Characterization of alpha contamination in lanthanum trichloride scintillators using coincidence measurements

The commercial availability of LaCl3:Ce scintillators has been much anticipated due to their significantly higher resolution relative to NaI(Tl). Our investigation of these scintillators for coincidence gamma-ray measurement applications revealed that the scintillators had a large, internal alpha contamination affecting the gamma-ray energy range from 1700 to 3000 keV. One passive method of identifying contaminants relies on exploiting coincident signatures. Aided by a coincidence signature library developed at PNNL, we determined that the parent contaminant is Ac-227 via an alpha-gamma coincidence measurement. In this paper, we characterize the level of contamination and describe our coincidence measurement technique. The Ac-227 concentration was approximately 0.13 ppt. We demonstrate that this coincidence technique measures minimum detectable activities much lower than single gamma-ray spectroscopy. We also discuss gamma- and beta-contamination in these scintillators.