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Estimating Beta Dose Rates Using Portable
Thin-window Gas Detectors

 


Robert L. Coleman 
Oak Ridge National Laboratory 

 

A common issue in radiation protection is the estimation of dermal, or skin, dose rates from beta emitters using hand-held thin-window Geiger-Mueller (GM) and gas proportional detectors. The most common of these are known as "pancake" detectors with an entrance window area of about 15 cm2, however some GM detectors are fairly large and some gas proportional varieties are much larger.

Estimates can be formulated by coupling raw detector efficiency data with published dose factors. As an example, assume that the 4-pi detection efficiency for 32P is 0.5 cps per Bq at near contact with a source. If a conversion for 32P at 8 mg/cm2 tissue depth is quoted as 2E-02 Sv/y per Bq/cm2 (Kocher et al 1987) and all activity is assumed to be evenly distributed across 5 cm2, then an estimated detector conversion factor would be 670 cpm per mrad/h (1100 cps per mSv/h).

Measurements have been made for a variety of isotopes using methods similiar to the previous example and also by more direct means using extrapolation chamber measurements (Coleman 1993). The following chart summarizes the expected count rate (counts/minute) per mGy/h as a function of average beta energy[1] for a GM-pancake detector with a typical operational configuration. The data represents an assumed full irradiation of the probe active surface area (15 cm2) with an equal amount of exposed tissue area. Sources with active areas less than the detector size would yield lower counts per unit dose. For example, if the active area of the source were only 1.5 cm2, then the dose per observed count would be an order of magnitude higher.

References

Kocher, D. C.; Eckerman, K. F. Electron Dose-rate Conversion Vactors for External Exposure of the Skin for Uniformly Deposited Activity on the Body Surface, Health Physics 53\:135; 1987.

Coleman, R. L. Beta Dose Rate Evaluations with a Geiger-Mueller Pancake Detector (pdf), University of Tennessee, Knoxville, August 1993. 

 

 

 Last Updated:  04/25/2000

 

 

 



[1]  Average beta energy represents the average expected for typical beta emission spectra (e.g., C-14, P-32, SrY-90, etc.)