A System to Monitor Displacements of
the Inner Tracking System
T. Humanic, B. S. Nilsen, D. Truesdale
The Ohio State University
M. Cherney, J. Fujita, Y. Gorbunov, R. Thomen
Creighton University
June 12, 2007
Abstract
The ALICE ITS Alignment Monitoring System, ITSAMS, is a simple
geometrical optics system using off-the-shelf PC CCD cameras and USB
communication protocols. The system consists of, a computer to monitor the
signals, eight multi-mode fibers for signal transport, a CAT5 USB 1.1 bridge,
motor control systems, four spherical mirrors each with a piezoelectric motor for
remote angular adjustment, and four Active Sensors. Each Active Sensor contains
a USB CCD camera, a 0.8 mW laser diode light source, a fiber optic collimator,
an optical USB signal extender, an electronics driver board, and two piezoelectric
motors for remote adjustment. The laser diode produces a light source which is
then collimated into a beam and projected onto a spherical mirror mounted on the
ITS SSD cone. The spherical mirror, having a focal point located on the CCD
imager, then focuses the beam back onto the Active Sensor's CCD array. A
movement of either the mirror or the Active Sensor causes the beam spot on the
CCD array to move. This automatically produces a resolution of relative
movement between the two pieces on the order of 1/ 12 of the CCD pixel size.
Using the geometry of the placement of the four mirrors and Active Sensor pairs,
it is possible to reconstruct the motion of the ITS, as a rigid body, in 3dimensional space to micron level accuracy.
1 Introduction
The Inner Tracking System (ITS) of the ALICE relativistic heavy ion detector,
pictured in Figure 1, is made up of 2198 silicon detectors arranged in six cylindrical
layers, with resolutions approaching 20 micronsa. The ITS has been designed to greatly
improve ALICE's momentum resolution, primary and secondary vertex resolution,
measure event multiplicity, and to extend ALICE's particle identification and tracking
capabilities beyond that attainable with the ALICE Time Projection Chamber aloneb.
Figure 1: The ITS SDD and SSD ladders and support cones. The SSD ladders are
shown in green and the SDD ladders are shown in orange. The SDD cone is in blue and
the SSD cone is in light-blue. The SPD staves are mounted also on a carbon fiber
support structure which is placed inside the SDD support cylinder, and is supported by a
second set of cones outside of the SDD and SSD cones. The holes in these cones allow
the necessary cabling and cooling services between the detectors and the rack of
electronics. Note: The SSD central cylinder is not shown.
The ITS silicone detectors are mechanically organized into a set of ladders †.
With the exception of the Pixel Detector, these ladders hold a line, along the z-axis, of
detectors which are staggered up and down, such that there is some overlap between
†
Called Staves for the SPD layers.
each detector on the ladder. A series of ladders are arranged into one of six cylinders
around the beam pipe, again staggered, up and down, so that there is some overlap
between each detector around this cylinder. Each cylinder is at a different radius starting
from nearly 4 cm out to nearly 44 cm. The ladders are supported by one of three carbon
fiber/foam support cones. The outer two cones are mechanically attached and each is
separated by a foam cylinder covered in two layers of carbon fiber. The Silicon Pixel
ladders are attached to their own support cone which is attached to the outer most part
of the outer Silicon Strip support cone, as shown in Figure 2. These ITS support
structures are attached to three mounting brackets which match up with two support
rings. These support rings are cantilevered off of the TPC support structure at the ends
of the TPC.
Figure 2: A cross section view of the ITS, RB 26 side, showing the layers of the ITS,
the support cones, cabling and other services, and other detectors. Also shown is the
Inner TPC cylinder and the ITS support structure. The ITS will hang on ``hooks''
attached to the ITS support structure and the outer, SSD, support cone. The mounting is
only at three points to constrain the position of the ITS without inducing any possible
stresses.
The great spatial resolution of the ITS detectors makes it critical to have, for the
proper operation of the ITS and the physics goals of ALICE, great knowledge of the
positions of each of these detectors. There are three ways in which this will be
accomplished. The first is through a series of surveys carried out through out all stages
of the ITS construction, assembly, and installation. The second is by track
reconstructionc. Finally, the third is by a system of monitors. It is this ITS Alignment
Monitoring System (ITSAMS) which is described here.
2 Concept
The ITS Alignment Monitoring System (ITSAMS) is intended to monitor the ITS
for any distortions that may occur during operations, and down times‡. It is not practical
to monitor most of the components, therefore the system will monitor a few accessible
and strategic points on the ITS, TPC, and various support structures. Any changes
detected at these positions will indicate that some distortions in the positions of the
different ITS elements will have occurred. Most of these elements will be operational
during installation and de-installation of the ITS so that any distortions can be detected
and, hopefully, compensated for. A conceptual diagram of this system is shown in
Figure 3.
Figure 3: A conceptual drawing of the ITSAMS Active Sensor and spherical mirror. On
the left, mounted to the TPC end plate is the Active Sensor, which includes a CCD array
and the collimated beam source. On the right is a spherical mirror mounted to the SSD
cone. The optical path is shown in red.
2.1 Design Theory
The ITSAMS is based on the properties of a spherical mirror. The key feature
being that parallel light rays striking the surface will be focused to a single point called
the focal point. Thus, any movement of the vertex of the mirror will result in an
identical movement of the focal point. By designing the mirrors to focus the beam
directly on the CCD imager, the movement of the focal point can be measured. Thus,
the motion of the spherical mirror relative to the CCD imager can be determined. As the
ITSAMS system utilizes a CCD camera with pixels measuring 7.4 microns, pixel
averaging should yield an absolute movement precision of 2-3 microns.
Given the slight incline of the optical path away from the SSD cone toward the
TPC, it becomes possible to calculate the translational motion in 3-dimensions and the
rotational motion about all three axises using only three mirror-sensor pairs. As the
mirror-sensor pairs will be placed at 90° increments starting from 45° off vertical, the
formula for all six forms of relative motion in terms of the first three mirror-sensor pair
coordinates are:
‡
Including installation and de-installation.
x 
y 
z 
X B  X C
2
X B  X A
2
Y A   C
2  Sin 
360
X B  X A  YC  YB 
4R  Sin 
360
X B  X C  YB  YA 
 y 
4R  Sin 
360
 X A  X C 
 z 
4r  Sin 
These equations use only sensors A, B, and C. Other iterations exist for any other three
sensor combination. Assuming that the angle between the beam axis and the optical
beam is   3.86  , we can approximate that δx and δy can be resolved to 2-3 microns, δz
to 30-45 microns, δθx and δθy to 0.68-1.01 millidegrees§, and δθz to 2.54.0 millidegrees**.
 x 
Figure 4: A diagram of the four Active Sensors as seen from the center of the ITS. The
dotted lines indicate the coordinate axises of each CCD.}
§
R=2536 mm, the distance from the center of the ITS to the plane of the TPC end plate.
r=645 mm, the distance from the beam path to the CCD imagers.
**
2.2 Data Transmission
The ITSAMS uses the USB 1.1†† protocold standard for data transmission. This
allows the use of off-the-self, digital web-cameras, which are small and cheap. In order
to overcome the five meter distance barrier inherent in USB, it is necessary to use an
active repeating extender. USB fiber bridges are being used to carry the image data out
from the Active Sensor located on the TPC and to send camera configuration data and
triggers from the computer back to the Active Sensor. This reduces cable thickness, the
likelihood of signal corruption, and grounding issues at the expense of powering the
Active Sensor locally. The fiber bridges carry the ITSAMS signals from the TPC end
plate to a rack located in the pit near the detector. Once in the rack space, the data is
converted back into a standard USB 1.1 signal and combined with USB signals from
other Active Sensors into a single signal. That combined signal is then sent through a
CAT 5 bridge up to a PC in one of the counting houses. This is conceptually shown in
Figure 5. There, the signal is analyzed by the computer which also commands all
cameras and interfaces with the ALICE slow controls signaling if and when some
distortion has been detected. This PC will also allow for on-line direct monitoring of
this system.
Figure 5: A conceptual drawing of the data path for ITSAMS. Note that data,
commands, and triggers can flow in both directions along this path.
2.3 Optical Circuits
The light beams needed for the ITSAMS are produced in a 0.8 mW laser diode
module mounted onto a power board located on the Active Sensor mount. The laser
light is transmitted to a collimator via a flexible single mode optical fiber. A beam is
created with the collimator and is then sent out of the Active Sensor. In order to bend
around the TPC onto which the Active Sensor is mounted, the beam reflects off a mirror
suspended from the ITSAMS TPC mount. The beam then traverses the distance to the
spherical mirror and back again impacting on the sensor's CCD array. This necessitates
a clear optical path between the inner edge of the TPC end plate and the SSD cone
wings on the RB24 side of the detector.
††
This is a limit of the protocol version supported by the USB Optical extender system.
2.4 Dynamic Range
A key issue dealing with the mounting of both the Active Sensors and the mirrors is
the system's projected dynamic range. The chief limits of the system's dynamic range
are the physical sizes of the mirror and the ¼ inch CCD imager. The mirror must be
aimed to focus perfectly on the center of the CCD to within an angular divergence of
0.0454° and a linear divergence of 1.58 mm. The Active Sensor must also be aimed at
the center of the mirror to within an angular divergence of 0.2728° and a linear
divergence of 1.58 mm. Since the system will be inaccessible after the ITS and TPC
have been installed, a method the system is capable of remote adjustment of the
mounting system.
2.5 Design
The design of the ITSAMS focused on six design priorities: high precision, small
physical size, low thermal output, low weight, relatively large dynamic range, and low
cost. Since the optical geometry already provides for high precision, provided the
physical pixel size is small, the actual design centered primarily about the latter five
priorities. The optics board requires only slightly more space than the camera board and
optical extender board, so it is also used as the board mounts to save space yielding a
final design size of no more than 70×33×40 mm3 for the Active Sensor. In order to
reduce power consumption in the thermally sensitive areas of the detector, all of the
optical components, except for the CCD array, USB optical extender, and laser diode
are passive. The imager is a CCD, which is inherently radiation tolerant. The camera
and optical extender are both off-the-shelf commercial technology.
2.6 Mounting System
The ITSAMS is, by design, extremely sensitive to extraneous motion. Therefore,
each Active Sensor and spherical mirror must be mounted securely to the objects they
are to monitor. To that end, wings have been added to the ITS support cones to act as
mounting locations for the ITSAMS. The wings, pictured in Figure 6, will serve as
mounts for both spherical mirrors on the ITS. In addition, both the TPC and SSD cone
mounts are made from aluminum and have multiple attachment points to their
respective detectors.
Figure 6: A preliminary drawing of the ITS support cones with an emphasized view of
the mounting wings.
The limited dynamic range of the system further requires that some form of remote
adjustment be built into the mounting system. To accomplish this, the spherical mirror
mount is hinged. The angle of the hinge is determined by rotating a threaded linear
piezoelectric motor, allowing the mirror's angle with respect to the beam axis to be
adjusted. In addition, the TPC mount incorporates two piezoelectric motors. One can
drive the Active Sensor up to 15 mm in either direction angularly around the TPC. The
other can change the orientation of the mirror used to bend the optical path. All three
motors for each mirror-sensor pair are being controlled from the counting house where
the computer is located‡‡.
2.7
Electrical Systems and Power Requirements
The ITSAMS has powered components in three different locations. The
counting house contains a PC with a USB port and connections to the ALICE slow
controls and on-line monitoring. It also contains the drive and power electronics to
control the remote adjustment motors. The ITSAMS also a CAT 5 USB bridge unit
located in the rack space down in the pit. It requires a DC current supplied at +5 V.
Each Active Sensor on the ITS also requires a DC current supplied at +9 V. Each
Active Sensor has an electronics power board attached to the TPC mount. This board
supplies power to the USB driven electronics in the Active Sensor and drives the laser
diode module. Table 1 gives a more detailed list of the power requirements of the
ITSAMS components.
Table 1: Detailed Power Requirements of ITSAMS Components.
Component
Current [mA]
Voltage [V]
Power [mW]
Personal Computer
Line voltage
450000
CAT 5 USB 1.1 Bridge
400 (max)
+5 DcV
200 (max)
CCD Imager
100ξ
+5 DcV
500ξ
ξ
USB Fiber Bridge
50-70
+5 DcV
250-350ξ
‡‡
Computer control of the motor drive systems will be attempted.
Laser Diode Module
ξ Value per Active Sensor.
30ξ
+2.2 DcV
250-350ξ
2.8 Components
The components of the ITSAMS and their power requirements are listed in
Table 2. The camera selected for the system is a Panasonic GP-KR651 US web-camera
with a ¼ inch CCD imager. These cameras are no longer made by Panasonic.
Consequently thirty cameras have been purchased for testing and development,
production, and replacement uses. The Optics Fiber Optic USB Extender and the fiber
optic collimator are still being produced; consequently there are no supply problems for
these components. Since these are the three most critical items about which the Active
Sensor is designed and built. This Active Sensor optics board and the spherical mirror
are specially manufactured to required specifications as needed.
Table 2: List of active components.
Component
Quantity
Power
Location
Requirement
Active Sensor
16
750-850 [mW]
On TPC
ζ
Eliptec X15G Pizo 1 per Sensor
[mW]
On TPC
motor
Laser Diode
1 per Sensor
On TPC
Spherical Mirror
1 per Sensor
On ITS SSD cone wing
Siggle Pizo Motors
[mW]ζ
On ITS SSD cone wing
e
Icron Ranger 410
1
2000 [mW] max
Rack Space
Personal Computer
1
450 [W]
Counting House
ζ Powered on only during initial calibration/alignment.
Table 3: Active Sensor Subcomponents.
Component
GP-KR651 US PC Web-camera, Panasonic
M2-100 USB Extender, Opticisf
CFS-T-2-VIS Fiber Collimator, Optics for Resarch
Power Requirement
500 [mW]
250-350 [mW]
-
2.9 Supporting Software
The ITSAMS is controlled by a personal computer running ITSAMS
comparison software that triggers each camera at regular intervals and compares images
received with earlier images in order to determine any differences. A preliminary
version is pictured in Figure 7. This is done by subtracting pixel intensity values of the
previous image from the newly received image. The computer will also interface with
the ALICE slow controls in a manner that has not yet been determined. The software
can compute relative motion of any of the spherical mirrors via a subtraction algorithm.
The software application, written in Windows using .net and Windows 7 C++ compiler,
can also take advantage of interference fringes to vastly increase resolution in the plane
perpendicular to the optical path. A simple mean of the image intensities gives the shift
of the mirror, as can be seen in Figure 8, or a Gaussian fit to the spot shape-intensity can
be implemented. In addition, a direct interface between this software and the USB
camera has yet to be developed. At present, a second piece of software must take an
image and write it to a file for processing by this application.
Figure 7: A preliminary version of the comparison software. In the Subtraction Image,
pixels with positive intensity values are red and negative values are green.
Figure 8: This plot shows the tests done in the plane perpendicular to the optical path.
The average slope (Red) compares well, especially over short distances, with the
theoretical slope (Green).
3 Tests
Figure 9: Here is an old active sensor module with the CCD camera mounted on it and
the fiber optic collimator, beam splitter, and adjustable flat mirror mounted on it. Not
shown is the Optical fiber USB extender card which can be mounted behind the camera
board.
Figure 10: This is the test set up used, showing the active module and a 2 inch high
quality spherical mirror. This mirror will be replaced with one specially made to our
specification, which will include focal length size shape, and material. In this test set up,
the mirror can be moved, using micrometer screws horizontally (x) or along the beam
direction (z).
4 Radiation Testing
The radiation environment at different places inside the ALICE experimental
cavity an around the ALICE detectors has been the subject of extensive simulation and
study. There are basically 4 different scenarios under which the radiation levels have
been estimated. 1) Continues running. 2) Beam miss-injection producing a grazing
injection energy beam. 3) Beam miss-injection producing an injection energy beam
sweep of the cavity. 4) Full energy beam dump into the ALICE detector system. Since
no ALICE sub system is expected to survive this last scenario, it will not be considered
any further.
4.1 The radiation environment
Scenarios 2 and 3 are covered by the ALICE internal note “Radiation in ALICE
from a misinjected beam to LHC” by Paolo Giubellino, Andreas Morsch, Lars Leistam
from CERN, Geneva Switzerland, and Blahoslav Pastirčák from the Institute of
Experimental Physics SAS, Košice, Slovak republic h. The first case is covered by “To
be found”.
Parts of the ITSAMS are located in one of 3 different regions. 1) On the wings
of the RB24 side SSD cone. Here are mounted passive first surface mirrors. These
mirrors can be adjusted by two pizo-electric motors (New Scale Technologies, SQL100-N) so that the beam from the camera subsystem will be focused and returned to the
camera for detection. 2) At the inner support ring of the TPC is mounted the laser light
source, USB camera, USB to fiber optic converter, and 2 pizo-electric motors (Elliptic,
X15G) to adjust the camera position on this TPC inner ring, and some power regulation.
3) in rack number A12g, is located the power supplies for this system (same as the SDD
for low voltage power supplies), the fiber back to USB converters, a USB 4 port hub
and cat-5 long distance bridge, and a patch panel to bring these signals up to the DCS
computer for real time analysis.
In Table 4 is listed an estimate of the radiation dose for a given miss-injection
and for continues running for the 3 different regions. These values were taken from
figures 5, 7, and 8 and table 2 from the paper by Giubellino, Morsch, Leistam, and
Pastirčákh.
Table 4: Energy deposition (rad) in the 3 different regions where the ITSAMS subsystem will be located for the 3 different scenarios.
Continues
Miss-injection
Miss-injection
running
grazing
sweep
SSD cone (mirrors)
10.0-1.0
1.0-0.1
TPC end plate (camera)
1.0-0.1
0.1-0.01
ITS-SDD Rack
10.0-1.0
10.0-0.1
4.2 Exposure of Components
During November 2004, an ITS test beam was conducted during which the
ITSAMS camera and its USB to fiber optics board were placed in the beam. The beam
consisted of 120 GeV pion beam. This beam consisted of 214.2857 spills/hour, 2.57E6
particles/spill, and lasted from 15:00 Nov. 14 2007 to 08:01 Nov. 15 2007 with 8
minutes of down time. This is 9.30E9 minimum ionizing particles in total. Since a 120
GeV pion deposits about 2.1 MeV cm^2/g of energy in Si this is 1.95E10 MeV cm^2/g
of energy depositedi. Recalling that 100Rad=6.24E12 MeV/kg (or 1 Rad = 6.24E7
MeV/g)j. This gives a radiation level applied to these components of 313 Rad cm^2§§.
§§
Beam size is of the order of 1cm2.
During this test, there were no malfunctions with the exception of 8 bad CCD
pixels being found. After a time powered off, none of the bad pixels could be found.
5 Conclusions
The ITSAMS currently under development can yield high precision monitoring of
the relative motion of various strategic points on and around the ITS to a 10-6 order of
magnitude resolution in the plane perpendicular to the optical path and a 10-5 order of
magnitude resolution in the plane parallel to the optical path. The entire system is
inexpensive and physically small for space issues near the ITS. It also has mostly
passive components, which limits grounding and thermal issues, and requires power at
only one of only two locations per strategically monitored point. In these regards, the
ITSAMS is a better option then comparable systems currently available for alignment
monitoring purposes, and is ideal for the alignment monitoring requirements of the
ALICE ITS.
6 References:
The ALICE Collaboration, “Inner Tracking System”, CERN/LHCC 99-12, ALICE
TDR 4, 18 June 1999, Table 1.3
a
The ALICE Collaboration, “Inner Tracking System”, CERN/LHCC 99-12, ALICE
TDR 4, 18 June 1999, Chapter 1.2.3
b
The ALICE Collaboration, “Inner Tracking System”, CERN/LHCC 99-12, ALICE
TDR 4, 18 June 1999, Chapters 6.2 and 6.3.
c
“Universal Serial Bus Specification”. Compaq Computer Corporation, Intel
Corporation, Microsoft Corporation, NEC Corporation. 1998
d
Icron Systems Inc. “USB Ranger Models 110 and 410 User Guide”. Burnaby, British
Columbia Canada: Icron Systems Inc, 2000.
e
Opticis Co. “M2-100/210/10S/21S Data Sheet”. Richmond Hill, Ontario Canada:
Opticis North America Inc, 2002.
f
Flavio Tosello, Private communications, To Bjorn S. Nilsen, e-mail Subject “Re:
Chassis for Rack in Cave (quick question)”, January 22 2007.
g
Paolo Giubellino, Andreas Morsch, Lars Leistam, and Blahoslav Pastirčák, “Radiation
in ALICE from a misinjected beam to LHC”, ALICE Internal Note # ALICE-INT2001-03 v.1, January 11 2001.
h
The Particle Data Group, “Review of Particle Physics”, Journal of Physics G. V 33
2006, figure 27.3
i
The Particle Data Group, “Review of Particle Physics”, Journal of Physics G. V 33
2006 Section 29.
j