# Material Budget Calculation of the new Inner Tracking System, ALICE

WANCHALOEM POONSAWAT  School of Physics, Institute of Science, Suranaree
University of Technology
Nakhon Ratchasima 30000, Thailand
<EMAIL_ADDRESS>CHINORAT KOBDAJ111Corresponding author School of
Physics, Institute of Science, Suranaree University of Technology
Nakhon Ratchasima 30000, Thailand
<EMAIL_ADDRESS>MARIO SITTA  Universit$\acute{a}$ del Piemonte Orientale,
Alessandria and INFN
Torino 10125, Italy
<EMAIL_ADDRESS>YUPENG YAN  School of Physics, Institute of Science,
Suranaree University of Technology
Nakhon Ratchasima 30000, Thailand
<EMAIL_ADDRESS>
(Day Month Year; Day Month Year)

###### Abstract

The ALICE Collaboration aims at studying the physics of strongly interacting
matter by building up a dedicated heavy-ion detector. The Inner Tracking
System (ITS) is located in the heart of the ALICE Detector surrounding the
interaction point. Now, ALICE has a plan to upgrade the inner tracking system
for rare probes at low transverse momentum. The new ITS composes of seven
layers of silicon pixel sensor on the supporting structure. One goal of the
new design is to reduce the material budget ($X/X_{0}$) per layer to 0.3$\%$
for inner layers and 0.8$\%$ for middle and outer layers. In this work, we
perform the simulations based on detailed geometry descriptions of different
supporting structures for inner and outer barrel using ALIROOT. Our results
show that it is possible to reduce the material budget of the inner and outer
barrel to the value that we have expected. The manufacturing of such
prototypes are also possible.

###### keywords:

Detector design and materials; Overall mechanics design (support structures
and materials).

PACS numbers: 29.40.Gx, 29.90.+r

##  1 Introduction

A Large Ion Collider Experiment (ALICE) is designed to studies the property of
Quark-Gluon Plasma (QGP), the deconfined state of strongly interacting matter.
There are eighteen systems within ALICE detector. In this work, we are
interested in a central part of ALICE detector called the Inner Tracking
System. The present ITS is made of six layers exploiting three different
silicon technologies. The main duty of ITS is to detect the primary (where the
collision occurs) and secondary vertices (where some of the unstable heavy
particles decay after a flight distance of some hundreds of micrometers). The
innermost layers are needed to be high resolution devices to record both x and
y coordinates for each passing particle.[1]

The precision of the present ITS for charm mesons is insufficient at low
transverse momentum ($<$ 1 GeV/c) and even worse for charm baryons. In case of
charm baryon, the lowest-mass charm baryon is the $\Lambda_{c}$ with a rest
mass of 2286.46 $\pm$ 0.14 MeV/c2.[2] The important channel of its measurement
is the decay of $\Lambda_{c}\to pK^{-}\pi^{+}$ with a branching ratio equal to
5.0 $\pm$ 1.3 $\%$. The mean proper decay length ($c\tau$) of $\Lambda_{c}$ is
only 60 $\upmu$m with a very short lifetime that turns out to be shorter than
the impact parameter resolution of the present ITS. Therefore, the charm
baryons are partially accessible by the ALICE detector in central Pb-Pb
collisions.

The current measurements in Pb-Pb collisions are characterized by a very small
signal-over-background ratio, which calls for large statistics. Thus, to reach
the goal of heavy quark measurement, the ALICE ITS must be upgraded.[3]

##  2 Current and upgrade Inner Tracking System

The current ITS is composed of six concentric barrels with three different
technologies of detector surrounding the beam-pipe. The Silicon Pixel
Detectors (SPD) are used in the first two innermost layers. The next two
layers are made of Silicon Drift Detectors (SDD) and the two outermost layers
are equipped with the Silicon Strip Detector (SSD).[3]

Due to the upgraded beam-pipe radius reduction from 2.94 cm to 1.98 cm, this
allows us to place the innermost layer 9.6 mm closer to the interaction point.
An extra layer is also introduced to increase tracking efficiency of the new
ITS. This new design is expected to improve the impact parameter resolution by
a factor of three [3]. The use of Monolithic Active Pixel Sensors (MAPS) for
the seven layers of ITS upgrade can reduce the material budget by a factor of
7 (50 $\upmu$m instead of 350 $\upmu$m) per layer.[4] For data readout, the
analog front-end timing and data transfer architecture will be developed to
increase frequency greater than 50 kHz for Pb-Pb interaction and about 7 MHz
for p-p interaction.[5] The new cooling system also is improved for a better
heat transfer.

The design of the new ITS composes of the inner barrels (layers 0 to 2), the
middle barrels (layers 3 to 4) and the outer barrels (layers 5 to 6) (see in
Fig. 1).

Figure 1:  Schematic cross section layout of the inner barrel (left) and outer
barrel (right).[6]

The layout of the new design are shown in Table 2 with the expected material
budget.

The layout of the upgrade scenario for the ITS inner barrel. The numbers in
brackets refer to the case of current detector.  Layer Radius (cm) Material
budget ($\%X_{0}$) inner barrel 1 pixel (pixel) 2.2 (3.9) 0.3 (1.14) 2 pixel
(pixel) 2.8 (7.6) 0.3 (1.14) 3 pixel (none) 3.6 (-) 0.3 (-) mid barrel 4 pixel
(driff) 20.0 (15.0) 0.8 (1.13) 5 pixel (driff) 22.0 (23.9) 0.8 (1.26) outer
barrel 6 pixel (strip) 41.0 (38.0) 0.8 (0.83) 7 pixel (strip) 43.0 (43.0) 0.8
(0.83)

###  2.1 The new stave structure of upgrade ITS

The stave is a composite element of the upgraded ITS layers devised to be a
support structure to be a support structure. It consists of the following
components:

Space Frame: a lightweight wound truss structure made of carbon fibre for
mechanical supporting.

Cold Plate: a carbon ply with embedded cooling units.

Hybrid Integrated Circuit: an electronic circuit with pixel sensor chip.

In this work, both the inner and outer barrels have been studied and their
material budgets have been calculated. The first three inner layers are built
with the identical staves. The space frame of each stave is made of a light
filament wound carbon. It is obtained by winding a M60J 3K (588GPa) carbon
rowing respect to the stave axis with an angle of 45∘. The winding angle and
the number of helices have been optimized to achieve the best compromise
between material budget and stiffness.

For the cooling system, cold plate is used to remove the heat dissipated from
the pixel chips. The cold plate is made of a high thermal conductive carbon
fibre laminated on top of which the silicon chips are glued. The heat is
conducted into the cooling pipes or microchannel embedded in the cold plate
and is removed by the coolant. In order to maximise the cooling efficiency,
the cold plates have been considered in four different models for the various
geometry design and the thermal constraints.

The mechanical support structure is designed under the technical constraints
such as the requirement of detector layout, specific sensor chip size and the
cooling technology. The stave structures have been separated into several
models.

Model 0: The wound truss structure with cooling pipes at the vertices

The winding carbon filaments with 5 mm diameter are used to construct the
support structure. K13D2U is used for wound truss structure instead of M55J 6K
carbon fiber for a better thermal conductive of two embedded pipes as shown in
Fig. 2(a). The use of K13D2U carbon type can reduce the bending radius against
M55J 6K. The carbon fiber are wound with an angle of 23 degrees along the
stave axis to reduce the breaking of production process. This prototype can
reduce the weight of structure while preserving a good stiffness. The
pyramidal structure made by the carbon fiber filaments will be strong enough
to support the silicon sensors and the layer structure. The overall structure
is suitable with low power consumption.

Model 1: The wound truss structure with polyimide microchannel cooling

For this model, the same technique described for wound truss structure has
been used but the cooling pipes have been replaced by a cold plate for a
better distributed heat transferring over the pixel chip. A monophase or
biphase refrigerant fluid is used in the cold plate flowing into 0.16 mm2 of
polyimide microchannel cooling section (see in Fig. 2(b)).

Model 2: The wound truss structure with uniform carbon plate and polyimide
tubes in the middle

This model, obtained with the same technology described above, uses a uniform
carbon plate for transferring heat to two embedded tubes. These polyimide
tubes are placed in the middle at the base of the stave structure. However,
the addition of a carbon plate may cause the increase of material.

In this model, two different outer radii, 0.15 mm and 0.10 mm, of cooling
pipes are separately simulated for the material properties.

Model 3: The wound truss structure with silicon microchannel

This model is the wound truss structure with cold plate, made of a silicon
substrate (see Fig. 2(e)). Based on etching technology, the microchannel can
be created on the silicon plate where the cooling fluid flows inside. The
microchannels are fabricated in a clean room of the Center of
Micronanotechnology, Ecole Polytechnique Federale de Lausanne and Thai
Microelectronics Center. Due to some complication in its fabrication process,
the prototype is just being developed with substantial cost.

Model 4: the wound truss structure with uniform carbon plate and polyimide
tubes in the middle (revisited)

The concept of this model is similar to Model 2 with 0.10 mm of outer radius
but the structures were completely rewritten to make them more systematic and
similar to those of in the outer barrel. Some inconsistencies such as the
element sequence and empty space are corrected (see in Fig. 2(f)).

(a) Model 0

(b) Model 1

(c) Model 2.1

(d) Model 2.2

(e) Model 3

(f) Model 4

Figure 2:  Assembly of the inner barrel stave prototype composed of space
frame, cooling structure and sensor chip. For material analysis the different
structures are being considered a) The wound truss structure with cooling
pipes at the vertices, b) The wound truss structure with polyimide
microchannel cooling, c) The wound truss structure with uniform carbon plate
and 0.15 mm outer radius polyimide tubes in the middle, d) The wound truss
structure with uniform carbon plate and 0.10 mm outer radius polyimide tubes
in the middle, e) The wound truss structure with silicon microchannel and f)
The wound truss structure with uniform carbon plate and polyimide tubes in the
middle (revisited).

By coding a detailed geometry description of all models in ALIROOT, the
complete calculation of material budget of all stave models can be performed.

All models have been considered and proceeded to calculate their material
budget distribution. Remarks and comments for each models have been passed to
the prototype production in the ALICE ITS upgrade.

##  3 Toward the material budget ($X/X_{0}$) reduction of inner barrel

The energy lost of a particle passing through matter is related to properties
of materials. The radiation length ($X_{0}$) is the mean path length required
to reduce the energy of relativistic charged particles by the factor $1/e$
related to its energy loss [7]. The radiation length of material can be
approximated by the expression of the atomic number and atomic weight of the
nucleus. In case of compound material or mixture, the radiation length can be
estimated with the combination of all compound radiation length multiplied by
the mass fraction [8]. The properties of materials used in the ALICE ITS
upgrade scenarios are given in Table 3.

List of the stave components and their thickness used in ALIROOT simulation.
The numbers in brackets refer to the conceptual parameters in CDR.  Material
Model 0 Model 1 Model 2.1 Model 2.2 Model 3 Model 4 Thickness [$\upmu$m]
Filament00000000000 00Top CFRP M60J 3K 70 (70) 200 (120) 200 (100) 200 (100)
200 (120) 200 (100) 00000Bottom CFRP M60J 3K 70 (-) 200 (240) - - 200 (240) -
Cooling000000000000 Pipe Kapton00000 70 (70) - 130 (70) 130 (70) - 130 (70)
water00000000000 1450 (1450) - 1450 (1450) 940 (940) - 940 (940) Carbon
plate00000 - - 140 (140) 140 (140) - 140 (140) Sensor000000000000
Glue000000000000 125 (200) 250 (200) 100 (200) 100 (200) 250 (200) 100 (200)
Silicon chip000000 50 (50) Flex cable0000000 100 (-) 00000Polyimide
Microchannel - 100 (100) - - - - 00Silicon Microchannel - - - - 40 (40) -
water00000000000 - 200 (200) - - 160 (160) -

The material budget of each stave prototypes can be calculated from the
radiation length of their components. It either depends on thickness
($X/X_{0}$) or percentage of covered surface ($X/X_{0}(\%)$). The computation
of the material budget in ALIROOT is done using fake particles, called
“geantino”s. They can be shot straight to the sensors, and having no charge
they suffer neither deviation nor any energy loss. They are tracked as all
other particles in each step. The user can restrict the volumes crossed by
geantinos by setting the minimum radius $R_{\rm{min}}$ and the maximum radius
$R_{\rm{max}}$ , the minimum and maximum $\phi$ values, and the range in $Z$,
between $−Z_{\rm{min}}$ and $+Z_{\rm{max}}$ of the travelling region. The
material budget can be determined in essentially two ways. In the first one
the geantino tracks are generated perpendicular to the $Z$ axis of the staves,
hence, the “actual” material budget can be determined. In the second way, all
geantinos come from the interaction point, so the material budget is the same
as seen by real particles coming from the collisions depending on the provided
$\eta$ range.

The stave design accounts for material budget requirement which is limited to
0.3$\%$ for the inner barrel and 0.8$\%$ for the outer barrel [3]. The
simulations include essentially all material properties used in each model as
shown in Table 3 and Table 3.

List of the stave components and their contribution to the radiation length.
The numbers in brackets refer to the approximation value in CDR.  Material
Model 0 Model 1 Model 2.1 Model 2.2 Model 3 Model 4 Thickness [$\upmu$m]
Filament Top CFRP M60J 3K 19 (25) Bottom CFRP M60J 3K 19 (25) 19 (25) - - 19
(25) - Cooling Pipe Kapton 28.4 (28.6) - 28.4 (28.6) 28.4 (28.6) - 28.4 (28.6)
water 35.8 (36.1) - 35.8 (36.1) 35.8 (36.1) - 35.8 (36.1) Amec Thermasol FGS
003 - - 27 (25) 27 (25) - 27 (25) C Fleece - - 106 (25) 106 (25) - 106 (25)
Sensor Glue 44.37 (44.37) Silicon chip 9.35 (9.36) Flex cable 13.3 (13.3)
Polyimide Microchannel - 28.4 (28.6) - - - - Silicon Microchannel - - - - 9.35
(9.36) - water - 35.8 (36.1) - - 35.8 (36.1) - K13D2U 2K - - 26 (25) 26 (25) -
26 (25) C Fleece - - 106 (25) 106 (25) - 106 (25)

The results of our calculations for material budget of different models of
inner barrel are shown in Fig. 3-7. The highest peaks represent the
overlapping between each stave. In model 2.2, the location of the cooling
pipes around the middle of the stave can help to enhance the thermal
conductivity despite the higher material budget than model 0. The second
highest peaks in Fig. 5 and Fig. 6 are due to the polyimide cooling pipes
filled with water. Instead of cooling pipes, using the microchannel filled
water in model 1 or Freon in model 3 as a coolant can serve as a better
average way to dissipate the heat from sensors [5]. However, model 1 has a
higher material budget than model 2. While in model 3, the silicon micro
channel is still in the R$\&$D phase and not ready to be implemented yet.

A summary of material budget calculation for all stave prototypes are given in
Table 3.

Figure 3:  The material budget distribution of Model 0.

Figure 4:  The material budget distribution of Model 1.

Figure 5:  The material budget distribution of Model 2.1. Figure 6:  The
material budget distribution of Model 2.2.

Figure 7:  The material budget distribution of Model 3.

Expected overall material budget obtained from ALIROOT simulation for
different stave prototypes compared to the possible theoretical calculation in
CDR [3].  Stave prototype $X/X_{0}$ [CDR] $X/X_{0}$ [ALIROOT] ($\%$) ($\%$)
Model 0 0.26 0.284 Model 1 0.30 0.334 Model 2.1 0.31 0.344 2.2 - 0.303 Model 3
- 0.249

##  4 Outer barrel stave

The conceptual design of the outer barrel stave is similar to the inner
barrel. However, the bottom of the supporting part is split longitudinally
into two half-staves on the azimuthal direction, i.e. along the Phi angle as
shown in Fig. 8. In each half-staves consist of seven sensor chips with
integrated cooling pipes and cold plates. These two pipes have 2.67 mm inner
diameter and are filled with water.

Figure 8:  Schematic layout of the mechanical and cooling structure of the
outer barrel stave. [6].

The staves of the outer barrels have been designed to achieve the required
stiffness and thermal properties as expected in inner barrels. Several
components, similar to inner barrel, are used to prototype the outer barrel
stave with seven-pixel chips. The estimate of the outer barrels stave to the
material budget contribution are reported in Table 4.

List of the outer stave components and their thickness and the estimated
contributions to the material budget [6].  Stave element Component Material
Thickness $X/X_{0}$ $X/X_{0}$ [$\upmu$m] [cm] [$\%$] Module FPC Metal layers
Aluminium 50 8.896 0.056 FPC Insulating layers Polyimide 100 28.41 0.035 FPC
Insulating layers Polyimide 100 28.41 0.035 Module plate Carbon fibre 120
26.08 0.046 Pixel Chip Silicon 50 9.369 0.053 Glue Eccobond 45 100 44.37 0.023
Power Bus Metal layers Aluminium 200 8.896 0.225 Insulating layers Polyimide
200 28.41 0.070 Glue Eccobond 45 100 44.37 0.023 Cold Plate Carbon fleece 40
106.80 0.004 Carbon paper 30 26.56 0.011 Cooling tube wall Polyimide 64 28.41
0.013 Cooling fluid water 35.76 0.105 Carbon plate Carbon fibre 120 26.08
0.046 Glue Eccobond 45 100 44.37 0.023 Space Frame Carbon rowing 0.080 Total
0.813

Table of element composition of all the materials used in IB and OB stave.
Mixture material Density g/cm3 Element $\%$ Weight Kapton 1.42 H 0.0226362 C
0.69113 N 0.07327 O 0.209235 Air 1.20479E-3 C 0.000124 N 0.755267 O 0.231781
Ar 0.012827 Water 1.0 H 0.111894 O 0.888106 Flex cable 1.0 C 0.520089 H
0.019839 N 0.055137 O 0.157399 Al 0.247536

Materials and property used in IB and OB stave.  SMaterial Density g/cm3 Glue
(C) 1.93/2.015 K13D2U 2K (C) 1.643 M60J 3k (C) 2.21 FGS 003 (C) 1.6 T300 (C)
1.725 C-fleece (C) 0.4

The results of the simulation of the material distribution across the outer
barrel staves are shown in Fig. 9. The half-staves are partially superimposed
surround the detector, thus giving rise to the peaks around 1.25$\%X_{0}$. The
highest peaks are due to the polyimide cooling pipes, filled with water,
embedded in the cold plate. The estimated overall material budget is within
reach of the expected 0.8$\%$.

Figure 9:  The material budget distribution of outer barrel prototype. The
highest peaks correspond to the polyimide cooling pipes, filled of water,
embedded in the cold plate [6].

##  5 Conclusions

The ITS upgrade design is aimed to improve the tracking efficiency of ALICE
Detector and reduce the statistical uncertainty for low transverse momentum
scattering. The construction of stave prototypes and the use of new cooling
system will achieve a reduction of material budget for the new ITS. Presently,
the ALIROOT simulation shows encouraging results on the material budget
estimation. The results indicate the possibility of reducing the material
budget to 0.3$\%$ per layer for Model 2.2 and 3. The simulation performed so
far guarantee the conceptual design used in new ITS will qualify for ALICE
Upgraded requirement.

The final decision for the inner barrel stave structure goes for the revised
model 2.2 or know as the current model 4. Currently, the stave production is
ongoing to final assembly of the detector module.[6]

## Acknowledgements

We would like to thank L. Musa and also the ALICE ITS upgrade team for their
assistance during at CERN. This work was supported by Suranaree University of
Technology (SUT), The office of the higher Education Commission under NRU
project of Thailand (SUT-PhD/07/2556), National Research Council of Thailand
(SUT1-105-58-24-03) and National Science and Technology Development Agency
(CPMO P-15-50416). C. Kobdaj and Y. Yan acknowledge support from SUT and CHE-
RNU project (SUT-COE: High Energy Physics $\&$ Astrophysics).

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  * [4] R. Lemmon, Nuclear Physics A 904, 937C (2013).
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