Answer to Question #11577 Submitted to "Ask the Experts"

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?

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, CHP