| Summary: | 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.
|
|---|
