[^1]

Abstract\-- The PHENIX muon spectrometers at RHIC incorporates cathode
strip chambers (CSC) as the tracking detectors. The design requirements
for the spectrometers was such that the mass resolution for the vector
mesons would allow for a clear separation of the individual states. This
lead to the requirement that the CSC's have a resolution of 100 µm and
the intermediate detector station have a radiation length less than
0.1%. The octant shaped chambers were constructed of a honeycomb panel
structure for the first and last station and the middle station used
etched 25 µm metallized mylar foils held under tension by thick aluminum
frames. The largest chambers, 3.5 m x 3.5 m, are at station 3. Both
spectrometers have now been fully instrumented and data taking has been
very successful. We have shown that the chambers have met design
requirements. A description of the design and performance of the
chambers for both cosmic rays and real data is given.

# INTRODUCTION

The PHENIX experiment^1^ at the Relativistic Heavy Ion Collider at the
Brookhaven National Laboratory consists of a central region with two
electron arms and muon spectrometers in each of the forward regions. The
primary goals of RHIC are to produce a deconfined state of matter called
the quark-gluon plasma (QGP) with the heavy ion beams and to probe spin
structure of the nucleon with polarized proton beams. An important
confirming signature for the quark-gluon plasma (QGP) is a measure of
J/Ψ suppression in Au-Au collisions. The two muon spectrometers are
designed to measure this important signature by detecting the muons from
the J/Ψ decay.

# Muon Spectrometers

One of the primary measurements of the muon tracking system is to
measure the muons from the decay of the vector mesons in collisions
ranging from pp to Au-Au with sufficient precision to determine their
rates of production. For the J/Ψ this translates to a mass resolution of
\~ 150 MeV so that the J/Ψ and the Ψ′can be separated. Similarly, to
separate the upsilon from the Υ(1S) and the Υ(2S+3S), the mass
resolution of the tracking system should be \~ 200 MeV. The central
magnet steel in front of the muon spectrometers limits the mass
resolution of the J/Ψ to about 150 MeV due to multiple scattering in the
steel. However, the resolution of the upsilon is dominated by the
momentum resolution in the spectrometer and this leads to constraints on
the design and resolution of the cathode strip chambers. For the three
position measurements in the center of the magnet at station 2, the
resolution must be 100 µm and furthermore the radiation length of the
station 2 assembly must be less than 0.1%.

The muon tracking systems consist of two large magnetic spectrometers
located on opposite ends of the interaction region. Each is a radial
field magnet with three detection stations of cathode strip chambers
(CSC). The field causes the charged muons to bend in the phi direction.
See Figure 1.

![](media/image1.png){width="3.5in" height="2.3618055555555557in"}

Figure 1 The PHENIX detector showing the two muon spectrometers.

The north muon magnet is the larger of the two spectrometers and covers
1.1 to 2.4 units of rapidity (10° to 35°) with 2π coverage in phi, the
rotation angle about the axis of the beam line The south muon magnet is
1.5 meters shorter due to the requirement that it must be able to move
away from the central magnet. To limit saturation in the south magnet
piston more steel was required and consequently the rapidity coverage
for the south magnet is 1.1 to 2.25 (12° to 35°) with 2π in phi. Both
magnets were designed to have a p~t~ kick of 200 MeV/c.

Cathode strip chambers were chosen as the baseline detectors for each
measurement station in the spectrometers. Because of the difference in
lengths of each spectrometer only the first measurement station, station
1, had identical detectors for each spectrometer. Stations 2 and 3 have
different dimensions in the north and south spectrometers.

#  Cathode Strip Chambers

The three measurement stations in the north and south spectrometers each
presented unique design requirements. To cover 360° in phi without
serious overlap or loss in acceptance the chambers at stations 2 and 3
were designed to be in the form of octants and at station 1 in the form
of quadrants. The cell geometry for all chambers was the same;
alternating ground and anode wires on the anode planes with 1 cm anode
to anode spacing, 20 µm Au plated W anode wires, 75 µm copper-beryllium
ground wire, 3.175 mm anode-to-cathode spacing, 5 mm wide cathode strips
with every other strip read out. The anodes are not read out. Stations 1
and 2 each have 3 CSC chambers and station 3 has 2 CSC chambers. Both
cathodes in each chamber are readout for a total of 6 (4) measurement
points. Three (two) of the cathode planes have strips oriented normal to
the anode wire and they provide the high resolution coordinate points.
The other cathodes have strips oriented at an angle between 3.75° and
11.25° to the perpendicular strips and provide stereo information to
help determine the radial coordinate and to help in eliminating ghost
hits.

In all types of chambers that read out the cathode charge and
interpolate to get the coordinate position, any inclined track that
deposits its charge along the anode wire will suffer degradation in the
resolution due to energy loss fluctuations in the gas that cause a non
uniform distribution of the charge along the anode wire. To minimize
this effect we chose to limit the angle of inclination from the normal
to the face of the chamber to ± 11.25°. This was accomplished by laying
the anode wires on the chord in each half octant. A special wire winding
machine was developed that moved the wires in an arc where the wires
were captured by fiberglass pins on each edge and one in the center of
the octant. The resultant continuous wire had an angle change in the
center of the octant of 22.5°. The intrinsic resolution of the chamber
at normal incidence is expected to be 100 µm and at the edge of the
octant, 150 µm.

Another cause for resolution degradation is the Lorentz angle due to the
magnetic field in the magnet causing the charge to spread along the
anode wire. This was minimized by selecting a "fast gas" for use in the
chambers. A tertiary gas mixture, 50% Ar + 30% CO2 + 20% CF4, was
chosen. For the magnetic fields we estimate that the Lorentz angle is
less than 5°. The operating voltage is \~1850 V and the design gain is
\~ 2 x 10^4^ .

## Station 1 CSC Chambers

The station 1 chambers are closest to the interaction region and are
therefore the smallest.^2^ Two station 1 quadrants are shown in figure
2.

![](media/image2.jpeg){width="3.498611111111111in"
height="2.623611111111111in"}

Figure 2 Two station 1 quadrants. HV cabling from the edges of the
chamber are visible.

The anode HV cables are connected to individual anode boards on either
radial edge of the chambers and the cathode connectors exit on the outer
circumference. While the station 1 chambers are quadrants mechanically
they are octants electrically with the anode wires and cathodes
electrically isolated in the center of each quadrant.

![](media/image3.png){width="3.5194444444444444in"
height="3.4319444444444445in"}

Figure 3. Cross section of the cathode end of the station 1 chambers.

Maximizing the active area was the reason for making quadrants because
the acceptance loss was minimized in the center. We were able to
maintain an active area of 95%. The chambers consist of 4 planes of
honeycomb sandwich panels with spacers on the edge to form the three CSC
gaps. (figure 3). A schematic of the station 1 chamber showing how the
anode wires change direction is shown in figure 4.

![](media/image4.png){width="3.5180555555555557in"
height="2.5840277777777776in"}

Figure 4. Schematic of station 1 quadrant with the anode wires shown.

The honeycomb sandwich consists of nomex core with copper clad FR4
laminated on either side. Each quadrant honeycomb panel had two octants
of the copper clad on each surface. The outer copper clad skin was solid
copper and the inner copper clad skins were made with a standard
photo-etching process to make the relevant strip orientation for each
octant. The 3.175 mm thick anode PC boards are glued to two edges of the
octant and solid FR4 3.175 mm thick are glued on the other edges to make
the spacer for the half gap. The mating spacer on the mating panel is
solid FR4 with an o-ring groove. The solid FR4 and the anode cards were
not continuous around the quadrant so the spaces between the separate
cards were filled with epoxy to help ensure a good gas seal. A total
channel count for the cathode readout for station 1 is 4608.

Ribs 5 mm wide, 3.175 mm thick with 0.5 mm FR4 pins every 5 mm for the
entire radius were located at the center of the quadrant and in the
middle of each octant ( every 11.25°) to guide the anode and ground wire
during the winding process and were left in place. All wires were glued
at each end and soldered to the anode boards. The inner to outer radius
of the station 1 chambers was 1.2 m. To ensure that the chambers were
assembled to an accuracy of 25 μm, an assembly fixture with 6 precision
guide holes was used to drill precision holes in all planes and
precision pins were installed in the final assembly. As in all of the
CSC chambers precision pins were used to translate the coordinates from
inside to the outside of the chambers.

## Station 2 CSC Chambers

The station 2 design was perhaps the most ambitious because the
radiation length was required to be less than 0.1% to achieve the
required momentum resolution.^3^ To achieve this we constructed station
2 to consist of thin etched copper coated cathode foils for all cathodes
and thin exit and entrance gas windows.^4^ A completed station 2 chamber
is shown in figure 5.

![](media/image5.jpeg){width="2.4451388888888888in"
height="2.863888888888889in"}

Figure 5. The station 2 CSC mounted on a test wall. Visible are the
large Al support frames, the entrance metalized gas window and the anode
and cathode cables and connectors on the lrft and top of the octant. The
radiation length is 8.5x10^-4^

All planes including the anodes, cathode, ground and spacer frames were
3.175 mm thick. The anode wire planes and the cathode foil planes were a
laminate of Al extrusions and FR4 PC boards. The entire frame was made
of many FR4 pieces so just like in station 1 we filled all cracks
between the separate pieces with epoxy. The aluminum extrusion had an
o-ring on the back side to contain the gas. The 25 µm thick copper
coated mylar foils were attached to the cathode PC boards by glue and
the electrical connection by conducting epoxy. The copper coating was
600 Angstroms thick. For the anode planes the anode and field wires were
glued and soldered to the anode boards. An FR4 rib, 5 mm wide and 3.175
mm thick, with 0.5 mm FR4 pins every 5 mm supported the anode wires in
the middle of the chamber. It was fixed at the outer diameter of the
anode plane and left to float in a groove at the bottom of the frame.
Allowing the support rib to float was necessary because the support rib
stretches under the tension due to the wires and the cathode foils cause
the upper support frame to deform under load. All anode, cathode, and
support frames had 9 precision holes , 3 on each long edge, drilled with
the same drill jig to hold the alignment pins that were used in the
assembly process. The accuracy of the hole locations were kept to 25 µm.

A cross sectional view of the anode side of the station 2 octants is
shown in figure 6. The thick 28.4 cm Al plates on the top and bottom
provide the mechanical support for all of individual anode and cathode
planes. During the etching process for the cathodes and the wiring
process for the anodes the thin 3.175 mm frames were held under tension
by these

![](media/image6.png){width="3.4944444444444445in"
height="2.5506944444444444in"}

Figure 6. Cross section view of the anode side of the station 2 octant.
The planes are labeled W for window foil, C for cathode foil plane, A
for anode plane, and G for ground plane. An o-ring was required for
every plane and two for the spacer frames for a total of 15 o-rings.

large support frames with the precision pins holding the frames in
place. Each cathode strip chamber consisted of an anode plane with two
cathode planes on either side. A ground plane was placed between each
CSC chamber. The gas window was a metallized mylar foil. The physical
size of the octants varied between the north and south arm due to
differences in location within the magnets and differences in size of
the central piston. In addition, within each arm 4 of the octants were
displaced downstream so that the octant edges could overlap. The north
arm spectrometer station 2 octant sizes are 2240x2140 mm^2^ and
2174x2068 mm^2^. The south arm spectrometer octant sizes are 1938x1740
mm^2^ and 1872x1650 mm^2^. The total channel count for cathode readout
for station 2 is 7680 channels for the south arm and 9216 for the north
arm.

## Station 3 CSC Chambers

The largest chambers in each of the PHENIX muon spectrometers are the
station 3 octants that are attached to the rear wall of the magnet. A
finished station 3 octant is shown in figure 7.

Like the station 1 chambers the station 3 chambers use honeycomb cores
with 1.59 mm FR4 copper clad on either side. The copper clad on the
cathode sides were mechanically routed to make the cathode strips. A
cross section of the station 3 chambers is shown in figure 8.

The edges of the gaps were a laminate of FR4 spacers and anode and
cathode PC boards just like in the station 1 construction. A 2 cm wide
rib with FR4 pins at 5 mm spacing spanned the center of the octant from
the outer radius to the inner radius and glued in place. A thick brass
bar along the edge of the assembly provided a good electrical connection
and spread the load to fully compress the o-rings.

The south station 3 octants were 2.4 m x 2.4 m and the north octants
were 3.3 m x 3.3 m. Precision alignment pins and a precision drill jig
were used to drill and assemble the planes.

![](media/image7.jpeg){width="3.5in" height="2.625in"}

Figure 7. A station 3 finished octant. Visible is the HV cabling coming
from the anodes, right, and the cathode connectors on the top. The
copper extensions on the top hold the CD cameras used in the optical
alignment system.

![](media/image8.png){width="3.497916666666667in"
height="2.683333333333333in"}

Figure 8. Cross section of station 3 octant. Because of the increase in
size for station3 the honeycomb core is 21.65 mm thick.

The routed cathode sheets were laminated in the commercial factory so
precision placement of the cathode skins onto the honeycomb core was not
possible. Therefore, an accurate survey of each cathode was made to
accurately reference the cathode strips to the alignment pins. The total
channel count for cathode readout for the north station 3 was 10240 and
for south station 3 was 8192.

# Results and Performance

One of the first station 2 octants out of the production factory was
placed in a cosmic ray test stand to incorporate the production
electronics in a chain test and to determine the resolution of the
chambers. The results of that test is shown in figure 9 which indicates
a chamber resolution of 100 µm.

![](media/image9.jpeg){width="3.4944444444444445in"
height="2.3694444444444445in"}

Figure 9. Residuals from the test of the station 2 octant with the
production electronics. The sigma of 130 µm corresponds to an intrinsic
resolution of the CSC's of \~100 microns.

During this test the data were limited to cosmic rays normal to the
chamber. Since no offsets between the three chambers were necessary,
this also gave us confidence that the assembly procedures were accurate
to much less than 100 µm. It also demonstrated that the production
electronics noise levels were at or better than design specifications (
\<3000 electrons).

The south muon arms were fully instrumented in the PHENIX detector for
Run II in the winter and spring of 2002.^5^ Initially all HV channels
were operational but gradually during the run 10-15% of the HV channels
dropped out due to excessive leakage currents, \> 1 uA. This problem was
traced to high humidity in the Brookhaven area affecting the exposed
anode boards, even though parylene coating was used on the anode boards
during the manufacturing process and additional conformal coating was
added during the Q/A process after the chambers were assembled. After
the run was completed, dry air circulation was implemented to keep the
local humidity in the vicinity of the anode boards low. This was
successful and most of the high leakage boards were operational. Another
failure occurred with the surface mount coupling capacitors on the anode
boards. Capacitors rated for 4 kV were used and all boards were tested
during the Q/A process for bad capacitors but during the run a few
developed high leakage paths that indicated a breakdown in the
capacitors. In some cases we were able to remove the capacitors but in
others we were not so some HV channels were turned off. In general the
tracking efficiency did not suffer because a sufficient number of hits
on the track remained even if only four out of the six cathodes
remained.

During the summer of 2002 the north arm spectrometer was fully
instrumented in preparation for the run III in the winter and spring of
2003. All chambers were thoroughly tested prior to installation and
conformal coating was used on the anodes PC boards. Again as was the
case for the south arm some HV coupling capacitors developed problems
during the run. Despite these problems the number of active HV channels
during the run III was 96% for the north arm and 98.7% for the south arm
. The tracking efficiency remained high at \~95%. A good indication of
the performance of both north and south muon spectrometers is the mass
peak of the J/Ψ. While the J/Ψ mass peak is dominated by multiple
scattering in the central arm magnet, any major deviation from the
design specifications would show up in a broadening of this peak. Shown
in figure 10 is the north J/Ψ mass peak from the recent run III.

![](media/image10.png){width="3.5076388888888888in"
height="2.645138888888889in"}

Figure 10. The J/Ψ mass peaks for the north muon spectrometer .

These are preliminary data and the absolute normalization of the data
has not been determined. Nevertheless, the peak exhibits a resolution of
168 MeV for the north arm, close to the values predicted from Monte
Carlo simulations. To obtain these results the chamber resolution should
be close to design goals, the electronics noise level must be close to
design goals, and the alignment of all stations must be accurately
known.

# References

1.  H. Akikawa et. al, "PHENIX Muon Arms", Nuclear Instruments and
    Methods in Physics Research A 499 (2003) 537 -- 548.

2.  D.E. Fields, et. Al, "Large, Light, High Acceptance CSC Chambers for
    the PHENIX Muon Detector Using a Honeycomb Panel Design", 1996 IEEE
    NSS Conf Rec, Vol. 1, 1997, pp 527-529.

3.  D. M. Lee, et. al., "Large CSC Chamber for the PHENIX Muon Detector
    with Ultra Thin Cathode Foils", 1996 IEEE NSS Conf Rec, Vol. 1,
    1997, pp 274-278.

4.  C.L Morris, , et. al., " Electric discharge etching of thin
    metalized plastic films", Nuclear Instruments and Methods in Physics
    Research A 379 (1996)pp. 243-246.

5.  \" J/Ψ production from proton-proton collisions at sqrt(s) = 200
    GeV\",

    S.S. Adler et al., submitted to Physical Review Letters July 8 2003

[^1]: ^^Manuscript received October 22, 2003. This work was supported
    in part by the U.S. Depart­ment of Energy and the RIKEN Institute.

    David M. Lee is with Los Alamos National Laboratory, MS H846, Los
    Alamos, NM 87545 USA (telephone 505 667-8888, email: dlee@lanl.gov