Use of Portable Gamma Spectrometers for Identifying Persons Exposed in a Nuclear Criticality Event

At the Y-12 National Security Complex, triage-style assessments are used to identify persons potentially exposed to high doses from criticality accident radiations using portable instruments by assessing the presence of activated sodium atoms in a person's blood. Historically, simple handheld Geiger-Mueller (G-M) probes were used for these purposes although it was recognized that since these instruments contain no information on incident photon energy, it was impossible to differentiate between photons emitted by contamination on the potentially exposed worker from activation of sodium in the person's blood. This work examines the use of a portable gamma spectrometer for assessing blood sodium activation. Irradiations of a representative phantom were performed using two neutron source configurations (unmoderated and polyethylene-moderated ²⁵²Cf), and measurements were made using the spectrometer and a G-M detector following irradiation. Detection limits in terms of personnel neutron dose are given for two neutron fields representing metal and solution criticality spectra. Both G-M and spectrometer results indicate a low minimum detectable neutron dose indicating that both instruments are useful as an emergency response instrument. The spectrometer has the added benefit of discriminating between surface contamination and blood sodium activation.