  LaBr3:Ce crystals have good scintillation properties for X-ray spectroscopy.
Initially, they were introduced for radiation imaging in medical physics with
either a photomultiplier or SiPM readout, and they found extensive applications
in homeland security and gamma-ray astronomy. We used 1 inch round LaBr3:Ce
crystals to realize compact detectors with the SiPM array readout. The aim was
a good energy resolution and a fast time response to detect low-energy X-rays
around 100 keV. A natural application was found inside the FAMU experiment, at
RIKEN RAL. Its aim is a precise measurement of the proton Zemach radius with
impinging muons, to contribute to the solution to the so-called proton radius
puzzle. Signals to be detected are characteristic X-rays around 130 KeV. A
limit for this type of detector, as compared to the ones with a photomultiplier
readout, is its poorer timing characteristics due to the large capacity of the
SiPM arrays used. In particular, long signal falltimes are a problem in
experiments such as FAMU, where a prompt background component must be separated
from a delayed one (after 600 ns) in the signal X-rays to be detected.
Dedicated studies were pursued to improve the timing characteristics of the
used detectors, starting from hybrid ganging of SiPM cells; then developing a
suitable zero pole circuit with a parallel ganging, where an increased
overvoltage for the SiPM array was used to compensate for the signal decrease;
and finally designing ad hoc electronics to split the 1 inch detector SiPM
array into four quadrants, thus reducing the involved capacitances. The aim was
to improve the detectors timing characteristics, especially falltime, while
keeping a good FWHM energy resolution for low-energy X-ray detection.
