293 found. From artificial radionuclides only Cs was observed with no doubts. After these measurements the samples were ashed and radiochemical analyses for the presence of 238, 239, 240, 241Pu,24IAm,244AmandI54-155Eu were performed. Initial measurements of ^Sr in two samples were performed on a liquid scintillation spectrometer at Budapest. The project is still in progress and will be conducted till the middle of the year 2000, being supported by a grant No 6P04G09014 from the State Committee for Scientific Research.

Cs-137 in Tree Rings from Region (, ^

P.L. Urban1, P. MazurVandJ.W. Mietelski PL9902586 1 Polish Children's Found, Warszawa, Poland

Two sets of samples of annual of tree rings: six of common pine (Pinus silvestris L. ) and six of European birch (Betula pendula L.), along with bark samples from the same trees, were separated from the trunks. The trees were growing in the Opole region (Silesia). This is the area with relatively high Chernobyl Cs deposition (about 40 kBq/m2). Masses of samples were between 100 g and 200 g. Initial direct measurements of radiocesium with a low-background HPGe gamma spectrometer have shown that in most cases the radioactive concentrations in bulk samples were close to the detection limit. Further, the samples were ashed in 400°C during two days in the oven. The ashes (about 0.5 g of each sample) were transferred to polyethylene cups and soaked with acetone. After evaporation of acetone the ashes formed a thin layer in the bottom of cup, what improved the geometric conditions of measurements. Ashes were again measured with a low-background gamma spectrometer, typical measurement time was two days. The obtained results showed that the most of radiocesium is present in bark, and the birch's bark was far more active then the pine's one. The tree rings from 1986 (the year of the Chernobyl accident) did not show increased activity, and generally the activity is increasing from the inside of tree to the bark (see Fig. 1). This suggests the diffusion from bark as the main process, which is reflected in the radiocesium profile within the trunk. The project has been extended to the analysis of potassium absorption in timber of those species. Scholarships of the Polish Children's Fund are gratefully acknowledged by two of us (P.L. Urban and P.Mazur).

0.04-1 Emu birch fHHpine 0.03- O)

0.02-

0.01-

(TT3 0.00- • E=l _IH1 _ EH nil m 81-83 84-86 87-89 90-92 93-95 96-98 bark sample

Fig. 1: Radioactive concentration of Cs in pine and birch timber samples (annual rings and bark).