Electron-hole pairs can also be generated by thermal energy at the surface of the material or within the depleted bulk. This leads to a leakage current and is observable as noise when measuring a particle signal.
Applying a very high reverse bias voltage causes a so called breakdown in which a high leakage current occurs so that the sensor is badly damaged and signal detection is not possible anymore.
3.4 The effects of radiation on silicon
Silicon has a crystalline structure which determines the behaviour of the material. This lattice structure as well as additional doping can be affected by a high radiation dose and hence change the behaviour of silicon. Due to these lattice defects of the material the efficiency in detecting particles decreases. The main effects causing such damages inside the silicon structure are explained in the following. Damages due to radiation can occur inside the sensor bulk and at the surface of the sensor. As the first is more crucial regarding the efficiency of particle detection merely defects inside the bulk are considered here.
A not reversible bulk damage is the displacement of a single silicon atom. For this a minimum energy of 25 eV is needed. This can be achieved by an electron with a minimal energy of 260 keV or a proton or neutron with a minimal energy of 190 eV. If the incoming particle has even more than the minimal energy the recoiling atom can induce further damage inside the sensor. With an energy of the silicon atom of more than 2 keV it can even damage a complete region inside the lattice and thus produce a cluster defect [28].
Point defects which come from radiation are for example silicon vacancies and interstitials.
The latter are atoms inside the material which are not included inside the lattice structure but in between. Vacancies describe a missing atom inside the regular lattice structure. Depending on the temperature both defects can move through the lattice and may recombine with other defects. Still they can have an effect on the space charge inside the depletion region: They produce additional energy levels. If these are located inside the band gap of silicon the defects act as generation-recombination centres and increase the leakage current.
Another side effect of defects is charge trapping in which a signal charge is kept for a longer time than the usual charge collection time needed to produce the full signal. As a result, the final signal height measured at the electrodes is smaller than in an unirradiated sensor. After a dose of 1015 neq/cm2 the effect becomes relevant as about 50% of the signal will be lost due to charge trapping.
Moreover, charge defects have an impact on the reverse bias. To fully deplete the sensor after irradiation, a higher operation voltage is necessary. Hence, the leakage current increases and therefore the power dissipation is higher as well. This heats up the sensor which in turn leads to a higher leakage current. As a consequence, a higher operation power is needed which would again increase the leakage current. This cycle which further increases the operation voltage and the leakage current is called thermal runaway and makes the cooling of silicon inevitable.
Radiation also impacts the effective doping concentration. The ratio of donor- and acceptor-like states changes due to defects. Inside an n-type doped bulk the majority charge carriers are electrons which can be removed easier than holes inside a p-type bulk. Radiation damages thus alter the original n-type bulk to a p-type one. As a consequence of this type inversion the pn-junction moves to the n+-segmented pixel side and the growth direction of the depletion zone changes.
To describe the radiation damages independent of the particle type all deposited energy except the energy used for creating electron-hole pairs can be expressed by Non-Ionising Energy Loss
3 Particle interaction with matter
(NIEL). The NIEL caused by a certain flux of an arbitrary type of particle is normalised to the NIEL caused by 1 MeV neutrons. The unit is the neutron equivalent fluence neq/cm2.
Due to thermal energy defects are able to move through the sensor. To prevent this the sensors are cooled down in normal detector operation. If the sensor is exposed for several hours to higher temperature of about 30 to 60◦C the annealing effect is increased. Differently charged defects can recombine due to thermal energy and become inactive. This is observable as the effect of trapping is decreasing after that time. In contrast, a longer annealing time of several weeks increases the effects of radiation damages. In this case the impact of defects in combination with higher temperature changes the crystal lattice in a more complex way. This effect is called reverse annealing [34]. Even in periods during which the sensor is not exposed to high radiation cooling is needed to avoid reverse annealing.
The described radiation damages have to be considered during detector operation with high irradiation and silicon as sensor material and lead to a necessary upgrade of the detector which is further explained in Chapter 6.
4 Experimental setup
4.1 The Large Hadron Collider
The Large Hadron Collider (LHC) is a proton-proton collider with a nominal centre of mass energy of 14 TeV and is located at CERN1 in Geneva. A schematic drawing of the ring can be seen in Figure 4.1. The LHC is located inside a tunnel which is about 100 m underground and
Figure 4.1: A schematic of the LHC ring [35]. Shown are the pre-accelerators, the two proton injection points and the four main experiments ATLAS, LHCb, CMS and ALICE.
has a circumference of 27 km. It is built inside the former tunnel from the previous accelerator machine Large Electron-Positron (LEP) Collider which was in operation from 1989 until 2000.
With a complex system protons are injected into a subsequent order of different machines to accelerate the particles up to 7 TeV. The protons themselves are taken from a hydrogen bottle.
After stripping off the electrons from the hydrogen the protons are accelerated up to an energy of 50 MeV by the 30 m long linear accelerator 2 (LINAC2). From there they are injected into the Proton Synchrotron Booster (PSB) and brought up to an energy of 1.4 GeV. In the following Proton Synchrotron (PS) and Super Proton Synchrotron (SPS) they are further accelerated to
1Conseil Europ´een pour la Recherche Nucl´eaire
4 Experimental setup
an energy of 25 GeV and 450 GeV, respectively. All three synchrotrons are circular accelerators with increasing circumferences of 50 m, 628 m and 7 km [36].
Finally, the protons are injected as bunches in two opposite directions into the LHC ring at the injection points TI 8 and TI 2 [37]. If the LHC is completely filled up each beam consists of 2808 bunches. Each bunch contains 1.5×1011 protons. With this setup every 25 ns a collision between two bunches takes place. This leads to a bunch crossing frequency of 40 MHz and determines the clock frequency for detector readout.
In the LHC the protons are accelerated by 16 cavities and focused with 858 quadropole magnets to counteract their electromagnetic repulsion. To keep the bunches on a circular track 1,232 dipole magnets are used. All these magnets are superconducting and have to be cooled down to temperatures of about 1.9 K. A drawing of a dipole magnet is shown in Figure 4.2.
Figure 4.2: A drawing of an LHC dipole [38]. The main elements are the two beam pipes which are enclosed by superconducting coils, a non magnetic collar and an iron yoke to gain a magnetic field of 8.3 T. To cool the superconducting magnets down to 1.9 K a helium-II vessel is used. An additional vacuum vessel surrounds the magnet and keeps an insulating vacuum [39].
Additionally, a heat exchanger and a thermal shield provide temperature compensation. Several bus bars interconnect the individual magnet elements.
The four main experiments making use of the collisions of the LHC are the ATLAS2-, CMS3, LHCb4- and ALICE5 detectors. The first three mentioned record the data of proton-proton collisions.
2A Toroidal LHC Apparatus
3Compact Muon Spectrometer
4Large Hadron Collider beauty
5A Large Ion Collider Experiment