Environmental Engineering Reference
In-Depth Information
2
Physics of Nuclear Fusion: the Source of all Solar-Related Energy
Energy from the sun is entirely produced by nuclear fusion, so it is important to
understand how this works. There are twomain aspects of a complex process that are
basic. First is the strong nuclear attractive force leading to reactions of the elementary
proton particles and to formation of more stable nuclear products, such as
4
He (see
Figure 1.5). Since this combination of the four nucleons (two protons and two
neutrons) is more stable, a large kinetic energy
Q
is released. The strong interaction
binding in
4
He amounts to around 9MeV per particle. The strong nuclear force has a
short range, on the order of a femtometer
f ¼
10
15
m. This force acts equally binding
protons and neutrons independent of the electric charge. The individual particles
have charge radii on the order of femtometers. The charge radius is the size within
which the electric charge is contained, about 1.2 f for a proton, directly measured by
electron scattering at high energy. (These experiments actually show substructures of
the proton and neutron, each built up from three smaller particles, called quarks,
within the overall radius 1.2 f. These details are not needed for our discussion.) Nuclei
in general have a radius, measured by high-energy scattering experiments, which fit
an empirical rule
R ¼ R
o
A
1
=
3
;
ð
2
:
1
Þ
where
R
o
is the basic nuclear size parameter, and
A¼Z þ N
is the number of
nucleons, protons plus neutrons. The value of
R
o
is about 1 f; values 1.07 f, 1.2 f, and
1.44 f have been used for slightly different circumstances. Formally, the radius of the
proton,
A¼
1, is just
R
o
, and that of the deuteron,
A¼
2, is 1.26
R
o
.
In themost important single reaction in the sun, the joining of two protons tomake
a deuteron, new particles, a positron and a neutrino, are produced. Initially, an
unstable
2
He or pp
þ
2
species is brie
y formed, and in a small fraction
T
Tof its decays
generates the
first important fusion product D
¼
2
H via
p
þ
p
!
D
þ
e
þ
þn
e
:
ð
1
:
10
Þ
The neutrino is neutral and has nearly zero mass, and is hard to detect because it
rarely interacts withmatter. In spite of this dif
culty, the neutrino
flux from the sun at
the earth has been measured and is now in agreement with solar models. The
