Berthold LB 444 - Exposure When Installing the Shielding; Exposure When Replacing Point Sources

Density Meter LB 444 SECTION 9. RADIATION PROTECTION
88
The dose in a distance of 50 cm of a Co-60 source with an activity of 350 MBq
and a time of 30 minutes in this distance has to be calculated. The source is
shielded by a shielding with an shielding factor of 30:
D
Sv
=
⋅⋅⋅
⋅⋅ ⋅
MBq m h
m h MBq 30
=
350 0 35 0 5
05
2
2
..
(. )
µ
8.2 Sv
µ
µµ
µ
9.2.1 Exposure when Installing the Shielding
This calculation can be simplified, and the accuracy is still OK, by using the
given doserate value in 1 meter distance from the shielding.
Shieldings for measuring systems are usually designed such that, regardless of
the activity and the type of source (point or rod source), the limit of the controlled
area with 7.5 µSv/h is in a distance of max. 1 meter around the shielding. This
value can be taken for granted when the mean distance for mounting the
shielding or operating the lock is 0.5 m. As a result, we get a doserate of max.
30 µSv
at this point. Provided that any work in the vicinity of the shielding is pre-
pared well, it will not take more than 20 minutes (= 1/3 hour) and from this we
can calculate a dose of
10 µSv for working a shorter time in the vicinity of
the shielding
.
If you compare this dose with the permissible annual doses of 5 mSv for oper-
ating staff, this work can be carried out
500 times per year
by one and the
same person. Due to the very low radiation exposure it is not necessary to carry
a person dosimeter for this work, since the lower detection limit of these meas-
uring systems lies just above this dose.
9.2.2 Exposure when Replacing Point Sources
It is important to calculate the possible radiation exposure in advance. An exact
calculation is possible using the equation above.
The anticipated working hours should be spilt up in work in the direct 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 source
holder. The dose obtained while working in the vicinity of the shielding and the
dose obtained while working with the unshielded source have to be calculated
separately and added up.
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 ra-
diation and the time you are working with the unshielded source of 6 minutes (=
1/10 hour), the radiation exposure can be calculated for different activities (A) as
follows:
Dose D = A * 0.15 at Co-60
Dose D = A * 0.04 at Cs-137
Enter the activity in
MBq
and the dose is calculated in
µSv
.