8.5.4 1/15
8.5.4. The downstream Cherenkov
The rejection of electrons will be provided by the downstream aerogel
Cherenkov detector. In principle, this detector should not be affected by background from
the RF cavities, as x-ray energies will rarely be high enough to produce electrons that
give Cherenkov light in the aerogel (i.e. above 2 MeV). The RF noise at 201 MHz has a
skin depth of 6 m in aluminium. However, a ferromagnetic shielding is however
necessary to be protected it against the stray magnetic field.
8.5.4.1 Description and Performance
In the momentum range of interest to MICE, aerogel (1.01 < n < 1.06) appears to
be the only adequate radiator from which to build a threshold Cherenkov blind to the
passage of muons. The choice of the appropriate index of refraction for the radiator is
governed by the relative light yields of electrons and muons, and their respective
detection efficiencies assuming a fixed detection threshold. The goal is to maximize the
response to electrons while minimizing possible contributions from higher energy muons.
The cylindrical radiator is made of aerogel with an index of refraction n = 1.03 and a total
thickness of 10 cm. The diameter of the radiator is about 80 cm. This geometrical
aperture is deliberately chosen to be sufficiently large as to avoid losses of good muons.
The upstream face of the aerogel radiator will be located at z= 70 cm from the
last correction coil and the downstream face of the Cherenkov vessel at z= 116.7 cm. The
useful transverse size has been evaluated on the basis of GEANT4 simulations (Figure 846). The simulations take into account the presence of a thick magnetic shield.
Figure 8.5-11. The beam spot of good muons before and after the CKOV2 detector. The
red circle is the outline of the aerogel radiator, and the green circle represents the useful
(free) aperture at the particle exit window.
8.5.4 2/15
1000
N
Electrons Abs2
Electrons Tr1
Electrons Tr2
100
Muons
10
Theta (degrees)
1
0
10
20
30
40
50
60
70
Figure 8.5-12 The angular divergence with respect to the beam axis for electrons and
good muons entering the CKOV2 detectors. The various electron distributions
correspond to muons decays at different positions along the beam axis: in the second
absorber (Abs2), in the first tracker (Tr1) and in the second tracker (Tr2).
The angular divergence of about 30 degrees of the particles entering the CKOV2
detector essentially affects the optical acceptance of the light collection setup.
8.5.4 3/15
8.5.4.2 Mechanical design
Figure 8.5-13. Transverse cut of the fully equipped Cherenkov detector through the axes
of photomultipliers (Dimensions are expressed in millimeters).
The mechanical design of CKOV2 is based on a regular 12-sided polygonal
"cylinder" aligned along the beam axis, and circumscribed in a circle of 134 cm diameter.
This vessel is constructed with 15-mm thick plates and specially designed massive corner
pieces. All elements are made of low carbon steel to protect the sensitive equipment
against the stray magnetic field. The pieces are welded together and the whole box is then
plated with chemical nickel to protect against corrosion. The inner faces and the
entrance/exit flanges are foreseen with O-rings.
8.5.4 4/15
Figure 8.5-14. 3D-view of the CKOV2 vessel.
8.5.4.3 Photodetector assemblies (PM tubes, Winston cones, optical windows)
All sides of the polygonal vessel are equipped with identical optical assemblies.
Each assembly is made of a partially aluminized optical glass window, a compound
parabolic reflector (Winston cone) and a magnetically shielded photomultiplier (Figure 850).
The system uses EMI 9356 KA 20-cm (8")-diameter photomultipliers, with a
standard bialkali photocathode. These very low noise PMT's are those used by a former
gas Cherenkov detector built for the HARP experiment. This PMT has the advantage of a
rather large gain, 3 x 106, which is well matched to the low light yield of aerogel
radiators. The PMT's are mounted in volumes separated from the radiator section. This
8.5.4 5/15
allows easy access to the PMT's without disturbing the dry environment of the aerogel
and avoids possible damage to the mirrors.
Figure 8.5-15. Constitutive elements of each optical assembly.
The low background EMI 9356 KA photomultiplier has a diameter of 200 mm. It
is shielding with a double layer of ferromagnetic materials: an inner 1-mm thick mumetal
sheet and an outer 5-mm thick iron cylinder. The magnetic shielding also serves as a
support for the other optical elements.
8.5.4 6/15
Figure.8.5-16. A whole optical assembly showing the double layer magnetic
shielding. It encloses the Winston cone and the optical window.
Figure 8.5-17. Cut of an optical assembly through the axis of a photomultiplier
showing the various optical and shielding elements.
The square optical windows are made 10-mm thick Schott B270 glass. It has a
good transmission in the visible and near-UV wavelength range.
The optical windows are maintained against O-rings with four special aluminium
clamping pieces (two of them visible on figure 8-52). These pieces will be covered with
Lexan reflective sheets at assembly time.
8.5.4 7/15
Figure 8.5-18. Transmission of the Schott B270 optical glass used for the optical
windows of CKOV2. Original Schott data for 25 mm thick B270 plates.
The windows are partially covered with a special reflective coating (outside the
acceptance area of the Winston cones) reaching a reflectivity of about 96% in the near
UV wavelength range.
Figure .8.5-19. Reflectivity of the multilayer coating (A. Braem, CERN,
private communication)
8.5.4 8/15
Figure 8.5-20. 3D-view of the CKOV2 vessel fully equipped with optical windows
and shielded photomultipliers.
The compound parabolic reflectors (“Winston cones”) have an acceptance angle
of about 30 degrees. They are CNC-machined out of full size PMMA blocks of about 350
mm x 350 mm x 100 mm. Their inner surfaces are coated with the same reflective layer
as the ones described for the optical windows.
8.5.4.4 Particle entrance and exit windows
The particle entrance and exit windows are based 10-mm thick composite panels.
They are made of a Nomex honeycomb core (phenolic impregnated paper) and have one
skin of 0.9 mm thick fiberglass-epoxy. For both windows the skins facing the inside of
the vessel are made of 1-mm thick aluminium sheet: a smooth surface is needed to get
gas tight contacts with the O-rings of the steel vessel.
The particle entrance window supports the aerogel box and a front mirror, while
the particle exit window supports a reflecting pyramidal mirror.
8.5.4 9/15
Figure 8.5-21. The honeycomb entrance window (dark green) supporting the aerogel box
and the annular front mirror. The beam direction is along the z-axis.
8.5.4.5 The aerogel radiator and its support box
The radiator is contained in a lightweight honeycomb box attached to the center
of the particle entrance window. The downstream face of this box will left open (or will
be closed by a very thin Mylar sheet to keep the aerogel tiles from falling). The radiator
box is assembled by epoxy-gluing the side panels.
The aerogel radiator material is manufactured by Matsushita-Japan. It is built
stacking standard tiles of 130 mm x 130 mm x 10 mm made of a new brand of
hydrophobic silicon oxide with a density of about 1.01 kg/m3. The refractive index will
be between 1.02 and 1.05 depending on the optical light collection efficiency and on the
performance for separating electrons from muons.
The aerogel radiator is 10 cm thick and covers an effective circular area of
diameter 80 cm.
8.5.4.6 The pyramidal mirror
Many different optical configurations to reflect sideways the Cherenkov light
have been tried. The simplest one, a dodecagonal reflecting pyramid with 12 flat faces
inclined at 45 degrees, was finally discarded since it generates a large average number of
reflections on the inner sides of the vessel. In the end, the most promising shape is
8.5.4 10/15
sketched on figure 8-57. All faces of the reflecting structure are parts of identical
cylindrical surfaces with a radius of curvature of 843 mm, and an opening angle of about
50 degrees.
Figure 8.5-22. Sketch showing the geometry of the pyramidal structure for bending
sideways the Cherenkov light from the radiator towards the photomultipliers on the sides
of the main vessel.
The mirror sheets are obtained from 3-mm thick polycarbonate (Lexan) sheets.
After cutting to the proper shape, the individual pieces are thermally preformed to the
proper radius of curvature before the vacuum evaporation of the reflecting multilayer.
The pyramid is then obtained by gluing (Rhodorsil silicon glue with a primer) the
pieces (“curved triangles”) on a lightweight honeycomb structure attached to the particle
exit window (not shown on figure 8-57).
8.5.4 11/15
Figure 8.5-23 The 12-sided reflecting pyramidal structure used to reflect the Cherenkov
light towards the photomultipliers. The honeycomb particle exit window is the partially
hidden green polygon in the picture.
8.5.4.7 Optical Performances
The assessment of the optical performances of CKOV2 uses the electron files
simulated with GEANT4 [TJR05].
Since the electrons reaching the downstream detectors are all ultrarelativistic, the
theoretical light yield and Cherenkov angle depend only on the thickness of the aerogel
radiator and of its index of refraction. The figure 8-59 shows the distribution of
Cherenkov angles for the indices n=1.02 and n=1.05.
8.5.4 12/15
Figure 8.5-24. Distributions of Cherenkov angles for two indices of refraction of the
aerogel radiator.
For the same two indices, the figure 8-60 shows the photoelectron yields, using
the standard formula for "normal response" bialkali photocathodes [Particle data book]
(convoluted with the spectral Cherenkov emission) and assuming 100% light collection
efficiency in CKOV2.
Figure 8.5-25. Distributions of the photoelectron yield as a function of the index of
refraction of the aerogel.
The tracking of the photons inside CKOV2 was performed in two steps:
- generation of the Cherenkov cones corresponding to the simulated electron files.
It takes into account the variable path length of electron inside the radiator. This
is done with a Mathematica v.3 program with the electron files as input, and outputting
photoelectron text files for the next step.
8.5.4 13/15
-
tracking of the "photoelectron rays" using the ZEMAX-EE software.
It allows a very realistic description of all surfaces, coatings, reflections and
refractions within the CKOV2 envelope.
8.5.4.8 The detector support
The whole CKOV2 detector is surrounded with a simple bolted frame built with
standard iron profiles. The vessel itself is suspended to this frame with several threaded
rods, allowing for fine position adjustment. It is also possible to get access to or to
replace any faulty photomultiplier without perturbing the whole setup (Figures 8-61 and
8-62 ).
Figure 8.5-26. Support frame of CKOV2 as seen from the axis of the last solenoid.
8.5.4 14/15
Figure 8.5-27. 3D view of the whole CKOV2 and its support.
8.5.4 15/15
Table 8.5-7. Overall dimensions of CKOV2
Directions
along Oz (beam axis)
along Oy (vertical)
along Ox (horizontal)
Dimensions (mm)
465
2590
2119
Table 8.5-8 Tolerances
Item
Support frame
Steel vessel
PMT magnetic shielding
Optical parts
Surface flatness (mirrors
and windows)
Leak rate
Tolerances (mm)
± 2.5 mm
± 0.5 mm
± 0.1 mm
± 0.1 mm
± 0.01 mm
About 1 liter He NPT/day