Determining Isotopic Concentration Using Delayed  -rays From Active Inspection Techniques For Nuclear Materials Safeguards And Maybe Other Applications

Dr. Alan W. Hunt
Idaho State University
Accelerator Center

There has been a substantial research and development effort into active inspection technologies that can nondestructively detect, identify and quantify special nuclear materials for advanced safeguards and nuclear forensics applications.  These active inspection technologies use a probing radiation source to stimulate fission reactions and then monitor for secondary emissions as a direct signature of fissile or fertile materials.  In this presentation, results of delayed ‑ray spectroscopy measurements will be demonstrated from the active inspection of samples containing 235U, 239Pu or a combination of the two.  On average, seven delayed ‑rays are emitted per fission reaction on timescales that range from hundreds of milliseconds out to years.  Since the fission fragment distribution is dependent on the fissioning isotope, the discrete delayed g‑ray lines provide a fingerprint of the interrogated material, allowing the determination of isotopic content.  Fission in the targets was induced by thermalized neutrons created by a pulsed electron linac with maximum energy of 25 MeV and a 9Be neutron converter.  A mechanically cooled HPGe detector collected delayed ‑ray spectra between irradiation periods.  Using advanced accelerator controls, the irradiation/detection periods could be varied from tens of milliseconds to hours, thereby emphasizing the shorter or longer lived fission fragments.  In particular, the goal was to find irradiation/detection time structures, which produced spectra with pronounced differences in the discrete ‑ray lines from the fission of 235U and 239Pu.  Spectra from the longer time structures (e.g. 15/15 min irradiation/detection) had a plethora of discrete delayed ‑ray lines from Rb isotopes, indicating the fission of 235U but lacked strong lines emphasizing the fission of 239Pu.  With shorter time structures (e.g. 90/90 s irradiation/detection), the presence of 239Pu was identified by discrete ‑rays from the decay of 106Tc.  Using these unique ‑rays, the isotopic content of the target containing the two different isotopes can be determined.