Development of thermal neutron shield with low radioactivity

The second phase of the Advanced Mo-based Rare process Experiment (AMoRE-II) searches for a neutrino-less double-beta decay of ¹⁰⁰Mo. A background level in the region of interest of 3034 ± 10 keV is required to be lower than 10^-4 count/(keV kg year). Neutrons can generate background signals by gamma-ray emitting reactions with AMoRE-II detector materials. Thermal neutron shields will be installed inside and outside the lead shield to reduce the neutron flux in the shielding system. The background radioactivity of the inner thermal neutron shield must be low because AMoRE-II requires a low background level. A thermal neutron shield with a high shielding efficiency and low radioactivity was produced by using boric acid powder and silicone rubber. The shielding efficiency was tested to be {99.2 ± 0.25(stat)_-0.53^+0.47(syst)}%, and the radioactivities of ⁴⁰K, ²²⁶Ra, ²²⁸Ac, and ²²⁸Th were measured to be < 9.2 at 90% C.L., 14.2 ± 1.5, < 1.6 at 90% C.L., and 2.83 ± 0.80 mBq/kg, respectively.