Photoelectron anticorrelations and sub-Poisson statistics in scintillation detectors

Abstract

The performance of scintillation detectors for x rays and gamma rays is limited fundamentally by the statistics of the scintillation light and the resulting photoelectrons. This paper presents a new experimental approach to studying these statistics by observing correlations in the signals from two photodetectors. It is shown that the Fano factors (ratios of variance to mean), both for the number the photoelectrons produced on the photocathode of the photomultiplier and for the underlying number of scintillation photons, can be deduced from these correlations. For LaBr3(Ce) and 662 keV gamma rays, the photopeak signals obtained by photomultipliers on opposite faces of a thin sample are negatively correlated, and the Fano factor for the photoelectrons is significantly less than one. The inferred Fano factor for the optical photons is very small, indistinguishable from zero within experimental error.

Estimation of Fano factor in inorganic scintillators

“…In essence, the new method is a version of the Hanbury Brown and Twiss experiment, which played a key role in differentiating between classical and quantum-mechanical light. The results confirm those in [1] and may provide new insights into the basic physics of scintillators.…”

“…The mathematics in that paper shows that sub-Poisson light ( F < 1) will produce negative correlations in the photodetectors, and conversely the correlation coefficient can be used to infer the Fano factor of the light. In [1] we showed that F was very small in LaBr 3 (Ce), in fact indistinguishable from zero within experimental error.…”

“…Inspired by these observations, we developed a new method for measuring the Fano factor in scintillators and published it in this journal [1]. The premise is that Poisson-distributed light (Fano factor F = 1) should produce statistically independent photoelectrons when two or more photodetectors receive light from the same scintillation event.…”

“…In the present paper, we present additional experimental results for other scintillators with the same methodology as in [1], and we present a completely new method for inferring the Fano factor from optical correlations. In essence, the new method is a version of the Hanbury Brown and Twiss experiment, which played a key role in differentiating between classical and quantum-mechanical light.…”

Impact of the Fano Factor on Position and Energy Estimation in Scintillation Detectors

“…Light from scintillation crystals has been reported to have Fano factors ranging from sub-Poisson to super-Poisson [2], [3]. …”