= Nuclear gamma-ray multipole selection rules
{title2=$|J_i-J_f|\leq L\leq J_i+J_f,\quad L\geq1$}
A real <photon> emitted in <gamma decay> carries a radiation multipole of integer <angular momentum> $L\geq1$. The triangle rule gives the displayed range. An electric multipole $EL$ requires $\pi_f/\pi_i=(-1)^L$, while a magnetic multipole $ML$ requires $\pi_f/\pi_i=(-1)^{L+1}$. There is no real single-photon $E0$ decay; an electric monopole transition can instead proceed through <internal conversion> or, above threshold, pair creation. In the long-wavelength regime $kR\ll1$, increasing $L$ usually strongly suppresses the rate, but nuclear transition matrix elements and collective enhancement can make an allowed lower multipole weaker than a higher one.
Back to article page
