In experimental nuclear physics, often your experiment
is only as good as your target. Especially at low
energies, solid targets suffer from problems such as
substantial energy loss effects and background effects
deriving from the backing material of thin targets.
Static gas cells have been used as alternatives to
alleviate these problems, but at very low energies
energy losses through the cell foil, in addition to
energy and angle straggling, preclude the use of such
cells. A windowless high density gas jet has none of
these problems, and allows high precision measurements
of cross sections and polarization observeables at low
energies.
At TUNL, we now have such a target. We acquired the jet-target that was developed and used at the University of Erlangen-Nürenberg during the 70's and 80's. At the time, it produced targets with some of the highest densities reported by such a device. We have been refurbishing and upgrading it for the past few years.
The target consists of a high-precision scattering chamber as well as the necessary apparatus to pump, recycle, clean, and compress the target gas. Reported thicknesses achieved with this system are 15 μg/cm2 for hydrogen, 0.5 mg/cm2 for nitrogen, and 1.4 mg/cm2 for argon. While in operation, the input gas (after forming the target) is differentially pumped by three pumping stages consisting of combinations of roots and auxiliary pumps. The system is also pumped by a 6000-l/s oil diffusion pump, which keeps the beam line pressure at 10-4 to 10-6 mbar. Currently the exhausts of the roots and auxiliary pumps are vented outside the building. The system has been designed to be transported relatively easily despite its large weight (around 7 tons) since it can be broken down into 4 pieces. This will allow for the target to be used at both the Low-Energy Beam Facility as well as with the FN tandem accelerator.
We are designing and building a gas-recylcing system which will allow the exhaust to be recompressed and then cleaned. The exhausts of the pumps will be recompressed by a two-stage compressore. The gas is then cleaned by a filter chain that should result in a purity of better than 1%.
