Everything about The Decay Scheme totally explained
The
Decay scheme of a
radioactive substance is a graphical presentation of all the transitions occurring in a decay, and of their relationships.
These relations can be quite complicated; a simple case is shown here: the decay scheme of the radioactive
cobalt isotope Cobalt-60.
60Co decays by emitting an
electron (
beta decay) with a
half-life of 5.26 years into an
excited state of
60Ni, which then decays very fast to the ground state of
60Ni, via two gamma decays.
It is useful to think of the decay scheme as placed in a coordinate system, where the ordinate axis is energy, increasing from bottom to top, and the abscissa is the proton number, increasing from left to right. The arrows indicate the emitted particles. For the
gamma rays (vertical arrows), the gamma energies are given; for the
beta decay (oblique arrow), the maximum beta energy.
Nickel is to the right of cobalt, since its proton number (28) is higher by one than that of cobalt (27). In beta decay, the proton number increases by one. For a positron decay and also for an alpha decay (see below), the oblique arrow would go from right to left since in these cases, the proton number decreases.
Since energy is
conserved and since the particles emitted carry away energy, arrows can only go downward (vertically or at an angle) in a decay scheme.
A somewhat more complicated scheme is shown here: the decay of the
nuclide 198Au which can be produced by irradiating natural gold in a
nuclear reactor.
198Au decays via
beta decay to one of two excited states or to the ground state of the
mercury isotope 198Hg. In the figure, mercury is to the right of gold, since the
atomic number of gold is 79, that of mercury is 80. The excited states decay after very short times (2.5 and 23 ps, resp.; 1
picosecond is a millionth of a millionth of a second) to the ground state.
While
excited nuclear states are usually very short lived, decaying almost immediately after a beta decay (see above), the excited state of the
technetium isotope shown here to the right is comparatively long lived. It is therefore called "
metastable" (hence the "m" in
99mTc). It decays to the ground state via gamma decay with a half life of 6 hours.
Here, to the left, we now have an
alpha decay. It is the decay of the element
Polonium discovered by
Marie Curie, with
mass number 210. The
isotope 210Po is the penultimate member of the uranium-radium-
decay series; it decays into a stable
lead-isotope with a half life of 138 days. In almost all cases, the decay is via the emission of an alpha particle of 5.305
MeV. Only in one case of 100000, an alpha particle of lower energy appears; in this case, the decay leads to an excited level of
206Pb, which then decays to the ground state via gamma radiation.
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