A possible mechanism for anomalous pulses observed in sodium iodide crystals

Low energy pulses with anomalously low time constant have been observed recently in sodium iodide crystals. A model is presented based on partial energy deposits from alpha particles passing out of the crystal from a thin contaminated layer, which would generate events of similar characteristic. The predicted 1/E2 form of the spectrum is consistent with observation. The required level of contamination is estimated both for U/Th impurities and for radon decay products but is found to be higher than would be expected from the crystal fabrication process and history. Possible experimental tests of the model are suggested.