Biomedical Engineering Reference
In-Depth Information
Electron Capture or K Capture
An orbital electron, usually from the inner shell (K shell), may be captured by the nucleus
(as if a proton captured an electron and converted itself into a neutron). An electron capture
process produces a different element by decreasing Z by 1, with A being the same (similar to
bþ
decay). Indeed, some radionuclides have definite probabilities of undergoing either posi-
tron decay or electron capture (such as iron 52, which decays with about 42 percent EC and
58 percent
emission). Further, there could be an associated gamma emission with electron
capture. For example,
51
Cr (chromium 51) transforms to
51
V (vanadium 51), with about
90 percent going directly to the ground state. The remaining 10 percent goes to an excited
state of
51
V followed by transition to ground state with the emission of photons. An electron
capture also causes a vacancy in the inner shell, which leads to the emission of a characteristic
x-ray or Auger electron. Absence of high-energy electrons (beta particles) in EC causes a low
dose of absorbed radiation to the tissue.
bþ
Isomeric Transition and Internal Conversion
Radionuclides at a metastable state emit only
rays. The element remains the same with
no change in A (isomeric transition). The atomic mass number of the isomer is, therefore,
denoted by Am. For example,
99
mTc (technetium 99m) decays to
99
Tc. However, there is a
definite probability that instead of a photon coming out, the energy may be transferred to
an inner orbital electron. This is known as internal conversion, and the internally converted
electrons are close to monoenergetic beta particles. For example, barium 135m (decaying by
IT) emits about 84 percent IC electrons. They also create a vacancy in the shell, consequently
leading to the emission of characteristic x-rays and Auger electrons.
g
Nuclear Fission
Usually a heavy nuclide will break up into two nuclides (more or less equal fragments).
This may happen spontaneously, but is more likely with the capture of a neutron. The
uranium fission products mostly range between atomic numbers 42 and 56. A number of
medically useful radionuclides are produced as fission products, such as Xenon 133, which
may be extracted by appropriate radiochemical procedures.
15.2.4 Radioactive Decay
The reduction of the number of atoms through disintegration of their nuclei is known as
radioactive decay and is characteristic of all radioactive materials. Unaffected by changes
in temperature, pressure, or chemical combination, the rate of the decay process remains
constant with the same number of disintegrations occurring during each interval of time.
Furthermore, this decay process is a random event. Consequently, every atom in a radio-
active element has the same probability of disintegrating.
As the decay process continues, it is clear that fewer atoms will be available to disinte-
grate. This fraction of the remaining number of atoms that decay per unit of time is called
the decay constant (
l
).
