First tests of a gamma-blind fast neutron detector using a ZnS:Ag-epoxy mixture cast around wavelength-shifting fibers

There are many applications for a fast neutron detector with a very low gamma sensitivity such as homeland security, nuclear nonproliferation, and, the main focus of this work, spent nuclear fuel rod characterization and post irradiation examination (PIE). Currently, gamma-blind detectors are often He-3 based, but the scarcity of He-3 has created a strong demand for alternatives. In this work, a novel detector design is presented which can help to meet this demand. The detector consists of silver-activated zinc sulfide (ZnS:Ag) as a scintillator with embedded wavelength shifting fibers. This was implemented by casting a scintillator-epoxy mixture into molds with the fibers already in place. A small prototype was constructed using 56 fibers and an active volume of 3 x 10 x 30 mm(3). It was shown that the count rate of the detector at a gamma flux of up to 2.3.10(7) s(-1) from a Co-60 source can be reduced to < 1 s(-1) while still having a detection efficiency of 0.7% for the fast neutrons emitted by a Cf-252 source. This makes the presented design a promising candidate for fast neutron detection in high gamma environments. Most of the Co-60 counts were found to be caused by a pileup of gamma interactions, which gives the possibility of decreasing the gamma sensitivity further by segmenting the detector. Additionally study of the time behavior of the scintillation light decay of ZnS:Ag, when excited by recoil protons, was performed. Overall, this work demonstrates the basic feasibility of this detector type, gives first insights into its physics, and lays a basis for future development and optimization of the approach for a wide range of applications.