A new liquid hydrogen target will be installed as part of the spallation programme at the GSI research centre near
Experiments such as Spaladin choose to accelerate heavy nuclei in the form of ions, and make them interact with hydrogen nuclei to obtain a spallation reaction. The liquid hydrogen is contained in a target that the heavy ion beam strikes. Parasite reaction between the heavy ions and the hydrogen chamber materials – windows – must be limited to the absolute minimum. The physicists have chosen to reduce the thickness of the target and the windows, whilst maintaining an adequate interaction rate.
Dapnia's personnel have perfected a new liquid hydrogen target 3 mm in thickness delimited by ultra-thin windows made of aluminised mylar 6 microns in thickness and 25 mm in diameter. The target is in a vacuum, and this makes it necessary to work with a very low hydrogen pressure of 150 hPa which corresponds to a temperature of 15.2 K.
The design of the target's filling and draining system is original. The buffer volume that contains the gas is deformable and this allows this pressure difference to be maintained, whatever the state of the system (initial vacuum, liquefaction, operation, return to atmospheric pressure).
To control the operation of this new target, an electronic and computerised control system is used. It manages the movement of the cryostat so as to move the different targets into the beam's axis: Two "liquid" targets and two "ghost" targets fixed on the lower part of the thermal shield, and an "empty" position. This system also regulates the temperature of the cryogenerator with a stability of 0.01 K. Finally, it manages the alarms and security of the installation in the event of faults (vacuum, hydrogen pressure, loss of temperature regulation or a broken window)
The new target will be used in the next Spaladin experiments that have been accepted by GSI's scientific recommendation committee, and later in the R3B project on the future FAIR machine.
