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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
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.
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