In the target station, fast, high-energy neutrons are released by spallation from the target consisting of neutron-rich material, a heavy metal, when a high-energy beam of protons from the accelerator impinges on it. The neutrons, which are travelling at 10 per cent of the speed of light, are then slowed down to roughly the speed of sound, using moderators and reflectors, to provide intense pulses of neutrons at velocities and energies that are useful for experiments, and can be delivered to the instrument suites through beam ports radiating from the target area.
Key features of the target station are the target itself, the neutron moderator, pre-moderator and reflector system, and the beam-extraction system. The target station must also incorporate a powerful cooling system able to absorb the heat generated by the powerful 5 MW proton beam hitting the target. A large amount of radioactive isotopes and radiation generated by the spallation process and by general activation of components. The target will be surrounded by steel shielding in the form of a cylindar 6-m radius target monolith to prevent unwanted ionising radiation escaping into the surrounding environment from the target station.
The baseline for the target design has been one of the major technical decisions for the ESS. the decision to propose a rotating tungsten target was taken by ESS in May 2011, and endorsed by the Steering committee in September 2011. This allowed further detailing of the target station design update (TSDU), including moderators and the beam extraction, but most important, the main parts of the licensing documentation could be completed. The beam extraction must be engineered and the moderators specified during 2012.
A decision on the cooling has been a recent major focus of the efforts for the target Division. Following the Steering committee recommendations, both water cooling and helium cooling have been studied in parallel. A water cooling task force, consisting of experts from partners in our collaboration, was established to look at the key technical and safety issues. A licence to use helium cooling has already been applied for. in February 2012, a baseline conceptual design for helium cooling, including the necessary containment systems and barriers, was evaluated by the TAC as the most viable option.