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The absorption of beta-rays in plant leaves and in wood


, : The absorption of beta-rays in plant leaves and in wood. Vestnik Kral Ceske Spolectnosti Nauk Tr Mat Prirod (9)

The paper gives the results of the measuring of the penetrability of biological material by [beta]-rays. The penetrability was detd. by the photographic, ionometric and counting-tube methods. For the expts., 2 prepns. of radium D made by Dr. J. Petrova were available. They were dried on to the bottom of flat dishes and covered with a mica lamella, 70 [mu] thick, which absorbed the a radiation of polonium and the B radiation of Ra D, so that in the expts. only the B radiation of Ra E took effect. The photographic method is very suitable for the comparison of the absorption and penetrability with regard to the B radiation, and shows clearly how differently B radiations penetrate according to the structure of the material. The photographic method is simple. Wooden wedges were placed directly on the photographic plate and were exposed to the rays. The leaves were exposed in a copying frame. The structure was always well depicted, though the contours were, of course, not so sharp as in a roent-genogram. In each case the small penetrability of the veins of the leaves was marked, and therefore the venation of the leaf showed dark in the copies; even very fine veins can be clearly and distinctly traced and sometimes they contrast more than in contact copies made by the light of an electric bulb. In the living, tissue water has the major role in the absorption of the [beta] rays. Photographs of living and dry leaves prove this clearly. On roentgenographs made with X-rays and on B radiographs there appears occasionally a variegation of the leaves; this is evidently due in the first place to differences in the thickness of the parts of the leaf. In the photographic examination of the penetrability of wood by B rays the same wooden wedges were used which had proved suitable also for other kinds of radiation. The structure of the wood was traced very distinctly and the darker bands of the spring wood always contrasted well on the plate with the lighter bands of the summer wood. The measurements mentioned in the paper were made by Dr. J. Petrova in the Radiol. Inst. in Prague. Ionometric measurement gave the following result: In leaves of Tradescantia the thickness of the green parts of the leaves was about 250 to 610 [mu] and the values of the absorption measured oscillated between 21 and 26 cm. The white parts of the leaf were regularly a little thinner (230-460 [mu]), but the absorption coeff. computed corresponded to the values ascertained in the green parts of the leaf (19-29/cm.). In the leaves of Aucuba japonica, Abutilon thomp-soni, and Pplysmenes sp., no difference was found in thickness between the white and the green parts of the leaves, nor did any difference appear in the measurement of the absorption. Wood is far more permeable to B rays than are living leaves and its permeability is about the same or less than that of dry leaves. The differences in the penetrability of the different samples of wood are due to their structure. Examples of the absorption coeffs. of different kinds and different samples of wood are given. Measurement with the counting tube after the principle of the Geiger-Muller tube is shown in the graph fig. 2. For the leaf of the fern Adianthum capillus veneris and for the inner epidermis of the onion-skin of A Allium cepa it shows a regular increase of the penetrability with the drying of the tissue; further it shows that also the absorption of one layer of cells can be followed well, though the absolute values are of course somewhat high considering the very thin layer. It is obvious from the examples given that the measured values of the absorption coeffs. of different samples of wood oscillate far more than could be accounted for by errors in observation; hence, different samples of wood differ materially from each other, and the main value of the described method lies in the fact that detn. of the absorption coeff. makes possible the numerical characterization of these differences. The expts. with the counting tube show that we can readily follow the loss of water from a piece of tissue too small to be weighed exactly. The penetrability of plant tissue by y rays is about 500 times greater than that by [beta] rays. The absorption coeffs. of different kinds of wood for B radiation Ra E oscillates between [mu] = 8 [long dash] 22 cm.-1; the values u/v, where v is the wt. of volume of the wood, correspond roughly to the values n/a given by Fournier and Guil-lot for light elements.

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