Carbon Fibre Spherical Mirrors for the LHCb RICH 1 Detector

1
Carbon Fibre Spherical Mirrors for the LHCb RICH
1 Detector
Kenneth Lessnoff
The Reflective Coating.
Radiation Hardness and Resistance to C4F10.
The Need for a RICH Detector.
The RICH, or Ring Imaging Cherenkov, detectors
play a crucial role in the LHCbs ability to
distinguish between different particles. A
charged particle travelling through a given
medium, or radiator, faster than the speed of
light in that medium emits Cherenkov radiation.
The angle at which it is emitted depends on the
particles velocity, information which, combined
with a knowledge of the particles momentum,
allows
RICH 1 uses a C4F10 radiator it is important,
therefore, that the mirrors do not degrade in
quality in C4F10. Similarly, the mirrors must be
able to survive the high radiation environment.


The spherical mirrors need a reflectivity of gt
85 in the wavelength range 250-600nm and gt 70
in the range 200-250nm. This has been achieved
with a coating of aluminium, protected by a layer
of MgF2. Applying a suitable coating is not an
easy procedure and reflectivity measurements had
to be made on many different samples before we
could be confident the mirrors would receive an
suitable coating. Early coatings had poor
reflectivities due to impurities in the coating
due to a slowness of the coating evaporation, as
well as insufficiently thick coatings.
The two mirrors used for aging tests.
The effect of exposure to radiation and C4F10
was tested on two small spherical carbon fibre
mirrors.
it to be identified. An example of the use of
particle identification is shown to the right, in
reducing background to Bd0-gtp p decays. There
are two RICH detectors using a total of three
radiators to identify particles over a momentum
range of 1 to 100GeV/c.
The Spherical Mirrors
Cherenkov light emitted in the RICH 1 detector is
reflected via a series of mirrors onto photon
detectors. Spherical mirrors are used to focus
the light. The mirrors must be light weight,
because of constraints on the material budget,
and have a high reflectivity. Carbon fibre
mirrors meet these criteria.
Before and after various exposures to C4F10 and
radiation, measurements were made of the radius of
Above reflectivities of early coatings. The
drop at low wavelengths is due to absorption from
unwanted oxygen in the sample. Below
reflectivity of the final coating.
curvature, reflectivity, surface roughness,
mechanical properties and D0, the diameter of the
circle into which 95 of the light intensity is
reflected. No changes have been observed after
10kGy of radiation exposure or 11 months in C4F10.
Schematic of RICH 1