. Biophysical research methods. Biophysics -- Research. XIV. X RAYS AND X IRRADIATION 459 varies approximately as the fourth power of this mnnher. Since the atomic mimber of lead is high, a comparatively thin lead screen may give effective protection against X rays {55). In tissue, the absorp- tion can be represented b}^ a weighted averaging of the atomic num- bers of the atoms composing it. Another complicating factor in this absorption correction is that of the wavelengths of the X ra3^s. It is not practical to obtain high intensity monochromatic radiation with X rays for irradiating the lar
. Biophysical research methods. Biophysics -- Research. XIV. X RAYS AND X IRRADIATION 459 varies approximately as the fourth power of this mnnher. Since the atomic mimber of lead is high, a comparatively thin lead screen may give effective protection against X rays {55). In tissue, the absorp- tion can be represented b}^ a weighted averaging of the atomic num- bers of the atoms composing it. Another complicating factor in this absorption correction is that of the wavelengths of the X ra3^s. It is not practical to obtain high intensity monochromatic radiation with X rays for irradiating the large areas encountered in most biological work. X-ray tubes give out a continuous band of general radiation having wavelengths rang- ing from 10 A. to some limiting value dependent upon the voltage put Characteristic A'-series rodiation Fig. 4. Variation of intensity of general X radiation with wavelength. Characteristic radi- ation is superimposed. Dashed Une shows intensity distribution possible by using proper metal filter. a z o o LlI (/I \ in z UJ UJ o tr Absorption edge of proper metal filter Exciting voltage limit to wave engtti Ctiaracteristic L- series radiation. General radiation .1 WAVELENGTH, angstrom units on the tube. A graph showing the absolute intensity as a function of wavelength for general radiation is shown in Figin-e 4. General radiation shows a continuous spectrum of wavelengths from a low intensity value in the long wavelengths, rising to a modal value, and then dropping more sharply to zero intensity at the short wavelength cut off. This limiting wavelength in A. is given by 12,395/F, where V is the voltage. With 20 kv. on the tube, the wavelength limit would be about A.; with 200 kv., A. Besides general radiation there is superimposed radiation charac- teristic of the metal used as target m the X-ray tube. Characteristic. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabili
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