Biomedical Engineering Reference
In-Depth Information
of an alpha particle in air and the logarithm of the emitter's half-life
T
. The relation
can be expressed in the form
-
ln
T
=
a
+
b
ln
R
,
(3.22)
where
a
and
b
are empirical constants.
To conclude this section, we briefly consider the possible radiation-protection
problems that alpha emitters can present. As we shall see in Chapter 5, alpha par-
ticles have very short ranges and cannot even penetrate the outer, dead layer of
skin. Therefore, they generally pose no direct external hazard to the body. Inhaled,
ingested, or entering through a wound, however, an alpha source can present a haz-
ard as an internal emitter. Depending upon the element, internal emitters tend to
seek various organs and irradiate them. Radium, for example, seeks bone, where it
can become lodged and irradiate an individual over his or her lifetime. In addition
to the internal hazard, one can generally expect gamma rays to occur with an al-
pha source, as is the case with radium. Also, many alpha emitters have radioactive
daughters that present radiation-protection problems.
3.4
Beta Decay (
β
-
)
In beta decay, a nucleus simultaneously emits an electron, or negative beta particle,
0
-1
0
0
ν. Both of these particles are created at the moment
of nuclear decay. The antineutrino, like its antiparticle
7)
β
, and an antineutrino,
the neutrino,
0
ν,hasno
chargeandlittleornomass;
8)
they have been detected only in rather elaborate
experiments.
As an example of beta decay, we consider
60
Co:
60
27
Co
60
28
Ni +
0
-1
+
0
ν
→
β
.
(3.23)
In this case, the value of
Q
is equal to the difference between the mass of the
60
Co
nucleus,
M
Co,N
, and that of the
60
Ni nucleus,
M
Ni,N
, plus one electron (
m
):
Q
=
M
Co,N
-(
M
Ni,N
+
m
).
(3.24)
The nickel atom has one more electron than the cobalt atom. Therefore, if we ne-
glect differences in atomic-electron binding energies, Eq. (3.24) implies that
Q
is
7 The Dirac equation predicts the existence of
an antiparticle for every spin-
2
particle and
describes its relationship to the particle.
Other examples include the positron,
of an electron-positron pair.
Particle-antiparticle pairs can annihilate, as
electrons and positrons do. A bar over a
symbol is used to denote an antiparticle: for
example,
+1
β
,
antiparticle to the electron; the antiproton;
and the antineutron. Creation of a spin-
2
particle is always accompanied by creation of
a related particle, which can be the
antiparticle, such as happens in the creation
. Several kinds of neutrinos
have been found—electron, muon, and tau.
8 Experimentally, the neutrino and
antineutrino masses cannot be larger than
about 30 eV.
ν
,
ν
