Page 81 - Radiochemistry and nuclear chemistry

70 Radiochemistry and Nuclear Chemistry

4.5. Gamma emission and internal conversion

The c~- and//-decay may leave the daughter nucleus in an excited state. This excitation
energy is removed either by 'y-ray emission or by a process called internal conversion.
The a-emission spectrum of 212Bi is shown in Figure 4.3. It is seen that the majority of
the a-particles have an energy of 6.04 MeV, but a considerable fraction (-30%) of the et-
particles have higher or lower energies. This can be understood if we assume that the decay
of parent 212Bi leads to excited levels of daughter 2~ This idea is supported by
measurements showing the emission of 'y-rays of energies which exactly correspond to the
difference between the highest a-energy 6.08 MeV, and the lower ones. For example, an
-0.32 MeV 'y accounts for the 5.76 MeV ~ (6.08 - 5.76 = 0.32). The excited levels of
2~ are indicated in the insert in Figure 4.3.
Gam~ rays produce very low density ionization in gases so they are not usually counted
by ionization, proportional, or Geiger counters. However, the fluorescence produced in
crystals such as sodium iodide make scintillation counting of 'y-rays efficient. Gamma ray
spectra can be measured with very high precision using semiconductor detectors (Ch. 8).
Figure 4.4 shows such a spectrum for the decay of various excited states of 197Au. The
7/2+, etc. symbols are explained in w
In the great majority of cases the emission of the 'y-ray occurs immediately after c~- or
/3-decay, i.e. within <_ 10-12 s, but in some instances the nucleus may remain in the higher
energy state for a measurable length of time. The longer-lived exited nuclei are called
isomers. An example is 60mCo, which decays with a half-life of 10.5 rain to the ground
state of 6~ The decay is referred to as isomeric transition.
The decay energy in 'y-emission is distributed between the 'y-ray quantum (E~) and the
kinetic energy of the recoiling product nucleus (Ed). We can therefore write

Q~, = E d + E~ (4.33)

The distribution of energy between the 'y-ray and the recoiling daughter, according to

E d = E~2/(2md c 2) (4.34)

shows that E d < 0.1% of ET. The amount of kinetic energy of the recoiling nuclide is
therefore so trivial that it may be neglected when only the 'y-ray energy is considered; cf.
exercise 4.4.
Gamma rays can interact with the orbital electrons of other atoms, so that the latter are
expelled from that atom with a certain kinetic energy (see Ch. 6). A different process,
called internal conversion, can occur within the atom undergoing radioactive decay. Because
the wave function of an orbital electron may overlap that of the excited nucleus, the
excitation energy of the nucleus can be transferred directly to the orbital electron (without
the involvement of a 'y-ray), which escapes from the atom with a certain kinetic energy E e.
No "y-ray is emitted in an internal conversion process; it is an alternate mode to -y-ray
emission of de-excitation of nuclei.
Internal conversion can be represented symbolically as

-. + _~ _. (4.35)