PNNL

Abstract

The performance of two large NaI(Tl) scintillation detectors has been determined as a function of detector type and as a function of temperature. One detector had dimensions of 4?4?16 in.3 with a stainless steel shell while the other detector was 2?4?16 in.3 with an aluminum shell. Absolute counting efficiencies for photopeaks and total counts were measured at 0.46 m and 2.0 m for gamma sources ranging in energy from 25 keV to 2500 keV. Photopeak resolutions were measured over the same energy range. The changes in pulse height and photopeak resolution were measured as a function of temperature over the range -50?C to +60?C. As expected from prior literature data, the scintillator light output decreases at both higher and lower temperatures compared to room temperature. However, the maximum peak height in this work occurred at 0?C whereas the literature gives the maximum light output at about 40?C. This difference is attributed to the fact that in this work, the phototubes and preamplifiers were heated and cooled along with the scintillator. Both detectors continued to function successfully over the entire temperature range studied in this work. The pulse height decreased by about 33% at -50ºC and about 25% at +60?C compared to its maximum at 0?C for both detectors. The resolution of both detectors degraded from about 8% to about 10% at -50?C for the 662-keV peak in 137Cs, but it did not change significantly at elevated temperatures.