Answer to Question #11577 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

Can measurements from a flow-through ion chamber used for detecting tritium in air be affected by the presence of helium-3 (3He), tritium's decay product? Both installed and portable ion chambers are used in this application, where old, off-gassing, tritium hydride storage beds are sampled. The beds also contain significant concentrations of 3He. Can the 3He contribute to errors in the tritium measurement?

A

I believe the answer to your question depends on what you mean by "significant concentrations of 3He." If "significant" implies volume percentage values on the order of 10% or more then, yes, the presence of 3He may produce a change in the response (current per unit tritium concentration). This is related to the fact that the amount of energy deposition required, on average, to produce an ion pair (the W-value) in helium is greater than that for air—about 43 electronvolts per ion pair (eV/ip) for helium and 34 eV/ip for air. The increased W-value leads to a reduction in the ion chamber current compared to what one might get for pure air. A 10% volume concentration of 3He in air would yield an expected current about 2% lower than what one would expect for pure air; i.e., (0.10)(34/43) + 0.9 = 0.979.

I do not know the specific characteristics of the storage beds that you mention, but it seems unlikely that the amount of 3He being generated would be sufficient to produce a measurable effect on the ion chamber response. As a crude example, if we assumed an initial stored activity of 3.7 × 1013 becquerels (Bq), the 3He produced by a year of decay would represent about 1.13 × 1021 atoms, 1.88 × 10-3 gram-atomic-weights, or an approximate volume at normal temperature and pressure (NTP) of 46 centimeters3 (cm3) of 3He, based on an assumption of 2.45 × 104 cm3 per gram-atomic-weight. Presumably this would be mixed with a much larger volume of air, and sampling of the atmosphere would yield a negligible concentration of 3He in the volume that would be directed to the flow-through ion chamber.

If the tritium hydride was in a sealed space so that the helium could not escape and the tritium decayed completely (say, over more than 60 years), the 1.13 × 1013 Bq would yield 1.95 × 1022 atoms, equivalent to about 790 cm3 at NTP. Unless the associated air volume in the storage location was very small, less than about 10 liters (L), I would not expect the presence of 3He to affect the ion chamber's performance. Of course, the activity numbers are all fictitious, based on my assumptions, and used only as an example. You are in a better position to judge the applicability of my assumptions and what you might expect for the actual conditions at your facility.

George Chabot, PhD

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