Scintillation studies of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ in the millikelvin temperature range

We extended the multiphoton counting technique to investigate the scintillation characteristics of crystals at millikelvin temperatures using ${}^3\mathrm{He}∕{}^4\mathrm{He}$ dilution refrigerator. The temperature dependence of the scintillation time constant and the light response of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ were measured down to $20\mathrm{mK}$. The scintillation light response is found to be independent of temperature below $10\mathrm{K}$—so far taken as an assumption in many cryogenic scintillator experiments, but yet previously unproved. The decay kinetics of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ and its temperature dependence are analyzed in terms of a simple three-level model of the emission center. The model agrees well with the data over the temperature range $(0.02–350\mathrm{K})$ investigated, and is used to determine the parameters of the relaxed excited state.

Figure 1

Temperature dependence of scintillation light response of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$.

Figure 2

(Color online) Scintillation decay curves of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ measured at $T=295$ and $77\mathrm{K}$ using $\gamma$ (1) and $\alpha$ (2) excitations. The solid line shows the best fit to the experimental data using two exponentials. Parameters of the fit are listed in Table .

Figure 3

(Color online) Scintillation decay curves of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ measured at $T=4.2$ and $0.02\mathrm{K}$ using $\gamma$ (1) and $\alpha$ (2) excitations. The solid line shows the best fit to the experimental data using two exponentials. Parameters of the fit are listed in Table .

Figure 4

(Color online) Temperature dependence of the slow scintillation decay time constant of $\mathrm{Ca}\mathrm{W}{\mathrm{O}}_4$ (excitation with ${}^{241}\mathrm{Am}$ $\alpha$ particles). The solid curve displays the result of the best fit to the experimental data using the three-level model shown in the inset. Parameters of the fit are $k_1=3.0\times {10}^3{\mathrm{s}}^{-1}$, $k_2=1.1\times {10}^5{\mathrm{s}}^{-1}$, $D=4.4\mathrm{meV}$, $K=8.6\times {10}^9{\mathrm{s}}^{-1}$, and $\Delta E=320\mathrm{meV}$.