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19 July 2008

Answer to Question #457 Submitted to "Ask the Experts"

Category: Radiation Basics — Neutrons

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

We have a cute little thermal neutron check source which consists of a 200 mCi AmBe neutron sealed source in a one-gallon paint can filled with paraffin. At what distance is it safe to be around this thing when it is on the shelf not being used? What would the approximate dose be when someone is actually using it to check a detector (probably a couple or three feet away)? Thanks.

While the one-gallon paint can provides only a modest degree of shielding, the source strength is relatively low (as portable neutron sources go), and the source can easily be handled safely with simple precautions. For the 0.2 Ci source cited and assuming about three inches of paraffin around the source, I would estimate the neutron dose rate to be about 25 to 30 mrem/hr at the surface of the can. This would likely drop to about 2 mrem/hr at about one foot from the source and around 0.1 mrem/hr at two feet. If the source is taken out of the paraffin can the comparable dose rates would expectedly be 2.5 to 3 times greater. Individuals working directly with the shielded source or frequenting the area where the source is stored should not receive any significant dose as long as they maintain a reasonable distance (a couple of feet or more) from the source and exercise sensible control of exposure times. George E. Chabot, Jr., CHP, Ph.D.

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