. Biological effects of radiation; mechanism and measurement of radiation, applications in biology, photochemical reactions, effects of radiant energy on organisms and organic products. Radiation; Biology. MEASUREMENT OF X-RAYS AND RADIUM 63 water, and body tissue over a wide range of wave-lengths), and (6) for convenience and ready reproducibility. Therefore, the internationally accepted unit of X-ray quantity is determined by the ionization produced in gm. of air. The definition accepted by the United States (56) differs from the international definition only in being less ambiguous
. Biological effects of radiation; mechanism and measurement of radiation, applications in biology, photochemical reactions, effects of radiant energy on organisms and organic products. Radiation; Biology. MEASUREMENT OF X-RAYS AND RADIUM 63 water, and body tissue over a wide range of wave-lengths), and (6) for convenience and ready reproducibility. Therefore, the internationally accepted unit of X-ray quantity is determined by the ionization produced in gm. of air. The definition accepted by the United States (56) differs from the international definition only in being less ambiguous for voltages above 200 kv., and reads as follows: "The roentgen is the quantity of X-radiation which, when the secondary electrons are fully utilized and the effects of all scattered radiation avoided, produces in 1 cubic centimeter of atmospheric air at 0°C. and 76 cm. mercury pres- sure such a degree of conductivity that 1 of charge is measured under saturation ; The problem of realizing the roentgen is then resolved, essentially, into two parts: (a) segregating a known volume of air which is ionized |l|-HHli-. Fig. 12.—Parallel-plate standard ionization chamber. by the beam in question, and (6) measuring the number of ions produced in that volume. Actually neither of these quantities is obtainable directly, although the indirect methods to be described below do not introduce any uncertainties, at least up to 200 kv. Standard Free-air Ionization Chamber.—The roentgen may be realized by means of the conventional ionization chamber shown diagrammati- cally in Fig. 12—(the figure is drawn out of proportion in order to empha- size certain essential features). The chamber proper consists of a flat plate H which is maintained at a suitable potential by a battery. The ion collector C is connected to an electrometer of such type that when in operation, C is maintained at ground potential (cf. page 72). G, G are guard plates on each side of C along
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