Page 456 - Schaum's Outline of Theory and Problems of Applied Physics

CHAPTER 36

Nuclear Physics

NUCLEARSTRUCTURE

The nucleus of an atom is composed of protons and neutrons whose masses are, respectively,
m p = 1.673 × 10 −27 kg = 1.007277 u
m n = 1.675 × 10 −27 kg = 1.008665 u
As we saw in Chapter 20, the value of the atomic mass unit is u = 1.660 × 10 −27 kg. The energy equivalent
of this amount of mass is 931 megaelectronvolts (MeV). The proton has a charge of +e, and the neutron is
uncharged. The atomic number of an element is the number of protons in the nucleus of one of its atoms. Protons
and neutrons are jointly called nucleons.
Although all the atoms of an element have the same number of protons in their nuclei, the number of neutrons
may be different. Each variety of nucleus found in a given element is called an isotope of the element. Symbols
for isotopes follow the pattern
A
Z X
where X = chemical symbol of element
Z = atomic number of element = number of protons in nucleus
A = mass number of isotope = number of protons + neutrons in nucleus

FUNDAMENTAL FORCES
The force between nucleons that holds an atomic nucleus together despite the repulsive electric forces its protons
exert on each other is the result of what is known as the strong interaction. This is a fundamental interaction in
the same sense as the gravitational and electromagnetic interactions are: None can be explained in terms of any
of the others. The strong interaction has only a very short range, unlike the gravitational and electromagnetic
interactions, and is effective only within nuclei.
Another interaction involving nuclei is the weak interaction, which helps govern the compositions of atomic
nuclei. The various fundamental interactions and some of their properties are listed in Table 36-1.

SOLVED PROBLEM 36.1
The largest stable nucleus is that of the bismuth isotope 209 Bi. Why are larger nuclei unstable?
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The range of the strong interaction, which provides the attractive forces that hold nucleons together, is quite
short, whereas the electric repulsive forces that act between protons have unlimited range. Hence, beyond a certain
size the repulsive forces become comparable with the attractive ones, and such nuclei are unstable.
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