RADIOACTIVITY METHODS 



989 



there is no change in A. In other words, the element changes into its next 

 higher neighbor in the periodic table. Each beta decay is thought to be 

 accompanied by the emission of a hypothetical particle of zero charge and 

 very small rest mass, the neutrino.f The energy Emax released by the decay 

 is divided between the ^ and the neutrino, so that the /? may have any energy 

 between zero and E^ax- The approximate shape of the energy spectrum is 

 shown in Figure 603, The spectrum is 

 continuous, with a definite upper limit 

 (Emax)- The average energy is approxi- 

 mately 1/3 E,„ax. 



'E, 



ENERCY- 



max. 



Fig. 603 — Continuous spectrum of a 



fl emitter. 



sec/g 

 ~1.4- MEV 



(3) K-capture. — The most uncom- 

 mon mode of decay is the capture of the 

 innermost atomic electron (K-electron) 

 by the nucleus, resulting in a decrease in Z 

 by one unit, with A unchanged. For exam- 

 ple, potassium^" decays to argon^°, its next 



lower neighbor in the periodic system. The K-capture is followed by x-ray 

 emission as the atomic electrons fall into the vacated lower orbits. 



A radioactive nucleus frequently has the opportunity of decaying in 

 more than one of these ways. Again, we may use potassium as an example 

 (Figure 604). A given nucleus of this element has an uncertain future. 



It may change to calcium^" by /3-decay, 

 or capture a K-electron and decay into 

 argon'**'. It is thought that the known 

 potassium gamma-ray is emitted by the 

 excited argon nucleus. The different 

 types of decay are not equally likely. 

 In the case of potassium^'', twice as 

 many nuclei decay by /^-emission as by 

 K-capture, but in other nuclei the 

 decay is much more unequally divided. 

 For instance, bismuth-^"* (RaC, see 

 Figure 605) prefers /3-decay and 

 transforms into polonium^i^, 2500 times more often than into thallium^i", 

 by a decay. 



Absorption 



The three types of radiation are all absorbed in matter, but each to a 

 different degree. Because of their great ionizing power, alphas are the 

 first absorbed. A sheet of paper is sufficient to stop them completely and 

 they travel only two or three inches in air. Natural alphas have a limited 

 range of energies: the weakest known (samarium) has an energy of 2 

 Mev, while the most energetic (ao from Po^^^ in the Th series) carries 10.6 



Fig. 604. — Decay scheme of potassium^". 

 Occurrence of the "silent" electron capture 

 (E.C.) transition from K*" to A^" is now in 

 doubt. (Modified after Ahrens and Evans, 

 Phys. Rev. 74, 279, 1948.) 



t C. W. Sherwin, "The Neutrino," Nucleonics 2, No. 5, 16-24, May, 1948. 



