Answer to Question #10431 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

Which instrument has the best efficiency for counting 55Fe wipe samples, a liquid scintillation counter (LSC) or a windowless gas proportional counter (GPC)?

A

Unfortunately, the answer to your question is not absolute; the relative efficiencies depend, at least in part, on the specific characteristics of the wipe samples and other possible considerations. For example, do the wipes contain significant extraneous material—e.g., dust, dirt, grease, or chemical contaminants—and/or has the radioactivity penetrated the collection surface to any degree? These questions are important because they can markedly affect the ability of certain radiations to reach the active volume of the respective detector and the ability to produce a pulse above the detection threshold. Before elaborating on these considerations, I will note that if you had an ideal sample, which presented no significant self-absorption of radiations and produced no significant quenching of pulse sizes, I would expect the liquid scintillation counter (LSC) to yield the highest counting efficiency.

In order to judge the validity of this conclusion, we must recognize the types and energies of radiations produced by the decay of 55Fe. This radionuclide decays by electron capture with the emission of low-energy Auger electrons and characteristic x rays. The significant electrons have individual energies of about 5.2 keV and a yield of about 0.60 electrons per disintegration. The dominant x rays are 5.9 keV in energy, with a total yield of about 0.25 x rays per disintegration. There are also some lower-yield (0.032 x rays per disintegration) 6.5 keV x rays. When the wipe sample is placed in a vial containing scintillation fluid, the counting geometry is something approaching 4 pi, implying that radiations emitted in all directions are intercepted by the detector (scintillation fluid). If the windowless gas proportional counter (GPC) is a standard 2 pi system in which the wipe sits in a suitable planchette at the bottom of the detector, approximately one-half of the emitted radiations will move in the direction of the detector. Thus, considering no other factors, we would expect a reduction in counting efficiency by about a factor of two in the typical GPC compared to the LSC. The use of a less common 4 pi GPC would increase efficiency for the x rays, but the Auger electrons would still be subject to 2 pi geometry because those emitted in the direction of the wipe medium likely would be completely attenuated.

Because the energies of all the radiations from 55Fe are low, a significant number will not be detected because they fail to reach the active volume of the detector or they do not produce a pulse above the discrimination level of the respective counting system. Detection efficiencies for 55Fe in a sample with little negative influencing factors (e.g., small aqueous sample dissolved in scintillation fluid) may yield a counting efficiency for 55Fe in excess of 50 percent, but this efficiency can decrease rapidly for less ideal samples. A wipe generally falls into this latter category. Even if the wipe contains no obvious dirt or grime and even if all the collected contamination is on the surface of the wipe, the body of the wipe itself will present some attenuation of radiations that are emitted from the surface into the wipe medium, especially the low-energy Auger electrons. The more translucent the wipe is in scintillation fluid, the better will be the counting efficiency since light produced within the wipe medium will be more likely to be detectable. If the wipe is dirty and/or if the 55Fe contamination has penetrated the surface of the wipe, a large fraction of the Auger electrons may be attenuated in the wipe medium, and such an effect will often have a greater negative effect on counting in the windowless GPC than in the LSC.

The bottom line is that I would expect that the counting efficiency for counting wipes would generally be higher for the LSC than for the typical windowless GPC. This may not be the only criterion for choosing a counting system, however. If the GPC is a low-background system, as many are, the background count rate may be considerably lower than that in the counting window for the LSC. Since the background rate determines the detection limit and minimum detectable activity, this can enter into a choice of counters. Another very significant factor is the added difficulty in a windowless GPC of keeping the counting chamber from becoming contaminated. When counting wipes containing dirt and dust that might get released under counting conditions that include gas purging of the chamber and gas flow during counting, such contamination is often a concern. This may be cause for reducing use of the GPC in some instances.

George Chabot, PhD

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