The Investigation of Mechanisms of Fast Neutron Registration in Oxide Scintillators

Abstract

The contributions of gamma rays from the inelastic scattering reaction and the resonant reaction in the process of slowing down fast neutrons inside the oxide detector volume are investigated. For this the countable efficiency of the ZnWO4, CdWO4, Bi4Ge3O12 oxide scintillators in terms of pulses/neutron during the registration of the fast neutrons from a 239Pu-Be source was measured. It is assumed that the response of detectors during neutron moderation in oxide scintillators with an effective thickness of about 40—50 mm is formed by instantaneous gamma rays from inelastic, resonant inelastic scattering reactions, as well as delayed gamma rays from the capture reaction of resonance neutrons. The parameters of the nuclei, which determine the detector response — the density of the nuclear levels of the compound nuclei being formed, the widths of the resonance regions, the lifetimes of the excited nucleus state were considered. It was found that the registration of a cascade of gamma rays from the discharge of excited levels leads to a significant increase in the countable efficiency of the detector and, as a consequence, an increase in the sensitivity of the detector to fast neutrons. The measured response in terms of pulses/neutron for the ZWO detector — 64, for CWO — 36, for BGO — 2.5. The response of the detectors was recorded by the broadband tract with a time feedback of τ~```0.7 ns. The measured values of the efficiency are explained by the fact that, in our case, the reaction of inelastic scattering is the starting process, which starts the process of discharging nuclear long-lived (~``` 1–1000 ns) states excited in both inelastic scattering and in resonance capture reaction. The registration of the gamma-quanta from discharge leads to an increase the countable efficiency of the detector. The observed increase of the countable efficiency of the secondary gamma quanta is realized when neutrons are moderated inside the oxide detectors with a thickness of 40—50 mm or more. The measurement error of the registration efficiency was about 7%.

The Threshold of Detection of Fission Materials by ZnWO4 and Bi4Ge3O12 Scintillation Detectors

“…In the BGO scintillator of the indicated sizes [6,7,9,10], CGQ are practically not observed, which is confirmed by the results of measuring the maximum detection distance of the monitor in different modes -photon counting and spectrometric. Thus, for BGO-type scintillators, an increase in statistics (photon counting mode) practically does not lead to an increase in sensitivity, since the conditions for the extraction of CGQ are not realized in BGO.…”

“…In the ZWO scintillator [6,7] arise the additional CGQ which associated with the primary gamma quantum from the inelastic scattering reaction, which results in the case of their efficient isolation and registration (single photon counting mode) to increase the statistics of signals related to one input particle and, as a result, to increase the sensitivity of the system. At the same time, measurements carried out in the spectrometric mode on a ZWO scintillator did not give an increase in sensitivity, since CGQ (with low energies ~ 0.2-1 keV) are suppressed in this mode.…”

“…The source is placed inside a lead ball Ø100 mm with a well Ø20 mm. The lead ball simultaneously attenuates the accompanying gamma radiation from the 239 Pu-Be source [6,7]. We add an additional lead shield -5 mm to protect the detector from background gamma radiation.…”

“…Also, the physical efficiency (impulse×s -1 ×cm -2 /neutr -1 ×cm -2 ) in spectrometric mode does not exceed 1. In [6,7] the first results of counting efficiency studies with ZWO scintillators and BGO. When using the mode of counting single photons in the ZWO scintillator, an increased (up to 60 pulse×s -1 ×cm -2 /neutron×s -1 ×cm -2 ) was detected, compared to the BGO scintillator (2.5 pulse×s -1 ×cm -2 /neutron×s -1 ×cm -2 ), which was explained by the registration of cascade quanta arising in the scintillator nuclei.…”

Counting Efficiency and Neutron/Gamma Ratio for KDP: Tl+ and UPS-923A Scintillators in a Single Photone Detection Mode

“…In view of the increased radiation resistance (~ 10 10 neutron/cm 2 ) of the obtained new crystals, an urgent task is to study the interaction of neutron radiation with the substance of KDP: TL + . As shown in our previous works [1][2][3][4][5][6][7][8][9][10], neutron radiation can be detected using scintillation crystals of potassium dihydrogen phosphate KH 2 PO 4 (KDP), which are doped with thallium ions Tl + [5]. This paper presents the experimental results of a study of the efficiency of fast neutron detection in water-soluble crystals in the photon-counting mode and pulse timefiltering mode.…”