PRINCIPLES OF RADIOLOGICAL PHYSICS 49 



motion and smear out the characteristic absorption frequencies. Mole- 

 cules and atoms of the same element have identical X-ray absorption 

 because this absorption is due to internal electrons which are unaffected 

 by chemical bonds. 



Each kind of atomic nucleus possesses a characteristic absorption 

 spectrum. This spectrum consists in general, hke the spectrum of an 

 atom, of a succession of "Hnes" pertaining to the excitation of various 

 stationary states. The lines correspond to the absorption of definite 

 photon energies which range from about 100 kev upward. The spectrum 

 of light nuclei consists of a number of lines followed by a broad band 

 which corresponds to the break-up or disintegration of the nucleus. (The 

 nuclei of mass numbers up to 4 and of mass number 9 have no excited 

 stationary state and correspondingly no absorption lines.) The spectrum 

 of heavier nuclei shows an extremely large number of lines which get 

 closer and closer at large energies and finally blend into a broad absorp- 

 tion band at an energy of the order of 5 to 10 Mev. Maximum absorp- 

 tion seems to obtain at an energy of the order of 20 Mev. Detailed data 

 on the absorption spectra of nuclei are still extremely scarce. 



Table 1-3 summarizes the elementary processes which occur in atoms 

 or molecules under the influence of electromagnetic radiation of various 

 frequencies. 



2-4. ACTION OF CHARGED PARTICLES ON ATOMS AND MOLECULES 



When a charged corpuscular radiation traverses matter, the radiation 

 particles exert an electric disturbance over all the atoms and molecules 

 which surround their paths. Most of the atoms thus disturbed exhibit 

 no aftereffect due to the passage of radiation. Observable aftereffects, 

 for example, excitations or ionizations, appear here and there, at random. 



The atoms which experience an excitation or an ionization need not 

 have been traversed directly or approached very closely by the path of a 

 particle. A close approach by a particle causes a particularly strong dis- 

 turbance and therefore a particularly high probability of excitation or 

 ionization. Nevertheless, an intrinsic randomness governs the actual 

 occurrence of definite processes in one or another atom, as explained in 

 the first part of Sect. 2. 



The interaction of an incident charged particle with an atom or group 

 of atoms can yield three types of effects, namely, elastic collisions, radia- 

 tive collisions, and inelastic collisions. 



Elastic colhsions are those in which the atom or molecule recoils 

 bodily and the incident particle is deflected, but no energy is transferred 

 to the internal motions within the scatterer. These colhsions result 

 mostly from the electric force between the incident particle and the 

 atomic nuclei. They are particularly apt to cause a deflection of the 

 incident particle, but unlikely to absorb most of its energy, owing to the 



