Silicon Drift Detectors for High Resolution Radioxenon Measurements

The detection of radioactive xenon is an important component of both national and international efforts to detect nuclear explosions. Existing radioxenon detection systems have high sensitivity, but a better discrimination of Xe isotopes is desirable to better distinguish the radioxenon background created by nuclear power plants and medical isotope facilities from a nuclear explosion. Existing systems are also very complex, requiring cryocoolers to reach the operating temperatures for germanium detectors or multiple detectors for beta/gamma coincidence rejection. In the proposed effort, silicon detectors will be explored as a possible alternative to existing radioxenon detector systems. Silicon detectors are sensitive to beta radiation; have very high resolution for X-ray energies that the existing systems can not clearly distinguish; and as compact, solid state devices can be made into simpler systems. On the other hand, their detection efficiency is lower, so that the Phase I goal was a careful study of the improvements and drawbacks compared to the existing detector system. Prototypes of silicon detectors will be developed and built in Phase II. In Phase I, radiation interactions in Si were simulated and test measurements were performed with reference sources to obtain resolutions and efficiencies at key energies as well as detector backgrounds. These results were used to compute the minimum detectable concentration for Xe isotopes (e.g. 0.13 mBq/m3 for 133Xe) and compare them to specifications for existing systems (1.0 mBq/m3). Following discussions with detector manufacturers, design concepts for Phase II prototype detectors were established. In Phase II, several prototype Si detectors will be built, operated with existing electronics, and evaluated for their performance through test measurements and quantitative spectrum analysis. This includes design and manufacture of the prototypes, firmware/software upgrades for the electronics and development of analysis routines that process the raw energy spectra to compute concentrations of Xe isotopes. Commercial applications and Other Benefits: The technology to be developed in this project, if successful, will support the efforts by US agencies and the International Monitoring System established by the Comprehensive Nuclear-Test-Ban Treaty to monitor radioxenon from nuclear explosions and background sources by giving them a more precise tool to detect and distinguish Xe isotopes. The proposed detector would be mainly used in the monitoring stations established by the Comprehensive Nuclear-Test-Ban Treaty, in related radioxenon research labs, in mobile equipment for on-site inspections, and potentially for exhaust stack monitoring of nuclear power plants and medical isotope facilities.