4.2.3 Caesium isotopes 
The most exact statement about the distribution of the radioactivity 
input is only possible by means of a radiochemical, nuclide-specific 
determination. As tracer isotopes for this, the Caesium isotopes 137 
and 134 are ideally suitable, and which were set free in an activity 
ratio of 2 : 1. The Gamma-spectroscopy investigations of the seawa- 
ter samples showed that the nuclide composition of the Chernobyl in- 
put - within the accuracy of measurement - did not change in seawa- 
ter by physical or chemical processes. Merely the radioactive de-cay 
effected, with time, a shifting of the isotope structure (Figs. 8 to 
13). 
For the period of time to the middle of the year, it should there- 
fore be possible to conclude from the radiochemical results of the 
Caesium determinations, those of the remaining Gamma-emitting 
nuclides. 
The differentiation between the "Chernobyl input" and the contami- 
aation already previously in the North Sea and the Baltic Sea from 
Sellafield Works and, to a lesser extent, from La Hague is possible 
rhrough the portion of the shorter lived Cs 134 (half-life 754.2 
jays). The activity portion of Cs 134 in the entire Radiocaesium 
hitherto from these sources in the North Sea was to be found in the 
order of magnitude of maximum 10 %. In Fig. 17, from the isolines 
the Cs 134 part of the "Chernobyl input” in the North Sea can be 
identified. 
4.2.4 Strontium 90 
As a further important nuclide to be monitored by radiochemical se- 
paration, Sr 90 was determined to a great extent. In the routine 
nethod applied in the laboratory, Sr 90 is detected, by means of its 
daughter product Y 90 (half-life 64.8 hrs.) in radiochemical equili- 
brium. Hereby, Yttrium is extracted from the water sample and, after 
further radiochemical working-up, is finally brought to ß-measure- 
ment as Yttrium oxide. The extraction of the water samples after the 
input from Chernobyl, nevertheless led to the fact that a series of 
short-lived nuclides (e. g. Y 91, Ce 141, Ce 144, La 140, Nd 147) 
were likewise extracted in the organic phase and thereby falsified 
the determination of Sr 90. A mathematical procedure, by which the 
decay curve could be resolved into its components, did not always 
lead to a satisfactory result of the Sr 90 activity, because the 
Sr 90 - in comparison to the disturbing isotopes - was only present 
in a slight activity share. For that reason, a chemical process must 
be applied, by which in the first instance the Strontium must be re- 
noved from the water sample as carbonate precipitation. After ad- 
justment of the radiochemical equilibrium between Sr 90 and Y 90, 
after circa 14 days the determination of Sr 90 could be completed. 
Ihis process was considerably more time and work consuming than