Retrospective Dosimetry and Nuclear Assay
The research efforts in these laboratories utilize a large suite of instrumentation to answer and delve into many nuclear nonproliferation research questions. Generally the research focuses on answering questions related to the operations in a nuclear facility which are not shared publically. Typical questions being answered could be simplified to, “what nuclear materials were present, how much, when and for how long” are answered by the research taking place in these laboratories.
The discipline of retrospective dosimetry at NC State primarily uses thermoluminescence, optically stimulated luminescence and electron paramagnetic resonance. In particular, this is done by measuring the historical dose distributions to materials from the vicinity where nuclear materials had once existed. Taking spatial distributions of dose deposition measurements enable reconstructing the historical locations of nuclear material. Taking dose depth profiles from a core sample enable measuring the mass energy absorption coefficient so that from this profile, the type (alpha, beta, gamma) of radiation and its gross energy distribution can be measured. A single exponential decay profile interprets to a single gamma energy group or the superposition of decaying exponentials can be deconvolved into multiple energy groups from which a radiological materials isotopics can be inferred. Having measured the dose and knowing the maximum time the source could have been present then gives the minimum dose rate from the source and so likewise its minimum activity. Typical materials of interest include, bricks, tiles, cinderblock, bone, candy, plastic, sheetrock, concrete aggregate, porcelain (or put simply dry non-conducting materials).
The current focus for this research is related to air samples taken downwind of the effluent from a nuclear facility. This laboratory utilizes portable high purity germanium, phoswich, liquid scintillation and surface barrier detectors to characterize nuclear materials obtained by air samples. Air samplers are used (of the same kind as those employed by federal radiological emergency response teams) to obtain samples for the testing and development of theories for radioactive aerosol effluent and its characterization. The entire lifecycle of an air sample is currently under study to improve quality via rigorous uncertainty characterization and throughput optimization. This entails radon progeny mitigation, plume modeling integration along with concomitant accuracy and precision improvements. Probabilistic evaluation of counts, aerosol evolution effects and radon progeny dynamics typically require sophisticated numerical and statistical analysis approaches but are similarly rich with physical meaning.