Page 70 - Radiochemistry and nuclear chemistry

Unstable Nuclei and Radioactive Decay 59

The time dependence of radioactive decay is expressed in terms of the half-life (hh),
which is the time required for one-half of the radioactive atoms in a sample to undergo
decay. In practice this is the time for the measured radioactive intensity (or simply,
radioactivity of a sample) to decrease to one-half of its previous value (see Fig. 1.1).
Half-lives vary from millions of years to fractions of seconds. While half-lives between a
minute and a year are easily determined with fairly simple laboratory techniques, the
determination of much shorter half-lives requires elaborate techniques with advanced
instrumentation. The shortest half-life measurable today is about 10 -18 s. Consequently,
radioactive decay which occurs with a time period less than 10 -18 s is considered to be
instantaneous. At the other extreme, if the half-life of the radioactive decay exceeds 1015
y, the decay usually cannot be observed above the normal signal background present in the
detectors. Therefore, nuclides which may have half-lives greater than 1015 y are normally
considered to be stable to radioactive decay. However, a few unstable nuclides with
extremely long half-lives, >_ 1020 y, have been identified. It should be realized that 1015 y
is about 105 times larger than the age of the universe.
Radioactive decay involves a transition from a definite quantum state of the original
nuclide to a definite quantum state of the product nuclide. The energy difference between
the two quantum levels involved in the transition corresponds to the decay energy. This
decay energy appears in the form of electromagnetic radiation and as the kinetic energy of
the products, see Element and Nuclide Index for decay energies.
The mode of radioactive decay is dependent upon the particular nuclide involved. We
have seen in Ch. 1 that radioactive decay can be characterize~ by a-, B-, and ~-radiation.
Alpha-decay is the emission of helium nuclei. Beta-decay is the creation and emission of
either electrons or positrons, or the process of electron capture. Gamma-decay is the
emission of electromagnetic radiation where the transition occurs between energy levels of
the same nucleus. An additional mode of radioactive decay is that of internal conversion
in which a nucleus loses its energy by interaction of the nuclear field with that of the orbital
electrons, causing ionization of an electron instead of 7-ray emission. A mode of
radioactive decay which is observed only in the heaviest nuclei is that of spontaneous fission
in which the nucleus dissociates spontaneously into two roughly equal parts. This fission
is accompanied by the emission of electromagnetic radiation and of neutrons. In the last
decade also some unusual decay modes have been observed for nuclides very far from the
stability line, namely neutron emission and proton emission. A few very rare decay modes
like 12C-emission have also been observed.
In the following, for convenience, we sometimes use an abbreviated form for decay
reactions, as illustrated for the 238U decay chain in w 1.3"

238U(ot) 234Th(/3-) 234pa(/3-) 234U(~), etc.,

or, if half-lives are of importance:

238U(c~, 4.5 x 109 y)234Th(~-, 24 d)234pa(/~-, 1.1 rain)234U(ot, 2.5 x 105 y), etc.

In the following chapter we discuss the energetics of the decay processes based on nuclear
binding energy considerations and simple mechanics, then we consider the kinetics of the
processes. In Ch. 11, where the internal properties of the nuclei are studied, the