Neutron cross-section measurements at the n_tof facility at CERN

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1 Nuclear Instruments and Methods in Physics Research B 213 (2004) Neutron cross-section measurements at the n_tof facility at CERN N. Colonna *, The n_tof Collaboration Istituto Nazionale Fisica Nucleare, Dipartimento di Fisica, Sezione di Bari, Via Amendola 173, V. Orabona 4, I Bari, Italy Abstract A neutron Time-of-Flight facility (n_tof)has recently become operative at CERN. The innovative features of the neutron beam, in particular the high instantaneous flux, the wide energy range, the high resolution and the low background, make this facility unique for measurements of neutron-induced reactions relevant to the field of emerging nuclear technologies, as well as to Nuclear Astrophysics and fundamental Nuclear Physics. The n_tof facility is here described, together with the main features of the experimental apparata used for crosssection measurements. The results of the first measurement campaign, which have confirmed the innovative aspects of the facility, are presented. The measurement plan of the n_tof collaboration, in particular with regard to implications to ADS, is briefly discussed. Ó 2003 Elsevier B.V. All rights reserved. PACS: Fc; Ec; kw; Dz Keywords: Neutron cross-section; Accelerator driven systems; Neutron time-of-flight facility 1. Introduction New developments and ideas in nuclear technology have recently raised the need for high-accuracy neutron cross-section data on a variety of isotopes. At present, available data are in fact insufficient for new medical and industrial applications, and in particular for the development of innovative concepts, such as Accelerator Driven Systems (ADS), currently being investigated as a promising solution for safer energy production and nuclear waste incineration [1 4]. A reliable * Corresponding author. Tel.: ; fax: address: nicola.colonna@ba.infn.it (N. Colonna). design and development of ADS requires highprecision experimental determination of crosssections for neutron capture, neutron-induced fission and neutron inelastic scattering on several isotopes, mainly radioactive. Capture and fission data are needed for fertile and fissile isotopes involved in the Th-cycle, such as 232 Th, 231 Pa, 233 U, 234 U and 236 U. Similarly, the design of ADS for nuclear waste incineration requires reliable experimentally determined capture, fission and (n,xn) cross-sections for transuranic isotopes, in particular 237 Np, 238;240;241 Pu, 241;243 Am and 244;245 Cm, while the incineration scheme of long lived fission products requires accurate data on capture reactions for 79 Se, 99 Tc, 129 I, 135 Cs, 151 Sm, etc... Finally, cross-section for structural material being considered as neutron-production target or as X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi: /s x(03)

2 50 N. Colonna / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) coolant, are still far from being accurately known, especially at high neutron energy. Many of the aforementioned needs can be consistently addressed at a neutron time-of-flight facility recently set-in-operation at CERN: n_tof. Based on an idea of Rubbia et al. [5], an extremely intense pulsed neutron beam, with white spectrum in the range 1 ev 250 MeV, is produced by spallation of 20 GeV protons from the PS accelerator on a lead target. The high instantaneous neutron flux, low duty cycle, high resolution and low background make this facility unique for cross-section measurements relevant to many fields of applied and fundamental Physics. Thanks to the high instantaneous neutron flux, n_tof results particularly suited for capture measurements on radioactive isotopes, nearly impossible to measure at other existing facilities, while the wide energy range allows one to extend the present knowledge of the fission and inelastic cross-sections to energy regions still largely unexplored. Furthermore, the new facility is expected to produce significant advances in the knowledge of capture processes involved in Stellar Nucleosyntesis, allowing one to address a number of still open questions, mostly involving radioactive species or isotopes with very low cross-sections. A large international collaboration, supported by the European Commission V Framework Program, has recently started a vast experimental program on capture, fission and inelastic reactions at n_tof [6]. The main objective is the determination of long needed neutron cross-sections of primary importance for the design of Accelerator Driven Systems, with a precision and completeness that would meet the industrial requirements. In parallel a theoretical activity is being carried out by the n_tof Theory Group, with the aim of producing evaluated cross-sections data ready for use by the scientific and industrial community. The facility, experimental apparata and first results at n_tof are here presented. 2. The n_tof facility Neutrons in the wide energy range (1 ev 250 MeV)are generated by spallation of 20 GeV protons from the PS accelerator complex on a lead target, cm 3. A 5 cm water layer surrounds the target, acting both as coolant and as a moderator, to produce an isolethargic neutron flux distribution over a wide energy range. Neutrons emerging from the target propagate in a vacuum tube mounted inside a time-of-flight tunnel 200 m long. Along the flight path, two collimators placed at 135 and 180 m from the spallation target, with aperture of 13.5 and 2 cm diameter, respectively, are used to shape the neutron beam to the desired dimension. Charged particles travelling along the vacuum tube are deflected outside the beam by a magnet located 40 m upstream of the experimental area. For an efficient background suppression, several concrete and iron walls are mounted inside the tunnel, following the spallation target, the two collimators and the sweeping magnet. While concrete provides an efficient shielding to neutrons and c-rays, iron walls of several meters of thickness are necessary to attenuate positive and negative muons abundantly produced in the spallation process, which would generate an intense prompt flash in the detectors or result in a large neutron and gamma background, following capture of negative muons in the experimental area. A measuring station is located inside the n_tof tunnel, centred at m from the spallation target, and delimited by two walls 7.5 m apart. An escape lane, 12 m long and ending in a polyethylene block, ensures a negligible background from backscattered neutrons and capture c-rays. The main features of the n_tof neutron beam, as currently available to experimenters, are summarized in Table 1. The high proton intensity of p/pulse, combined with the large number of neutrons produced by spallation of high-energy protons, results in an instantaneous neutron flux more than two orders of magnitude higher than the one available at other facilities. This feature results particularly useful for cross-section measurements of radioactive samples, as it improves considerably the signal-to-background ratio, in particular for samples with high specific activity. Other important features of the n_tof neutron beam are the high resolution and the low level of ambient background, that are expected to result in a significant improvement of the quality and

3 N. Colonna / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) Table 1 Main features of the n_tof facility Neutron energy range 1 ev 250 MeV Proton beam energy and intensity 20 GeV/c; p/pulse Pulse repetition frequency 0.25 s 1 (average in dedicated mode) Neutron flux at m (uncollimated) n/cm 2 /pulse Neutron flux with ¼ 1:9 collimator n/cm 2 /pulse Fraction of flux in 1 ev 1 MeV range 2/3 Resolution ev; kev Background (fluence out/in beam)10 5 accuracy of existing experimental data over a wide energy range. Finally, the availability of high-energy neutrons (up to several hundred MeV)is of great importance for studies of fission and inelastic processes involved in the operation of spallation neutron sources in Accelerator Driven Systems. 3. The experimental setup Different detection systems have been developed by the n_tof Collaboration for measurements of capture, neutron-induced fission and inelastic reactions. Monitoring of the neutron beam up to 1 MeV is performed by a low-mass system, based on a thin Mylar foil with a 6 Li deposit placed in the beam, surrounded by an array of silicon detectors placed outside the beam, in which tritons and alpha particles from the 6 Li(n,a)reaction are detected. The small amount of material in the beam ensures a negligible level of scattered neutrons, while minimization of the neutron-induced c-background has been achieved by constructing the scattering chamber with carbon fibre. The measurement of neutron-induced fission processes is performed with a stack of several position sensitive Parallel Plate Avalanche Counters (PPAC), with thin entrance windows. The measured isotopes are deposited on thin Mylar or Al backings, placed in between PPACÕs so that both fission fragments from the reaction can be detected. Together with the position information, the coincidence requirements allows one to reject spurious reactions, in particular the a-particles from the natural radioactivity of the samples and the charged products of neutron elastic and inelastic reactions with the windows and sample backing. Another advantage of PPAC consists in the fast charge collection time, particularly useful at n_tof, as it minimizes the probability of pile-up events. Monitoring of the neutron flux in fission measurements is performed with 235 U sample, covering the whole energy range, and by 238 U and 209 Bi for the high-energy region. Together with PPAC, a multi-layer Fission Ionization Chamber has also been constructed and will be used for ADS-related measurements at n_tof. In this case, the optimisation of the gas pressure and of the electrode distance leads to a good discrimination of fission fragments from other competing reactions. Measurements of capture cross-sections in the first phase of the n_tof project are being performed with a set of C 6 D 6 -based liquid scintillator detectors, characterized by a low sensitivity to lowenergy neutrons. The measurements are based on the detection of c-rays emitted in the de-excitation cascade following neutron capture. At present, two types of detectors are being used: commercially available Bicron cells BC537, modified to reduce the neutron sensitivity, and specifically designed detectors with carbon fibre container, characterized by an extremely low neutron sensitivity that makes them suited even for samples with a large scattering to capture ratio. A remotely-controlled carbon fibre sample changer connected to the n_tof vacuum tube and hosting up to 10 samples allows one to perform periodically background and reference measurements, needed for the accurate determination of the capture cross-section. The measurement of capture cross-sections with C 6 D 6 detectors relies on the use of a pulse height weighting function technique, which consists of modifying by software the detectorõs response so

4 52 N. Colonna / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) that the overall efficiency does not depend on the details of the cascade (for example the multiplicity), but only on the total capture energy [7]. Weighting functions for each sample are obtained by means of detailed Monte Carlo simulations of the detector response, validated against the well-known capture cross-sections for 197 Au, 109 Ag and nat Fe. Due to the high instantaneous neutron flux, several events are generally produced inside the detectors for a single neutron bunch, giving rise in some case to pile-up between signals. In these conditions standard acquisition systems are mostly inadequate. A new type of DAQ has been therefore specifically designed at n_tof, based exclusively on the use of high-frequency Flash ADC. At present, commercially available modules, capable of 1 GS/s sampling rate, and equipped with 16 MB buffer memory for each channel are being used. To reduce the amount of data stored on disk, software zero suppression is applied, so that only a small number of points, including a few pre- and postsamples for baseline determination, are kept for signals above a given threshold. A suitable procedure for signal reconstruction allows one to extract all pertinent information, such as the time-offlight, amplitude, charge and, for liquid scintillator detectors, the n/c discrimination [8]. 4. First results at n_tof: beam characterization The first measurement campaign, performed in 2001, was dedicated to the determination and experimental validation of the n_tof neutron beam. In particular, the activity concentrated on the measurement of the neutron flux, beam profile, energy resolution and background. Several detection systems were used for the measurement of the neutron flux. A calibrated fission chamber with standard 235 U and 238 U deposit [9], and a stack of PPAC with 235;238 U and 209 Bi deposits, provided accurate information on the whole energy range, while further information in the low-energy region were obtained from the silicon-based flux monitor, and from Au foil activation measurement (at 4.9 ev). Fig. 1 shows a comparison of the different results, together with the predictions obtained from an accurate simulation of the spallation process and of the neutron transport through all elements of the time-of-flight Fig. 1. Simulated and measured neutron intensity in the measuring station (normalized to the nominal proton bunch of p). The different curves represent the results obtained in different measurements.

5 N. Colonna / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) facility. All measurements agree within their associated uncertainty, and indicate a neutron flux only slightly smaller than expected. The results clearly demonstrate the innovative features of the n_tof neutron beam, characterized by a white energy spectrum spanning over more than 10 orders of magnitude, with high instantaneous flux. The neutron background inside the experimental area was measured with 3 He detectors, while C 6 D 6 were employed to characterize the c-background. After the first measurements, which revealed an unexpectedly high ambient background produced by capture of negative muons in the experimental area, an additional iron shielding was mounted in the tunnel, thus restoring the expected level, approximately one order of magnitude smaller that the one present at other facilities. The low-background and high-resolution features of the n_tof neutron beam are evident in Fig. 2, which shows the comparison between the 235 U(n,f)reaction, measured with the PPAC fission setup, and the tabulated reaction cross-section, from the ENDF-B/VI database. An excellent agreement between experimental and evaluated cross-sections is observed, which testifies the high quality of the neutron beam and the reliability of the measured data. A more accurate indication of the resolution of the n_tof neutron beam was obtained from the Fig. 2. Comparison between 235 U(n,f)data, measured at n_tof with Parallel Plate Avalanche Counter (histogram)and evaluated cross-sections from the ENDF-B/VI database. A perfect matching between experimental results and tabulated data demonstrate the high-accuracy achievable at the n_tof neutron beam. analysis of the 1.15 kev resonance in the Fe(n,c) reaction. Fig. 3 shows the measured capture yield, obtained from the C 6 D 6 after applying the Pulse Height Weighting Function technique. A fit of the resonance, performed with the code SAMMY [10] and including Doppler broadening, indicates that less that 1 ev of the measured resonance width can be attributed to the n_tof resolution functions, corresponding to a DE=E < 10 3, as expected for Fig. 3. Weighted capture resonance in Fe(n,c)reaction, measured with C 6 D 6 detectors for a 1.5 mm thick Fe sample (symbols). The curve shows the result of a fit performed with the code SAMMY. A neutron beam energy resolution <10 3 has been estimated from the fit.

6 54 N. Colonna / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) the low-energy region. An analysis of the resolution at higher neutron energy is in progress. 5. Conclusions A vast program on the measurements of long needed neutron cross-sections for ADS-related studies has started at the innovative neutron timeof-flight facility (n_tof), recently set-in-operation at CERN. Capture, neutron-induced fission and inelastic cross-sections will be studied taking advantage of the innovative features of the facility and of the high-performance detection and DAQ systems set up by the n_tof Collaboration. The first experimental results have confirmed the expectations for the facility, in terms of neutron flux, resolution and background, thus opening the way to the planned program on several isotopes relevant to energy production, nuclear waste transmutation and Stellar Nucleosyntesis. Acknowledgements This work is supported by the European Commission under the contract no. FIKW-CT References [1] C. Rubbia et al., CERN/AT/95-44, CERN, [2] C.D. Bowman, Ann. Rev. Nucl. Part. Sci. 48 (1998)505. [3] S. Leray, Nucl. Instr. and Meth. B 113 (1996)495. [4] M. Salvatores et al., Nucl. Instr. and Meth. A 414 (1998) 5. [5] C. Rubbia et al., CERN/LHC/98-02, CERN, [6] S. Abramovich et al., CERN/SPSC 99-8, SPSC-P-310, CERN, [7] F. Corvi, G. Fioni, F. Gasperini, P.B. Smith, Nucl. Sci. Eng. 107 (1991)272. [8] S. Marrone et al., Nucl. Instr. and Meth. A 490 (2002) 299. [9] D.B. Gayther, Metrologia 27 (1990)221. [10] N.M. Larson, ORNL/TM-9179/R5, 2001.