Multiphoton coincidence technique and studies of scintillation properties over wide temperature range

The multiphoton counting technique (MPC) is a technique developed by our group to measure the scintillation parameters of samples over a wide temperature range [1, 2].

The MPC method works by recording a sequence of single photon pulses produced by a PMT due to the photons from a scintillation event. Each pulse in the sequence corresponds to an individual photon impinging on the photocathode of the PMT. The distribution of arrival times of the photons provides information on the decay characteristics of the scintillation process, while the number of photons recorded per event is proportional to the light yield of the scintillator. Thus, by recording a large number of scintillation events (10³ – 10⁴) one can obtain the decay time characteristics and the light output in a single measurement.

A schematic of this setup is shown to the right. The PMT signal is passed to an integrating amplifier that produces a signal that is a measure of the total energy detected; this is fed into a single channel analyser (SCA). The discrimination thresholds of the SCA reduce the number of pulses with low and very high amplitudes which are associated with electronic noise and spurious events caused by cosmic muons, respectively. The logic output pulses of the SCA produce a trigger for the transient recorder (TR).

All scintillation events recorded by the transient recorder are acquired and analysed using custom-made DAQ software. A key aspect of the MPC technique is that it can discriminate between single and multiple events via statistical analysis of the arrival times of individual scintillation photons. The algorithm used for searching for and eliminating multiple events is based on the idea that the decay time constant τ obtained for each event should be noticeably dissimilar between single events and multiple events. The statistical analysis consists of a combination of a cut on the number of photons, a Shapiro-Wilk likelihood cut and a Poisson statistics cut for the distribution of arrival times of the photons.

Using MPC we have investigated the temperature dependence of scintillation light output and decay time of CaWO₄ down to 20 mK [3]. The scintillation light yield was shown to be independent of temperature below 10 K (fig. 2); as assumed in many cryogenic scintillator experiments, but until this study unproven. This finding provides a long-awaited support for the assumption that data from studies above 10 K can be used to assess the suitability of scintillation materials for applications at much lower temperatures. 

The decay kinetic of CaWO₄ and its temperature dependence was analysed within the framework of a simple three-level model of the emission centre with one metastable level. Subsequently the formula that describes the temperature dependence of the decay time constant τ_r was derived as follows:

This model successfully describes the features of the decay kinetics, giving the parameters of the relaxed excited state of the emission centre (see Fig. 3).

Figure 1. Schematic of MPC setups for measurements of scintillation characteristics down to 10 mK in a ³He/⁴He dilution cryostat. PMT – photomultiplier, PA – preamplifier, TR – transient recorder, Amp – linear amplifier, SCA – single channel analyser.