Angel Mae G. Bolaño BSRT 1-A
What is Radioactive Decay/ Radioactivity?
Radioactive decay, also known as nuclear decay or radioactivity, is the process
by which a nucleus of an unstable atom loses energy by emitting particles of ionizing
radiation. A material that spontaneously emits this kind of radiation—which includes the
emission of energetic alpha particles, beta particles, and gamma rays—is considered
Modes of Radioactive Decay
1. Alpha Decay - Alpha decay, or α-decay, is a type of radioactive decay in which
an atomic nucleus emits an alpha particle and thereby transforms (or 'decays')
into an atom with a mass number 4 less and atomic number 2 less.
Ex: 238U → 231Pa + α
2. Negatron Emission (Beta Decay)-A beta particle is often an electron, but can
also be a positron, a positively-charged particle that is the anti-matter equivalent
of the electron. If an electron is involved, the number of neutrons in the nucleus
decreases by one and the number of protons increases by one.
Ex: 60Co → 60Ni + e− + ν¯
3. Positron Emission - Nuclides that are imbalanced in their ratio of protons to
neutrons undergo decay to correct the imbalance. Nuclei that are rich in protons
relative to their number of neutrons can decay by conversion of a proton to a
neutron, emitting a positron (01e+) and a neutrino (ν). Positrons are the
antiparticles of electrons, therefore a positron has the same mass as an electron
but with the opposite (positive) charge. In positron emission, the atomic number
Z decreases by 1 while the mass number A remains the same.
Ex:22Na → 22Ne + e+ + ν
4. Electron Capture - is a type of radioactive decay where the nucleus of an atom
absorbs a K or L shell electron and converts a proton into a neutron. This
process reduces the atomic number by 1 and emits gamma radiation and a
Ex: 147Dy → 146Tb + e+ + ν + p
5. Isomeric Transition - is a form of radioactive decay where a gamma photon is
emitted by a nucleus in an excited metastable state. The photon is emitted when
the energy of the excited nucleus drops to the lower, ground state.
Ex: 137mBa → 137Ba + γ(662 keV)
6. Internal Conversion- a transition from a higher to a lower electronic state in a
molecule or atom. It is sometimes called "radiationless de-excitation", because
no photons are emitted. It differs from intersystem crossing in that, while both
are radiationless methods of de-excitation, the molecular spin state for internal
conversion remains the same, whereas it changes for intersystem crossing. The
energy of the electronically excited
state is given off to vibrational modes
of the molecule or phonons. The
excitation energy is transformed into
Internal conversion is another
electromagnetic process which can
occur in the nucleus and which
competes with gamma emission.
Sometimes the multipole electric fields
of the nucleus interact with orbital
electrons with enough energy to eject
them from the atom. This process is
not the same as emitting a gamma ray
which knocks an electron out of the
atom. It is also not the same as beta
decay, since the emitted electron was
previously one of the orbital electrons,
whereas the electron in beta decay is
produced by the decay of a neutron.
Properties of a Radioactive Material
1. Decay Constant (λ) - The constant ratio for the number of atoms of a
radionuclide that decay in a given period of time compared with the total
number of atoms of the same kind present at the beginning of that period. Also
called disintegration constant, radioactive constant.
Half-life - is the amount of time required for a quantity to fall to half its value
as measured at the beginning of the time period. While the term "half-life" can
be used to describe any quantity which follows an exponential decay, it is most
often used within the context of nuclear physics and nuclear chemistry—that is,
the time required, probabilistically, for half of the unstable, radioactive atoms in
a sample to undergo radioactive decay.
The original term, dating to Ernest Rutherford's discovery of the principle in
1907, was "half-life period", which was shortened to "half-life" in the early
1950s.Rutherford applied the principle of a radioactive elements' half-life to
studies of age determination of rocks by measuring the decay period of radium
Half-life is used to describe a quantity undergoing exponential decay, and is
constant over the lifetime of the decaying quantity. It is a characteristic unit for
the exponential decay equation. The term "half-life" may generically be used to
refer to any period of time in which a quantity falls by half, even if the decay is
not exponential. The table on the right shows the reduction of a quantity in
terms of the number of half-lives elapsed.
3. Activity - The quantity which expresses the degree of radioactivity or the
radiation producing potential of a given amount of radioactive material is activity.
The curie was originally defined as that amount of any radioactive material that
disintegrates at the same rate as one gram of pure radium. The curie has since
been defined more precisely as a quantity of radioactive material in which 3.7 x
1010 atoms disintegrate per second. The International System (SI) unit for
activity is the Becquerel (Bq), which is that quantity of radioactive material in
which one atom is transformed per second. The radioactivity of a given amount
of radioactive material does not depend upon the mass of material present. For
example, two one-curie sources of Cs-137 might have very different masses
depending upon the relative proportion of non-radioactive atoms present in each
source. Radioactivity is expressed as the number of curies or becquerels per unit
mass or volume.
The concentration of radioactivity, or the relationship between the mass of
radioactive material and the activity, is called "specific activity." Specific activity
is expressed as the number of
curies or becquerels per unit
mass or volume. Each gram of
cobalt-60 will contain
approximately 50 curies.
Iridium-192 will contain 350
curies for every gram of
material. The shorter half-life,
the less amount of material that
will be required to produce a
given activity or curies. The
higher specific activity of iridium
results in physically smaller
sources. This allows technicians
to place the source in closer
proximity to the film while maintaining geometric unsharpness requirements on
the radiograph. These unsharpness requirements may not be met if a source
with a low specific activity were used at similar source to film distances.