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 bmin=0.77 (i.e. a threshold momentum pth(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.
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
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
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
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
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.