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Multipolarity of gamma radiation
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, of course).
Electric dipole, quadrupole, octupole… radiation is also designated as E1, E2, E3,… radiation. (generally: 2pole radiation).
Similarly, magnetic dipole, quadrupole, octupole… radiation is designated as M1, M2, M3,… radiation (generally: 2pole radiation).
There is no monopole radiation ()[1].
In quantum mechanics, angular momentum is quantized. The various multipole fields have particular values of angular momentum: E radiation carries an angular momentum in units of ; likewise, M radiation carries an angular momentum in units of . The conservation of angular momentum leads to selection rules, i.e., rules defining which multipoles may or may not be emitted in particular transitions.
To make a simple classical comparison, consider the figure, and imagine the electric dipole to oscillate. This would produce electric field lines travelling outwards, intertwined with magnetic field lines, according to Maxwell's equation. 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.