Page 54 - Radiochemistry and nuclear chemistry

Nuclear Mass and Stability 43

relative to those of gallium (31Ga, 2 stable isotopes), and arsenic (33As, 1 stable isotope).
The same pairing stabilization holds true for neutrons so that an even-even nuclide which
has all its nucleons, both neutrons and protons, paired represents a quite stable situation.
In the elements in which the atomic number is even, if the neutron number is uneven, there
is still some stability conferred through the proton-proton pairing. For elements of odd
atomic number, unless there is stability due to an even neutron number (neutron-neutron
pairing), the nuclei are radioactive with rare exceptions. We should also note that the
number of stable nuclear species is approximately the same for even-odd and odd-even
cases. The pairing of protons with protons and neutrons with neutrons must thus confer
approximately equal degrees of stability to the nucleus.

3.2. Neutron to proton ratio

If a graph is made (Fig. 3.1)1 of the relation of the number of neutrons to the number
of protons in the known stable nuclei, we find that in the light dements stability is achieved
when the number of neutrons and protons are approximately equal (N = Z). However, with
increasing atomic number of the element (i.e. along the Z-line), the ratio of neutrons to
protons, the N/Z ratio, for nuclear stability increases from unity to approximately 1.5 at
bismuth. Thus pairing of the nucleons is not a sufficient criterion for stability: a certain
ratio N/Z must also exist. However, even this does not suffice for stability, because at high
Z-values, a new mode of radioactive decay, ca-emission, appears. Above bismuth the
nuclides are all unstable to radioactive decay by ca-particle emission, while some are
unstable also to B-decay.
If a nucleus has a N/Z ratio too high for stability, it is said to be neutron-rich. It will
undergo radioactive decay in such a manner that the neutron to proton ratio decreases to
approach more closely the stable value. In such a case the nucleus must decrease the value
of N and increase the value of Z, which can be done by conversion of a neutron to a
proton. When such a conversion occurs within a nucleus, ~- (or negatron) emission is the
consequence, with creation and emission of a negative fl-particle designated by/~- or _ ~
(together with an anti-neutrino, here omitted for simplicity, see Ch. 4). For example:
116 116,-,
49In ~ 50~n + _0e-

At extreme N/Z ratios beyond the so called neutron drip-line, or for highly excited nuclei,
neutron emission is an alternative to/~- decay.
If the N/Z ratio is too low for stability, then radioactive decay occurs in such a manner
as to lower Z and increase N by conversion of a proton to neutron. This may be
accomplished through positron emission, i.e. creation and emission of a positron (B+ or
+~ or by absorption by the nucleus of an orbital electron (electron capture, EC).
Examples of these reactions are:

I16~i., ''~ 50an + +~ and l~5Au + _0e- EC 195v,
78 IL
~
51ou
I16,~
1
In graphs like Fig. 3.1, Z is commonly plotted as the abscissa; we have here reversed the axes to conform with
the commercially available isotope and nuclide charts.