High spatial resolution fiber based miniature detector for neutron flux mapping in a subcritical assembly and neutron generator

Abstract

Accurate neutron flux mapping of reactors and neutron generators is important from the point of view of determining system characteristics. In these applications, the flux measurement with high spatial resolution and in very narrow spaces is required such as in between fuel rods assemblies or shielding materials. In this paper, we describe the design and development of high spatial resolution ( $\sim \text{mm}$ ) fiber-based detectors for neutron flux mapping in a typical subcritical assembly – BRAHMMA. These detectors utilize neutron convertor-scintillator coupled to the tip of an optical fiber at one end with its other end coupled to a photomultiplier tube. We have used scintillators such as ${}^6\text{LiF}+\text{ZnS}$ (Ag) for thermal neutrons and ${\text{ThO}}_2+\text{ZnS}$ (Ag) for fast neutrons. The detectors were calibrated for $3\times {10}^{-4}\text{cps}$ and $3.6\times {10}^{-6}\text{cps}$ per unit neuron flux of thermal and fast neutron respectively. Small size ( $\sim \text{mm}$ ) of these detectors is well suited for measurement of thermal neutron flux profile in narrow experimental channels of the subcritical assembly BRAHMMA. The measured thermal neutron flux profile was compared with those obtained from a miniature ³He detector used in the past. Periodic variation in the flux profile has been observed at the fuel lattice positions of the subcritical assembly, which was not observed with the ³He detector. We also present a neutron emission profile measurement of a D-T neutron generator using the developed fast neutron detector.