The tungsten target and moderator-reflector are surrounded by a radiation shielding system of 6,000 tonnes of steel in order to contain the extreme level of highly penetrating gamma and fast neutron radiations created in the target and its vicinity. The beam extraction system provides intense slow neutron beams through beam tubes going across the target shielding. At the surface of the shielding, the neutrons are delivered to be used at the neutron scattering instruments. The proton beam window separates the high vacuum in the accelerator from the inert Helium gas. All of these systems sit inside a large vessel. Together they form the Target Station monolith, a large cylinder 8 m high and 6 m across.
A rotating tungsten wheel is the baseline option for the target, which distributes the irradiation over a large volume of target material. After periods of operation the target emits significant amounts of ‘after-heat’, gradually decaying in time. The tungsten wheel is a new technology for spallation sources, none of the established target designs being adequate for the higher power level of ESS.
The ESS Target Station will contain two liquid-hydrogen moderators with a volume of about 2.5 l each, partially surrounded by water pre-moderators of comparable volume. The moderators are placed inside an inner reflector of about 1 m3 of beryllium. These components will be kept at their desired operational temperature by dedicated cooling systems. These systems will not emit significant after-heat.
The beam-extraction system will consist of more than 40 beam tubes arranged in four sectors with a 60 to 65° horizontal angular spread. Each beam tube will be equipped with a beam shutter within the target monolith to assure that the residual radiation escaping through the closed beam line when the target station is not in operation is reduced to safe working levels at the wall of the target monolith. This monolith will be surrounded by a combination of integrated and individual radiation shielding for each beam line, which guarantees safe working access to the areas outside of these shielding structures all the time, including during full power operation.