Tests of Irradiated Semiconductor Detectors for ATLAS Upgrade

Transcription

Univerzita Karlova v Praze
Matematicko-fyzikálnı́ fakulta
Bakalářská práce
Pavel Novotný
Testovánı́ ozářených křemı́kových
detektorů pro experiment ATLAS
Upgrade
Ústav částicové a jaderné fyziky
Vedoucı́ bakalářské práce: RNDr. Peter Kodyš, CSc.
Studijnı́ program: Fyzika
Studijnı́ obor: Obecná fyzika
Praha 2014
Charles University in Prague
Faculty of Mathematics and Physics
BACHELOR THESIS
Pavel Novotný
Tests of Irradiated Semiconductor
Detectors for ATLAS Upgrade
Institute of Particle and Nuclear Physics
Supervisor of the bachelor thesis: RNDr. Peter Kodyš, CSc.
Study programme: Physics
Specialization: General Physics
Prague 2014
I would like to thank to my supervisor RNDr. Peter Kodyš, CSc. for his leading
of my bachelor thesis and for discussions about particle detection.
I am extremely grateful to the Prague National Library of Technology for
help me to gain a literature concerning the physics and engineering of radiation
detection.
I declare that I carried out this bachelor thesis independently, and only with the
cited sources, literature and other professional sources.
I understand that my work relates to the rights and obligations under the Act
No. 121/2000 Coll., the Copyright Act, as amended, in particular the fact that
the Charles University in Prague has the right to conclude a license agreement
on the use of this work as a school work pursuant to Section 60 paragraph 1 of
the Copyright Act.
In Prague, 8 May 2014
signature of the author
Název práce: Testovánı́ ozářených křemı́kových detektorů pro experiment ATLAS
Upgrade
Autor: Pavel Novotný
Katedra: Ústav částicové a jaderné fyziky
Vedoucı́ bakalářské práce: RNDr. Peter Kodyš, CSc., Ústav částicové a jaderné
fyziky
Abstrakt: Hlavnı́m cı́lem této práce byla účast na vývoji radiačně odolného detektoru částic pro experiment ATLAS. Spolupráce na tomto projektu vyžaduje
pochopenı́ principů detekce částic v experimentech fyziky vysokých energiı́. Problematika souvisı́ s oblastmi, jako je interakce zářenı́ s hmotou, provoz a konstrukce
polovodičových detektorů a zpracovánı́ signálů. Zvláštnı́ pozornost byla věnována
tématu radiačnı́ho poškozenı́ křemı́kových detektorů. Pomocı́ laserového testu,
provozovaného za nı́zkých teplot, byly studovány vybrané elektrické vlastnosti
ozářených detektorů. Aktivně jsem se podı́lel na jeho přı́pravě, realizaci i vyhodnocenı́. V neposlednı́ řadě, nový systém vyčı́tánı́ detektorů HSIO byl za mého
přispěnı́ zprovozněn v pražské detektorové laboratoři Ústavu částicové a jaderné
fyziky. Posloužı́ k práci na dalšı́m vývoji detektorů částic pro experiment ATLAS.
Klı́čová slova: laserové testy, polovodičový detektor, křemı́kový detektor
Title: Tests of Irradiated Semiconductor Detectors for ATLAS Upgrade
Author: Pavel Novotný
Department: Institute of Particle and Nuclear Physics
Supervisor: RNDr. Peter Kodyš, CSc., Institute of Particle and Nuclear Physics
Abstract: Participation on development of radiation hard particle detectors for
the ATLAS experiment was main goal of this bachelor thesis. Collaboration on
the detector development requires understanding of principles of particle detection in HEP experiments. It contains topics of radiation interaction with matter, operation and construction of semiconductor detectors and signal processing.
Special attention was paid to radiation damage of silicon detectors. Laser test
of irradiated detectors, where operation at low temperature was necessary, were
performed. Finally, basic electric characteristics of the detectors were obtained.
I participated in preparation and realization of measurement and analysis of the
results. Further, the new readout system HSIO was installed with my assistance
at the detector laboratory of Institute of Particle and Nuclear Physics in Prague.
It will be used in further development of particle detectors for the ATLAS experiment.
Keywords: laser test, semiconductor detector, silicon detector
Contents
Introduction
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1 Nuclear Radiation
1.1 Properties of Selected Types of Radiation .
1.1.1 Photons . . . . . . . . . . . . . . .
1.1.2 Protons . . . . . . . . . . . . . . .
1.1.3 Alpha Particles . . . . . . . . . . .
1.1.4 Electrons . . . . . . . . . . . . . .
1.1.5 Positrons . . . . . . . . . . . . . .
1.1.6 Neutrons . . . . . . . . . . . . . . .
1.2 Irradiation from the Perspective of Particle
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Physics
2 Radiation Interactions
2.1 Interaction of Photons with Matter . . . . . . .
2.1.1 Photoelectric Effect . . . . . . . . . . . .
2.1.2 Compton Scattering . . . . . . . . . . .
2.1.3 Pair Production . . . . . . . . . . . . . .
2.2 Interaction of Charged Particles with Matter . .
2.2.1 Heavy Charged Particles . . . . . . . . .
2.2.2 Light Charged Particles . . . . . . . . .
2.3 Interaction of Neutrons with Matter . . . . . . .
2.4 Simulation of Particle Interactions with Matter
3 Semiconductor Detectors
3.1 Structure of Semiconductors . . . . . . . . . .
3.2 Charge Carriers in Semiconductors . . . . . .
3.2.1 Electron-Hole Pair Production . . . . .
3.2.2 Trapping and Recombination . . . . .
3.2.3 Migration of Charge Carriers . . . . .
3.3 Intrinsic and Doped Semiconductors . . . . .
3.3.1 Intrinsic Semiconductors . . . . . . . .
3.3.2 Doped Semiconductors . . . . . . . . .
3.4 pn-Junction . . . . . . . . . . . . . . . . . . .
3.4.1 Characteristics of the Depletion Region
3.4.2 Reverse Biased Junction . . . . . . . .
3.5 Semiconductors as Particle Detectors . . . . .
3.5.1 Germanium . . . . . . . . . . . . . . .
3.5.2 Silicon . . . . . . . . . . . . . . . . . .
3.6 Operation of Semiconductor Detectors . . . .
3.6.1 pn-Diode . . . . . . . . . . . . . . . . .
3.6.2 Specific Semiconductor Detectors . . .
3.6.3 Position Sensitive Detection . . . . . .
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4 Radiation Damage
4.1 Defects in Silicon . . . . . . . . . . . . .
4.1.1 Damage Mechanism . . . . . . .
4.1.2 NIEL Scaling . . . . . . . . . . .
4.1.3 Hardness Factor . . . . . . . . . .
4.1.4 Annealing . . . . . . . . . . . . .
4.2 Detector Properties after Irradiation . .
4.2.1 Leakage Current . . . . . . . . .
4.2.2 Type Inversion . . . . . . . . . .
4.2.3 Depletion Voltage . . . . . . . . .
4.2.4 Charge Trapping . . . . . . . . .
4.3 Electronic Components after Irradiation
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5 Signal Processing
5.1 Preamplification . . . . . . . . . .
5.2 Amplification and Pulse Shaping
5.2.1 CR-RC Pulse Shaping . .
5.3 Filtering . . . . . . . . . . . . . .
5.4 Discrimination . . . . . . . . . . .
5.5 Digital Processing . . . . . . . . .
5.6 Signal Transmision . . . . . . . .
5.7 Modular Instruments . . . . . . .
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6 ATLAS Experiment
6.1 Description of the ATLAS Experiment . . . . . . . . . . . . . . .
6.2 ATLAS Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . .
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7 Tests of Irradiated Detectors
7.1 Description of the Experiment . . . . . . . . . . . . . . . . . . . .
7.2 Results of the Measurement . . . . . . . . . . . . . . . . . . . . .
7.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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8 HSIO Development Board
8.1 Description of the HSIO . .
8.2 HSIO Instalation in Prague
8.2.1 Hardware . . . . . .
8.2.2 Software . . . . . . .
8.3 Operation of the HSIO . . .
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Conclusion
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Bibliography
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List of Figures
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List of Tables
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List of Abbreviations
61
2
Introduction
Semiconductor detectors become important type of detection technique in high
energy physics experiments. All major experiments at CERN use silicon sensors
for tracking and vertexing.
However, operation of the semiconductor detectors in environment of intense
radiation dose leads to damage of these devices. Radiation induced changes in
semiconductor material cause changes of detection capabilities of the detectors.
Developing physics research requires modification of the LHC accelerator - increase of centre of mass energy and peak luminosity. In this case, development
of radiation hard semiconductor detectors becomes necessary for further physics
exploration.
This bachelor thesis is focused on the principles of particle detection in the
view of high energy physics experiments. At the beginning, the most common
types of nuclear radiation are briefly introduced. Second chapter describes fundamental mechanisms by which particles interact with matter. It is important
topic because the function of radiation detector is based on the way of particle
interaction with the material. Chapter 3 forms main part of this thesis. There
are discussed general properties of semiconductors and usage of them in the radiation detectors. Special attention is paid to radiation damage of silicon detectors
which is described in next separate chapter. Signal processing, fundamental task
for extracting information from detectors, is also mentioned. Chapter 5 introduces basic devices used for analog and digital processing. Description of the
ATLAS experiment and its upgrade is mentioned in chapter 6.
Chapter 7 contains description, results and discussion of tests of irradiated
detectors. I participated in preparation and realization of this measurement and
analysis of the results. Installation of new apparatus (HSIO) for detectors testing
is described in chapter 8.
3
4
1. Nuclear Radiation
Nuclear radiation is term used for a flux of micro objects like elementary particles,
ions or atomic nuclei [12, page 416]. Even if it is not an accurate expression (it
does not always have to do with nuclei) it is very often used in practise.
Nuclear radiation can be divided into some categories according to several
perspectives.
• capability to ionise atoms and molecules of an irradiated matter
– ionizing radiation
∗ directly ionizing radiation (charged particles)
∗ indirectly ionizing radiation (photons, neutrons)
– nonionizing radiation
• harmful effects of radiation on the human body1
– harmful radiation
– harmless radiation
• origin of source of radiation
– natural sources (cosmic radiation, terrestrial radiation - radioactive
elements)
– man-made sources (industry, medical machines, reactors)
1.1
Properties of Selected Types of Radiation
In the following part of this chapter, there the properties of the most important
types of nuclear radiation will be mentioned briefly. A selection covers the most
common used particles in radiation detection development.
1.1.1
Photons
A photon is one of the fundamentals particles with zero rest mass and zero electrical charge. It represents one quantum of electromagnetic energy [1, page 31].
As we can see in the picture 1.1, the spectrum and the energy2 range of
photons are wide. Particular types of electromagnetic radiation have different
properties and sources.
1
A boundary between harmful and harmless radiation is not well defined. This type of
radiation sorting depends strongly on a level of knowledge of an effects of radiation and on the
progress of human civilisation in general.
2
In particle physics, a special unit of energy - electronvolt (eV) - is used. It is equal to the
kinetic energy gained by an electron if it accelerates through a potential difference of one volt.
This unit do not belong to SI but it is widely used in certain branches of physics. Its small
value is suitable for description properties of elementary particles. Moreover, this unit relates
directly with methods of measurements of energy of particles [3, page 158].
5
X-ray photons are produced in the x-ray tube or as a synchrotron radiation. LASER (Light Amplification by Stimulated Emission of Radiation) uses a
quantum mechanical phenomenon (stimulated emission of photons) for emitting
intense, highly collimated and coherent beam of light [1, page 36]. Most of the
current LASERs have spectra in or around the visible part of electromagnetic
waves [1, page 39].
High energy photons (γ photons) are also emitted by some radioactive elements. Emission of an γ quantum accompanies transition of excited nucleus (it
can become excited as a result of beta decay) into stable state. Photons are emitted when a charge particle interacts with matter - bremsstrahlung, Cherenkov
radiation. These phenomena are described in the following chapter in more detail. Gamma rays follow some nuclear reactions and gamma radiation is also
generated in the annihilation process [7, pages 10 - 13].
Figure 1.1: Electromagnetic spectrum, taken from [1, page 31]
1.1.2
Protons
A proton is a subatomic stable particle. Its rest mass is 938.27 MeV/c2 [16] and
its value of electrical charge is equal to the elementary charge3 . According to the
Standard Model theory, protons are particles made up of three quarks (two up
quarks and one down quark).
Proton beams have found many applications in practise and also in fundamental research. Protons are mostly generated from hydrogen atoms and then
they are accelerated in an electrical field [1, page 47].
3
The elementary charge equals 1.602 × 10−19 C [16].
6
1.1.3
Alpha Particles
Alpha particles are helium nuclei which are composed of two protons and two
neutrons. It is an extremely stable configuration which is held together by the
strong nuclear interaction [1, page 54]. Its rest mass is 3.727 GeV/c2 and its
electrical charge equals double of the elementary charge [1, page 53]. Because of
high mass and electrical charge, alpha particles do not penetrate as deep as other
charged particles (protons, electrons) do [1, page 53].
Alpha particles are mostly emitted by heavy radioisotopes whose nuclei have
too many nucleons [8, page 3]. It is also possible to produce them during collisions
of particles with fixed target materials [1, page 54].
1.1.4
Electrons
An electron is one of the elementary particles. It is a negative charged particle
whose charge is equal to the negative elementary charge. It has small rest mass
511 keV/c2 in comparison with remaining constituents of an atom [16].
According to [1, page 42], two main ways of production of electron beams
exist. First is called an electron gun. In this device, there are used processes in
which an electron gains sufficient kinetic energy to break away from a material
[1, page 42]. Electrons can obtain energy in the form of heat (thermionic electron
gun) [1, page 42]. They can also break away from the material using a high
electric field (field emission electron gun) or after an illumination of material by
photons (photo emission electron gun) [1, page 42].
Certain radioisotopes are other sources of electrons [1, page 45]. In that case,
electrons are called beta particles according to the beta decay. It is one type of
radioactive decays in which a neutron converts into a proton and an electron e−
and an electron antineutrino ν̄ is emitted:
A
ZX
−
→A
Z+1 X + e + ν̄.
(1.1)
Spectrum of beta particles is continuous [12, page 256]. As an atom is not mainly
in ground and stable state after the beta decay, emission of a beta particle is
mostly associated with emission of γ rays [1, page 45].
1.1.5
Positrons
As a positron is the anti-particle of an electron, it has the same properties as
an electron except the polarity of the electrical charge [1, page 45]. Positrons are
produced using the process of pair production or β + decay (a proton converts
into a neutron and a positron e+ and a neutrino ν̄ is emitted) [1, page 45 and
46].
1.1.6
Neutrons
Together with protons, neutrons are particles which make atomic nuclei up [6,
page 236]. Their rest mass is similar to protons value - 939.55 MeV/c2 , they
are also constituted by three quarks (two down quarks and one up quark) but
neutrons have zero charge [1, page 49]. Thanks to their zero charge, neutrons
7
can penetrate material deeper than charged particles and this property is widely
used in many applications [1, page 49].
Neutrons are produced in spallation sources or in nuclear reactors and they
also accompany some fusion reactions [1, pages 49 - 51].
1.2
Irradiation from the Perspective of Particle
Physics
Nowadays, many particles are known. Current knowledge of particles is summarized by the Standard Model theory. Elementary matter constituents are leptons
and quarks4 . Hadrons are constituted by quarks (baryons - 3 quarks, mesons 2 quarks) [29].
Table 1.1 summarizes sorting of selected particles into groups. It is compiled
using information from [12] and [29]. Standard notation of the particles is used.
Table 1.1: Sorting of particles
Groups of particles
Leptons
Fermions
Baryons
Hadrons
Mesons
Bosons
Force carriers
Particles
e, νe , µ, νµ , τ, ντ
p, n, Λ...
π, K, ρ...
γ, W− , W+ , Z0 , g
Figure 1.2: The Standard Model of particle physics, taken from [28]
4
There are six leptons and six quarks (up, down, charm, strange, top, bottom) [29].
8
2. Radiation Interactions
Function of a radiation detector is based on a way of a particle interaction with
matter. Detectors use some of those interactions to generate a signal. Properties
of radiation are determined from this signal. So it is essential to understand fundamental mechanisms of the interaction and energy loss of radiations in matter.
2.1
Interaction of Photons with Matter
Photons (X-rays or γ radiation1 ) interact with material in several ways. But photoelectric effect, Compton scattering and pair production are the most important
ones for radiation measurements [7, page 47].
2.1.1
Photoelectric Effect
The photoelectric effect is a manifestation of the wave-particle duality (light
sometimes behaves as particles) [1, page 83]. Description of the effect is simple: electrons can be emitted from a material when light shines on it. Emission
of electrons depends on the frequency of light, not on its intensity [1, page 83].
Some certain value of cut-off frequency exists for every material [6, page 212].
If the frequency of incident light is lower than a cut-off value, no electrons are
emitted.
Figure 2.1: Photoelectric effect, taken from [1, page 85]
The effect can be viewed simplified as a conversion of a photon into an electron.
This process is possible to write as
γ + X → X+ + e
(2.1)
using the nuclear reaction formalism (X represents a target atom which is ionized
after the incident photon γ knocks off the electron e from the atomic shell) [1,
page 84].
1
Both X-rays and γ radiation have the same character. They are classified according to the
manner of generation: γ radiation is generated during nuclear processes; X-rays accompany
electron transitions and interactions [11, page 28].
9
Figure 2.1 shows a schematic diagram of the photoelectric effect in a free atom.
If the incident photon has sufficient energy and the K-shell electron is knocked
off, the gap may be filled by an electron from higher energy level. This leads to
the emission of characteristic x-rays (photons with energy equal to the difference
of the two energy levels) [1, pages 85 and 86].
2.1.2
Compton Scattering
Compton scattering is an inelastic collision of photons with free or loosely bound
electrons (schematic diagram of this effect is shown in the picture 2.2). After the
interaction, energy of the photons decreases (wavelength increases) [6, page 220].
Relation between wavelengths of the incident and scattered photons can be derived from the laws of energy and momentum conservation:
λ = λ0 +
h
[1 − cos θ],
m0 c
(2.2)
where λ0 is wavelength of the incident photons, λ is wavelength of the scattered
photons, m0 is the rest mass of electron, θ is the angle between incident and
scattered photons [1, page 87]. The formula (2.2) can be rewritten in energy
terms as:
−1
Eγ0
(1 − cos θ)
.
(2.3)
Eγ = Eγ0 1 +
m0 c
The relation (2.3) shows that Compton scattering is not isotropic (energy of
the scattered photons depends on the incident photon energy and also on the
scattering angle) [1, page 89].
Figure 2.2: Compton scattering, taken from [1, page 88]
2.1.3
Pair Production
In this process, an incident photon converts into an electron-positron pair [1,
page 95]. It can be describe as a conversion of energy into mass according to Einstein relation E = mc2 . Process of pair production is possible only with another
particle X (usually atomic nucleus [11, page 35]) in the vicinity of the photon
(momentum conservation) [1, page 95]. Pair production can be represented in
the nuclear reaction formalism as:
γ + X → e + e+ + X ∗ .
10
(2.4)
Other necessary condition for pair production comes from energetic balance. Energy of the photon must be at least equal to the rest masses of electron and
positron (both 511 keV) [1, page 95].
Figure 2.3: Relative importance of the three major types of photon interaction,
taken from [7, page 51]
2.2
Interaction of Charged Particles
with Matter
Charged particles interact with matter mainly through the Coulomb electromagnetic force [10, page 188]. Thanks to that, atoms of matter receive energy from
the particles. These processes are called ionisation and excitation [11, page 15].
Ionisation means that an orbital electron receive enough energy to leave an atom.
If the energy is not sufficient, the electron is only transferred to higher energetic
level of the atomic shell and this process is called excitation.
When a charged particle moves through a material at a speed higher than
the velocity of light in that medium, Cherenkov radiation is emitted [8, page 35].
Transition radiation accompanies passage of a charged particle through inhomogeneous media.
Next more detailed description of charged particle interactions is restricted to
electrons, protons and charged particles formed from nucleons. Acording to [11,
page 17] or [1, page 105], it is essential to divide these particles into two groups by
their masses: heavy charged particles (particles with mass number equal or higher
than 1 - protons, α-particles) and light charged particles (electrons). Mechanisms
of interaction are similar for both groups; however certain differences occur thanks
to different masses of the particles [11, page 17].
In general, energy losses of the particles are expressed by the linear stopping
power2 of the material. This quantity is defined as the differential energy loss
2
It is also called specific energy loss or “rate” of energy loss[7, page 31].
11
divided by the differential path length [7, page 31]. The dependence of the linear stopping power on the distance of penetration is called the Bragg curve [7,
page 32].
2.2.1
Heavy Charged Particles
Ionisation and excitation (a result of an inelastic scattering of charged particles
with orbital electrons) are crucial processes used for detecting heavy charged
particles [7, page 30]. But elastic interaction of a particle with nuclei such as
Rutherford scattering (sometimes called Coulomb scattering according to the
Coulomb interaction between incident particle and target nucleus) is also possible [1, pages 105 and 106]. Rutherford scattering leads to a deflection of the
particle from its original direction of motion. As the particle collides with many
atoms on its path, the process is called multiple scattering. In case of heavy
charged particles, the total deflection of the particle’s direction is mostly not
large and it is inverse proportional to a square of a kinetic energy of the particle
[12, page 417].
The linear stopping power of heavy charged particles is described by the BetheBloch formula:
4πe4 z 2
2me v 2
v2
dE
=
N
Z
ln
−
ln
1
−
−
dx
me v 2
I
c2
"
!
v2
− 2 ,
c
#
(2.5)
where e is the elementary charge, c the speed of light, v and ze are the velocity
and charge of the primary charged particle, N and Z are the number density and
the atomic number of the target, me is the rest mass of the electron and I is
average excitation and ionisation potential of the absorber which is determined
experimentally [7, page 31]. This formula covers energy losses caused by ionisation
and excitation only.
In case of heavy charged particles, the Bragg curve has maximum at the end
part of the particle track [1, page 118].
2.2.2
Light Charged Particles
A way of an electron interaction with matter depends on its energy. According to
[1, pages 122 - 124], the dominant process for lower energies is ionisation, other
modes of the interaction are:
• Moeller interaction (elastic scattering of an electron from another electron),
• Bhabha scattering (scattering of an electron from a positron),
• electron-positron annihilation (both of an electron and a positron annihilate
and produce two photons at least).
At higher energies, the emission of Bremsstrahlung 3 dominates [1, page 122].
According to classical electrodynamics, this type of radiation is emitted if a
charged particle accelerates. That happens when the charged particle passes
close to a nucleus and it is affected by the Coulomb force.
3
It is German expression for braking radiation which is also used in English [6, page 178].
12
According to [1, page 128], ionisation losses of electrons are described by the
formula:
q
2πZe4 ρ
me v 2 E
dE
=
ln
−
ln
2
2
−
1 − β2 − 1 + β2 +
dx
me v 2
2I 2 (1 − β 2 )
2 #
q
1
2
.
1 − 1 − β2
+ 1−β +
8
"
!
(2.6)
Similarly, the following equation from [1, page 128] expresses radiative losses
of electrons:
dE
Z(Z + 1)e4 ρE
2E
4
−
=
4 ln
−
.
(2.7)
dx
137m2e c4
me c2
3
In both equations (2.6) and (2.7), there the notation of quantities is the same
as in the chapter 2.2.1 (equation (2.5)). In addition, ρ means the density of the
absorber, E is energy of an electron and β ≡ vc .
An elastic interaction between electrons and atoms of a material also occurs.
Rutherford scattering has analogous description as in heavy charged particle case.
However, large angle scattering is more probable with respect to smaller mass of
an electron [11, page 24]. Large deviation in the electron path is consequence of
that effect [7, page 42].
Figure 2.4: Bremsstrahlung, taken from [6, page 179]
2.3
Interaction of Neutrons with Matter
Neutrons as chargeless particles do not interact with an electric atomic field [1,
page 137]. However, they are affected by the strong nuclear force near nuclei.
Neutrons interact with nuclei in several ways [1, page 137]:
• elastic scattering (a target nucleus do not change its state),
• inelastic scattering (a target nucleus is left in an excited state),
• spallation reaction (fragmentation of a nucleus into several parts by high
energy neutron),
13
• transmutation (a reaction which leads to change of an element into another
one),
• radiative capture (a neutron is absorbed by a nucleus and it takes the nucleus into excited state - transfer to a stable state is accompanied by emission
of γ quantum),
• fission (a slow neutron is captured by a heavy nucleus which is taken into
an excited state and it is split into fragments).
2.4
Simulation of Particle Interactions
with Matter
Interaction of nuclear radiation with matter can be studied using a computer
simulation program. Such software contains physical description of passage of a
particle through a material mentioned above. GEANT44 is one of these Monte
Carlo simulation programs. It is standardized computer tool used in the detector
development in high energy physics. It has also found application in nuclear,
particle and accelerator physics or medical and space science [21].
GEANT4 is built on C++ programming language and it utilises the objectoriented technology. This software package is possible to run on several operating
systems - Windows, Mac OS X, Linux or UNIX. It is required to have installed two
additional packages for building and linking the program: CLHEP (Class Library
for High Energy Physics) and STL (Standard Template Library for fundamental
classes like C++ containers and strings) [27].
User controls the program via a GUI (Graphical User Interface), an interactive command line or a macro-based system. Knowledge of C++ and OOP
(object-oriented programming) is not required for an end user but an application programmer, a person who implements simulation task, needs programing
abilities [27].
4
Name of the program is derived from the words GEometry ANd Tracking.
14
3. Semiconductor Detectors
Semiconductor detectors became an important type of detection technique in
HEP (high energy physics) experiments. In this chapter, general properties of
semiconductors will be mentioned first. Then the usage of semiconductors as
radiation detectors will be discussed and this type of detectors will be compared
to other ones.
3.1
Structure of Semiconductors
A periodic lattice of crystalline solids leads to formation of an energy band structure. It consists of a valence band, a conduction band and a forbidden energy gap 1
between them [8, page 216]. Both of the bands represent plenty of very closely
spaced energy levels which are derived from the quantum mechanical models of
atoms [1, page 250].
Electrons in the valence band are closely bound to lattice atoms [8, page 216].
On the other hand, electrons in the conduction band are almost free to move
around the whole crystal [1, page 250]. These electrons are responsible for an
electric current in a material when an electric field is applied.
Figure 3.1: Energy band structure (simplified), adapted from [8, page 216]
The energy band configuration is not typical of just semiconductors however
insulators and conductors have similar band structure. Width of the energy gap
causes differences in (electrical) properties of these types of solids [1, page 250].
In case of conductors, the energy gap is absent and electrons can easy move
through the conduction band. Insulators have very large energy gap so electrons
are normally in the valence band only. As semiconductors have not large band
gap, even small excitation can cause that electrons jump up to the conduction
band [1, page 250].
3.2
Charge Carriers in Semiconductors
An electron leaves a positive charge in the valence band when it is excited to the
conduction band. This charge is called hole and it behaves like a real positive
1
It is also called band gap [1, page 250] or bandgap [7, page 366].
15
particle [1, page 250]. Other valence electron can easy jump to this hole and it
effectively moves through the crystal. Movement of free electrons in the conduction band as well as movement of holes in the valence band contributes to an
electric current in a semiconductor [8, page 217].
3.2.1
Electron-Hole Pair Production
An electron and a vacancy (hole) left behind are called electron-hole pair together.
It can be understood that this pair is roughly analogue of ion pair in gases [7,
page 367]. As it was mentioned above, charge carriers are produced when a
valence electron is exited to the conduction band. This effect is caused by various
processes: thermal excitation, optical excitation or ionisation by charged particles
[18, page 17].
Thermal excitation is undesirable process which leads to noise increase. Detectors which are made of semiconductors with small band gap should be cooled
to reduce this effect [18, page 18]. Optical excitation is used in photodiodes and
solar cells [18, page 18].
Radiation which passes through the semiconductor material can deposit energy in three different modes:
• lattice excitation - radiation increase the lattice vibration,
• ionisation - production of electron-hole pairs,
• atomic displacement - nonionizing effect [1, page 255].
The atomic displacement is responsible for radiation damage and it will be
discussed in the following chapter in detail. From perspective of semiconductor
detectors, the ionisation process is the most important phenomenon. However,
the lattice excitations also influence statistics of electron-hole pair production [1,
page 256].
Radiation which delivers energy above a threshold2 value is able to create
electron-hole pairs in the material [1, page 256]. Average energy for creation
e-h pair does not depend on type of radiation but it depends on the material
and its temperature [1, page 257]. The processes which lead to generation of
electron-hole pairs by nuclear radiation have been mentioned in the previous
chapter (Radiation Interactions).
3.2.2
Trapping and Recombination
Processes of recombination and trapping are opposite to the generation of electronhole pairs. Effectively, it leads to loss of charge carriers in semiconductor and it
reduces their average lifetime.
Impurities in crystal form recombination centres. The energy band structure
is changed by adding new energy levels to the band gap [8, page 219]. These
states are capable of capturing both electrons and holes. It causes annihilation
of an electron and a hole.
2
The threshold is higher than the value of band gap energy because some energy goes into
crystal excitation [1, page 256].
16
Trapping centres are also consequence of some impurities. However, these
states are able to capture one kind of charge carriers only [8, page 219]. This
effect does not lead to annihilation. Captured charge carriers are released back
after a characteristic time so they do not contribute to the measured pulse [7,
page 375].
3.2.3
Migration of Charge Carriers
Free electrons move through the lattice in a random direction. They gain thermal
energy of lattice vibrations ( 12 kT per degree of freedom where k is the Boltzmann’s
constant and T is the absolute temperature) [13, page 19].
New transport phenomenon of charge carriers occurs when an electric field is
applied. It is called drift. Both electron and holes are accelerated by an electric
force. The drift velocity v is proportional to the applied electric field E
vh = µh E,
(3.1)
ve = µe E,
(3.2)
where proportionality constant µh or µe is hole or electron mobility [7, page 368].
Its value depends on the material [1, page 265]. The relations (3.1) and (3.2)
are valid only in case of weak electric field. At higher electric field values, the
increase of the drift velocity is slower and it finally saturates [7, page 369].
Diffusion is another process which also contributes to generation of an electric
current in semiconductors [13, page 21]. It is caused by nonhomogeneous spatial
distribution of charge carriers. Both electrons and holes move from a point of
high concentration to a point of low concentration [13, page 21]. It creates flux
Fh , Fe which is described by formulae
Fh = −Dh ∇nh ,
(3.3)
Fe = −De ∇ne ,
(3.4)
where De , Dh is the diffusion coefficient and ∇n is the concentration gradient
[18, page 16].
The total current density J which is created as a combination of the drift and
diffusion effect is described by equation
J = ene µe E + enh µh E + eDe ∇ne − eDh ∇nh ,
(3.5)
where e is the elementary charge, ne , nh is number of electrons, holes and rest
symbols are the same as in equations (3.1) - (3.4) [13, page 22].
In a presence of a magnetic field B, charge carriers are affected by the Lorentz
force
F = q (E + v × B) ,
which bends a trajectory of a particle [18, page 17].
17
(3.6)
3.3
3.3.1
Intrinsic and Doped Semiconductors
Intrinsic Semiconductors
The energy bands mentioned above in the section 3.1 corresponds to an ideal or
an intrinsic semiconductor. It means that there are either no impurities or they
do not change conduction properties of this material [1, page 251]. A number of
free electrons is equal to a number of holes in the intrinsic material [8, page 250].
Density of states of the conduction band Nc and also the valence band Nv depends
on the temperature T according to T 3/2 [1, page 252]. The whole intrinsic charge
concentration is described by relation
ni = [nc nv ]1/2 exp −
Eg
,
2kT
(3.7)
where nc and nv are charge concentrations in the conduction and the valence band,
k is the Boltzmann’s constant and T is the absolute temperature [1, page 252].
3.3.2
Doped Semiconductors
Conductive properties of semiconductors are changed when some impurities are
added to the material [1, page 252]. Process of adding impurities is called doping
and resulting material is an extrinsic semiconductor.
During the doping, small quantity of an element with different number of
valence electrons is added to the bulk of the original material [1, page 252]. It
creates new energy levels between the valence and the conduction band. Their
location depends on a type of impurity.
Doping with Acceptor Impurity
When the trivalent impurity (element from a group III of the periodic table, according to [8, page 221] gallium, boron and indium are mainly used) is added to a
silicon lattice, it leads to creation of a hole [7, page 373]. Acceptor impurity creates abundance of positive charges and also new energy levels which are situated
near the valence band [1, page 252]. Electrons are easily excited from the valence
band into these new levels and they also leave other holes behind. Abundance
of holes decreases concentration of free electrons - holes become majority charge
carriers, electrons minority charge carriers [8, page 221]. Such a material is called
p-type semiconductor.
Doping with Donor Impurity
As the impurity atom is pentavalent (element from a group V of the periodic table,
according to [8, page 221] arsenic, phosphorous, antimony are mainly used), one
electron does not make covalent bond with lattice atoms and it becomes free
to move around [1, page 255]. These extra free electrons fill up holes and as a
consequence the hole concentration is decreasing [8, page 221].
This type of impurity is called donor impurity because it donates free negative charge carriers. It creates also new energy levels which are situated near
the conduction band. Doped semiconductor materials where the electrons are
majority charge carriers are called n-type semiconductors [8, page 221].
18
(a) Addition of an acceptor impurity (boron)
(b) Addition of a donor impurity (phosphorus)
Figure 3.2: Doped Semiconductors, taken from [1, page 254 and 256]
3.4
pn-Junction
A pn-junction is created when p-type and n-type semiconductors are joined together [1, page 284]. Simply pressing of these two types of material together is
not sufficient however special techniques must be used to obtain required junction [8, page 223]. These junctions became important elements from which many
electronic components are made of. They are also used in radiation detection
technologies.
When two semiconductor types are connected, an imbalance in charge concentrations across the junction is automatically compensated by a flow of charges
(electrons move towards the p-type and holes diffuse towards the n-type) [1,
page 284]. This process continues until the Fermi levels of the two parts are not
equal. As electrons and holes neutralise each other, the depletion region lack of
an electrical charge is formed [1, page 284]. An electrostatic contact potential
across the junction is created because recombination of electrons and holes causes
charge build-up on each side of the junction [8, page 223].
The depletion region is fundamental for operation of semiconductor detector because there are produced electron-hole pairs by incident nuclear radiation.
19
These free charges form an electric current signal which can be detected and
measured.
Figure 3.3: Schema of creation of a pn-junction and courses of the charge density
and the electric field, taken from [8, page 223]
3.4.1
Characteristics of the Depletion Region
A width of the depletion zone (depletion depth) is not huge and it depends on
the concentration of crystal impurities [8, page 224]. It can be calculated from
Poisson’s equation
ρ(x)
d2 V
=−
,
(3.8)
2
dx
where is the permittivity and ρ(x) is a charge density distribution [8, page 224].
In case of the uniform charge distribution, we can find the total depletion width d
d=
2V0 (NA + ND )
e
NA ND
!1/2
,
(3.9)
where e is the elementary charge, V0 is the contact potential, ND and NA are
the donor and acceptor impurity concentrations [8, pages 224 and 225]. In fact,
relation (3.9) is a sum of the widths of the depletion regions on p- and n-side [1,
page 285].
In case of absence of radiation, the depletion region acts as an insulator between a positive and a negative electrode [1, page 290]. This configuration corresponds to a capacitor. For a planar geometry, its capacitance C is
A
C= ,
d
(3.10)
where A is the area of depletion region and d is depletion depth [8, page 226].
3.4.2
Reverse Biased Junction
The depletion region serves as an active medium for production of electron-hole
pairs [1, page 286]. However the intrinsic electric field is not intense enough for
20
efficient charge collection and the depletion depth is not sufficient for stopping
high energy particles. Small thickness of the depletion zone also causes an increase
of a noise [8, page 226].
Detection properties of the pn-junction are improved if a reverse bias voltage
is applied to the junction. It enlarges the depletion region - sensitive volume
for radiation detection and it also leads to the more efficient charge collection
[8, page 227]. Partially depleted junction is situation when the depletion region
does not reach either surface and some part of volume remains undepleted [7,
page 383]. If the depletion region extends to the whole volume, fully depleted 3
junction is created [7, page 384]. Maximum operating voltage must be below
the breakdown voltage which is limited by the resistance of semiconductor [8,
page 227].
One can calculate the depletion depth using formula (3.9) where the contact
potential V0 is replaced by sum of the contact potential V0 and the bias voltage
VB . As in general V0 << VB , there can be substitute V0 by VB only in (3.9) [8,
page 227].
Figure 3.4: Reverse biased junction, taken from [1, page 286]
In the figure 3.4, there is displayed enlargement of the depletion zone. On the
right, there is current-voltage characteristic of the pn-junction. In case of forward
bias, current increases with applied voltage. On the other hand at reverse bias,
only small leakage current4 is observed and its value is constant for any applied
voltage.
3.5
Semiconductors as Particle Detectors
Usage of a solid state as the detection medium brings certain advantages in many
applications [7, page 365]. In HEP experiments, semiconductor detectors prove
to be high-resolution particle track detectors [8, page 215].
• As the average energy for creation of electron-hole pair is 10 times smaller
than for gas ionisation, more charge carriers are produced [8, page 215]. It
is a consequence of small value of the band gap.
• A high density causes that semiconductor detectors have higher stopping
power than gas ones [8, page 215]. Thanks to the high density, they are
also smaller than equivalent gas-filled detector [7, page 365].
3
Totally depleted is also used [7, page 384].
In fact, the depletion region is not completely devoid of free charges - crystal imperfection
and impurities or thermal agitation can cause production of e-h pairs [1, page 285].
4
21
• Semiconductor detectors have a short dead time5 and they provide fast and
accurate time information thanks to the high mobility of charge carriers
[18, page 26].
• Energy resolution of semiconductor detectors is better than resolution of
other detectors (e.g. scintillation counters) [7, page 365].
• Other advantages of semiconductor detectors are a compact size [7, page 365]
and a mechanical rigidity [18, page 27].
• Semiconductor detectors can be easily integrated with electronics because
semiconductors are widely used in production of electronic components [18,
page 27].
• On the other hand, their disadvantage is relatively high susceptibility which
leads to a degradation from radiation damage [7, page 365].
Not all semiconductors are suitable for nuclear radiation detectors. Traditionally, intrinsic germanium (Ge) and silicon (Si) are used. Other perspective
materials are gallium arsenide (GaAs) and cadmium-zinc-tellurim (CdZnTe). Requirement for radiation hard semiconductor detectors leads to the development
of more complex semiconductor structures [1, page 266]. Doped semiconductors
are also used [18, page 28].
Table 3.1 shows a basic properties of germanium and silicon, common material
for particle detectors.
Table 3.1: Properties of silicon and germanium, information is from [7, page 368]
Atomic number
Density (300 K) [g · cm−3 ]
Forbidden energy gap (300 K) [eV]
Forbidden energy gap (0 K) [eV]
Intrinsic carrier density (300 K) [cm−3 ]
Intrinsic resistivity (300 K) [Ω · cm]
Electron mobility (300 K) [cm2 · V−1 · s−1 ]
Hole mobility (300 K) [cm2 · V−1 · s−1 ]
Electron mobility (77 K) [cm2 · V−1 · s−1 ]
Hole mobility (77 K) [cm2 · V−1 · s−1 ]
Energy per electron-hole pair (300 K) [eV]
Energy per electron-hole pair (77 K) [eV]
3.5.1
Silicon
14
2.33
1.115
1.165
1.5 × 1010
2.3 × 105
1350
480
2.1 × 104
1.1 × 104
3.62
3.76
Germanium
32
5.32
0.665
0.746
2.4 × 1013
47
3900
1900
3.6 × 104
4.2 × 104
2,96
Germanium
Germanium detectors are used in a gamma ray spectroscopy because they have
high resolution and wide dynamic range. However, germanium did not become
5
According to [7, page 121], it is a minimum time which must separate two events in order
to detect them as two isolated pulses. Its value is limited either by processes in the detector or
by electronics.
22
common material for particle tracking application [1, page 275]. As the forbidden
energy gap is low, intrinsic noise is significant. Operation of germanium detectors
required an intensive cooling (liquid nitrogen temperature) [18, page 27].
3.5.2
Silicon
Silicon is the most commonly used material for production of radiation detectors.
It is cheap and it is available in a purified form [1, page 267]. Also the technology
of manufacturing of silicon is highly developed thanks to its application in electronics and electrical engineering [18, page 27]. Although silicon detectors can be
operated at a room temperature, they are operated mainly at low temperatures
[1, page 269]. It leads to widening of the forbidden energy gap and decrease of
the noise caused by thermal agitation [1, page 268].
3.6
Operation of Semiconductor Detectors
There are many configurations of semiconductor detectors. This chapter mentions
only some of them which play important role in HEP experiments.
3.6.1
pn-Diode
A pn-diode is straight application of the pn-junction which is mentioned above
in this chapter. It can be operated in two different basal modes - photovoltaic
and photoconductive [1, page 294].
Photovoltaic Mode
The diode is not supplied from an external voltage source in this mode of operation. The depletion region is formed only by the intrinsic electric field. Incident
radiation creates electron-hole pairs which form a charge pulse. Either voltage
pulse or current pulse is measured in dependence on a way of wiring [18, page 30].
Low noise is advantage of the photovoltaic mode of operation [1, page 295].
However, small depletion region and non-linearity in response is disadvantage therefore it cannot be used for spectroscopic applications [1, page 295]. Photovoltaic mode is not use very often; measurement of a level of radiation is one
application [18, page 30].
Photoconductive Mode
Radiation detectors are mostly operated in this mode where a high reverse biased
voltage is applied across the junction [1, page 295]. The principle of the reverse
biased junction is described above in the section 3.4.2.
An incident particle produces electron-hole pairs along its track and this
charges form an electrical current. Measured signal is proportional to energy
deposited by the incident radiation [1, page 296].
Reverse biasing the pn-diode brings certain advantages; increase of the depletion region, increase of the signal to noise ration and decrease of the capacitance
are three main ones [1, page 295].
23
Figure 3.5: Reverse biased pn-diode - production of e-h pairs, taken from [1,
page 296]
Schema of production of electron-hole pairs in the depletion region of the
reverse biased diode is displayed in the picture 3.5. Current-voltage characteristic
shows increase of the measured current with increase of a flux of incident nuclear
radiation.
3.6.2
Specific Semiconductor Detectors
Semiconductor detectors can be built in different geometry and structure configurations. Ahmed presents some of them in [1, page 301]: construction of PIN diodes
(p-type, intrinsic type, n-type) allows large depletion region without high reverse
bias. A Schottky diode uses a junction of semiconductor and metal. Avalanche
photodiodes increase the signal using multiplying of charge carriers therefore they
can be used for detection light of low level. Surface barrier detector also uses a
metal-semiconductor junction.
3.6.3
Position Sensitive Detection
Requirement of a high spatial resolution led to development of specific types of
position sensitive detectors. These detectors are used in particle tracking system
in HEP experiments. Precise imagining in a various sectors of industry is other
application of the position sensitive detectors.
Figure 3.6: Position sensitive detector based on resistive charge division technology, taken from [8, page 236]
24
Resistive charge division (figure 3.6) is the simplest form of the construction
of the position sensitive detector. A resistive electrode is put on the front face
of the diode [8, page 235]. Figure shows a sketch of this type of detector. When
a particle passes through the diode, the charge collected at the contact B is
proportional to the energy of the particle and the resistance of the electrode
between the contact and the point of incidence [8, page 236]. At the contact C,
the signal is proportional to all energy [8, page 236]. Position x is finally get from
B
x=L ,
C
(3.11)
where L is the length of the resistive layer [8, page 236].
Division of semiconductor into independent segments or strips is an alternative
way of the construction of position sensitive microstrip detector. Each segment
behaves as a separate detector [1, page 301]. They are mainly operated in the
full depletion mode. A fast response time is offered therefore these detectors can
be used as triggering devices for HEP experiments [8, page 238].
Building such detectors has certain limitations. Fact that each strip needs
its own readout electronics is one of them [1, page 301]. It influences possible
number of segments and also their pitch 6 [18, page 44].
6
Centre to centre distance between two segments [1, page 301].
25
26
4. Radiation Damage
Passage of radiation through matter leads up to two main ways of energy losses.
Non-destructive ionization, in which electron-hole pairs are produced, is first of
them [7, page 397]. This is a fully reversible process which leaves no damage
[7, page 397]. NIEL (Non Ionizing Energy Loss) is a second way which covers
processes of a displacement of atoms in a crystalline lattice or nuclear reactions
[1, page 302].
Although the effects of radiation damage occur in various materials, the following part of this chapter is focused especially on silicon because it is the most
important material for semiconductor detectors.
4.1
Defects in Silicon
In silicon, there are formed surface or bulk damages. Properties of silicon detectors for HEP experiments are influenced mostly by bulk damages because they
change electrical properties of detectors [9, page 31].
4.1.1
Damage Mechanism
Radiation induced bulk damage in silicon is generated when a heavy particle
(proton, neutron) displaces a PKA (Primary Knock-on Atom) out of its lattice
site to an interstitial one [9, page 31]. Enough imparted energy is a necessary
condition for the process of displacement. It must be higher than the displacement
threshold energy which is approximately 25 eV (in case of silicon) [7, page 398].
The displaced atom, together with the vacancy left behind, creates site where
charge carriers are trapped [7, page 398]. This is the most fundamental type of
bulk radiation damage and it is called Frenkel defect [7, page 398].
Atomic displacements away from each other are sometimes called point defects [1, page 302]. On the other hand, cluster defect means larger region where
crystalline damages are close to each other. Clusters are produced by the PKAs
along their tracks if the PKAs have sufficient energy [7, page 398].
However process of bulk damage is not only about displacing of atoms. Interstitials and vacancies can move through the crystal lattice of silicon and some of
Frenkel pairs annihilate [9, page 33].
4.1.2
NIEL Scaling
Interaction of various types of particles with silicon differs to each other. This
problematic is discussed in the chapter Radiation Interactions in more detail.
Briefly, charged particles interact mainly through the Coulomb interaction. It
mostly causes ionisation which does not make real damage of the material. On
the other hand, neutrons interact only with nuclei. NIEL hypothesis brings description of radiation damages produced by different kinds of interaction with
respect to the changes observed in the material [9, page 33].
27
According to the NIEL hypothesis, radiation damage is linear proportional to
non-ionizing energy loss of incident particles and this depends linearly on energy
which is used to displace lattice atoms [17, slide 17].
Radiation damage can be expressed by the damage function 1 defined as
D(E) =
X
ν
σν ·
Z E max
R
0
fν (E, ER )P (ER )dER ,
(4.1)
where index ν represents all possible interaction between an incident particle
with energy E and silicon atoms which lead to the displacement in the lattice,
fν (E, ER ) says the probability of generation the PKA with recoil energy ER by
the particle with energy E, σν is the cross section for interaction with index ν
and P (ER ) is Lindhard partition function [9, page 33]. The integrand in the (4.1)
is integrated over all possible values of recoil energy ER [9, page 33].
Figure 4.1: Displacement damage functions D(E) normalized to 95 MeVmb for
neutrons, protons, pions and electrons, taken from [9, page 34]
The figure 4.1 displays function D(E) for neutrons, protons, electrons and
pions. In case of low energetic protons, there is significant contribution of the
Coulomb interaction and the values of D(E) are higher for protons than for
neutrons with the same energy [9, page 35]. On the other hand, the damage
function is very similar for high energetic (GeV) protons and neutrons because
nuclear reactions dominate in this energetic range and they are similar for both
particles [9, page 35]. Neutrons with energy lower than ≈ 10 MeV cause smaller
damage than protons with the same energy do. It is thanks to zero charge of
neutrons which results in easier penetration of a material without making defects.
1
It is also called displacement damage cross section and its unit is MeV mb [9, page 33].
NIELs unit is keVcm2 /g. Relation 100 MeVmb = 2.144 keVcm2 /g is valid for silicon [18,
page 67].
28
The fluctuating part of damage function for neutrons (≈ 100 keV to 10 MeV)
describes nuclear resonation. In the low energy range (energies lower than 100
eV), there is obvious that the damage function of neutrons is increasing with
decreasing energy. It is consequence of neutron nuclear reactions like nuclear
fission. In the high energy range, there the values of D(E) are smaller for pions
than for proton [9, page 35]. Damage potential of electrons is higher for particles
with higher energy (≈ GeV) than for the slower ones because of small rest mass
of electrons.
4.1.3
Hardness Factor
Hardness factor κ allows comparison of damage efficiency of different types of
nuclear radiation. It is defined as
R
κ=
D(E)φ(E)dE
R
,
D(En = 1M eV ) · φ(E)dE
(4.2)
where D(E) is the damage function and φ(E) represents energy spectra of certain
source of nuclear radiation [9, page 35]. In the equation 4.2, there damage caused
by certain radiation is compared with damage which would have been caused by
monoenergetic neutrons of 1 MeV (D(En = 1M eV ) is set to 95 MeV mb) and
the same fluence [9, page 35].
Then the equivalent 1 MeV neutron fluennce is defined as
Φeq = κΦ = κ
Z
φ(E)dE,
(4.3)
where Φ is a real fluence [9, page 35].
4.1.4
Annealing
Annealing is the process in which the radiation damage in a semiconductor material decreases2 by time [1, page 305]. It can be used for ”healing” of irradiated
detectors.
According to [1, page 305], fundamental process responsible for this effect is
not exactly known yet but it is sometimes explained as a consequence of migration
of defects [18, page 75]. However the strong temperature dependence of the
annealing process is known [1, page 305]. Parameters of the ideal annealing are
80 minutes and temperature equal to 60 degrees Celsius [18, page 75]. Figure 4.2
shows time evolution of the typical annealing process. It is obvious that radiation
damage starts to increase (reverse annealing) after first beneficial annealing.
4.2
Detector Properties after Irradiation
The defects in silicon caused by radiation are responsible for changes of an electrical properties of detectors (and change of detection capability is its consequence).
An increase in a leakage current, a type inversion, an increase in the depletion
2
Radiation damage can also increase by time - this process is called reverse annealing. [18,
page 75]
29
Figure 4.2: Annealing - time evolution of effective space charge density (conductivity type inverted n-type silicon detector), taken from [5, page 8]
voltage and a charge trapping are the most important consequences of the radiation damage. They will be discussed in the following part of this chapter in more
detail.
4.2.1
Leakage Current
Change in the reverse bias current is one of effects of the radiation damage. It is
caused by increase or decrease of free charge carriers in the depletion region [1,
page 303].
Figure 4.3: Dependence of leakage current on fluence3 , taken from [9, page 99]
3
Abbreviations in the picture refer to different processes of crystal pulling: CZ = Czochralski
method, FZ = float zone method and EPI - epitaxy method [26]. Sheet resistance (units Ωcm)
is used as a measure of resistance of thin materials [36].
30
The leakage current induced by radiation damage depends on an integrated radiation dose, an exposed volume of the detector and the temperature [1,
page 303]. At certain temperature, the dependence can be expressed by following
formula
∆il = αV Φ,
(4.4)
where V is the exposed volume of detector, Φ is particle fluence [1, page 303].
According to [18, page 70], the damage coefficient α is equal to 8 × 10−17 Acm−1 .
Increase of leakage current does not depend on a resistivity, semiconductor type
or other material properties [9, page 98].
The temperature dependence of the leakage current after irradiation ir is
described by the relation
E
(4.5)
ir ∝ T 2 e− 2kT ,
where T is the absolute temperature, k is the Boltzmann’s constant and E is the
activation energy of the material [1, page 303].
Increase in the leakage current causes undesirable performance of detector.
Increase in a noise is the most significant one (deterioration of a signal to noise
ratio is a consequence of it) [1, page 303]. Thanks to the dependence of the leakage
current on the temperature (4.5), the decrease of an operating temperature can
be used as a compensation of this effect [1, page 303].
4.2.2
Type Inversion
The type inversion is other effect caused by radiation damage of material. An
n-type semi-conductive material changes into a p-type and vice versa during this
process [1, page 304].
Figure 4.4: Change of the depletion voltage and the effective doping concentration
after irradiation, taken from [9, page 111]
This effect is consequence of change of the effective dopant concentration in
material after irradiation [1, page 304]. Interstitials and vacancies, which were
formed in material by radiation, modify a ratio between donors and acceptors [18,
page 70]. In fact, radiation damage increases the charge carriers of the opposite
31
sign [1, page 304]. When the donor level is equal to the acceptor level, material
is changed into an intrinsic semiconductor4 . Further irradiation changes material
into a non-intrinsic (opposite type) one [1, page 304].
4.2.3
Depletion Voltage
Increase of the depletion voltage is next consequence of change of the effective
dopant concentration. As detectors are operated in the fully depleted mode, the
bias voltage for irradiated detectors must be increased to overcome this effect [1,
page 305].
4.2.4
Charge Trapping
Interaction of radiation with matter can lead to creation of an energy levels which
trap free charge carriers [1, page 305]. It causes deterioration of charge collection
efficiency of detector and nonlinearity in its response [1, page 305].
4.3
Electronic Components after Irradiation
Not only detectors are exposed to irradiation in detection devices. They contain
many other electronic components (front-end electronics) whose properties can be
also influenced by radiation damage. According to [2], resistors become unstable
above an exposure dose of 1014 cm−2 . Transistors are very sensitive to radiation
which reduces a current amplification coefficient. TTL (transistor-transistor logic) integrated circuits tolerate radiation doses up to 1014 cm−2 , at 1015 cm−2 they
are heavily damaged. Ionizing radiation can also cause degradation of isolators
which leads to break down of capacitors [2].
4
According to [18, page 70], it happens when the fluence equals 1012 cm−2 .
32
5. Signal Processing
Signal processing is a fundamental task for extracting information from detectors.
According to [1, page 463], there are two types of information - an amplitude and
a timing of the pulse. The amplitude is important in a spectroscopy application.
On the other hand, the precise timing is needed in particle tracking.
Figure 5.1: Analog signal processing, taken from [7, page 596]
Generally, two approaches of signal processing exist. Signal is either process
using various types of an analog circuit or it is digitized [1, page 463]. Some
analog devices are also needed even in case of a digital conversion [1, page 463].
Figure 5.1 shows fundamental devices in the analog signal processing chain.
Figure 5.2: Signal terminology, taken from [8, page 250]
5.1
Preamplification
As the output pulse from detector has very narrow width and amplitude, it is not
possible direct connection with analog or digital circuits without preamplification
[1, pages 464]. A preamplifier is a simple and efficient device which amplify weak
output signal. It is mounted close to the detector [8, page 277].
According to [1, pages 465 - 471], there are various types of preamplifier construction - voltage sensitive preamplifier, current sensitive preamplifier or charge
sensitive preamplifier. Voltage sensitive one is the most typical preamplifier which
is used in a radiation detection system.
33
5.2
Amplification and Pulse Shaping
A main amplifier provides amplification of the signal from the preamplifier and
shapes the pulse into a form suitable for further processing and measurement [8,
page 280]. A pulse shaping amplifier transforms narrow preamplifier signal into
broader pulse with and rounded maximum [1, page 480]. Increase a signal to
noise ratio and increase a pulse pair resolution are goals of that transformation.
Various methods of pulse shaping are mentioned in [1, pages 480 - 490] - delay
line, CR-RC, semi-Gaussian or semi-triangular pulse shaping. Simple CR-RC
method is widely used for shaping preamplified detector pulses.
5.2.1
CR-RC Pulse Shaping
This pulse shaping device is made of a CR differentiator and a RC integrator.
According to [1, page 483], it behaves like a combination of a high-pass filter
and a low-pass filter - a band-pass filter. First, pulse from the preamplifier goes
through the differentiator where noisy low frequencies are attenuated. Then the
integrator cleans high frequency noise components from the signal and only low
frequency clean signal can pass through.
Figure 5.3: Simple CR-RC shaper and its response to two input pulses, taken
from [7, page 596]
The figure 5.3 shows a circuit diagram of the simple CR-RC shaper and its
response to a step input pulse and an exponentially decaying input pulse which
is a more realistic preamplifier output [1, page 483]. It is obvious, that there is
produced an undershoot in case of an exponentially decaying input pulse. This
can be a problem because it can lower effective height of next pulse but it can be
solved using a special pole-zero cancellation circuit [1, page 483 - 485].
5.3
Filtering
Generally, filters block certain frequency range in the signal and therefore they
are used for a noise reduction. There is a low-pass filter, a high-pass filter and its
combination a band-pass filter [1, page 490]. Partially, it was already discussed
in previous section because pulse shapers also provide the filtration function.
34
5.4
Discrimination
A discriminator selects shaped pulses for further processing [7, page 596]. It
compares the input voltage to a present threshold one [1, page 494]. Criteria
can be different - for example, pulse height above the threshold, peak amplitude
between upper and lower threshold [7, page 596].
Selected events are processed by other circuits [7, page 596]. A counter counts
the number of pulses over a measured time. A multichannel analyser is more
complex machine which produces an energy spectrum.
5.5
Digital Processing
Digital processing becomes an alternative to analog signal processing in radiation detection technology as well as in other several applications. It brings many
advantages - logical components are faster, cheaper and more efficient, digital
systems are more flexible, it does not suffer from traditional problems of analog circuits. On the other hand, digital processing is not advantageous for all
applications [4, page 4].
First, pulses are preamplified even in case of digital processing. Signal is
digitized using an ADC (analog to digital converter) and it is further processed
in the digital units. They can be constructed as hardware device however digital
processing using computer software is more frequent [1, page 507].
There are various types of ADC construction. Quality of the ADC is described by parameters such as conversion time, dead time, resolution, linearity
and stability [1, pages 498 and 499].
5.6
Signal Transmision
An interconnection of signal processing devices to each other is important task
for signal processing. Various types of standard cables are used in a radiation
detection systems. Coaxial cables or twisted pair cables provide shielding from
the outside electromagnetic field and they are used to transport detector signal.
On the other hand, unshielded flat ribbon cables serve to transport power and
digital signals [1, pages 474 - 479].
5.7
Modular Instruments
Modular instruments are standardized electronic devices used in research and industry. Standardisation leads to reduction of the cost and it eases installation and
operation of these systems. In nuclear electronics, NIM (Nuclear Instrumentation
Methods), CAMAC (Computer Automated Measurement and Control) and VME
(Versa Module Europa) are most commonly used systems [1, pages 721 - 729].
Using of PC based modules is new trend in signal processing and DAQ. Devices are connected to the PC via standard input/output interface (serial port,
parallel port, USB). Use of PCI (Peripheral Component Interconnect) bus slot or
TCP/IP (Transmission Control Protocol/Internet Protocol) is other possibility
of the interconnection [1, pages 729 - 733].
35
36
6. ATLAS Experiment
6.1
Description of the ATLAS Experiment
ATLAS (A Toroidal LHC Apparatus) is a general-purpose particle detector built
at the particle accelerator LHC (Large Hadron Collider) at the CERN laboratory.
It is in operation since September 2008. The ATLAS experiment has ample and
huge program of a fundamental physics research. It studies conditions of the
Universe just after the Big Bang which relates with the question of antimatter
and dark matter, ATLAS also tries to discover new and unknown physics - new
processes and particles, extra dimensions of space, unification of fundamental
forces1 or evidence for string theory [23].
c 2013 CERN, taken
Figure 6.1: The ATLAS detector (ATLAS Experiment from [24])
The ATLAS detector has four main parts: an inner detector, a calorimeter,
a muon spectrometer and a magnet system [22]. The inner detector is made of
three sections: pixel detector, SCT (Semiconductor Tracker) and TRT (Transition Radiation Tracker) [22]. It measures tracks and momentum of each charged
particle. Momentum is derived from the curvature of particle tracks. They are
bent by magnetic field of 2 T provided by the magnet system. Energies of charged
and neutral particles are measured in the calorimeter. It consists of metal absorbers and sensing elements. There are produced a ”showers” of particles in
the absorber. Those particles are detected using sensing elements. The muon2
spectrometer is last main part of ATLAS detector system. It measures energy of
muons - particles which produce no signal in the calorimeter.
1
gravitational, electromagnetic, strong nuclear and weak nuclear interactions
Muons are elementary particles similar to electrons but they are approximately 200 times
heavier.[15]
2
37
Information from the detectors is further processed using other devices. A trigger system selects interesting events, DAQ (data acquisition) system transfers
data from the detectors to the storage and a computing system is used for data
analysing [22].
6.2
ATLAS Upgrade
Upgrade of the ATLAS detector is planned in the context of modifications of
the LHC accelerator. Current LHC repair will enable operation of the accelerator with its design parameters (centre of mass energy and peak luminosity3
1034 cm−2 s−1 ) [14, page 5]. Next LHC upgrade is planned in 2018. The peak
luminosity will be again increased to 2 − 3 × 1034 cm−2 s−1 [14, page 5]. In addition, all present plans of LHC upgrade are compatible with program of HL-LHC
(High Luminosity LHC) where the instantaneous luminosity should be about
5 − 7 × 1034 cm−2 s−1 and the integrated luminosity about 3000 fb−1 [14, page 5].
Figure 6.2: Simulations of the inner tracker, taken from [30])
The illustration 6.2 shows simulations of the inner detector. On the left, there
are 5 superimposed collisions (it corresponds to current luminosities of LHC).
Second picture shows an event with 400 simultaneous collisions (it corresponds
to expected luminosities at HL-LHC) [30].
Phase-I of ATLAS Upgrade will enable utilize new opportunities of physics
research caused by modifications of the LHC. It will provide ideal conditions
for measurement of Higgs boson properties or studies of electroweak symmetry
breaking mechanism [14, page 5]. Further, searching for new particles and new
phenomena will continue [14, page 6].
New conditions in the LHC accelerator will set new requirements to the ATLAS detector. Because of radiation dose received by detector will increase, whole
tracking system will be replaced with a new one [25]. Development of radiation
hard sensors is fundamental task of this operation. Trigger and DAQ are the
other parts of ATLAS which will be also upgraded [25].
3
In accelerator physics, it means number of particles per unit area per unit time times the
opacity of the target [35].
38
7. Tests of Irradiated Detectors
Institute of Particle and Nuclear Physics in Prague participates in development
of particle detectors for HEP experiments. In the clean laboratories of IPNP,
there are run tests with a laser or a radioactive source at room temperature.
Measurement at low temperatures improved abilities of strip detector laser testing. It allowed operation of tests of irradiated detectors and also participation
on development of radiation hard detectors.
Testing of detectors was performed in cooperation with Ing. Jan Böhm, CSc.
and Bc. Tomáš Jindra at laboratory of Institute of Physics ASCR, v. v. i.,
Na Slovance 2, Prague 8. Detailed report, which is available on the web [20], was
written out.
7.1
Description of the Experiment
Central part of the experimental equipment is a metal box where a detector and a
front-end electronics is situated. It is wired with a high and a low voltage supply.
High voltage source with an ammeter (1 or 3 kV) provides a bias voltage for the
detector, low voltage source (with output voltage levels -5V/GND/GND/+5V)
powers circuits of a preamplifier.
Figure 7.1: Block schema of the experiment
The preamplifier has been developed for the detector laboratory of Institute
of Particle and Nuclear Physics by Ing. Jan Scheirich (a circuit diagram is in the
figure 7.2a, photo with description is in the figure 7.2b). The preamplifier has four
channels therefore only four strips of the detector are connected and measured.
Four data cables connect outputs of the preamplifier with inputs (voltage limit
1 V) of a digital oscilloscope Evaluation board DRS4 V2.
39
(a) Circuit diagram of the preamplifier, made by Ing. Jan Scheirich
(b) Photo with description of the preamplifier, made by Ing. Jan Scheirich
Figure 7.2: Preamplifier
40
Whole box is situated in a freezer where temperature is about minus 30 degrees
Celsius. Temperature is measured by thermometer whose probe is close to the
detector.
A laser generator of infrared light ray (wavelength 1066 nm) is connected
with the central box using optical fiber. Distance between head of the laser and
the detector is about 12 mm and light beam is not completely focused for easier
lightening all measured strips. The laser works in a trigger mode and it is driven
by pulses from a generator (frequency 1 kHz, width 22.5 - 50 ns, shape of pulses
can be controlled using the adjunct oscilloscope). The generator also triggers the
digital oscilloscope.
The Evaluation board DRS4 is connected to the computer via USB. Data are
recorded using DRS Oscilloscope software with frequency 200 - 300 Hz.
(a) Central box, black cable - high voltage supply, RS232 - low voltage supply, on the right four data cables, on the top - optical fibre
(b) Signal generator and laser
(c) Evaluation board DRS4 V2, on the left USB output and trigger input, on the right input of channels 1 - 4
Figure 7.3: Devices used in the experiment, photo by Tomáš Jindra
A type of the performed measurement is called laser test. A detector is lighted
by the laser and its response is measured. The bias voltage is changed during this
test. If it is possible, measurement is done for both of increasing and decreasing
bias voltage. Leakage current is also recorded.
7.2
Results of the Measurement
Detectors W12-Z6-P22 (irradiation 0) and W12-BZ4D-P22 (irradiation 4·1014 cm−2 )
were measured by Tomáš Jindra, I measured the others by myself.
41
Detector W12-Z6-P22
• Irradiation: 0 (non-irradiated detector)
• Temperature: -22.0 ◦ C/-22.3 ◦ C and -30.6 ◦ C/-30.7 ◦ C
• Bias voltage: 10 - 350 V, step 20 V, increasing, decreasing
• Leakage current: nearly non-measurable (0 - 20 nA)
Detector W12-BZ4D-P22
• Irradiation: 4 · 1014 cm−2
• Temperature: -21.6 ◦ C/-22.4 ◦ C
• Bias voltage: 20 - 700 V, step 20 V, increasing, decreasing
Detector W13-BZ4A-P04
• Temperature: -31.2 ◦ C/-31.3 ◦ C
• Bias voltage: 100 - 1370 V, step 50 V, increasing, breakdown 1370 V
Detector W13-BZ4B
• Temperature: -27.6 ◦ C/-27.8 ◦ C
Detector W13-BZ4D
• Temperature: -28.7 ◦ C/-28.8 ◦ C
Detector W13-BZ5-P11
• Temperature: -31.2 ◦ C/-31.3 ◦ C
42
43
Figure 7.4: Results of detectors tests, dots are values of amplitude of response [mV], triangle is symbol of leakage current [µA], information
of each detector has the same colour
Final results (dependence of the amplitudes of response on the bias voltage
- dots and dependence of the leakage current on the bias voltage - triangles)
are displayed in the graph 7.4 where are values from one channel of each tested
detector. The same colour is used for both dependences of each detector. All of
measured data is available in the report [20, pages 18 - 24].
Plateau which indicates depletion region was found only in case of nonirradiated detector. The depletion region was not reached for remaining detectors.
The dependence of the response was always straight linear. Irradiated detectors
were tested only in case of increasing the bias voltage because of all of them broke
down.
7.3
Discussion
A temperature stabilisation was achieved during measurements. Temperature
was almost constant and it fluctuated only about decimals of degrees Celsius)
thanks to a good freezer. Each detector was fixed into the experimental setup
and it was cooled during at least 24 hours to temperature about minus 30 ◦ C.
Importance of the temperature stabilisation is obvious from the results in the
figure 7.4. In case of non-irradiated detector, the amplitude of response differs
for both temperatures.
It was necessary to the increase bias voltage slowly during the measurement
because of Joule heating produced in electronic circuits. One can see detailed
results of measurement of the detector W12-BZ4D-P22 which was not performed
carefully. As increasing of the bias voltage was hasty, the freezer did not suffice
to cool apparatus properly.
Data recording using the Evaluation board DRS4 V2 and software DRS Oscilloscope enabled to gain a statistical huge file of data. On the other hand, it
was not possible to record the same quantity of data in each measurement.
Increasing of the bias voltage was done manually although it is generally
possible to control the voltage source remotely. However, the DRS Evaluation
board and DRS software do not enable a remote control. Use of another device
would be necessary in case of the pertinent automation of the experiment
Impossibility of setting the laser in the same conditions for each measurement
was the cardinal disadvantage of this measurement setup. Therefore, it is not
possible to calculate collected charge from acquired data.
In view of chapter 4.2 about detector properties after irradiation, increase of
the leakage current and increase of the depletion voltage was observed. However,
increased value of depletion voltage was not found because of breakdown of the
detectors. According to the theory, depletion region should be reached for the bias
voltage approximately 2 kV. However, ATLAS SCT will be operated at lower bias
voltage level (approximately 1 kV). Therefore, detectors with high response were
chosen for further development. They cannot be operated in the full depletion
mode but the response is at least higher than a noise.
44
8. HSIO Development Board
The HSIO (High Speed Input Output) board is an apparatus developed originally
for Linac Coherent Light Source facility at SLAC laboratory [31]. However it
has found application in the ATLAS SCT upgrade project thanks to its wide
capability in signal processing. HSIO should become new standardized design for
testing ATLAS pixel and strip detectors.
8.1
Description of the HSIO
The HSIO board is a device with large FPGA (Field-programmable Gate Array)
and modern standart input-output interfaces (RJ-45, SFP, XFP) for ethernet or
USB comunication [33]. It was designed in accord with the ATCA (Advanced
Telecommunications Computing Architecture) specification [19, page 5].
The front view of the HSIO board is displayed in the figure 8.1. There the
most important parts are marked with yellow sign and number. The detailed
description of those components continues below the picture.
Figure 8.1: HSIO Development Board
Main parts of the HSIO
1. JTAG (Joint Test Action Group) configuration port (J13)
It serves for configuring the FPGA using Xilinx programing cable [19,
page 15].
45
2. Virtex-4 FPGA
3. Rotary switch (S1)
S1 is a rotary switch with 16 positions. It selects interface type: position 0
corresponds to SFP (U15) and position 1 sets RJ-45 (J12) communication [32].
4. USB 2.0 port (J10)
This port enables data transfer between peripheral and computer (speed up
to 480 Megabyte per second) [19, page 15].
5. SFP fiber transceivers (U18, U19)
They are two optical transceiver which support high speed serial links over
multimode optical fiber (signaling rates up to 4.25 GBd) [19, page 19].
6. SFP fiber transceivers (U15, U16)
This two SFP transceivers interface a device to a fiber optic or copper
network cable [19, page 17].
7. XFP fiber transceivers (U20)
The HSIO has one XFP interface. It is high speed, hot-swappable, protocolindependent optical transceiver which can be used for various types of network connection [19, page 20].
8. General purpose SMA/LEMO connectors (J1 - J8)
Eight connectors are used such as triggers and interrupts [19, page 22].
9. USB 1.0 port (J9)
This port enables data transfer between peripheral and computer (speed up
to 1 Megabyte per second) [19, page 15].
10. RJ-45 connector (J12)
Standart RJ-45 connector is used for integrated 10/100/1000 Gigabit Ethernet interface [19, page 13].
11. Push buttons (SW1, SW2)
These two push buttons serve booting the FPGA, SW1 (right) is cold boot
of it, SW2 (left) is used for code reload [19, page 14]. According to [32],
SW2 button should not be used as it gets the HSIO into a strange state.
12. Power and FPGA status indicators (D1)
Two green LEDs (Light-Emitting Diode) are situated next to the Character
displays. The lower LED is a power indicator and the upper one shows
FPGA status [19, page 10].
13. Character displays (U12, U43)
Two 4 character dot-matrix displays are provided [19, page 22]. They show
build-info and firmware version at firmware boot/reset and status info during normal use [32].
14. Input power connector (JP2)
Molex Mini-Fit JR (type 5569 right angle PC mount, 4 pins) is used as
input power connector [19, page 10].
46
15. On-board power supplies
DC-DC converters provide various voltages to power the components on
the board [19, page 10].
16. 12 V DC (direct current) output (SP10, SP13)
17. Interface board
An IB (Interface Board) is a feature which is used to connect the system
with front-end detector electronic. It has installed specific connectors for
this purpose [19, page 5].
8.2
HSIO Instalation in Prague
One HSIO board was instaled at the detector laboratory of Institute of Particle
and Nuclear Physics in Prague (picture 8.2e). During instalation the instructions
mainly from the TWiki ATLAS websites [32] and [34] have been followed.
8.2.1
Hardware
Some electronic equipment is necessary for operation of HSIO - power supply
(48 V DC, >50 W), PC with free network card and Xilinx programing cable [32].
Also several simple hardware arrangement is needed before starting using the
HSIO system.
• Standoffs - add them to the HSIO and IB.
• Power connector - HSIO is connect to a power supply unit using a molex 4
pins connector (picture 8.2a). Yellow cables are connected to 48 V power
point, black ones to 0 V.
• Fan for cooling of DC-DC converter (picture 8.2b). It is supplied from 12 V
output on the HSIO board (pins SP10, SP13).
8.2.2
Software
DAQ PC
For proper working of system, it is neccesary to install following software on the
control PC [34]. Computer with operating system Windows 7 Professional was
used in our laboratory.
1. Microsoft Visual C++
• version 2008 or 2010, free Express edition is sufficient
• http://www.microsoft.com/visualstudio/cze/
downloads#d-2010-express
2. ROOT
• use recommended version (probably the last one)
• http://root.cern.ch/drupal/content/downloading-root
47
3. Wireshark
• an auxiliary program for monitoring network comunication
• http://www.wireshark.org/download.html
4. WinPCAP packet driver and C++ headers
• version 4.1.1, not the latest version
• http://www.winpcap.org/
5. Xilinx ISE and Impact
• software for programming FPGAs
• http://www.xilinx.com/support/download/index.htm
SCTDAQ Setup
FPGA firmware and SCTDAQ software is available at http://svnweb.cern.ch/
cern/wsvn/atlasupstrip/.
1. FPGA Configuring
• Xilinx programming cable is necessary for configuring the FPGA on
the HSIO board [32]. It is connect to the JTAG configuration port
J13 (picture 8.2c). The HSIO must be powered when it is configured.
• Two flavours of firmware are provided:
– .bit is for direct configuration but it is lost when the HSIO is
rebooted,
– .mcs is loaded into the PROM (Programmable Read Only Memory) and the FPGA is configured from it after any reboot [32].
• Software for FPGA configuration is called Impact. It loads a JTAG
chain - there are only two devices in this case (PROM, FPGA). Then
the proper file is assigned to a device. Finally, the device is configured.
2. SCTDAQ Software Installation
• A gile extension should be changed to .zip. After unpacking, we copy
content of folder trunk to C:/sctdaq.
• According to the manual [34], we open hsio access.sln project and we
set compilation with the HSIO flag (instead of Debug or Release).
Include path of the WpdPack headers and location of the WpdPack
library should be changed. Finally, we rebuild the project.
– Include path of the headers is set in the Project → Properties →
Configuration Properties → C/C++ → Additional Include Directories [34].
• Similarly, we open sctdaq hsio.sln project and we set compilation with
the HSIO flag. The location of ROOT and National Instruments libraries needs to be changed [34]. We also redefine the HV number in
the file sct hardware.h (SCTHV NUMBER change to 0) [34]. Finally,
we rebuild the project.
48
(a) Molex connector
(b) DC-DC converter and its cooling
(c) FPGA and Xilinx programator
(d) Reset buttons, LED indicators, character
displays and power connector
(e) HSIO at the detector laboratory of Institute of Particle and Nuclear Physics
Figure 8.2: HSIO instalation
49
8.3
Operation of the HSIO
The HSIO is prepared for operation after installation which is described in the
previous section. First, we select interface type on the rotary switch S1 and
we connect the HSIO with computer’s network card. Then we turn the power
supply on. We run SCTDAQ on the computer and we start the application using
command .X ST.cpp. Alternatively, the application with new GUI can be started
using .X Stavelet.cpp command.
The HSIO system has been successfully installed (including compiling of
source codes) at the detector laboratory of Institute of Particle and Nuclear
Physics in Prague. Data acquisition was tested in partner laboratory in Freiburg
twice. Now, testing module for Prague work place is being prepared. Subsequently, tests and measurements using this new device will be launched.
50
Conclusion
Overview of radiation detection was compiled in this thesis. It consists of description of ways how certain types of particles interact with material, principles
and operation of semiconductor detectors, radiation damage of semiconductor
detectors and signal processing.
Measurement of the irradiated silicon detectors was carried out. It required
operation at low temperatures. Dependence of the amplitudes of response on
the bias voltage was obtained. However, the depletion region was not reached
in case of highly irradiated detectors. For further development, detectors with
high response were chosen. Description and results are described in the chapter
7 which covers main experimental part of this thesis. Detailed discussion of
obtained results can be found in the section 7.3.
The HSIO, new readout system for particle detectors, was successfully installed at the detector laboratory of Institute of Particle and Nuclear Physics.
It becomes new standardized design for testing ATLAS strip detectors. It will
be used for further tests (with a laser and radioactive sources) of radiation hard
sensors developed for the ATLAS SCT upgrade program.
However, the HSIO cannot be used for real testing yet. Some other necessary
devices are being prepared now and whole system for detector testing is being
installed at the laboratory. Therefore no experimental results have been obtained.
51
52
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55
56
List of Figures
1.1
1.2
Electromagnetic spectrum . . . . . . . . . . . . . . . . . . . . . .
The Standard Model of particle physics . . . . . . . . . . . . . . .
2.1
2.2
2.3
2.4
Photoelectric effect . . . . . . . . . . .
Compton scattering . . . . . . . . . . .
Relative importance of the major types
Bremsstrahlung . . . . . . . . . . . . .
3.1
3.2
3.3
3.4
3.5
3.6
Energy band structure . . . . . . . . . . . . . . . .
Doped Semiconductors . . . . . . . . . . . . . . . .
Schema of creation of a pn-junction . . . . . . . . .
Reverse biased junction . . . . . . . . . . . . . . . .
Reverse biased pn-diode - production of e-h pairs .
Position sensitive detector - resistive charge division
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
4.1
4.2
4.3
4.4
Displacement damage functions . . . . . . .
Annealing . . . . . . . . . . . . . . . . . . .
Dependence of leakage current on fluence . .
Change of depletion voltage after irradiation
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
9
10
11
13
.
.
.
.
.
.
.
.
.
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.
.
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15
19
20
21
24
24
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
28
30
30
31
5.1
5.2
5.3
Analog signal processing . . . . . . . . . . . . . . . . . . . . . . .
Signal terminology . . . . . . . . . . . . . . . . . . . . . . . . . .
Simple CR-RC shaper . . . . . . . . . . . . . . . . . . . . . . . .
33
33
34
6.1
6.2
The ATLAS detector . . . . . . . . . . . . . . . . . . . . . . . . .
Simulations of the inner tracker . . . . . . . . . . . . . . . . . . .
37
38
7.1
7.2
7.3
7.4
Block schema of the experiment
Preamplifier . . . . . . . . . . .
Devices used in the experiment
Results of detectors tests . . . .
.
.
.
.
39
40
41
43
8.1
8.2
HSIO Development Board . . . . . . . . . . . . . . . . . . . . . .
HSIO instalation . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
49
57
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. . . . . . . . . . . .
. . . . . . . . . . . .
of photon interaction
. . . . . . . . . . . .
6
8
.
.
.
.
.
.
.
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.
.
.
58
List of Tables
1.1
Sorting of particles . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.1
Properties of silicon and germanium . . . . . . . . . . . . . . . . .
22
59
60
List of Abbreviations
ADC
Analog to Digital Converter
ATCA
Advanced Telecommunications Computing Architecture
ATLAS
A Toroidal LHC Apparatus
CAMAC
Computer Automated Measurement and Control
CERN
Conseil Européen pour la recherche nucléaire (European Organization for Nuclear Research)
CLHEP
Class Library for High Energy Physics
DAQ
Data Acquisition
DC
direct current
FPGA
Field-programmable Gate Array
GUI
Graphical User Interface
HEP
High Energy Physics
HL-LHC
High Luminosity LHC
HSIO
High Speed Input Output
IB
Interface Board
JTAG
Joint Test Action Group
LASER
Light Amplification by Stimulated Emission of Radiation
LED
Light-Emitting Diode
LHC
Large Hadron Collider
NIEL
Non Ionizing Energy Loss
NIM
Nuclear Instrumentation Methods
OOP
Object-oriented programming
PCI
Peripheral Component Interconnect
PKA
Primary Knock-on Atom
PROM
Programmable Read Only Memory
SCT
Semiconductor Tracker
SFP
Small Form Factor Pluggable
STL
Standard Template Library
61
TCP/IP
Transmission Control Protocol/Internet Protocol
TRT
Transition Radiation Tracker
TTL
Transistor-transistor logic
USB
Universal Serial Bus
VME
Versa Module Europa
XFP
10 Gigabit Small Form Factor Pluggable
62

Tests of Irradiated Semiconductor Detectors for ATLAS Upgrade

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