Study on possible proton-induced background of LaBr3(Ce) scintillator in a low-earth orbit

Abstract

Cerium-doped Lanthanum Bromide [LaBr₃(Ce)] is a relatively newly developed inorganic scintillator with high performance for X/gamma-rays. It has high light yield, good energy resolution, and suitable time response for X/gamma- ray spectroscopy in comparison with NaI(Tl) conventionally used for spaceexperiments. Although NaI(Tl), especially using a cleaved surface forthe entrance window, was regarded as the most suitable scintillatorfor the spectroscopy of photons in the hard X-ray to gamma-ray band, a large amount of induced background from radio-activated iodinepolluted observed spectra, which were caused by charged particleirradiation in a high radiation region such as South Atlantic Anomalyin space experiments (e.g., Murakami et al. 1989). LaBr₃(Ce) could have comparable or even better performance toNaI(Tl). There is, however, very limited knowledge about radio-activated LaBr₃(Ce) estimated in a low-earth orbit. To verify the degradation of performance by high charged particle radiation field estimated in a low-earth orbit, we performed proton irradiation experiments using a synchrotron accelerator at TheWakasa Bay Energy Research Center. We performed irradiation experiments of LaBr₃(Ce) using proton beams with 20, 70, 140 MeV. The crystal has the size of 0.5 inch in diameter and a thickness of 0.5 inch; the thickness is the same as the gamma-ray burst detector flown after for the CALET experiment (Yamaoka et al. 2013). The measurement has been done to take gamma-ray spectra from the radio-activated LaBr3(Ce) crystal during the period from 7 minutes to 3.8 days after the proton irradiation. As a result, we detected 24 gamma-ray peak sand identified 19 nuclides by activation. We also found that the nuclides generated by the proton irradiation can be divided into two types; short-lived nuclides (half-life shorter than 24 hours) and long-lived nuclides (half- life longer than a day). From the results, we estimated that the background rate of LaBr₃(Ce) in a low-earth orbitexpected for such as the International Space Station was several counts per second, which was smaller than that for Cosmic X-ray background from an opening angle of about a pi steradian. The contamination suffered from radio-activation of LaBr₃(Ce) crystal was sufficiently small for spectroscopy with a large field-of-view taken forsuch as gamma-ray transient observations.