| 1. |
- Chmill, Valery, 1963-
(författare)
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Radiation tests of semiconductor detectors
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates the response of Gallium Arsenide (GaAs) detectors to ionizing irradiation. Detectors based on π-υ junction formed by deep level centers doping. The detectors have been irradiated with 137Cs γ-rays up to 110 kGy, with 6 MeV mean energy neutron up to approximately 6 · 1014 n/cm2, with protons and mixed beam up to 1015 p/cm2. Results are presented for the effects on leakage currents and charge collection efficiencies for minimum ionizing electrons and alpha particles. The signal from minimum ionizing electrons was well separated from the noise even after the highest delivered exposures and the diodes are thus still operational as detectors. Saturation of the effects of radiation damage is observed in both the I-V characteristics and charge collection efficiency measurements.The requirements for detectors e.g. at present and planned hadron colliders is very high in terms of radiation hardness. Detectors for tracking applications close to the interaction point will receive charged particle doses in the range of 110 kGy and fast neutron fluences of 1014 n/cm2 during the lifetime of an experiment. In this thesis it is confirmed that GaAs detectors are radiation resistant to neutron irradiation for fluences up to 1015 n/cm2 and that GaAs detectors are feasible as inner trackers. Most of this work was performed in the framework of the RD8 collaboration at CERN.
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| 2. |
- Ersmark, Tore, 1977-
(författare)
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Geant4 Monte Carlo Simulations of the International Space Station Radiation Environment
- 2006
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A detailed characterization of the proton and neutron induced radiation environment onboard Columbus and the International Space Station (ISS) has been carried out using the Geant4 Monte Carlo particle transport toolkit. Dose and dose equivalent rates, as well as penetrating particle spectra corresponding to incident trapped protons, GCR protons, SPE protons and cosmic ray albedo neutrons are presented. These results are based on detailed Geant4 geometry models of Columbus and ISS, comprising a total of about 750 and 350 geometry volumes, respectively. Additionally, the physics models of Geant4 have been validated with respect to space radiation shielding applications. Geant4 physics configurations based on the “Binary Cascade” and “Bertini Cascade” models of hadronic reactions were found to adequately model the particle interactions of the relevant space radiation fields. Other studied Geant4 models of hadronic reactions were found to be unsatisfactory for this application. Calculated trapped proton dose rates are found to be strongly dependent on ISS altitude. Dose rates for different locations inside the Columbus cabin are presented, as well as for different models of the incident space radiation flux. Dose rates resulting from incident anisotropic trapped protons are found to be lower, or equal to, those of omnidirectional models. The anisotropy induced by the asymmetric shielding distribution of Columbus/ISS is also studied. GCR proton dose rates are presented, and it is demonstrated that the presence of thick shielding may increase the dose rate. A possible problem using Geant4 for future studies of effects induced by high-energy GCR ions is discussed. The dose rate due to cosmic ray albedo neutrons is demonstrated to be negligible. The calculated trapped proton dose rates are 120 μGy/d and 79 μGy/d for solar minimum and maximum conditions, respectively. GCR dose rates are estimated based on calculated GCR proton dose rates to 161 μGy/d and 114 μGy/d, respectively. These dose rates are found to be compatible with experimental measurements.
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| 3. |
- Grahn, Karl-Johan, 1981-
(författare)
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Measurement of Multi-Jet Production in Proton-Proton Collisions at 7 TeV Center-of-Mass Energy and Hadronic Calibration Studies with the ATLAS Detector at CERN
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ATLAS experiment – situated at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva – took its first collsion data in 2010. Physics topics include finding the Higgs boson, heavy quark physics, and looking for extensions of the Standard Model of Particle Physics such as supersymmetry. In this thesis, inclusive multi-jet production has been studied with the ATLAS detector in proton-proton collisions at a center-of-mass energy of 7 TeV, using an integrated luminosity of 17 nb−1. The anti-kt algorithm with distance parameter ℝ = 0.6 is used to identify jets. The inclusive multi-jet cross section is measured, as well as the ratio ofcross sections for inclusive production of n − 1 and n jets for n ≤ 6. The differential cross sections of the first, second, third and fourth leading jets as a function of transverse momentum, and the differential cross section as a function of the scalar sum of the pT of selected jets, HT, for different jet multiplicities are presented. The ratio of the differential cross section as a function of HT for 3-jet and 2-jet events is also measured. The results are compared to expectations based on leading order QCD, which agree with the data. In addition, a new method for calibrating the hadron response of a segmented calorim-ter is developed and successfully applied to 2004 ATLAS combined beam test data. It is based on a principal component analysis of the calorimeter layer energy deposits, exploiting longitudinal shower development information to improve the measured energyresolution. For pion beams with energies between 20 and 180 GeV, the particle energy is reconstructed within 3% and the energy resolution is improved by 11% to 25% comparedto the response at the electromagnetic scale. Multi-mode optical readout cables for the ATLAS liquid argon calorimeters, about one hundred meters in length, were installed between the main ATLAS cavern and the counting room in the USA15 cavern. Patch cables were spliced onto the ribbons and the fiber attenuation was measured. For 1296 fiber pairs in 54 cables, the average attenuation was 0.69 dB. Only five fibers were found to have losses exceeding 4 dB, resulting in a failure rate of less than 2 per mill. In the ATLAS liquid argon barrel presampler, short circuits consisting of small pieces of dust, metal, etc. can be burned away in situ by discharging a capacitor over the high voltage lines. In a burning campaign in November 2006, seventeen existing short circuits were successfully removed. An investigation on how to implement saturation effects in liquid argon due to high ionization densities resulted in the implementation of the effect in the ATLAS Monte Carlo code, improving agreement with beam test data. The timing structure of hadronic showers was investigated using a Geant4 Monte Carlo. The expected behavior as described in the literature was reproduced, with the exception that some sets of physics models gave unphysical gamma energies from nuclear neutron capture.
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| 4. |
- Hansson, Per, 1980-
(författare)
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First Determination of the Electric Charge of the Top Quark and Studies of the Top Quark Pair Background to New Physics
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with experimental investigations of properties of the top quark and processes involving this particle. In the first part of the thesis, the first determination of the electric charge of the top quark is presented. The measurement was made using top quark pair events produced in proton-antiproton collisions recorded by the D0 detector at the Fermilab Tevatron. It is based on the reconstruction of the charge of the top quarks decay products from the dominant decay to a W boson and a b-quark. The method uses a jet charge algorithm, calibrated with data, to discriminate between b- and antib-quark jets. A constrained kinematic fit is also performed to resolve the ambiguities of the pairing of the top quark decay products and to extract the top quark electric charge. The result is in good agreement with the Standard Model top quark electric charge of 2e/3 and an upper limit of 0.8 at 90\% confidence level on the fraction of exotic quarks with charge 4e/3 in the data sample is obtained. The second part of the thesis concerns the estimation of the top quark pair background to searches for new physics, such as supersymmetry, with the ATLAS experiment at the CERN Large Hadron Collider. These searches will require a robust estimation of standard model backgrounds in order to make any claims of discovery or to exclude models of new physics. For searches with a final state signature characterized by two isolated charged leptons, multiple jets and large missing transverse energy the largest source of background is expected to be top quark pairs with leptonic decay of the two W bosons from the top quarks in the event. A data-driven method to estimate this contribution based on full kinematic reconstruction of the top quark pair events is studied using simulated proton-proton collisions. It is shown that the method is capable of estimating the top quark pair background to within 12% using data corresponding to approximately 1fb-1. The systematic uncertainty is of the order of 20% and, depending on the model, the contamination of signal events can potentially be large.
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| 5. |
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| 6. |
- Kuwertz, Emma, 1988-
(författare)
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A Search for Squarks and Gluinos in Final States with At Least Two Leptons with the ATLAS Detector
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A search for squarks and gluinos in nal states with at least two leptons (electrons or muons) is presented. Events are selected using the Razor variables to discriminate between signal and background processes. This analysis is performed over the full 2012 dataset of proton-proton collisions at 8 TeV centre-of-mass energy, totalling an integrated luminosity of 20:3 fb-1 recorded by the ATLAS experiment at the CERN Large Hadron Collider. In the absence of any signicant excess with respect to the Standard Model expectation, exclusion limits are derived in the context of several supersymmetry-inspired simplied model topologies involving the direct production of squarks and gluinos.
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| 7. |
- Larsson, Oscar, 1982-
(författare)
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Analysis of the Radiation Environment on Board the International Space Station Using Data from the SilEye-3/Alteino Experiment
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents an analysis of the radiation environment on board the Russian section of the International Space Station (ISS) using data from the SilEye-3/Alteino experiment. As part of the analysis the efficiency and response of the SilEye-3/Alteino detector was studied. The relative nuclear abundance is generally in agreement with expected results. The presence of odd Z nuclei is significantly increased when compared with measurements outside the ISS. However, in ISS-y (Starboard-Ports) and z (Nadir-Zenith) directions an underabundance of carbon and oxygen nuclei is seen, whereasin x (Forward-Aft) there seemes to be an overabundance. One possible explanation is the absence of high-Z material in the ISS module wall for y and z . Whereas in x, most of the main body of the ISS is in front of the detector and the amount of high-Z material (i.e. aluminium) is large. The nalysis of fragmentation of iron into a range of secondary nuclei (15≤Z ≤25) indicates an aluminium hull equivalent thickness of 8-9 cm in y- and z-directions. For x the aluminium hull equivalence amounts to about 17 cm. Flux, LET, dose and dose equivalent rates present a clear anisotropy in the different orthogonal directions of the ISS, with rates consistently lower in x. This effect is more pronounced for the heavy-ion component (LET >50 keV/μm). Measureddose rates vary from 25 μGy/day to 75 μGy/day, depending on location, orientationand configuration of the detector. The dose equivalent varies from 50 μSv/day toalmost 470 μSv/day.The shielding effect of the polyethylene amounts to 25-37% dependent on loca-tion and orientation inside the ISS. The majority of the reduction occurs duringpassages through the SAA. A Geant4 comparison with the Phits simulations code have been preformed as an initial survey into the treatment of hadronic physics for heavy ions in Geant4.
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| 8. |
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