Characterization of mixed Bi$_4$(Ge$_x$Si$_{1-x}$)$_3$O$_{12}$ for crystal calorimetry at future colliders

Bismuth germanate (BGO) is a well known high density scintillating material widely used in many applications such as high energy physics and medical imaging. Bismuth silicate (BSO) features properties similar to BGO in terms of stopping power and Cherenkov photon yield with a lower scintillation light output but faster decay time, thus being more attractive for applications in high-rate environments. Mixed crystals such as Bi$_4$(Ge$_x$Si$_{1-x}$)$_3$O$_{12}$ (BGSO, with 𝑥 varying from 0 to 1) make it possible to optimize decay time and light yield based on the detector needs. A characterization campaign of the optical and scintillation properties of two sets of BGSO mixed crystals with Ge fraction varying from 0 to 100% was performed. A coincidence time resolution (CTR) at 511 keV of 208 ±2 ps FWHM was measured for a 2 × 2 × 3 mm$^3$ pixel with 40% Ge, while the optimum value obtained for the effective decay time is 49.9 ±1.8 ns for a 6 × 6 × 0.7 mm$^3$ plate-shaped sample with 30% Ge. Furthermore the smallest slow decay time component achieved is 101 ±2 ns and is obtained for the plate-shaped sample with 30% Ge, while the largest is 236 ±5 ns for a pure BGO sample with the same geometry. In addition we demonstrated the possibility to efficiently separate the Cherenkov and scintillation light produced in a pure BSO sample. Such a technique could be exploited in a crystal-based dual-readout calorimeter to improve the energy resolution for hadronic showers and jets.