Characterizing Helium-3 Nuclear Spin Relaxation in Vessels of Glass and Metal

Historically, high-pressure gaseous polarized Helium-3 targets at Jefferson Lab (JLab) have utilized “target cells” fabricated entirely out of glass. With the 12 GeV upgrade nearing completion, experiments requiring significantly higher luminosities are planned, and to meet the challenge, it is desirable to produce metal end windows through which the electrons will enter and exit the target cell. The polarization technique used in 3He targets at Jefferson Lab utilizes spin-exchange collisions between 3He atoms and alkali-metal atoms polarized through optical pumping. Unfortunately, relatively few studies have investigated the spin relaxation of nuclear-polarized noble gases on metal surfaces, particularly under the conditions in our targets. We have tested various cells incorporating both glass and metal and have found that acceptable spin-relaxation rates can be obtained by electroplating gold coatings on OFHC copper substrates. Initial tests using titanium substrates, an alternative material for end windows, have not yet been as successful as copper ones. Various types of glass (borosilicate glass, aluminosilicate glass, and uranium glass) were used in the construction of these cells. These studies have produced multiple cells that demonstrate the viability of 3He targets incorporating metal windows into their design.