Calibration of the Fast Neutron Imaging Telescope (FNIT) Prototype Detector

The paper describes a novel detector for neutrons in the 1 to 20-MeV energy range with combined imaging and spectroscopic capabilities. The Fast Neutron Imaging Telescope (FNIT) was designed to detect solar neutrons from spacecraft deployed to the inner heliosphere. However, the potential application of this instrument to Special Nuclear Material (SNM) identification was also examined. In either case, neutron detection relies on double elastic neutron-proton (n-p) scattering in liquid scintillator. We optimized the design of FNIT through a combination of Monte Carlo simulations and lab measurements. We then assembled a scaled-down version of the full detector and assessed its performance by exposing it to a neutron beam and an SNM source. The results from these tests, which were used to characterize the response of the complete FNIT detector to fast neutrons, are presented herein.

I. Introduction

The Fast Neutron Imaging Telescope (FNIT) is an instrument developed specifically to study neutrons in the 1 to 20-MeV energy range. The structure of FNIT has evolved through a number of significantly different design layouts. After a series of Monte Carlo simulations and lab tests, the initial design of this instrument, developed in 2005 and consisting of a tower of position sensitive plastic scintillator modules [1], was abandoned in favor of a detector based on liquid scintillator tubes arranged in a radially symmetric pattern (Fig. 1). A prototype science model, simplified version of this latter design layout, was recently assembled and underwent initial performance testing at the University of New Hampshire (UNH). The FNIT prototype was successively calibrated at the pulsed neutron source of Crocker Nuclear Laboratory, UC Davis (CNL) and tested by exposing it to a weapon grade plutonium (WGP) fission neutron source at Pacific Northwest National Laboratory (PNNL). Finally, data collected in these campaigns were used to characterize the response of the FNIT detector to 1 to 20-MeV neutrons. Due to the modular structure of FNIT, the design from Fig. 1 may be enlarged or compacted as necessary to meet specifications for individual applications. This affects efficiency and exposure times, but not the basic performance parameters such as energy and angular resolution. Determining these parameters was the goal of our prototype calibration. Fig. 1.

Schematic view of the FNIT detector design and the double elastic n-p scattering of an incident neutron. Liquid scintillator bars are arranged in a radially symmetric pattern and every bar is coupled to two PMTs, one at each extremity. By measuring the positions, time difference and energies of two recoil protons, the kinetic energy of the incident neutron can be determined and its arrival direction restricted to the mantle of a cone. An “event circle”, such as the one shown in this figure and with an uncertainty arising from measurement errors in the kinematic parameters of the neutron, can then be drawn.