Performance of a neutron imaging detector based on the μPIC micro-pixel gaseous chamber

A new detector employing a time projection chamber built on the micro-pixel gaseous chamber (μPIC) and an FPGA (Field Programmable Gate Array)-based data acquisition system has been developed as a thermal-neutron imaging detector for small-angle neutron scattering (SANS) and neutron radiography. A small amount of ³He added to the gas mixture is used as the neutron absorber, giving a detection efficiency for thermal neutrons of up to 30%. An improved FPGA program permits the simultaneous measurement of the track length and energy deposition for the resulting proton-triton pairs, allowing the separation of the proton and triton for improved position resolution. Here we report the preliminary results of several test experiments performed at the NOBORU beam line at the J-PARC Spallation Neutron Source in November 2009 and June 2010. During these experiments, we observed time-averaged data rates up to 4.5 MHz, and with a time resolution on the order of 1 μs per neutron event, we were able to cleanly separate the individual neutron pulses and determine neutron energy by time-of-flight. Using images of cadmium test charts, the position resolution of our detector was estimated as 315 ± 15μm (preliminary). We also successfully observed resonance absorption in various metals, SANS diffraction from spherical SiO₂ nanoparticles, and Bragg edge transmission of a welded steel plate.