46 RADIATION BIOLOGY 



The scattering of radiation may also produce a localized effect on a molecule 

 by modifying its internal motion. The "Raman effect" is a well-known phe- 

 nomenon of this type in which the scattering of light causes a change in the vibra- 

 tion or rotation of the atoms of a molecule with respect to one another. 



On the whole, the localized effects of radiation scattering other than 

 the Compton effect do not seem important for the action of radiation on 

 biological materials. Therefore attention will be confined here to the 

 absorption of radiation. 



Atoms and molecules behave like antennas whose internal electric 

 charges are tuned to oscillate with any one of the characteristic fre- 

 quencies Vc pertaining to the changeover from the initial to another sta- 

 tionary state (see Sect. 2-la). The internal charges "resonate" when 

 driven by a radiation whose frequency coincides with one of the values 

 Vc. The intensity of the induced currents is then limited by the dissipa- 

 tion of energy recjuired to raise atoms from one to another stationary 

 state. Within a population of atoms subjected to this action, individual 

 atoms suddenly appear to have actually absorbed one radiation photon 

 and changed over to a new stationary state. (A photon is emitted if the 

 changeover releases energy instead of absorbing it.) 



After an atom or molecule has been excited it can return to the 

 fundamental state (see Sect. 2-lc) by releasing energy in one or more 

 steps through two main channels: 



(a) "Fluorescence," i.e., emission of radiation of one of the character- 

 istic frequencies of the atom equal to or smaller than the frequency of the 

 absorbed radiation. 



(b) Internal rearrangements in which the excitation energy is trans- 

 ferred from one to other particles and is split up in smaller fractions. 



The fundamental state eventually attained need not be identical with 

 the state preceding the absorption of radiation, because the excitation 

 may have induced a lasting rearrangement of atoms ("photochemical 

 reaction"). 



The atomic electrons can also resonate under the influence of any 

 radiation whose photons have sufficient energy to remove an electron 

 from the atom or molecule. As indicated in Sect. 2-la, oscillating cur- 

 rents accompany transitions from stationary to ionized states. The 

 transition to an ionized state under the influence of electromagnetic 

 radiation takes place simultaneously with the absorption of a photon and 

 constitutes a photoelectric effect (see Sect. l-3b). 



An atom or molecule remains ionized until it acquires a new electron. 

 The eventual capture of an electron releases energy which had been 

 originally provided by the photon absorbed at the time of the ionization. 

 (The remaining portion of the energy of that photon is carried away as 

 kinetic energy of the ejected electron.) 



