Gamma ray coincidence and multipoles

[miss_mayhem]

Dear Physics Forums,

I am currently conducting an experiment on gamma ray coincidence from Co-60. Co-60 decays to an excited state, then de-excited by emitting two gamma rays. The aim is to deduce the angular momentum L of the first excited state.

Apparently L indicates the type of multipole that the first state is. I can't seem to find any information on this, but what I know so far is that L=1 is a multipole, L=2 is a quadrupole etc.. But I do not understand how this is calculated from L. Is it something to do with spin? I'm currently imagining that spin degeneracy is 2 for a photon, and so if L=2 then 2x2=4 which tells us that this state is quadrupole. I'm a bit fuzzy on this.

Also, I am not entirely sure what 'multipole' is meant to mean - something to do with magnetic moment? I would be extremely grateful for a brief explanation of this, I hope I'm not being too demanding!

Thanks guys!

 

[fzero]

Multipole in this context refers to the Fourier expansion of an arbitrary function into spherical harmonics.

f(\theta,\phi) = \sum_{\ell,m} c_{\ell, m } Y_{\ell m} (\theta,\phi).

If we were to expand a dipole interaction in this way, we'd find that only the \ell=1 term contributes. A quadrapole corresponds to the \ell =2 term, etc. So this terminology is used to refer to the moments that let us calculate the

c_{\ell,m}\sim \int d\Omega Y^*_{\ell m} f(\theta,\phi).