Identification of internal radioactive contaminants in elpasolites (CLYC, CLLB, CLLBC) and other inorganic scintillators

We report the results from an experiment conducted to identify the isotopic contaminants that produce measurable background emission in a family of inorganic scintillation crystals known as elpasolites, namely Cs2LiYCl6:Ce (CLYC), Cs2LiLaBr6:Ce (CLLB), and Cs2LiLa(Br,Cl)(6):Ce (CLLBC), and in other inorganic scintillation crystals, such as Li co-doped NaI:Tl (NaIL). Elpasolites have high light yield in response to ionizing radiation, excellent energy resolution for gamma-ray spectroscopy, and can detect and discriminate neutrons (from gamma rays) using pulse shape discrimination methods. There are, however, internal contaminants associated with certain constituents of these crystals that result in a measurable background signal. The signal from some of these contaminants is well understood; for instance, lanthanum-containing crystals demonstrate a prominent 1436 keV gamma-ray line and beta-particle continuum in the internal background spectrum. However, they also demonstrate spectral features indicative of alpha-particle decay. Given that alpha-particle decay schemes are often complicated, determining the constituents is a thorny problem. One method to understand the contaminants is to measure the gamma-ray emission associated with alpha-particle decay in coincidence with an external detector that can perform high-quality spectroscopic measurements. We performed such measurements using a high-purity germanium (HPGe) detector abutted to varying-sized crystals, seeking out coincident alpha-gamma events in the pair. A scatter plot of the energy deposited by the gamma ray in the HPGe coincident with the energy deposited by the alpha particle in the scintillation crystal allows one to identify the constituent isotope causing the contamination. This methodology was previously demonstrated by others [1]. We will show the results from measurements with elpasolites and other inorganic scintillators, measured in coincidence with a HPGe detector, in terms of a 2-d scatter of deposited energy in the HPGe and scintillator, and the methods that allowed for isotope identification for each of the inorganic scintillation crystals tested.