Biomedical Engineering Reference
In-Depth Information
Fig. 3.3
Average energy per nucleon as a function of atomic mass number.
binding than their immediate neighbors. These nuclei are all “multiples” of the al-
pha particle, which appears to be a particularly stable nuclear subunit. (No nuclides
with
A
= 5
exist for longer than
∼10
-21
s.
3)
The loss of mass that accompanies the binding of particles is not a specifically
nuclear phenomenon. The mass of the hydrogen atom is smaller than the sum of
the proton and electron masses by
1.46
10
-8
AMU. This is equivalent to an energy
×
10
-8
AMU
931
MeV/AMU
-1
10
-5
MeV
1.46
×
×
=
1.36
×
=
13.6
eV, the binding
energy of the H atom.
We turn now to the subject of radioactivity, the property that some atomic species,
called radionuclides, have of undergoing spontaneous nuclear transformation. All
of the heaviest elements are radioactive;
20
83
Bi is the only stable nuclide with
Z
>82.
All elements have radioactive isotopes, the majority being man-made. The various
kinds of radioactive decay and their associated nuclear energetics are described in
the following sections.
3 Various forms of shell models have been
studied for nuclei, analogous to an atomic
shell model. The alpha particle consists of
two spin-
2
protons and two spin-
2
neutrons
in s states, forming the most tightly bound,
“inner” nuclear shell. Generally, nuclei with
even numbers of protons and neutrons
(“even-even” nuclei) have the largest binding
energies per nucleon.
