The Deep Virtual Compton Scattering (DVCS) is studied at Jefferson Lab (Newport News, VA, USA) using the CEBAF 6 GeV polarized electron beam and the CLAS detector. One of the aims of the experiment is the measurement of the beam spin asymmetry in the reaction ep->ep + γ. It requires the detection of the photons in a compact electromagnetic calorimeter with high energy and space resolutions. The responsability of the R&D Detection group is the mechanical aspects of the design, fabrication, and also the integration of the electronic and optical equipment.

The calorimeter will be composed of 424 lead tungstate tapered crystals (13 × 16 × 160 mm). It is positionned in CLAS detector at 80 cm from the target. The tapered shape permits the crystals to focus on a particle downstream the beam.

Different array configurations were discussed and physics simulations were performed by the R&D Detection group. GEANT 4 simulations helped in the choice of the support structure by showing the influence of the mechanics on the energy resolution.

The readout is done by an Avalanche Photo Diode (Si APD S8664-55 from Hamamatsu), glued on the back of the crystal and connected to a preamplifier designed by the IPN Orsay electronics department (SEP).

Each crystal is wrapped in VM2000 multilayer polymer.

A prototype (100 crystals) was constructed in 2002 to validate the concepts and tested on beam on October 2003

The R&D Detection group took in charge the mechanical study of the support structure, the thermal cooling and the electronic and optical fiber integrations. For the crystal supports, where a global piling up has to be avoided, different options have been studied. One was the design and construction of 2×2 matrix alveoles in carbon fibers of 200 μm thick. The second one, finally used and validated by the prototype, is composed of aluminium frames stacked one upon each other. Each frame is loaded with a row of crystals on a stainless steel foil (120 μm thick) stretched between the frames sides.

A layer of wrapped scintillator crystals

In order to keep their performances, the PbWO4 crystals and the APDs must operate at a temperature stabilized with a precision of 0.1 °C. The thermal power to evacuate is 120 W. The calorimeter is equipped with copper thermal screens cooled with serpentines in which flows the water coolant at 17.6 °C. The insulation is made with a 12 mm thick styrene foam covered with an aluminized mylar foil. A chiller supplies the cooling circuit with a flow of 2 liters/min. The temperature measurements are performed with 20 Pt100 sensors recorded with a National Instruments data acquisition system. The thermal stabilization was measured to be around 0.05 °C for an external drop of 0.5 °C.

The assembly of the calorimeter in USA was performed with Russian and American teams and took about one month. The crystals were first installed on their support frames. During the crystals stacking, each row of preamplifiers was tested. The calorimeter was installed in front of a solenoid magnet and connected to the CLAS detector set-up. Its position has been controlled and fixed by the survey department with a laser tracker.

The first prototype was successfully tested at the CEBAF beam in 2003. The experiment started on March 2005 and lasted two months. Although there is no direct measure of the resolution on a single particle, an estimation of the resolution could be made with the invariant mass of the two photons coming from the ?0 annihilation. At 1 GeV, the resolution is slightly better than 5 %.