Scintillation properties of Bi${}_4$Ge${}_3$O${}_{12}$ down to 3 K under $\gamma$ rays

Bismuth germanate (BGO) has been widely used as a room-temperature scintillator in many applications for decades. Interest in it has recently increased as a low-temperature scintillator to be used in bolometers for rare-event detection. We present our time-resolved-scintillation studies of BGO down to 3 K under $\gamma$-ray excitation. Our multiple-photon-counting-coincidence-based setup allows clear identification of $\gamma$-line energies at least as low as 122 keV down to base temperature and the measurement of the light yield and decay-time constants as a function of temperature. We also discuss the time structure of the pulses and report a previously unappreciated but significant, very slow component assigned to afterglow. Finally, we demonstrate that nonlinearity of the light yield as a function of energy persists at low temperatures.

Figure 1

Typical setup scheme. The PMTs are well-adapted for photon counting with a low dark-count level and work in coincidence mode. The digitizer has a sampling rate of 1 GHz.

Figure 2

Example of a scintillation event of BGO-Q with ${}^{22}$Na at 300 K, recorded with a sampling rate of 1 GS/s. The algorithm then recognizes the time and the amplitude of the photons (red diamonds) and in particular the first photon (green star). The inset shows a magnified view (with the same units).

Figure 3

Evolution of the scintillation-decay time of BGO-Q as a function of temperature between 300 K and 3.4 K. Time constants are reported on the ordinates with vertical error bars. The LY contribution of each component is represented by the area of the circles. (b) A main contributor to the LY is identifiable. (c) A faster component only contributes a small fraction of the light. Moreover, (a) a long component, attributed to afterglow, contributes significantly below 20 K (up to $\sim$50% of the light below 10 K).

Figure 4

Average events of BGO-Q with ${}^{22}$Na (511 keV for $\gamma$ particles only) at temperatures of (a) 300 K, (b) 30 K, and (c) 3.4 K. The insets show magnified views (with the same units). The pulses are fitted with sums of exponentials whose numbers of components vary with the temperature. The decay-time constants obtained are: (a) 54.9 $\pm$ 4 ns, 412 $\pm$ 12 ns, 1.02 $\pm$ 0.25 $\mu$s; (b) 13 $\pm$ 6 ns, 22.6 $\pm$ 0.2 $\mu$s, 64 $\pm$ 12 $\mu$s; and (c) 12 $\pm$ 2 ns, 1.22 $\pm$ 0.09 $\mu$s, 135 $\pm$ 5 $\mu$s, 332 $\pm$ 16 $\mu$s.

Figure 5

Evolution of the main decay-time constant of BGO-Q and BGO-L under $\gamma$ particles between 300 K and 3 K. The combined data set has been fitted with a three-level model of Bi${}^{3+}$. The fit curve (solid green) shows very good agreement with the result obtained under $\alpha$ excitation (dashed black curve) elsewhere.[17]

Figure 6

Numeric spectrum of ${}^{22}$Na at 511 keV $\gamma$ in BGO-L at 3 K. This histogram has been obtained from the number of photons counted for each individual event. The full energy peak is clearly visible with a mean value of 141.6 $\pm$ 0.2 detected photons (as fitted by a Gaussian function).

Figure 7

Evolution of the light yield in BGO with the temperature between 300 K and 3 K. The light yield increases by a factor of about 5.5 and 5 for the two crystals tested under $\gamma$ particles in this work, noticeably more than the factor of 3.5 observed for another sample under $\alpha$ particles elsewhere.[17] The LY has been normalized to its room-temperature value.

Figure 8

Nonlinearity of the LY of BGO-L for different temperatures. The LY is defined here as the ratio of the number of detected photons over the deposited energy. The data are normalized to 662 keV. Data at 10 K and 3 K show good agreement with the nonlinearity observed elsewhere at room temperature and 77 K (Ref. 28) though the 30-K data appear relatively linear.