Answer to Question #7770 Submitted to "Ask the Experts"

Category: Radiation Basics — Beta Particles (negative and positive) and Electrons

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


While checking a 90Sr beta cell with a radiation counter, I found a high count around the source door. How can I know if the count is beta radiation or x rays generated by bremsstrahlung production? Also, should I be more concerned with secondary x rays generated since the 90Sr is a sealed source confined to the interior of the cell?


If you have made measurements on this cell on previous occasions and have not observed elevated readings, and source and cell configurations have not changed, then I might suspect that the source is leaking. If you have not made such measurements previously, then I would judge that there is a significant probability that the elevated readings are associated with the beta-induced bremsstrahlung radiation, which is inevitably present and measurable to varying extents, depending on the type and thickness of material present between the source and measurement location.

You state in your question that the 90Sr in the cell is in the form of a sealed source. In most instances license requirements impose leak-testing criteria associated with the use of sealed sources. If such testing has not been done recently, taking wipes close to the closed door and measuring them in an appropriate beta-sensitive counter would certainly be desirable to rule out the possibility that the source is leaking and material has been finding its way out of the cell.

If contamination outside the cell is not a problem, you might make other measurements to assess the nature of the radiation. The beta radiation from the 90Y daughter of 90Sr is high energy and produces a fairly energetic distribution of bremsstrahlung radiation that is quite penetrating. The beta radiation can be effectively stopped by 1 cm of Lucite, and if you take a measurement (#1) at a location 1 cm or slightly more from the place where you have already made measurements, and then make a second measurement at the same location when the detector is shielded by an added 1 cm of Lucite, the reading should drop dramatically, perhaps being reduced by more than 95 percent compared to the unshielded reading if the original reading was due to direct beta radiation. If reading #1 was due to bremmstrahlung radiation, the presence of the 1 cm Lucite shield will have a much lesser effect, expectedly reducing the reading by about 10 percent. It is also possible that the original reading was due to both beta radiation and bremmstrahlung, in which case quantitative results as to contributions of each radiation type might be indeterminate by this method. More sophisticated measurements involving other detectors may be useful in such instances, although may not be warranted. I would think that the "door" of the source holder or "cell" would be sufficiently thick to stop all the direct beta radiation so that one would not expect to see such outside the holder as long as the source was not leaking.

A decision as to how concerned you should be about bremsstrahlung radiation and whether additional action is required depends on the levels of radiation to which workers may be exposed and ultimate doses that might ensue. Radiation streaming through a narrow gap that produces elevated dose rates may not be as great a concern as a broader beam of radiation penetrating a shield with the potential for producing exposure to large portions of the body. Making specific recommendations requires more detailed knowledge of the magnitude of the readings and the potential for dose to workers, either whole-body or localized doses.

Good luck in your evaluations.

George Chabot, PhD, CHP

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