An Underappreciated Radiation Hazard from High Voltage Electrodes in Vacuum.

The use of high voltage (HV) electrodes in vacuum is commonplace in physics laboratories. In such systems, it has long been known that electron emission from an HV cathode can lead to bremsstrahlung x rays; indeed, this is the basic principle behind the operation of standard x-ray sources. However, in laboratory setups where x-ray production is not the goal and no electron source is deliberately introduced, field-emitted electrons accelerated by HV can produce x rays as an unintended hazardous byproduct. Both the level of hazard and the safe operating regimes for HV vacuum electrode systems are not widely appreciated, at least in university laboratories. A reinforced awareness of the radiation hazards associated with vacuum HV setups would be beneficial. The authors present a case study of a HV vacuum electrode device operated in a university atomic physics laboratory. They describe the characterization of the observed x-ray radiation, its relation to the observed leakage current in the device, the steps taken to contain and mitigate the radiation hazard, and suggested safety guidelines.

Portal monitor characterization for internally and externally deposited radionuclides.

In this evaluation, both the internal and external radionuclide detection efficiencies for a portal monitor were evaluated as a function of photon energy using an anthropomorphic phantom. Pass-through and static measurements were completed using 241Am, 57Co, 133Ba, 137Cs, 60Co, 109Cd, and 54Mn in various locations both external and internal to the phantom. Other parameters, such as single detector uniformity, total detector uniformity, background linearity, and activity linearity have been analyzed. It was found that the minimum detectable activity for internally deposited 137Cs in the abdomen was approximately ten times higher for pass-through versus static measurements. Additionally, it was found that the minimum detectable activity for 137Cs in the abdominal region for both internal and external pass-through scenarios are nearly equivalent. In general, if the expected radionuclide source term is primarily non-transuranic, the pass-through mode offers sufficient sensitivity to identify potential overexposures while providing much greater personnel throughput. However, minimum detectable committed effective doses for transuranics such as 241Am, show potential for personnel over exposure if the radionuclide mixture contains a significant fraction of transuranics. It is therefore recommended that nuclear facilities evaluate their radionuclide source term in order to bound potential personnel doses.