Chemistry Reference
In-Depth Information
Fig. 1.1
The fusion of four
protons (
1
H) affording a
4
He
nucleus (
4
1
H
He
1
H
α
-particle)
12
C
γ
15
N
13
N
e
+
ν
e
+
ν
15
O
13
C
γ
γ
14
N
1
H
1
H
electron-emitter with a half-life of 5730 years. We will examine briefly each of these
four carbon nuclides.
First,
starting with the stable
12
C, which accounts for 89.9 % of all carbon atoms, it
is formed in the interior of stars by the triple-alpha process. It is no longer a mystery
how three
α
-particles interact in the nucleus despite the electrostatic repulsion (two
-particles cannot form a stable
8
Be nuclide): it was recently calculated that in the
ground state of the
12
C nucleus, three
α
-particles form a triangle cluster, whereas with
low-energy excitation they form a kinked obtuse-angle cluster (so-called Hoyle state)
shaped like a bent arm (Epelbaum et al.
2011
; Lee et al.
2012
). Pairing of nuclear
spins for protons and neutrons in even-even nuclei such as
α
2
He,
12
C,
16
O results in
such nuclei to be devoid of magnetic moments.
In the sun and other sun-like stars, fusion of hydrogen nuclei to helium in-
volves a process discovered by Hans Bethe, who was honored for it in 1967
with the Nobel Prize for Physics. A catalytic cycle involving
12
C,
13
C,
13
N,
14
N,
15
N,
and
15
O nuclei allows this fundamental process to occur; 2 positrons, 2 neutri-
nos, and 3 gamma photons carrying away the fusion energy are also released
(Fig.
1.1
). In the Bethe cycle there are four stable nuclides (
12
C,
13
C,
14
N,
15
N) and two
positron
+
neutrino-emitters:
13
N(
T
½
=
10 min) and
15
O(
T
½
=
2 min). Positrons
undergo rapid annihilation on encountering electrons, further increasing the emitted
fusion energy during the life-time of the star.
An important characteristic of the
12
C and
13
C nuclei is their small cross-section
for neutron capture. Thus, despite their higher mass than that of deuterons or protons,
which could thermalize better the fast neutrons released during the fission of
235
U,
the first nuclear reactor was built by E. Fermi and coworkers in Chicago by using pure
graphite as moderator. This use of graphite has several drawbacks: (i) small amounts
of impurities reduce substantially its efficiency, and this is why Hitler's Germany
opted for using heavy water as moderator in the 1940s; (ii) nuclear radiations cause
lattice defects (“E. P. Wigner defects”) which may release large amounts of energy,
as it happened in 1957 in the Windscale reactor fire in UK; (iii) at higher temperature
