[en] The introduction of impurities in the silicon crystal can affect the radiation hardness. The effective density of doping, the inverse current and the charge collection efficiency are the most important electrical parameters which are changing in function of the fluence. This thesis presents a detailed study of these parameters changes in function of the hadron fluence and the time after irradiation, for silicon crystals with impurities. A model of the electric field distribution is proposed to explain the charge collection properties of the irradiated detectors. (A.L.B.)

[en] The LHCb Vertex Locator (VELO) is designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. This is a silicon strip detector system operating at the closest distance from the interaction region among all the currently installed detectors at the CERN LHC. The geometry of the sensors implies that every detector region is exposed to a different dose of radiation depending mainly on the radial distance from the beam line. In the first 2 years of operation they have been exposed to a range of fluences up to a maximum value of about 4.5×10¹³ 1 MeV neutron equivalent (n_eq) cm⁻². At the end of 2012 a maximum of about 1.5×10¹⁴ n_eq has been reached; data analysis is ongoing. Sensor currents, full depletion voltage and charge collection efficiencies have been studied at various fluences. Observation of type inversion of the n-type silicon sensors has been made (about after 10–15×10¹² n_eq). The n-type bulk sensors have been compared with the only p-type bulk detectors installed in the LHC. A reduction in the charge collection efficiency due to charge losses to a second metal layer (used for routing the signal to the readout electronics in LHCb-VELO sensors) has been measured. Possible solutions to this undesirable effect are planned to be studied in the framework of the LHCb-VELO upgrade

[en] Silicon detectors have risen to a predominant role in high energy physics experiments since their introduction just over thirty years ago. Their outstanding capabilities of high resolution, low mass and fast response to ionising radiation have given silicon detectors the role of device of choice for the inner regions of collider experiments. Their evolution over the years has been notable, and it is possibly accelerating in the present times with the impulse coming from stringent requirements of future experiments and from developments in the microelectronics industry. Recent advancements of silicon detectors are reviewed and reported from the perspective of future challenges

[en] The degradation of the electrical properties of silicon detectors exposed to 24 GeV/c protons were studied using pad diodes made from different silicon materials. Standard high-grade p-type and n-type substrates and oxygenated n-type substrates have been used. The diodes were studied in term of reverse current (I_r) and full depletion voltage (V_fd) as a function of fluence. The oxygenated devices from different suppliers with a variety of starting materials and techniques, all show a consistent improvement of the degradation rate of V_fd and CCE compared to un-oxygenated substrate devices. The degradation of un-oxygenated n-type and p-type diodes is very similar under proton irradiation. The presence of high oxygen concentration in the substrate allows the reduction of the damage factor of N_eff by a factor of three compared to un-oxygenated material. It is foreseen that the benefits obtained with n-type silicon can be extended to p-type substrate using the same oxygenation technique. The use of both oxygenated material and p-type substrates together, therefore looks a very promising route to a silicon detector able to survive radiation levels as high as 1.10'1'5 protons cm^-2. (authors)

[en] Following earlier work on 'oxygenated' detectors in terms of charge collection efficiencies after proton irradiation, full-size detectors for the LHC have been processed with n-side read-out on oxygen enhanced n-type silicon substrates. Two hundred-micron-thick detectors have been inhomogeneously irradiated up to doses of 7x10¹⁴ p/cm² using 24 GeV protons from the CERN PS. Results are presented on the charge collection efficiencies as a function of operating voltage for regions of the detectors irradiated to different doses, using LHC speed analogue read-out electronics. The measurements confirm the expectations which led to our original proposal of such detectors which are now being envisaged for the silicon-based detector systems at the LHC designed to withstand the greatest doses. The possibilities for survival at an upgraded luminosity LHC (Super-LHC) are also briefly discussed

[en] Light pulses emitted by a red (670 nm) Light Emitting Diode (LED) have been used to study the charge collection properties of non-irradiated and irradiated n-type silicon detectors. The advantages of red LED light pulses, compared to low-range alpha particles, are the availability of an external trigger and a very shallow distribution of the created e-h pairs (<10 μm). These features, combined with the use of a fast current amplifier and a 2.5 Gs/s sampling oscilloscope, allow the study of the evolution of the electric field in irradiated detectors. Evidence of a sensitive region on both sides of the detector has been observed. The model of the diode that depletes from the n⁺ junction side after conductivity-type inversion is discussed and the electric field distribution in the inverted detector is presented. A first evaluation of the strength of the electric field in the undepleted bulk of the detector is proposed

[en] The subject of radiation damage to silicon detectors induced by 24-GeV/c protons and nuclear reactor neutrons has been studied. Detectors fabricated on single-crystal silicon enriched with various impurities have been tested. Significant differences in electrically active defects have been found between the various types of material. The results of the study suggest for the first time that the widely used nonionizing energy loss (NIEL) factors are insufficient for normalization of the electrically active damage in case of oxygen- and carbon-enriched silicon detectors. It has been found that a deliberate introduction of impurities into the semiconductor can affect the radiation hardness of silicon detectors

[en] The electrical characteristics of non-irradiated and irradiated n-type silicon detectors (p⁺-n-n⁺ diode) are extracted by fitting a charge transport model to a set of experimental data obtained from the measurement of the current pulse response induced by α and β particles in non-irradiated and irradiated silicon detectors. The detectors have been irradiated either with ∼1 MeV neutrons up to a fluence of 11.2x10¹³ n/cm² or with 24 GeV/c protons up to a fluence of 10.6x10¹³ p/cm². After n to p-type inversion, a small junction on the p⁺ side of the detector is introduced to fit the experimental data and therefore to account for the evolution of the electrical characteristics of the detectors with fluence

[en] Recent results on the radiation hardness of silicon detectors fabricated on epitaxial and float zone bulk silicon enriched by various impurities, such as carbon, oxygen, tin and platinum are reported. A new methodology of measurements of electrical properties of the devices has been utilized in the experiment. It has been shown that in the case of irradiation by protons, oxygen enriched silicon has better radiation hardness than standard float zone silicon. The carbon enriched silicon detectors, on the other hand, exhibited significantly inferior radiation hardness compared to standard detectors. This study shows for the first time, a violation of the widely used normalization technique of the various particle irradiations by NIEL coefficients. The study has been carried out in the framework of the RD48 (ROSE) collaboration, which studies the radiation hardening of silicon detectors

[en] The electrical characteristics of silicon detectors (standard planar float zone and MESA detectors) as a function of the particle fluence can be extracted by the application of a model describing the transport of charge carriers generated in the detectors by ionizing particles. The current pulse response induced by α and β particles in non-irradiated detectors and detectors irradiated up to fluences PHI ∼ 3 · 10¹⁴ particles/cm² is reproduced via this model: i) by adding a small n-type region 15 μm deep on the p⁺ side for the detectors at fluences beyond the n to p-type inversion and ii) for the MESA detectors, by considering one additional dead layer of 14 μm (observed experimentally) on each side of the detector, and introducing a second (delayed) component to the current pulse response. For both types of detectors, the model gives mobilities decreasing linearily up to fluences of about 5·10¹³ particles/cm² and converging, beyond, to saturation values of about 1050 cm²/Vs and 450 cm²/Vs for electrons and holes, respectively. At a fluence PHI ∼ 10¹⁴ particles/cm² (corresponding to about ten years of operation at the CERN-LHC), charge collection deficits of about 14% for β particles, 25% for α particles incident on the front and 35% for α particles incident on the back of the detector are found for both type of detectors