Optical Pumping and Polarization
In order for these measurements to be performed the radionuclide beam must be spin-polarized, and for this the laser spectroscopy beamline is used. First, the ion beam is neutralized by charge exchange in a sodium vapour cell. A magnetic field of ~10 G is applied to the beamline by spaced rings of electric coils in order to dominate perturbations by ambient geomagnetic fields and provide a defined spin-axis. This field splits the degeneracy of the electron-orbital hyperfine states by the Zeeman effect, giving each axial angular momentum eigenstate its own slightly-shifted energy level. A 671 nm dye laser, with which the beam has been Doppler-tuned to precise resonance, is then used to excite the atomic valence electron up to its P1/2 orbital. The laser is passed through an electro-optical modulator that adds frequency sidebands at ±382 MHz, which allows both hyperfine levels of the ground state to be excited at once. Because the laser is circularly polarized, each photon absorbed by an atom adds one unit ħ to its axial angular momentum. When the excited state decays, on the other hand, it is equally likely to gain or lose a unit of angular momentum. Therefore, exciting and de-exciting every atom many times while in flight will result in the majority of them ending up near the highest-magnitude axial angular momentum eigenstate. This is known as optical pumping. The helicity of the laser beam determines whether this final state is aligned or anti-aligned with the applied magnetic field. Inside the atom, the angular momentum of the valence electron in turn creates a strong magnetic field which polarizes the spin-state of the nucleus to align or anti-align with the pre-defined spin axis. This nuclear polarization remains while the beam is re-ionized in a helium gas cell and then directed for implantation into the β-NQR sample.