A study on photomultiplier afterpulses in TL/OSL readers

In this work we present results of investigations of afterpulses in the photomultiplier tube (PMT) EMI 9235QA built into the Daybreak TL/OSL reader. The amplitudes of pulses and the time intervals between subsequent pulses were recorded using an ultra-fast data acquisition board and a laboratory made multichannel time/amplitude analyzer. We have recorded afterpulses following background (i.e. dark) pulses, pulses induced by single light photons and by high energy photonic radiation from a number of radioactive sources (²⁴¹Am, ¹³⁷Cs, ⁶⁰Co, ⁹⁰Sr/⁹⁰Y) with different X/γ energy ranges. The measurements were performed at varying PMT temperatures. Distributions of time intervals between pulses show that there are several kinds of afterpulses, accompanying primary pulses caused by ionizing radiation, that follow with delays between 10^-8-10^-4 s. In comparison to pulses induced by a single photoelectron, the amplitude of afterpulses varies over a wide range from less than a single photoelectron to around 10 photoelectrons. In the investigated PMT for single light photon pulses we detected 7.5% of afterpulses and for background pulses up to 31% of afterpulses. Random series of afterpulses following primary pulses change the statistical distribution of the number of pulses recorded using a photomultiplier. If the number of primary pulses follows a Poisson distribution, then total counts number has another distribution with its variance greater than the mean, clearly showing an over-dispersion.