammas and electrons produced in interaction of neutrons with silicon detectors. Expected threshold is 1-2 MeV (M. Wielunski et al., NIMA 517 (2004) 240). II.2. Micromegas From the results of CERN n_tof experiments, it has been demonstrated that a Micromegas detector previously developed for high energy physics [1], is also excellent for neutronic studies at a very large energy range from thermal up to 100 MeV. The main advantage of Micromegas is its robustness, its high resistance to radiations and its non-sensitivity to gamma background. These qualities have been exploited to develop a new neutron detector and in particular for the use in nuclear reactor environment [2, 3]. We propose to develop a new neutron detector for monitoring the neutron beam of NFS project. 41

42 In summary, the Micromegas is a gas detector. The detection principle is simple: the gas volume is separated in two regions by thin micromesh, the first one where the conversion and drift of the ionization electrons occur, and the second one, micron thick, where the amplification takes place. Ionization electrons are created by the energy deposition of an incident charged particle in the conversion gap. In the amplification region, a high field (40 to 70 kv/cm) is created by applying a voltage of a few hundred volts between the micromesh and the anode plane, which collects the charge produced by the avalanche process. The anode can be segmented into strips or pads. The positive ions are drifting in the opposite direction and are collected on the micromesh. For neutronic experiment, an appropriate neutron/charged particle converter has been used [4,5]: (i) 6 Li(n,α) for the neutron having an energy up to 1 MeV, (ii) H(n,n')H and 4 He(n,n') 4 He from the energy threshold of the detector to high energy over 10 MeV. For NFS project we plan to extend this method using a fissionable element such as 235 U for neutron/charged converter. The 235 U element has chosen because its fission cross section is well known from thermal to several MeV. The use of the new bulk technique [6], makes it possible to use relatively thin materials (kapton and copper), thus minimizing the disturbance of the neutron beam. A compact micromegas detector associated with a fast preamplifier and an appropriate acquisition system permit to measure praticaly on line the neutron flux of NFS. References: [1] Y. Giomataris et al.,, Nucl. Intr. and Meth. A, 376(1996)29. [2] S. Andriamonje et al., Nucl. Instr. and Meth. A, 562 (2006) [3] J. Pancin et al to be published to Nucl. Instr. and Meth. A (2008) [4] S. Andriamonje et al., Nucl. Instr. and Meth. A, 481 (2002) 36. [5] J. Pancin et al., Nucl. Instr. and Meth. A, 524 (2004) [6] I. Giomataris et al., Nucl. Instr. and Meth. A, 560 (2006)

43 Appendix 7: Safety issues and proposed solutions for NFS Contribution from CEA Saclay, DSM/IRFU/SENAC 1. External radiation exposure risks External radiation exposure risks results mainly from neutron production when the particle beam (proton or deuteron) interacts with the target (respectively Lithium or Beryllium), or γ rays emitted by the different activated materials when beam is off. The external prompt dose rate due to neutron production has been studied on the basis of a preliminary conceptual design of a deuteron- beryllium converter. The results of calculations are described in this document and the safety and radioprotection issues discussed in terms of recommendations. Residual γ dose rates have not been yet studied but should be significant in the converter room after operation. The access to this room is required to perform maintenance of equipments and to prepare the irradiations of materials by using proton or deuteron beams directly supplied the LINAC when the converter is not used. These experiences are also envisaged in the frame of nuclear data programs. To satisfy this need, residual dose must be limited (ALARA concept) and the design has to be optimized in order to minimize the neutron activation of the different materials of the equipments in the room. Figure 1: preliminary design of NFS experimental rooms In order to evaluate the external dose rate when beam converter is operated, a preliminary study has been performed by calculating the neutron production due to a pulsed deuteron beam (40 MeV, 50µA) interacting with a fixed (not rotating) target of beryllium (cylinder of 2 cm in diameter and 1 cm in thickness). This type of converter located in the neutron production room (as represented in the preliminary design on Figure 1 below) can produce a neutron flux (~1x10 8 n/cm 2 /s) with an energy range of 1 kev 40 MeV needed for the ntof measurements in the experiment hall. The neutron flux produced by the particle target interaction is illustrated on Figure 2 as a function of energy for different types of configuration. 43

44 Figure 2: Neutron flux produced for different particle target configurations The neutron production and transport have been calculated with MCNPX. The Isabel-Dresner model has been selected as the best one compared to available experimental data in terms of energy and angular distributions. Figure 3: Neutron production at 0 degrees for 9 Be(d,Xn) and 12 C(d,Xn) reactions at 60 MeV and 50MeV respectively The experimental data available from literature are related to neutron energy and angular distributions used for 9 Be(d,Xn) and 12 C(d,Xn) reactions respectively at the energy of 60 MeV and 50 MeV (ie slightly higher than the deuteron beam energy delivered by the LINAC). The calculated neutron production rate is compared to experimental data as a function of energy and is presented in Figure 3. The neutron rate is correctly evaluated and, as required in such a safety study, remains conservatively estimated. As illustrated on Figure 4, the response of Isabel-Dresner model appears fairly good for neutrons emitted to the front way (0 degree distribution). 44

45 Figure 4: neutron energy distribution emitted at 0 degree for 9 Be(d,Xn) reaction à 60 MeV Figure 5: Modelling of NFS experimental area The preliminary design represented in Figure 1 has been modeled as illustrated in Figure 5. The converter cave room is surrounded by a biological shielding consisting of a 2m thick lateral wall and 1.5 m thick roof. The ntof hall is shielded by a 0.5 m thick walls and roof. All walls are made of ordinary concrete (density of 2.3 and hydrogen content of 1.39 % in weight). The hydrogen content is a very sensitive parameter in terms of neutron flux attenuation through the wall. Consequently it must be correctly chosen in order to prevent any underestimation of wall thickness needed to comply with the targeted dose rate outside the rooms. The external dose rate has been evaluated from the neutron flux distribution represented in Figure 6. 45

46 Figure 6: 3D neutron flux distribution in the converter cave room and in the experimental ntof hall As seen on Figures 7 and 8, the neutron dose rate do not comply with the targeted dose rate (max value of 0.5 µsv/h for public area). Consequently, the assumed wall thickness is not sufficient for the converter cave and for the ntof experimental hall. The lateral wall thickness of the converter vault should be increased from 2 to 2.5 m and the thickness of the ntof hall wall should also be enlarged from 0.5 m to approximately 1 m. Figure 7: Neutron dose rate in the different NFS rooms When particle beam converter is operated, all access to the rooms must be prohibited and strictly controlled by an interlock system. The neutron dose rate on the roof of the converter vault is about 100 µsv/h. As a consequence, access on the roof must also be controlled during beam operations. Neutron backscattering from the neutron beam dump can be observed in the TOF hall. The efficiency of the beam dump has to be studied in order to minimize external dose rate outside and 46

47 also the backscattering effect to prevent materials in the TOF hall from additional activation. Figure 8: neutron dose rate attenuation from beryllium converter to outside If the increase of ordinary concrete walls cannot be envisaged (for instance for economical reasons) then a local shielding surrounding the converter is recommended and could be studied in terms of efficiency to minimize the activation of the different equipments in the converter room and then to optimize the residual dose rate and to limit the production of nuclear wastes. 2. Nuclear solid wastes and liquid or gaseous effluents Radioactive solid wastes result from neutron activation of materials in the converter cave and in the ntof hall. Activation of equipments in the converter cave has been studied on the basis of the preliminary design used for the neutron dose rate evaluation. Calculations have been performed by using the CINDER 90 code for the following materials: - the deflecting magnet made of steel and which could be used for a Deuteron Lithium converter, staying in the vault during the use of a deuteron Beryllium converter, - the Beryllium target of the converter, - the ordinary concrete of walls. The Be converter is assumed to be operated during 90 days at a constant deuteron beam of 50 µa at 40 MeV. Calculation results are presented in. These results have not been yet analysed in terms of activity concentration of the different nuclear wastes to be managed during the operation of the facility (replacement of equipments for instance) and in view of its future decommissioning (cleaning and dismantling of the structures). Meanwhile the results show a significant level of activity generated during operation and slowly decreasing after beam stop (one decade in one year for the major structures). 47