Section 12 Radiation Protection
113
12.3.1 Exposure when Installing the Shielding
Provided that any work in the vicinity of the shielding is prepared
well, it will not take more than 20 minutes. With an assumed dose
rate of 30 microSv/h, the calculated dose is 10 µSv.
If you compare this dose with the permissible annual doses of
5 mSv for operating staff, this work can be carried out 100 times
per year by one and the same person.
12.3.2 Exposure when Replacing Rod Sources
It is important to calculate the possible radiation exposure even
before mounting or dismantling rod sources. An exact calculation is
possible using the equation above.
The anticipated working hours should be spilt up in work in the di-
rect vicinity of the shielding during mounting and dismantling the
source holders and work with the unshielded source while fixing
and dismantling the source and the Ms extension bar.
Again, a rather simplified estimation is possible when the work is
prepared well. Based on the assumptions of a mean distance of
0.5 m for the whole body radiation and the time you are working
with the unshielded source of 6 minutes (= 1/10 hour), the radia-
tion exposure can be calculated for different activities (A) as fol-
lows:
Dose D = A * 0.15 at Co-60
For multi-part rod sources, the estimated radiation exposure has to
be multiplied with the number of source parts.
Calculation example
A single part rod source with an activity of 400 MBq (approx.
11 mCi) has to be replaced. Using the above assumptions concern-
ing distance and time and the above equation, we get the following
result:
D = 400 * 0.15 = 60 µSv
The radiation exposure in the vicinity of the shielding was already
calculated to be 10 µSv. The total exposure including mounting
and dismantling can then be estimated as being 70 µ
µµ
µSv for a sin-
gle part source.
Note: If the above assumptions do no apply, the calculations have
to be corrected accordingly. Actually, it can only be another work-
ing time which has a proportional effect on the result of the calcu-
lated dose rate.
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