BERTHOLD TECHNOLOGIES Uni-Probe LB 490, User Manual

Page: 55 / 730

Uni-Probe LB 490
BERTHOLD TECHNOLOGIES GmbH & Co. KG
1 – 55
Volume 1 7 Source Replacement
1
7.4
Radiation Exposure during Source
Replacement
It is important to calculate the potential radiation exposure before
mounting or dismantling point or rod sources. An exact calculation
is possible using the equation below.
The anticipated working hours should be split 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 cal-
culated separately and added up.
A rather simplified estimation is possible when the work is prepared
well. Based on the assumptions of a mean distance of 0.5m for the
whole body radiation and the time you are working with the
unshielded source of 6 minutes (= 1/10 hour), the radiation expo-
sure can be calculated for different activities (A) as follows:
Dose D = A x 0.15 at Co-60
Dose D = A x 0.04 at Cs-137
Enter the activity in MBq and the dose is calculated in Sv. the dose
is calculated in µSv.
For multi-part rod sources, the estimated radiation exposure has to
be multiplied with the number of source parts.
IMPORTANT
Using a pocket dosimeter with direct reading, measure the accu-
rate radiation exposure during this work, even if the radiation
exposure lies below the detection limit of dosimeters.
Calculation example
A single part rod source with an activity of 400MBq (approx.
11mCi) has to be replaced. Using the above assumptions concern-
ing distance and time and the above equation, we get the following
result:
D = 400 x 0.15 = 60µSv
The radiation exposure in the vicinity of the shielding was already
calculated to be 10µSv. The total exposure including mounting and
dismantling can then be estimated as being 70µSv for a single part
source.
IMPORTANT
If the above assumptions do not apply, the calculations have to be
corrected accordingly. Actually, it can only be another working time
which has a proportional effect on the result of the calculated dose
rate.