As a matter of fact, gamma emission always results from some other form of nuclear transformation (as described in Section 4.4.6). However, there are two cases when the emission of photons is separated from the preceding nuclear trans­formation either in time or in space, so the patient’s radiation exposure is limited to that of photons:

• The atomic kernel may get into a metastable (excited in contrast to ground) state after beta decay, from which it can later decay to the ground state by emitting gamma photon (s). We need to separate the metastable element from the parent radionuclide with a suitable solvent and administer it to the patient after binding it to a selected molecule.

• Electron capture is a special case of positive beta decay, in which (instead of emitting a positron) the kernel captures an electron from the K shell, thus reducing the atomic num­ber (the number of protons; see Section 4.4.3). This can be accompanied by emitting gamma photon(s) as well, but more importantly, an electron from a higher-energy state will always “drop” into the hole left in the K shell, emitting the energy difference between the two shells in the form of characteristic X-rays. In the case of heavier atoms, the photon energy may be high enough to allow imaging by a gamma camera (see Section 12.2.5).