Osiris - A Spectrum-Blind Gamma-Ray Spectroscopy System for On-Site Inspections under the Comprehensive Nuclear-Test-Ban Treaty 

A. J. Caffrey, Ph.D.
Idaho National Laboratory

Under the Comprehensive Nuclear-Test-Ban Treaty (CTBT), high resolution gamma spectroscopy measurements during an on-site inspection (OSI) may be restricted to only seventeen “CTBT-relevant” radioisotopes, rather than performing full spectral analysis.  The On-Site Inspection RadioIsotopic Spectroscopy— Osiris —system includes software filters that limit the display of spectral data to radioisotopic information relevant to CTBT OSIs, e.g., 131I.  To accomplish this, the Osiris software actually hides the measured spectrum from the user, in effect, energy-calibrating and analyzing the spectrum “blind”.

To evaluate Osiris performance a set of over 150 OSI-relevant spectra was produced via measurements and simulation.  The test spectral compositions include non-nuclear-explosion scenarios, e.g., a severe nuclear reactor accident, and nuclear-explosion scenarios such as a vented underground nuclear test; and at many levels of activity relative to natural background.  Using this set of gamma-ray spectra, the Osiris software correctly identified over 95% of the treaty-relevant fission-product isotopes, as compared to hand-analysis by an expert spectroscopist.  

Currently, the Osiris hardware is being evaluated in laboratory environmental chamber tests and outdoor experiments at INL to determine its reliability and energy-calibration stability under field conditions.