A Method to Determine the Minimum Energy Threshold and Coarse Low Energy Calibration for PVT Based Detectors Using the Lower Level Discriminator. ALISON EARNHART (Juniata College Huntingdon, PA 16652) DR. JIM BORGARDT (Pacific Northwest National Laboratory, Richland, WA, 99352)

Radiation portal monitors (RPMs) play a critical role in ongoing attempts to intercede illicit radiological sources. RPMs widely utilize polyvinyl toluene (PVT) based detectors which have relatively high durability and low cost, but intrinsically poor energy resolution. However, a pulse height analyzer does provide some spectroscopic information allowing broad energy windowing (EW) algorithms to be employed as a complement to less specific gross count measures when detecting radiation. Quantifying the low energy sensitivity and low energy calibration of these detectors presents unique obstacles since the photon interaction in PVT is dominated by Compton scattering events, masking full energy peaks in the energy range of interest, and driving an increase in the count rate in the low energy region. A method to gauge the sensitivity of PVT detectors and generate a coarse low energy (~5keV to ~80keV) calibration by varying the lower level discriminator (LLD) setting and tracking the net count rate from a source has been investigated for several commercial PVT-based RPM systems using 55Fe, 109Cd and 241Am. Parameters in a Fermi function were adjusted to fit the resulting integral pulse height data and the derivative of the generated fit function, representing the differential pulse height spectrum, was determined. The presence of a peak in the derivative of the count rate for a source signified the presence of a photopeak, and provided a means of assessing the low energy sensitivity of the detector. Additionally, observed peaks in such differential pulse height curves for multiple sources with peaks in the 5keV to 80keV range allowed a coarse low energy calibration to be established.