Termination effect on pulse shape discrimination

NE213 liquid scintillation detectors have been used in mixed radiation fields with great success due to their pulse shape discrimination (PSD) ability. Numerous PSD techniques using various analog equipments had been proposed and developed by various individuals in past years. A common method for PSD is to evaluate the fall-time of the voltage across a resistor terminating the output of the photomultiplier tube (PMT) attached to the NE213 cell. The product of the terminating resistor and the terminating capacitance is known as the time- constant. Additionally, the combination of the terminating resistance and capacitance create a high-pass filter whose characteristics depend upon the value of the time constant. The greater the time-constant the less attenuation of the longer frequencies in a given signal occur. This paper will present a quantitative comparison of the fall time PSD technique using various terminating resistors. Specifically, 50 ohm, 500 ohm and a 1 kohm termination schemes are tested. Furthermore, due to nonlinearities in the system, a linear PSD spectrum may not be possible to obtain. In such cases, a traditional figure of merit (FOM) may not be usable to quantify the PSD capability of the system. A modified version of FOM is explored and used to describe the PSD capability of the current system.