Electron detection efficiency of a Nal(Tl) scintillator for electrons and for gamma rays above a few keV

Abstract

A technique is developed for the analysis of photoelectron pulse amplitude distributions from gamma rays or electrons of selected energy observed with a GaP first dynode photomultiplier, taking into account the time-spread of photon emission from the Nal(Tl) scintillator. The observed amplitude distribution is decomposed into a unique sum of experimentally based, constructed amplitude distributions (with relative intensities f n ), one for each number n of photoelectrons emitted in each detected event. It is experimentally verified that, for unique energy deposition in the scintillator, the pulse amplitude distribution gives rise to a number distribution f n , which is Poisson P m ( n), and also that the loss of detected events above a very low discrimination level is accurately given by the Poisson prediction for events which fail to release even one photoelectron, i.e., P m (0). For a spectrum of energies actually deposited in the crystal due to various degrees of back-scattering of an impinging electron with given energy, the derived f n distribution below n = 10 is analyzed into a quasi continuum of weighted Poisson distributions. The efficiency loss for any given discrimination level can be obtained from this continuous function. The method is shown to work down to an intrinsic detection efficiency of 10% for very low energy gamma rays and down to at least 5 keV for electrons. Reasons for disagreement between observation and expectation in the range 2.6-5 keV for electrons are discussed.