[A method of continuous water content measurements on leaves using beta-radiation absorption].
A brief introduction to the beta-ray gauging technique is given with its application in determining the water content of leaves.Some aspects for the choice of the radioactive isotope, the detector and the recording system are discussed in respect to their utilization and precision. A short review of the instrumentation used until now is given. The construction of a new beta-ray gauge is described.Krypton-85 (E max=0.67 mev) with an activity of 0.35 mc served as the radiation source (active diameter=1 mm). This point-shaped and relatively energyrich beta-ray source permits a working distance between source and detector of several centimeters and allows within certain limits a choice of test surface area. A halogen-quenched Geiger-Müller-counter with a window of 2 cm diameter and 1.5-2.0 mg/cm(2) was used. In the construction of the measuring set-up special attention was paid to the reproducibility of the geometry between source, sample and detector. The impulses which reach the detector were recorded by an electronic scaler-timer-system.Count rate measurements were taken at various distances between emitter, absorber and detector. Furthermore the dependence of beta-ray absorption on the nature of the sample was examined. Comparative measurements were made with homogeneous materials (aluminium foils, plastic sheets and filter paper) and heterogeneous materials (leaves with varying anatomical structure). Large differences were found between the amounts of beta-radiation absorbed by the same weights per unit area of these materials. These differences are discussed.A calibration method is described which allows an accurate determination of the dependence of count rate on water content of the tested leaf area after the experiment. A difference in weight per unit surface area of 0.1 mg/cm(2) can be exactly determined.Leaves of the same species were examined under natural conditions in the habitat of the plants. With these continuous measurements of beta-radiation absorption great daily fluctuations in leaf water content were found according to the special weather conditions.
Botanisches Institut II der Universität Würzburg, Würzburg, Deutschland.
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Buschbom