Measurement result and performance parameters are presented for fast neutron detectors exploiting the scintillation of natural helium at high pressure. This detection medium has a very low electron density, minimizing the sensitivity to gamma radiation and thus enabling neutron detection also in high gamma radiation environment. Contrary to proportional counters, scintillation detection enables fast (nanosecond) timing and pulse shape discrimination, a technique that enables a lower neutron detection threshold. In this work, the basic principles of the detector are described, followed by a study of gamma rejection capabilities. Methods to calibrate the detector are discussed. Finally, a brief description of a 4 He scintillation based detector system including data acquisition electronics is given.

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He NE-213 Scintillation Gamma-rays Fast neutrons Pulse-shape discrimination a b s t r a c t A first comparison has been made between the pulse-shape discrimination characteristics of a novel 4 He-based pressurized scintillation detector and a NE-213 liquid-scintillator reference detector using an Am/Be mixed-field neutron and gamma-ray source and a high-resolution scintillation-pulse digitizer. In particular, the capabilities of the two fast neutron detectors to discriminate between neutrons and gamma-rays were investigated. The NE-213 liquid-scintillator reference cell produced a wide range of scintillation-light yields in response to the gamma-ray field of the source. In stark contrast, due to the size and pressure of the 4 He gas volume, the 4 He-based detector registered a maximum scintillationlight yield of 750 keV ee to the same gamma-ray field. Pulse-shape discrimination for particles with scintillation-light yields of more than 750 keV ee was excellent in the case of the 4 He-based detector. Above 750 keV ee its signal was unambiguously neutron, enabling particle identification based entirely upon the amount of scintillation light produced.

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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.

In this paper we report the experimental study of high-pressure xenon used as a scintillator, in the context of developing a gamma ray detector. We measure a light yield near 2 photoelectrons per keV for xenon at 40 bar. Together with the light yield, we also measured an energy resolution of ∼9% (FWHM) at 662 keV, dominated by the statistical fluctuations in the number of photoelectrons.

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