Biomedical Engineering Reference
In-Depth Information
and therefore require no additional energy from external sources. Artificial radionuclides,
on the other hand, are essentially man-made and are produced by bombarding so-called
stable nuclides with high-energy particles. Both types of radionuclides play an important
role in emission scanning and nuclear medicine. The average life or half-life, the mode of
transformation or decay, and the nature of emission (type and energy of the ionizing radia-
tion) constitute the basic characteristics of a radionuclide.
All radioactive materials, whether they occur in nature or are artificially produced, decay
by the same types of processes: emitting alpha, beta, and/or gamma radiations. As just dis-
cussed, the emission of alpha and beta particles involves the disintegration of one element,
often called the parent, into another, the daughter. The modes or phases of transformation
in the process of a radionuclide passage from an unstable to stable condition may be
divided into six different categories:
a
(alpha) decay/emission,
b
- (negatron) decay,
bþ
(positron) decay, electron capture (EC), isomeric transition (IT), and fission.
Alpha (a) Decay
Alpha particles are ionized helium atoms (
2
H) moving at a high velocity (recall
Z
I notation).
If a nucleus emits an alpha particle, it loses two protons and two neutrons, thereby reducing
Z by 2 and A by 4. Several naturally occurring heavy elements undergo such decay. For
example, a parent nucleus
238
92
U emits an alpha particle, thereby changing to a daughter nucleus
234
90
Th (Thorium). In symbolic form, this process may be written as
238
234
4
2
He
Note the following facts about this reaction: (1) the atomic number (number of protons) on
the left is the same as on the right (92
92
U
!
90
Th
þ
2), since charge must be conserved, and (2) the
mass number (protons plus neutrons) on the left is the same as on the right (238
¼
90
þ
¼
234
þ
4).
When one element changes into another, as in alpha decay, the process is called
transmu-
tation
. In order for alpha emission to occur, the mass of the parent must be greater than the
combined mass of the daughter and the alpha particle. In the decay process, this excess
mass is converted into energy and appears in the form of kinetic energy in the daughter
nucleus and the alpha particle. Most of the kinetic energy is carried away by the alpha par-
ticle because it is much less massive than the daughter nucleus. That is, because momentum
must be conserved in the decay process, the lighter alpha particle recoils with a much
higher velocity than the daughter nucleus. Generally, light particles carry off most of the
energy in nuclear decays.
EXAMPLE PROBLEM 15.2
Radium,
226
88
Ra, decays by alpha emission. What is the daughter element formed?
Solution
The decay can be written symbolically as
226
88
Ra
4
2
He
!
X
þ
