Improved fission neutron energy discrimination with 4He detectors through pulse filtering

Abstract

This paper presents experimental and computational techniques implemented for 4 He gas scintillation detectors for induced fission neutron detection. Fission neutrons are produced when natural uranium samples are actively interrogated by 2.45 MeV deuterium-deuterium fusion reaction neutrons. Fission neutrons of energies greater than 2.45 MeV can be distinguished by their different scintillation pulse height spectra since 4 He detectors retain incident fast neutron energy information. To enable the preferential detection of fast neutrons up to 10 MeV and suppress low-energy event counts, the detector photomultiplier gain is lowered and trigger threshold is increased. Pile-up and other unreliable events due to the interrogating neutron flux and background radiation are filtered out prior to the evaluation of pulse height spectra. With these problem-specific calibrations and data processing, the 4 He detector's accuracy at discriminating fission neutrons up to 10 MeV is improved and verified with 252 Cf spontaneous fission neutrons. Given the 4 He detector's ability to differentiate fast neutron sources, this proof-of-concept active-interrogation measurement demonstrates the potential of special nuclear materials detection using a 4 He fast neutron detection system.

Used Fuel Storage Monitoring Using Helium-4 Scintillation Fast Neutron Detectors and Neutron Spectral Analysis

“…The two PMTs on the ends of the gas chamber are set up in coincidence mode so as to suppress the recording of PMT noise signals. PMT calibration was performed to ensure the two PMTs have matching gain factors and the amplified scintillation signals do not exceed the limited dynamic range of the digitizer [5]. The scintillation light can be separated into two components: a fast component which lasts on the scale of tens of nanoseconds and a slow component with a much longer de-excitation time scale of microseconds (see Figure 8).…”

“…Events above the black cut-off line will be regarded as neutrons and used for building the detector response matrix. In addition, pulse filtering algorithms [5] are applied to remove pile-up events (i.e. two events within the same event window) and other unreliable events.…”

Timing Characterization of Helium-4 Fast Neutron Detector with EJ-309 Organic Liquid Scintillator

“…Scintillation photons are then be generated from the de-excitation process of the helium excimers. The first version of the Helium-4 detector has two Hamamatsu R580 photomultiplier tubes (PMT) on the both ends of the gas chamber are set up in coincidence mode so as to suppress the recording of PMT noise signals [4].…”

“…They also have fast response time on the order of nanoseconds and low sensitivity to γ-rays. In addition, neutrons and γ-rays can be well separated because of the good pulse shape discrimination capability [4]. These properties make the Helium-4 detector especially attractive for neutron detection in ultra-high γ-ray background.…”

“…8 shows the experimental setup for measuring the PMTbased Helium-4 fast neutron detectors. The PMT high voltage was set as 1356V (for node A003E) and 1538V (for node A003D) respectively to match the pulse amplitude based on previous calibration [4]. A Cobalt-60 gamma source was placed at the center of the detector's active volume and was 3 cm to the face of both detectors in order to achieve a higher count rate.…”