Answer to Question #11941 Submitted to "Ask the Experts"

Category: Policy, Guidelines, and Regulations — Regulations and Guidelines

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

Q

I'm hoping to get an expert's opinion or interpretation of applying American National Standards Institute (ANSI)/Health Physics Society (HPS) standard N13.12 to gross alpha/gross beta surveys for release.

The way I read it is that using, say, thorium-232 (232Th) and progeny as a Group 1 radionuclide, one would take the 600 disintegrations per minute (dpm) per 100 centimeters2 (cm2) limit and break it out into alpha and beta components. Given 10 decays (4 beta and 6 alpha), one would have a gross alpha release limit of 360 dpm alpha per 100 cm2 and 240 dpm beta per 100 cm2 (or 180 dpm beta per 100 cm2, assuming the energy of the radium-228 [228Ra] beta is too low to measure).

I've heard differing thoughts on ANSI/HPS N13.12, including that the 600 dpm per 100 cm2 applies to each daughter individually. I would be interested in hearing an expert's opinion.

A

The following answer is based on the current version of the standard, ANSI/HPS N13.12-2013 (approved in 2013). Footnote d in Table 1 of the standard states that "the screening levels represent the total activity (i.e., the activity of the parent plus the activity of all progeny) present." I presume your question refers to 232Th plus all progeny in equilibrium. In that case, you are correct that the screening level (in the example, 600 dpm per 100 cm2) applies effectively to the 10 radionuclides in the decay chain. And, as you indicated, there are effectively six alpha emitters and four beta emitters; I say "effectively" because there are more radionuclides, but polonium-212 (212Po) and thallium-208 (208Tl) would be present at lower relative concentrations due to the branching at bismuth-212 (212Bi). Thus, it is appropriate to consider two limits, one for gross alpha and one for gross beta, based on their relative contributions to the total activity screening level.

So, for the example given, it would be appropriate to use a gross alpha limit of (600 dpm per 100 cm2) × 0.6 = 360 dpm per 100 cm2, and similarly a gross beta limit of (600 dpm per 100 cm2) × 0.4 = 240 dpm per 100 cm2. You also proposed making the beta limit 180 dpm per 100 cm2 to account for the instrument's inability to detect the low-energy betas of 228Ra. This could be done, but it should be clearly described that this is not truly a gross beta limit, as some betas are not being measured or reported. Alternatively, in making surveys to meet these limits, the instrument and surface efficiencies for each of the radionuclides should be considered (which in some cases, like for the 228Ra betas, may be an efficiency of zero) in determining an overall efficiency. However, since the use of the ANSI/HPS N13.12 standard has not been endorsed by all regulators in the United States (e.g., the Nuclear Regulatory Commission staff has not generally endorsed it at this time), I recommend that you consult with the appropriate regulatory authority to obtain approval for your specific case.

Margaret Cervera

Reference

Health Physics Society. Surface and volume radioactivity standards for clearance. McLean, VA: Health Physics Society; ANSI/HPS N13.12-2013; 2013.

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