very heavy nuclei (A > 238) spontaneous fission becomes an important mode of decay. In this process,
complete fission of the nucleus into two roughly equal parts occurs.
For a given mass number there is a known, stable neutron to proton ratio which varies from 1 to 1.5 as
the mass increases (Figure 10.2). Any nucleide whose ratio falls outside of these values will be unstable
and decay so as to obtain a more stable ratio. In doing so, it will interconvert neutrons and protons and
will typically emit positrons, , or negatrons,. Decay processes will involve the
emission of a high energy particle in a primary event, closely followed (apart from a few important
exceptions) by the emission of γ-rays as the new nucleus adjusts to its ground state. The important
modes of radioactive decay are summarized below.
Figure 10.2
Stable nucleides and the stability line.Alpha Decay
This involves the loss of a helium nucleus from a heavy 'overcrowded' nucleus and is accompanied by
the emission of γ-rays. The process may be summarized by