Several phenomena may be studied if nuclei are orientated in external or atomic fields at very low temperatures. We will discuss here the influence of such orientation on the/~-and y-rays emitted by radioactive nuclei. In the first place, the directional distribution of the emitted radiation may differ from spherical symmetry once the nuclei are orientated. Another phenomenon is that the radiation may show polarization phenomena. Both effects may occur for/~-as well as for y-radiation.
In certain cases we can readily show by simple arguments that the E-rays emitted by orientated nuclei may be polarized. Assuming non-relativistic energies for the emitted electron, the orbital and spin angular momentum of the electron can be considered separately. We know that for allowed transitions the electron is emitted without orbital angular momentum, while the neutrino takes the angular momentum ~. If the initial nuclei are completely orientated (ml = i~; ]'i and ~' I characterize the nuclear spin of the initial and final nucleus respectively), it follows from the law of conservation of angular momentum that the /~-rays must be totally polarized if ~' i = it + 1 (cf. Fig. ). We have given a quantitative treatment of the polarization of E-rays in allowed transitions in an earlier paper 1), and showed further that the directional distribution of the /~-radiation remains spherically symmetric for allowed transitions 1). However, for certain forbidden transitions the directional distribution may differ from spherical symmetry (namely those forbidden transitions for which also /~--y directional correlation may occur if a y-ray exists 2)). I