Realization of a thin boron layer neutron beam monitor

Abstract

A neutron beam monitor can play an important role in a neutron experiment when the spatial and temporal distribution of the incident neutrons should be exactly known. In this study, we propose a method to realize a neutron beam monitor with a thin boron layer, which is necessary for easily setting an optimal threshold to separate the neutron signals from those of gamma rays and electronic noise with the aid of atomic layer deposition (ALD) technique. By applying the B2O3 layer forming process and the ZnO layer forming process, boron layers with thicknesses ranging from 16.8 nm to 472.7 nm are successfully prepared to form the beam monitors with a multiwire proportional chamber (MWPC) as the readout. A test is conducted at the compact pulsed hadron source (CPHS) at Tsinghua University. The results show that a slope of 0.7%/100 V of the counting plateau can be achieved for the high voltage region of 250 V, with the spatial resolution being better than 3.09 mm. The B2O3 film with a minimum mass thickness of 0.38μg/cm 2 enables a maximum neutron flux of 9.7 × 107 n/cm 2/s to be monitored with this beam monitor, with an accompanying attenuation for penetrating incident neutrons of 0.35%, significantly smaller than that of typical state-of-art beam monitor.