Page 85 - Radiochemistry and nuclear chemistry

74 Radiochemistry and Nuclear Chemistry

G. Flerov found that 238U in addition to or-decay also had a competing mode of radioactive
decay termed spontaneous fission. In this mode two heavy fragments (fission products) are
formed in addition to some neutrons. The reaction may be written

alX + ~X 2 + m
~X "~ ZI 1 (4.38)
where v is the number of neutrons, usually 2 - 3. The half-life for spontaneous fission of
238U is very long, about 8 • 1015 y. This means that about 70 fissions occur per second
in 1 kg of 238U, which can be compared with the simultaneous emission of 45 • 109
c~-particles.
With increasing Z, spontaneous fission becomes more common; i.e. the half-life for this
decay process decreases. For 2944~ it is 1.2 • 1011 y; for 2~Cm, 1.4 • 107 y; for 252~c
98""
66 y; and for 256- 3 • 10 -4 y. In fact, spontaneous fission becomes the dominating decay
100 rm,
mode for the heaviest nuclei (see Fig. 3.3).
Spontaneous fission is in some ways similar to fission induced by bombardment with low
energy neutrons (w 14.7).

4.7. Rare modes of decay

Radioactive decay by proton emission is a very seldom observed decay mode for very
neutron deficient nuclides because decay by/3+ or EC normally has a very much shorter
+
partial half-life (w Decay by p has been observed for 53m Co (Ep 1.55 MeV, tth 0.25
s, ~ 1.5 %). However,/~+ decay sometimes leads to a proton-unstable excited state which
immediately (< 10-12 s) emits a proton. Several/3+ emitters from 9C to 41Ti with N =
Z - 3 have/3+ delayed proton emission with half-lives in the range 10 -3 - 0.5 s. Also
radioactive decay by simultaneous emission of two protons has been observed for a few
proton rich nuclides, e.g. 16Ne ' tt,~ ~. 10-2~ s.
Among the very neutron rich nuclides, e.g. some fission products,/3- delayed neutron
emission is observed. This decay mode is similar in nature to the/3 + delayed p + emission.
Delayed n-emission is important for the safe operation of nuclear reactors, see Ch. 19.
Decay by emission of particles heavier than o~, e.g. 12C, 160, is energetically possible for
some heavy a-emitters and has been observed in a few cases.

4.8. Decay schemes and isotope charts

Information on the mode of decay, the decay energy, and the half-life are included in the
nuclear decay scheme. A number of simplified decay schemes are shown in Figure 4.5.
Figure 3.8 explains a more detailed decay scheme, for A=99.
Using Figure 3.8 as a guide, the decay schemes in Figure 4.5 are easily understood.
Figure 4.5(c) shows the/3- decay of the neutron. The decay scheme of 137Cs (Fig. 4.5(0)
differs somewhat from what we would expect from the curve in Figure 4.1. The reason is
that in the electron spectrum of Figure 4.1, a small fraction of electrons (8 %) emitted with
an energy of 1.20 MeV could not be detected because of the insensitivity of the magnetic
spectrometer used. It is common for nuclei to decay through different competing reactions,
as in this case which involves different/3-rays. If the higher energy t3-decay had been as