Answer to Question #10434 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:

What is the proper useful range to describe the lowest scale of the RO-2 or RO-2A?

In theory, if the ionization chamber always behaved ideally, any reading above zero on either the RO-2 or 2A instrument could be interpreted as real, although likely uncertain because of normal statistical fluctuations and limited reading accuracy. Unfortunately, it is difficult to ensure that ideal behavior applies during routine use.

This is especially true because of the very low currents induced by low intensity photon fields and the ability to ferret these out of nonradiation-induced currents. The specifications for the RO-2 instrument designed by Eberline, Inc. (now under Thermo Fisher Scientific, Inc.) cite an expected current of 9.65 x 10-14 A for a gamma exposure rate of 5 mR h-1. A nominal external radiation exposure background of about 0.01 mR h-1 would yield an expected current of about 1.9 x 10-16 A, a quantity that could not be measured with either of the cited instruments. A reading of 0.2 mR h-1 would be associated with a current of about 3.9 x 10-15 A, a small current but measurable, with expected statistical fluctuations, in an ideal normal background situation.

The common type problems that make such low-current measurements unreliable include such things as changes in temperature that affect the “zero” on the instrument and changes in humidity that can produce excessive leakage current in the chamber, yielding falsely high readings. The ion chambers you cite use a replaceable solid dessicant to control humidity entering the open-to-atmosphere chambers, but it is easy to neglect to change the dessicant when it is timely, and the likelihood of excess charge leakage becomes more significant. Also, any contaminants that might get on the insulators in the chamber can increase leakage currents. Given these kinds of influences, I would prefer not to accept any readings below 0.5 mR h-1 on either the RO-2 or 2A instruments as legitimate, and I would not hold strong confidence in any reading less than 1 mR h-1.

George Chabot, PhD

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