Principles of radiological physics 33 



relative likelihood of different kinds of disintegration is discussed sepa- 

 rately in Sect. 2-ld. 



The excitation of the motion of a whole atom in a molecule or crystal 

 with respect to its neighbors is also apt to be subdivided quickly among 

 many atoms. However, an atom which belongs to a small molecule has 

 a good chance of breaking loose at once if it acquires sufficient energy. 

 The separation of an atom from its molecule is called a "dissociation." 



When the excitation of an atomic structure does not suffice to overcome 

 the bonds which hold the structure together, the excitation normally 

 can subside through the emission of electromagnetic radiation. The 

 nucleus, atom, or molecule "drops" from an excited stationary state to 

 another stationary state of lower energy. The energy difference radiates 

 away as a photon of electromagnetic radiation. The radiation arises 

 from the oscillating current of characteristic frequency which accom- 

 panies the changeover from one pattern of internal motion to another. 

 According to Eq. (12a) one photon of the emitted radiation amounts 

 to an energy hv exactly equal to the energy difference AE between 

 the initial and final states of the atom. 



The excitation may subside in one or more steps depending on whether 

 the atomic structure returns to its normal state directly or through a suc- 

 cession of intermediate stationary states. Alternately, the excitation 

 energy may be released by direct transfer to an adjacent atomic structure. 

 A direct transfer is quite possible not only in liquids and solids but also in 

 gases unless the density is so low that intermolecular colUsions become 

 exceedingly rare. Nuclei, being more sheltered from the environment 

 and less subject to external disturbances, find it difficult to transfer their 

 excitation directly to other atomic structures. 



The excitation of the external atomic electrons subsides through the 

 emission of electromagnetic radiation within a time ranging from 10~* sec 

 upward. Direct energy transfers compete effectively with the emission 

 of radiation if they take place at a faster rate. 



The larger the energy amount to be released, the higher is the frequency 

 of the oscillating current and the higher is the rate of emission of radiation 

 (see Sect. 1-3). Therefore the excitation of the motion of atoms within a 

 molecule, which involves smaller amounts of energy, requires a time 

 longer than 10~* sec to subside through the emission of radiation. On 

 the contrary, the excitation of internal electrons may lead to the emission 

 of high-energy X rays within a time as short as 10~^* sec. 



The release of nuclear excitation energy through the emission of y rays 

 requires a time which is most frequently of the order of 10~^^ sec. Occa- 

 sionally the release of y rays may require a period of seconds or hours. 

 (An unusually long duration of an excitation is more frequently observed 

 in nuclei than in the electronic portion of atoms and molecules because the 

 chance of direct energy transfer from nuclei is smaller. A long excitation 



