Physics and Radiobiology of Nuclear Medicine

Electron Capture

Decay by electron capture (EC) is an alternative to the b+-decay for pro-
tonrich radionuclides with N/Zlower than that of the stable nuclide. In EC
decay, an electron from an extranuclear shell, particularly the Kshell
because of its proximity, is captured by a proton in the nucleus, forming a
neutron accompanied by the emission of a neutrino for conservation of
energy. Thus,

p+e−→n+

In this process, the atomic number of the daughter nuclide is lowered by

1. The EC process occurs usually in nuclides having excitation energy less
   than 1.02 MeV. In nuclides having excitation energy greater than 1.02 MeV,
   both EC and b+-decay can occur, although the probability of b+-decay
   increases with higher excitation energy. The decay scheme of^111 In is shown
   in Figure 2.7. The EC decay is indicated by a right-to-left arrow. Some exam-
   ples of EC decay follow:

111

49 In +e

−→ 111

48 Cd +

67

31 Ga +e

−→ 67

30 Zn +

125

53 I +e

−→ 125

52 Te +

57

27 Co +e

−→ 57

26 Fe +

123

53 I +e

−→ 123

52 Te +
In EC decay, analogous to the situation in internal conversion, a vacancy
is created in the shell from which the electron is captured. It is filled in by

Electron Capture 19

Fig. 2.7. Decay scheme of^111 In illustrating the electron capture process. The abun-
dances of 171-keV and 245-keV g-rays are 90% and 94%, respectively.