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.
A pair of experiments called "PREX" and "CREX" are currently underway in Hall A using a custom-built target system developed by the JLab Target Group and the experimental collaborations. It features two arrays (or "ladders") of solid target foils such as 208Pb and 48Ca that are mounted to copper frames inside a vacuum-insulated scattering chamber. One frame is oriented in the horizontal plane and is cooled to a temperature of approximately 20 K using ultra-cold helium gas. The second is oriented at an angle of 45 degrees from horizontal and is maintained at room temperature using water as its cooling agent.
The JLab Target Group is developing a new, dynamically polarized target of protons and deuterons for the CLAS12 detector system in Hall B. The target is designed to operate 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.