The CsI photodetector

The photodetector, filled with pure methane at ambient temperature and pressure, is closed on one side by an end-flange where six independent CsI photocathode boards of size 64x40 cm², segmented into pads of 8x8.4 mm², are installed. On the opposite side, a honeycomb panel supports three C₆F₁₄ radiator vessels placed at a distance of 80 mm from the photocathode plane. The gas tightness is ensured by soft O-rings placed in grooves in the chamber frames.  

A view of the HMPID module-1 during HV tests

The assembly procedure of module elements ensures the wire chamber gap (2 mm anode–cathode distance) with a precision of 50 mm and the parallelism between the radiator trays and the photon detector to better than 100 mm. The MWPC anode plane is made of gold-plated tungsten–rhenium wires of 20 mm diameter, with 4.2 mm pitch, soldered on a G-10 printed board with a precision of 0.1 mm and a tension of 47 g. On both edges of the anode plane, thicker guard wires are installed to resist the boundary discontinuity of the electrostatic field. At half-length of the module an insulating support structure is placed between the pad cathode and the anode plane in order to minimize the sag of the sensing wires due to the electrostatic force.

Cathode and collection wire planes are made out of 100 mm diameter gold-plated Cu/Be wires, stretched at 200 and 50 g/wire, respectively, by using crimping pins. A positive voltage of 2050 V applied to the anodes, while cathodes are grounded, provides a total gas gain of 8x10⁴, allowing the detection of ionizing particles and Cherenkov photons with single e^- detection efficiency of 90%.  

The anode wires and the 
Macor support lines

The pad cathode is built by gluing together two printed circuit boards to achieve a stiff and flat plane made out of light materials. The photosensitive layer is obtained by depositing 300 nm of CsI onto the Cu-clad printed circuit boards, electrolytically coated by 7 mm thick Ni layer followed by a 0.5 mm thick Au layer and accurately pre-polished through mechanical and chemical treatments.  

The photocathode pad plane

The photocathodes are prepared at CERN in the large evaporation station, equipped with four DC heated tungsten crucibles operated simultaneously to achieve a uniform CsI layer.

The CsI photocathode coating station