The fast X-ray detector system of the FAMU experiment at RAL

The FAMU experiment at RAL has been designed to study the hyperfine splitting (HFS) of muonic hydrogen and thus measure the Zemach radius of the proton, with a precision better than 1 %. The HFS transition is excited by a tunable MIR laser at ~ 6790 nm and is recognized by delayed ($\mu$O) X-ray emission around 130-170 keV. The fast X-ray detection system is based on 34 scintillating LaBr3:Ce crystals and one HPGe detector for inter-calibration.