Effect of Temperature on Counting Measurements in a Uranium Enrichment Monitor Based on a NaI(Tl) Spectrometer and Transmission Source

We have studied the stability of a NaI(T1) scintillation detector and a commercial, general-purpose, digital, multichannel analyzer (MCA) used in uranium enrichment monitoring and have evaluated their combined sensitivity to temperature. The method used to calculate enrichment is very sensitive to any error that may be introduced by the instrument when measuring the intensity of the transmission peak corrected by the dead time (DT). An error as small as 0.1% in the attenuation of the transmission-peak count rate results in about a 3% error in the enrichment calculation. The demand for such an unusually high level of accuracy challenges the capabilities of an off-the-shelf instrument. The experimental data show that a variation of +/-10degC in the room temperature can cause a 0.4% change in the count rate of the transmission peak, even though the gain stabilization system holds the transmission peak at a specified channel. We have observed that the temperature changes the MCA's pileup rejector efficiency and, thus, the DT correction. This is because the shapers in the fast and slow channels have different temperature characteristics. We have utilized a custom-designed detector preamplifier that stabilizes the preamplifier pulse shape over the working temperature range.