The Nucleus and Nuclear Radiation - Atoms, Radiation, and-Radiation Protection - page 66

A number of beta emitters have no accompanying gamma rays. Examples of
such pure beta emitters are 3 H, 14 C, 32 P, 90 Sr, and 90 Y. Mixed beta-gamma emit-
ters include 60 Co, 137 Cs, and many others. A number of radionuclides emit beta
particles in decaying to several levels of the daughter nucleus, thus giving rise to
complex beta spectra. A few radioisotopes can decay by emission of either an alpha
or a beta particle. For example, 21 83 Bi decays by alpha emission 36% of the time and
by beta emission 64% of the time.
Beta rays can have sufficient energy to penetrate the skin and thus be an external
radiation hazard. Internal beta emitters are also a hazard. As is the case with 60 Co,
many beta radionuclides also emit gamma rays. High-energy beta particles (i.e.,
in the MeV range) can emit bremsstrahlung, particularly in heavy-metal shielding.
The bremsstrahlung from a beta source may be the only radiation that escapes the
containment.
3.5
Gamma-Ray Emission
As we have seen, one or more gamma photons can be emitted from the excited
states of daughter nuclei following radioactive decay. Transitions that result in
gamma emission leave Z and A unchanged and are called isomeric ; nuclides in
the initial and final states are called isomers .
As the examples in the last two sections illustrate, the gamma-ray spectrum from
a radionuclide is discrete. Furthermore, just as optical spectra are characteristic of
the chemical elements, a gamma-ray spectrum is characteristic of the particular
radionuclides that are present. By techniques of gamma-ray spectroscopy (Chap-
ter 10), the intensities of photons at various energies can be measured to determine
the distribution of radionuclides in a sample. When 60 Co is present, for example,
photons of energy 1.173 MeV and 1.332 MeV are observed with equal frequency.
(Although these are called “ 60 Co gamma rays,” we note from Fig. 3.6 that they are
actually emitted by the daughter 60 Ni nucleus.) Radium can also be easily detected
by its gamma-ray spectrum, which is more complex than indicated by Fig. 3.4.
Since individual photons are registered in a spectrometer, gamma rays from infre-
quent modes of radioactive decay can often be readily measured. Figure 3.7 shows
a more detailed decay scheme for 226 Ra, which involves three excited states of the
daughter 222 Rn nucleus and the emission of photons of four different energies.
Transitions from the highest excited level (0.601 MeV) to the next (0.448 MeV) and
from there to ground are “forbidden” by selection rules.
Example
Like 6 27 Co, another important gamma-ray source is the radioisotope 13 55 Cs. Consult
Appendix D and work out its decay scheme.