Caesium 137 in Finnish Grass and Cow’s Milk
3ANEN
In the previous issue of these Annales (1) an investigation on contamination of Finnish grass and cow’s milk by radionuclides is described. Collection
of representative grass and milk samples from 9 experimental farms between latitudes 60°—69°N and longitudes 23°—27°E and the results of
Sr determinations made from them are described in the above-mentioned
publication.
We havedetermined the "Cs contents of the same samples by y-spectrometry. Determinations were made from milk and grass ash prepared below
450°C as described by Paakkola et al. (1), the spectrometric method
being similar to procedures described in several earlier papers, e.g. by Low
and Edvarson?) and by Lillegraven’).
The detector used in this work was a sodium iodide (thallium activated)
crystal 144 in diam. x 1 in. with a phototube 6292, manufactured by Du
Mont. Ash samples were placed in an aluminium container (wall thickness
1.0 mm) round the crystal to make the geometric efficiency as high as
possible. The volume of the container used was about 100 cm® and the
weight of the ash samples varied between 7 and 15 g. The shielding system,
made from the barrel of an old canon, has been described by A. Vuorinen’.
Background was checked occasionally during the measuring period and in
the energy range 50 keV—1.8 MeV it was about 45 cpm. Pulse amplitude
spectra were analysed with a Hutchinson-Scarrot type 70—100 channel
pulse height analyser manufactured by Sunvic Control in Britain.
Before and after every count of a sample the energy calibration was
made with a #8’Cs + Co preparation. After a counting time of 5—6 hours
the drift of photopeaks of Cs and Co was not more than + 1 channel.
Activity calculation of the milk samples was done in the usual way.
After background subtraction a correction was made for the potassium in
the sample. Calibration was done with a standard sample of 8’Cs and
with KCl. The results are given in Table 1. The accuracyis estimated to
be better than + 20%.
The calculation of the ®’Cs activity of the grass samplesis less easy than
in the case of the milk samples. Figure 1 illustrates a differential pulse