Answer to Question #12319 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

At what distance should GM surveys of sealed sources in storage be performed?

A

The answer depends, at least in part, on the conditions that prevail in the area of the stored source(s). For example, if the source(s) are being stored temporarily while awaiting transport per Department of Transportation (DOT) regulations, measurements may be made at 30 cm from the source surface to identify whether a high radiation area exists. Measurements at one meter from the shield surface will also be required to show that the dose rate does not exceed 0.1 mSv per hour (see 10CFR20.1601(e)(2)). The requirements for monitoring radiation levels from shielded sources held in long-term storage (or otherwise being used) are intended to provide information used to implement measures to control potential doses to exposed personnel. Such control measures may range from a simple posting to alert personnel entering the area of possible radiation levels to sophisticated mechanisms that reduce dose rates automatically when a particular area is entered.

For example, according to 10CFR Part 20.1601(a)(1), the regulation that specifies means for controlling access to high radiation areas, the radiation intensity is specified at a distance of 30 cm from the source or from the surface of any material through which the radiation penetrates, which would be the shield surface in your case. Further, the deep-dose equivalent is limited to 1 mSv in one hour at that distance. While the dose rate at 30 cm from a shield surface may exceed 1 mSv in one hour that would require posting and special control measures to ensure that no one is exposed at this level. When such a source is stored in a room, perhaps along with other sources, the entire room may not be required to be posted as a high radiation area. In that case it would be necessary to identify and post a boundary within the room that identified where the high radiation level did apply.

Similarly, when measurements are made to identify the boundaries of a radiation area (external radiation levels exceeding 0.05mSv in one hour), the criteria are that the measurements be taken at 30 cm from a given source or at 30 cm from any surface penetrated by the source radiation. From a regulatory perspective, this 30 cm distance appears to be the appropriate distance at which measurements should be made for purposes of identifying the potential for exposure and for specifying and posting areas as appropriate.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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