Nuclear Instruments and Methods in Physics Research A 418 (1998) 65—79
First experience and results from the HERA-B
vertex detector system
C. Bauer , I. Baumann , M. Bräuer , M. Eberle , W. Fallot-Burghardt ,
E. Grigoriev , W. Hofmann , A. Hüpper , F. Klefenz , K.T. Knöpfle , G. Leffers ,
T. Perschke , J. Rieling , M. Schmelling , B. Schwingenheuer , E. Sexauer , L. Seybold ,
J. Spengler , R. StDenis *, U. Trunk , R. Wanke , I. Abt, H. Fox, B. Moshous,
K. Riechmann, M. Rietz, R. Ruebsam, W. Wagner
Max-Planck-Institute for Nuclear Physics, Postfach 10 39 80, D-69029 Heidelberg, Germany
Max-Planck-Institute for Physics (Werner Heisenberg Institute), Fo( hringer Ring 6, D-80805 Mu( nchen, Germany
Abstract
The HERA-B collaboration is building a detector to realize the ambitious goal of observing CP violation in decays of
neutral B-mesons. A central element of the apparatus is the silicon vertex detector used to selectively trigger on these
decays in a high charged particle multiplicity background environment and to reconstruct secondary vertices from such
decays with high precision. The vertex detector, the supporting infrastructure and first results using prototype detectors
are described. Results include imaging of the proton interaction region on the HERA-B target, hit distributions in the
detector planes, and alignment of the detectors with each other and the target. 1998 Elsevier Science B.V. All rights
reserved.
1. Introduction
The HERA-B experiment [1,2] makes use of
interactions between protons stored in the HERA
storage ring and an internal target to generate
* Corresponding author. E-mail: [email protected].
Expanded version of a talk presented at the 6th International Workshop on Vertex Detectors VERTEX 97 (Rio de
Janeiro, September, 1997).
On leave of absence from ITEP, Moscow.
Now at MPI-München.
Now at Mainz University.
Now at Articon, Kirchheim.
B-mesons at a relatively large rate. Interactions are
detected, particles are identified and their momenta
analyzed in a multiparticle spectrometer with the
ultimate goal of observing CP-violation in the neutral B-system. Central to achieving this goal is the
silicon Vertex Detector System (VDS). These physics goals require that the VDS have minimum material to allow for accurate reconstruction of the
B-meson vertices and that they operate close to the
beam in a high radiation enviroment so that the
interaction rate is sufficient to produce enough
reconstructed B-mesons to measure CP-violation.
Material considerations lead to a double-sided detector design and operation inside the vacuum of
0168-9002/98/$19.00 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 7 1 8 - 9
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C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
Fig. 1. Cross-sectional view through the detector (not to scale).
the machine. This, together with the desire to be as
close to the beam as possible leads to a complicated
mechanical design. Minimization of radiation damage puts strict requirements on cooling. The materials supporting the detectors must have a high heat
conductivity but minimum material.
Although the experiment is scheduled to begin
physics data taking in 1998, prototypes of the silicon vertex detector as well as the outer tracker,
transition radiation detector, high p trigger cham
bers, electromagnetic calorimeter and muon system
have been installed in 1996 and 1997. A description
of the VDS and first results from VDS data taken
are presented in this report.
2. Detector modules
Double-sided silicon microstrip detectors have
been developed for use in the ATLAS [3] and
HERA-B experiments. The maximum expected fluence from pions during one year of operation in the
HERA-B experiment will be 3;10cm\; the energy of 80% of these pions exceeds 1 GeV [1,2].
This would require a bias voltage of about 300 V
for full depletion for the 280 lm VDS detector
operated at a temperature of 10° C [4].
At the semiconductor laboratory of the MPI
Munich, double-sided silicon microstrip detectors
with a special guard ring structure have been developed (Fig. 1) to work up to significantly higher
bias voltages [5]. These structures consist of about
20 guard rings on each side of the detector. Each
guard ring has a width of 20 lm and surrounds the
active area on both sides. The detector is biased
using reach through structures [6]. For diagnostic
purposes the guard ring closest to the bias ring is
enlarged to the bias ring width of 60 lm and used to
allow separate measurements of the current
through the guard ring structure and current
through the active area of the detector. The detector is made on n-type silicon and has capacitively coupled strips consisting of n> implants on
one side and orthogonal p> implants on the
other side [6,7]. The n> implants are isolated
by a spray-on p sheet [8]. The strip pitch is
about 25 lm and the readout pitch is about 50 lm
with the exact pitch determined from the number
of readout strips and guard rings. There are
1024 readout strips on the p-side and 1280 readout
strips on the n-side giving an active area of
32.8 cm.
Radiation tests using Rutherford scattering of
a 21 MeV proton beam into a small 1.2 cm;1.2 cm
and 5 cm;5 cm prototypes [9,10], as well as tests
of a single-sided 5.3 cm;7.3 cm prototype [11]
have been performed. These tests show that the
C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
67
Fig. 2. View of the main components of a detector module installed in 1997 (not to scale).
detectors can be operated to bias voltages well in
excess of 300 V.
The detector wafers are assembled into 19 cm
long by 7 cm wide modules (Fig. 2) consisting of
E carriers for holding the devices,
E microadaptors for taking the strip signals to the
electronics that are mounted 10 cm away from
the beam in order to keep radiation exposure
below 100 kRad/yr,
E printed circuit boards (Al O hybrids) that hold
the APC pipelined readout chips [12], distribute
power and timing signals to the chips, bring the
bias voltage to the detector, and carry the two
analog output signals per detector side.
The p-side of the detector has strips which are 7 cm
long, and, in the 1996/1997 setup provide measurement of track coordinates radial to the proton
beam. The p-side is glued to one side of the carrier
and the other side of the carrier has a microadaptor
with gold traces matching the readout pitch and
bonded to the p-side of the detector. These traces
carry the signals to the readout electronics located
about 4 cm away from the detector. The n-side
strips are orthogonal to the p-side and are also
connected to a second microadaptor mounted next
to the detector and carries the signals to the
readout hybrid. The 1024 p-readout strips and 1280
n-readout strips on each detector are bonded to the
microadaptors and the microadaptors are in turn
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C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
bonded to the APC pipelined readout chips. Together with the bonding of the printed circuit hybrid to the Kapton cable used to carry the clock,
power and analog signals, an excess of 4700 bonds
are required per module. One module can be
bonded in one to two days with an automatic
bonding machine.
Three detector modules installed in 1996 were
positioned 65 cm, 1, and 1.5 m from the target and
26, 28, and 54 mm from the beam axis (locations
303, 403, and 503 as indicated in Fig. 5). These
detectors were studied over a period of nine months
and then removed so that a second set of detectors
could be installed in 1997. The sets of modules are
quite similar; however, instead of using aluminum
nitride for the carrier, the 1997 modules implement
a new carrier substrate fabricated from a composite
structure of ultra-high modulus graphite fibers
(THORNEL K1100X) laminated in an epoxy
matrix. For the same thickness the product of the
thermal conductivity and radiation length of this
new substrate is larger than for the aluminum nitride carrier by a factor of eight. To further reduce
the radiation length within the acceptance the carrier substrate has been cut to the shape of a fork
underneath the silicon detector and the gold traces
on the microadaptor were replaced by aluminum.
One of the main deficiencies of the 1996 modules
was the huge leakage currents of the detectors.
Currents up to 700 lA at 100 V depletion voltage
and 10C were measured. It turned out that this
was an effect of the glue which was used to fix the
silicon detector to the carrier substrate. After extensive tests a silicone glue has been found to work
suitably [13]. The 1997 modules have dark currents of the order of a few microamps at room temperature.
The 1997 modules also feature a new strip detector layout where the strips of both sides have
been rotated by 2.5 with respect to the HERA-B
coordinate system to allow for stereo view tracking
when placing two of these detectors back to back
within a so called superlayer. The stereo detectors
led to a slightly modified design of the microadaptors which serve as a fanout between the silicon
detector and the readout chips. Finally, the 1996
detectors had only every second strip bonded on
five of the six sides. The 1997 detectors had every
strip bonded on five of the six sides with only 75%
of the strips bonded on one of the sides.
To minimize multiple scattering, the vertex detector modules are installed in a roman pot system
inside a large vacuum tank. An internal target consisting of movable wires arranged in a square
around the beam is also in the vacuum tank. The
vertex vessel in its 1996/1997-configuration is
shown in Fig. 3. The intricate manipulator mechanics discussed in Section 3 is clearly visible in the
photograph. Under stable running conditions these
devices allow one to bring the detector modules to
within 1 cm of the beam. An RF-shield installed
inside the vessel closely surrounds the beam to
prevent disturbances of the bunched proton beam
by wake fields created in the cavity of the vertex
tank. The silicon detectors are held in a secondary
vacuum where aluminum caps with wall thicknesses around 400 lm shield the detectors against
residual pickup from the beam. The shielding caps
and the RF-shield are discussed in detail in Sections 3 and 5, respectively. Cooling of the detector
modules is reported in Section 4. In 1996/1997, the
silicon detector was read out with APC pipeline
chips and a first version of the FED-Sharc chain as
described in Section 6. Results from the 1996 running period are presented in Section 7 and the 1997
running period is described in Section 8.
3. Vertex detector mechanics
The infrastructure for the VDS is a complex
system of mechanics, vacuum and cooling. Elements of this system that were installed for running
in 1996 and 1997 include
E the 2.6 m long stainless steel vacuum vessel
(Fig. 3),
E a 3 mm thick exit window having a 1 m diameter
fabricated from aluminum and welded to a 6 m
long tapered aluminum beam pipe of 0.5 mm
wall thickness,
E a prototype primary and secondary vacuum system with a vacuum manifold serving the top and
right-hand detector quadrants,
E six motorized manipulators with pots and shielding caps for the mounting of detectors in the top
C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
69
Fig. 3. The VDS vessel equipped with one Ti-sublimation pump and six manipulators.
and right-hand quadrants of superlayers 5—7,
and
E a preliminary version of the RF-shield and (see
Section 5),
E a prototype 400 lm thick aluminum shield cap
separating the detectors from the primary vacuum,
E three silicon detectors installed in the positions
303, 403 and 503 as indicated in Fig. 5.
In March 1996 the vessel equipped with pumps,
manipulators, RF-shield and target system was
transported together with the protected beam pipe
(see Fig. 4) into the West Hall for installation in the
HERA proton ring. With the secondary vacuum
system connected and three water-cooled detector
modules installed, the complete setup has been operated since then with no adverse impact on the
HERA machine operation.
The manipulators (Fig. 5) allow the detectors of
each quadrant to be moved radially with respect to
the beam so that they may be retracted to safe
positions during proton injection in the machine.
Lateral displacement of the detectors also allows
distribution of the radiation load over a larger
detector area.
The vertex vessel is a part of the primary machine vacuum and is separated from the secondary
vacuum of the detector modules by aluminum
shielding caps. This vessel is isolated from the
HERA vacuum by fast acting shutter valves that
automatically close in case of a failure. Pumping
the systems down to their respective vacuum levels
occurs in three main stages. The first stage takes
2 h. The two vacuum systems are evacuated to
a level of 1 mbar and are maintained to have a pressure difference no greater than 1 mbar to keep the
mechanical stress on the aluminum caps below
tolerance. The second stage takes 8 h and the pressure is brought to below 10\ mbar. At this point
the secondary vacuum has reached its proper value.
In the third stage it takes 2 days before the primary
vacuum is brought to the required 10\ mbar.
A perspective view of the shielding cap used in
the 1996/1997 setup is shown in Fig. 6. The top
part produced by cold extrusion [14] is welded to
a bottom piece that has been fabricated from
a single aluminum block by wire erosion at a workshop of the Heidelberg university. The wall thickness of 500 lm at the bottom of the cap tapers to
400 lm at the top and has been deliberately chosen
to be large for safety. Caps with thicknesses of
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Fig. 5. Locations of manipulators 103/203 (top) and 303 (bottom) for moving the detectors of superlayers 1—3/4 and 5, respectively. Pot names and positions of superlayers 1—8 are indicated.
‘x’ " (0,3,6,9) denotes the respective quadrant at 0,3,6 and
9 o’clock. Vessel flanges for pumps and the target stations are
labeled as ‘P’ and ‘T’, respectively.
150 lm have been fabricated and will be installed in
October 1997.
4. The VDS cooling system
To reduce the leakage current, noise and reverse
annealing of the silicon detector, the operating temperature is set to 10C or less. The silicon detector
and the readout chips are positioned inside the
geometrical acceptance region and in the secondary
vacuum of 10\ mbar of the HERA-B vertex detector. The heat load of the readout chips chosen
䉳
Fig. 4. The setup for transporting the vertex vessel with
manipulators, pumps and the 6 m long beam pipe into the West
Hall. The overall length is about 10 m.
C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
71
of 10C. The temperatures on each module were
measured with three Pt-100 sensors whose locations are indicated in Fig. 2. The temperatures are
monitored with a TempScan [15] system and
stored in a data base once a minute. A hardware
interlock system that cuts power to the detector
modules in case the cooling system fails ensures safe
operation of the detectors.
For the 1997 running, the aluminum nitride substrate has been replaced by a composite structure of
ultra-high modulus graphite K1100X-fibers
laminated in an epoxy matrix. The heat conduction
of this material [16] has been measured to be about
450 W/mK. Berylium was rejected from consideration as a carrier material because the product of the
heat conduction and radiation length for the carbon fiber composite is about two times larger. The
electrically conducting substrate is insulated from
the detector with a 50 lm thick piece of Kapton; the
Kapton does not affect the thermal contact between the detector and the carrier.
Fig. 6. Perspective view of the shielding cap used in the
1996/1997 run.
for the 1996/1997 running is about 1 W per one half
detector module. It is removed by the heat conduction through the detector carrier material (Fig. 2)
to a cooling block mounted onto the roman pot
outside the geometrical acceptance region and
cooled by forced convection.
The material of the heat bridge has to meet
several requirements, the most important of which
are large heat conductivity j, large radiation length
X , low outgasing rate and high mechanical stabil
ity. The material chosen in 1996 was aluminum
nitride. A good thermal and mechanical contact
between the aluminum nitride substrate and the
stainless steel cooling block is provided by a copper
plate that is glued to the cooling block. Bellows
welded between the cooling block and the water
pipes decouple the detector module mechanically.
For safety reasons stainless steel is used for water
pipes, cooling blocks and bellows inside the vacuum. A water—alcohol mixture cooled by a water
chiller to about 0C is pumped through the cooling
block. The cooling system was stable in time and
the detectors could be operated at the temperature
5. RF shielding
The 820 GeV HERA proton beam excites strong
wake fields inside the vertex vessel. These RF-fields
are responsible for single-bunch instabilities which
lead to unacceptable power loss in the vessel, and
resonant multi-bunch effects can cause global instabilities in the HERA proton ring. Therefore an
RF-shield surrounding the proton beam has to be
installed. To reduce multiple scattering close to the
silicon detectors the shielding has to be as light as
possible. The shielding must have a radius close to
the entering beam pipe radius so that resonances
are not excited at the ends of the tank, and it must
also carry the mirror charge from the beam.
Extensive calculations [17] have been done using
the MAFIA simulation software [18]. The calculations in the frequency domain and in the time
domain lead to a better understanding of the RFproperties of the vertex vessel. Shielding schemes
with a tube with holes (as installed for the 1996 run)
and four thin steel strips (as installed for the 1997
run) were studied. The different schemes were compared in terms of the shunt impedances predicted
for the eigenmodes of the vessel in a frequency
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Fig. 7. View into the vertex vessel showing the target cage and the RF-shielding tube. The left photograph depicts the RF-shield in the
region of the exit window of the vertex vessel. Clearly visible are the pumping holes in the tube which are needed to evacuate the vessel to
the same level as the HERA machine. The picture on the right shows the target region with its eight forks for the target wires. The proton
beam travels from right to left.
range up to 1 GHz. The configuration with four
steel strips provided the best shielding effect, with
some uncertainty due to the fact that the skin effect
could not be completely incorporated in the calculation.
In addition to numerical studies, the power
spectra in a model of the RF cavity for all shielding
models were compared at the INFN in Naples with
an improved coaxial line method [19,20]. In the
frequency range up to 2 GHz all shielding models
show a reduction by a factor 10—100 over a vertex
vessel having no shielding.
During the 1996 HERA operation an aluminum
pipe with a wall thickness of 150 lm and 40 mm
diameter, close to the beam pipe diameter of
45 mm, was installed in the vertex vessel (Fig. 7).
The aluminum has a thickness that is in excess of
ten skin depths. A small loop antenna was installed
at the point where the vertex vessel flares out into
a cone. The B component of the electromagnetic
(
fields were observed to measure TM modes exited
by the beam during a normal HERA luminosity
run.
To reduce the scattering length the tube has been
replaced by a shielding solution consisting of thin
steel strips. In contrast to the aluminum pipe, the
thickness of the strips is only about to of a skin
depth. In December 1996 a first version of the strip
shielding, consisting of three stainless steel strips
(width 12.7 mm, thickness 5 lm) stretched along the
C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
beam direction was installed during a short break
in the machine operation. A dedicated proton run
was used to test the new shielding. No instabilities
were detected in the machine and for the 1997
luminosity run period a shield with four strips
separted by 45 mm was used. The recorded power
spectra (Fig. 8) shows that there is less power in the
frequency modes excited in the 1997 setup with
steel strips than that in 1996 with the metal pipe.
6. Data acquisition
The readout chain of the silicon vertex detector
consists of readout chips for the silicon strip detector signals, analog optical transmission lines
from the detector to the control room, front-end
driver boards (FED) for digitization and event buffer boards. A prototype FED was developed at the
University of Heidelberg [21] while digital signal
processors from Analog Devices (ADSP 21060) are
used for the event buffers [22].
The design bandwidth of the chain is chosen to
allow for a 100 kHz event rate. The prototype FED
and the APC readout chip were used in 1996/1997.
The silicon detector is read out after a first level
trigger (FLT) has been issued for a given HERA
bunch crossing. In this case the fast control system
(FCS) sends a trigger signal, an 8-bit bunch crossing number and a 16-bit event number to the
front-end-driver boards. The FEDs send the trigger
with the correct latency to the readout chips. For
the 1996/1997 running, a trigger control board
(TCB) was constructed [11] to emulate the behavior of the FCS system.
The APC readout chips contain a 32-deep pipeline for the analog detector signals. The 128 input
channels of each APC chip are multiplexed to
a single analog output that is then daisy-chained in
two groups of 4 chips on the p-side and two groups
of 5 chips on the n-side. Thus only two analog
output signals are required per detector side. Timing control of the APC is provided by a programmable digital sequencer. This sequencer and the TCB
board operate together to accept triggers, provide
clocking signals of the pipeline between triggers
and allow the APC pipeline to be halted and the
serial readout to occur.
73
The prototype FED, Fig. 9, is a VME board
containing the hardware for 12 channels. It is controlled by a field programmable gate array (FPGA)
Xilinx XC4013-4. Two state machines are realised
on the FPGA. The first one operates at the bunch
crossing frequency (10.4 MHz) for communication
with the fast control system. For the 1996/1997
running, this clock was run at half the bunch crossing frequency. This state machine contains a FIFO
for the first level triggers in order to facilitate deadtimeless readout despite the fact that the time between two triggers can be smaller than the readout
time for one event.
The second state machine runs with the 1 MHz
clock of the readout chip multiplexer and controls
the data transfer to the event buffer. An event
record consists of a header, the digitized data and
a trailer. Header and trailer are added to ensure
event integrity and contain information such as the
FLT number and the bunch crossing number of the
trigger.
The analog input is digitized with a 10-bit flash
ADC with a sampling rate of 1 MHz for the
1996/1997 operation. Since the ADSP processor
used as event buffer has 4-bit wide link ports for
asynchronous data transfer the 10-bit ADC result
has to be multiplexed. For multiplexing in situ
erasable programmable logic devices (EPLD) are
used (Lattice ispLSI1032-80).
The ADSP processor has 4 Mbits of on-chip
memory which is dual ported and can therefore be
accessed by the core processor and a DMA processor simultaneously. In 1996/1997 data received
from the FED were read out directly via VME bus.
7. Results from the 1996 running
In 1996 the first data were taken with doublesided VDS detector modules. From October to
December about 360 runs with typically 4000 triggers each were taken and analyzed. Although only
three detector modules with a total of 3968 bonded
strips were installed, the pipeline readout with
a depth of 32 cells made it necessary to keep track
of 126 976 active readout channels.
Combining hits from the two sides of each detector module one obtains candidates for spatial
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C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
Fig. 9. Schematic layout of the front end driver board.
75
Fig. 11. Overlay of data from runs with different single target
wires. The transverse coordinates of track candidates from the
vertex detector are plotted at the plane of the target wires. The
clusters correspond to the individual wires.
Fig. 10. Display of an event with track candidates originating at
the target wire and being observed in all three detector modules.
䉳
Fig. 8. The RF power spectrum recorded in the VDS vacuum
tank with an RF shielding consisting of a tube (top plots)
compared to that for an RF shielding made from four thin steel
bands (bottom plots).
Fig. 12. Distribution of the extrapolated track positions to the
target plane for events produced from a single wire target. The
data were taken with the 1996 detector prototypes. The twodimensional distribution is shown together with its projections
along (upper right) and orthogonal to the wire (lower left).
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Fig. 13. Real-time single event display of pulse heights vs. strip for each of the two sides of the three modules installed in 1997 after
baseline and pedestal subtraction. A clear signal from a track passing through the detector is seen. The signal is positive on the n-sides
(sides 0, 2, 4) and negative on the p-sides (sides 1, 3, 5) of the detector.
coordinates, which can be used to form track candidates. Studying the transverse coordinates of those
candidates at the z-position of the target wires, one
observes a clear signal over some combinatorial
background from tracks originating at the wire.
Fig. 10 shows an event with tracks from one wire
which pass through all three detector modules.
An overlay of target positions measured by projecting tracks from different events to the target
plane in runs with different single target wires is
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77
Fig. 14. (a) Residuals in x and (b) y as a function of position for the tracks extrapolated from modules 303/403 to 503 before alignment.
Position dependence of the residuals indicates a tilt and the difference from zero the offset. (c) Images of the interaction region on the
target wire determined from tracks found by associating hits in pairs of planes 303/403, 303/503 and 403/503 before alignment. Multiple
spots appearing as one large spot here indicate the misalignment of the detector. (d)—(f) are the same as (a)—(c) but with alignment.
given in Fig. 11. The elongated forms of the hit
distributions on the wires are clearly visible. From
the rms width of the projection orthogonal to the
wires the intrinsic resolution for the target position
was measured to be around 300 lm for the 1996
setup, in agreement with Monte-Carlo estimates for
the 1996 geometry. Along the wires the width of the
distribution is dominated by the width of the beam
profile, which has an rms width around 500 lm.
Data from one wire with its two projections is
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shown in Fig. 12. The Gaussian profile in both
views indicates that the target wires are scraping
the beam in order to produce the required interaction rates.
Analyzing runs with more than one target wire
allow direct determination of the distribution of
hits on different wires. Thus, the VDS serves to
monitor the performance of the target control system in its task of equalizing the interaction rate on
all wires.
The vertex detector system thus not only demonstrated its functionality in the 1996 running but was
also successfully employed as a diagnostics tool for
the HERA-B wire target.
8. Results from the 1997 run
New module prototypes with less material and
a 2.5 tilt in the strips were installed together
with a steel band RF shield in January 1997. The
Fig. 16. Signal-to-noise distributions for strips belonging to
events with a single track passing through all detectors.
Fig. 15. Distribution of residuals for the tracks extrapolated
from modules 303/403 to 503 after alignment.
analysis from 1996 was enhanced and integrated
into the online environment. Online analysis features pedestal and baseline determination for each
pipeline cell of each strip, cluster finding and writing of clusters in a sparse data format, simple tracking and alignment.
An example of a single event from the 1997
detectors is shown in Fig. 13. A cluster of strips in
each detector side is clearly visible.
The alignment of the detector in x- and y-projections is done by extrapolating hits from pairs of
planes to the third plane and requiring that the
difference between the expected hit position and the
measured hit be zero for all strips in both projections. Residuals of the extrapolation of tracks from
planes 303 and 403 to plane 503 are shown before
(Fig. 14a—c) and after (Fig. 14d—f) alignment as
a function of the strip position. The strip dependence indicates a tilt of the detectors with respect to
one another that has been removed with the alignment procedure. Alignment of the detector is
C. Bauer et al. /Nucl. Instr. and Meth. in Phys. Res. A 418 (1998) 65—79
checked by extrapolating pairs of hits in the three
combinations of modules (303/403, 303/503 and
403/503) to the target plane. Before alignment, the
three combinations show three interaction regions
(Fig. 14c) that appear as one large spot. After alignment, the three spots lie on top of one another
(Fig. 14f). The final distribution of residuals is
shown in Fig. 15.
Using tracks that pass through all the three detectors, distribution of the signal-to-noise ratio is
shown for each of the detector planes in
Fig. 16 and have means lying between 12 and 17.
9. Summary and conclusions
Two sets of three planes of prototype HERA-B
VDS detectors have been operated with the
HERA-B wire target for approximately six weeks.
The infrastructure supporting the positioning of the
detectors close to the HERA proton beam with
minimal material has functioned well. Specifically,
the two vacuum systems have been operated without failure for a total of 17 months. The RF shielding within the vacuum tank has not affected the
operation of the HERA protons. The cooling of the
detector and the interlock system have functioned
properly.
The detectors have been operated with failure in
less than 1% of the bonded strips. Imaging of the
target interaction region, monitoring of the target
control system, cluster finding and data sparsification, and alignment have all become part of the
routine online operation of the detector. The pointing resolution of tracks is dominated by multiple
scattering and is at the level expected by the material installed.
Acknowledgements
We wish to thank our colleagues from the accelerator divisions for the successful operation of the
79
HERA machine. We would like to express our
gratitude to Pavel S[ olc at MPI-Munich and Jeff
Bizzell at Rutherford Appelton Laboratory for
their bonding artistry and Günther Tratzl at MPIMunich for the module assembly.
References
[1] T. Lohse et al., HERA-B an experiment to study CP
violation in the B-system using an internal target at the
HERA proton ring, Proposal, DESY-PRC 94/02, 1994.
[2] E. Hartouni et al., An experiment to study CP violation in
the B system using an internal target at the HERA proton
ring, Technical Design Report, DESY-PRC 95/01, 1995.
[3] ATLAS Technical Proposal, LHCC/P2. CERN/LHCC/
94-43.
[4] K. Riechmann et al., Nucl. Instr. and Meth. A 377 (1995)
276.
[5] A. Bischoff et al., Nucl. Instr. and Meth. A 326 (1993) 27.
[6] P. Holl et al., IEEE Trans. Nucl. Sci. NS 36 (1988) 251.
[7] M. Caccia et al., Nucl. Instr. and Meth. A 260 (1987) 124.
[8] J. Kemmer et al., Nucl. Instr. and Meth. A 326 (1993) 209.
[9] I. Abt et al., IEEE Trans. Nucl. Sci. NS 43 (1996) 1113.
[10] R. Rübsam, Untersuchung von doppelseitigen SiliziumStreifendetektoren mit neuartiger Schutzringstruktur, Diplomarbeit, München, November 1995, MPI-PhE/96-07.
[11] M. Eberle, Untersuchung eines typ-invertierten SiliziumStreifenzhlers mit der Auslesekette des HERA-B Vertexdetektors des Jahres 1996, Diplomarbeit, Heidelberg,
Mai 1997.
[12] R. Horisberger, D. Pitzl, Nucl. Instr. and Meth. A 326
(1993) 92.
[13] I. Abt et al., MPI-PhE/97-20.
[14] In collaboration with Fa. Unglaub, Ubstadt-Weiher.
[15] TempScan/1000, IOtech, Ohio, USA, 1994.
[16] L. Seybold, Thermische Modellierung der HERA-B Vertexdetektormodule, Diplomarbeit, Heidelberg, 1996.
[17] F. Klefenz, Die Hochfrequenz Abschirmung des HERA-B
Vertexdetektors, Diplomarbeit, Heidelberg, 1997.
[18] T. Weiland, Part. Accel. 15 (1984) 245.
[19] V.G. Vaccaro, INFN-TC-94-23 Report, 1994.
[20] L.S. Walling et al., Nucl. Instr. and Meth. A 281 (1989) 433.
[21] C. Polenz, Aufbau eines Prototypen für die Auslese der
MSGC’s bei HERA-B Diplomarbeit, Heidelberg, 1996.
[22] R. Wurth, Status of Sharc Board Development, Zeuten
1995, HERA-B internal note no. 95, 1995.
II. FIXED TARGET VERTEX DETECTORS