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Rapid Neutron Activation Analysis of Fissile Material Traces at the High Flux Isotope Reactor

Neutron Activation Analysis (NAA) has a long history in forensic analysis of trace elements in materials [1] and exhibits many advantages for evidentiary forensics. Sample materials are introduced to the intense neutron field at the High Flux Isotope Reactor where elements within them become radioactive, giving up measurable radiation that is often unique to the elements themselves. The High Flux Isotope Reactor offers highly thermalized neutron irradiation of samples in two pneumatic transfer systems. Thermal neutron flux values for the PT-1 and PT-2 systems are 4.3E+14 n cm-2 s-1 and 4.0E+13 n cm-2 s1, respectively. The PT-2 facility connects to a sensitive neutron counter array (Figure 1) which is used to quantify neutrons given up by fission products created during the irradiation of traces of uranium. The measurement requires only a few minutes and yields a 235U detection limit of about 20 picograms. Because NAA may be performed without sample dissolution or separation chemistries in many cases, the analysis of refractory solids is facilitated. NAA has been demonstrated by the National Institutes of Standards and Technology (NIST) to be capable of accuracy and precision commensurate with the demands of primary ratio methods such as Isotope Dilution Mass Spectrometry (IDMS) [2]. Furthermore, all of the sources of uncertainty are known and may be quantified [3]. The process of Neutron Activation Analysis may be modeled using the SCALE modeling suite [4] and detector response can be modeled using the Monte Carlo transport code MCNP [5], thereby facilitating virtual experiments in support of real ones.