Time response of an undoped LaCl3 crystal

We have characterized a detector equipped with an undoped LaCl${}_3$ truncated cone crystal with dimensions of 22.5 mm in the largest diameter, 16 mm in the smallest diameter, and 16 mm in height, coupled to a fast Photonis XP2020/URQ photomultiplier tube (PMT). Its time response at 511 keV (²²Na) and ⁶⁰Co photon energies has been measured against a reference detector using a fast digitizer module by digital signal processing methods based on a genetic algorithm. The time resolution was optimized by the choice of the photomultiplier bias voltage and the fine-tuning of the digital parameters of the time pickup algorithm. The de-convoluted full width at half maximum (FWHM) time resolution is found to be 250 ± 10 ps at ⁶⁰Co energies, and 444 ± 8 ps using positron annihilation $\gamma$-rays from ²²Na, providing good prospects for the use of the crystal in applications requiring a fast time response.