So we know gamma decays are directionally symmetric, but assume we have two detectors and we want to know the likelihood of two γs hitting each detector at the same time as a function of the angle between the line connecting the first detector and the source and the line connecting the second detector and the source. Assume the detectors are equidistant from the source. Obviously, the likelihood has to be related to the size of the detector and to the rate at which the γs are being produced, since if the detectors were infinitesimally sized the only coincidences would be either a result of two separate decays or would occur at [itex]\theta[/itex]=[itex]\pi[/itex] radians. How might we go about doing this?
Check out Perturbed Angular Correlation Spectroscopy (PAC). I'll post a good link when I find one... [edit] Oh, I forgot: When you orient the nuclei, the gamma emissions are not isotropic (directionally symmetric) anymore. But you need very low (milliKelvin) temperatures for that.
To observe PAC you don't necessarily need very low temperatures. That you need only if you want to observe an asymmetry of the uncorrelated emissions. This one is quite good, but you need a bit of background knowledge in condensed matter physics. http://physik2.uni-goettingen.de/research/2_hofs/methods/pac This one give more of the gory details. http://www.ias.ac.in/pramana/v70/p835/fulltext.pdf