Annual Report 2009/2010 - JUWEL - Forschungszentrum Jülich

In the drum, 4 ‘hot spots’ for Cs-137 and 1 ‘hot spot’ for Co-60 were identified. Two Cs-137-
‘hot spots’ are positioned at a drum height of about 76 cm with the same radial position (13
cm) but different angular positions (210° and 300°). The two other Cs-137-‘hot spots’ and
the Co-60-‘hot spot’ are positioned at a drum height of about 60 cm with different radial and
angular positions: (7 cm, 210°) and (11cm, 120°) for Cs-137 and (13 cm, 300°) for Co-60. It
may be noted that the maximum of the count rate for a ‘hot spot’ occurred at a drum rotation
angle - , which corresponds to the closest distance between the ‘hot spot’ and the
detector during drum rotation. A second maximum may be observed at -
depending on the ‘hot spot’ activity and position, the -ray energy and the attenuation
properties of the matrix. The radioactive sources with an activity lower than 1 MBq are not
detected because of the lead shielding structures. The mean thickness of the lead shielding
in the drum segments containing the ‘hot spots’ was found to be 4 cm. The resulting activities
of Cs-137 and Co-60 in the segments of the drum are shown in Fig. 92. They reflect well the
axial distribution of the isotopes within the drums.
Drum Segment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
10 11 Drum Height [cm]
10 10
Cs-137
Co-60
10 9
10 8
Activity [Bq]
10 7
10 6
10 5
10 4
10 3
10 2
4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 70 74 78
Fig. 92: Activity of Cs-137 and Co-60 in the segments of the drum with an apparent waste
density of 2.63 g/cm 3 . The activity is calculated for a non uniform distribution of the
isotopes and a heterogeneous sample matrix (see Fig. 3).
The results obtained with the improved method compared to the true activity and to the
activity calculated by the conventional method which assumes a homogeneous distribution of
activity and matrix inside the drum segments are summarized for Cs-137 in Tab. 23. The
mean ratio of calculated to true activity is shown for the different methods applied in Fig. 93.
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