The cluster-jet target

For current experiments involving accelerator beams, which require high event rates and precise measurements of the resulting particles and detector responses, cluster-jet targets are ideally suited as internal targets with densities of 10¹⁴ – 10¹⁵ atoms/cm². Examples include the MAGIX experiment in Mainz, the KOALA experiment at GSI and the future PANDA experiment, in which the Khoukaz research group is involved.

A cluster is defined as a collection of several tens to thousands of atoms or molecules held together by van der Waals forces. Hydrogen gas is generally used to produce the cluster jets, thereby effectively providing a proton target for a wide variety of experiments. This gas is forced through a Laval nozzle at a pressure of approximately 18 bar. Both the Laval nozzle and the gas are cooled to temperatures of around 20–30 K. Temperature, pressure and the design of the nozzle are responsible for the formation of the clusters. However, as only a portion of the hydrogen gas forms clusters, the cluster jet is separated from the residual gas by an aperture known as the skimmer. The final shape of the cluster jet is defined by a second aperture, the collimator.

The Münster cluster-jet targets can be divided into three main parts. The first part, the cluster source, contains the nozzle, the skimmer and the collimator. The scattering chamber represents the subsequent point of interaction with an accelerator beam. It is precisely at this point that the cluster beam should have the highest possible density. At the end of the assembly, the cluster beam reaches a beam dump. There, the clusters break apart and are extracted to prevent them from being reflected back into the scattering chamber.

The Bachelor’s and Master’s students and PhD candidates working on the Münster cluster-jet targets are not only involved in the design and construction of the targets, but are also working on the continuous optimisation of the entire system. This includes optimising the setup, the vacuum system and the characteristics of the cluster beam, as well as developing a wide variety of programmes to analyse the properties of the cluster beam. Some of this work takes place at various particle accelerators.

The investigation of the properties of the cluster beam, in particular its density within the scattering chamber, is especially important for its use in accelerator experiments. To determine the density, key parameters such as the extent and velocity of the cluster beam are measured. There are two different methods for determining the cluster velocity. Firstly, the clusters are ionised by a pulsed electron gun and detected at the end of the drift path. The time between ionisation and detection is measured, so that the velocity of the cluster beam can be calculated given the known flight distance. Secondly, the velocity can be determined optically from the flight path of the clusters on a shadowgraph image, on which they are illuminated by a laser.