Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

ATOMIC AND NUCLEAR STRUCTURE 77
where A, the atomic mass number, represents the number of nucleons within the
nucleus. For the case of 17 O, the binding energy is 131.7 MeV and the average
binding energy per nucleon is 7.75 MeV.
The binding energy per nucleon is very low for the low-atomic-numbered
elements but rises rapidly to a very broad peak at binding energies in excess of
8 MeV/nucleon and then decreases very slowly until a value of 7.58 MeV/nucleon
is reached for 238 U. Figure 3-5, in which the binding energy per nucleon is plotted
against the number of nucleons in the various isotopes, shows that, with very few
exceptions, there is a systematic variation of binding energy per nucleon with the
number of nucleons within the nucleus.
The most notable departures from the smooth curve are the nuclides 4 He, 12 C,
and 16 O. Each of these nuclides lies above the curve, indicating that they are very
strongly bound. The 12 C and 16 O isotopes, as well as 20 Ne, which has more binding
energy per nucleon than either of the nuclides that flank it, may be thought of
as containing three, four, and five subunits of 4 He, respectively. The exceptional
binding energies in these nuclei, together with the fact that 4 He nuclei, as alpha
particles, are emitted in certain modes of radioactive transformation, suggest that
nucleons tend to form stable subgroups of two protons and two neutrons within the
nucleus.
The fact that the binding energy curve (Fig. 3-5) has the shape that it does explains
why it is possible to release energy by splitting the very heavy elements and by fusing
two very light elements. Since the binding energy per nucleon is greater for nuclei
in the center of the curve than for nuclei at both extremes, any change in nuclear
Figure 3-5. Variation of binding energy per nucleon with atomic mass number.