The radiator

The most suitable radiator for the momentum range to be covered by the HMPID is the liquid perfluorohexane, C₆F₁₄. The index of refraction of C₆F₁₄ at l=175 nm is  n = 1.2988, corresponding to b_min=0.77 (i.e. a threshold momentum p_th(GeV/c) = 1.21 m, with m equal to the particle mass in GeV/c²). 

The liquid radiator containers consist of trays of 1330x413 mm² made of a glass-ceramic material (NEOCERAM), thermally compatible (thermal coefficient 0.5 x 10^-6 °C^-1) with the fused silica plates used as UV transparent windows.

The thickness and dimensions of the tray elements have been carefully studied to reach the best compromise between minimizing the detector total radiation length and maximizing the mechanical strength to withstand the perfluorohexane hydrostatic pressure.

The result is that the quartz window is 5 mm thick, while the NEOCERAM base plate is 4 mm thick. They are strengthened by 27 cylindrical spacers glued between the NEOCERAM bottom plate on one side and the quartz window on the other side. Spacers consist of fused silica rods with a diameter of 10 mm placed in three rows of  nine equi-spaced elements.

Radiator trays are supported by a stiff and light composite panel (less than 0.02 X₀) made out of a sandwich of a 50 mm thick plate of Rohacell 51 (density 0.0513 g/cm³) between two 0.5 mm thin foils of aluminium.

The composite panel for the HMPID radiator trays 

The radiator trays of the HMPID proto-2

Radiator connections to the external liquid circuit are made via stainless steel bellows glued to the inlet and outlet points. The inlet (lowest point) and outlet (highest point) are located in opposite corners of the NEOCERAM tray. A liquid circulation system is required to purify bulk C₆F₁₄, fill, recirculate, and empty the 21 radiator trays independently, remotely and safely. Because of the inaccessibility of the detector during running and the fragility of the radiator trays, a system based on the gravity flow principle has been chosen, owing to its safe nature.  Since C₆F₁₄ is not available in a high-purity grade form, filters are implemented in the circulation system in order to remove contaminants (mainly dissolved water and oxygen) and achieve the best transparency in the UV region where the RICH detector operates.