M.M. Bourne

New generation plastic scintillators for fast neutron spectroscopy and Pulse Shape Discrimination

New generation of plastic scintillators have been developed at RMD for fast neutron detection technology. These plastics have peak emission wavelength ~ 440 nm, fast scintillation decay <; 10 ns, light output ~ 13,000 photons/MeV, and excellent Pulse Shape Discrimination (PSD) between gamma rays and neutrons. We have achieved a Figure-of-Merit (FOM) of 2.3 at 1.0 MeVee electron energy threshold for a 2 inch diameter right cylinder sample. At RMD, comparative measurements were made between the plastic scintillator and Eljen liquid scintillator EJ309 both 1 inch diameter × 1 inch length. RMD plastic showed competitive performance. Additionally, in an experiment performed at the University of Kentucky 7 MV Van De Graaff accelerator, RMD plastic scintillator was irradiated with mono-energetic fast neutron beam energies up to 20.8 MeV. The results from this experiment confirm fast neutron spectroscopy capabilities. These results and effects of different electronic systems on the PSD measurements are discussed in this paper.

Fast multiplicity counter featuring stilbene detectors for special nuclear material assay

A fast neutron multiplicity counting system based on organic scintillators, i.e. EJ-309 and stilbene, has been developed and experimentally tested at the University of Michigan. The system is able to detect correlated photon and neutron multiplets emitted by the fission reaction, within a gate time of tens of nanoseconds. This counting strategy is exploited to quantify fissile mass, without the need of complex electronic circuitry and unfolding procedures, otherwise required in moderated systems. Moderated systems are traditionally based on helium-3 detectors and feature a gate time of hundreds of microseconds. Measurement precision is thus negatively affected by accidental coincidences. A prototypal version of the proposed well-shaped counter was assembled and tested in the laboratory, using a spontaneous fission and an (α, n) neutron source, i.e. 252Cf and PuBe respectively. Measured results show excellent agreement with the simulated model of the system. The viability of the system to discriminate time-correlated fission neutrons from random, uncorrelated neutrons was proved. Preliminary results of an experimental campaign, carried out at INL (Idaho National Laboratory), to characterize plutonium metal samples are also shown. Results show a monotonic increasing trend for the range of measured 240-Pu effective masses, i.e. 0.024-0.5 kg, enabling the measurement of the mass of an unknown sample.

Measurement of detector resolution for neutral particle detection with liquid scintillators

The characterization of the photon energy resolution of a liquid scintillation detector is desirable for simulating photon pulse height distributions in Monte Carlo codes such as MCNPX-PoliMi. Previous studies in literature determined a function characterizing the photon energy resolution for four photon detectors using the wide-angle Compton coincidence technique. The goal of this work is to utilize this technique to measure the photon resolution for the EJ-309 organic scintillation detector. The photon energy resolution was measured for an EJ-309 organic scintillation detector using a high-purity germanium (HPGe) detector and a 100-μCi 137Cs source. The photon energy resolution of the EJ-309 was measured to be between 16 and 28% for photon energies between 390 and 280 keV.

Time-of-flight neutron spectrum of 239 Pu/ 9 Be source

Neutron sources whose neutrons can be tagged according to their time of emission are extremely useful because time-of-flight spectroscopy can be used to accurately measure their neutron energy spectrum. Sources utilizing the ⁹Be(α, n)¹²C reaction exhibit neutron groups corresponding to the states of the ¹²C residual nucleus. Reactions in which the ¹²C nucleus is left in an excited state are characterized by the emission of an energetic gamma ray in addition to the neutron. These gamma rays were used to tag the fast neutrons emitted from a 1-Ci ²³⁹Pu/⁹Be source and time-of-flight spectroscopy was employed to measure the neutron spectrum from 0.75 MeV to 6.3 MeV. Minimum and maximum taggable neutron energies were determined and validated, and prominent spectral peaks identified and related to peaks in the ⁹Be(α, n)¹²C cross section.