Laboratory of Fast Neutron Generators of the NPI

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1 Laboratory of Fast Neutron Generators of the NPI Mitja Majerle Nuclear Physics Institute of the CAS Ref near Prague Czech Republic Martin Ansorge, Pavel Bern, Milan Cihak, Pavel Krist, Miloslav Gotz, Jan Novak, Zdenek Pulec, Eva Sirneckova, Milan Stefanik, Jan Stursa, Zafar Yasin, Vaclav Zach Nuclear Physics Institute of the CAS Ref near Prague Czech Republic ABSTRACT The Nuclear Reactions Department of the NPI operates neutron sources with neutron energies extending up to 35 MeV. The cyclotron U-120M provides protons in the energy range of 6-36 MeV. These are directed to a thin Li or Be foil (quasi-monoenergetic neutrons) or to a thick Be target (continuous neutron spectrum). The available neutron fluxes are up to 10 9 njcm 2 /s for quasimonoenergetic neutrons and loll njcm 2 /s for continuous neutron spectrum. The produced neutrons are used in a wide scale of activities connected to accelerator driven technologies and fusion. This contribution focuses on cross-section measurement and benchmarks, the measurements of the produced neutron spectra, the development of the (n,cp) chamber and online gamma measurements. The planned extensions of the neutron sources are also mentioned. 1 INTRODUCTION The experimentally measured neutron data above 20 Me V are becoming increasingly important in the future energy production. New reactor concepts, fusion and accelerator driven systems (ADS) utilize more energetic neutron spectrum than conventional reactors. The experimentally measured cross-sections data for neutrons with energies above 20 Me V are rare and many dedicated facilities are operating or are in construction for this purpose. The need for neutron data at higher energies is also reflected in the extensions of the commonly used libraries to higher energies. In the past decades, several procedures to gather knowledge on neutron reactions were standardized. Irradiation with the known neutron spectrum and the subsequent activity measurement is commonly used to determine the reaction cross-section. Monoenergetic neutron beams from the d+t reaction are commonly used below 20 MeV neutron energy, while above this energy only quasimonoenergetic (QM) beams are available, with the p+li reaction as the most used QM neutron source. Integral validation of cross-sections with continuous neutron spectra (p+thick Be at NPI) provide another proof of the correctness of cross-section evaluations. The uncertainties of the measured data with QM neutrons are usually above 10-15%, mostly because of not well known neutron spectra. At the NPI facility this can be partly overcome by the measurement of the 7Be production in the lithium target after the irradiation and the neutron spectral shapes measurements by the Time-Of-Flight method (TOF)

2 203.2 The gas production due to neutron irradiation presents one of the main concerns for the structural material performance in the future power reactors. Experimental cross-sections above 20 MeV are scarce and estimations of gas production in neutron spectra of future power reactors have large uncertainties. The data on gas production can be obtained either by the direct measurement of the produced charged particles or by the measurement of the radioactive residual nuclei. Data from both approaches are complementary and necessary for a good quality cross-section evaluation. The measurements of the radioactive residual nuclei are already performed at the NPI, the direct production of charged particle is planned in near future. Recently, the collimator was constructed and is already in use with the present neutron generators. In near future, the neutron generators shall be placed closer to the collimator and well collimated neutron beams of high intensities will be provided. New types of detectors are being constructed around the collimated neutron beam aiming mainly at the production of the charged particles in reaction with neutrons and at the cross-section measurements of the isotopes with ms decay times and prompt gammas. The capabilities of both detector systems will be significantly extended after the planned U-120M cyclotron upgrade, when a bunched proton beam for TOF with repetition rates 1 MHz will become available. Intensive sources of medium energy neutrons are interesting also in the electronics industry. A new cyclotron TR-24 was recently installed and will provide significantly higher proton currents and neutron fluxes on planned continuous neutron source. Many detector parts from larger detector complexes (eg. CERN) were already tested against high dose rates of medium energy neutrons. The new neutron facility shall extend the capabilities in this field. 2 PRESENT NEUTRON GENERATORS A basic experimental facility of the NPI is the isochronous Cyclotron U-120M. It provides protons, deuterons, 3 He ions and alphas. In the negative ion mode of acceleration, the protons and deuterons with energies of 6-36 MeV and MeV with good beam-current stability (10-15 µa) are obtained and used for neutron production at the suitable targets. The protons are mainly used for neutron production (deuterons and 3 He particles were tested as well), and are extracted from the beam using the stripping foil. The proton beam is directed to the target installed at the end of the beam pipe. These are equipped with the pneumatic post system, which allows the transport of the irradiated samples from the irradiation place to the detector laboratory in ca. 8 seconds. 2.1 Quasi-monoenergetic neutron source based on the p+li reaction For quasi-monoenergetic neutron production, the p+ 7 Li source reaction is used; the 2 mm thin lithium foil together with the ethanol cooled carbon beam-stopper is bombarded by the protons of energies up to 36 MeV and intensities of 8 µa. The generated quasi-monoenergetic neutron fluxes present the power-tool for cross-section data measurement in the neutron energy range of MeV. The neutron flux is studied with MCNPX calculations and online detection techniques. The calculated neutron spectral flux is validated against both the TOF measurement with the scintillation detector and the proton recoil telescope, the predicted number of monoenergetic peak neutrons is compared with the residual activity of 7 Be in the lithium foil. The TOF measurements are performed at each irradiation with the 2 x2 NE213 scintillator placed on the beam axis at a distance ranging from 3.5 m to 4.5 m from the target front. The pulse shapes from the scintillator are recorded simultaneously with the cyclotron radio-frequency (RF) sig-

3 203.3 nal using a fast digitizer. For each recorded pulse, the time phase related to the RF signal and the signal surface integrals in two time windows are determined (long and short gate for n/γ discrimination). The scintillator efficiency is calculated with the dynamic threshold method. The method operates only with protons scattered to the limited angle. This approach removes the necessity to know the whole response function of the scintillator, the results are based only on the cross-section of elastic scattering of neutrons on hydrogen [1]. The number of produced neutrons in the monoenergetic peak corresponds to the number of the residual 7 Be nuclei in the lithium target [2] and can be determined with good accuracy using the γ-spectroscopy. The ratio of forward directed monoenergetic neutrons can be determined using the equation R = E 4 p E 3 p E 2 p E p [3]. Combining the two methods, the MCNPX predicted spectra are modified in order to include the accurate normalization based on residual 7 Be activity and the modification of the spectra at energies below the peak based on the TOF measurements, see Figure 1. Another option to produce quasi-monoenergetic neutrons is the reaction of protons with a thin Be foil (0.5 mm). The higher melting point of beryllium allows higher proton beam currents and higher neutron intensities at the irradiation position, but at the cost of slightly worse characteristics of the neutron spectrum (broader monoenergetic peak). This type of the neutron source is currently under development at the NPI MeV Neutron flux [n/mev/sr -1 /C] TOF spectrum MCNPX energy spectrum MCNPX TOF spectrum, 3ns MCNPX, LE corr Energy [MeV] Figure 1: Experimentally determined Time-Of-Flight spectra and the comparison with the MCNPX simulated spectra. The number of neutrons in the monoenergetic peak is normalized to residual 7 Be activity for all spectra. The simulations exhibit a slight underestimation of the neutron flux below the monoenergetic peak, the blue curve shows the spectrum after the applied correction.

4 I γ (%) High-Flux White Neutron Source Based on p+be Reaction The thick beryllium target station of the NPI utilizes higher proton beam current (10-12 µa) at the energy of 35 MeV. During the operation, the beryllium target with thickness of 8 mm and diameter of 50 mm is cooled by the ethanol. The spectrum is continuous with the integral flux of n/cm 2 /s in the closest position, 1.5 cm from the target front. The neutron spectrum was estimated with the MCNPX simulation and validated with the standard multi-foil activation method. Eleven activation foils (Al, Nb, Y, MnNi, Co, In, Lu, Au, Ti, Fe, Bi) were used at two standard irradiation positions (15 mm and 156 mm). The induced activities of the irradiated foils were investigated by the nuclear γ-spectrometry technique and the reaction rates were obtained. A modied version of the SAND-II code with the nuclear data from the EAF-2007 library up to 55 MeV was used to unfold the neutron spectrum [4]. The obtained spectra and MCNPX predictions are shown in Fig. 2. The spectral shape is in good agreement with the TOF measurement of other Fig. authors 3. Neutron at lowerfields protonofenergies NG-2 generator (e. g. [5]), at andtwo the irradiation energy rangepositions corresponds measured to the predicted by main energy multi-foil range activation of the IFMIF technique facility at[6]. NPI. The p+be neutron spectra are convenient for crosssection data validation in the energy range relevant to the IFMIF, for testing the radiation hardness of electronics, and for the ADS research program. isotopes sities of aturated nge corrections, plied. To SAND-II the EAF-. For the pectrum sing the ployed. positions sed. The reaction g. 5, and Fig. 4. Neutron spectra of NG-2 generator at two irradiation positions displayed in Figure 2: Neutron spectra of the p+be (35 MeV) neutron source. The MCNPX simulated spectra log log scale. were adjusted using the SAND-II procedure on the reaction rates measured with the standard multifoil activation method. 3 CROSS-SECTION Table 2 MEASUREMENTS The C/E ratios for adjusted neutron spectra. The activation method is used to measure the cross-sections. The samples are first irradiated with the known Reaction neutron spectrum (quasi-monoenergetic), Position 15later mmthe activity of theposition produced 156 radioisotopes mm is measured by the γ-spectrometry. The studied samples in the form of thin metal foils are placed 40 mm 27 from Al(n,α) the 24 target Na front inside a polyethylene 0.91 transportation container. 0.94After the irradiation 93 Nb(n,2n) 92m Nb Proceedings 93 Nb(n,4n) of the International 90 Nb Conference Nuclear Energy for New Europe, Portorož, 0.90 Slovenia, September 5-8, Nb(n,α) 90 Nb Au(n,2n) 196 Au Au(n,3n) 195 Au

5 203.5 with the duration of ca. 5 minutes, the container with the sample is pneumatically transported to the spectroscopy laboratory, disassembled and the samples are measured with the HPGe detector. Typical time between the end of the irradiation and the start of the measurement with the HPGe is around 20 seconds. The cycle irradiation-measurement is repeated several times for each sample (a new sample for each repetition). The pneumatic transport system is then removed and new samples of the same materials are irradiated at the distance of mm for ca. 6 hours to measure longer lived isotopes. Seven irradiations are usually performed with proton energies 20, 22.5, 25, 27.5, 30, 32.5, and 35 MeV. The radioactive products are detected in the irradiated samples using offline γ-spectroscopy employing two calibrated HPGe detectors with 50% efficiency and good energy resolution. The decay spectra are measured during the cooling period from minutes up to 100 days. The isotopes with decay times ranging from tens of seconds to years are detected. The p+li source neutron spectra at the locations of the irradiated samples indicate that monoenergetic peaks account for 30-50% of the total flux, the rest being the low energy neutrons. In such complex spectra, a modified version of the SAND-II [4] code is used to extract the activation crosssection curves. The procedure is described in [7], the examples of the obtained cross-sections are shown in Fig. 3. The irradiations of selected materials are performed also with the continuous neutron spectrum. The obtained activities are compared to the calculations based on the knowledge of the neutron spectrum and cross-sections to obtain the calculated/experimental ratios. These ratio are used to measure the quality of the cross-section evaluation. The continuous neutron spectrum from p+be reaction is used as one of the standard benchmarks of fusion cross-section evaluations. nat Fe(n,x) 52 V nat Cr(n,x) 48 V Cross-section [mbarns] EAF-2010 This work - 40 mm Cross-section [mbarns] EAF-2010 This work - 87 mm This work - 48 mm Energy [MeV] Energy [MeV] Figure 3: The cross-sections of nat Fe(n,x) 52 V and nat Cr(n,x) 52 V reactions extracted from the NPI/Řež experimental data with the evaluated data from the EAF-2010 database [8]. The red points were obtained with short time irradiation runs (5 min) in the position 40 mm from the Li target front, the blue and the green points were obtained with long time irradiation in the positions 48 mm and 87 mm from the target front. The cross-section curves EAF-2010 were obtained by the sum of all reactions leading to the given residual which are included in the EAF-2010 evaluation. The discrepancy between the experimental and evaluated cross-sections for the reaction nat Fe(n,x) 52 V suggest the underestimation of the (n,p+α) reaction channel in the EAF-2010 evaluation. Similar discrepancies were observed also on elements Cr, Cu and V [9].

6 TOTAL CROSS-SECTION MEASUREMENTS FOR REACTION WITH NEUTRONS Using a quasi-monoenergetic neutron beam and the TOF measurements with the NE213 scintillator, the decrease of the monoenergetic neutron peak after the insertion of the material between the neutron source and detector can be determined. The decrease is due to the (n,tot) reaction and allows the direct calculation of this cross-section. This technique is currently used to measure the cross-section for the reaction 16 O(n,tot) in the energy range MeV. Liquid oxygen (thickness 20 cm) in the polystyrene container is used in the collimated neutron beam. Oxygen is commonly present element in today s reactors (in water) and these values will be measured for the first time above the energy of 20 MeV. Figure 4: The experimental setup for total cross-section measurement with the QM neutrons. The polystyrene container contains the liquid oxygen. The collimated neutron beam with the diameter of 3 cm is used. 5 DETECTOR SYSTEMS IN CONSTRUCTION The measurements of cross-sections are so far based on the gamma activity measurement of the residual products with decay times of tens of seconds and longer. In 2015, the neutron collimator (Fig. 4, 5) was constructed allowing the on-beam measurements with neutrons and opening new measurement possibilities. The collimator with the length of 170 cm is integrated in the iron door dividing the cyclotron hall from the measurement/storage hall. The design was proposed after the study of existing collimators and Monte Carlo calculations and is based on the alternating layers of polyethylene and iron. The collimator opening can be selected between the diameters of 3 or 8.5 Proceedings of the International Conference Nuclear Energy for New Europe, Portoroˇz, Slovenia, September 5-8, 2016

7 203.7 cm, the neutron beam can be further pre/post-collimated with transportable polyethylene collimators. The TOF tests that were performed on the collimated beam show good agreement with the expected beam parameters. After the ongoing modernization of the neutron generators, the newly acquired neutron beam will have properties similar to those in the SPIRAL2/NFS facility. Two new types of the detectors are being constructed on this neutron beam. Figure 5: The schematic drawing of the experimental setup with the collimated neutron beam. The neutron generators are installed at their old position 5 m from the collimator. They will be placed in front of the collimator in the near future. 5.1 Chamber for charged particle production measurements The first detector system is dedicated to the direct detection of charged particles produced in reactions with neutrons. It consists of a large vacuum chamber with a thin foil of studied material placed in the center, where it is irradiated with the collimated neutron beam. The charged particle detectors are mounted outside the reach of the neutron beam at several angles on a rotating table, and collect the flux of the charged particles exiting from the irradiated foil, their type, energy and angle in respect to the neutron beam, Fig. 6. Similar chamber was operating in past in TSL Uppsala and is currently being moved to SPIRAL2/NFS facility [10]. 5.2 HPGe-array γ-spectrometry The repeating time structure of the cyclotron beam (based on duty cycle, Hz) allows the on-beam detection of the ms isotopes. The HPGe detector will be placed next to the foil irradiated with the collimated neutron beam. After a short pulse of neutrons (few ms), a time window of 15 ms will open for the γ-spectrometry measurement. In future, the detector system will be upgraded with 3 more HPGe detectors placed at various angles relative to the neutron beam allowing the detection of the prompt gammas from the reactions with neutrons. The final goal is to implement the bunching of the cyclotron beam which would enable the neutron time of flight measurement focused on the energy region MeV making this detector system complementary to similar arrays at GELINA, IRMM and ntof, CERN. Similar HPGe detector array is developed at the SPIRAL2/NFS facility [11]. 6 DEVELOPMENT OF THE BUNCHING SYSTEM FOR THE CYCLOTRON BEAM Spectrometry of products of the neutron-induced reactions using the neutron TOF methods is based on the pulse characteristics of the accelerated beam. In order to eliminate overlapping of

8 203.8 Figure 6: The schematic drawing of the vacuum chamber for charged particle detection in the reactions with the collimated neutron beam. The de-e telescopes based on sillicon detectors are installed on the rotating table around the thin foil of studied material irradiated with the neutrons. detected events from different time separated bunches of neutrons, the interval between bunches has to be in the order of 1 µs. The corresponding bunch frequency of 1 MHz differs from the usual operating frequency of the cyclotron accelerators (tens of MHz). In the case of linear accelerators, a selector of individual pulses is technically conveniently installed as a part of an ion source section. In the case of the isochronous cyclotron, the selection of pulses in the ion source is complicated because of the orbit mixing during the acceleration process. Development of the bunching system for the proton beam of the U-120M cyclotron originates from the classical deflection-bunching system tested on the isochronous cyclotron in Karlsruhe [12], which is uniquely combined with extraction of the negative accelerated ions. The advantage of this combination is to create a controlled time interval between pulses without loss of intensity. A new neutron target based on p+be reaction will be installed next to the cyclotron, as close as possible to the place where protons will be extracted to conserve the good time resolution of the pulse. A new neutron collimator on the axis of the newly acquired beam at the distance of 7 m is also planned. The predicted parameters of the pulsations with the p+be neutron source are comparable with the parameters of the SPIRAL2/NFS proton-neutron source, Fig. 8.

9 203.9 Figure 7: The photo of the HPGe detector array at GELINA, IRMM. 7 CYCLOTRON TR-24 Continuous neutron source at the NPI is currently the only facility for irradiation of samples by medium energy neutrons in the EU. An extensive program for construction of effective neutron generators on compact cyclic accelerators with proton or deuteron beams and with currents in the ma range has been announced. Neutron sources of a very high power are intended for the study of radiation hardness of materials of the first walls of the future fusion reactors (ITER, DEMO). The proposed IFMIF (International Fusion Material Irradiation Facility) and DONES (Demo Oriented Neutron Source) systems are based on deuteron linear accelerators (40 MeV, 125 ma) with envisaged start of operation after In order to reduce costs and operational risk of the ITER / DEMO programs, simpler projects with a lower performance are also considered. In accordance with these projects, a new program based on a compact new TR-24 cyclotron (24 MeV, 300 µa) has started at the NPI. The goal is to provide a neutron field with a high flux up to n/cm2 /s, by about one order of magnitude higher than the current neutron flux at the U-120M cyclotron. The power and spectrum of fast neutrons of the projected neutron source, based on the TR-24 cyclotron, provide a suitable tool for experimental investigation of transmutation processes induced by fast neutrons on nuclear waste components generated in fission reactors. Such data are requested by the neutronics of future concepts of transmutation and processing of long-lived, highly radiotoxic nuclear waste. Parameters of a neutron source at the TR-24 cyclotron will also provide a methodical base for radiation hardness Proceedings of the International Conference Nuclear Energy for New Europe, Portoroˇz, Slovenia, September 5-8, 2016

10 Figure 8: The upper part of the figure shows the schematic plan of the experimental setup around the bunched neutron beam. The collimated neutron beam will be usable at the distance 3-6 m and m. The lower part of the figure compares the average flux of powerful neutron sources with the predicted spectrum of the planned source at the NPI. tests of the selected ITER facility components and electronic components to be used under exposure to intermediate energy neutrons (eg. data acquisition at LHC). The tests will be performed under more appropriate conditions compared to experiments carried out at the experimental fission reactors. 8 CONCLUSION The existing neutron generators at the NPI present a unique facility in the frames of the EU. With similar facilities shutting down (UCL Louvain-la-Neuve, TSL Uppsala), delays in construction and high beam time costs of SPIRAL2/NFS, other fusion research neutron sources (IFMIF) in distant future, the NPI neutron generators play an important role in the nuclear research dedicated to the ADS and fusion. The upgrades and new detector systems scheduled in next years assure that they will keep this position also in future.

11 ˇ z on the left and the design drawing Figure 9: The photo of the TR-24 cyclotron installed at the NPI Reˇ of the thick beryllium target for neutron production to be installed at the end of the beam pipe. ACKNOWLEDGMENTS Measurements were carried with the support of the Fusion for Energy (F4E) and International ˇ z supported Atomic Energy Agency (IAEA) grants at the CANAM infrastructure of the NPI ASCR Reˇ ˇ through MSMT project No. LM REFERENCES [1] M. Majerle et al., Nuc. Phys. A 953 (2016) , DOI: /j.nuclphysa [2] S.D. Schery et al., Nucl Instrum Meth 147 (1977) 399, EXFOR ID: B0127. DOI: / X(77) [3] Y. Uwamino, T.S. Soewarsono et al., High-energy p-li neutron field for activation experiment, Nucl Instrum Meth A 389 (1997) , EXFOR ID: E1826, DOI: /S (97) [4] SAND-II-SNL: Neutron Flux Spectra Determination by Multiple Foil Activation-iterative Method, RSICC Shielding Routine Collection PSR-345, Oak Ridge 1996 [5] H. J. Brede et al., Neutron yields from thick Be targets bombarded with deuterons or protons, Nucl Instrum Meth A 274 (1989) 332, DOI: / (89) [6] U. Fischer et al., Neutronics and nuclear data for the IFMIF neutron source, Fus Eng Design 63 (2002) 493, DOI: /S (02) [7] M. Majerle, Final Report F4E-2010-GRT-056 Action2 Task 4.2, available as INDC(CZR)-0002 on [8] M. Majerle, Final Report F4E-FPA Task 4.1, available as INDC(CZR)-0003 on [9] M. Majerle, ND2016 conference proceedings, in preparation. [10] R. Bevilacqua et al., Medley spectrometer for light ions in neutron-induced reactions at 175 MeV, Nucl Instrum Meth A 646 (2011) , DOI: /j.nima Proceedings of the International Conference Nuclear Energy for New Europe, Portoroˇz, Slovenia, September 5-8, 2016

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