New spectrometer dedicated to metrological characterization of radionuclide-based neutron sources

This work presents a new system dedicated to the measurement of energy spectra from radionuclide-based neutrons sources (ex. AmBe, PuBe,
²⁵²
Cf). The experimental device consists of a large-volume polyethylene container that is equipped with a central channel accommodating the source to be measured and 12-measurement channels (in a spiral formation) around the source, into which detectors can be placed. The container is filled with water in order to moderate neutrons emitted from the source and to reduce the sensitivity of the detectors to the external environment. Measurements have been performed with
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Li-doped plastic scintillators (PS) developed in-house and optimized for the simultaneous detection of fast neutrons, thermal neutrons, and gamma rays, through signal processing based on Pulse Shape Discrimination (PSD). This novel approach solves the underdetermined problem as observed in the case of classical neutron spectrometers (ex. Bonner sphere systems with limited experimental data). Through the use of
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Li-doped PS information, the increased experimental data enable the spectrum of interest to be recreated without the need to introduce a "default spectrum" in the unfolding process. The reconstruction is performed with an iterative Maximum-Likelihood Expectation–Maximization algorithm (ML-EM) throughout the detector's responses matrix calculated with the MCNP6 code. An optimized version of this ML-EM algorithm with a regularization step has been also tested. The design, methodology and preliminary results are presented. Different types of neutron sources were measured and results are consistent, with a clear benefit when using experimental data taking into account the measured fast neutron component instead of using only thermal neutron counting.