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Multipolarity of gamma radiation

Electric dipole field lines

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. In particular, these are called electric and magnetic multipole radiations. (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: 2^$l$pole radiation).

Similarly, magnetic dipole, quadrupole, octupole… radiation is designated as M1, M2, M3,… radiation (generally: 2^$l$pole radiation).

In quantum mechanics, angular momentum is quantized. The various multipole fields have particular values of angular momentum: E$l$ radiation carries an angular momentum $l$ in units of $\hbar$; likewise, M$l$ radiation carries an angular momentum $l$ in units of $\hbar$. Since angular momentum is conserved, this 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.

Quantum numbers and selection rules

Simplified decay scheme of 60Co, with angular momenta and parities shown

A state of a nuclide is described by its energy above the ground state, by its angular momentum J (in units of $\hbar$), and by its parity, i.e., its behaviour under reflection. Since the spin of nucleons is 1/2 (in units of $\hbar$), and since orbital angular momentum has integral values, J may be an integral or half integral number.

Electric and magnetic multipole radiations of the same order $l$ (i.e., dipole, or quadrupole...) carry the same angular momentum $l$ (in units of $\hbar$), but differ in parity. The following relations hold:

Parity of electric multipole radiation= (-1)^$l$

Parity of magnetic multipole radiation= -(-1)^$l$

An example: in the simplified decay scheme of ⁶⁰Co to the right, the angular momenta and the parities of the various states are shown (A + sign means even parity, a - 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.