Radiation length estimation of the SCT Barrel section using Geant4

Makoto ASAI

a

, Itsuo NAKANO

b

, Yoshinobu UNNO

c

, Tomohiro YAMASHITA

b

a

Hiroshima Institute of Technology, 2-1-1 Miyake, Saeki-ku, Hiroshima, Hiroshima 731-5193 Japan

b

Okayama university, 3-1 Tsushimanaka, Okayama, Okayama 700-8530, Japan

c

KEK, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan

Abstract

Radiation length of the materials in the four cylinders of thebarrel section of the silicon microstrip SCT system has beenevaluated by using a modern detector simulation program,Geant4. The objects of modules, rows of 12 modules, coolingpipes, power tapes, and support cylinders are implemented inthe program. A virtual particle, geantino, generated uniformlyover the pseudo rapidity range calculated the radiation lengthsof total and individual contributions of the geometry elements.Although the result is preliminary, it is revealing usefulness inevaluating contribution from localized components such as hy-brids.

I. I

NTRODUCTION

The ATLAS detector, exploring the high energy regime inthe large hadron collider (LHC) where the so-called Higgs par-ticles can be produced, has been under construction from 199?,aiming for starting the experiment in 2005 [1]. The detector hasbeen designed to be generous for many reactions at the centre-of-mass collision energy of 14 TeV, for the production of heavyparticles, such as the Higgs particle, which are generating high-ly transverse high-momentum particles, and of relatively lightparticles, such as B-mesons, which are flying toward beam di-rections generating transversely low-momentum particles. Inorder to measure all the range of particle, the detector is com-posed of many detection elements.

One of the detection element is a charged particle trackingsystem with high position resolution in a solenoidal magneticfield near the interaction point. The system is further composedof three elements near from the interaction point: Silicon pixeldetectors (Pixel), Silicon microstrip semiconductor detectors(SCT), and a straw drift chambers with transition-radiation ca-pability (TRT). These detection elements are designed to belight and transparent for electromagnetic particles such as elec-trons, positrons, and photons, in order not to disturb their trajec-tories and energies. This lightness and transparentness isexpressed in the unit of radiation length. The charged particletracking system, called the inner detector, has presented itstechnical design report (TDR) and near its production [2].

After a development phase of a new detector simulation pro-gram based on the object-oriented programming using C

++

lan-guage, a code system named Geant4 has been released for usersin the high energy physics community [3]. Using this moderntool, the radiation length of the inner detector can be evaluatedwith a better precision than the estimation done in the TDR. Theestimation in the TDR is more or less averaging the materials

uniformly over the surface and volume. This is good to under-stand the material in a global view. A more precise evaluation,realizing localized masses, becomes possible not only in the de-sign-wise but also in the modern evaluation tool-wise.

Out of an effort implementing the geometry of the TDR de-sign, as a reference, the radiation length of the barrel section ofSCT was evaluated and is reported in this note.

II. I

MPLEMENTATION

OF

THE

BARREL

Implementation of the barrel was done in a sequence of

1. defining an object of “a module”,

2. defining an object of “a block of 12 modules in a row”,

3. defining other objects of “cooling pipe”, “power tape”, andfour “support cylinders” (cylinder 1, 2, 3, and 4), and

4. combining the objects on to four cylinders, setting the ob-jects in angles, called “tilt” angles, in order compensatingthe drifting angles of electrons and holes in the Siliconwafers in the solenoid magnetic field, and mechanicallytiling the modules.

A global construction of the objects are presented first. Thedescription of components of the object is given in the sectionIII.

A. Module

The SCT silicon strip module is made of

1. four single-sided detectors with the p-strip readout on then-type Silicon wafer of a thickness of 300

µ

m (the latestdesign is 285

µ

m in the outer 3 cylinders and of 260

µ

min the innermost cylinder, which will be implemented inthe future versions),

2. a pair of detectors being aligned and electrically connectedto form a 126.090 mm long strip with an insensitive regionof 2.090 mm in the middle,

3. the two pairs being back-to-back glued sandwiching abaseboard, with +20 and -20 mrad rotated along the axisof the module

4. the readout electronics hybrids being placed near the cen-tre of the module, one on the topside and the other on thebackside, and

5. the baseboard, mechanical and highly heat conducting, be-ing made of Thermal Pyrolytic Graphite (TPG) with BeOceramics facings attached where the hybrids are glued onand a cooling pipe is to be contacted.

The constructed “Module” object is shown in Figure 1,where the Silicon wafers, electronics chips, hybrid circuits, hy-brid substrate, BeO facings, and baseboard are coloured in gray,yellow, blue, blue, green, and red, respectively.

B. Row of modules

On a cylinder of the barrel, 12 modules are aligned in a rowin such a way as

1. rotate each module by +20 or -20 mrad in order to form ax-ially aligned strips on a side of the module and +40 or -40mrad stereo strips on the other side, in the cylinders 1 and3, or the cylinders 2 and 4, respectively, and

2. stagger the elevation of the adjacent modules by 2 mm andset the location of the modules along the row so that thestrips of adjacent modules have an overlap of 500

µ

mwhen pointed from the 2 sigma offset points of the inter-action, i.e,

±

112 mm.

The object of “Row of 12 modules” is shown inFigure 2.

C. Cylinder

A cylinder is composed of

1. a number objects of “Row of 12 modules” on

2. an object of “support cylinder”,

3. together with service elements of one “cooling pipe” andone “power tape” object per “Row of 12 modules”.

The support cylinder is a mechanical support for the ele-ments on the cylinder. It is to be made of a honeycomb sand-wich structure, however, in this implementation, it isapproximated with a single layer of carbon of an equivalent ra-diation length. The cooling pipe is a tube to carry a cooling cool-ant and the power tape is the electrical power supply lines. (Indetail, one module has its own power tape. In the middle of the

“Row” there is one thickness of power tape, but at the ends thereis 6 thicknesses of power tapes. Only one thickness is imple-mented in this simulation.) The dimensions of these objects aregiven in the section III.

In the barrel, there are 4 layers of cylinders: the 1st, 2nd, 3rd,and 4th cylinders have 32, 40, 48, and 56 objects each of “Rowsof 12 modules”, “cooling pipe”, and “power tape”. The “Row of12 modules” and “cooling pipe” are placed in a circle with thetilt angle of 10 degrees. The “power tape”s are placed on thesurface of the support cylinder in flat.

A section of a cylinder is shown in Figure 3. The coolingpipes are coloured in red and the power tapes are coloured ingreen. The support cylinder is coloured in cyan but drawn in awire-frame so that the power tape and the backside of the mod-ules can be seen.

III. C

OMPONENT

DIMENSIONS

A. Module

The most complex object in the implementation is the mod-ule. In order to show the construction, a 3 dimensional view ofthe module is shown in Figure 4 where a vertical scale is ex-panded.

Figure 1: Barrel module, where

Silicon wafers, electronics chips, hybrid circuits, hybrid substrate, BeO facings, and baseboard are coloured in gray, yellow, blue, blue, green, and red, respectively

Figure 2: Row of 12 modules

This figure shows the design feature of the hybrid elements.The hybrid is made of three components: 12 chips (yellow), 2circuits (blue), and 2 substrates (blue). The hybrid substrate hasa step so that the hybrid is bridging over the detector with aclearance between the hybrid and the Silicon wafer.

The top and the bottom Silicon wafers are separated by sand-wiching the baseboard (red) where the baseboard is profiled notto cover the full area of the wafers in order to save material. Thedetail dimensions of the elements are described in the followingsections.

1) Silicon wafers

Figure 5 shows the dimension of a pair of silicon microstripdetectors (Silicon wafers). One silicon microstrip detector has acharacteristics of

1. outer dimensions of 63.560 mm x 63.960 mm x 0.30 mm,

2. strip length of 62.000 mm, and

3. readout strip number of 768 with a strip pitch of 80

µ

m.

The pair of detectors is aligned,

1. with the space between two wafers of 0.13 mm, and then

2. the active strip length of 126.090 mm with an insensitiveregion in the middle of the strip of 2.090 mm.

2) TPG baseboard

The dimensions of the baseboard is shown in Figure 6. Thebaseboard is designed to have the minimum required area to

cool the Silicon wafers, made of a highly thermally conductivegraphite with a thermal conductivity of 1700 W/m/K, e.g.,Thermal Pyrolytic graphite (TPG) manufactured by AdvancedCeramics Co. Ltd.

3) BeO facings

The baseboard is covered with Beryllia (BeO) ceramics fac-ings where the hybrids are attached and a cooling pipe is con-tacted. These are for electrical isolation and for the mechanical

Figure 3: A view of the inner-most cylinder. In addition to the row of modules, the cooling pipe, power tape, and support cylinder (wire-frame) are shown in colour, red, green, and cyan, respectively.

Figure 4: Barrel module in a 3d view

reinforcement. Figure 7 shows the dimensions of each facing.From left to right, each facing is named as Facing-CU, Facing-CL, Facing-FL, Facing-FU, with “C” and “F” representing thecooling side and the far side, respectively, and “U” and “L” theupper and the lower side of the baseboard, respectively. Theshape of all facings is trapezoid board with a thickness of 0.025cm.

4) Hybrid

Figure 8 shows the dimensions of the hybrid. One hybrid iscomposed of 6 chips, a circuit part, and a substrate with step (orcalled “bridge”). The dimension of the chip is 8.4 mm (length)

x 6.6 mm (width) x 0.35 mm (thickness). The chips are distrib-uted with a space of 3.7 mm in-between. The dimension of thehybrid circuit and substrate is 28 mm (width) x 74.6 mm(length). The thickness of the circuit is set to be 0.2 mm and theradiation length is approximated with Gold with a reduced den-sity to count the mixture of metal and insulator.

B. Cooling pipe

The dimensions of the cooling pipe is shown in Figure 9. Analuminium of a thickness of 0.15 mm is assumed for the coolingpipe, and water is assumed for the coolant inside, although thelatest coolant expected is a fluorocarbon (C

3

F

8

) for an evapora-tive cooling.

Figure 5: Aligned pair of silicon microstrip detectors (Silicon wafers), with dimensional unit in mm

Figure 6: TPG baseboard, the left side being the cooling side and the right the far side, with dimensional unit in cm

63.5

60

61.6

80

63.960

62.000

63.960

62.000

126

0.130

2.5

6.0

8.95

1.05

4.65

5.25

3.7

8.76

4.2

5.7

7.86

0.04

Figure 7: BeO facings of the baseboard, from left to right, Facing-CU, Facing- CL, Facing-FU, and Facing-FL, with dimensional unit in cm

Figure 8: Hybrid made of three elements: chips (yellow), circuit (blue), and substrate with steps (blue), with dimensional unit in cm

1.76

1.64

6.0

1.7

1.58

6.0

0.54

0.46

3.6

0.53

3.6

0.6

0.0

25

0.0

25

0.0

25

0.0

25

0.005 0.005

0.52

0.090.035

0.02

7.46

1.14

0.370.66

2.8

0.84

C. Power tape

The dimensions of the power tape is shown in Figure 10. Thepower tape is approximated with a thin plate of Aluminium of2.0 cm wide and 0.0125 cm thick. The density of power tape isreduced to count the space between the Aluminium traces andplastic covers.

D. Support cylinder

The dimension of support cylinders is shown in Figure 11.The structure of the design is a honeycomb core sandwich withcarbon-fibre-reinforced-plastic (CFRP) skins. The structure istoo complex to implement in the simulation and is approximat-ed with a layer of carbon of a thickness of 0.0564 cm. The innerradii of the cylinders are 28 cm, 35.3 cm, 42.7 cm, and 50 cmfor the 1st, 2nd, 3rd, and 4th one, respectively.

E. List of materials

The materials used in the implementation are summarized inTable 1. In the column of “Material”, when two names are givenin a cell, they express the major components and the first one isused for the material with reduced density to approximate thecomplex. When two thicknesses are given in the column of“Thickness”, they represent the ones of thinner and thicker sec-tions. The column of “Unit radiation length [cm]” is the thick-ness of material to have an unit radiation length and the columnof “Radiation length [%X0]” is the percent of the radiationlength of the listed thickness of the material.

IV. E

XTRACTING

R

ADIATION

L

ENGTHS

Once the geometries and materials are implemented in theGeant4 program, the calculation of the radiation length is done

straight-forwardly by generating a virtual particle, geantino,from the interaction point. The geantino accumulates the radia-tion length of the materials along its passage. Each componentcan be switched on and off so that individual contribution of theradiation length is also calculated in a simple way.

In order to evaluate the radiation length as a function of pseu-do-rapidity,

η

, the geantino’s were produced uniformly in thepseudo-rapidity unit. The geantino’s can be generated accord-ing to the distribution of the interaction points, however, in thisnote, they were produced only from the coordinate origin.

A. Total radiation length

The sum of the radiation length of the four cylinders fully

Figure 9: Cooling pipe, with dimensional unit in cm

0.9

0.015

0.9

0.0150.3

147.8

Figure 10: Power tape, with dimensional unit in cm

Figure 11: Support cylinder, with dimensional unit in cm

2.0

0.0125

2.0149.8

150.0

0.0564

Inner diameter

70.6 (2nd layer)

100.0 (4th layer)

56.0 (1st layer)

85.4 (3rd layer)

equipped with modules, cooling pines, power tapes, of the bar-rel is shown Figure 12. Several peaks of radiation length are ob-served at

η

’s about 0.18,1.2 and 1.4. As seen in the followingsections, these peaks are mainly caused by the hybrids, becausethey are lumped and aligned. Specially at about 0.18, three lay-ers of hybrids were passed through.

B. Contribution of individual components

In order to clarify contributions of the individual componentsto the peaks, the radiation lengths of Silicon wafers, baseboardswith facings, hybrids, cooling pipes, power tapes and supportcylinders are plotted in Figure 13 to Figure 18, respectively.Out of the components, Silicon wafers, cooling pipes, powertapes, and support cylinder are expected to show smooth varia-tion of the radiation length in

η

’s only affected by the incidentangles.

As seen in Figure 13, Figure 16, Figure 17, and Figure 18,the radiation length of the Silicon wafers, cooling pipes, powertapes, and support cylinders are smoothly varying in

η

’s, wherethe radiation lengths at

η

=0 are about 2.8%, 0.75%, 0.18%, and1.9% Xo, respectively. In total, about 5.6% Xo is a constant inthe radiation length in

η

.

Lumped in

η

are the radiation length of the baseboards withfacings and hybrids as seen in Figure 14 and Figure 15, where

the radiation lengths at

η

around 0.18 are about 2.2% and 5.5%Xo, respectively. The hybrid only gives the same amount of thecontribution as of the constant. The contribution of hybrids andother lumped ones spread out in higher

η

’s because of less si-multaneous overlapping.

V. P

LANS

FOR

FUTURE

Many elements are still need to be implemented relevant forthe barrel:

1. different Silicon wafer thickness of each cylinder,

2. a branch of power tape from the main power tape to themodule,

3. opto hybrid on the branch of the power tape,

4. support bracket of each module,

5. extra cooling block mating the high elevation module,

Table 1Materials in the simulation

Mate-rial

Density

[g/cm

3

]

Thick-ness [cm]

Unit radia-tion length [cm]

Radia-tion length [%X0]

Silicon wafer

Silicon 2.33 0.03 9.37 0.32

Base board

Carbon 3.513 0.04 12.15 0.33

Facings BeO 2.86 0.025 14.44 0.17

BridgeBeO 2.86 0.038,

0.09014.44 0.26,

0.63

CircuitAu / PbO

4.31 0.02 5.70 0.35

Chip Silicon 2.33 0.035 9.37 0.37

Cool-ing pipe

Alu-minium

2.70 0.015 8.89 0.17

Coolant Water 1.00 0.27 36.05 0.75

Power tape

Alu-minium

2.50 0.0125 9.60 0.13

Support Carbon 3.513 0.0564 12.15 0.46

Figure 12: Total radiation length of the four fully equipped barrel cylinders

Figure 13: Radiation length of Silicon wafers

0

2

4

6

8

10

12

14

16

18

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

6. individual power tape for each module,

7. cooling manifolds,

8. thermal enclosure,

9. service elements from each cylinder to the outer world,and

10. tuning the parameters to reflect the design correctly.

In order to complete the SCT system, similar elements are re-quired to be implemented for the forward components. The fullevaluation of the radiation length of the SCT system is to followafterward. The evaluation of the full inner detector is to be doneafter the implementation of the Pixel and the TRT system to-gether.

VI. S

UMMARY

Radiation length of the materials in the four cylinders of the

Figure 14: Radiation length of the baseboards (TPG plates + BeO facings)

Figure 15: Radiation length of the hybrids

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

0

1

2

3

4

5

6

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

Figure 16: Radiation length of cooling pipes

Figure 17: Radiation length of power tapes

Figure 18: Radiation length of support cylinders

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Rad

iatio

n L

engt

h( %

X0)

pseud rapidity

barrel section of the silicon microstrip SCT system has beenevaluated by using a modern detector simulation program,Geant4. The objects of modules, rows of 12 modules, coolingpipes, power tapes, and support cylinders are implemented inthe program. A virtual particle, geantino, generated uniformlyover the pseudo rapidity range calculated the radiation lengthsof total and individual contributions of the geometry elements.

This note is only to show the capability of the program sincethe parameters in the program still need to reflect the latest de-sign correctly. Although the result is preliminary, it is revealingusefulness in evaluating contribution from localized compo-nents such as hybrids.

The implementation of elements are further required to eval-uate the full SCT system together with the forward elements,and to evaluate the full inner detector with the Pixel and theTRT systems.

VII.

References

[1] ATLAS technical proposal, ???

[2] Inner detector technical design report, ???

[3] Geant4 user’s manual, ???