NE213 Scintillator Characterization Using n/gamma Digital Pulse Shape Discrimination

Abstract

NE213 scintillation detectors are excellent tools for use in a mixed gamma and neutron field due to its established pulse shape discrimination ability. With proper pulse shape discrimination, gamma or neutron responses may be obtained in a mixed radiation field. The neutron response needs to be deconvolved from the detector response function to obtain an energy spectrum. To perform unfolding of neutron spectra, mono-energetic responses are needed and the responses may be obtained via experiment or simulation of the NE213 detector.

In this work, response functions were tested with the unfolding of a Cf-252 spectrum. Particularly, experiments were performed at the Los Alamos National Laboratory where Cf-252 spectra were obtained. During the experiment, different pre-amplifier set-ups were tested. Namely, the pulse shape discrimination ability of the system using 50 ohm, 500 ohm, and 1000 ohm termination resistors were compared. However, linear system responses were not observed with the different settings. Thus, the Amoeba Simplex fitting routine was used to augment the charge integration pulse shape discrimination technique to separate the neutron and the gamma signals. Furthermore, a new figure of merit scheme was explored to quantify the pulse shape discrimination ability of the said non-linear system. Alongside the Cf-252 spectra experimental measurements, the program Scinful was used to generate mono-energetic neutron responses needed for unfolding. Consequently, both the Cf-252 neutron spectra and the Scinful responses were used with the program FERD-PC for unfolding.

Among the different termination resistors compared with the new figure of merit scheme, the 50 ohm resistor setting was observed to be superior. The resultant unfolded spectrum using the 50 ohm termination resistor shows excellent agreement between 2-10 MeV. However, even with the Amoeba Simplex method, results below 2 MeV are not accurate due to the poor pulse shape discrimination limit of the system used. Results above 10 MeV were not obtained due to the energy range of the used Cf-252 source. The excellent agreement for the unfolded spectrum between 2-10 MeV does allow for the confirmation of Scinful's responses' accuracy for that particular energy range. However, Scinful's accuracy for energy ranges outside of this bound is not confirmed in this work.