Measurements with the hellhound
First use of the CERBEROS detector setup
At the entrance of Hades, the Greek underworld, three-headed hellhound Cerberus guards the coming and going. So far the HADES experiment at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt had to get by without a watchdog, but this has changed since May 2014. The new measurement system CERBEROS was commissioned and is able to exactly detect pion particles. It is also in use in the currently running beamtime.
At HADES (High-Acceptance Di-Electron Spectrometer) scientists analyse the collisions of accelerated particles onto a target material. They are especially interested in so called pions. They belong to the group of mesons, consisting of a quark and an antiquark. As they don't naturally occur on earth, but only in space, they have to be produced artificially. For that purpose the scientists accelerate nitrogen with the GSI accelerators and collide it with long rods of beryllium. In the passage pions are produced and can be sorted and focused with magnets. Thanks to the upgrade of the GSI ring accelerator for FAIR pions can now be produced in large numbers for the experiments. Inside the HADES setup they are collided with other particles to produce even rarer exotic particles.
"The momentum of the pions is crucial in this collision", explains professor Laura Fabbietti from the Technical University Munich, who built up the CERBEROS system. "We have to determine the momentum with a precision of one per mill. This is why we have developed CERBEROS. It has three heads, like the hellhound. And it's positioned before HADES, so we chose the name." The three heads of the system are three detector components at different places: a diamond counter and two semiconductor detectors made from silicium, one at the pion production and one at the HADES setup.
The electronics connected to the semiconductor detectors is something special. Usually a detector requires an external signal to tell when an interesting event takes place and it should start the measurements. This is called "triggering". A novel microchip called n-XYTER built into CERBEROS is able to trigger itself. It decides wether an event is relevant or uninteresting and only measures the important events. It is combined with the readout electronics TRB3, which is very fast and precise. The diamond counter consists of an area of approximately 200 square millimeters and nine monocrystalline diamonds, which are very difficult to produce. All components have been developed by researches from GSI's detector laboratory and experiment electronics departments in cooperation with different universities.
It took three years to assemble CERBEROS. In Mai 2014 the system was ready and commissioned in a beamtime at the GSI accelerator. The successful measurements are now continued in the current experiments in July and August. As the concept of self-triggering readout electronics in not limited to the measurement of pions, but very flexible, parts of the system will also be used at the future international FAIR facility, e. g. in the experiments with antiprotons PANDA and with compressed nuclear matter CBM. FAIR is currently under construction and will be connected to the existing GSI accelerators. Also research facilities in Russia and Japan have already signaled their interest in the system.