Jefferson Lab's Target Group is responsible for the design, construction, and operation of most of the targets used in Jefferson Lab's four experimental halls. These include large "cryotargets" of liquid hydrogen, some capable of dissipating hundreds of watts from the electron beam with little or no discernable boiling. We are also experts in the field of dynamically polarized solid targets, which operate in extremely high magnetic fields and at temperatures less than a degree above absolute zero. Some of the group's current and future targets are described below.
The MOLLER experiment in Hall will measure the parity-violating asymmetry in electron-electron (Møller) scattering to unprecedented accuracy. A key component in this experiment is an 125 cm long target of liquid hydrogen currently being developed by the JLab Target Group. Operating with 70 µA of electron beam current, the target must dissipate 4.5 kW of heat with no discernible boiling and make MOLLER the most powerful liquid hydrogen target ever constructed.
The JLab Target Group has designed and constructed a new dynamically polarized target of protons and deuterons for the CLAS12 detector system in Hall B. The target operates at a temperature one degree above absolute zero and in a magnetic field of 5 tesla. This project is in collaboration with Christopher Newport University, Old Dominion University, and the University of Virginia.
Experiments in Hall C frequently utilize one of Jefferson Lab's two high-power cryotarget systems (the second system is used in Hall A). Each system comprises up to three cryogenic fluids circulating through thin-walled aluminum containers (target cells) and helium-cooled heat exchangers that remove the heat generated when JLab's electron beam interacts with the fluid. Close to one kilowatt of beam heat can be removed with little to no boiling of the fluid. Liquid hydrogen and liquid deuterium are the most frequently utilized target fluids, with helium-three and helium-four also available. The cryotargets also include a wide array of thin, solid foils for background and other ancillary measurements.
The GlueX experiment in Hall D uses a much smaller target of liquid hydrogen. Because the Hall D beam of tagged photons generate little heat in the target liquid, the JLab Target Group designed and constructed a compact system for condensing liquid hydrogen into a 30 cm long target cell made of thin, polyimide film. The target, which uses a pulse-tube cryocooler to condense hydrogen gas into liquid, operates independently of JLab's vast cryogenic facilities, and requires very little user intervention.
The Target Group is currently performing R&D towards the first polarized He-3 target that will operate inside a high field magnetic spectrometer like CLAS12 in Hall B. Polarized He-3 is utilized in scattering experiments because its spin is almost entirely generated by the sole neutron inside its nucleus, thus making it an effective polarized neutron target. We are utilizing Metastability Optical Pumping (MEOP) to polarize the nuclei of low-density He-3 gas atoms in magnetic fields as high as 5 tesla. The atoms will then be cooled close to absolute zero to increase their density for scattering experiments with electron beams. This project is a partnership between the JLab Target Group, MIT, the University of Tennessee, and Shandong University.
