2.4 Transition Between Nuclear Energy Levels …
21
E γ = E 1 − E 2 = hν
where
E γ = Energy of electromagnetic radiation
E 1 = Energy of excited state
E 2 = Energy of ground state
=
(or lower excited state than E 1 )
h =
Plank’s constant
ν =
Frequency of radiation.
While the emission of γ -radiation usually takes place very rapidly, there are
occasions when the excited nuclei take a very long time for such emission. Such a
state of nuclei is called as metastable state and decays slowly. The decay process
of
137m Ba is one example of such decay process. This type of decay is known as an
isomeric transition (I.T.).
137m Ba
2.4 min
− −−−− →
137 Ba.
Sometimes excess nuclear energy, instead of being emitted as electromagnetic
radiation, is transferred to one of the orbital electrons, usually in the K -shell. This
absorption of energy causes the ejection of electrons from the atom with energy
equal to the difference between decay energy and binding energy of the concerned
electron. This type of decay is known as internal conversion. The ejected electrons
are monoenergetic, in contrast to the heteroenergetic β-particles. An X -ray radiation
is also emitted, as the vacancy in K -shell is filled by the electrons falling from higher
orbitals. This type of emission may lead to X -ray emission of different energies due
to a series of falls of electrons from various higher orbitals to next lower energy
orbitals and finally to the K -shell orbital. Approximately, 12% of the disintegration
of Barium-137m is by internal conversion (Figs. 2.3 and 2.4). Along with γ -rays
(Fig. 2.3B), some X -ray (Fig. 2.3A) is also emitted. Decay by an internal conversion
may have interesting consequences if the atom concerned is covalently bonded to
another atom, as one of the electrons forming the bond may fill the vacancy in the
inner orbital. In this case, the chemical bond would be ruptured.
These discussions thus also suggest that practically in every decay process, irrespective of its kind, there is a possibility of γ -ray as well as X -ray emission. Because
it is very unlikely that after the decay of the radioactive isotope by any of the processes discussed earlier, there would not be some excess mass left (i.e., mass of the
isotope being more than required for becoming stable isotope) with the daughter
nuclei, this excess mass is not enough to emit, for example, α- or β-particles and
hence the excess mass is emitted in the form of either γ -rays or X -rays.
In fact, the emission of γ -ray is advantageous to us, because the energy of γ -ray is
very specific for a radioactive isotope and thus can help to detect the presence of isotope in the sample by measuring γ -rays (Fig. 2.4C) rather than β-particle (Fig. 2.4B)
or X -rays (Fig. 2.4A).
21
E γ = E 1 − E 2 = hν
where
E γ = Energy of electromagnetic radiation
E 1 = Energy of excited state
E 2 = Energy of ground state
=
(or lower excited state than E 1 )
h =
Plank’s constant
ν =
Frequency of radiation.
While the emission of γ -radiation usually takes place very rapidly, there are
occasions when the excited nuclei take a very long time for such emission. Such a
state of nuclei is called as metastable state and decays slowly. The decay process
of
137m Ba is one example of such decay process. This type of decay is known as an
isomeric transition (I.T.).
137m Ba
2.4 min
− −−−− →
137 Ba.
Sometimes excess nuclear energy, instead of being emitted as electromagnetic
radiation, is transferred to one of the orbital electrons, usually in the K -shell. This
absorption of energy causes the ejection of electrons from the atom with energy
equal to the difference between decay energy and binding energy of the concerned
electron. This type of decay is known as internal conversion. The ejected electrons
are monoenergetic, in contrast to the heteroenergetic β-particles. An X -ray radiation
is also emitted, as the vacancy in K -shell is filled by the electrons falling from higher
orbitals. This type of emission may lead to X -ray emission of different energies due
to a series of falls of electrons from various higher orbitals to next lower energy
orbitals and finally to the K -shell orbital. Approximately, 12% of the disintegration
of Barium-137m is by internal conversion (Figs. 2.3 and 2.4). Along with γ -rays
(Fig. 2.3B), some X -ray (Fig. 2.3A) is also emitted. Decay by an internal conversion
may have interesting consequences if the atom concerned is covalently bonded to
another atom, as one of the electrons forming the bond may fill the vacancy in the
inner orbital. In this case, the chemical bond would be ruptured.
These discussions thus also suggest that practically in every decay process, irrespective of its kind, there is a possibility of γ -ray as well as X -ray emission. Because
it is very unlikely that after the decay of the radioactive isotope by any of the processes discussed earlier, there would not be some excess mass left (i.e., mass of the
isotope being more than required for becoming stable isotope) with the daughter
nuclei, this excess mass is not enough to emit, for example, α- or β-particles and
hence the excess mass is emitted in the form of either γ -rays or X -rays.
In fact, the emission of γ -ray is advantageous to us, because the energy of γ -ray is
very specific for a radioactive isotope and thus can help to detect the presence of isotope in the sample by measuring γ -rays (Fig. 2.4C) rather than β-particle (Fig. 2.4B)
or X -rays (Fig. 2.4A).