- Email: [email protected]
Position-sensitive neutron detector
Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Position-sensitive neutron detector A.V. Kuznetsova,*, D.N. Vakhtina,b, Z. Radivojevichb, I.D. Alkhazova, V.G. Lyapina, W.H. Trzaskab b
a V.G. Khlopin Radium Institute, 2-Murinski pr., 28, 194021 St. Petersburg, Russia Department of Physics, Accelerator Laboratory, University of Jyvaskyl a, a, . . P.O. Box 35, FIN-40351 Jyvaskyl . . Finland
Abstract A position-sensitive neutron detector has been developed for use in nuclear physics research. The detector consists of a +5.5 cm 100 cm long quartz tube filled with liquid scintillator viewed from both ends by photomultipliers and enclosed in a light-tight titanium container. The properties of the detector were determined both experimentally and by Monte Carlo simulations (EFEN code). A time resolution of 0.4 ns was reached resulting in the position resolution of less than 4 cm. The neutron registration efficiency varies from 36% to 20% within neutron energy range 1–10 MeV and is practically independent of the position along the detector length. Good n–g separation is achieved for neutron energies greater than 0.5 MeV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Mc; 29.30.Hs Keywords: Neutron detector position sensitive
1. Introduction The demand for position-sensitive neutron detectors (PSND) is generated by two contradicting trends. On one hand, there is a growing need to cover large solid angle and to provide high spatial granularity of the measurements and, on the other hand, the requirement to limit the costs and complexity of multi-detector arrays. For instance, a PSND like ours, with granularity equal to 10 individual detectors, costs about 5 times less and needs only 20% of the electronics and data acquisition parameters as compared to a traditional 10-detector system. In addition, PSND is
easier to tune and monitor due to the reduced number of photomultipliers. Recently, there have been a number of publications [1–3] in which scintillator-based PSNDs were described. These detectors, however, were often made for use in a single in-beam experiment, and were abandoned after that. In this article, we report on the recent upgrade of our PSND which is being successfully and extensively used since 1994 in various nuclear physics experiments ranging from nuclear dynamics studies to the applications of neutron beams.
2. Construction and principles of operation of PSND *Corresponding author. Tel.: +7-812-2470173; fax: +7-8122478095. E-mail address: [email protected] (A.V. Kuznetsov).
The detector consists of a +5.5 cm 100 cm long quartz tube filled with liquid scintillator
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 8 3 3 - 2
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
373
Fig. 1. Scheme of the PSND.
Fig. 3. Principle of time and position determination: tFtime of flight; t1 ; t2 Fmean times needed by light to reach PMs; XFposition of the interaction point; LFlength of the detector; nFrefractive index; cFspeed of light in vacuum.
Time of flight, position and n–g separation are obtained in the following ways: *
Fig. 2. Scheme of the PSND electronics: PM1, PM2Fphotomultipliers; CFDFconstant fraction discriminators; n–gF neutron–gamma separation schemes; QDCFcharge to digit converter; TDCFtime to digit converter.
* *
* *
NE-213 viewed from both ends by photomultipliers (PMs) PHILIPS XP2020, and enclosed in a light-tight titanium container (see Fig. 1) [4,5]. The construction of the holders and the expansion volume allows both horizontal and vertical mounting of the PSND. Two signals from anodes of both PMs are processed by a dedicated signal processor (SP) which occupies one standard CAMAC slot. SP includes two constant fraction discriminators and two pulse-shape analysis schemes similar to those described in Ref. [6]. These analogue pulse-shape schemes allow to avoid multiple gates which are usually needed for particle identification in liquid scintillators. The resulting signals are digitized by fast multi-channel TDCs and QDCs producing timing information (t1 ; t2 ), fast and total pulse components (E1;fast ; E1;total ; E2;fast ; E2;total ). In case of a multi-detector array, SP block’s outputs can be plugged into a low-cost QDCs triggered with common gate (Fig. 2).
(t1 þ t2 )/2 ) TOF (time of flight), t1 t2 ) X (position), E1;total =E2;total ) X (additional position information), E1;total ; E1;fast ; E2;total ; E2;fast ) n–g separation, Etotal ; TOF ) n–g separation.
The method of TOF and position extraction is further illustrated in Fig. 3.
3. Monte-Carlo calculations with EFEN code Before any physically meaningful data can be obtained from measurements with a scintillatorbased PSND, one should determine the intrinsic efficiency of the detector, which can be significantly less than 100%. One should keep in mind that the path of the neutron inside the active volume of the detector depends on the geometry of the experiment; this means that for any possible geometry the efficiency should be determined separately, which makes the experimental determination of the intrinsic neutron registration efficiency a difficult and time-consuming task. Using a well-known cross-sections for neutron interaction with the NE-213 ingredients as the basis of numerical simulation, the EFEN computer simulation code has been written [7]. The EFEN
374
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 4. Mean traveled path of neutrons scattered in the NE213 before the first detectable interaction, calculated by EFEN simulation. Resonances at certain energies correspond to resonances in cross-sections for neutron interaction with constituents of the NE213 scintillator (hydrogen and carbon).
Monte-Carlo simulation is based on ENDF/B VI data library and calculates the efficiency of the PSND for any geometry and neutron registration threshold. As a complete 3D neutron-transport code, the simulation allows extraction of a lot of useful variables, e.g. in order to estimate the time error in neutron’s TOF caused by the finite size of the PSND, the EFEN simulation calculated the mean value of the traveled paths for scattered neutrons in the active volume of the detector. Fig. 4 shows the energy dependence of the mean traveled path of a neutron in the scintillator before the first detectable interaction. The curve saturates at 2.55 cm, which means that the first interaction takes place close to the axis of symmetry of the detector (which is 5.5 cm in diameter). This in turn reduces the uncertainty of the flight path of a neutron due to the finite size of the detector. The results obtained with EFEN for DEMON [8] detectors are in good agreement with original MENATE [9] calculations. Any good positionsensitive detector should have the same efficiency (and other properties) across its active surface. Most common PSDs show a drop in efficiency close to the edges. Fig. 5 demonstrates the dependence of the efficiency on the position where neutron hits the scintillator.
Fig. 5. Intrinsic efficiency of the PSND as function of the position (the middle of the detector is at 50 cm). The peripheral segments have a little larger efficiency than the central parts. This is the result of competition between the geometrical effect of the source-detector and light attenuation process in the scintillator.
4. Characteristics of the PSND Before putting NE-213 scintillator in the airtight container of the PSND, the liquid is ‘‘bubbled’’ for 40 min with pure argon. The transparency of the liquid and quality of the light collection is monitored by collimating 60Co gamma-source at different points along the detector and determining positions of the photo-peaks at both PMs. The refractive index of quartz walls is similar to that of the scintillator, which allows to improve light collection due to the inclusion of light reflected from the quartz-air border. The position resolution of the PSND was measured directly by collimating a flux of gamma rays from 60Co source on a small spot (3 mm) on the detector. Fig. 6 shows the time difference spectrum (t1 t2 ) obtained in this experiment. Since the total length of the PSND on t1 t2 spectrum is 10 ns, the width of the peak st ¼ 0:4 ns corresponds to position resolution sL ¼ 4 cm of an equivalent neutron energy 3.25 MeV. st is influenced by transit time spread of two PMs (250 ps for PHILIPS XP2020), light collection properties of the detector, and errors of the electronics (less than 100 ps). By putting gates on energy loss spectrum in the scintillator, it is possible to
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 6. Testing of the position resolution for new PSND viewed by two PHILIPS XP2020 PMTs. The collimated 60Co gamma source was in the middle of the PSND. Start was one PMT and Stop the second one. The variance of the measured gamma line is composed from variance of the time jitter in scintillator (sS ) and two PMTs (sPM ). The measurement indicates sS ¼ 0:245 ns assuming that both XP2020 PMTs had equal transition time spread 0.25 ns. The position resolution obtained was 4.3 cm.
375
Fig. 8. Neutron–gamma separation obtained by pulse shape analysis in 16O+204Pb at E ¼ 160 MeV experiment. The threshold for neutron detection was at 500 keV (proton recoil equivalent).
Another important property of the PSND is n–g separation based on the properties of NE-213 liquid scintillator. Normally, the quality of the separation depends on the volume and geometrical properties of the detector (see e.g. Ref. [10]). In case of the PSND, the off-line analysis of total and fast pulse components is carried out for ten 10 cm long parts of the detector separately, which allows to achieve good quality of separation (Fig. 8).
5. Performance of PSNDs in recent in-beam experiments
Fig. 7. Experimentally measured position resolution of a PSND as a function of incident neutron energy.
determine the dependence of the position resolution on the energy deposited by an incident neutron in the scintillator (Fig. 7). At large incident neutron energies, the contribution of the light collection properties become small, and position resolution depends mostly on the quality of the PMs and reaches 3 cm.
PSNDs have widely been used in experiments at HENDES [11] facility at the Accelerator Laboratory, University of Jyv.askyl.a. An example of neutron angular distributions obtained in the reaction 40Ar+180Hf at four beam energies with four PSNDs is given in Fig. 9. PSNDs allowed us to cover 2p in-plane angle and make a reliable multiple-source fit to distinguish between neutrons emitted from the compound nucleus (pre-scission) and fission fragments (post-scission). PSNDs have also been used to measure the flux of neutrons from 65 MeV d+9Be reaction. The experiment was conducted with sub-nanoampere
376
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 9. Measured angular dependence of neutron multiplicities from the reaction 180Hf(40Ar, fission) at Elab =180, 190, 216 and 249 MeV. The results of the multiple-source fit are shown by lines: full linesFtotal; dashed linesFpost-scission; dotted linesFprescission neutron multiplicity.
beam in order to estimate the neutron field in the reaction area and to provide guidelines for the production of neutron beams at JYFL. By using three PSNDs, we were able to measure energy spectra of neutrons and their angular distribution from 01 to 601 (see Fig. 10).
Acknowledgement This work has been supported in part by the Academy of Finland. References
Fig. 10. The differential yields d2 YðE; YÞ=dEdO produced in the (d; n) reaction on thick 9Be target for different angles as function of neutron energy.
[1] P.D. Zecher, et al., Nucl. Instr. and Meth. A 401 (1997) 329. [2] P. Netter, et al., Nucl. Instr. and Meth. A 185 (1981) 165. [3] L. Ludemann, . et al., Nucl. Instr. and Meth. A334 (1993) 495. [4] A.V. Kuznetsov, I.D. Alkhazov, D.N. Vakhtin, V.G. Lyapin, V.A. Rubchenya, W.H. Trzaska, K. Loberg, A.V. Daniel, Nucl. Instr. and Meth. A 346 (1994) 259. [5] I.D. Alkhazov, A.V. Daniel, V.D. Dmitriev, E.M. Kozulin, A.V. Kuznetsov, Yu.E. Penionzhkevich, V.A.
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
[6] [7] [8] [9]
Rubchenya, JINR Rapid Comm. 6(57) (1992) 80, Dubna. J. Toke, et al., Nucl. Instr. and Meth. A 334 (1993) 653. Z. Radivojevic, M.Sc. Thesis 1998, Jyv.askyl.a, Finland. G. Bizard, et al., Nucl. Phys. News 1 (5) (1991) 15. P. Desesquelles, IReS-Strasbourg, private communication.
377
[10] M. Moszynski, et al., Nucl. Instr. and Meth. A350 (1994) 226. [11] W.H. Trzaska, et al., In: J.L. Duggan, I.L. Morgan (Eds.), Proceedings of XIV International Conference on the Application of Accelerators in Research and Industry, Denton, TX, USA, November 6–9, 1996, AIP Conference Proceedings 392, Vol. 2, 1997, p. 1059.
Position-sensitive neutron detector A.V. Kuznetsova,*, D.N. Vakhtina,b, Z. Radivojevichb, I.D. Alkhazova, V.G. Lyapina, W.H. Trzaskab b
a V.G. Khlopin Radium Institute, 2-Murinski pr., 28, 194021 St. Petersburg, Russia Department of Physics, Accelerator Laboratory, University of Jyvaskyl a, a, . . P.O. Box 35, FIN-40351 Jyvaskyl . . Finland
Abstract A position-sensitive neutron detector has been developed for use in nuclear physics research. The detector consists of a +5.5 cm 100 cm long quartz tube filled with liquid scintillator viewed from both ends by photomultipliers and enclosed in a light-tight titanium container. The properties of the detector were determined both experimentally and by Monte Carlo simulations (EFEN code). A time resolution of 0.4 ns was reached resulting in the position resolution of less than 4 cm. The neutron registration efficiency varies from 36% to 20% within neutron energy range 1–10 MeV and is practically independent of the position along the detector length. Good n–g separation is achieved for neutron energies greater than 0.5 MeV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 29.40.Mc; 29.30.Hs Keywords: Neutron detector position sensitive
1. Introduction The demand for position-sensitive neutron detectors (PSND) is generated by two contradicting trends. On one hand, there is a growing need to cover large solid angle and to provide high spatial granularity of the measurements and, on the other hand, the requirement to limit the costs and complexity of multi-detector arrays. For instance, a PSND like ours, with granularity equal to 10 individual detectors, costs about 5 times less and needs only 20% of the electronics and data acquisition parameters as compared to a traditional 10-detector system. In addition, PSND is
easier to tune and monitor due to the reduced number of photomultipliers. Recently, there have been a number of publications [1–3] in which scintillator-based PSNDs were described. These detectors, however, were often made for use in a single in-beam experiment, and were abandoned after that. In this article, we report on the recent upgrade of our PSND which is being successfully and extensively used since 1994 in various nuclear physics experiments ranging from nuclear dynamics studies to the applications of neutron beams.
2. Construction and principles of operation of PSND *Corresponding author. Tel.: +7-812-2470173; fax: +7-8122478095. E-mail address: [email protected] (A.V. Kuznetsov).
The detector consists of a +5.5 cm 100 cm long quartz tube filled with liquid scintillator
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 8 3 3 - 2
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
373
Fig. 1. Scheme of the PSND.
Fig. 3. Principle of time and position determination: tFtime of flight; t1 ; t2 Fmean times needed by light to reach PMs; XFposition of the interaction point; LFlength of the detector; nFrefractive index; cFspeed of light in vacuum.
Time of flight, position and n–g separation are obtained in the following ways: *
Fig. 2. Scheme of the PSND electronics: PM1, PM2Fphotomultipliers; CFDFconstant fraction discriminators; n–gF neutron–gamma separation schemes; QDCFcharge to digit converter; TDCFtime to digit converter.
* *
* *
NE-213 viewed from both ends by photomultipliers (PMs) PHILIPS XP2020, and enclosed in a light-tight titanium container (see Fig. 1) [4,5]. The construction of the holders and the expansion volume allows both horizontal and vertical mounting of the PSND. Two signals from anodes of both PMs are processed by a dedicated signal processor (SP) which occupies one standard CAMAC slot. SP includes two constant fraction discriminators and two pulse-shape analysis schemes similar to those described in Ref. [6]. These analogue pulse-shape schemes allow to avoid multiple gates which are usually needed for particle identification in liquid scintillators. The resulting signals are digitized by fast multi-channel TDCs and QDCs producing timing information (t1 ; t2 ), fast and total pulse components (E1;fast ; E1;total ; E2;fast ; E2;total ). In case of a multi-detector array, SP block’s outputs can be plugged into a low-cost QDCs triggered with common gate (Fig. 2).
(t1 þ t2 )/2 ) TOF (time of flight), t1 t2 ) X (position), E1;total =E2;total ) X (additional position information), E1;total ; E1;fast ; E2;total ; E2;fast ) n–g separation, Etotal ; TOF ) n–g separation.
The method of TOF and position extraction is further illustrated in Fig. 3.
3. Monte-Carlo calculations with EFEN code Before any physically meaningful data can be obtained from measurements with a scintillatorbased PSND, one should determine the intrinsic efficiency of the detector, which can be significantly less than 100%. One should keep in mind that the path of the neutron inside the active volume of the detector depends on the geometry of the experiment; this means that for any possible geometry the efficiency should be determined separately, which makes the experimental determination of the intrinsic neutron registration efficiency a difficult and time-consuming task. Using a well-known cross-sections for neutron interaction with the NE-213 ingredients as the basis of numerical simulation, the EFEN computer simulation code has been written [7]. The EFEN
374
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 4. Mean traveled path of neutrons scattered in the NE213 before the first detectable interaction, calculated by EFEN simulation. Resonances at certain energies correspond to resonances in cross-sections for neutron interaction with constituents of the NE213 scintillator (hydrogen and carbon).
Monte-Carlo simulation is based on ENDF/B VI data library and calculates the efficiency of the PSND for any geometry and neutron registration threshold. As a complete 3D neutron-transport code, the simulation allows extraction of a lot of useful variables, e.g. in order to estimate the time error in neutron’s TOF caused by the finite size of the PSND, the EFEN simulation calculated the mean value of the traveled paths for scattered neutrons in the active volume of the detector. Fig. 4 shows the energy dependence of the mean traveled path of a neutron in the scintillator before the first detectable interaction. The curve saturates at 2.55 cm, which means that the first interaction takes place close to the axis of symmetry of the detector (which is 5.5 cm in diameter). This in turn reduces the uncertainty of the flight path of a neutron due to the finite size of the detector. The results obtained with EFEN for DEMON [8] detectors are in good agreement with original MENATE [9] calculations. Any good positionsensitive detector should have the same efficiency (and other properties) across its active surface. Most common PSDs show a drop in efficiency close to the edges. Fig. 5 demonstrates the dependence of the efficiency on the position where neutron hits the scintillator.
Fig. 5. Intrinsic efficiency of the PSND as function of the position (the middle of the detector is at 50 cm). The peripheral segments have a little larger efficiency than the central parts. This is the result of competition between the geometrical effect of the source-detector and light attenuation process in the scintillator.
4. Characteristics of the PSND Before putting NE-213 scintillator in the airtight container of the PSND, the liquid is ‘‘bubbled’’ for 40 min with pure argon. The transparency of the liquid and quality of the light collection is monitored by collimating 60Co gamma-source at different points along the detector and determining positions of the photo-peaks at both PMs. The refractive index of quartz walls is similar to that of the scintillator, which allows to improve light collection due to the inclusion of light reflected from the quartz-air border. The position resolution of the PSND was measured directly by collimating a flux of gamma rays from 60Co source on a small spot (3 mm) on the detector. Fig. 6 shows the time difference spectrum (t1 t2 ) obtained in this experiment. Since the total length of the PSND on t1 t2 spectrum is 10 ns, the width of the peak st ¼ 0:4 ns corresponds to position resolution sL ¼ 4 cm of an equivalent neutron energy 3.25 MeV. st is influenced by transit time spread of two PMs (250 ps for PHILIPS XP2020), light collection properties of the detector, and errors of the electronics (less than 100 ps). By putting gates on energy loss spectrum in the scintillator, it is possible to
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 6. Testing of the position resolution for new PSND viewed by two PHILIPS XP2020 PMTs. The collimated 60Co gamma source was in the middle of the PSND. Start was one PMT and Stop the second one. The variance of the measured gamma line is composed from variance of the time jitter in scintillator (sS ) and two PMTs (sPM ). The measurement indicates sS ¼ 0:245 ns assuming that both XP2020 PMTs had equal transition time spread 0.25 ns. The position resolution obtained was 4.3 cm.
375
Fig. 8. Neutron–gamma separation obtained by pulse shape analysis in 16O+204Pb at E ¼ 160 MeV experiment. The threshold for neutron detection was at 500 keV (proton recoil equivalent).
Another important property of the PSND is n–g separation based on the properties of NE-213 liquid scintillator. Normally, the quality of the separation depends on the volume and geometrical properties of the detector (see e.g. Ref. [10]). In case of the PSND, the off-line analysis of total and fast pulse components is carried out for ten 10 cm long parts of the detector separately, which allows to achieve good quality of separation (Fig. 8).
5. Performance of PSNDs in recent in-beam experiments
Fig. 7. Experimentally measured position resolution of a PSND as a function of incident neutron energy.
determine the dependence of the position resolution on the energy deposited by an incident neutron in the scintillator (Fig. 7). At large incident neutron energies, the contribution of the light collection properties become small, and position resolution depends mostly on the quality of the PMs and reaches 3 cm.
PSNDs have widely been used in experiments at HENDES [11] facility at the Accelerator Laboratory, University of Jyv.askyl.a. An example of neutron angular distributions obtained in the reaction 40Ar+180Hf at four beam energies with four PSNDs is given in Fig. 9. PSNDs allowed us to cover 2p in-plane angle and make a reliable multiple-source fit to distinguish between neutrons emitted from the compound nucleus (pre-scission) and fission fragments (post-scission). PSNDs have also been used to measure the flux of neutrons from 65 MeV d+9Be reaction. The experiment was conducted with sub-nanoampere
376
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
Fig. 9. Measured angular dependence of neutron multiplicities from the reaction 180Hf(40Ar, fission) at Elab =180, 190, 216 and 249 MeV. The results of the multiple-source fit are shown by lines: full linesFtotal; dashed linesFpost-scission; dotted linesFprescission neutron multiplicity.
beam in order to estimate the neutron field in the reaction area and to provide guidelines for the production of neutron beams at JYFL. By using three PSNDs, we were able to measure energy spectra of neutrons and their angular distribution from 01 to 601 (see Fig. 10).
Acknowledgement This work has been supported in part by the Academy of Finland. References
Fig. 10. The differential yields d2 YðE; YÞ=dEdO produced in the (d; n) reaction on thick 9Be target for different angles as function of neutron energy.
[1] P.D. Zecher, et al., Nucl. Instr. and Meth. A 401 (1997) 329. [2] P. Netter, et al., Nucl. Instr. and Meth. A 185 (1981) 165. [3] L. Ludemann, . et al., Nucl. Instr. and Meth. A334 (1993) 495. [4] A.V. Kuznetsov, I.D. Alkhazov, D.N. Vakhtin, V.G. Lyapin, V.A. Rubchenya, W.H. Trzaska, K. Loberg, A.V. Daniel, Nucl. Instr. and Meth. A 346 (1994) 259. [5] I.D. Alkhazov, A.V. Daniel, V.D. Dmitriev, E.M. Kozulin, A.V. Kuznetsov, Yu.E. Penionzhkevich, V.A.
A.V. Kuznetsov et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 372–377
[6] [7] [8] [9]
Rubchenya, JINR Rapid Comm. 6(57) (1992) 80, Dubna. J. Toke, et al., Nucl. Instr. and Meth. A 334 (1993) 653. Z. Radivojevic, M.Sc. Thesis 1998, Jyv.askyl.a, Finland. G. Bizard, et al., Nucl. Phys. News 1 (5) (1991) 15. P. Desesquelles, IReS-Strasbourg, private communication.
377
[10] M. Moszynski, et al., Nucl. Instr. and Meth. A350 (1994) 226. [11] W.H. Trzaska, et al., In: J.L. Duggan, I.L. Morgan (Eds.), Proceedings of XIV International Conference on the Application of Accelerators in Research and Industry, Denton, TX, USA, November 6–9, 1996, AIP Conference Proceedings 392, Vol. 2, 1997, p. 1059.