The high luminosity antiproton beam which will be delivered by FAIR (Facility for Antiproton and Ion Research) at GSI (Darmstadt, Germany) will open a large field of hadronic physics measurements. The PANDA detector is an internal target detector which allows high resolution spectroscopy of neutral and charge particles on a nearly 4π coverage. It consists of a target spectrometer surrounding the interaction region and of a forward spectrometer with a second magnet for the coverage in the most forward angles.

The PANDA detector comprises an electromagnetic calorimeter composed of 3 parts: the "barrel" (2.5 m long, 0.57 m radius, 11 520 crystals), the forward and backward "endcaps" (816 and 6864 crystals). The crystals are in lead tungstate (PbWO4) cut in truncated pyramid shapes (front face 21×21 mm², 200 mm long, around 1 kg each). Each crystal is wrapped with a reflective multilayer polymer (ESR). The photons are detected with a large avalanche photodiode (APD of 1 cm² from Hamamatsu), glued on the crystal back face and connected to a charge preamplifier. The R&D Detector Department (RDD) took in charge the mechanical and thermal R&D for the design of the calorimeter and its full integration. The preliminary concepts are detailed in the technical proposal report.

The crystal support concept is based on the CMS and Babar experiments. The crystals are inserted by pack of 4 in carbon alveoles of 180 μm thickness for a total distance between crystals of 600 μm. The trapezoidal alveoles are made precisely (20 μm tolerance) in an aluminium mould. Their good rigidity allows holding them by the back extremity with an aluminium insert and then avoiding materials in front of crystals. The design was validated by mechanical simulations carried out with SAMCEF and prototypes. The electronics preamplifier is integrated in the insert screwed on the cooled back plate.

Simulation of a deformation

In order to reach the performances required for PANDA, the PbWO4 crystal and the APDs must be cooled at -25°C and stabilized with a precision of
+/- 0.1°C. These constraints lead to a mechanical design close to a cryostat for which the main requirements are:
- Consideration of the thermal expansion.
- Research on thermal screens of low thickness (vacuum screen).
- Optimisation of the electronic integration and cables sections.
- Running under dry gas to avoid frost.

During the R&D phase, a prototype was developed to validate the analytical model and the computer CFD analysis results (with the Flotherm software). The cooling is performed by serpentines placed around the set-up. They are supplied by a 500 W cryostat. The temperatures are measured with an Agilent data acquisition system equipped with thermocouples and Pt100 sensors.

In order to assess the behaviour of the barrel calorimeter and to validate the principles developed during the R&D phase, a representative part of the barrel composed of 60 crystals has been designed in detail in 2006 and realize in 2007.

This system has been done in collaboration between the IPN Orsay for mechanical design and construction, for the integration and cooling, the Giessen University for the APD and crystals, the Basel University for the electronic preamplifiers, and the KVI (Gröningen) for the optical fiber calibration device.

This design made with CATIA CAD includes PbWO4 crystals wrapped with ESR reflectors and enclosed in carbon alveoles. They rely on a horizontal aluminium base plate to allow an easy access from the back for maintenance. The preamplifiers are connected to the APDs and are linked to the outside by a printed circuit board which drives all the power supply and signals. The laser optical fibers are inserted by the back and inject light in the crystal through a special hole done in the insert. All these equipments are surrounded by thermal screens made of copper plates and serpentines in which circulates Syltherm cooling at -25°C stabilized at +/- 0.1°C and by an insulation box made of 40 mm of foam and 5 mm of plastic plates for nitrogen sealing (to avoid any humidity). This setup is connected to many devices as for example, the Julabo chiller, the high voltage unit or the data acquisition system.