Measurement of radioactive xenon in the atmosphere is one of several techniques to detect nuclear weapons testing. For high
sensitivity, some existing systems use beta/gamma coincidence detection to suppress background, which is very effective, but
increases complexity due to separate beta and gamma detectors that require careful calibration and gain matching. In this
paper, we will describe the development and evaluation of a simpler detector system, named PhosWatch, consisting of a CsI(Tl)/BC-404
phoswich well detector, digital readout electronics, and pulse shape analysis algorithms implemented in a digital signal processor
on the electronics, and compare its performance to existing multi-detector systems.