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 alpha-particles from the intrinsic Uranium and Thorium contamination in the scintillator. The beta-alpha and the alpha-alpha cascade decays from the U/Th decay chains were used to select alpha-particles. The contamination levels of U/Th were consequently measured to be (5.54 +/- 0.15) x 10(-11) g/g, (1.45 +/- 0.01) x 10(-10) g/g and (1.07 +/- 0.01) x 10(-11) g/g for Th-232, U-238 and U-235, respectively, assuming secular equilibrium. The stopping power of alpha-particles in the liquid scintillator was simulated by the TRIM software. Then the Birks constant, kB, of the scintillator for alpha-particles was determined to be (7.28 +/- 0.23) mg/(cm(2).MeV) by Birks' formulation. The response for protons is also presented assuming the kB constant is the same as for alpha-particles.