Transitions between excited states (or excited states and the ground state) of a nuclide lead to the emission of gamma quanta. These can be classified by their multipolarity.[1] There are two kinds: electric and magnetic multipole radiation. Each of these, being electromagnetic radiation, consists of an electric and a magnetic field.
See main article: Multipole radiation. Electric dipole, quadrupole, octupole… radiation (generally: 2
\ell
\ell
Similarly, magnetic dipole, quadrupole, octupole… radiation (generally: 2
\ell
\ell
There is no monopole radiation (
\ell=0
In quantum mechanics, angular momentum is quantized. The various multipole fields have particular values of angular momentum: E
\ell
\ell
\hbar
\ell
\ell
\hbar
To make a simple classical comparison, consider the figure of the oscillating dipole. It produces electric field lines travelling outwards, intertwined with magnetic field lines, according to Maxwell's equations. This system of field lines then corresponds to that of E1 radiation. Similar considerations hold for oscillating electric or magnetic multipoles of higher order.
Conversely, it is plausible that the multipolarity of radiation can be determined from the angular distribution of the emitted radiation.
A state of a nuclide is described by its energy above the ground state, by its angular momentum J (in units of
\hbar
\hbar
Electric and magnetic multipole radiations of the same order
\ell
\ell
\hbar
\ell>0
Electric multipole radiation: Parity
=(-1)\ell
Here, the electric field has parity
=(-1)\ell
(-1)\ell
Magnetic multipole radiation: Parity
=(-1)\ell+1
Here, the electric field has parity
=(-1)\ell+1
(-1)\ell+1
The designation "electric multipole radiation" seems appropriate since the major part of that radiation is produced by the charge density in the source; conversely, the "magnetic multipole radiation" is mainly due to the current density of the source.
In electric multipole radiation, the electric field has a radial component; in magnetic multipole radiation, the magnetic field has a radial component.
An example: in the simplified decay scheme of 60Co above, the angular momenta and the parities of the various states are shown (A plus sign means even parity, a minus sign means odd parity). Consider the 1.33 MeV transition to the ground state. Clearly, this must carry away an angular momentum of 2, without change of parity. It is therefore an E2 transition. The case of the 1.17 MeV transition is a bit more complex: going from J = 4 to J = 2, all values of angular momentum from 2 to 6 could be emitted. But in practice, the smallest values are most likely, so it is also a quadrupole transition, and it is E2 since there is no parity change.
\ell