Estimation of iron-55 volumetric contamination via surrogates produced during Z-machine operations

Analysis of the radiation produced by Z-machine nuclear experiments at Sandia National Laboratory and the materials irradiated indicate that the majority of produced radionuclides can easily be detected. One significant exception is volumetric contamination of stainless steel by iron-55. Detecting iron-55 in Z-machine components presents a particular problem due to its low-abundance and the low-energy (5.9 keV) xray it emits. The nuclide is often below the minimum detectable activity (MDA) threshold and resolution criteria of many standard radiation detection devices. Liquid scintillation has proven useful in determining iron-55 presence in loose contamination at concentrations below that of regulatory guidelines, but determination of volumetric iron- 55 contamination remains a significant challenge. Due to this difficulty, an alternate method of detection is needed. The use of radioactive surrogates correlating to iron-55 production is proposed in order to establish an estimate of iron-55 abundance. The primary interaction pathways and interaction probabilities for all likely radionuclide production in the Z-machine were tabulated and radionuclides with production pathways matching those of iron-55 production were noted. For purposes of nuclide identification and adequate detection, abundant gamma emitters with half-lives on the order of days were selected for use as surrogates. Interaction probabilities were compared between that of iron-55 production and a chosen surrogate. Weighting factors were developed to account for the differences in the interaction probabilities over the range of the known energy spectra produced on the device. The selection process resulted in cobalt-55, cobalt-57 and chromium-51 as optimal surrogates for iron-55 detection in both deuterium and non-deuterium loaded interactions. A decay corrected correlation of the surrogates (chromium-51, cobalt-57 and cobalt-55) to iron-55 for deuterium and non-deuterium loaded Z-machine driven reactions was derived. The weighting factors presented here are estimates which are based on rough comparisons of cross-section graphs. Analysis considering factors such as energy spectrum criteria to provide refined weighting factors may be utilized in future work.