Past exam of the particle and nuclear physics course of the University of Cambridge / 2014 / Paper 1 / 4 / b

The symbols $EL$ and $ML$ mean electric and magnetic radiation multipoles of order $L$. The orders $L=1,2,3$ are called dipole, quadrupole and octupole. They describe the angular structure of charge/current radiation and the total angular momentum carried by the radiation; they do not change the photon's intrinsic spin from one. A real single photon has no monopole radiation mode.

An electric multipole has radiation parity $(-1{)}^L$ and a magnetic multipole parity $(-1{)}^{L+1}$. The radiation quantum numbers are thereforeEquivalently, an $EL$ transition requires ${\pi }_f/{\pi }_i=(-1{)}^L$, while an $ML$ transition requires ${\pi }_f/{\pi }_i=(-1{)}^{L+1}$.

For comparable transition energies and ordinary single-particle nuclear strengths in the long-wavelength regime, the usual qualitative ordering is $E1$, $M1$, $E2$, $M2$, $E3$, $M3$ from faster to slower. Each increase of multipole order is strongly suppressed, and magnetic radiation is usually weaker than electric radiation of the same order. More precisely,The nuclear reduced transition strength $B$ is essential: collective enhancement, cancellation and selection rules can overturn this benchmark. It is not a universal ranking between arbitrary transitions of different energies. The weak measured M3 branch is consistent with high-multipole suppression, but its absolute lifetime cannot be obtained from its arbitrary-unit intensity alone.^[ref]