Sound and Light from Fractures in Scintillators

Prompted by intriguing events observed in certain particle-physics searches for rare events, we study light and acoustic emission simultaneously in some inorganic scintillators subject to mechanical stress. We observe mechanoluminescence in ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$, ${\mathrm{CdWO}}_4$, and ${\mathrm{ZnWO}}_4$, in various mechanical configurations at room temperature and ambient pressure. We analyze the temporal and amplitude correlations between the light emission and the acoustic emission during fracture. A novel application of the precise energy calibration of ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ provided by radioactive sources allows us to deduce that the fraction of elastic energy converted to light is at least $3\times {10}^{-5}$.

Figure 1

Actuator-driven experimental setup with DCDC sample geometry.

Figure 2

10 s samples from observations of manually compressed DCDC specimens of ${\mathrm{CdWO}}_4$ and ${\mathrm{ZnWO}}_4$. In each figure, top to bottom: applied force, both AE signals, and light signal. Drops in the force indicating fracture are correlated to increased acoustic and photon activity. All signals are uncalibrated.

Figure 3

Waiting times between light events and between AE events during the indentation of a BGO sample. Both channels show similar behavior over the shared part of the range, with the shorter light pulses providing better timing separation than the longer AE ones.

Figure 4

Emission spectra from a BGO sample, first during fracture, then under x-ray excitation. The similarity of spectra indicates a common emission mechanism.

Figure 5

Energy calibration of a DCDC BGO scintillator using radioactive sources. Top: Spectra for various sources and fits to the full-collection and backscattering peaks. Bottom: Response of the system is fairly linear over this energy range and varies by $\approx 10\%$ after the fracture.

Figure 6

Zoom on the rupture of the DCDC BGO sample of Fig. 5 showing correlation between the various channels when the main fracture occurs around 23 s (corresponding to $\approx 17\min$ since the start of the procedure). Photon signal shows a wide range of amplitudes (the largest of which saturate the integrator).

Figure 7

Normalized cumulative number of events on AE and photon channels around the main fracture, for the BGO sample of Fig. 6. The AE and photon channels show similar behavior. The drop in force signaling the main fracture corresponds to a sharp increase in the event rates for photons and AE.

Figure 8

Amplitude distributions for the various channels of the BGO sample from Fig. 6. Amplitude of the light channel is proportional to energy. All distributions are fitted by power laws (up to saturation, for light).