The EUROBALL neutron wall – design and performance tests of neutron detectors

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Abstract

The mechanical design of the EUROBALL neutron wall and neutron detectors, and their performance measured with a 246,248Cm 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 0.30.

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, 2, 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:

  • Intrinsic efficiency for evaporation neutrons εI≅50%, Ω=1π, resulting in a typical total efficiency εn=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 Mn≅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 performance as measured with a 246,248Cm 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.

Section snippets

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 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

Photoelectron yields

The number of photoelectrons per energy unit (phe/MeV) was measured by the method of Bertolaccini et al. [12](see also Refs. 5, 6, 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 a137Cs 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

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 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 time-of-flight variance.

  • The low electronic threshold for neutron detection

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 EUROBALL neutron detectors. A full separation of γ- and neutron-events is observed down to 50 keV of 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

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