Investigation on the gamma-sensitivity of 6LiF-diamond thermal neutron detectors: measurement and Monte Carlo simulation

We have investigated the gamma-sensitivity of single-crystal chemical vapor deposited (CVD) diamond detectors coupled to a (LiF)-Li-6 thermal neutron converter. The responses of the detectors of different thicknesses to gamma-rays and thermal neutrons were simulated by using the PHITS Monte Carlo simulation code. The simulated responses were compared against the experimental data measured with two diamond detectors (CIVIDEV, Wien, Austria) of 25 and 140 mu m thicknesses. Prior to the response measurement, the detectors were fully characterized in terms of leakage current, charge collection efficiency (CCE), and energy resolution. The results of our study show that with a 25 mu m thick (LiF)-Li-6-diamond detector, thermal neutrons can be clearly measured by setting an energy gate on the triton peak of the Li-6(n,t)He-4 reaction, while this thickness exhibits low sensitivity to gamma-rays. An enhancement in thermal neutron detection efficiency is demonstrated by the PHITS simulations for a sandwich structure of the 25 mu m thick diamond detector and an increased (LiF)-Li-6 converter thickness. The 25-mu m-thick (LiF)-Li-6-diamond detector is a promising candidate for real-time neutron measurements in a strong background of gamma-rays, beyond a dose rate of 80 Gy/h, such as that in the Fukushima Daiichi nuclear power station.