Replacing NaI with CdZnTe in Radioxenon Detectors for Nuclear Explosion Monitoring

Radioxenon collection and monitoring provide valuable information about nuclear weapon testing and nuclear power accidents via 80 radionuclide detection stations around the world. Radioactive xenon is produced via nuclear reactions in nuclear reactors or nuclear weapons. However, the ratios of xenon isotopes vary based on the production mechanism, so by accurately quantifying these ratios, the source of the radioxenon can be ascertained. Current detection systems use sodium iodide (NaI) scintillators to detect photons in coincidence with a plastic scintillator to detect beta particles in order to reduce background. However, NaI detectors have relatively poor energy resolution which increases the likelihood of recording a false positive. Furthermore, the photomultiplier tubes used to read out NaI detectors are known to drift as a function of temperature, reducing the accuracy of the energy measurements, especially when the detectors are deployed in challenging environments. Semiconductor detectors inherently offer better energy resolution. CdZnTe, an alternative room- temperature semiconductor detector, has the same effective atomic number as NaI, and therefore the same efficiency per unit volume. The gamma-ray energy resolution of CdZnTe is seven times better than NaI at 662 keV which will reduce the minimum detectable concentration of radioxenon in a sample, improving international authorities’ ability to sense nuclear weapon detonations. In the first phase of this effort we will develop a drop-in direct replacement of NaI gamma-ray detectors with CdZnTe in existing radioxenon sensors. This will improve the sensitivity of the instruments to radioxenon, increasing authorities’ ability to detect smaller nuclear weapons tests. We will also examine alternate detector geometries for further improvements.