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Measurement of neutron energy spectra from 15 to 150 MeV using stacked liquid scintillators
Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
Measurement of neutron energy spectra from 15 to 150 MeV using stacked liquid scintillators A. Bufflera,*, F.D. Brooksa, M.S. Alliea, P.J. Binnsb, V. Dangendorfc, K.M. Langenb, R. Noltec, H. Schuhmacherc a
Department of Physics, University of Cape Town, Rondebosch 7701, South Africa b National Accelerator Centre, Faure, South Africa c Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
Abstract A multiple liquid scintillator system for measuring the energy spectrum of a neutron beam in the range 15–150 MeV is described. Two or more slabs of NE213 scintillator (13 13 7 cm3) are stacked behind one-another and only events in which a neutron interacts in the upstream scintillator are analysed. The system is designed to minimise the escape of forward recoil protons from the detecting media. Test measurements and Monte Carlo simulations of the detector response to quasi-monoenergetic neutron beams of energies 62.5 and 97.5 MeV are presented. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Neutron spectrometry; NE213 scintillator; Response function
1. Introduction Liquid organic scintillators such as NE213 and BC501A have been shown to be effective spectrometers for measuring neutron energy distributions in the range 2–30 MeV [1,2]. These spectrometers are less reliable at higher energies [2], firstly because neutron interactions with carbon nuclei in the scintillator become more important and secondly because the increased proton escape at higher energy distorts the response function of the spectrometer. The increase in n–C interactions is a *Corresponding author. Tel.: +27-21-650-3339; fax: +2721-650-3342. E-mail address: abuffl[email protected] (A. Buffler).
problem because detailed cross-sectional data that are required to calculate the contributions of these interactions to the detector response matrix [1,2] are not available. This problem can be bypassed, fortunately, if the component of the response function due to forward proton recoils from n–p elastic scattering in the scintillator can be isolated and analysed independent of contributions from nC interactions [2,3]. Unfortunately, however, the problem of proton escape is most acute for forward recoil protons, because these are naturally the highest energy component of the response function. We have investigated a method for overcoming this problem by the use of a multiple scintillator system. The principle of the method is similar to that previously employed [4] in a smaller
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A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
system designed to measure neutron spectra at energies in the range 1–15 MeV.
2. The stacked scintillator neutron spectrometer A schematic diagram of the stacked scintillator spectrometer used for neutron energies up to 100 MeV is shown in Fig. 1. A neutron beam (diameter o6 cm) is incident normally on a slab of NE213 liquid scintillator A of cross-section 13 13 cm2 and thickness 7 cm, held in a thinwalled (0.3 mm) iron container. A second identical NE213 scintillator, B, is mounted immediately behind A and a 3 mm thick plastic scintillator V vetoes any events in which charged particles escape downstream of B. Pulse shape discriminators (Link Model 5010) are connected to both A and B and the pulse height gains of the two scintillators are accurately matched. The two essential features of this system are, firstly, that only events in which an initial neutron interaction is detected in A are accepted and, secondly, that for more than 90% of such events, the full energy of charged reaction products produced in the event is captured, either in A alone, or else in A and B together. Pulse shape
discrimination (PSD) is used to reject g-rays and to determine whether protons or other charged particles detected by the scintillators are ‘‘stops’’ (come to rest in the scintillator) or ‘‘escapes’’ (escape from the scintillator). Two types of events are accepted for analysis: ‘‘single’’ events (A-only) and ‘‘coincidence’’ events (A+B). Single events are identified by a ‘‘stop’’ from pulse shape discriminator A and no coincident signal in B. Coincidence events (A+B) are identified by a ‘‘stop’’ from pulse shape discriminator B and an ‘‘escape’’ from pulse shape discriminator A. Thus, the single events correspond to neutron interactions for which the resulting charged particle (or particles) come to rest in A and the coincidence events are predominantly neutron events in which one or more recoils or charged reaction products escape from A into B. The spectrum of pulse heights LA from detector A, called the ‘‘Aspectrum’’, is determined for single events. The spectrum of the summed pulse heights ðLA þ LB Þ; called the ‘‘AB-spectrum’’, is determined for coincidence events. For incident neutron energies of o100 MeV, nearly all events arising from neutron interactions that are detected in A will fall into one or other of these categories. The ‘‘total spectrum’’ formed by summing the A- and AB-spectra is therefore a response function for neutron interactions in A, which is minimally affected by proton escape from the system.
3. Response function measurements
Fig. 1. Schematic diagram of the stacked scintillator spectrometer, showing NE213 liquid scintillators A and B, photomultiplier tubes PMT, light guides LG and plastic scintillator veto detector V.
The response function of the spectrometer was investigated using pulsed beams of quasi-monoenergetic neutrons emitted at an angle of 01 from the 7Li(p,n)7Be reaction. Protons of energy either 66 or 100 MeV from the k ¼ 200 cyclotron of the South African National Accelerator Centre) bombarded a 5 mm thick lithium target. Time-of-flight windows selected neutrons of energy either 62.5 or 97.5 MeV, corresponding to the unresolved (ground state+0.429 MeV) transition in this reaction. Fig. 2 shows plots of counts (vertical) as a function of pulse height LA and pulse shape parameter SA for single events (A-only) produced
A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
183
attributed to 3H or 3He ions, or to the simultaneous detection of two or more charged particles. Fig. 3 shows plots of counts versus pulse height and pulse shape obtained (a) from detector A and (b) from detector B, for coincidence events produced by 97.5 MeV neutrons. These distributions confirm that the coincidences correspond to protons originating in A which escape (locus e in (a)) into B and are brought to rest in B (locus p in (b)). The pulse shape discrimination cuts C1 shown in Figs. 2 and 3 are used to distinguish between the ‘‘escapes’’ and ‘‘stops’’ defined in Section 2. ‘‘Escapes’’ are identified by SðLÞoC1 ðLÞ and ‘‘stops’’ by SðLÞ > C1 ðLÞ: The PSD cuts are applied to select the events used to form the Aspectra and AB-spectra described in Section 2. The A-spectra for the single events measured at 62.5 and 97.5 MeV are shown in Figs. 4a and b,
Fig. 2. Counts (vertical) versus pulse height LA and pulse shape SA for single events in scintillator A when irradiated by neutron beams of energy: (a) 62.5 MeV and (b) 97.5 MeV. The labels show: loci attributed to protons (p), escaping protons (e) deuterons (d) and alphas ðaÞ; and pulse shape discrimination cuts (C1 and C2 ), as explained in the text.
by neutrons of energy: (a) 62.5 MeV; and (b) 97.5 MeV. The pulse height scale is calibrated in MeV-electron-equivalent (MeVee), based on measurements made using 4.43 MeV g-rays. The pulse shape parameters SA and SB were obtained from the Link pulse shape discriminators as described in Ref. [5]. Loci attributed to different charged particles released by n–p elastic scattering or by n–C interactions in the scintillator are identified as follows in Fig. 2: protons (p); charged particles (mainly protons) which escape from A and are not vetoed by B (e); deuterons (d); and a-particles ðaÞ: Events in the region between loci d and a can be
Fig. 3. Counts (vertical) versus pulse height L and pulse shape S from: (a) scintillator A and (b) scintillator B, obtained from coincidence events (A+B) produced by incident neutrons of energy 97.5 MeV. Loci and cuts are labelled as in Fig. 2.
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of this response function obtained using the code SCINFUL [6] is also shown (curve) in Fig. 4d. The AB-spectrum was not calculated for the events measured at 62.5 MeV incident neutron energy, because the number of (A+B) coincidence events is much smaller (o10%) than the number of Aonly events due to the smaller proton range at this lower energy. The A-spectrum alone (Fig. 4a) can therefore be taken as a good approximation to the spectrometer response function for 62.5 MeV neutrons.
4. Discussion and conclusions
Fig. 4. Pulse height spectra measured (histograms) and calculated using the code SCINFUL (curves) for the stacked scintillator system. Panels (a) and (b) show A-only spectra (single events) for incident neutrons of energy 62.5 and 97.5 MeV, respectively. Panel (c) shows the AB-spectrum (for (A+B) coincidence events) for 97.5 MeV neutrons. Panel (d), obtained by summing the spectra shown in (b) and (c), shows the response of the spectrometer to incident neutrons of energy 97.5 MeV. The calculated response functions are normalised to the experimental measurements in the pulse height region L > LT ; which may be attributed to n–p elastic scattering only.
respectively, together with simulations obtained using the code SCINFUL [6]. The simulations were carried out in cylindrical geometry (as required by SCINFUL) of the same cross-sectional area and thickness as the detector modules used and utilizing experimentally determined proton output light yield functions. The ABspectrum for coincidence events at 97.5 MeV is shown in Fig. 4c. Fig. 4d (histogram) shows the total spectrum obtained by summing the Aspectrum (Fig. 4b) and the AB-spectrum (Fig. 4c). This is the response function of the stacked spectrometer for 97.5 MeV neutrons. A simulation
The upper regions of the measured response functions (Figs. 4a and d), which correspond to n– p scattering only, are well reproduced in the simulated spectra, whereas agreement is poor in the lower pulse height regions, which include substantial contributions from n–C interactions. Thus, for monoenergetic neutrons, the neutron detection efficiency of the spectrometer may be determined, relative to the well-known n-p elastic scattering cross section, by using a threshold pulse height LT (see Figs. 4a and d) which selects only the upper, n–p region. The detection efficiencies for a lower threshold suitable for neutron spectrometry over a wide energy range, may then be estimated directly from the measured response functions [7,8]. Further measurements have been made, employing the same technique as used at 97.5 MeV, to determine response functions of the stacked scintillator spectrometer for neutrons of energy 118 and 148 MeV. The thickness of the B scintillator was doubled for these measurements, by adding a third scintillator module, identical to B, between B and V (Fig. 1). Measurements have also been made using a thick graphite target instead of the thin lithium target (in the proton beam). The graphite target provides a continuous neutron spectrum from which a series of discrete neutron energies may be selected off-line, by means of different time-of-flight windows, in order to determine the spectrometer response matrix over a wide energy range.
A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
In the analyses described above, the PSD cut C1 ðLÞ (Figs. 2 and 3) was set to select ‘‘stop’’ events (Section 2) corresponding to the detection of one or more protons or heavier particles. An alternative procedure in which ‘‘stops’’ correspond to the detection of single protons (events between cuts C1 ðLÞ and C2 ðLÞ in Figs 2 and 3) is also being tested. The simulation of response functions measured for events satisfying this constraint should be simpler and more accurate for the following reasons: the events selected will include a larger fraction of recoils from n–p scattering; many of the possible n–C interactions will be excluded leaving only a few better-known reactions such as 12C(n, p)12B and 12C(n, np)11B to be simulated; multiple interactions or scatterings in which two or more charged particles are detected will be excluded; and the only response-versusenergy function required for the simulation will be that for protons. In summary, we conclude that the stacked liquid scintillator system employing two scintillator modules, as in Fig. 1, is a useful neutron spectrometer for neutron energies up to 100 MeV and that a system of three such modules appears suitable for measurements up to 150 MeV. Some practical advantages associated with the modular design may also be noted: it is flexible insofar that the number of modules used can be tailored to match the energy range of interest; the combined system can operate linearly over a significantly wider range of pulse height than any one of its constituent modules; and the uniform design simplifies the task of matching pulse height and pulse shape discrimination responses of the differ-
185
ent modules, as required for the operation of the spectrometer.
Acknowledgements We thank the staff of the National Accelerator Centre for their help in carrying out the experiments, the staff of the UCT Physics Department workshop for constructing the equipment used and Drs. H. Klein and B. Wiegel for valuable discussions.
References [1] S. Guldbakke, H. Klein, A. Meister, J. Pulpan, U. Scheler, M. Tichy, S. Unholzer, in: H. Farrar, et al. (Eds.), Proceedings of the VIII. ASTM-EURATOM Symposium on Reactor Dosimetry Vail/USA (1993) 280, American Society for Testing and Materials, Philadelphia, 1994. [2] R. Nolte, H.J. Brede, U.J. Schrewe, H. Schuhmacher, Report PTB-N-9, Physikalisch-Technische Bundesanstalt, Braunschweig, 1993. [3] H. Schuhmacher, H.J. Brede, V. Dangendorf, M. Kuhfu, J.P. Meulders, W.D. Newhauser, R. Nolte, Nucl. Instr. and Meth. A 421 (1999) 284. [4] M.S. Dias, R.G. Johnson, O.A. Watson, Nucl. Instr. and Meth. 223 (1984) 532.M.S. Dias, Nucl. Instr. and Meth. A 321 (1992) 317. [5] F.D. Smit, F.D. Brooks, Nucl. Phys. A 465 (1987) 429. [6] J.K. Dickens, Report ORNL-6462, Oak Ridge National Laboratory, Oak Ridge, 1988. [7] L.C. Northcliffe, C.W. Lewis, D.P. Sailor, Nucl. Instr. and Meth. 83 (1970) 93. [8] K. Shin, Y. Ishii, Y. Uwamino, H. Sakai, S. Numata, Nucl. Instr. and Meth. A 308 (1991) 609.
Measurement of neutron energy spectra from 15 to 150 MeV using stacked liquid scintillators A. Bufflera,*, F.D. Brooksa, M.S. Alliea, P.J. Binnsb, V. Dangendorfc, K.M. Langenb, R. Noltec, H. Schuhmacherc a
Department of Physics, University of Cape Town, Rondebosch 7701, South Africa b National Accelerator Centre, Faure, South Africa c Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
Abstract A multiple liquid scintillator system for measuring the energy spectrum of a neutron beam in the range 15–150 MeV is described. Two or more slabs of NE213 scintillator (13 13 7 cm3) are stacked behind one-another and only events in which a neutron interacts in the upstream scintillator are analysed. The system is designed to minimise the escape of forward recoil protons from the detecting media. Test measurements and Monte Carlo simulations of the detector response to quasi-monoenergetic neutron beams of energies 62.5 and 97.5 MeV are presented. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.30.Hs; 29.40.Mc Keywords: Neutron spectrometry; NE213 scintillator; Response function
1. Introduction Liquid organic scintillators such as NE213 and BC501A have been shown to be effective spectrometers for measuring neutron energy distributions in the range 2–30 MeV [1,2]. These spectrometers are less reliable at higher energies [2], firstly because neutron interactions with carbon nuclei in the scintillator become more important and secondly because the increased proton escape at higher energy distorts the response function of the spectrometer. The increase in n–C interactions is a *Corresponding author. Tel.: +27-21-650-3339; fax: +2721-650-3342. E-mail address: abuffl[email protected] (A. Buffler).
problem because detailed cross-sectional data that are required to calculate the contributions of these interactions to the detector response matrix [1,2] are not available. This problem can be bypassed, fortunately, if the component of the response function due to forward proton recoils from n–p elastic scattering in the scintillator can be isolated and analysed independent of contributions from nC interactions [2,3]. Unfortunately, however, the problem of proton escape is most acute for forward recoil protons, because these are naturally the highest energy component of the response function. We have investigated a method for overcoming this problem by the use of a multiple scintillator system. The principle of the method is similar to that previously employed [4] in a smaller
0168-9002/02/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 4 2 7 - 9
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A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
system designed to measure neutron spectra at energies in the range 1–15 MeV.
2. The stacked scintillator neutron spectrometer A schematic diagram of the stacked scintillator spectrometer used for neutron energies up to 100 MeV is shown in Fig. 1. A neutron beam (diameter o6 cm) is incident normally on a slab of NE213 liquid scintillator A of cross-section 13 13 cm2 and thickness 7 cm, held in a thinwalled (0.3 mm) iron container. A second identical NE213 scintillator, B, is mounted immediately behind A and a 3 mm thick plastic scintillator V vetoes any events in which charged particles escape downstream of B. Pulse shape discriminators (Link Model 5010) are connected to both A and B and the pulse height gains of the two scintillators are accurately matched. The two essential features of this system are, firstly, that only events in which an initial neutron interaction is detected in A are accepted and, secondly, that for more than 90% of such events, the full energy of charged reaction products produced in the event is captured, either in A alone, or else in A and B together. Pulse shape
discrimination (PSD) is used to reject g-rays and to determine whether protons or other charged particles detected by the scintillators are ‘‘stops’’ (come to rest in the scintillator) or ‘‘escapes’’ (escape from the scintillator). Two types of events are accepted for analysis: ‘‘single’’ events (A-only) and ‘‘coincidence’’ events (A+B). Single events are identified by a ‘‘stop’’ from pulse shape discriminator A and no coincident signal in B. Coincidence events (A+B) are identified by a ‘‘stop’’ from pulse shape discriminator B and an ‘‘escape’’ from pulse shape discriminator A. Thus, the single events correspond to neutron interactions for which the resulting charged particle (or particles) come to rest in A and the coincidence events are predominantly neutron events in which one or more recoils or charged reaction products escape from A into B. The spectrum of pulse heights LA from detector A, called the ‘‘Aspectrum’’, is determined for single events. The spectrum of the summed pulse heights ðLA þ LB Þ; called the ‘‘AB-spectrum’’, is determined for coincidence events. For incident neutron energies of o100 MeV, nearly all events arising from neutron interactions that are detected in A will fall into one or other of these categories. The ‘‘total spectrum’’ formed by summing the A- and AB-spectra is therefore a response function for neutron interactions in A, which is minimally affected by proton escape from the system.
3. Response function measurements
Fig. 1. Schematic diagram of the stacked scintillator spectrometer, showing NE213 liquid scintillators A and B, photomultiplier tubes PMT, light guides LG and plastic scintillator veto detector V.
The response function of the spectrometer was investigated using pulsed beams of quasi-monoenergetic neutrons emitted at an angle of 01 from the 7Li(p,n)7Be reaction. Protons of energy either 66 or 100 MeV from the k ¼ 200 cyclotron of the South African National Accelerator Centre) bombarded a 5 mm thick lithium target. Time-of-flight windows selected neutrons of energy either 62.5 or 97.5 MeV, corresponding to the unresolved (ground state+0.429 MeV) transition in this reaction. Fig. 2 shows plots of counts (vertical) as a function of pulse height LA and pulse shape parameter SA for single events (A-only) produced
A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
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attributed to 3H or 3He ions, or to the simultaneous detection of two or more charged particles. Fig. 3 shows plots of counts versus pulse height and pulse shape obtained (a) from detector A and (b) from detector B, for coincidence events produced by 97.5 MeV neutrons. These distributions confirm that the coincidences correspond to protons originating in A which escape (locus e in (a)) into B and are brought to rest in B (locus p in (b)). The pulse shape discrimination cuts C1 shown in Figs. 2 and 3 are used to distinguish between the ‘‘escapes’’ and ‘‘stops’’ defined in Section 2. ‘‘Escapes’’ are identified by SðLÞoC1 ðLÞ and ‘‘stops’’ by SðLÞ > C1 ðLÞ: The PSD cuts are applied to select the events used to form the Aspectra and AB-spectra described in Section 2. The A-spectra for the single events measured at 62.5 and 97.5 MeV are shown in Figs. 4a and b,
Fig. 2. Counts (vertical) versus pulse height LA and pulse shape SA for single events in scintillator A when irradiated by neutron beams of energy: (a) 62.5 MeV and (b) 97.5 MeV. The labels show: loci attributed to protons (p), escaping protons (e) deuterons (d) and alphas ðaÞ; and pulse shape discrimination cuts (C1 and C2 ), as explained in the text.
by neutrons of energy: (a) 62.5 MeV; and (b) 97.5 MeV. The pulse height scale is calibrated in MeV-electron-equivalent (MeVee), based on measurements made using 4.43 MeV g-rays. The pulse shape parameters SA and SB were obtained from the Link pulse shape discriminators as described in Ref. [5]. Loci attributed to different charged particles released by n–p elastic scattering or by n–C interactions in the scintillator are identified as follows in Fig. 2: protons (p); charged particles (mainly protons) which escape from A and are not vetoed by B (e); deuterons (d); and a-particles ðaÞ: Events in the region between loci d and a can be
Fig. 3. Counts (vertical) versus pulse height L and pulse shape S from: (a) scintillator A and (b) scintillator B, obtained from coincidence events (A+B) produced by incident neutrons of energy 97.5 MeV. Loci and cuts are labelled as in Fig. 2.
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A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
of this response function obtained using the code SCINFUL [6] is also shown (curve) in Fig. 4d. The AB-spectrum was not calculated for the events measured at 62.5 MeV incident neutron energy, because the number of (A+B) coincidence events is much smaller (o10%) than the number of Aonly events due to the smaller proton range at this lower energy. The A-spectrum alone (Fig. 4a) can therefore be taken as a good approximation to the spectrometer response function for 62.5 MeV neutrons.
4. Discussion and conclusions
Fig. 4. Pulse height spectra measured (histograms) and calculated using the code SCINFUL (curves) for the stacked scintillator system. Panels (a) and (b) show A-only spectra (single events) for incident neutrons of energy 62.5 and 97.5 MeV, respectively. Panel (c) shows the AB-spectrum (for (A+B) coincidence events) for 97.5 MeV neutrons. Panel (d), obtained by summing the spectra shown in (b) and (c), shows the response of the spectrometer to incident neutrons of energy 97.5 MeV. The calculated response functions are normalised to the experimental measurements in the pulse height region L > LT ; which may be attributed to n–p elastic scattering only.
respectively, together with simulations obtained using the code SCINFUL [6]. The simulations were carried out in cylindrical geometry (as required by SCINFUL) of the same cross-sectional area and thickness as the detector modules used and utilizing experimentally determined proton output light yield functions. The ABspectrum for coincidence events at 97.5 MeV is shown in Fig. 4c. Fig. 4d (histogram) shows the total spectrum obtained by summing the Aspectrum (Fig. 4b) and the AB-spectrum (Fig. 4c). This is the response function of the stacked spectrometer for 97.5 MeV neutrons. A simulation
The upper regions of the measured response functions (Figs. 4a and d), which correspond to n– p scattering only, are well reproduced in the simulated spectra, whereas agreement is poor in the lower pulse height regions, which include substantial contributions from n–C interactions. Thus, for monoenergetic neutrons, the neutron detection efficiency of the spectrometer may be determined, relative to the well-known n-p elastic scattering cross section, by using a threshold pulse height LT (see Figs. 4a and d) which selects only the upper, n–p region. The detection efficiencies for a lower threshold suitable for neutron spectrometry over a wide energy range, may then be estimated directly from the measured response functions [7,8]. Further measurements have been made, employing the same technique as used at 97.5 MeV, to determine response functions of the stacked scintillator spectrometer for neutrons of energy 118 and 148 MeV. The thickness of the B scintillator was doubled for these measurements, by adding a third scintillator module, identical to B, between B and V (Fig. 1). Measurements have also been made using a thick graphite target instead of the thin lithium target (in the proton beam). The graphite target provides a continuous neutron spectrum from which a series of discrete neutron energies may be selected off-line, by means of different time-of-flight windows, in order to determine the spectrometer response matrix over a wide energy range.
A. Buffler et al. / Nuclear Instruments and Methods in Physics Research A 476 (2002) 181–185
In the analyses described above, the PSD cut C1 ðLÞ (Figs. 2 and 3) was set to select ‘‘stop’’ events (Section 2) corresponding to the detection of one or more protons or heavier particles. An alternative procedure in which ‘‘stops’’ correspond to the detection of single protons (events between cuts C1 ðLÞ and C2 ðLÞ in Figs 2 and 3) is also being tested. The simulation of response functions measured for events satisfying this constraint should be simpler and more accurate for the following reasons: the events selected will include a larger fraction of recoils from n–p scattering; many of the possible n–C interactions will be excluded leaving only a few better-known reactions such as 12C(n, p)12B and 12C(n, np)11B to be simulated; multiple interactions or scatterings in which two or more charged particles are detected will be excluded; and the only response-versusenergy function required for the simulation will be that for protons. In summary, we conclude that the stacked liquid scintillator system employing two scintillator modules, as in Fig. 1, is a useful neutron spectrometer for neutron energies up to 100 MeV and that a system of three such modules appears suitable for measurements up to 150 MeV. Some practical advantages associated with the modular design may also be noted: it is flexible insofar that the number of modules used can be tailored to match the energy range of interest; the combined system can operate linearly over a significantly wider range of pulse height than any one of its constituent modules; and the uniform design simplifies the task of matching pulse height and pulse shape discrimination responses of the differ-
185
ent modules, as required for the operation of the spectrometer.
Acknowledgements We thank the staff of the National Accelerator Centre for their help in carrying out the experiments, the staff of the UCT Physics Department workshop for constructing the equipment used and Drs. H. Klein and B. Wiegel for valuable discussions.
References [1] S. Guldbakke, H. Klein, A. Meister, J. Pulpan, U. Scheler, M. Tichy, S. Unholzer, in: H. Farrar, et al. (Eds.), Proceedings of the VIII. ASTM-EURATOM Symposium on Reactor Dosimetry Vail/USA (1993) 280, American Society for Testing and Materials, Philadelphia, 1994. [2] R. Nolte, H.J. Brede, U.J. Schrewe, H. Schuhmacher, Report PTB-N-9, Physikalisch-Technische Bundesanstalt, Braunschweig, 1993. [3] H. Schuhmacher, H.J. Brede, V. Dangendorf, M. Kuhfu, J.P. Meulders, W.D. Newhauser, R. Nolte, Nucl. Instr. and Meth. A 421 (1999) 284. [4] M.S. Dias, R.G. Johnson, O.A. Watson, Nucl. Instr. and Meth. 223 (1984) 532.M.S. Dias, Nucl. Instr. and Meth. A 321 (1992) 317. [5] F.D. Smit, F.D. Brooks, Nucl. Phys. A 465 (1987) 429. [6] J.K. Dickens, Report ORNL-6462, Oak Ridge National Laboratory, Oak Ridge, 1988. [7] L.C. Northcliffe, C.W. Lewis, D.P. Sailor, Nucl. Instr. and Meth. 83 (1970) 93. [8] K. Shin, Y. Ishii, Y. Uwamino, H. Sakai, S. Numata, Nucl. Instr. and Meth. A 308 (1991) 609.