HERA-B RICH light collection system

Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
HERA-B RICH light collection system
Daniel R. Broemmelsiek
Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
Abstract
The HERA-B RICH detector utilizes the recently developed Hamamatsu R5900 multianode photomultiplier. In order
to increase the light collection e$ciency, a novel scheme to optically magnify the photocathode is implemented. The
design and development of these optics are persented here. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction
The primary purpose of the HERA-B spectrometer is to measure CP violation in the B meson
system. The HERA-B RICH detector, [1] and
Fig. 1, is designed to provide excellent particle
identi"cation over the momentum range 4}80
GeV/c. Flavor tagging of B (or BM ) mesons by
e$cient kaon}pion separation for HERA-B is the
speci"c role of this RICH detector. The photosensitve component of the photon detector consists
of both the 4 and 16 anode versions of the
Hamamatsu R5900 multianode photomultiplier arranged on 36 mm centers as shown in Fig. 2.
A basic problem for any RICH detector is to
collect Cherenkov light with high e$ciency while
preserving the required position resolution. By fortuitous coincidence, the R5900 anode cell sizes,
9 mm;9 mm and 4.5 mm;4.5 mm for the 4 and
16 anode versions, respectively, are almost exactly
1/2 the cell size needed for su$cient position
resolution required by kaon}pion separation. By
E-mail address: daniel.broemmelsiek@desy.de (D.R. Broemmelsiek)
magnifying the photocathode a factor 2, not only is
the fraction of photosensitive area on the focal
plane improved, 25}95%, but the cell sizes are then
matched to the intrinsic angular resolution for
a single photon in the di!erent regions shown in
Fig. 2. An array of telescopes consisting of 2 acrylic
lenses is mounted in front of the PMT array in
order to give the correct magni"cation of the
photocathodes.
2. Telescope design
The primary consideration when designing such
a telescope is the angular distribution of Cherenkov
light at the focal plane surface. The focal plane
consists of 10 planar surfaces which approximate
a cylindrical surface. The cylindrical surface is
chosen to remove the shift in the photon angular
distribution at the focal plane due to the tilted
spherical mirror and the de#ection of charged
tracks by the 2 T m dipole magnet 7 m closer to the
target than the RICH mirrors [2]. Care was taken
that the focal plane remain within the mirror's
depth of focus and not further increase the intrinsic
single photon resolution.
0168-9002/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 2 9 4 - 6
D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
137
Fig. 1. HERA-B RICH detector schematic [3].
As seen in Fig. 1, the radiator occupies the "nal
25% of the distance from the target to the mirrors.
Therefore the Cherenkov photon emission angle
and position would imply a distribution of photons
in the range 0 40420 where 0 is the polar
angle with respect to the focal surface normal and
0 "52 mrad. Studies [2] using realistic produc
tion models and detector simulation indicate that
Cherenkov photons will have an angular distribution at the chosen focal surface of 04100 mrad.
2.1. A two lens solution
The basic design consists of a planoconvex "eld
lens, the planar surface being placed at the chosen
focal surface, and a biconvex circular lens as shown
in Fig. 3. UV transmitting acrylic can be molded
simply and e$ciently and makes an excellent
choice of material for the lenses.
By placing the biconvex lens at the focus of the
"eld lens, the required magni"cation factor of 2 implies that f "2f where f and f are the focal
lengths for the "eld and biconvex lenses, respectively. Equivalently, the total length from the focal
plane to photocathode will be 1.5f in the thin lens
approximation. A few initial test rays quickly show
that lenses of reasonable thickness, F number
(,f/lens diameter) &1.5, can be designed for the
choice of f "95 mm. The diameter of the bicon
vex lens is chosen to be 32 mm as a compromise
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D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
Fig. 2 . PMT map of the photon detector [3].
The R and f for each lens are taken to be
G
G
correlated through the uncorrected Lens Maker's
Formula. With the choice of f "95 mm the "ve
remaining parameters of the telescope to be determined are,
Fig. 3. A schematic of the proposed telescope solution.
between aperture and mechanical assembly considerations.
As previously noted, the Cherenkov light does
not arrive at the telescope parallel to the optical
axis. This causes image distortions at the photocathode. By using aspheric lenses, these distortions
may be minimized. The aspheric surfaces may be
parameterized by introducing a coe$cient, b where
G
i"1,2 for the planoconvex and biconvex lenses,
respectively, to the second order term in the expansion of a sphere with radius R. Suppressing the
constant term in such an expansion gives,
b o o
z"$
1# G
2R
4 R
G
G
as the equations for the curved surfaces in Fig. 3.
The R represent the `nominala radii of curvature
G
for each lens surface.
E b for i"1,2, i.e. the aspheric corrections for both
G
lenses,
E positions of the biconvex lens and photocathode
relative to the "eld lens,
E and f .
With the telescope parameterized in this fashion, all
that is now needed to optimize the telescope is
a reasonable measure of the imaging performance
properties pertinant to the RICH.
2.2. Optimization of parameters
Ray tracing is used to determine the focal properties of the telescope. The rays are generated on
a 2 mm grid at the planar surface of the "eld lens
according to a #at angular distribution in the range
0 40420 where 0 is the polar angle with re spect to the optical axis of the telescope. Chromatic
dispersion is a negligible contribution to the focal
properties of the lenses. The spectral sensitivity of
the PMTs is averaged and monochromatic rays
are generated. The refractive index of the acrylic
material for this average wavelength is n"1.51.
D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
139
Fig. 4 . RICH photon detector module [3].
Table 1
The telescope parameters found by the optimization scheme.
The distances d and d are shown in Fig. 3
b
b
f
d
d
!1.32
!1.52
30.27 mm
101.67 mm
47.85 mm
The focal point of the "eld lens and the "nal
focus at the PMT photocathode are of interest
when developing a reasonable measure of the imag-
ing performance. `Focusa is de"ned as
"q (z)!q( (z)"
G
N
where z is measured along the optical axis, q is the
transverse vector of a ray and q( is the average
transverse vector for the N generated rays. A set of
rays with the same incidence angle at each grid
point provides a measure of the "eld lens focal
point. A set of rays in the range 0 40420 at
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140
D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
Fig. 5. Transmission of injection molded lenses.
a single grid point provides a measure of the "nal
focus at the PMT photocathode. Clearly, the size of
these focal points is a minimum for the optimal
telescope.
Aperture losses at the biconvex lens and the
photocathode are also minimized to keep the
photon collection e$ciency high. Minimization of
the focal points and aperture losses does not necessarily imply a telescope with a magni"cation of 2.
A Gaussian term to regulate the magni"cation of
the telescope to 1% is needed to ensure an appropriate solution.
A simple summation of these positive de"nite
considerations on the telescope imaging performance is taken as the appropriate measure to minimize. The chosen measure re#ects a desire for large
collection e$ciencies and the lack of any a priori
knowledge of correlations. Table 1 gives the parameters which de"ne the telescope design in use at
the HERA-B RICH. The "eld and biconvex lenses
are 6.78 and 9.45 mm thick with minimum thicknesses of 1 and 2 mm, respectively. Minimum
thicknesses were adopted to simplify the lens
molds.
2.3. Telescope performance
Performance of the optimized telescope can also
be estimated by ray tracing. Rastor plots are used
to qualitatively assess the focusing aberrations of
the telescope. The path length of simulated Cherenkov rays and the re#ection losses at the lens surfaces are estimated to be 415 mm and 415%,
respectively. Manufacturer's data for acrylic
D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
141
Fig. 6 . 12;12 simulated photocathode array as viewed from 3 m.
indicates there is &1%/mm loss in total collection
e$ciency due to the absorption of detectable
Cherenkov photons.
2.4. Telescope assembly
The telescope assembly is shown in Fig. 4 mounted in front of photomultipliers. The photomultipliers are mounted in an iron frame set in molded
plastic which provides a rigid support frame and
magnetic shielding.
The lenses are mounted to an aluminum frame
made from 1 mm thick "ns which are also imbedded in molded plastic. At the position of the "eld
lens, the aluminum "ns are scalloped to allow the
lenses to be glued to both the surrounding lenses
and the framing. The biconvex lenses were placed
and glued directly onto the plastic molding. Positioning jigs were used to accurately place the lenses
for gluing to the frame. The result is a rigid assembly of 2;12 telescopes weighing &300 g.
3. Results
The designed lenses were injection molded by
Wahl Kunststo!optik, GmbH. The measured surfaces of the delivered lenses are well within the
0.01 mm tolerances given by the manufacturer. The
acrylic remembers the mold stresses as indicated by
an orientation dependent transmission, Fig. 5.
However, the transmission of the acrylic compares
well with the quantum e$ciency of the normal
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D.R. Broemmelsiek / Nuclear Instruments and Methods in Physics Research A 433 (1999) 136}142
Fig. 7 . 12;12 simulated photocathode array as viewed from 1.5 m.
bialkali Hamamatsu photocathode with a borosilicate window.
To test the "nal quality of the telescope assembly, a 12;12 array of simulated photocathodes
was backlit and viewed from 3 and 1.5 m,
Figs. 6 and 7, respectively. From Fig. 6, the
simulated photocathode array is seen to be magni"ed with high "delity and uniformity of cellularization. The framing losses are clearly seen to be at
the 5% level. Additionally, the inversion properties
of the telescope are explicitely shown by the lettering.
Each telescope is on 36 mm centers so that
Fig. 7 explicitely shows the aperture cuto! at
140 mrad ("216 mm/1500 mm) as expected from
the ray tracing simulations. The losses are due
primarily to the aperture of the biconvex lens and
do not represent collection losses at the photocathode.
4. Conclusion
The timely development of the Hamamatsu
multianode photomultipliers has enabled the
HERA-B RICH to deploy a large area, 3.25 m,
photocathode. An optical telescope, optimized for
the RICH, is an essential component.
References
[1] Samo Korpar, Nucl. Instr. and Meth. A 433 (1999) 128.
[2] Marco Staric, HERA-B Note 96-290.
[3] JoK rg Pyrlik provided these technically accurate drawings.