164 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 7 



b. Positron Emission (Beta Decay). A positron and a neutrino are emitted 

 simultaneously and share in any proportion the total discrete energy of the 

 level transition. The observed positron kinetic energy may take any value 

 from zero up to the well-defined total transition energy. The residual atom 

 is smaller in charge by one electronic unit, and its exact atomic weight is 

 diminished by an amount equal to the mass equivalence of the maximum 

 kinetic energy plus twice the electron rest mass. The statistics of the 

 nucleus remain unchanged, but the spin is altered by an integral multiple 

 of h/2ir. 



c. K Capture. An alternative process to positron decay, K capture is 

 often found to compete with positron emission for the same level transition 

 in the transformation proton — > neutron. Unlike positron decay, however, a 

 monoenergetic neutrino is emitted and K x-radiation characteristic of the 

 daughter substance is emitted. The residual atom is smaller in charge by 

 one electronic unit, and its exact atomic weight is diminished by the mass 

 equivalence of the kinetic energy carried off by the neutrino. The statistics 

 of the nucleus remain the same, but the spin is altered by an integral multiple 

 of h/2ir. 



d. Isomeric Transition (Gamma Emission). Gamma rays are always 

 monoenergetic for any one nuclear level transition, but several gamma rays 

 of different energy hv may be emitted in cascade in the decay of a single atom. 

 The charge of the residual nucleus remains unaltered, the spin is altered by 

 an integral multiple of h/2ir, and the exact atomic weight is reduced by the 

 amount hv/c 2 . 



e. Internal Conversion. Although internal conversion is not a nuclear phe- 

 nomenon in the decay process, it is frequently observed to accompany gamma 

 emission. Internal conversion (I. C.) is essentially the photoelectric absorp- 

 tion of a gamma ray by an orbital electron of the atom from whose nucleus 

 the gamma ray is emitted. K electrons are often observed to have the 

 highest probability for emission, L electrons next, etc. The electron is 

 ejected with a kinetic energy equal to the gamma-ray energy hv, minus the 

 binding energy (ionization potential) of the electron in the atom. Ejection 

 of conversion electrons is accompanied by characteristic K, L, M, and possibly 

 N, x-radiation. The probability for the process is given in terms of the 

 conversion coefficient a which is numerically equal to the ratio of the number 

 of conversion electrons ejected to the number of gamma rays emitted: 



N K + iV L + • • • 

 a = 



where N r , Nk, . • • are the observed numbers of gamma rays and K, 

 L, . . . electrons. 



