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Answer to Question #128 Submitted to "Ask the Experts"Category: Doses and Dose Calculations — Conversion of survey meter readings to dose The following question was answered by an expert in the appropriate field: Q
Okay, background is 200 cpm, in a room that is 30 square feet. My gamma source is 125I
and is in a container about 1 liter. Let's say the container has the
dimensions of a quart of beer and the source is approximately radiating
homogenously. If my Ludlum has an efficiency for 125I of 12 percent, how do I convert 1000 cpm, at a foot from the middle of the source, to mrem h-1?
A
Based on the information provided, it does not appear that the
instrument being proposed is designed for exposure measurements. It
sounds more like a low-energy x/gamma surface contamination monitor
(thin NaI) such as a Ludlum 44-3. If so, this type of detector is not
really designed for exposure measurements and the stated efficiency (12
percent) for 125I would be an absolute (4-Pi) efficiency
for surface activity estimation. Never the less, it is indeed possible
to calibrate the instrument for the stated use or to estimate exposure
coefficients by extrapolating from an absolute detection efficiency for
a specific radionuclide--however, the latter requires greater
calculational detail than what is easily provided on the "Ask the
Expert" Web site. A quick overview of such an answer is as follows and
can be applied to other types of detectors:
Assume a Ludlum 44-3 detector, or equivalent, with a 1-inch diameter
NaI crystal. Assume that the absolute efficiency at 35 keV (125I) is 12 percent and assume further that this efficiency was determined at near contact with a distributed 125I
calibration standard. The intrinsic detector efficiency is therefore
about 25 percent and the detector response will be about 1.2 counts per
photon/cm^2. To estimate dose rate per unit fluence, an absorption
coefficient of 35 cm^2/g is selected for water and a dose coefficient
of about 1.5E-10 rad per photon per cm^2 is calculated at 35 keV. These
two factors are ultimately combined to yield a calibration coefficient
of 1.3E-10 rad per count; or 8E-9 rad/h (water) per cpm. Considering a
background given count rate of about 200 cpm, the expected surficial
(1st centimeter) dose-equilavent rate at 1000 cpm is therefore
approximated to be 6E-6 rem/h. Note that this is the estimated dose
rate in the first centimeter and that the 1-cm "deep" dose rate would
be lower by about 20 percent.
Robert Coleman Oak Ridge National Laboratory
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