Improving the Time Resolution of Large-Area LaBr3:Ce Detectors with SiPM Array Readout

Abstract

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″ 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″ detector’s 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.

One inch LaBr3:Ce detectors, with temperature control and improved time resolution for low energy X-rays spectroscopy

“…The main experimental challenge in the FAMU experiment is the detection of low-energy X-rays around 130 keV separating a "delayed" signal component (after 600 ns) from a prompt one. The reported results: especially the ∼ 8% energy resolution at the 57 Co peak and the signal falltime (∼ 150 ns) show that the required target may be reached [7], [9]. Prelimary data taken at RIKEN RAL with 55 MeV muons from March 2023 confirm this assumption.…”

“…Afterwords, the four signals of the sub-arrays (2x2 cells) are summed together and inverted, to give a positive output. All is realized with a custom circuit realized with Nuclear Instruments srl, see reference [9] for additional details. In this way a good energy resolution is obtained and pulse timing (risetime/falltime) is reduced by a factor 2X.…”

Status of the detector setup for the FAMU experiment at RIKEN-RAL for a precision measurement of the Zemach radius of the proton in muonic hydrogen

“…They use Hamamatsu S14161-3050-AS and S14161-6050-AS SiPM arrays. While 1/2 ′′ detectors have a standard parallel ganging, 1 ′′ detectors use a custom 4-1 PCB developed with Nuclear Instruments srl, see reference [19] for more details. For construction details of the 1/2 ′′ and the 1 ′′ detectors see references [20] and [21].…”

“…Twelve 1/2 ′′ detectors were mounted on the downstream detector holder, whereas sixteen 1 ′′ detectors were positioned in the central (6 detectors) and upstream (10 detectors) rings, as shown in figure 4. The performances, measured in laboratory, of the 1/2 ′′ and 1 ′′ detectors are summarised in table 1 [19,20]. Energy resolutions on the beamline, obtained from the reconstructed muonic Ag peak at 141 keV for detectors in a typical run on the full experiment setup with 55 MeV/c impinging muons, is reported in figure 5.…”