2.4 Instrument Description
12
2.4 Instrument Description
2.4.1 Rod Source
Radioactive sources for industrial applications are always „encap-
sulated radioactive substances“ which are tightly welded into a
sturdy stainless steel capsule, so that the radioactive substance
cannot leak out. Contamination is therefore ruled out. Moreover,
any activation of the product being measured by the sources used
is not possible for physical reasons.
The following rod-shaped Gamma sources can be used for these
measuring configurations:
241
Am,
60
Co,
137
Cs,
85
Kr.
The following isotopes are primarily used for the radiometric bulk
flow measurement:
a)
60
Co has a relatively high energy of 1.17 and 1.33 MeV, respec-
tively. It is used for very high loads. Its half-life period is 5.27
years.
*
b)
137
Cs is the isotope most frequently used for radiometric bulk
flow measurement. Its energy of 0.660 MeV is sufficient to irra-
diate common belt loads. Due to the lower energy, the measur-
ing effect is better than with Co-60. Also, the shielding costs for
a Cs-137 source are lower than for a Co-60 source. The half-life
period of Cs-137 is approx. 30 years.
*
The rod-shaped Cs-137 source is assembled from several
pieces.
c)
241
Am has a very low energy (60 keV) which is much easier to
shield. Therefore, a much smaller lead shielding suffices. 241Am
is rarely used due to its minor radiation energy, mainly for ma-
terials such as tobacco and washing powder or for very low belt
loads.
Caution: Due to its low energy, it has to be borne in mind that
the attenuation of the radiation is dependent not only on the
bulk weight and height, but also on the atomic number. This
means that the result can be distorted when the chemical com-
position changes. How much has to be determined on an indi-
vidual basis.
*
According to NBS, half-life is defined as: Time for the activity of
any particular radioisotope to be reduced to half its initial value.
Please note the Radiation Protection Guidelines in the
Service Instructions on page 89!
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