Speaker
Description
The FAMU [1] experiment at RIKEN-RAL aims at a high-precision measurement of the hyperfine splitting of muonic hydrogen through the detection of an excess of characteristic X-rays emitted by muonic oxygen following laser-induced [2] excitation. The X-ray detection system is based on LaBr3:Ce scintillators read out by both photomultiplier tubes (PMTs) and SiPM arrays, requiring stable energy calibration over extended data taking periods.
SiPM-based detectors are known to exhibit gain variations correlated with temperature fluctuations and operating conditions, potentially degrading the energy resolution and affecting the stability of the reconstructed spectra. In this work, we present a calibration strategy based on the use of prompt X-ray flashes, intrinsically available during data taking, to monitor and correct gain drifts on a run-by-run basis. This method complements the standard offline calibration using radioactive sources and internal spectral lines.
The evolution of the detector response is studied over multiple data taking periods, showing residual gain variations at the percent level in absence of correction. The application of the prompt-based calibration allows for an effective stabilization of the energy scale and a significant improvement in the peak reconstruction consistency.
The resulting energy resolution is evaluated for SiPM-readout detectors and compared with that obtained using PMT-based systems. The performance is found to be competitive, with SiPM detectors achieving comparable or improved FWHM values in the energy range relevant for oxygen X-ray detection (~100–150 keV).
These results demonstrate the effectiveness of in-situ calibration using prompt signals for mitigating gain drifts in SiPM-based systems, representing a key element for stable long-term operation in high-rate X-ray spectroscopy experiments such as FAMU.
[1] Adamczak A., Bakalov D., Baldazzi G., Baruzzo M., Benocci R., Bertoni R., Bonesini M., Capra S., Cirrincione D., Clemenza M., Colace L., Danailov M., Danev P., Bari A.D., Vecchi C.D., Ferdinando D.D., Fasci E., Gaigher R., Gianfrani L., Hillier A.D., Ishida K., Lord J.S., Menegolli A., Mocchiutti E., Monzani S., Moretti L., Morgante G., Pizzolotto C., Pullia A., Pullia M., Ramponi R., Roman H.E., Rossella M., Rossini R., Sbrizzi A., Stoilov M., Suárez-Vargas J.J., Toci G., Tortora L., Vallazza E., Yokoyama K., Vacchi A., First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility, (2025) European Physical Journal A, 61 (12), art. no. 284, DOI: 10.1140/epja/s10050-025-01758-4
[2] Baruzzo M., Fasci E., Moretti L., Suárez-Vargas J.J., Cabrera H., Toci G., Capra S., Gianfrani L., Pizzolotto C., Mocchiutti E., Danailov M.B., Vacchi A., Long-term performance of a mid-IR laser system for muonic hydrogen spectroscopy in the FAMU experiment, (2026) Optics Communications, 608, art. no. 133082, DOI: 10.1016/j.optcom.2026.133082