An Alternative Approach to Estimating Instrument Decision Thresholds for Clearance of Personal Property From...

Personal property exposed to particle beams and stray radiation at high-energy particle accelerators may contain induced volumetric radioactivity. At both the Brookhaven National Laboratory's (BNL) Collider-Accelerator and National Synchrotron Light Source II Facilities this radioactivity contains both gamma-emitting and beta-emitting radionuclides. The US Department of Energy (US DOE) recently published Technical Standard 6004-2016. This standard provides radionuclide-specific volumetric screening levels (Bq g) below which accelerator materials are eligible for clearance and release from radiological control. The standard also establishes several approaches for decision-making relative to the clearance process implemented, one of which is the "Indistinguishable from Background" (IFB) approach. BNL implements the IFB approach for survey of potentially activated accelerator materials. Radiological control technicians perform on-contact measurements using portable scintillators that are sensitive to gamma and x-ray radiation. Instrument decision thresholds are usually estimated by measuring total background counts over a pre-determined counting interval using an integrating count-rate instrument and then applying an appropriate confidence level factor. Measurement results obtained in a low background area are then directly compared to these detection thresholds. This paper presents an alternative statistical approach using logistic regression for estimating instrument decision thresholds for small-mass items using the IFB method and compares them to established release criteria. On-contact Micro-R meter measurements are correlated with analytical data obtained for activated materials weighing 0.3 kg to 3 kg. Analytical sample results show that Co and Na accounted for more than 90% of total sample radioactivity. Co and Na emit high-energy gamma rays and are both group one radionuclides as defined in DOE-6004-2016. For this size material the results show that the probability of detecting residual volumetric radioactivity at the Group One screening level concentration of 0.11 Bq g under normal field conditions is about 68%. This increases to 95% at 0.16 Bq g. The 95% confidence interval is 0.09 Bq g to 0.23 Bq g. Grouping low-mass items during the survey process could mitigate this concern if all items are expected to have similar activity concentrations.