14 BIOPHYSICALLY ACTIVE X-RAYS 



The value of p for a rock salt crystal is 2.165, and its molecular weight 

 is 58.46. Hence the value of the " lattice constant " of NaCl is 



d = 2.814 X 1(T 8 cm 



Using this value of d in the Bragg equation when a crystal of rock 

 salt serves as a grating, one may compute the wavelengths for any 

 glancing angle 6 observed on the spectrometer table. If at the same 

 time the ionization current due to the absorption of the x-rays in the 

 ionization chamber is observed, one has a measure of the intensity of 

 the x-rays possessing that particular wavelength. The plot of these 

 values gives the curves shown in Figs. 1-4 and 1-5. Obviously this is a 

 tedious method, and for therapeutic and other practical uses such de- 

 tailed information is not necessary. 



A simplified method of obtaining practically the same information 

 will be outlined after the study of the law of x-ray absorption involved 

 in the procedure. 



Absorption 



The medical radiologist cannot neglect the phenomena associated 

 with absorption of x-radiation in both diagnostic and therapeutic 

 branches if quantitative reproducible results are to be obtained. In 

 therapeutic work where filters are constantly used to absorb the longer- 

 wavelength radiations, which, for large doses, are dangerous to the 

 patient, a knowledge of the processes involved in absorption is indis- 

 pensable. 



When a beam of x-radiation is incident upon any medium, several 

 effects may occur, all of which result in the reduction of its energy. 

 These effects are : 



1. The photoelectric effect. Loss by absorption with the resulting 

 emission of electrons. 



2. Scattering of the primary radiation and Compton scattering, 

 collectively classified as secondary radiation. Unfortunately, the re- 

 moval of energy from the incident beam is not a true additive property 

 of absorption and scattering, but to a good approximation we may write 



Energy loss = Loss due to absorption + Loss due to scattering 



If the energy loss of an x-ray beam passing through matter followed 

 a true absorption law and were dependent only on the thickness of the 

 absorbing layer, then for a given wavelength the energy would be de- 

 creased in each succeeding centimeter of absorbing layer by the same 

 fractional amount. For instance, if the incident beam were composed 

 of hard x-rays and the absorption in the first centimeter were 20 per 



