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To calculate the dose rate generated by a source, you need to know the gamma radiation
constant (k), in order to take into account the different energies of the various isotopes.
The following table lists the constants for the radioactive sources most frequently used in
industrial applications:
Isotope
k
old
mrem m
mCi
*
*
2
k
new
µSv m
h Mbq
*
*
2
Co-60 1.35 0.365
Cs-137 0.35 0.095
3. Half-value Layer
Half-value layer (HVL) or half-value thickness (HVT) specifies the material thickness nec-
essary to reduce the intensity of a certain type of radiation to half its original value. The
following table lists the half-value thickness of some materials:
Half-value thickness layer in mm
Isotope
Material Co-60 Cs-137
Water 157 110
Concrete 68 47
Steel 20 14
Lead 14 7
The attenuation factor “s” can be calculated from the half-value thickness.
Example:
Activity: 9250 MBq (250 mCi) 137-Cs
Shielding thickness: d = 50 mm lead
Distance: 0.6 m
Determine the dose rate in a distance of 0.6 m using the following formula:
mrem 1.73Sv 17.3
141.2*(0.6)
1*0.095*9250
s*a
t*k*a
D
22
====
µ
A = activity in MBq
k = radiation constant
t = time in h
a = distance from radioactive source in m
s = attenuation factor
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