Fast timing measurements of gamma-ray events in liquid xenon

We have used newly developed metal-channel photomultipliers (PMTs) from Hamamatsu Photonics to detect the fast scintillation light from liquid xenon. The PMTs are designed to operate within the liquid and to efficiently detect the 175 nm Xe light. Their small transit time spread makes them ideal for fast timing applications. To evaluate the timing characteristics of a liquid xenon detector, we tested a gridded ionization chamber equipped with two such PMTs. Both PMTs observe the same interaction and share the detected scintillation light about equally. At zero field the measured timing resolution is 565 ps FWHM for 122 keV from ⁵⁷Co and 227 ps FWHM for 662 keV from ¹³⁷Cs. For electric fields up to 4 kV/cm drift field the timing resolution remained constant. This timing resolution opens the possibility of a time-of-flight (ToF) measurement between two LXe detectors separated by a distance of 10-20 cm, which is of great interest for background reduction in a Compton telescope for γ-ray astrophysics.