The EUROBALL neutron wall design and performance tests of neutron detectors

Transcription

1 Nuclear Instruments and Methods in Physics Research A 421 (1999) The EUROBALL neutron wall design and performance tests of neutron detectors O. Skeppstedt, H.A. Roth, L. Lindström, R. Wadsworth, I. Hibbert, N. Kelsall, D. Jenkins, H. Grawe, M.Go rska, M. Moszyński *, Z. Sujkowski, D. Wolski, M. Kapusta, M. Hellström, S. Kalogeropoulos, D. Oner, A. Johnson, J. Cederkäll, W. Klamra, J. Nyberg, M. Weiszflog, J. Kay, R. Griffiths, J. Garces Narro, C. Pearson, J. Eberth Department of Experimental Physics, Chalmers University of Technology and Go( teborg University, S Go( teborg, Sweden Department of Physics, University of York, Heslington, York YO1 5DD, UK GSI, D Darmstadt, Germany Soltan Institute for Nuclear Studies, Department of Nuclear Electronics, PL Otwock S! wierk, Poland Heavy Ion Laboratory Warsaw University, PL Warsaw, Poland Division of Cosmic and Subatomic Physics, Lund University, S Lund, Sweden Royal Institute of Technology, Physics Department, S Stockholm, Sweden The Svedberg Laboratory, Uppsala University, S Uppsala, Sweden CCLRC Daresbury Laboratory, Daresbury, Warrington, Cheshire WA4 4AD, UK Physics Department, University of Surrey, Guildford GU2 5XH, UK IKP Universita( tzuko( ln, D Ko( ln, Germany Received 21 August 1998; received in revised form 15 September 1998 Abstract The mechanical design of the EUROBALL neutron wall and neutron detectors, and their performance measured with a Cm fission source are described. The array consists of 15 pseudohexaconical detector units subdivided into three, 149 mm high, hermetically separated segments and a smaller central pentagonal unit subdivided into five segments. The detectors are filled with Bicron BC501A liquid scintillator. Each section of the hexaconical detectors is viewed by a 130 mm diameter Philips XP4512PA photomultiplier while the sections of pentagonal detectors are viewed by Philips XP4312B PMTs. The tests of n γ discrimination performed by zero-crossing and time-of-flight methods show a full separation of γ- and neutron events down to 50 kev recoil electron energy. These tests demonstrate the excellent timing properties of the detectors and an average time resolution of 1.56 ns. The factors determining the efficiency of neutron detectors are discussed. The total efficiency for the full array for a symmetric fusion-evaporation reaction is predicted to be Elsevier Science B.V. All rights reserved. Keywords: EUROBALL neutron wall; Neutron detectors; n γ discrimination; Time-of-flight * Corresponding author. Tel.: # ; fax: # ; marek@ipj.gov.pl /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 ) X

2 532 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Introduction The combination of detectors for neutron and light charged particle detection with efficient γ- detector arrays has been proven to be a powerful tool in the spectroscopy of exotic neutron deficient nuclei close to N"Z and the proton dripline [1 3,17]. The application of a filter detector for the identification and suppression of events from the dominant charged particle evaporation channels is an essential prerequisite for the successful spectroscopy of proton rich nuclei. Future applications of such a filter device, may comprise combination with recoil detection, charged particle decay studies and μs-ms tagging devices for γ-ray spectroscopy at and beyond the proton dripline. The excellent timing characteristics of the liquid scintillators and their efficiency for γ-rays provide a good time reference which is important for example, in DC beam experiments. The leading design criteria for ancillary detectors are in general: large efficiency, high granularity and minimum interference with γ ray detection. The relevant specifications for the EUROBALL [4] neutron detector array are as follows: performance as measured with a Cm fission source in laboratory conditions. The performance tests were begun with a measurement of the number of photoelectrons produced in the PMT by γ-rays detected in the scintillator cell. This allows a comparative check of the quality of the assembled detectors to be made. Next, a full test of n γ separation by the Z/C pulse-shape discrimination and time-of-flight was carried out for some detectors. In the last section of the paper various factors determining the efficiency of the EUROBALL neutron wall are discussed. 2. Detector design The geometry of the Ge cluster/bgo shield end cap of EUROBALL was chosen to ensure optimum compatibility with the EUROBALL mechanical design. The array consists of 15 pseudohexaconical detector units in two rings (Fig. 1b) and a central pentagonal unit. Each hexagonal unit is subdivided Intrinsic efficiency for evaporation neutrons ε 50%, Ω"1π, resulting in a typical total efficiency ε "25 30%, dependent on reaction kinematics. High granularity to limit count rate per detector and n γ double hits. Minimum cross talk from neutron scattering, obtained by suppressing next neighbour hits and using time analysis of neutron events of fold M 2. Combination of zero/crossover (Z/C) pulse shape and time-of-flight (TOF) analysis. Minimum interference with γ-ray detection. Cluster cap design of EUROBALL. Some of the above requirements are related to the mechanical design of the neutron wall while others are determined by the quality of the detectors, their performance in n γ discrimination and fast timing. The aim of this work is to report on the design and assembly of the neutron detectors and their Fig. 1. The neutron wall mounted to the EUROBALL frame (a) and segmentation of the neutron detector array (b) as viewed from the target.

3 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) into three hermetically separated segments, each viewed by a 130 mm diameter PMT. The subdivision is made in two ways (Fig. 2) in aider to maximize the symmetry of the detector array. The smaller central pentagonal unit is subdivided into five segments, each viewed by a 75 mm diameter PMT. This amounts to a granularity of 50 segments at a distance of 510 mm from the focal point of the neutron detector array Mechanics In Fig. 2 the base plates and an isometric view of both types of hexagons are shown. The effective height of the cells was chosen as 149 mm, yielding about 50% intrinsic efficiency for neutrons evaporated in heavy ion fusion reaction at an acceptable time-of-flight variance [10,16]. Both types of hexagon have a volume of 9.70 l, that is, three sections each of 3.23 l, with an allowance for a variation of 2 4% in the segments due to the irregular shape of the hexagons. The central pentagons have a total volume of 5.37 l consisting of five segments each of 1.07 l. The detector cans have 25 mm high collars around their circumference, allowing the 16 detector units to connect to each other in a selfsupporting way. Scintillation light passes through high-quality glass windows into 130 mm diameter PMTs. The PMTs are enclosed in light tight, black anodised aluminium covers containing magnetic shielding made from 25 μm VITROVAC 6025X metglass tape. The photocathode area of the PMT corresponds in both types of hexagonal detectors to an average of 47% of the exit wall face of the hexagonal segments with a variance of 2% arising from the geometry of the individual segments. The corresponding PMT coverage of the pentagonal segments is 50%. The inner walls of the Al detector cans are painted with three layers of TiO (BC622A) reflector paint which together with the pyramidal shape of the cells ensures good light collection at the photocathode of the PMT. The interface of the neutron wall to the EURO- BALL structure is made via the Ge-Clover section flange (Fig. 1a). The central pentagonal unit will be mounted independently of the other n-detectors on a separate stand. This ensures that the two halves of EUROBALL may be moved perpendicular to the beam line in the normal way. The beam line and its supports, before and behind the EUROBALL frame, support the target chamber which is normally a Si-ball chamber. In experiments in which the 0 pentagonal detector is used, the beam will be stopped inside the target chamber Liquid scintillator For a good n γ discrimination the Bicron BC501A liquid scintillator was chosen. It has been demonstrated in Ref. [5] that this material provides the best performance in this respect. The scintillating cells were filled with BC501A liquid and bubbled throughly with argon gas to remove traces of oxygen. The BC501A scintillator is found to yield optimum pulse-shape discrimination properties when used in conjunction with the XP4512B PMT [5 7,11] Photomultipliers The 130 mm diameter Philips XP4512PA photomultipliers were chosen for use with the hexagonal detectors. They have been used previously in the DEMON neutron detectors [6,7] and have been found to be the best available 130 mm diameter PMTs in a comparative study [6]. The XP4512 PMTs are characterized by a high quantum efficiency and very good photoelectron collection efficiency. A high gain first dynode and a low time jitter of 1 ns ensure a good timing and pulse-shape discrimination in conjunction with liquid scintillators [6]. For the pentagonal detector, Philips XP4312B photomultipliers were used. According to the manufacturers, Philips Photonics, these 75 mm diameter PMTs have similar properties to those of the XP4512B. All the photomultipliers of both types have, according to the manufacturer, a blue photocathode sensitivity of μa/lm F. The high voltage dividers for the PMTs have been designed by Philips Photonics for interaction rates in each singledetector cell of well above 30 khz.

4 534 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 2. Baseplates, sections and isometric views of the two types (a, b and c) of hexagonal detectors.

5 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Electronics For the EUROBALL neutron wall integrated electronics consisting of the n γ discriminator, the time-of-flight circuit and an energy output has been built [8]. The present study, however, was carried out with the NIM electronics previously used in the NORDBALL array [9,10] and tested recently as described in Ref. [11] for comparison with previous studies. Fig. 3 shows a block diagram of the experimental arrangement. The anode signal of the tested detector was sent to the n γ discriminator [9] and constant fraction discriminator. The output signals from both units are sent to a time-to-pulse height converter (TPHC) to observe the zero-crossing time distribution. The other TPHC started by another hexagon detector and stopped by the tested detector allows the recording of the time-of-flight spectrum. The anode signal of the tested detector was picked up from the n γ discriminator [9] and sent through a spectroscopy amplifier to provide an energy signal. The output signals were sent to the GSI multiparameter data acquisition system, which was triggered by the coincidence signal of the timeof-flight. To observe the n γ separation due to both PSD and TOF a fission Cm source was placed at the reference detector, about 40 cm away from the tested detector. 3. Performance tests 3.1. Photoelectron yields The number of photoelectrons per energy unit (phe/mev) was measured by the method of Fig. 3. Block diagram of electronics used to test n γ separation by means of zero-crossing pulse-shape discrimination and time-of-flight methods.

6 536 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Bertolaccini et al. [12] (see also Refs. [5 7,13]). In this method the number of photoelectrons is measured directly by comparing the position of the Compton-edge of the 662 kev γ-rays from a Cs source to that of a single photoelectron peak, which determines the gain of the PMT. Measurements have been made for all 45 segments of the assembled hexagonal detectors. Fig. 4 shows the distribution of the measured numbers of photoelectrons presented in bins of 100 phe for the 45 detector segments. The photoelectron yields from all segments were found to be between 1000 and 1600 phe/mev with an average of 1300 phe/mev and a variance of σ"150 phe/ MeV. The observed spread of the photoelectron yield cannot be ascribed to the PMTs since the blue photocathode sensitivity is very similar for all the tubes. It has been demonstrated previously that for PMTs of the same type the number of photoelectrons is proportional to the blue photocathode sensitivity [6]. A simple check of the efficiency of light collection with the Cs source placed at the front and back of one of the scintillator cell shows about 15 20% change of the signal amplitude from front to back, reflecting a good light collection. Fig. 4. The distribution of the measured number of photoelectrons/mev for 45 assembled detector segments. Probably the observed spread of the photoelectron number is associated with a non-uniformity of the custom-made scintillator cells, for example, the PMT coupling to the windows. The observed average photoelectron number of 1300 phe/mev can be compared to that measured with the DEMON detectors of 4 l volume [5,7]. For a cylindrical cell of 16 cm diameter and 20 cm height filled with BC501A liquid scintillator viewed by a XP4512B PMT the photoelectron number of 1070 phe/mev was measured for three cells manufactured by Bicron [6 8]. Note that the photocathode area of the 130 mm diameter PMT corresponds to about 66% of the scintillator exit face. In the case of the EUROBALL hexagonal detectors the fraction of the exit face seen by the photocathode is significantly lower (47%). Thus, we conclude that the high light output arises from the pyramidal shape of the EUROBALL detector cells and their somewhat smaller volume Neutron gamma ray discrimination N γ discrimination tests were carried out for a typical detector section with a measured photoelectron number of 1330 phe/mev. The reference detector used in the time-of-flight measurements had a photoelectron yield of 1270 phe/mev. A Cm fission source was used as its neutron energy spectrum resembles closely that of fusionevaporation neutrons. Fig. 5 presents the results of the measurements using the zero-crossing (Z/C) and the time-of-flight (TOF) methods. Fig. 5c shows the main spectrum of the Z/C time distribution versus the TOF. In this spectrum the recorded events are controlled by the threshold of the constant fraction discriminator in the tested detector, which was set at 50 kev of the recoil electrons energy. The log arithmic scale for the Z-axis in Fig. 5 demonstrates the excellent separation of the neutron and γ-ray events for this low energy threshold. At the level corresponding to a fraction of a percent, in relation to the height of γ-ray peak, there is no overlap of neutron and γ-events. Fig. 5a and b represent the Z/C time and TOF spectra versus energy. In this case an ADC threshold corresponding to about 100 kev of recoil electron energy defines the energy threshold.

7 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 5. The n γ discrimination by simultaneous measurement of the zero-crossing time and time-of-flight, measured with a Cm fission source: the Z/C time versus energy (a), the time-of-flight versus energy (b), the Z/C time versus the time-of-flight (c). Note that the analysis of the 2D spectrum of the Z/C versus the time-of-flight allows to select very clearly both the neutron and γ-events. Fig. 5d presents the energy spectrum of neutrons separated by the TOF. Fig. 6 shows the n γ discrimination spectra corresponding to energy gates set at 110, 240, 480 and about 1 MeV of recoil electron energies in the 2D spectrum of Fig. 5a. To quantify the n γ discrimination power at a given energy, a figure of merit, M is used (see Ref. [14]) defined as M"peak separation/(fwhm #FWHM ). (1) γ The M values are given in Fig. 6 for each spectrum and can be compared to those reported in Ref. [7] for the DEMON detectors (see Table 1). Despite the fact that for DEMON detectors the digital

8 538 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Table 1 Figure of merit, M, measured for EUROBALL and DEMON detectors Energy (kev) EUROBALL DEMON see Ref. [7], Am Be neutron source. source in Ref. [7]), the quality of n γ discrimination reflected by the M factor is clearly comparable. Note that the gate at 110 kev of recoil electrons corresponds to about 500 kev of neutron energy. Thus, the tested neutron detectors assure very good n γ discrimination down to at least 400 kev of neutron energy, particularly when using the combined PSD and TOF separation methods (see Fig. 5c). The design aim, that the PSD system with the energy threshold set at 400 kev neutron energy implies less than 10% loss of the total efficiency, has been fulfilled with a large margin. Fig. 7 presents the 2D spectrum of Z/C time versus TOF measured with a Co γ-ray source. The two peaks have been obtained in the spectrum by changing a delay in the TOF by 50 ns. It shows a clean region where neutrons should be observed, free of the tails observed in Fig. 5c. High energy γ-rays, in the previous measurements, overload the n γ discriminator and shift the Z/C time (see Fig. 5a). Fig. 8 presents the time spectrum measured for Co γ-rays. The FWHM of the spectrum is equal to 2.20 ns. Since both the hexagonal detectors were adjusted in the same way, with the energy threshold set at 50 kev, the time resolution of one detector is calculated to be 1.56 ns. Fig. 6. The n γ discrimination spectra for gates set at 110, 240, 480 kev and 1 MeV of recoil electron energies from the 2D spectrum of Fig. 5a. The upper spectrum presents the total n γ discrimination spectrum. charge comparison method of pulse-shape discrimination was used and the fact that the two experiments were performed with somewhat different energy gates and neutron sources (Am Be 4. Efficiency of the neutron detectors The efficiency of the EUROBALL neutron detectors is determined by The solid angle Ω subtended by the detectors, which is limited by the forward 1π section of

9 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 7. The 2D spectrum of Z/C time versus TOF measured with γ-rays from a Co source. Fig. 8. Time spectrum measured with two EUROBALL sections of detectors for Co γ-rays. EUROBALL, whose 30 individual Ge detectors contribute only 1.5% to the total 10% efficiency of EUROBALL minimising γ-efficiency losses. The detector thickness, which was chosen as 149 mm, yielding about 50% intrinsic efficiency for evaporation neutrons at an acceptable timeof-flight variance. The low electronic threshold for neutron detection with good n γ discrimination, which is below 400 kev neutron energy.

10 540 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Losses due to simultaneous n- and γ-interactions in one detector cell, which are minimized by a high granularity of 50 individual subunits; losses due to wrong Z/C pulse-shape discrimination are assumed to be 410% of the total efficiency for M γ "15. Losses due to suppression of nearest neighbor neutron detector hits to minimize detector cross talk from neutron scattering are estimated to be less than 17% of the total efficiency for a multiplicity M "2 The design aim for the overall intrinsic efficiency is ε 50% for evaporation neutrons, which in their energy distribution compare well to neutrons from a fission source like Cf or Cm used in the present test. Taking into account the kinematical focussing in a symmetric fusion evaporation reaction this should imply a total efficiency ε "30%. The efficiency of the array has been estimated in Monte Carlo simulations and by extrapolation from smaller arrays, namely the NORDBALL [10,15] and OSIRIS systems [16]. The Monte Carlo calculations were found to reproduce the properties of these arrays very well. The efficiency of the detectors for neutrons can be written: ε "P (Ω) Ω/4π ε /(1#ε ) (2) γ where P (Ω) is the kinematic focussing, ε is the intrinsic efficiency and ε is a reduction factor due γ to double n γ hits. For a typical reaction of Ni# Cr at 230 MeV, the kinematic focussing can be estimated as P (Ω) The intrinsic efficiency of +50% results from the 149 mm thick- ness of the detectors assuming a 0.5 MeV threshold of the electronics and a reduction due to double n γ hits estimated to be equal to The probability of n γ double hits was estimated based on earlier experience with the OSIRIS [16] and NORDBALL [10] neutron detector arrays. The results are shown in Table 2. The high granularity of the EUROBALL neutron wall reduces the probability of n γ double hits to the acceptable level of The result of tests of n γ discrimination presented in Section 3.2. shows that losses due to wrong Z/C pulse-shape discrimination and time-of-flight are negligible for the fullenergy range of neutrons. Table 2 The probability of n γ double hits in different neutron detector arrays Array Number of detectors Solid angle OSIRIS [16] 16 2π 0.27 OSIRIS 7 1π 0.36 NORDBALL [10] 15 1π 0.24 EUROBALL 50 1π 0.06 Probability of n γ double hits γ-multiplicity M γ "15, Compton scattering included [16]. In a commissioning experiment the efficiency of a subarray (five hexagons plus pentagon), covering about the same solid angles the final array, was determined in the reaction Ti( Ni, 4pn) Pd at 228 MeV to 27%. This is in excellent agreement with the design data considering the reduced granularity and therefore increased n γ double hit probability. The performance of the neutron detector array is strongly affected by cross talk from scattered neutrons, simulating higher multiplicities leading to misidentification of extremely neutron deficient exit channels and by excessive background from the abundant neutron evaporation channels. Two methods are considered for application in the EUROBALL array, which have been tested earlier with the NORDBALL array [15]. The first one uses the rejection of hits in adjacent detectors leading to a reduction of ε by 17% for 50 detector sections. The other method involves gating on the time difference between neutron hits [15]. The results of a Monte Carlo simulation presented in Fig. 9 show a much narrower time distribution for direct (non scattered) events. Combination of the two methods with the NORDBALL neutron wall led to a reduction of scattered events from a true 1n channel in fold M "2 spectra by a factor of Conclusion The n γ discrimination tests carried out using Z/C pulse-shape discrimination and time-of-flight methods confirm the high quality of the EURO- BALL neutron detectors. A full separation of γ- and neutron-events is observed down to 50 kev of

11 O$. Skeppstedt et al./nucl. Instr. and Meth. in Phys. Res. A 421 (1999) Fig. 9. Monte Carlo simulation of time distributions from direct hits and scattered events in the NORDBALL neutron wall [15]. recoil electron energy. These tests have also demonstrated the excellent timing properties of the detectors. This is reflected in the measured time resolution of 1.56 ns FWHM. The design of the EUROBALL neutron wall, presented in this work combined with the performance of the detectors from our tests, the expected high efficiency of detection and the high rejection of the cross talk caused by scattered neutrons, taken together indicate that the EUROBALL neutron wall will be a very powerful experimental tool for nuclear structure physics. References [1] H. Grawe et al., Progr. Part. Nucl. Phys. 28 (1992) 281. [2] R. Schubart et al., Z. Phys. A 352 (1995) 373. [3] M. Lipoglavs\ ek et al., Phys. Rev. Lett. 76 (1996). [4] H. Grawe et al., Meeting on Inner Balls and Neutron Detectors for EUROBALL, Rossendorf, [5] M. Moszyński et al., Nucl. Instr. and Meth. A 350 (1994) 226. [6] M. Moszyński et al., Nucl. Instr. and Meth. A 307 (1991) 97. [7] M. Moszyński et al., Nucl. Instr. and Meth. A 317 (1992) 262. [8] D. Wolski et al., Meeting on Inner Balls and Neutron Detectors for EUROBALL, Rossendorf, [9] J. Bia"kowski et al., Nucl. Instr. and Meth. A 275 (1989) 525. [10] S.E. Arnell et al., Nucl. Instr. and Meth. A 300 (1991) 303. [11] D. Wolski et al., Nucl. Instr. and Meth. A 360 (1995) 584. [12] M. Bertolaccini et al., Proc. Nucl. Electr. Symp.,Versailles, France, [13] M. Moszyński et al., IEEE Trans. Nucl. Sci. NS-44 (1997) [14] G.F. Knoll, Radiation Detection and Measurements, Wiley, New York, [15] J. Cederkäll et al., Nucl. Instr. and Meth. A 360 (1995) 584. [16] D. Alber et al., Nucl. Instr. and Meth. A 263 (1988) 401. [17] M. Lipoglavs\ ek et al., Z. Phys. A 356 (1996) 239.