Measuring $^{3}$He Cell Density Using a Tunable Laser

Compton scattering experiments using polarized $^{3}$He targets and the High Intensity Gamma Ray Source at the Duke Free Electron Laser Laboratory have been designed to investigate the spin polarizabilities of $^{3}$He and the neutron. The targets will consist of high-pressure $^{3}$He gas in glass cells that has been polarized by spin-exchange with optically pumped rubidium vapor. A precise knowledge of the number density of $^{3}$He in the cells is necessary for the experiments; however, the cells can neither be opened nor contain pressure sensors. The polarization technique requires a small amount of rubidium and N$_{2}$ in the cell along with the $^{3}$He. The width of the D1 and D2 absorption lines in rubidium are linearly related to the density of $^{3}$He gas in the cells. Observing this widening of the absorption spectrum is achieved by using a tunable laser controlled by LabView software and measuring the power transmitted over a range of wavelengths. Preliminary results will be obtained and analyzed before the end of the summer.