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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.
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