The effect of detector geometry on EJ-309 pulse shape discrimination performance

In a liquid scintillator detector such as EJ-309, neutrons and gamma-rays give rise to different light pulse shapes due to the way they give up their energy but, in addition to this, there are fluctuations in the shape of the light pulse that varies with the point of interaction in the detector volume. The way in which light is dispersed and reflected throughout a detector volume introduces small fluctuations in the shape of the light pulse. This has an impact on the spread of pulses from neutrons and gamma rays and hence impacts the ability to perform discrimination of the two radiation types by pulse shape. This paper investigates, by experimentation, how the spread in the pulses produced by neutrons and gamma rays varies as the detector volume varies from 350 cubic centimeters to 6,570 cubic centimeters. The effect that this has on pulse shape discrimination performance is described. Also investigated are the effects of right-cylindrical geometries versus cuboid geometries. Pulse-shape discrimination of digitally captured neutron and gamma-ray event pulses was performed using the charge integration technique. A figure of merit at 100 keVee is seen to degrade by 25.2 ± 1.2% as the detector volume is increased from 350 to 6,570 cubic centimeters. At 1450 keVee this degradation increases to 30.9 ± 3.8%. In these experiments, cubic geometries appear to have similar pulse shape discrimination performance to right-cylindrical geometries. A 102 mm cube is calculated to have a figure of merit of 4.1 ± 0.7% better than a 102 × 102 mm cylinder at 100 keVee and on average about 4% better across the energy range 100 to 1450 keVee.