Estimation of low and high-energy photon spectra from a Phoswich spectrum of an in vivo actinide monitor using digital pulse shape discrimination technique

Abstract A Phoswich detector with an analog pulse shape discrimination (PSD) system is used for the in vivo monitoring of actinides in radiation workers. This detector has a diameter of 200 mm, with a 3 mm thick NaI(Tl) primary detector, followed by a 51 mm thick CsI(Tl) secondary detector optically coupled to common photomultiplier tubes. The system works in an anti-coincidence mode to record low-energy photon pulses from the NaI(Tl) detector and reject the high-energy pulses from the CsI(Tl). In this study, a digitizer was used to record the waveform data of the pulses generated by the Phoswich detector. The gate widths for the PSD were optimized using the figure of merit analysis. A charge comparison PSD technique was applied for the segregation of pulses based on their temporal profiles, to generate NaI(Tl) and CsI(Tl) pulse height spectra. This enables simultaneous quantification of the low and the high-energy photon emitters. The accuracy in the measurement of low-energy photon emitters will also improve with the additional information on high-energy photon emitters. The detector efficiencies and resolutions for Phoswich and NaI(Tl) detectors were comparable to corresponding analog values for the point sources and lung sources in the phantom. Thus, the method practically enhances the monitoring capability to estimate both the actinides and high-energy photon emitters with a compact and simplified setup.