Faculteit WetenschappenVakgroep Fysica en Sterrenkunde

Design of a cosmic test bench for the quality

control of Resistive Plate Chamber detectors

for the CMS experiment

Tom Cornelis

Promotor: dr. M. Tytgat

Masterproef ingediend tot het behalen van de academische graad

Master in de Fysica en Sterrenkunde

Academiejaar 2010–2011

Acknowledgements

I would like to thank Michael Tytgat for supervising and proof-reading my thesis. I would

also like to express my thanks to Gerard, Michiel, Patrick and Philippe with whom I have

worked together to prepare the Ghent RPC lab. It was a pleasure for me to work in this nice

and friendly environment. Very special thanks go to Andrey Marinov for the pleasant times

during the GEM test beams, his work on the small cosmic setup at CERN and for driving

me to/from CERN or the airport at very early/late hours. Finally, I want to thank all my

friends at CERN and the INW for the many nice moments we had this year.

i

Contents

1 Introduction 1

2 Resistive Plate Chambers 3

2.1 Avalanche formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Time resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.3 Resistive plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.4 Streamer or avalanche mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.5 Applications and use of RPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 The Compact Muon Solenoid 7

3.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.1 LHC as a proton collider . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1.2 LHC as an ion collider . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.3 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 The Compact Muon Solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2.1 Coordinate conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2.2 Physics and detector requirements . . . . . . . . . . . . . . . . . . . . 10

3.2.3 Parts of the CMS detector . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Resistive Plate Chambers at CMS 17

4.1 The CMS RPC system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2 Design of the endcap RPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.3 Upgrade of the RPC forward system: RE4 . . . . . . . . . . . . . . . . . . . . 20

5 The Ghent RPC lab 23

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2 Proposed quality control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2.1 Tests at the gas gap level . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2.2 Tests at chamber level . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.3 Design of the high voltage test . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.4 Design of the cosmic ray test . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

iii

5.4.1 Trigger system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.4.2 Tracking of events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.4.3 RPC control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

5.4.4 RPC readout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5.4.5 Pre-assembly tests on scintillators and PMTs . . . . . . . . . . . . . . 36

6 Conclusions 43

7 Nederlandstalige samenvatting 45

A Qt and ROOT 47

A.1 Qt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

A.2 ROOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

B Manual for the HVtest program 49

B.1 Starting up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

B.2 Main window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

B.3 The A1526 Control window . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

B.4 Browsing the rootfiles/graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

B.5 Changing program parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 55

B.6 Upgrade of the HV test program . . . . . . . . . . . . . . . . . . . . . . . . . 55

C Manual for the cosmic DAQ 57

C.1 Starting up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

C.2 Main window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

C.3 The A1526 and A1513B Control window . . . . . . . . . . . . . . . . . . . . . 59

C.4 The SY527 Control windows . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

C.5 The V812 Control window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

C.6 The V1190A Control window . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

C.7 The StartDAQ window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

C.8 V792 configuration files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

C.9 Data saved by the program . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

C.10 View the TDC results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

C.11 Changing program parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 64

C.12 Upgrade of the cosmic DAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

D How to use the histogram.C macro 67

Chapter 1

Introduction

In the 20th century, physicists have developed the standard model of particle physics. It

explains how matter is build up from elementary particles and how the interactions between

these particles are mediated by the electromagnetic, weak and strong force. It predictions

are confirmed through experiments to a very high precision. Unfortunately, the existence

of the Higgs boson, which is predicted by the Higgs mechanism that explains how mass is

generated, is still not confirmed experimentally. The standard model does not incorporate

gravity and some theoretical issues, such as the hierarchy problem or the strong CP puzzle,

are still unsolved. Therefore, one commonly believes new physics will appear at the TeV

scale. Since 2010, the Large Hadron Collider (LHC) is providing collisions at the TeV scale

and physicists are searching for signs of new physics.

The Compact Muon Solenoid (CMS) is one of the experiments at the LHC. Next to tracking

detectors and calorimeters, it contains also a dedicated muon system, as many of the predicted

decay modes of new particles are expected to involve muons. One type of gas detectors used

in the muon system are resistive plate chambers (RPC), which determine precisely the time

of passage of the muons. They give a fast trigger signal to identify the muon track, measure

its transverse momentum and relate it to the correct bunch crossing of the LHC. At present,

the RPC system is located in 5 barrel wheels and 3 endcap disks on either side of the detector.

Due to budget reasons, a fourth layer of the RPC endcap system was staged and must still

needs to be built. A part of the new RPC chambers will be assembled in Ghent. Different

quality control tests will be performed on the chambers before sending them to CERN.

The main subject of this thesis is the design and preparation of the quality control tests,

in particular the cosmic ray test. In February 2011, we started to build the experimental

setup used for the pre-assembly tests of the scintillators which will be used in the cosmic test

bench. The first parts of the cosmicdaq program were written and by around half of March

we had a working setup. Once all the bugs were removed from the program, we could start

the measurements of the attenuation lengths. The same setup was used for the calibration

1

and HV scan of the PMT. While these pre-assembly tests were going on, the program for

the HV test was written. This program is completely finished and the test can be performed

automatically. Finally, the development of the final program for the cosmic ray test was

started, based on the earlier version of the cosmicdaq and HVtest programs. This program

is not completely finished and could not yet be tested, because the final experimental setup

is not fully built yet. Therefore, some extra work should be done before this test can be

performed.

In chapter 2 we discuss the detector layout and working principles of resistive plate chambers.

Chapter 3 contains two parts: the first part presents the LHC and the second part gives a

short description of the CMS detector. Chapter 4 will focus on the resistive plate chambers

of CMS and the foreseen upgrade of the RPC system. In chapter 5 we present the RPC lab

in Ghent and we discuss the quality control tests that will be performed on the RPCs. The

design of the high voltage test and the cosmic ray test are discussed in detail. The results of

the pre-assembly tests on the scintillators, which will be used as a trigger in the cosmic ray

test, are presented. A manual for the HV test program can be found in appendix B and the

manual for the cosmic ray test is included in appendix C.

Chapter 2

Resistive Plate Chambers

The Resistive Plate Chamber (RPC) is a gaseous parallel plate detector which was developed

in 1981 by R.Santonico and R. Cardarelli. It consists of two parallel plates made of a material

with high resistivity, separated by a gap of a few millimeters filled with atmospheric-pressure

gas. These plates are painted with a graphite coating. This is used to apply an electric field

on the gap such that the passage of a charged particle initiates a Townsend avalanche. This

charge movement induces a signal on external read-out strips which are separated from the

graphite coating by an insulating layer. RPCs can achieve a good time and space resolution.

The simple design and low cost makes them attractive for large area detectors. This chapter

is based on references [1–7]

Figure 2.1: A schematic image of a RPC gap [2]

2.1 Avalanche formation

When a charged particle traverses the gap between the plates, it will ionize the gas molecules.

The charges are collected by applying an electric field on the gap. During the migration of

the electrons and ions to their respective collecting electrodes, many collisions occur with

the neutral gas molecules. The electrons are easily accelerated by the applied field and

their energy may become larger than the ionization energy of the neutral gas molecules. As a

3

result, additional ion pairs will be created in the collisions. The average energy of the electron

between collisions increases with increasing electric field. This secondary ionization will start

above a threshold value. RPC’s are operating above this threshold value. This process will

repeat itself and takes the form of a cascade, known as a Townsend avalanche. The fractional

increase in the number of electrons per unit path length is given by the Townsend equation

dn

dx= αn (2.1)

where α is called the first Townsend coefficient for the gas. This coefficient is zero below the

threshold and increases with increasing field strength above this minimum. RPC detectors

have a parallel plate geometry, which involves a spatially constant electric field and a constant

Townsend coefficient. Therefore the electron density grows exponentially with the distance:

n(x) = n0eαx (2.2)

However, this is a simplified model: the avalanche carriers can distort the electric field in the

gas gap. As a consequence, the drift velocity and the Townsend coefficient may vary with the

position in the gas gap.

2.2 Time resolution

In the parallel plate configuration the avalanche amplification starts immediately after the

initial ionization by the incident particle, as opposed to wire based gaseous detectors. In

wire based detectors, the Townsend coefficient is variable and only non-zero close to the

wires. Therefore all avalanches will have the same total charge and there is no information

about how far the electron had to travel before initiating the avalanche. This introduces a

time jitter and limits the time resolution to a few nanoseconds. RPC’s doesn’t suffer this

problem: by using a parallel plate geometry with high electric field, the avalanche is starting

immediately after the charge deposit and the fluctuations in time of the arriving electrons can

be eliminated. It has been demonstrated that RPC’s with small gap dimensions can achieve

time resolutions down to 50ps [2, 6].

2.3 Resistive plates

At least one of the electrodes is fabricated from materials such as Bakelite, plastic or glass

that have a significant electrical resistivity. By using resistive plates, the discharge is limited

to the local area around the primary avalanche and the remaining counter area is still sensitive

to particles. When a charge Q0 enters, the charge on the resistive electrode surface will follow

Q(t) = Q0e−t/τ with τ = ρ�0�r (2.3)

where ρ is the resistivity of the material, �0 is the dielectrical constant and �r is the relative

permittivity of the resistive material. The relaxation time τ is a measure for the time needed

to remove the avalanche charge from the surface of the resistive plate. The detector has a

blind spot at the place where the avalanche reaches the resistive plate for a time of the order

τ , but the remaining area is still sensitive for particles. The resistivity ρ is typically 109 to

1013 Ωcm, which leads to relaxation times of the order milliseconds to seconds.

2.4 Streamer or avalanche mode

RPCs can be operated in streamer mode or in avalanche mode. During the progression of an

avalanche, excited molecules are formed by electron collisions in addition to the secondary

ions. Within a few nanoseconds, these molecules return to their ground state through emission

of photons (mostly in the UV region). These photons could be reabsorbed elsewhere in the

gas or by the cathode, creating a new free electron. In avalanche mode, these electrons are

prevented from triggering a new avalanche. This is done by keeping the avalanche small

(using a smaller electric field across the gap) or by adding a quench gas that absorbs the

photons without additional electron release. In streamer mode, the avalanches are allowed to

propagate in a confined region. By using gas mixtures that strongly absorb the photons, the

formation of avalanches far from the original site is prevented. The new avalanches will grow

in the form of a narrow streamer, a conducting path in the gap.

Streamer mode produces signals that are a factor of 100 larger compared to avalanche mode.

Therefore RPCs is streamer mode don’t need preamplification and the signals and the read-

out is quite simple. However, they need a longer recovery time and as a consequence the

counting rate per unit area is limited to 100 Hz/cm2.

2.5 Applications and use of RPCs

Because of their excellent time resolution, RPCs are mostly used for trigger and Time-Of-

Flight (TOF) applications. Streamer-mode RPCs have been used in L3 at CERN, BABAR at

SLAC, BELLE at KEK and OPERA at LNGS. Avalanche-mode RPCs are used in the CMS,

ATLAS and LHCb experiments for triggering muons. The ALICE and HARP experiments

use RPCs with multiple small gaps for their TOF detectors [2, 8]. There has also been some

interest in applying RPCs to gamma rays and slow neutrons [1].

Figure 2.2: A schematic image of a RPC gap with applied field E0 when an avalanche is developing.

a) A charged particle ionizes gas atoms and an avalanche is started. b) The avalanche

size is sufficiently large to influence the electric field in the gas gap. c) The electrons reach

the anode, the ions are still drifting to their cathode. d) The ions reach the cathode. [2]

Figure 2.3: A schematic image of a RPC gap with applied field E0 when a streamer is developing.

a) An avalanche is developing as in figure 2.2. b) The avalanche charges lead to a high

field deterioration in the gas gap and photons start to contribute, causing a rapid spread

of the avalanche. c) A weak spark may be created and the local area is discharged. d)

There is a strong decrease of the electric field around the spot of the avalanche. [2]

Chapter 3

The Compact Muon Solenoid

The Compact Muon Solenoid (CMS) detector is a multi-purpose apparatus operating at the

Large Hadron Collider (LHC) at CERN. The main references for this chapter are [3–5,9–11]

3.1 The Large Hadron Collider

The Large Hadron Collider (LHC) is a superconducting hadron accelerator and collider. It

is installed in a tunnel of about 26.7 km which was designed for the former Large Electron-

Positron (LEP) collider and constructed between 1984 and 1989 at the Franco-Swiss border

near Geneva. The number of events per second generated in the LHC is given by:

Nevent = Lσevent (3.1)

where L is the machine luminosity and σevent is the cross section for the event under study.The luminosity is given by:

L =γfnbN

2b

4π�nβ∗F (3.2)

where γ is the Lorentz factor, f is the revolution frequency, Nb is the number of particles

per bunch, nb is the number of bunches, �n is the normalized transverse beam emittance, β∗

is the beta function at the collision point and F is the reduction factor due to the crossing

angle.

3.1.1 LHC as a proton collider

In the LHC, protons are accelerated in both directions to 7 TeV, allowing centre-of-mass ener-

gies up to 14 TeV. Because anti-protons are produced by a process with very low efficiency, it

was decided to use proton-proton collisions in the LHC instead of collisions between protons

and antiprotons. Hence a particle-anti-particle configuration of a common vacuum and mag-

net system for both circulating beams is excluded (particles and anti-particles can be guided

7

Figure 3.1: The CERN accelerator complex [12]

by the same magnets because, although they travel in opposite directions, they have opposite

charges). Thus, in order to collide two counter-rotating proton beams, opposite magnetic

dipole fields are needed in both rings. When one wants to accelerate protons (with rest mass

m0c2 = 938 MeV) to an energy of T = 7 TeV, a magnetic rigidity 1 of

χb =1

qc

√T (T + 2m0c2) (3.4)

= 23353 Tm (3.5)

is needed. The LHC ring accommodates 1232 dipole magnets which have a bending radius of

2803.98 m. Therefore, we need a nominal field of

B =χbρ

(3.6)

= 8.33 T (3.7)

1The magnetic rigidity χb of a particle is defined as the momentum per unit charge of a particle:

χb =p

q=mv

q= Bρ (3.3)

where the last equality can be found by using the equilibrium between the centrifugal force (mv2ρ−1) and the

centripetal Lorentz force (qvB) when a particle is travelling along a circular trajectory with bending radius ρ

in a uniform magnetic field B.

which is reached using the technology of superconducting magnets operating at 1.9K in

superfluid helium. In addition, quadrupole and higher-order magnets are used for beam

focusing and small corrections.

The proton beams consist of compact bunches of 1011 protons each, 25ns apart, leading

to a collision rate of L = 1034 cm−2s−1. This design luminosity leads to around 1 billionproton-proton interactions per second.

3.1.2 LHC as an ion collider

When the LHC is operating in heavy ion mode, fully stripped lead ions (208Pb82+) are accel-

erated. The energy of these lead ions can be calculated from formula (3.4), using the same

magnetic rigidity, and is 574 TeV or 2.759 TeV per nucleon yielding a total centre-of-mass

energy of 1.15 PeV. Bunches can contain up to 7 · 107 ions and the bunch separation is 100 nsleading to a luminosity of L = 1027 cm−2s−1.

3.1.3 Experiments

Seven experiments are taking place at the LHC [13–15]:

• ALICE (A Large Ion Collider Experiment), is a dedicated heavy-ion detector to exploitthe physics potential of nucleus-nucleus interactions at LHC energies. It aims to study

the physics of strongly interacting matter at extreme energy densities, where the forma-

tion of the quark-gluon plasma is expected. Proton-proton collisions are also studied,

both as a comparison with lead-lead ion collision and in physics areas where Alice is

competitive with other LHC experiments. The peak luminosity is L = 1027 cm−2s−1 fornominal lead-lead ion operation. ALICE is located at interaction point 2.

• ATLAS (A Toroidal LHC Apparatus), is designed as a general purpose detector.ATLAS is located at point 1 and aims a peak luminosity of L = 1034 cm−2s−1 forproton operation.

• CMS (Compact Muon Solenoid), is also a general purpose detector and aims the samephysics goals as ATLAS, but is constructed differently. It is located at interaction

point 5 and aims a peak luminosity of L = 1034 cm−2s−1 for proton operation. Thisexperiment will be described more in depth below.

• LHCb (LHC Beauty), is a dedicated detector for heavy flavour physics. By doing pre-cision measurements of CP violation and rare decays of beauty and charm hadrons, it

looks for indirect evidence of new physics. As opposed to ALICE, ATLAS and CMS

which have full 4π coverage around the interaction point, LHCb is a single-arm spec-

trometer and has a forward angular coverage form approximately 10mrad tot 300mrad.

This geometry is chosen based on the fact that at high energies both b- and b̄-hadrons

are predominantly produced in the same forward or backward cone. LHCb is located

at interaction point 8 and is designed to run at luminosity of L = 1032 cm−2s−1.

• LHCf (LHC forward), is an experiment which consists of two small calorimeters eachone placed 140m away from the ATLAS interaction point. The purpose is to measure

the energy and number of neutral pions in proton-proton collisions at extremely low

angles in order to understand better the origin of ultra-high-energy cosmic rays.

• MoEDAL (Monopole and Exotics Detector At the LHC ), is a small experiment search-ing for magnetic monopoles and other highly ionizing Stable (or pseudo-stable) Massive

Particles (SMPs) at the LHC. It is installed at interaction point 8, together with LHCb.

• TOTEM (Total Cross Section), Elastic Scattering and Diffraction Dissociation, con-sists of two times three small sub-detectors, positioned at distances between 10 and

220m before and after the CMS interaction point. It aims precision measurements

of the proton-proton interaction cross section. It is also dedicated to study the pro-

ton structure, which is still poorly understood. It is aiming a peak luminosity of

L = 1029 cm−2s−1.

3.2 The Compact Muon Solenoid

3.2.1 Coordinate conventions

The nominal collision point inside the CMS experiment is used as the origin of the coordinate

system. The x-axis is pointing towards the centre of the LHC, and the y-axis is pointing

vertically upward. The z-axis points along the beam direction toward the Jura mountains

from the CMS interaction point (counter-clockwise if the LHC is seen from above). The

azimuthal angle φ is measured from the x-axis in the x-y plane, while the polar angle θ is

measured from the z-axis. As in other high energy particle experiments, pseudorapidity is a

more useful alternative to the polar angle. The pseudorapidity is defined as η = − ln tan θ2 .The transverse momentum pT and energy ET , are computed from the x and y components.

The imbalance of energy measured in the transverse plane is denoted by EmissT .

3.2.2 Physics and detector requirements

Physics goals

The main physics goals of the CMS experiment are:

• Precision tests of the standard model

• Search for the Higgs boson

• Search for supersymmetric particles

• Search for new massive vector bosons

• Search for extra dimensions

• Study aspects of heavy ion collisions

Detector requirements

The design of CMS is based on the following detector requirements which are chosen to meet

the goals of the LHC physics programme:

• As muons are expected to be produced in the decay a number of potential new particles(for example in Higgs decays), we need good muon identification and resolution and the

ability to determine the charge of muons.

• A high quality central tracking system with good momentum resolution and recon-struction efficiency.

• A high resolution method to measure the energy and direction of electrons and photonsand a correct localization of the primary interaction vertex.

• In order to have a precise measurement of the EmissT and the dijet mass, hadroniccalorimeters with a large hermetic geometric coverage and fine lateral segmentation are

required.

3.2.3 Parts of the CMS detector

Superconducting magnet

A magnet is used to bend the paths of the charged particles. A large bending power is needed

to measure precisely the momentum of charged particles: the more momentum a particle has,

the less its path will be curved by the magnetic field. The CMS magnet is a superconducting

solenoid which generates a field2 of 4 T and should be operated at 4.6 K. It has an inner

radius of 5.9 m and a length of 12.5 m.

The flux of the magnetic field is returned through the iron yoke which also stops all remaining

particles except muons and weakly interacting particles, such as neutrinos. The iron yoke

weights 10000 tons and is composed of 5 barrel wheels and 6 endcaps.

2The CMS collaboration has decided the operate the magnet at a magnetic flux density of 3.8 T until the

aging of the coil is better understood. [16]

Figure 3.2: Exploded view of the CMS detector [5]

Inner tracking system

The inner tracking system surrounds the interaction point and measures the trajectories of the

charged particles emerging from the LHC collisions. It also provides a precise reconstruction of

secondary vertices. The tracker is operating in a very dense track environment and therefore

requires a high granularity such that the trajectories can be identified. Because the LHC

bunch crossing are only 25 ns separated from each other, the tracker also needs a fast response

to attribute the trajectories to the correct bunch crossing. The tracker should also be able to

withstand the intense particle flux which can cause severe damage to the readout electronics.

Because of these requirements, the tracker design is entirely based on silicon technology and

contains about 200m2 of active silicon area, making it the largest silicon tracker ever built.

Close to the interaction vertex, where the particle flux is the highest, 1440 hybrid pixel

detectors with size of about 100× 150µm2, giving an occupancy of about 10−4 per pixel andLHC bunch crossing (1% in heavy-ion running). They are placed in three layers located at

mean radii of 4.4 cm, 7.3 cm and 10.2 cm and in four disks, at | z |= 32.5 cm and 46.5 cm oneach side, covering a radius from 6 cm to 15 cm. The total area of the pixel detector is 1 m2

The spatial resolution is measured to be about 10µm for the r − φ measurement and about20µm for the z measurement.

In the intermediate region (20 cm < r < 55 cm) and the outermost region (55 cm < r

110 cm), the particle flux is low enough to use silicon microstrip detectors. The smallest

microstrips in the intermediate region, surrounding the pixel detector, cover 10 cm × 80µmand in the outermost region larger strips are used up to 25 cm × 180µm. They provide aspatial resolution which varies from 23 to 52µm.

Figure 3.3: The CMS coil (at insertion)

[17]

Figure 3.4: The tracker [17]

Electromagnetic calorimeter

The electromagnetic calorimeter (ECAL) is needed to measure with high accuracy the energies

of electrons, positrons and photons which interact through the electromagnetic interaction.

When an electron traverse the ECAL, it will lose most of his energy through bremsstrahlung,

resulting in a photon. The major interaction for a photon is pair production, leading to an

electron and a positron. This process is repeated and both particles will result in an avalanche

of secondary electrons, positrons and photons, known as an electromagnetic shower. Finally,

the particles will interact with the scintillators of the ECAL and the total deposited energy

will be that of the incident particle.

The ECAL is a hermetic homogeneous calorimeter made of 61200 lead tungstate ( PbWO4)

crystals mounted in the central barrel part, closed by 7324 crystals in each of the two endcaps.

The high density (8.28 gcm−3), short radiation length3 (0.89 cm) and small Molière radius4

(2.2 cm) of these PbWO4 crystals result in a fine granularity and a compact calorimeter.

When electrons, positrons and photons traverse the ECAL, a green-blue scintillation light is

emitted by the crystals. Most (80%) of the scintillation light is emitted after a delay of 25 ns,

which is the same as the nominal time between two LHC bunches. The relative low light

output varies with temperature and is detected by silicon avalanche photodiodes (APDs) in

3High-energy electrons predominantly lose energy in matter by bremsstrahlung, and high-energy photons

by e+e− pair production. The characteristic amount of matter traversed for these related interactions is called

the radiation length X0. It is both the mean distance over which a high-energy electron loses all but 1/e of

its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. [7]4The Molière radius gives a measure for the scale of the transverse dimension of the electromagnetic showers

initiated by an incident electron or photon. It is a characteristic constant of the material. On average, 90% of

the shower’s energy lies inside the cylinder with radius RM . [7]

the barrel and by vacuum phototriodes (VPTs) in the endcaps. The APDs and VPTs convert

the scintillation light to an electrical and amplified signal. They are designed to withstand the

high magnetic field, but they are also temperature dependent. The crystals are not pointed

exactly to the interaction point but are a little bit tilted, because otherwise straight particle

trajectories of photons could be aligned with the non-active areas between the crystals.

There is also a preshower detector placed in front of the endcap crystals. Incoming electrons

and photons initiate electromagnetic showers in lead radiators whilst silicon strip sensors

placed at depths of 2 X0 and 3 X0 after each radiator measure the deposited energy and

the transverse shower profiles. It is used to identify neutral pions which can fake a photon

signal. It also improves the position determination of electrons and photons and helps the

identification of electrons against minimum ionizing particles.

Figure 3.5: The ECAL (endcap) [17] Figure 3.6: The HCAL (endcap) [17]

Hadron calorimeter

The Hadron calorimeter (HCAL) is used to measure the energy of the hadron jets which

interact through the strong interaction. It is located between the outer extent of the ECAL (up

to 1.77 m) and the inner extent of the magnetic coil (starting from 2.95 m. It is also important

for the measurement of missing transverse energy EmissT which is a result of neutrinos or new

exotic particles which are not detected.

The hadron barrel (HB) and the hadron endcap (HE) calorimeter are organised using al-

ternating layers of absorber and scintillation materials. The light of the scintillator tiles is

delivered to hybrid photodiodes (HPDs) by wavelength shifting fibres. A couple of additional

scintillator layers are installed around the coil. This is the hadron outer (HO) calorimeter,

which increase the effective thickness of the hadron calorimetry to over 10 interaction lengths

and improves the missing energy resolution. The hadron forward (HF) calorimeters are placed

at 11.2 m of the interaction point and provides the coverage between pseudorapidities of 3.0

and 5.2 using a Cherenkov-based, radiation-hard technology.

Forward detectors

In addition to the hadron calorimeters, additional dedicated calorimeters deliver a higher

forward coverage. The CASTOR (Centauro And Strange Object Research) detector is a

Cherenkov based sampling calorimeter, similar in concept to the HF. It consists of a layer of

tungsten plates as absorber and silica quartz plates as active medium for both the electro-

magnetic and hadronic section (they only differ in thickness). It is installed at 14.38 m from

the interaction point, covering the pseudorapidity range between 5.2 and 6.6. In heavy-ion

running it will search for exotic objects with unusual longitudinal shower profile properties.

Two identical zero degree calorimeters (ZDCs) are located between the two LHC beam pipes

at about 140 m on each side of the CMS interaction region. They cover very forward region of

η ≥ 8.3 for neutral particles and the design is again similar to the HF and CASTOR. Duringheavy ion running it should allow the reconstruction of the energy of 2.75 TeV spectator

neutrons with a resolution of 10 to 15%.

The muon system

Muons play an important role in the decays of many potential new particles. Because of their

greater mass, muons emit almost no bremsstrahlung and are more penetrating than electrons.

As a consequence, they are not stopped in the electromagnetic calorimeter. Tau-leptons are

not seen in the muon system because of their small lifetime. There is a small background from

neutrons, which are not always fully stopped in the calorimeters, especially in the endcaps

where they cannot be absorbed by the iron yoke. The muon system tracks the path of the

muons after they traversed the other parts of the detector. The momentum and charge of the

muon is determined by the bending of his path in the magnetic field. Three types of gaseous

detectors are used: drift tubes (DTs) in the barrel (installed between the wheels of the iron

yoke), cathode strip chambers (CSCs) in the endcaps and resistive plate chambers (RPCs) in

both the barrel and endcaps, resulting in a total area of 25000 m2 of detection planes.

The drift tubes are used in the barrel region where the neutron-induced background is small,

the muon rate is low and the magnetic field is uniform and mostly contained in the iron yoke.

A drift tube chamber is made of 3 (our two for the outer station where the middle one is

not present) superlayers, each made 4 layers of rectangular drift cells staggered by half a cell.

Each cell is a tube with a cross section of 42 mm × 13 mm and a 2.4 m wire in the middle.Hence, the maximum drift path is 21 mm, corresponding to a maximum drift time of 380 s

in the used gas mixture. A superlayer gives excellent time-tagging capability, with a time

resolution of a few nanoseconds. The drift time is used to track the position of the muon

with respect to the wire. A single-point resolution of 200µm is achieved. The wires in the

two outer superlayers are parallel to the beam line and provide the track measurement in the

magnetic bending plane while the middle superlayer, the wires are orthogonal to the beam

line measuring the track position along the beam.

The cathode strip chambers are used in the endcap where there is more neutron-induced

background. The last layer of chambers also catches particles created inside the hadron

forward calorimeter and the hadron endcap calorimeter. The CSCs can operate at high

rates and in large and non-uniform magnetic fields. Each CSCs consists of 6 gas gaps with

cathode strips running radially (they have constant ∆ϕ spacing) and a plane of closely spaced

wires running across, making it a fast detector. A precise position measurement is made by

determining the centre-of-gravity of the charge distribution induced on the cathode strips.

The CMS RPCs will be discussed in the next chapter.

Figure 3.7: Slice through CMS showing particles incident on the different sub-detectors [17]

Chapter 4

Resistive Plate Chambers at CMS

In this chapter we describe the present RPC system at CMS and the design of the endcap

RPCs. We will also present the upgrade plans for the RPC forward system. This chapter is

based on [4, 5, 18–20].

4.1 The CMS RPC system

Figure 4.1: Layout and η coverage of one quarter of the present CMS muon system [5]

17

Figure 4.2: Installed RE2 chambers in the CMS detector [17]

Because of their fast response, the resistive plate chambers in the CMS experiment are used

for the first level muon trigger. They can determine the bunch crossing (BX) from which the

muons emerged and help measuring the transverse momentum of the muons, ranging from a

few GeV/c to 1 TeV/c.

Double-gap RPCs are used in both the barrel and endcap regions. The present system is

shown in figure 4.1. A total of six layers of RPCs are embedded in the barrel iron yoke: in

the inner two muon stations (MB1 and MB2), the RPCs are mounted on both sides of the

DTs, in the last two stations (MB3 and MB4) only on the outside of the DTs. In the endcap,

3 layers of RPCs are built, covering a pseudo-rapidity up to 1.6. Each endcap layer consists

of 2 rings of trapezoidal RPC chambers, as can be seen in figure 4.2.

4.2 Design of the endcap RPCs

Figure 4.3: A double-gap CMS RPC [18,21]

Figure 4.4: A cross-section diagram of a gas gap [18]

Gaps

The CMS RPCs are double-gap RPCs (figure 4.3). The gap at the bottom covers the entire

chamber, and the top gap is cut into a narrow and wide part in order to allow signal extraction

from the strips placed between the two layers. By using the double-gap configuration, the

gaps can be operated at a lower high voltage with an effective detector efficiency higher than

for a single-gap. The layout of a CMS gas gap is shown in figure 4.4. The resistive plates

are made out of Bakelite and have a bulk resistivity of 1010 − 1011 Ωcm. The outer Bakelitesurfaces are coated with graphite to form HV and ground electrodes. A 190µm thick polyester

(PET) sheet is used to insulate the graphite surface.

The uniform thickness of the gas volume (2 mm) is maintained by insulating spacers, which

are placed uniformly at a distance of 100 mm.

The CMS RPCs are operated in avalanche mode in order to ensure high counting rate capa-

bility (1 kHz/cm2)

Read out

The read-out copper strips are placed in between on a mylar sheet and hence the induced

signal is a sum of the two single-gap signals. The 32 strips are running along the radial

direction and are radially segmented in three trigger sections (figure 4.5). The read-out strips

are connected to the three Front-End Boards (FEBs). Each FEB houses four front-end chips1 which are made of 8 identical channels, each consisting of the following elements:

• an amplifier

• a constant fraction discriminator (CFD) using the zero-crossing technique which makes

1The barrel RPCs have two front-end chips

the timing response independent of amplitude2

• a one-shot circuit which produces a pulse shaped at 100 ns in order to mask the possibleafter-pulses that may follow the avalanche pulse.

• a low voltage differential signaling (LVDS) driver

1265

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56

VUB DEPT:ELEMPLEINLAAN 2 - 1050 BRUSSELTEL:32-2-6293231 FAX:00-32-2-6293816

TITLE:StripsRE4-Ring3

ASSEMBLY:RPC-RE4-Ring3

ISO-SYMBOL:

Design by: Luc Van Lancker

Verify by: LVL

Datum:10-Nov-10

PLAN NR.RE4-Ring3-011 -- V1

REPLACE:RE4-Ring3-011

SCALE:1/10- 1/5

On top 32 strips - parallel space of 2mmStrips width 35.9368 mm eachFiducial mark in middle strips

On bottom 32 strips - parallel space of 2mmStrips width 25.4147 mm eachFiducial mark in middle strips

Middle FM

Bottom FM

Top TMSEE DETAIL A

SEE DETAIL B

SEE DETAIL D

SCALE 0,500DETAIL A

SEE DETAIL C

SCALE 0,500DETAIL B

SCALE 1,000DETAIL C

SCALE 0,500DETAIL D

Figure 4.5: Strip layout of a RE4/3 chamber [22]

4.3 Upgrade of the RPC forward system: RE4

At each endcap, 4 RPC stations were foreseen in the initial design, as shown in figure 4.6.

The fourth station, called RE4, was staged due to insufficient funding availability. With 4

stations instead of 3, the trigger efficiency becomes much better, as is shown in figure 4.7. It

is therefore important to complete the forward RPC system to 4 layers to allow an efficient

and robust trigger operation when the LHC is running at design luminosity. The goal is to

install this station at the CMS upgrade during the LHC shutdown of 2013. The RPCs will

be placed in two concentric rings (RE4/2 and RE4/3) on each endcap disk like the previous

2An overview of different techniques involved in pulse timing can be found in chapter 17 of “Radiation,

detection and measurement” of G.F. Knoll [1]

Figure 4.6: Layout and η coverage of the planned forward RPC system [23]

Figure 4.7: simulated trigger efficiency vs. η-coordinate as function of the numbers of layers [20]

stations. Each ring is composed of 36 RPCs, resulting in a total of 144 RPCs which should

be assembled and tested for the RE4 station. There will be a production of 56 additional

RPCs, which will be kept as spares for the RPC forward system [20].

Production process

The production of Bakelite sheets, which are actually high pressure plastic laminates (HPL),

takes place at the Puricelli firm in Milan3. The pilot production was done in June 2010,

but the resistivity of those sheets showed instability in time. Stability measurements on new

prototypes, which are produced in a different way, should be completed by May 2011 allowing

the start of a preproduction by June 2011. A quality control will be pursued at the Pavia

INFN site. Validated sheets are shipped to RIVA (Milano, Italymuim) for cutting procedures

and to GT (Frosinone, Italy) for surface cleaning [19,20].

In order to produce 200 RPCs, we need 200 gaps of each type (narrow, wide and bottom). We

also need 10% additional production to replace the gaps which fail the quality assurance at

the chamber assembly sites. There are two possible gap assembly sites: the Korean Detector

Laboratory (KODEL) or GT in Italy. The assembly site will be chosen in June 2011 and

preproduction of the gaps should start by July 2011 [19]. The FEB boards will be produced

in Pakistan.

The RPC chambers will be assembled at three sites: Ghent (Belgium), Mumbay (India) and

CERN. The first gaps will arrive at the assembly sites by August 20114. The first RPC

chamber should be assembled and fully tested by September 2011, 9 other chambers are

foreseen to be completed in 2011. In March 2013, the last chamber is supposed to leave the

assembly sites.

Future plans

In a second phase, one is aiming to complete the high η region (1.6

Chapter 5

The Ghent RPC lab

5.1 Introduction

The Ghent RPC lab compromises a 70 m2 area for assembly and testing, located in the old

accelerator hall of the institute. It has an easy access for detector transports and there is

also a crane available for heavy lifting. In this area, a 19 m2 air conditioned hut is installed

in order to have a controlled environment for chamber testing1. We also have an additional

R&D room (45 m2), which was used for the pre-assembly tests of the scintillators.

The this chapter, we present a short overview of the proposed quality control (QC) tests, and

the design of the high voltage and cosmic test.

5.2 Proposed quality control

5.2.1 Tests at the gas gap level

Leakage test and spacer test

In order to ensure the absence of gas leaks a leakage test is performed. An overpressure of

20 mbar is slowly applied on each gap and the pressure is monitored for two hours. A gap

is accepted if the loss of the applied pressure is less than 0.8 mbar/hr [18]. In addition, one

has to ensure there are no popped spacers in the gap, because it is import to have a constant

distance between the electric plates in order to have a uniform electric field in the gap. he

spacers are tested by pressing the gas gaps on the positions where the spacers are located2

while the pressure is monitored using a pressure sensor connected to a PC. After pressing all

the spacers, one can check the graph of the monitored pressure (see figure 5.2). When a gap

is pressed at the location of a spacer, the spacer will keep the gap at the same volume and

no to big pressure difference should be monitored. If the gap is pressed at a place without

1More information about this air conditioned hut can be found in Michiel’s thesis [24]2A template is used in order to know the positions of the spacers

23

spacers or at the location of a broken spacer, the volume of the gap will deform and a peak

is noticed in the graph. Details about this setup can be found in Michiel’s thesis [24].

Figure 5.1: A top wide gap with the template for the spacer test

High voltage scan and dark current

The gas gaps that pass the leakage and spacer tests will be placed under high voltage in

order to measure the dark current of the gaps. When this test was performed during former

productions [25, 26], the gaps were operated with a gas mixture of Freon or R134a (95.5%

- 96.5%) and Iso-butane (3.5% - 4.5%) under 2 mbar pressure at a flow rate of 5 L/hr. At

former productions [18], the test was started by applying 2 kV to check any disconnection

or electrical shortage. Then, the high voltage was raised to 8.5 kV, the voltage at which gas

avalanches begin to develop, with steps of 1 kV over 5 hours. The gap was kept at 8.5 kV

Figure 5.2: A spacer test on an old gap: at the end, a reference point was taken by pressing the gap

at a place without spacers and broken spacers will show a similar peak [24]

for 12 hours to observe the behaviour of the ohmic dark currents of the gas gaps. The high

voltage was then increased with steps of 100 V to the operating voltage of 9.4 kV and the

current behaviour was monitored for 36 hours. If the dark current is too high, the gap was

rejected and not used for further assembly. The test will follow a similar schedule in the

Ghent RPC lab, the length of the test may change as a function of the production schedule.

The current limits used for the RE2/2 production are given in table 5.1 3.

This test will take place in the temperature and humidity controlled environment and the

temperature and humidity values will be measured. An effective high voltage Veff , which

corrects for temperature and pressure variation, can be calculated according to

Veff = VappP0P

T

T0(5.1)

where Vapp is the applied voltage, P0 is 1010 mbar and T0 is 293 K.

8.5 kV 9.4 kV

Top wide gaps 2.0µA 3.0µA

Top narrow gaps 3.0µA 5.0µA

Bottom gaps 5.0µA 8.0µA

Table 5.1: The current limits at 8.5 and 9.4 kV for qualified RE2/2 gas gaps [18]

3This values are from reference [18], but according to reference [25] the value of 5µA was used for all gaps

at operating voltage

5.2.2 Tests at chamber level

High voltage scan and dark current

After the gap testing, the RPC chambers are assembled using the narrow, wide and bottom

gaps. The chambers are again subject to a high voltage scan to check if the gaps were not

damaged during assembly.

Cosmic ray test

In the cosmic ray test stand, several RPCs are placed horizontally. When a cosmic muon

traverses the test stand, it is triggered by scintillators4 The cosmic ray test should be able to

study the following characteristics:

• The chamber efficiency which is the ratio between the number of events in whichan RPC has at least one fired strip in the trigger window (100 ns) and the number of

triggers. The value of efficiency should be greater than 95% at operating voltages [25].

For good RPC performance of an RPC, its efficiency should extend over 300 V. One

can correct the efficiency for spurious hits:

ε =

NobsNtr− Ps

1− Ps(5.2)

where Nobs is the number of observed events, Ntr is the number of triggered events and

Ps is the probability for spurious hits. This probability can be determined by counting

the hits in a time window delayed 100 ns after the trigger. Efficiency scans will be

performed for a number of voltages in the range of 8.5 kV to 9.6 kV and for a number of

chamber threshold values in the range of 180 mV to 300 mV [27]. If the muon trajectory

is precisely known, the individual strip efficiency can be calculated.

• The strip response profiles which is used to ensure all readout strips are active andworking as they are supposed to. If the chamber has more than 2 dead or noisy strips,

the chamber will be rejected [25].

• The chamber cluster size which is defined as the average value of the cluster-sizedistribution. This value should be less than 3 [25].

This test should be done three times for each chamber: for the bottom and top gaps separately

(the high voltage is only applied on the top or bottom gap) and for both gaps together. This

will allow us to get efficiency measurements for every gap separately.

4A detailed discussion about scintillation detector principles can be found in [1]

Figure 5.3: Schematic view of a cosmic test stand [21]

5.3 Design of the high voltage test

[28, 29] The high voltage power supply to the RPCs is provided by the CAEN SY1527LC

system, where two CAEN A1526N boards are installed. The parameters of the SY1527LC and

the boards installed inside it, can be controlled via Ethernet (TCP/IP). The high voltage of

the A1526N can go up to 15000 V, although a maximum voltage can be programmed through

software for safety reasons. The A1526 has 6 output channels, thus a total of 12 gaps or 4

chambers can be put under HV.

A C++/QT/ROOT-based program was developed in order to control the high voltage system

and monitor the current. The user can set the voltage and maximum current of the A1526N

boards through a graphical user interface. It is also possible to change these values at fixed

time intervals so that measurements at different voltage levels can be done during nights or

weekends. The monitored values and status of the channels are displayed on the screen and

are saved in different graphs:

• The programmed and monitored voltage vs. time

• The monitored current vs. time

• The monitored current vs. programmed voltage

The program is written using Qt and ROOT, which should be installed on the computer

(for more information, see appendix A). The program makes use of the CAEN HV Wrapper

library which permits the control of a generic CAEN power supply system. A manual is

included in appendix B.

Figure 5.4: Two A1526 and three A1513B modules are installed inside the SY1527LC crate

Figure 5.5: Example of a graph showing the monitored current vs. time (note: this graph was taken

during a test without any device connected to the HV module)

Figure 5.6: Example of a graph showing the monitored current vs. programmed voltage (note: this

graph was taken during a test without any device connected to the HV module)

5.4 Design of the cosmic ray test

In this section, we will describe the different components of the experimental setup and how

these are controlled through our cosmic DAQ program. A detailed manual for this program

can be found in manual C. At the end of this section, the results of the pre-assembly tests

are presented.

All modules, except the ones concerning power supply, are installed in the VME crate. A

SIS1100/3100 PCI to VME interface card is used to communicate with the modules inside

the VME crate. A Linux driver is installed on the computer to control this card. This driver

contains routines to read (write) 16 or 32 bit words from/to the VME modules, which are

accessed through their base address. An overview of the routines, which are included in our

program through the sis3100_vme_calls.h header file can be found in reference [30].

5.4.1 Trigger system

Scintillators

The scintillators are configured in 4 planes: two planes above the RPCs (one in x-direction

and one in y-direction) and the same configuration is repeated below the RPCs (see figures

5.8 and 5.9). In this way, it will be possible to provide a rough tracking for the cosmic muons

when they are going through the cosmic test stand.

The scintillator strips were ordered in 2009 from the Institute of Particle Physics (IHEP)

in Protvino, Russia. They have dimensions 200 × 10 × 1.5 cm3, which will allow a triggersurface of 200 × 160 cm2 when 20 scintillators are used in one direction and 16 in the other

Figure 5.7: The computer with the SIS1100/3100 PCI to VME interface card (called charpac in the

network), the VME crate which contains the modules discussed below and the SY527

crate used for the HV of the PMTs

Figure 5.8: Schematic view of the cosmic test stand (simplified, the QDC modules are not shown) [31]

Figure 5.9: Schematic view of the scintillators in the cosmic test stand (from above) [31]

direction. The first six scintillators were delivered in December 2010, one of them was used

to for pre-assembly tests, described below. The final batch of 79 scintillators was delivered in

April 2011.

Photomultiplier tubes and fibers

The scintillation light has to be collected. In October 2010, some tests with multi-pixel

photon counters (MPPC) were done, but these devices require dedicated electronics which

would make the readout of the entire setup rather complex. Therefore we decided to use

photomultiplier tubes (PMT) which are more simple to use. We will use Philips XP1911

PMTs with a diameter of 19 mm, which were used before in the HERMES RICH detector5. The power supply of the PMTs is provided by the SY527 crate [32], which is controlled

through a High Speed Caenet network via the V288 VME module [33]. To avoid reflections,

the H.S. CAENET line is terminated by a 50Ω impedance on the SY527 crate.

To transport the light in an efficient way from the scintillators to the PMTs, Bicron BCF-91A

multi-clad wavelength shifting fibers are used. Four grooves are cut on each scintillator, to

put in the fibers. The first six scintillators were delivered without grooves. Because we don’t

have the machines to cut the grooves precisely enough and it is a lot of work, the other 79

scintillators were delivered with the grooves already in.

Event triggering

The output signals of the PMTs will be connected to the CAEN V812 constant fraction

discriminators (CFD) [34]. Each V812 module has 16 input channels, which is perfect for

the planes with 16 scintillators. For the planes with 20 scintillators, we will have to connect

multiple signals at the same channel, although it is not yet clear how this will be done. The

thresholds and output widths of the V812 modules can be controlled with the cosmic DAQ

program. From each module, the ’OR’ output is taken as trigger signal. In other words, a

V812 module will give a signal when there is at least one PMT signal above the threshold in

the corresponding plane. The thresholds of each individual input channel can be set through

software, allowing to use different thresholds to adjust for differences in the amplitudes for

the PMTs. It is also possible to change the pulse width of the output signals and to assign a

dead time to the module.

The 4 ’OR’ outputs of the V812 module are connected to the 4 input channels of section 1 of

the V976 module [35]. Using the ’AND’ of this module, we can trigger if the event is seen in

all 4 planes. Alternatively, it is also possible to trigger if only 3 out of 4 planes see the event.

5HERMES was an experiment operated at the HERA accelerator in DESY. The HERMES RICH detector

contained 1934 of these PMTs and we only need 72

In this case, the internal rotary switch should be set to majority level 2 and the function

switch on the front panel must be on ’OR’ position.

5.4.2 Tracking of events

Because the scintillators are placed in both x and y directions, we will be able to track single

muon events with a pitch6 of 10 cm. This will allow us to do efficiency measurements on a

part of the chamber. As the RPCs are smaller than the trigger surface, the tracking is needed

to reject events which are not passing through the RPC surface.

The PMT signals which are send to the V812 modules are also send to the CAEN V792 charge

to digital converters (QDC) [36]. These modules allow to measure the charge of the PMT

pulses. The input current is integrated during a temporal window, common to all channels

of the V792 module, which is defined by the GATE signal. This GATE signal is provided

by the trigger signal. The QDC counts can be used to determine which scintillators were

responsible for triggering and the cosmic muon can be tracked. Because we have only two

position points in each direction, only single muon events could be tracked (otherwise we

would interchange their start and end positions).Therefore we need to ensure no tracking is

applied when multiple muons are triggering together. When the cosmic test stand is build,

it should be researched which cuts are appropriate. This can be done by analysing the QDC

values which are also saved into data files. It is possible to put the values of the pedestal

peaks in a file which will be used by the program to subtract these values from the QDC

values.

Each plane of scintillators is connected to one of the four V792 modules, hence not all channels

will be used. One can define in a configuration file which scintillator and input channel are

connected to each other.

5.4.3 RPC control

Power supply

The power supply is provided by the same SY1527LC crate used for the HV test. The A1526N

modules are again used to put a high voltage on the gap. There are also three A1513B modules

installed in the SY1527LC crate [28,29,37]. These will provide the low voltage for the FEBs.

The output range of an A1513B modules goes up to 10 V with 10 mV monitor resolution.

The control of both the A1526N and A1513B modules are implemented in the cosmic DAQ

program in the same way as was done for the HV test program.

6The standard deviation is defined as σ =pitch√

12

Controlling the FEBs of the RPCs

There is a Labview program developed at CERN, which can be used to change the FEB

thresholds.

Figure 5.10: Labview program for controlling the FEBs [38]

5.4.4 RPC readout

The RPC signals will be read out by CAEN V1190A time to digital converters (TDC)7 [39].

These modules accept LVDS signals and therefore we do not need LVDS-ECL converter

boards (which were used for previous RPC productions). When a trigger arrives, the V1190A

will search for hits in the trigger matching window, which can precede the trigger. The

time parameters of this trigger matching window can be controlled through our cosmic DAQ

program, more information can be found in appendix C.

The program will store the TDC values in ROOT files. It is not yet implemented to check the

muon trajectories and therefore one cannot use the total trigger surface at the moment. If less

scintillators are connected and the trigger surface is smaller, it should work already. A ROOT

macro8, RpcEff_Ghent.C, can be used to show histograms with the clustersize, stripprofile

and the time distribution. The efficiency will be displayed in the terminal. Figures 5.11, 5.12

and 5.13 show these histograms for a first small cosmic test at CERN in April 2011.

7These modules are not yet arrived in Ghent at the moment of writing this thesis8This macro is written by Andrey Marinov for a cosmic test with small scintillator paddles at CERN

Figure 5.11: Clustersize from a test on RE42-CERN-PP01 (assembled at CERN, first of the prepro-

duction) operating at 9400 kV and threshold at 215 mV

Figure 5.12: Strip profile from a test on RE42-CERN-PP01 (assembled at CERN, first of the pre-

production) operating at 9400 kV and threshold at 215 mV. The trigger was provided

by two small scintillator paddles, placed above a small part of the detector, were used

for trigger. This explains the strip profile. Only the strips of one read-out section are

shown.

Figure 5.13: Time distribution from a test on RE42-CERN-PP01 (assembled at CERN, first of the

preproduction) operating at 9400 kV and threshold at 215 mV

5.4.5 Pre-assembly tests on scintillators and PMTs

Two pre-assembly tests were done: the calibration of a XP1911 PMT and the measurement

of the attenuation length in a scintillator with or without using fibers. Both experimental

setups were using a “small” black box, were one scintillator could be placed. A PMT was

installed in the side of the box and in the signal was send to an input channel of a CAEN

V792 charge to digital converter (QDC). The input current is integrated during a temporal

window, common to all channels of the V792 module, which is defined by the GATE signal.

A V812 CFD was used to control the width of the gate. The V812 and V792 modules were

controlled through older versions of the cosmicdaq program (up to version 5), which are

similar to the latest version, which has a manual in appendix C. The V792 data was written

into text files, stored in the folder data, and a ROOT macro (histogram.C) was written in

order to create histograms which show the QDC spectra. The macro can be found in the

same folder as the cosmicdaq program and more information on how to use this macro can

be found in appendix D.

Calibration of a PMT

It was possible to find a relation between the number of photo-electrons on the photocathode

of the PMT and the QDC data channels. This was done with the experimental setup shown

in figure 5.15. A LED, which light output and pulses are controlled through a LED driver, is

placed in a black box. When a pulse is generated by the LED driver, a synchronous signal is

Figure 5.14: The scintillator and PMT in the black box

Figure 5.15: Experimental setup used for the calibration of the PMT [31]

emitted. A small delay is applied on this signal and send to the V812 CFD, which enabled us

to control the pulse width. This pulse was used for controlling the gate of the V792 module,

which took the QDC spectra of the PMT.

If the light output of the LED is too small, only the pedestal peak is seen in the QDC spectrum

(figure 5.16). When this light output is slowly turned up, a second peak will appear (figure

5.17). This peak is the single photo-electron peak. From the difference between the pedestal

and single photo-electron peak, we see that 1 photo-electron corresponds to 18 QDC channels.

Figure 5.16: QDC spectrum with only the pedestal peak (LED at 1.4 V)

Attenuation length

The cosmic ray stand will make use of 2 m long scintillators, where the light is collected

by PMTs on only one side. Therefore several attenuation processes, like reabsorbing of

scintillation light or light escaping out of the scintillator, could affect the light collection.

The number of photons reaching the PMTs will depend on the distance travelled by these

photons. The intensity of light I at a distance x from the original scintillation site will fall

off exponentially as

I = I0e− xλ (5.3)

where I0 is the intensity close to the site and λ is the attenuation length. If the attenuation

length is small, the number of photons in the scintillator is dropping fast and photons created

Figure 5.17: QDC spectrum with pedestal and single photo-electron peak (LED at 1.5 V)

at a scintillation site close to the PMT have a higher probability to reach this PMT. Hence,

events close to the PMT will generate more photo-electrons on the photocathode of the

PMT, creating a larger signal which will easier surpass the trigger threshold. It is therefore

very important to keep the attenuation length as long as possible to guarantee equal trigger

probability over the scintillator surface. We have made an experimental setup to study how

the use of fibers and reflecting aluminium foils can affects the attenuation length.

The experimental setup is shown in figure 5.18. Two small scintillators paddles, which were

before part of the H0 hodoscope in the HERMES experiment, are used to trigger cosmic

muons. There light is collected by PMTs and the signal is send to CAEN V812 CFDs where

a threshold is applied. The coincidence of those two trigger signals is taken by the CAEN

V976 module. The coincidence signal was send to another V812 module where we could

assign a constant output pulse width. This output pulse defined the gate of a V792 module.

The scintillator strip under study is placed in a black box and the PMT is installed in the

side of the box. A delay is assigned to the signal in order to get the output pulse in the time

window of the V792 module. This delay was determined by comparing the PMT signal with

the V812 output pulse on an oscilloscope. The signal is connected to an input channel of the

V792 module which integrates the input charge.

The scintillator paddles could be moved, allowing to trigger at different positions of the

scintillator strip under study. We took data at about 0, 50, 100 and 170 cm where we have

Figure 5.18: Experimental setup used for the measurement of the attenuation length [31]

taken the beginning of the scintillator on the side of the PMT as origin. The mean values

of the QDC spectra were converted to the number of photo-electrons at the photocathode

of the PMT. This values were plotted using MATLAB and a fit was done to determine the

attenuation length. The plots are shown in figures 5.19 and 5.20. They show the attenuation

length for 6 configurations:

• Only the scintillator in direct contact with the PMT (λ = 70 cm)

• The scintillator covered in kitchen aluminium foil (λ = 76 cm)

• The scintillator covered in kitchen aluminium foil and the light is collected throughfibers (λ = 175 cm)

• The scintillator covered in aluminized Mylar and the light is collected through fibers(λ = 247 cm)

• The scintillator covered in aluminized Mylar and the light is collected through fiberswhich are glued with BC-600 optical cement into the grooves of the scintillator (λ =

386 cm)

• The scintillator covered in 3M Mirror foil 1100 and the light is collected through fiberswhich are glued with BC-600 optical cement into the grooves of the scintillator (λ =

418 cm)

Although the use of fibers reduces the intensity for events with a scintillation site close to the

PMT, the use of glued fibers and 3M Mirror foil 1100 are already superior for scintillation

Figure 5.19: The attenuation length of a scintillator without using fibers [31]

Figure 5.20: The attenuation length of a scintillator with fibers [31]

sites above 50 cm. In this last setup, the difference in collected light intensity between both

ends of the scintillator is:

I

I0= e−

200 cm418 cm

= .62 (5.4)

High voltage scan for the PMT

The same experimental setup was also used to do a HV scan on the PMT which is shown in

figure 5.21.

Figure 5.21: HV scan on the PMT [31]

Chapter 6

Conclusions

The CMS experiment has put emphasis on the detection and identification of muons. Resistive

plate chambers are used in the muon trigger system and they determine precisely the beam

crossing from which the muons emerge. Four layers of RPCs on each endcap disk have been

foreseen in the initial design. However, due to insufficient budgets, the fourth endcap disk

was staged and is yet to be built.

An international collaboration, including the UGent, was formed to build the 4th RPC station.

The new RE4 chambers will be assembled and tested in Ghent, Mumbai and and CERN. The

Ghent RPC assembly and test facility is presently being set up. The RPC components are

produced abroad before being transported to the different assembly sites. At each site, all

components will undergo a dedicated quality control. A leakage and spacer test is performed

on the gas gaps in order to check for leakages a popped spacers. The dark current of the

gas gap is measured during a HV test. After assembling the RPC chamber, the HV test

is repeated. Finally, the RPCs will be placed inside a cosmic test bench to measure their

efficiency, strip profile and clustersize.

In this work, the DAQ software for the HV test, which controls the HV on the gas gaps, was

designed. The monitored current is automatically displayed in graphs by the program. This

program is, together with the leakage and spacer tests, ready for use.

A cosmic testbench using 4 planes of scintillators was designed. Before starting the construc-

tion of the testbench, several pre-assembly tests were performed with a single scintillator and

PMT. A calibration of the PMT and a HV scan was done. The attenuation length of the

scintillator was measured using different configurations. We obtained the best result using

fibers glued in grooves on the surface of the scintillator and the 3M Mirror foil, resulting in

an attenuation length of λ = 418 cm.

The DAQ and reconstruction software developed for the cosmic test bench can be used for

applying the first basic tests. This program should be extended to have a full working setup

which can track the muons and measure the local efficiency of a RPC.

43

Chapter 7

Nederlandstalige samenvatting

In de 20e eeuw werd het standaard model van de deeltjesfysica ontwikkeld. Dit model be-

schrijft hoe materie is opgebouwd uit elementaire deeltjes en hoe deze deeltjes met elkaar

interageren via de sterke, zwakke en elektromagnetische kracht. Het standaard model is door

verschillende experimenten met hoge precisie getest. Het Higgs boson, dat gepostuleerd werd

om de masa van deeltjes te verklaren, is echter nog niet waargenomen. Daarnaast zijn er nog

heel wat andere open vragen in de deeltjesfysica waar het standaard model geen antwoord

op kan geven. We verwachten daarom dat er nieuwe fysica zal gevonden worden wanneer

men botsingen bij hogere energieën onderzoekt. Sinds 2010 is dit mogelijk met behulp van

de Large Hadron Collider (LHC), dat protonen met een centrumenergie van momenteel tot

7 TeV met elkaar laat botsen.

De Compact Muon Solenoid (CMS) is één van de detectoren die gebruik maken van de LHC

om deze proton-proton botsingen te bestuderen. De belangrijkste componenten van CMS

zijn:

• de supergeleidende solenöıde die de baan van geladen deeltjes afbuigt in een magnetischveld om het impulsmoment van deeltjes te bepalen.

• de tracking detector die het pad van geladen deeltjes kan reconstrueren zodat de impulsen lading van het deeltje bepaald kunnen worden

• de elektromagnetische calorimeter die de energie van fotonen en elektronen bepaalt

• de hadronische calorimeter die de energie van hadronen bepaalt

• het muon detectiesysteem

Het muon detectiesysteem bestaat uit 3 types gasdetectoren, waaronder resistive plate cham-

bers (RPCs). Een RPC ’gap’ bestaat uit twee parallele platen waartussen zich een gas bevindt.

Als er hoogspanning over de gap wordt gezet zal er bij de passage van een geladen deeltje

45

een Townsend lawine worden gevormd. De parallele platen zijn gemaakt uit een materiaal

met hoge restiviteit zodat de ontlading lokaal blijft en de rest van het oppervlak nog altijd

gevoelig blijft voor andere deeltjes. Spacers zorgen ervoor dat platen op gelijke afstand van

elkaar blijven, wat belangrijk is voor de uniformiteit van het elektrich veld.

CMS maakt gebruikt van double-gap RPCs waarbij de uitleesstrips tussen de 2 gaps worden

geplaatst. De RPCs geven een snel signaal en worden daarom vooral gebruikt bij de triggering

van het muon systeem. Het huidig RPC systeem bestaat uit 6 lagen in het barrelgedeelte van

de CMS detector en 3 lagen in de endcap disks langs beide zijden. Een 4e endcap laag was

uitgesteld wegens budgetproblemen. Aangezien deze 4e laag de trigger efficientie aanzienlijk

kan verbeteren is het van groot belang om deze toch nog te realiseren. De UGent is betrokken

bij dit project en vanaf augustus 2011 zullen een deel van deze RPCs geassembleerd worden

in Gent. Tijdens de assemblage zullen verschillende kwaliteitscontroles uitgevoerd worden.

De verschillende componenten (gaskamers, elektronica,. . . ) zullen in het buitenland worden

geproduceerd en worden vervolgens opgestuurd. Voordat de RPC detectoren geassembleerd

worden, moeten de gaskamers getest worden op gaslekken en gebroken spacers. Vervolgens

worden ze onderworpen aan een hoogspanningstest. Nadat de RPCs geassembleerd zijn,

kan men terug een hoogspanningstest uitvoeren en uiteindelijk worden metingen met behulp

van kosmische muonen uitgevoerd om de efficientie en clustergrootte te bepalen. Voor deze

laatste test zal een kosmische testbank gebouwd worden waarbij scintillatoren in verschillende

vlakken gebruikt worden om de kosmische muonen te triggeren en een ruwe tracking uit te

voeren.

In het kader van deze thesis hebben we de software voor de hoogspanningstest en kosmische

test ontworpen en zijn er enkele pre-assamblage tests uitgevoerd. We hebben een testopstelling

gebouwd om de attenuatielengte van de scintillatoren te meten. Het scintillatielicht zal worden

verzameld door photomultiplier tubes (PMT). De PMT van onze testopstelling is gekalibreerd

en we hebben een HV scan uitgevoerd op de PMT.

Appendix A

Qt and ROOT

The software of the Ghent RPC lab makes use of the Qt 4.6.3 [40] and ROOT 5.28/00 [41]

frameworks, which should be installed on the computers.

A.1 Qt

Qt is cross-platform graphical user interface (GUI) toolkit written in C++. It also provides

the very useful signal and slots mechanism: a signal is emitted when a particular event occurs

and a slot is a function that can be called in response to a signal. A class will emit a signal

when something interesting occurs, but it doesn’t know or care which slots catch the signal

(if any slot catches the signal). Another feature is Qt is thread management: it is possible to

divide the functions across multiple threads. For example, saving the graphs in the HV test

program is a time consuming task and is running in a separate thread in order not to block

the user control and monitoring of the high voltage. In the program for the cosmic DAQ, a

separate thread is always checking the V792 QDCs for new data. A detailed documentation

of Qt can be found on their webpage [40].

To simplify the build process, the qmake tool is used. This tool generate a Makefile (which

can be quite long) based on the information in a project file, which has the extension .pro.

If one wants to extend the software of the Ghent RPC lab with new classes or libraries, they

should be added in the project files and the qmake command should be used to generate the

new Makefile.

A.2 ROOT

ROOT is a data analysis framework, designed for analysis in high energy particle physics.

In the software written for the Ghent RPC lab, ROOT is used for making the graphs and

histograms. In order to use ROOT together with Qt, it should be installed with the Qt-layer,

47

which can be done by adding --enable-qt after the .\configure command. In principle, it

should be possible to show the ROOT graphs directly in the HV test or cosmic DAQ programs.

Although this is easy to implement, we have chosen to leave it out of the program, because in

our experience, the program becomes slow and unstable when loading a graph or histogram.

More information about the Qt integration in ROOT can be found in reference [42].

Appendix B

Manual for the HVtest program

The HVtest program enables the user to control the CAEN A1526N HV module which is

placed inside the CAEN SY1527 crate. The monitored current and voltage values saved in

rootfiles and/or directly in a graphical format. The program connects to the crate through

an TCP/IP interface and can be displayed from any computer in the network. The manuals

describing the hardware/software libraries used for this program can be found in references

[28,29,43].

B.1 Starting up

The program, which can be found in the HVtest folder on the charpac computer, is started

from a terminal using the command:

~/HVtest/HVtest

If this, or another program which makes use of a connection with the SY1527, crashes and

the connection is not properly closed, the semaphore1 should be cleared before restarting the

program. To list the semaphores, open a terminal and type

ipcs -s

To remove the semaphore which controls the access to the SY1527 (probably the last one in

the list), type

ipcrm sem XXX

where XXX is the semaphore ID.

1A semaphore is a computer variable which controls access by multiple processes to a common resource

49

B.2 Main window

When the program is started, it will automatically try to set up the connection with the

SY1527 crate. If this connection fails, one should check:

• if the SY1527 is switched on

• if the router in the RPC lab is switched on

• if the login parameters are still valid, these can be changed in a dialog window whichwill retry to establish the connection after “OK” is pressed.

Figure B.1: The dialog window where the login parameters can be changed

If the connection is established, the main window will look like figure B.2, and has 4 buttons:

• A1526 Control, which opens the window which controls the A1526N HV boards

• Kill, which turns all channels off in the SY1527 crate

• Clear alarm, which allows to remove all alarm conditions

• Quit, which closes the application after properly closing the connection with the SY1527and saving the rootfiles/graphs.

B.3 The A1526 Control window

Figures B.3 and B.4 show the A1526N Control window, which contains the following fields

and buttons:

• The Channel (slot)-(channel) of the modules.

• The RPC code is used to construct the names of the rootfiles and graphs. This fieldis only changed after pressing “ENTER”, and it will become gray and read-only (like

channel 4-1 in figure B.3) for a while until everything is saved and the program is ready

to fill new graphs. When this field is empty, no graphs will be saved.

Figure B.2: The main window

• The on/off button switches the channel on/off. Note that there’s a little delay in theresponse of the A1526N module, thus the button and status will not change immediately.

• The status can show the following messages:

– ON or OFF

– UP or DOWN, when the channel is ramping up or down

– OVERCURRENT, OVERVOLTAGE or UNDERVOLTAGE, when the channel is in on of these

conditions, but still on

– MAXV, when the channel has reached the maximum voltage2

– EXTTRIP, when the channel is switched off due to external trip line signal

– INTTRIP, when the channel is switched off due to internal overcurrent condition

– EXT DIS, when the channel is disabled by the board interlock protection

• The Vset spinbox can be used to change the voltage between 0 V and 15000 V.

• The Vmon field shows the monitored voltage with an accuracy of ±0.3%± 2 V.

• The Iset spinbox can be used to change the maximum output current between thepossible limits.

2This maximum voltage level could be fixed through hardware or software, but this is not (yet) incorporated

in the program.

• The Imon field shows the monitored current with an accuracy3 of ±2%± 0.01µA.

• The start/stop button can be used to change the Vset and Iset at fixed times. Whenpressing the start button, a dialog window will show up asking for a file which contains

these settings (see figure B.5). An example settings file is showed below:

3

0 1 100 .10

100 1 200 .20

200 0 0 0

The number on the first line tells the program how many times the settings should

be changed. The next lines contain the time when the settings should be changed (in

seconds), the on/off status, the Vset value and the Iset value respectively. It is not

possible to change the other fields of the channel when a settings file is loaded.

Figure B.3: The A1526 Control window when the RPC code is changed

B.4 Browsing the rootfiles/graphs

The monitored current and voltage will be saved in graphs and rootfiles. The filenames are

constructed using the RPC code and one will find the following graphs:

• Vmon/Vset vs. time

• Imon vs. time

• Imon vs. time (for each Vset separately)3If the module is changed to the 1 mA range, the accuracy is ±2% ± 0.1µA.

Figure B.4: The A1526 Control window while the first 9 channels are following a fixed program and

the 10th channel is in overcurrent condition after manually changing the voltage while

the Iset is still at 0µA.

Figure B.5: The dialog window used to load a program

• Imon vs. Vset (the mean value of the previous graph is plotted against the Vset value)

When the program is restarted (or when a voltage is revisited for the separate Imon graphs),

the images will be overwritten. The older graphs can still be found in the rootfiles. To browse

through the graphs, open ROOT and type

TBrowser b;

Figure B.6: Example of the TBrowser in ROOT

Every rootfile is divided into folders with the filename constructed out of the date and time

when the program was started (in the MONTH_DAY_HOUR_MINUT format). When using the

TBrowser, one should be aware of some ROOT-settings and ROOT-bugs:

• When the Imon graphs are viewed, the draw option “ALP” should be selected4 (see alsofigure B.7)

• The canvas should be cleared (or a new canvas should be taken) if order to see theVmon graphs properly 5

• If a graph is not loaded by the TBrowser when clicking on it, restart ROOT and tryagain

4this is not needed for the Vmon graphs because the last one is an object of the TMultiGraph class instead

of the TGraph class5The Vmon graphs are objects of the TMultiGraph class, which suffer an axes problem if they are drawed

while the canvas is not cleared

Figure B.7: Select the draw option “ALP” in order to see the Imon graphs

B.5 Changing program parameters

Some parameters of the high voltage are stored in the A1526.dat file, which is loaded at the

start-up of the program. The file contains 5 values which represents respectively

• the slot of the first A1526N module

• the slot of the second A1526N module

• the output voltage limit (SVMax)

• the voltage ramp-up (RUp) speed

• the voltage ramp-down (RDWn) speed

Some program parameters (which are defined using #define) can be changed in the header

file include/A1526Control.h, the default values of the most important parameters are given

in table B.1. The current range and precision of the module can be changed by a dip-switch,

see page 15 in the manual [29].

B.6 Upgrade of the HV test program

Some additional features could be programmed if needed, but are not yet implemented:

• The trip (Trip) time, which is the maximum time an overcurrent condition is allowed,can be programmed

• The power ON option (Pon), which decides if a channel is restored after a power off orreset, can be enabled/disabled

UPDATE TIME 250 ms The monitored values on the screen are refreshed at this fixed interval

WRITE TIME 1 s The monitored values are written into the graphs at this fixed interval

SAVE TIME 300 s The graphs are saved at this fixed interval

FORMAT “png” The graphics format used for saving the graphs

MAXI 100 nA The maximum value which can be set by the Iset spinbox