12 



BIOPHYSICALLY ACTIVE X-RAYS 



The X-Ray Spectrometer 



An ionization spectrometer of the Bragg type may be used to obtain 

 curves similar to those shown in Figs. 1-4 and 1-5. 



The x-ray beam emitted by the target of the Coolidge tube T (Fig. 1-6) 

 is collimated by the narrow slits s. It strikes the surface of a three- 

 dimensional crystal grating G, is deviated, and enters the ionization 



Fig. 1-6. A diagrammatic representation of a Bragg x-ray spectrometer using 

 an ionization chamber to measure the intensity of the x-radiations. 



chamber C. The gas in the ionization chamber is ionized by the ab- 

 sorbed x-radiation, and this ionization current, which is proportional 

 to the intensity of the deviated beam, is recorded by the electrometer E. 

 The ionization chamber and the crystal may be rotated about the center 

 of the calibrated spectrometer circle. The wavelength of the x-radiation 

 entering the ionization chamber is then obtained from the relation 



nX = 2d sin 6 



where n is the spectral order, X the wavelength, 20 the angle between 

 direct and deviated beams obtained from the spectrometer scale, and 

 d the distance between the reflecting planes of the crystal. If we exam- 

 ine the first-order spectrum for which n = 1, the wavelength may be 

 obtained if the crystal constant d is known. 



Suppose that we use a rock salt (NaCl) crystal as the three-dimen- 

 sional grating. This salt crystallizes in a cubic form, the Na and CI 

 ions occupying alternate positions at the corners of elementary cubes 

 in the cubic lattice characteristic of this crystal. The arrangements 

 of the lattice are similar to the scheme shown in Fig. 1-7, which repre- 

 sents a horizontal plane of a three-dimensional array of diffraction 

 centers with the Na and CI ions located at the bright and dark 

 points. 



