Response of gadolinium doped liquid scintillator to charged particles: measurement based on intrinsic U/Th contamination

A measurement is reported for the response to charged particles of a liquid scintillator named EJ-335 doped with 0.5% gadolinium by weight. This liquid scintillator was used as the detection medium in a neutron detector. The measurement is based on the in-situ α-particles from the intrinsic Uranium and Thorium contamination in the scintillator. The β–α and the α–α cascade decays from the U/Th decay chains were used to select α-particles. The contamination levels of U/Th were consequently measured to be (5.54±0.15)× 10⁻¹¹ g/g, (1.45±0.01)× 10⁻¹⁰ g/g and (1.07±0.01)× 10⁻¹¹ g/g for ²³²Th, ²³⁸U and ²³⁵U, respectively, assuming secular equilibrium. The stopping power of α-particles in the liquid scintillator was simulated by the TRIM software. Then the Birks constant, kB, of the scintillator for α-particles was determined to be (7.28±0.23) mg/(cm²⋅MeV) by Birks' formulation. The response for protons is also presented assuming the kB constant is the same as for α-particles.