Elements of Modern Physics
318
Rutherford’s experiment (1911) indicated the existence of a heavy, positively
charged nucleus of very small dimensions near the centre of the atoms. It was
found that the scattering of α-particles i.e. ionized helium atoms, by a thin,
metal foil could be described by assigning a charge of Ze to this nucleus.
However, it was also noted that when the distance of closest approach was less
than 10
–14
m, the scattering showed deviations from Coulomb scattering,
indicating the extension of the nucleus over a distance of 10
–14
m. Indeed, the
descriptions of the details require the existence of a new, shortchange interaction
known as the strong interaction. The properties of the nuclei and their interactions
are quite different from those of an atom, and will be briefly discussed here.
Some of the important properties of the nucleus, such as its mass, size,
magnetic moment, etc. will be discussed first. This will be followed by an
analysis of nuclear forces and different models of the nucleus. Finally, stability
criteria, nuclear reactions and fission and fusion processes will be discussed. At
this stage, it may be mentioned that while modern experimental techniques
have provided quite detailed information about nuclear properties, an entirely
satisfactory framework for the quantitative prediction of these properties has
not been formulated. This is primarily because (i) nuclear force appear to be
structurally much more complicated than electromagnetic forces, and (ii) the
strength of nuclear forces is quite large which means that perturbative methods
cannot be used for calculation.
9.1 PROPERTIES OF THE NUCLEUS
Nuclear properties are most simply described in terms of the nuclear constituents.
Nuclear Constituents
The nucleus is made up of protons and neutrons. The proton is the nucleus of
the simplest atom, the hydrogen atom. It has a rest energy of 938.256 MeV or a
mass of 1.0072766 mu (1 atomic mass unit, mu, is equal to 1/12 of the
12
C mass
and corresponds to 931.478 MeV), a positive charge of e and spin /2. The
neutron has a rest energy of 939.550 MeV or a mass of 1.0086654 mu, zero net
charge and spin /2. Since protons and neutrons have half-integral spin, they
are fermions and satisfy Fermi-Dirac statistics. The near-equality of the neutron
and proton masses is an important property and it has a bearing on nuclear
interactions.
Both the proton and the neutron have magnetic moments given by
µ p = p
p
e
g m
s
(9.1)
µ n =
n
p
e
g m
−
s
318
Rutherford’s experiment (1911) indicated the existence of a heavy, positively
charged nucleus of very small dimensions near the centre of the atoms. It was
found that the scattering of α-particles i.e. ionized helium atoms, by a thin,
metal foil could be described by assigning a charge of Ze to this nucleus.
However, it was also noted that when the distance of closest approach was less
than 10
–14
m, the scattering showed deviations from Coulomb scattering,
indicating the extension of the nucleus over a distance of 10
–14
m. Indeed, the
descriptions of the details require the existence of a new, shortchange interaction
known as the strong interaction. The properties of the nuclei and their interactions
are quite different from those of an atom, and will be briefly discussed here.
Some of the important properties of the nucleus, such as its mass, size,
magnetic moment, etc. will be discussed first. This will be followed by an
analysis of nuclear forces and different models of the nucleus. Finally, stability
criteria, nuclear reactions and fission and fusion processes will be discussed. At
this stage, it may be mentioned that while modern experimental techniques
have provided quite detailed information about nuclear properties, an entirely
satisfactory framework for the quantitative prediction of these properties has
not been formulated. This is primarily because (i) nuclear force appear to be
structurally much more complicated than electromagnetic forces, and (ii) the
strength of nuclear forces is quite large which means that perturbative methods
cannot be used for calculation.
9.1 PROPERTIES OF THE NUCLEUS
Nuclear properties are most simply described in terms of the nuclear constituents.
Nuclear Constituents
The nucleus is made up of protons and neutrons. The proton is the nucleus of
the simplest atom, the hydrogen atom. It has a rest energy of 938.256 MeV or a
mass of 1.0072766 mu (1 atomic mass unit, mu, is equal to 1/12 of the
12
C mass
and corresponds to 931.478 MeV), a positive charge of e and spin /2. The
neutron has a rest energy of 939.550 MeV or a mass of 1.0086654 mu, zero net
charge and spin /2. Since protons and neutrons have half-integral spin, they
are fermions and satisfy Fermi-Dirac statistics. The near-equality of the neutron
and proton masses is an important property and it has a bearing on nuclear
interactions.
Both the proton and the neutron have magnetic moments given by
µ p = p
p
e
g m
s
(9.1)
µ n =
n
p
e
g m
−
s