Multi Sphere Neutron Spectrometer Based On Elpasolite Scintillators

The Multi Sphere or Bonner Sphere Spectrometer (BSS) is a widely used neutron spectrometry technique. Traditionally, a He-3 detector is coupled with polyethylene spheres that act as moderators. This method allows for use in a wide energy range and has an isotropic response. However, the usage of He-3 has challenges, such as the rising cost of He-3 and difficulty in using it in high fluence rate fields due to its relatively long dead time. This paper proposes the use of thermal neutron sensitive scintillators, such as the CLYC and CLLBC, to resolve these issues and allow the use of BSS in fields characterized by high fluence rates. These scintillators exhibit a fast response and good pulse shape discrimination. In this work, we first show that the simulated response of the CLLBC-based BSS to a 252 Cf source compares well to the measured one. After demonstrating the fidelity of the MCNP 6.2 model, we calculated the response matrix of the BSS to monoenergetic neutron sources, ranging from 10 -9 to 20 MeV. We used the calculated response matrix and a Bayesian unfolding algorithm to derive the spectra of various neutron fields, produced by DD and DT neutron generators, and 252 Cf and 241 AmBe sources, and compared the spectroscopy capability of He-3 and CLLBC-based Multi Sphere Spectrometer.