What is HADES?
The High Acceptance Di-Electron Spectrometer HADES is a detection device for lepton pairs in high resolution experiments investigating in-medium properties of vector mesons in hadronic and nuclear reactions. At present, this spectrometer becomes operationally at the Heavy Ion Synchrotron SIS at GSI (Gesellschaft für Schwerionenforschung) by a European collaboration involving groups from 19 universities and institutions in 10 countries.
The HADES detector is specifically designed to provide excellent mass resolution (δm/m=1 %) and a very large acceptance for comprehensive studies of the behaviour of ρ, ω and Φ mesons in the nuclear medium. These mesons are produced in nucleus nucleus (AA), proton-nucleus (pA) and pion-nucleus (πA) collisions at a few GeV/nucleon. They may partially decay within the nuclear fireball. Because of their weak final state interaction dilepton decay channels are suitable probes to study medium effects on these mesons, such as effective masses, decay widths etc. Also their fundamental electromagnetic properties are accessible.
Fig. 1a: HADES spectrometer (artist's view)
Fig. 1b: side view of HADES
HADES Drift Chambers
Fig. 2: schematic placement of the anode frames
inside a chamber module
The HADES tracking system consists of four sets of drift chambers (I-IV), two in front and two behind the field area of the super conducting toroidal magnet. Each set is composed of six trapezoidal modules and has the form of a frustum (see Fig. 1). A module covers 60° of azimuthal angle. There sizes range from 0.88 × 0.8 m2 to 2.8×2.3 m2 (height x larger baseline). A drift chamber module is composed of six drift cell layers, each consisting of a sandwich, made from two cathode frames and one anode frame in between. In Fig. 2 only these anode frames are shown. The drift cell sizes vary from 5×5 mm2 to 14×10 mm2 from detector I to IV to achieve a constant granularity. These cells are formed by interspersed sense and field wires in the anode frames and cathode wire in the cathode frames. The total number of drift cells is 27.000.
A wealth of information on the HADES driftchambers can be found on the HADES-MDC homepage at GSI Darmstadt, where the entire MDC project is coordinated. See the notes in the documentation pool. E.g. particularly helpful for orientation purposes are notes 3/98 (MDC reference system) and 4/98 (number of readout channels). The MDC project is a common effort of four different institutions:
|GSI Darmstadt, Germany
||JINR/LHE Dubna, Russia
||HZ Dresden-Rossendorf, Germany
||IPN Orsay, France
Rossendorf Drift Chambers MDC III
Fig. 3: current characteristic of the MDC III prototype
Fig. 4: counting characteristic of the MDC III prototype
|The final design of large trapezoidal low-mass drift chambers of the HADES tracking system has been developed at the detector laboratory of the Helmholtz-Zentrum Dresden-Rossendorf. These drift chambers build the second largest tracking plane (MDC III) of the spectrometer, which consists of 6 modules with an active area of roughly 2.2x2.0 m2 for each plane and a drift cell size of 8×12 mm2. Each module has 6 drift cell layers, i.e. 6 anode and 7 cathode frames. The anode frames contain alternately signal wires (tungsten gold coated, Ø=20 μm) and field wires (aluminium, Ø =100 μm) with a distance of 6 mm from each other. The wires in the 6 anode frames have orientations of -40°, +20°, -0°, +0°, -20° and +40°, respectively, whereas, the wires in the cathode frames (aluminium, Ø =80 μm) have always the same orientation +90° and a distance of 3 mm.
The Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, delivers one of the two large planes of drift chambers to the HADES experiments. Over the last years we have fixed all physical and technical problems of the design and production of the MDC III. The large scale production of the cathode- and anode-frames has started in summer 1999 and the first full equipped module was available for the HADES beam time in autumn 2000. A full size prototype was finished in February 2000, consisting of only one anode frame (orientation +0°) between two cathode frames and in addition the other 10 frames without wires. Thus, the total volume of this prototype (150 l) matches the volume of a complete MDC III layer. A mixture of helium and iso-butan (60 % He, 40 % i-C4H10) was used for test measurements, which should be applied in real experiments.
The results of first measurements are depicted in the following figures.
Fig. 3 shows the current versus the high voltage. It is obvious to see, that external light sources (e.g. fluorescent tubes) dramatically reduce the break down voltage, due to the emission of photo electrons from the Al wires.
The counting characteristic of the prototype is depicted in Fig. 4. A 90Sr source (50 kBq) was used to deliver electrons, which are additionally detected in a master scintillator telescope (2×2 cm2), a 1 mm thick scintillator in front of the prototype and a 5 mm thick scintillator behind. The signal from the anodes were amplified by a factor of 50 with a fast current sensitive preamplifier (I/U converter, ΔUa/ΔI0=500 mV/10 μA; Sr=4100 V/μs) and further processed with NIM- and CAMAC modules. Up to a high voltage of -2600 V the prototype works without break through. A large counting plateau of 500 V was found above -2100 V. In this region a counting efficiency of 99.9 % was reached.
In 1998 the HZDR hadron physics group participated in the preparation work for the first physics proposal for HADES. Initial studies using pp elastic scattering will be followed by electron pair detection in pp, pA and AA collisions. The emphasis will be on the low invariant mass region (100 MeV/c2 < Mee < 500 MeV/c2), where background contributions by η and Δ Dalitz decays have to be understood in detail.
People working on the HADES experiment of the Institute for Nuclear and Hadron Physics at Helmholtz-Zentrum Dresden-Rossendorf
created by Frank Dohrmann, 28-Jun-1999
changed by: Rugard Dressler, 14-Apr-2000
last update: R. Kotte, 16-Dec-2003