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  • 1.
    Abedi-Valugerdi, Manuchehr
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
    Mercury and silver induce B cell activation and anti-nucleolar autoantibody production in outbred mouse stocks: are environmental factors more important than the susceptibility genes in connection with autoimmunity?2009In: Clinical and Experimental Immunology, ISSN 0009-9104, E-ISSN 1365-2249, Vol. 155, no 1, p. 117-124Article in journal (Refereed)
    Abstract [en]

    Environmental and predisposing genetic factors are known to play a crucial role in the development of systemic autoimmune diseases. With respect to the role of environmental factors, it is not known how and to what extent they contribute to the initiation and exacerbation of systemic autoimmunity. In the present study, I considered this issue and asked if environmental factors can induce autoimmunity in the absence of specific susceptible genes. The development of genetically controlled mercury- and silver-induced B cell activation and anti-nucleolar autoantibodies (ANolA) production in genetically heterozygous outbred Institute of Cancer Research (ICR), Naval Medical Research Institute (NMRI) and Black Swiss mouse stocks were analysed. Four weeks of treatment with both mercury and silver induced a strong B cell activation characterized by increased numbers of splenic antibody-secreting cells of at least one or more immunoglobulin (Ig) isotype(s) in all treated stocks. The three stocks also exhibited a marked increase in the serum IgE levels in response to mercury, but not silver. More importantly, in response to mercury a large numbers of ICR (88%), NMRI (96%) and Black Swiss (100%) mice produced different levels of IgG1 and IgG2a ANolA (a characteristic which is linked strictly to the H-2 genes). Similarly, but at lower magnitudes, treatment with silver also induced the production of IgG1 and IgG2a ANolA in 60% of ICR, 75% of NMRI and 100% of Black Swiss mice. Thus, the findings of this study suggest that long-term exposure to certain environmental factors can activate the immune system to produce autoimmunity per se, without requiring specific susceptible genes.

  • 2.
    Almhagen, Erik
    Stockholm University, Faculty of Science, Department of Physics.
    Development and validation of a scanned proton beam model for dose distribution verification using Monte Carlo2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 3.
    Almhagen, Erik
    Stockholm University, Faculty of Science, Department of Physics.
    Development and validation of a scanned proton beam model for dose distribution verification using Monte Carlo2015Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Background and purpose: Although proton therapy is becoming increasingly common as a radiotherapy modality, facilities offering proton therapy are still scarce in comparison to photon therapy. Sweden's new proton therapy facility, Skandionkliniken, is scheduled to being operation during August 2015, employing the pencil beam scanning technique. Given Skandionklinikens unique stance as the only facility offering proton therapy in Sweden as of this writing, it is important to minimize the need for measurements during quality assurance to free up beam time for patients and other endeavors. It is the purpose of this work to create a foundation for a method whereby dose distribution verification is done via Monte Carlo simulation by developing and performing simple validation of a beam model. As input for simulating a dose distribution, log files storing a wide variety of data on how the dose distribution was delivered were used.

    Method: GATE, an open source Monte Carlo code and built on top of Geant4, was used for all simulations. A beam model parameterizing phase space at the nozzle exit was developed. The beam model development process made use of the beam data library and log file data. Using an in house developed code to convert log file data to treatment plans readable by GATE allowed simulation of delivered dose distributions. For validation, gamma index tests were performed comparing measured and simulated dose distributions.

    Results: The beam model was found able to predict the spot size in almost all cases within 0.2 mm. Likewise, the beam model was able to predict the proton range within 0.2 mm. The energy spread was found to be more difficult to estimate; comparisons of simulated and measured curves for at six points around the Bragg peak yielded a maximum deviation of 0.86 mm. Several difficulties prevented easy interpretation of the results of the gamma index tests. If allowance is made for certain data manipulation, pass rates of 90% or above using the global method can be achieved for all depths and for both treatment plans scanned.

    Conclusion: Although some complications arose during validation, the beam model performance appears capable of producing accurate results. To produce a full product suitable for routine patient specific quality assurance, further work will be necessary. Significant computing power would also be mandatory for routine use, necessitating the acquisition of a dedicated computer cluster or using GPUs.

  • 4.
    Andisheh, Bahram
    Stockholm University, Faculty of Science, Department of Physics.
    Improving the therapeutic ratio of stereotactic radiosurgery and radiotherapy2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    New methods of high dose delivery, such as intensity modulated radiation therapy (IMRT), stereotactic radiation therapy (SRT) or stereotactic radiosurgery (SRS), hadron therapy, tomotherapy, etc., all make use of a few large fractions. To improve these treatments, there are three main directions: (i) improving physical dose distribution, (ii) optimizing radiosurgery dose-time scheme and (iii) modifying dose response of tumors or normal tissues.

    Different radiation modalities and systems have been developed to deliver the best possible physical dose to the target while keeping radiation to normal tissue minimum. Although applications of radiobiological findings to clinical practice are still at an early stage, many studies have shown that   sublethal radiation damage repair kinetics plays an important role in tissue response to radiation.

    The purpose of the present thesis is to show how the above-mentioned directions could be used to improve treatment outcomes with special interest in radiation modalities and dose-time scheme, as well as radiobiological modeling. Also for arteriovenous malformations (AVM), the possible impact of AVM network angiostructure in radiation response was studied.

  • 5.
    Ardenfors, Oscar
    Stockholm University, Faculty of Science, Department of Physics.
    Secondary doses to healthy tissues from radiotherapy and modern imaging techniques2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The use of ionizing radiation for treatment of cancer diseases is continuously increasing giving rise to several new questions and concerns. One of the most important aspects of this is the associated dose imparted to healthy tissues. This unwanted dose contribution is a result of both radiotherapy procedures and diagnostic imaging. The dose deposition in healthy tissues from external radiotherapy mainly originates from the incident primary beam. However, the patient also receives non-negligible organ doses originating from secondary radiation produced in the treatment machine and within the patient. This secondary radiation, especially neutrons, can travel large distances and consequently deposit doses to organs located far from the primary treatment field. The dose contribution to healthy organs from diagnostic procedures is growing due to the increase in repeated imaging performed in conjunction with radiotherapy. Also, patients undergoing both external radiotherapy and radionuclide therapy with radioactive isotopes could receive a high combined dose burden to healthy tissues.

    The need to quantify the secondary dose contribution and the associated risk of radiation-induced cancer is a relevant matter as new techniques are continuously emerging both in the field of radiotherapy and imaging. The technical advances in modern treatment techniques such as intensity modulated radiotherapy, rotational therapy and ion therapy have contributed to the overall increase in patient survival. A parallel development in medical imaging has caused an increase in the use of cone-beam computed tomography for repeated image-guidance imaging providing better tumor localization and a reduction in high doses deposited in adjacent healthy tissues.

    The most accurate way of estimating the risk of radiation-induced secondary cancers is to conduct comprehensive epidemiological studies on an exposed population stretching over several decades. This has been done in the past using cohorts of survivors of the atomic bombings and other nuclear accidents and medical exposures. However, the implementation of these epidemiological data is complex as the types of exposure differ greatly from modern radiotherapy procedures. Also, the long latency associated with radiation-induced secondary cancers further complicate the use of epidemiological data.

    Thus, the goal of achieving a dose-response relationship for secondary cancers is not only a matter of assessing the dose to the patient but also on how this data should be analyzed. Today, the most popular way of achieving this is through theoretical risk models using patient-specific parameters including dose distributions and risk coefficients obtained for populations from epidemiological studies.

    Due to the difficulties associated with performing measurements of radiation-induced organ doses from treatment and imaging, the dose is often calculated either analytically using an algorithm employed in the clinical treatment planning system or through Monte Carlo simulations that offer the most accurate tool for such calculations. To allow for accurate Monte Carlo simulations of secondary radiation from external radiotherapy the beam model should be validated against measurements with regard to both the primary beam and the out-of-field secondary radiation.

    These aspects have been investigated in individual studies that make the object of the articles included in this thesis. Paper I presents a literature review of secondary doses from different treatment and imaging modalities. Paper II shows a comparison between the risks of radiation-induced cancer for patients treated for head and neck cancer using two different treatment techniques. Paper III deals with Monte Carlo simulations of doses to healthy tissues from radionuclide therapy given in conjunction with external radiotherapy. Paper IV presents the validation of a proton spot scanning Monte Carlo model.

  • 6.
    Azzouz, Hatim
    Stockholm University, Faculty of Science, Department of Physics.
    Creation and Detection of Single Photons2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    A growing number of technologies employ quantum properties in order to produce solutions that surpass the performance of conventional devices, or to execute operations that are fundamentally impossible with classical systems alone. In the field of optical quantum information science, photons are utilized to encode, communicate and manipulate information, making them vitally important. While photon production always constitutes the first step in any optical experiment, in the field of quantum information science, the recording of data through the process of photon detection is an equally crucial final step.

    This thesis deals with both the single photons generation (based on diamond color defects) and their detection, utilizing a novel type of superconducting detectors. In particular, part one of this thesis is devoted to the construction of custom designed microscope setup, and the development of laboratory experiments, to enable the generation of single photons as well as the investigation of the optical and spin properties of diamond color centers. Confocal microscopy is used for this purpose, as it allows for the identification and addressing of individual color centers that emit only single photons. This microscope also feature an integrated self-built microwave and magnetic hardware setup, which allows for a wide range of spin environment spectroscopy studies. Single photon emission is demonstrated through both photon anti-bunching and Rabi oscillations at room temperature.

    The second part of the thesis offers an exploration of superconducting single photon detectors through experiment. Since electronics are an essential part of these detectors, the possibility of using a novel alternative scheme based on capacitive readout combined with fast gating to enable simplified readout is demonstrated. This scheme overcomes the limitations of conventional readout schemes, which require large bandwidth amplification and complex counting electronics. Besides photon detection, the capabilities of these detectors are also expanded to include high-energy particles in the MeV energy range, and the detectors are demonstrated to not only detect single α- and β-particles, but to do so with near unity efficiency. Finally, a multipurpose testing station for superconducting detectors is demonstrated with a central objective of optimizing the coupling efficiency of light to the active area of the detector, as well as to allow for a fast exchange of the optical fiber, thereby facilitating an efficient characterization of the detector. The optimization of this coupling efficiency was demonstrated through proof-of-principle experiments.

  • 7.
    Bengtsson, Ingemar
    Stockholm University, Faculty of Science, Department of Physics.
    A Kochen-Specker Inequality2009In: AIP Conference Proceedings, ISSN 0094-243X, E-ISSN 1551-7616, Vol. 1101, p. 241-245Article in journal (Other academic)
    Abstract [en]

    I review a paper by Klyachko, Can, Binicioğlu, and Shumovsky, and explain a little of the background as I see it

  • 8.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Badziag, Piotr
    Stockholm University, Faculty of Science, Department of Physics.
    Cabello, Adan
    Pitowsky, Itamar
    Universality of State-Independent Violation of Correlation Inequalities for Non-Contextual Theories2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 5, p. 050401-Article in journal (Refereed)
    Abstract [en]

    We show that the state-independent violation of inequalities for noncontextual hidden variable theories introduced in [Phys. Rev. Lett. 101, 210401 (2008)] is universal, i.e., occurs for any quantum mechanical system in which noncontextuality is meaningful. We describe a method to obtain state-independent violations for any system of dimension d >= 3. This universality proves that, according to quantum mechanics, there are no "classical'' states.

  • 9.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Holst, Sören
    Stockholm University, Faculty of Science, Department of Physics.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Classics illustrated: limits of spacetimes2014In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 31, no 20, article id 205008Article in journal (Refereed)
    Abstract [en]

    We carefully study the em and e→0 limits of the Reissner-Nordström spacetime using Geroch's definition of limits of spacetimes. This is implemented by embedding the one-parameter family of spacetimes in anti-de Sitter space, and as a result we obtain metrically correct Penrose diagrams. For em two distinct limits are studied.

  • 10.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    A Toy Penrose Inequality and Its Proof2016In: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 48, no 12, article id 156Article in journal (Refereed)
    Abstract [en]

    We formulate and prove a toy version of the Penrose inequality. The formulation mimics the original Penrose inequality in which the scenario is the following: A shell of null dust collapses in Minkowski space and a marginally trapped surface forms on it. Through a series of arguments relying on established assumptions, an inequality relating the area of this surface to the total energy of the shell is formulated. Then a further reformulation turns the inequality into a statement relating the area and the outer null expansion of a class of surfaces in Minkowski space itself. The inequality has been proven to hold true in many special cases, but there is no proof in general. In the toy version here presented, an analogous inequality in (2+1)-dimensional anti-de Sitter space turns out to hold true.

  • 11.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Black holes: Their large interiors2015In: Modern Physics Letters A, ISSN 0217-7323, E-ISSN 1793-6632, Vol. 30, no 21, article id 1550103Article in journal (Refereed)
    Abstract [en]

    Christodoulou and Rovelli (CR) have remarked on the large interiors possessed by static black holes. We amplify their remarks, and extend them to the spinning case.

  • 12.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Senovilla, José M. M.
    Trapped surfaces in Oppenheimer-Snyder black holes2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 88, no 6, article id 064012Article in journal (Refereed)
    Abstract [en]

    The Oppenheimer-Snyder solution models a homogeneous round dust of cloud collapsing to a black hole. Inside its event horizon there is a region through which trapped surfaces pass. We try to determine exactly where the boundary of this region meets the center of the cloud. We present explicit examples of the relevant trapped (topological) spheres; they extend into the exterior vacuum region, and are carefully matched at the junction between the cloud and the vacuum.

  • 13.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Jose M M Senovilla,
    A Note on Trapped Surfaces in the Vaidya Solution2009In: Physical Review D. Particles and fields, ISSN 0556-2821, E-ISSN 1089-4918, Vol. 79, no 2, p. 024027-Article in journal (Refereed)
    Abstract [en]

    The Vaidya solution describes the gravitational collapse of a finite shell of incoherent radiation falling into flat spacetime and giving rise to a Schwarzschild black hole. There has been a question whether closed trapped surfaces can extend into the flat region (whereas closed outer trapped surfaces certainly can). For the special case of self-similar collapse we show that the answer is yes, if and only if the mass function rises fast enough.

  • 14.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kus, Marek
    'Classical' Quantum States2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, no 2, p. 022319-Article in journal (Refereed)
    Abstract [en]

    We show that several classes of mixed quantum states in finite-dimensional Hilbert spaces which can be characterized as being, in some respect, "most classical" can be described and analyzed in a unified way. Among the states we consider are separable states of distinguishable particles, uncorrelated states of indistinguishable fermions and bosons, as well as mixed spin states decomposable into probabilistic mixtures of pure coherent states. The latter was the subject of the recent paper by Giraud et al. [Phys. Rev. A 78, 042112 (2008)], who showed that in the lowest-dimensional nontrivial case of spin 1, each such state can be decomposed into a mixture of eight pure states. Using our method, we prove that in fact four pure states always suffice.

  • 15.
    Bengtsson, Ingemar
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Åminneborg, Stefan
    Anti-de Sitter Quotients: When Are They Black Holes?2008In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 25, p. 095019-Article in journal (Refereed)
    Abstract [en]

    e point out that the BTZ black holes, and their relatives, can be defined in a cleaner way than they originally were. The covering space can be taken to be anti-de Sitter space, period, while splits up into components due to Misner singularities. Our definition permits us to choose between two conflicting claims concerning BTZ black holes in (3 + 1) dimensions.

  • 16.
    Benmakhlouf, Hamza
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Andreo, Pedro
    Stockholm University, Faculty of Science, Department of Physics. Karolinska Institutet, Sweden.
    Ten years after: Impact of recent research in photon and electron beam dosimetry on the IAEA TRS-398 Code of Practice2011In: Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS): Proceedings of an International Symposium. V. 1 / [ed] Benmakhlouf, H.; Andreo, P., Vienna: International Atomic Energy Agency, 2011, , p. 14p. 139-152Conference paper (Refereed)
  • 17.
    Bergholtz, Emil
    et al.
    Max Planck Institute for the Physics of Complex Systems, Dresden.
    Karlhede, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum Hall Circle2009In: Journal of Statistical Mechanics: Theory and Experiment, ISSN 1742-5468, Vol. P04015Article in journal (Refereed)
    Abstract [en]

    We consider spin-polarized electrons in a single Landau level on a cylinder as the circumference of the cylinder goes to infinity. This gives a model of interacting electrons on a circle where the momenta of the particles are restricted and there is no kinetic energy. Quantum Hall states are exact ground states for appropriate short range interactions, and there is a gap to excitations. These states develop adiabatically from this one-dimensional quantum Hall circle to the bulk quantum Hall states and further on into the Tao–Thouless states as the circumference goes to zero. For low filling fractions a gapless state is formed which we suggest is connected to the Wigner crystal expected in the bulk.

  • 18.
    Blanchfield, Kate
    Stockholm University, Faculty of Science, Department of Physics.
    Configurations in Quantum Information2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Measurements play a central role in quantum information. This thesis looksat two types: contextual measurements and symmetric measurements. Contextualityoriginates from the Kochen-Specker theorem about hidden variablemodels and has recently undergone a subtle shift in its manifestation. Symmetricmeasurements are characterised by the regular polytopes they formin Bloch space (the vector space containing all density matrices) and are thesubject of several investigations into their existence in all dimensions.We often describe measurements by the vectors in Hilbert space ontowhich our operators project. In this sense, both contextual and symmetricmeasurements are connected to special sets of vectors. These vectors areoften special for another reason: they form congurations in a given incidencegeometry.In this thesis, we aim to show various connections between congurationsand measurements in quantum information. The congurations discussedhere would have been well-known to 19th and 20th century geometers andwe show they are relevant for advances in quantum theory today. Specically,the Hesse and Reye congurations provide proofs of measurement contextuality,both in its original form and its newer guise. The Hesse congurationalso ties together dierent types of symmetric measurements in dimension3called SICs and MUBswhile giving insights into the group theoreticalproperties of higher dimensional symmetric measurements.

  • 19. Bouarroudj, Sofian
    et al.
    Grozman, Pavel
    Leites, Dimitry
    Stockholm University, Faculty of Science, Department of Mathematics.
    DEFINING RELATIONS OF ALMOST AFFINE (HYPERBOLIC) LIE SUPERALGEBRAS2010In: Journal of Nonlinear Mathematical Physics, ISSN 1402-9251, E-ISSN 1776-0852, Vol. 17, p. 163-168Article in journal (Refereed)
    Abstract [en]

    For all almost affine (hyperbolic) Lie superalgebras, the defining relations are computed in terms of their Chevalley generators.

  • 20. Chapovalov, Danil
    et al.
    Chapovalov, Maxim
    Stockholm University, Faculty of Science, Department of Mathematics.
    Lebedev, Alexei
    Leites, Dimitry
    Stockholm University, Faculty of Science, Department of Mathematics.
    THE CLASSIFICATION OF ALMOST AFFINE (HYPERBOLIC) LIE SUPERALGEBRAS2010In: Journal of Nonlinear Mathematical Physics, ISSN 1402-9251, E-ISSN 1776-0852, Vol. 17, p. 103-161Article in journal (Refereed)
    Abstract [en]

    We say that an indecomposable Cartan matrix A with entries in the ground field is almost affine if the Lie (super) algebra determined by it is not finite dimensional or affine (Kac-Moody) but the Lie sub(super) algebra determined by any submatrix of A, obtained by striking out any row and any column intersecting on the main diagonal, is the sum of finite dimensional or affine Lie (super) algebras. A Lie (super) algebra with Cartan matrix is said to be almost affine if it is not finite dimensional or affine (Kac-Moody), and all of its Cartan matrices are almost affine. We list all almost affine Lie superalgebras over complex numbers with indecomposable Cartan matrix correcting two earlier claims of classification.

  • 21.
    Chapovalov, Maxim
    et al.
    Stockholm University, Faculty of Science, Department of Mathematics.
    Leites, Dimitry
    Stockholm University, Faculty of Science, Department of Mathematics.
    Stekolshchik, Rafael
    THE POINCARE SERIES OF THE HYPERBOLIC COXETER GROUPS WITH FINITE VOLUME OF FUNDAMENTAL DOMAINS2010In: Journal of Nonlinear Mathematical Physics, ISSN 1402-9251, E-ISSN 1776-0852, Vol. 17, p. 169-215Article in journal (Refereed)
    Abstract [en]

    The discrete group generated by reflections of the sphere, or the Euclidean space, or hyperbolic space are said to be Coxeter groups of, respectively, spherical, or Euclidean, or hyperbolic type. The hyperbolic Coxeter groups are said to be (quasi-) Lanner if the tiles covering the space are of finite volume and all (resp. some of them) are compact. For any Coxeter group stratified by the length of its elements, the Poincare series is the generating function of the cardinalities of sets of elements of equal length. Around 1966, Solomon established that, for ANY Coxeter group, its Poincare series is a rational function with zeros somewhere on the unit circle centered at the origin, and gave an implicit (recurrence) formula. For the spherical and Euclidean Coxeter groups, the explicit expression of the Poincare series is well-known. The explicit answer was known for any 3-generated Coxeter group, and (with mistakes) for the Lanner groups. Here we give a lucid description of the numerator of the Poincare series of any Coxeter group, the explicit expression of the Poincare series for each Lanner and quasi-Lanner group, and review the scene. We give an interpretation of some coefficients of the denominator of the growth function. The non-real poles behave as in Enestrom's theorem (lie in a narrow annulus) though the coefficients of the denominators do not satisfy theorem's requirements.

  • 22.
    Dumitru, Irina
    Stockholm University, Faculty of Science, Department of Physics.
    Exploring the Elegant Bell Inequality2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In quantum information, device independent certification is a matter of both practical and fundamental interest. In this thesis, we explore the use of a particular Bell inequality, known as the elegant Bell inequality, in device independent certification. We first characterize all states and measurements that can lead to a maximal violation of the elegant Bell inequality. It turns out that, in all cases, the state involved in a maximal violation is a generalized singlet state, and the measurements of one of the parties are always maximally spread out on the Bloch sphere, forming a complete set of mutually unbiased bases. The measurements involved on the other party form two pairs of symmetric informationally complete vectors. The elegant Bell inequality, then, can be used to certify the presence of these elements.

    We also explore the usefulness of the elegant Bell inequality in randomness certification, in particular in a protocol for certification of maximal randomness from one entangled bit.

    The last part of this thesis is dedicated to a study of some of the special geometricstructures involved in the maximal violation of the elegant Bell inequality, namely the symmetric informationally complete vectors. The problem of the existence and of the construction of these sets of vectors in Hilbert spaces of any dimension is open, but there are solutions available in many dimensions. We look at these structures from both a geometric and an algebraic number theory perspective, and conjecture a relation between vectors in different dimensions. We introduce the relation of "alignment" between such sets of vectors.

  • 23.
    Fager, Marcus
    Stockholm University, Faculty of Science, Department of Physics.
    Radiobiological plan optimization in Proton therapy for Prostate tumors using a Patched Integrated Edge [PIE] technique2013Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Purpose: A novel treatment planning technique using proton pencil beam scanning (PBS) is proposed that takes advantage of the increased Linear Energy Transfer (LET) at the distal edge of proton beams to deposit the increased biological effective dose related to the elevated LET protons within prostate tumors.

    Background: The availability of proton treatment for cancer has increased the latest decade and will continue to increase rapidly in the coming decade. The Relative Biological Effectiveness (RBE) of protons earlier considered to be 1.10 has started to be questioned in the latest decade. This thesis investigates what would be the effect of a variable RBE on the effective dose to the target.

    Method: Uniform dose distributions were planned using two different beam arrangements: (1) Full-Target Plans (FTP), with two lateral fields, each field targeting the entire target; (2) Patched Integrated Edge (PIE) plans, with 2, 4, and 7 fields, each field targeting only the respective proximal segment of the target. Dose distributions were calculated and optimized with Eclipse in order to deliver the same dose to the target as well as to maintain the same OARs dose constrains for all the plans. Beam properties (range, modulation, spot map and weights) were used to calculate dose and dose averaged LET distributions with Monte Carlo. The RBE for each plan was calculated using radiobiological models taking into account the dose and LETd distribution as well as published values of  for the irradiated tissues as input parameters. The RBE weighted dose (DRBE) was calculated for each planning approach and evaluated with respect to three different aims.

    Results: An increase of the number of fields using PIE increased the LETd within the target. The increased LETd resulted in an increase of the RBE weighted dose, DRBE, of up to 12.7 Gy (RBE) to the target, which is a 14% increase. However, if the same DRBE is to be delivered to the target with FTP and PIE the increase of LETd in the target implied a decrease of dose per fraction, d, of up to 0.21 Gy, a decrease of 13 %.

    Conclusions: A modified distribution of proton’s linear energy transfer in PBS allows to deposit highly effective biological dose by the elevated-LET protons within the target, which might help to increase the effectiveness of prostate radiotherapy. This might also serve as a platform to investigate how the physical prescribed dose can be reduced by increasing the LETd in the target in order to maintain a constant DRBE in the prostate. 

  • 24. Gallos, Lazaros K.
    et al.
    Rybski, Diego
    Liljeros, Fredrik
    Stockholm University, Faculty of Social Sciences, Department of Sociology.
    Havlin, Shlomo
    Makse, Hernan A.
    How People Interact in Evolving Online Affiliation Networks2012In: PHYSICAL REVIEW X, ISSN 2160-3308, Vol. 2, no 3, p. 031014-Article in journal (Refereed)
    Abstract [en]

    The study of human interactions is of central importance for understanding the behavior of individuals, groups, and societies. Here, we observe the formation and evolution of networks by monitoring the addition of all new links, and we analyze quantitatively the tendencies used to create ties in these evolving online affiliation networks. We show that an accurate estimation of these probabilistic tendencies can be achieved only by following the time evolution of the network. Inferences about the reason for the existence of links using statistical analysis of network snapshots must therefore be made with great caution. Here, we start by characterizing every single link when the tie was established in the network. This information allows us to describe the probabilistic tendencies of tie formation and extract meaningful sociological conclusions. We also find significant differences in behavioral traits in the social tendencies among individuals according to their degree of activity, gender, age, popularity, and other attributes. For instance, in the particular data sets analyzed here, we find that women reciprocate connections 3 times as much as men and that this difference increases with age. Men tend to connect with the most popular people more often than women do, across all ages. On the other hand, triangular tie tendencies are similar, independent of gender, and show an increase with age. These results require further validation in other social settings. Our findings can be useful to build models of realistic social network structures and to discover the underlying laws that govern establishment of ties in evolving social networks.

  • 25. Gandhi, Sohang
    et al.
    McAllister, Sohang
    Sjörs, Stefan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    A Toolkit for Perturbing Flux Compactifications2011In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 12, p. 053-Article in journal (Refereed)
    Abstract [en]

    We develop a perturbative expansion scheme for solving general boundary value problems in a broad class of type IIB flux compactifications. The background solution is any conformally Calabi-Yau compactification with imaginary self-dual (ISD) fluxes. Upon expanding in small deviations from the ISD solution, the equations of motion simplify dramatically: we find a simple basis in which the n-th order equations take a triangular form. This structure implies that the system can be solved iteratively whenever the individual, uncoupled equations can be solved. We go on to demonstrate the solution of the system for a general warped Calabi-Yau cone: we present an algorithm that yields an explicit Green’s function solution for all the supergravity fields, to any desired order, in terms of the harmonic functions on the base of the cone. Our results provide a systematic procedure for obtaining the corrections to a warped throat geometry induced by attachment to a compact bulk. We also present a simple method for determining the sizes of physical effects mediated through warped geometries.

  • 26.
    Gustafsson, Peter
    et al.
    Mälardalens högskola, Sweden.
    Jonsson, Gunnar
    Karlstads universitet, Sweden.
    Enghag, Margareta
    Stockholm University, Faculty of Science, Department of Mathematics and Science Education.
    The problem-solving process in physics as observed when engineering students at university level work in groups2013In: European Journal of Engineering Education, ISSN 0304-3797, E-ISSN 1469-5898, Vol. 40, no 4, p. 380-399Article in journal (Refereed)
  • 27.
    Hameedi, Alley
    Stockholm University, Faculty of Science, Department of Physics.
    Quantum Random Access Codes & their Applications2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Based on the principles of quantum mechanics, quantum information is a highly interesting and fast emerging field which refers to processing information encoded into the state of a quantum system and the subsequent use of such quantum systems for various information tasks. In this thesis, we have studied the role of single d-level quantum systems (qudits) as a quantum resource in the context of a communication task, commonly known as random access codes (RACs). We investigate the advantage of quantum random access codes (QRACs), employing quantum systems of arbitrary dimensions as means of communication between the parties, in terms of average performances over their classical counterparts. For this purpose, a class of QRACs with dimension d=4 is focused upon. Additionally, these higher dimensional QRACs have also been studied in terms of applications where we consider their potential in generation of true randomness. Furthermore, a parallel implementation of two parallel QRACs (employing qubits) is explored as a resource for test of non-classicality of a physical system.

    Our results obtained show that QRACs outperform their classical counterparts performance wise. Moreover, this advantage over classical resources can be extended further by use of higher dimensional QRACs. The high-level QRACs lead to higher average success probabilities and more generated randomness as compared to classical RACs or QRACs of lower dimensions. Finally, an experimental test of non-classicality is demonstrated which allows for arbitrarily low detection efficiencies and does not invoke extra assumptions as lack of shared randomness between devices.

  • 28.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Combinatorial and Geometrical Structures of Multipartite Quantum Systems2009In: Mathematical Physics Research Developments / [ed] Morris B. Levy, New York: Nova Science Publishers , 2009, 1, p. 589-604Chapter in book (Other (popular science, discussion, etc.))
  • 29.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Different classes of quantum gates entanglers2009In: International journal of quantum information, ISSN 0219-7499, Vol. 7, no 1, p. 279-285Article in journal (Refereed)
    Abstract [en]

    We construct quantum gates entanglers for different classes of multipartite states. In particular, we construct entangler operators for W and GHZ classes of multipartite states based on the construction of the concurrence classes. We also discuss in detail these two classes of the quantum gates entanglers for three-partite states.

  • 30.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Generalized controlled phase quantum gates entanglers2009In: International journal of quantum information, ISSN 0219-7499, Vol. 7, no 6, p. 1211-1216Article in journal (Refereed)
    Abstract [en]

    We construct a generalized controlled phased gate entangler for a multi-qubit state based on the geometrical structure of quantum systems. We also investigate the relation between the generalized controlled phase construction of a quantum gate entangler and graph state for two-qubit and three-qubit states.

  • 31.
    Heydari, Hoshang
    Stockholm University, Faculty of Science, Department of Physics.
    Selective Phase Rotation Quantum Gate Entangler2009In: Open systems & information dynamics, ISSN 1230-1612, E-ISSN 1573-1324, Vol. 16, no 4, p. 407-412Article in journal (Refereed)
    Abstract [en]

    We construct a quantum gate entangler for multi-qubit states based on a selective phase rotation transform. In particular, we establish a relation between the quantum integral transform and the quantum gate entangler in terms of universal controlled gates for multi-qubit states. Our result gives an effective way of constructing topological and geometrical quantum gate entanglers for multipartite quantum systems, which could also lead to a construction of geometrical quantum algorithms.

  • 32.
    Hidvégi, Attila
    Stockholm University, Faculty of Science, Department of Physics.
    FPGA-based Instrumentation for Advanced Physics Experiments2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Modern physical experiments often demand advanced instrumentation based on advances in  technology. This work describes four instrumentation physics projects that are based on modern, high-capacity Field-Programmable Gate Arrays, making use of their versatility, programmability, high bandwidth communication interfaces and signal processing capabilities.

    In the first project, a jet-finding algorithm for the ATLAS detector at the LHC experiment at CERN was developed and implemented, and different verification methods were created to validate the functionality and reliability. The experiment uses a three level trigger system, where the first level uses custom FPGA-based hardware for analysis of collision events in real-time.

    The second project was an advanced timing and triggering distribution system for the new European X-Ray Free Electron Laser (XFEL) facility at DESY in Hamburg. XFEL will enable scientists to study nano structures on the atomic scale. Its laser pulses will have the strongest peak power in the world with extremely short duration and a high repetition rate, which will even allow filming of chemical reactions. The timing system uses modern FPGAs to distribute high-speed signals over optical fibers and to deliver clocks and triggers with high accuracy.

    The third project was a new data acquisition board based on high-speed ADCs combined with high-performance FPGAs, to process data from segmented Ge-detectors in real-time. The aim was to improve system performance by greatly oversampling and filtering the analog signals to achieve greater effective resolution.

    Finally, an innovative solution was developed to replace an aging system used at CERN and Stockholm University to test vital electronics in the Tile Calorimeters of the ATLAS detector system. The new system is entirely based on a commercial FPGA development board, where all necessary custom communication protocols were implemented in firmware to emulate obsolete hardware.

  • 33.
    Huang, Ningdong
    et al.
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Nordlund, Dennis
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Huang, Congcong
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA.
    Tyliszczak, Tolek
    LBL, Adv Light Source, Berkeley, CA 94720 USA .
    Weiss, Thomas M.
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA .
    Acremann, Yves
    Stanford Synchrotron Radiat Lab, Stanford, CA 94309 USA .
    Pettersson, Lars G.M.
    Stockholm University, Faculty of Science, Department of Physics.
    Nilsson, Anders
    Stockholm University, Faculty of Science, Department of Physics.
    Schlesinger, Daniel
    Stockholm University, Faculty of Science, Department of Physics.
    Microscopic Probing of the Size Dependence in Hydrophobic Solvation2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 7, p. 074507-Article in journal (Refereed)
    Abstract [en]

    A dependence on solute size of the hydrophobic effect has been proposed based on theory and simulations, such that small apolar solutes leave the hydrogen (H-) bonding network in water intact or even strengthened, whereas hydration of larger, nanometer-sized apolar solutes breaks hydrogen bonds and creates a liquid-vapor-like interface around the solutes. Here we report the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in aqueous solutions of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N + (TAA) cations with increased side chain length by probing the H-bonding network in water through O K-edge x-ray absorption spectroscopy and the solute-solute interaction using small angle x-ray scattering. These results open for unique experimental opportunities to investigate hydrophobic effects for a range of important processes in chemistry and biology.

    We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (CnH2n+1)4N+ (R4N+) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.

  • 34.
    Hultqvist, Martha
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gudowska, Irena
    Stockholm University, Faculty of Science, Department of Physics.
    Secondary absorbed doses from light ion irradiation in anthropomorphic phantoms representing an adult male and a 10 year old child2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 22, p. 6633-6653Article in journal (Refereed)
    Abstract [en]

    Secondary organ absorbed doses were calculated by Monte Carlo simulations with the SHIELD-HIT07 code coupled with the mathematical anthropomorphic phantoms CHILD-HIT and ADAM-HIT. The simulated irradiations were performed with primary 1H, 4He, 7Li, 12C and 16O ion beams in the energy range 100–400 MeV/u which were directly impinging on the phantoms, i.e. approximating scanned beams, and with a simplified beamline for 12C irradiation. The evaluated absorbed doses to the out-of-field organs were in the range 10−6 to 10−1 mGy per target Gy and with standard deviations 0.5–20%. While the contribution to the organ absorbed doses from secondary neutrons dominated in the ion beams of low atomic number Z, the produced charged fragments and their subsequent charged secondaries of higher generations became increasingly important for the secondary dose delivery as Z of the primary ions increased. As compared to the simulated scanned 12C ion beam, the implementation of a simplified beamline for prostate irradiation with 12C ions resulted in an increase of 2–50 times in the organ absorbed doses depending on the distance from the target volume. Comparison of secondary organ absorbed doses delivered by 1H and 12C beams showed smaller differences when the RBE for local tumor control of the ions was considered and normalization to the RBE-weighted dose to the target was performed.

    General scientific summary. During light ion therapy, the production of nuclear fragments results in a complex secondary radiation field which the organs and normal tissues of the patient are exposed to. In the present work, the absorbed doses to out-of-field organs and the energy distribution of secondary particle fluences in anthropomorphic phantoms have been simulated by the Monte Carlo code SHIELD-HIT07 for brain tumor and prostate irradiation with approximated scanned beams of 1H, 4He, 7Li, 12C and 16O ions in the energy range 100–400 MeV/u, as well as with a simplified beam line for 12C irradiation. The evaluated organ absorbed doses were in the range 10−6 to 10−1 mGy per target Gy. The absorbed dose contribution from secondary neutrons dominated in the ion beams of low atomic number Z, while the produced charged fragments and their subsequent charged secondaries became increasingly important for the secondary dose delivery as Z of the primary ion increased.

  • 35.
    Hultqvist, Martha
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Gudowska, Irena
    Stockholm University, Faculty of Science, Department of Physics.
    Secondary doses in anthropomorphic phantoms irradiated with light ion beams2009In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 168, no 1, p. 123-127Article in journal (Refereed)
    Abstract [en]

    The mathematical anthropomorphic phantoms EVA-HIT and ADAM-HIT have been used in the Monte Carlo code SHIELD-HIT07 for simulations of lung tumor and prostate irradiation with light ions. Calculations were performed for 1H, 7Li, and 12C beams of energies in the range of 80 to 330 MeV/u. The secondary doses to organs, due to scattered primary ions and secondary particles produced in the phantoms, were studied taking into account the contribution from secondary neutrons, secondary protons, pions, and heavier fragments from helium to calcium. The doses to organs per dose to target (tumor) are of the order of 10-6 to 10-1 mGy Gy-1 and decrease with increasing distance from the target. In general the organ dose per target dose increases with increasing Z of the primary particle; however, for lighter primary ions (Z 3) and for organs close to the target, scattered primary particles show a nonnegligible dose contribution.

  • 36.
    Högås, Marcus
    Stockholm University, Faculty of Science, Department of Physics.
    On the consistency of multigravity theories2016Independent thesis Advanced level (degree of Master (Two Years)), 30 credits / 45 HE creditsStudent thesis
    Abstract [en]

    In this thesis a set of recently proposed multigravity theories is analysed. In the special case of bimetric gravity, the theory has been conclusively shown to be ghost-free. On the other hand, for multigravity theories in general, the ghost-issue has not been settled conclusively. Motivated by this fact, the main object of this thesis is to clarify what has been proven so far and what issues that still needs to be addressed. We also provide new calculations and results pointing in the direction that the multigravity theories must be restricted to a set of bimetric Hassan-Rosen couplings in a tree-type structure in order to be consistent. In particular, we prove that for a multivielbein theory of  interacting vielbeins, the Lorentz equations of motion is a set of  Deser-van Nieuwenhuizen conditions if and only if the theory consists of bimetric Hassan-Rosen couplings in a tree-type structure.

  • 37. Iyer, Uma N.
    et al.
    Lebedev, Alexei
    Leites, Dimitry
    Stockholm University, Faculty of Science, Department of Mathematics.
    PROLONGS OF (ORTHO-)ORTHOGONAL LIE (SUPER)ALGEBRAS IN CHARACTERISTIC 22010In: Journal of Nonlinear Mathematical Physics, ISSN 1402-9251, E-ISSN 1776-0852, Vol. 17, p. 253-309Article in journal (Refereed)
    Abstract [en]

    Cartan described some of the finite dimensional simple Lie algebras and three of the four series of simple infinite dimensional vectorial Lie algebras with polynomial coefficients as prolongs, which now bear his name. The rest of the simple Lie algebras of these two types (finite dimensional and vectorial) are, if the depth of their grading is greater than 1, results of generalized Cartan-Tanaka-Shchepochkina (CTS) prolongs. Here we are looking for new examples of simple finite dimensional modular Lie (super) algebras in characteristic 2 obtained as Cartan prolongs. We consider pairs (an (ortho-)orthogonal Lie (super) algebra or its derived algebra, its irreducible module) and compute the Cartan prolongs of such pairs. The derived algebras of these prolongs are simple Lie (super) algebras. We point out several amazing phenomena in characteristic 2: a supersymmetry of representations of certain Lie algebras, latent or hidden over complex numbers, becomes manifest; the adjoint representation of some simple Lie superalgebras is not irreducible.

  • 38. Iyer, Uma N.
    et al.
    Leites, Dimitry
    Stockholm University, Faculty of Science, Department of Mathematics.
    Messaoudene, Mohamed
    Shchepochkina, Irina
    EXAMPLES OF SIMPLE VECTORIAL LIE ALGEBRAS IN CHARACTERISTIC 22010In: Journal of Nonlinear Mathematical Physics, ISSN 1402-9251, E-ISSN 1776-0852, Vol. 17, p. 311-374Article in journal (Refereed)
    Abstract [en]

    The classification of simple finite dimensional modular Lie algebras over algebraically closed fields of characteristic p > 3 (described by the generalized Kostrikin-Shafarevich conjecture) being completed due to Block, Wilson, Premet and Strade (with contributions from other researchers) the next major classification problems are those of simple finite dimensional modular Lie algebras over fields of characteristic 3 and 2. For the latter, the Kochetkov-Leites conjecture involved classification of Lie superalgebras and their inhomogeneous with respect to parity subalgebras, called Volichenko algebras. In characteristic 2, we consider the result of application of the functor forgetting the superstructure to the simple serial vectorial Lie algebras known to us and their Volichenko subalgebras.

  • 39.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Black holes and trapped surfaces2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The study of black holes is an important part of general relativity. However, the very definition of black holes is not completely satisfactory. Alternative definitions are based on the concept of trapped surfaces. This licentiate thesis is based on work with the aim to better understand the behaviour of such trapped surfaces.

    The standard definition of a black hole and specific examples are reviewed, as well as the definition of trapped surfaces, various horizons related to trapped surfaces, and the trapping boundary. This serves as an introduction to two published papers. The first paper provides an exact model of a marginally trapped tube making a sudden jump outwards as matter is falling into the black hole. The second paper concerns the question of the location of the trapping boundary in the Oppenheimer-Snyder black hole.

  • 40.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    How trapped surfaces jump in 2+1 dimensions2013In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 30, no 6, article id 065022Article in journal (Refereed)
    Abstract [en]

    When a lump of matter falls into a black hole it is expected that a marginally trapped tube when hit moves outwards everywhere, even in regions not yet in causal contact with the infalling matter. But to describe this phenomenon analytically in 3+1 dimensions is difficult since gravitational radiation is emitted. By considering a particle falling into a toy model of a black hole in 2+1 dimensions an exact description of this non-local behaviour of a marginally trapped tube is found.

  • 41.
    Jakobsson, Emma
    Stockholm University, Faculty of Science, Department of Physics.
    Shapes of Spacetimes: Collected tales of black holes2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In theory, the existence of black holes is predicted by general relativity. In reality, there is a general consensus that they exist in space; in particular at the center of many galaxies. The theory of black holes has been around for decades, but there are still interesting questions calling for attention. This doctoral thesis and its four contributions touches upon some of these questions.

    One challenging theoretical aspect of black holes lies in their definition, the event horizon. For several reasons, this definition is not satisfactory in many contexts, and alternative horizons based on the concept of trapped surfaces have been suggested to take its place. The question raised in Paper I has to do with the location of such surfaces in a simple model of gravitational collapse, the Oppenheimer-Snyder model.

    A different scenario of gravitational collapse, that of a null shell of dust collapsing in flat spacetime, is the starting point of the original formulation of the Penrose inequality. By a reformulation, this inequality can be turned into a purely geometric relation in Minkowski space. In Paper IV we formulate and prove a (2+1)-dimensional version in anti-de Sitter space.

    The Penrose inequality sometimes goes under the name of the "isoperimetric inequality for black holes". In Paper III a different kind of isoperimetric inequality is discussed (with less rigour), namely that of the volume contained in a black hole with a given area.

    In Paper II, the subject of limits of spacetimes is visualized. Again, (2+1)-dimensional anti-de Sitter space finds its use, as a one parameter family of surfaces, capturing the geometry of charged black hole spacetimes, is embedded in it. Thus different limiting procedures are illustrated.

    Finally, interesting models can be constructed by cutting and gluing in spacetimes, but in doing so one needs to take care, in order to obtain a physically realistic model. With this background as motivation, a study of Lorentzian cones is given.

    Taken together, all of these contributions make up a collection of interesting aspects of black hole geometry, or, shapes of spacetimes.

  • 42.
    Jenei, István Zoltán
    Stockholm University, Faculty of Science, Department of Physics.
    Scanning electron microscopy (SEM) analysis of tribofilms enhanced by fullerene-like nanoparticles2012Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The beneficiary effects of WS2 inorganic fullerene-like nanoparticles in the lubrication industry were shown in recent years. The incorporation of the nanoparticles into lubricants (oils, greases) is however not straightforward.

    When two surfaces are sliding against each other and a lubricant is used, a thin layer (tribofilm) is formed on the contact area, which effects the friction process. Lubricants usually contain several additives. These additives can impair the friction reducing behaviour of the WS2 inorganic fullerene-like nanoparticles. This thesis investigates the effects of several additives in the lubrication process by analysing the tribofilms formed on the worn surfaces using energy-dispersive X-ray spectroscopy in a scanning electron microscope.

  • 43.
    Jenei, Zsolt
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Visbeck, Ken
    Lawrence Livermore National Laboratory.
    Cynn, Hyunchae
    Lawrence Livermore National Laboratory.
    Yoo, Choong-Shik
    Washington State University.
    Evans, William
    Lawrence Livermore National Laboratory.
    High temperature experiments using a resistively heated membrane driven diamond anvil cellManuscript (Other academic)
    Abstract [en]

     

    A reliable high-performance heating method using resistive heaters and a membrane driven diamond anvil cell (mDAC) is presented. Two micro-heaters are mounted in a mDAC and use electrical power of less than 150 W to achieve sample temperatures up to 1200 K. For temperature measurement we use two K-type thermocouples mounted near the sample. The approach can be used for in-situ Raman spectroscopy and x-ray diffraction at high pressures and temperatures. A W-Re alloy gasket material permits stable operation of mDAC at high temperature. Using this method, we made an isothermal compression at 900 K to pressures in excess of 100 GPa and isobaric heating at 95 GPa to temperatures in excess of 1000 K. As an example, we present high temperature Raman spectroscopy measurements of nitrogen at high pressures

  • 44.
    Jonsson, Cathrine
    Stockholm University.
    Advances in quantitative emission tomography: development and analysis of methods for validation and correction2000Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The nuclear medicine imaging techniques SPECT and PET are sensitive tools in mapping various physiological and biochemical processes in vivo. This work was devoted to investigations of methodological and physical properties of the SPECT technique as well as developing methods for validation and correction. Brain imaging was the major focus in this work, but the results are applicable to other examination areas. In the first part regional cerebral blood flow (rCBF) as measured by PET - the golden standard - and by SPECT was compared using a computerised brain atlas. Overall the differences were surprisingly small but revealed an apparent reduced grey-to-white matter ratio for SPECT, mainly due to the lack of scatter correction in SPECT, a lower spatial resolution and a non-linear extraction of the SPECT radiopharmaceutical. Furthermore, the reproducibility of resting state rCBF with SPECT was investigated, imperative information whenever scans are being repeated, as in activation studies, or when comparing an individual scan to a database of scans from healthy individuals. The results indicate that normalised flow data show high intra- and inter-individual reproducibility, expressed as standard deviations, ±1.3% and ±2.9%, respectively. The major error was addressed to the methodology, i.e. the scanning procedure and the atlas adaptation.

    At that stage of the study, using a clinical protocol, without scatter correction and proper attenuation correction there was no possibility to extract quantitative information from the SPECT images. Therefore, in the subsequent work, a phantom concept (the stack phantom) was designed aiming at experimental validation of the SPECT methodology, including effects of photon interactions. Extracting information where the primary (unscattered) photons are separated from the ones undergoing scattering or attenuation before detection, was, until now, only possible using Monte Carlo simulations. The basis of the novel phantom concept is to sample various 3D-activity distributions from a set of 2D samples. The 2D samples are ordinary paper sheets where the cross sectional radioactivity distribution is printed out using radioactive ink. Mounting the samples together with a certain equidistant axial spacing, using either tissue equivalent material or some low-density material, give possibilities to mimick the very same activity distribution clinically (with degrading photon interactions) as well as almost "pure" primary photon images. The phantom concept is very flexible, any activity distribution may be constructed, pathological patterns can easily be introduced and varied. Moreover, correction methods and software evaluation tools may be assessed and validated with the phantom. However, the phantom has not only applicability in research areas - recurrent quality assurance programs can as well make use of dedicated stack phantoms.

    In the last phase of this work a scatter correction algorithm was developed, making use of the stack phantom. The correction was developed using a multi-spectral acquisition system. This allowed for spectral acquisitions in each pixel in the projection images. After correcting for the scatter contribution in the upper half of the photo-peak using the shape of the scatter distribution from the Klein-Nishina cross section the resulting estimate of primary photons was mirrored (folded) over to estimate the primary photons in the lower half of the photo-peak. The scatter correction works in each spectrum and hence corrects for the scatter contribution locally. The correction method was validated both with Monte Carlo simulations and the stack-phantom, both indicating accurate results.

  • 45.
    Khan, Shehryar
    Stockholm University, Faculty of Science, Department of Physics.
    Combined Quantum Mechanical and Molecular Dynamics study of paramagnetic complexes: Towards an understanding of electronic spin relaxation2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The prime objectives of contrast agents in Magnetic Resonance Imaging (MRI) is to accelerate the relaxation rate of the solvent water protons in the surrounding tissue. Paramagnetic relaxation originates from dipole-dipole interactions between the nuclear spins and the fluctuating magnetic field induced by unpaired electrons. Currently Gadolinium(III) chelates are the most widely used contrast agents in MRI, and therefore it is incumbent to extend the fundamental theoretical understanding of parameters that drive the relaxation mechanism in these complexes. In compounds such as Gadolinium(III) complexes with total electron spins higher than 1 (in this case S=7/2) the Zero-Field Splitting (ZFS) plays a significant role in influencing the electron spin dynamics and nuclear spin dynamics. For this purpose, the current research delves into an understanding of the relaxation process, focusing on ZFS in various complexes of interest, using multi-scale modelling by combining quantum, semi-quantum and newtonian methods.

    We compare and contrast Density Function Theory (DFT) with multi-configurational quantum chemical calculation and find that DFT is highly functional dependent and unreliable in accurately reproducing experimental data for the static ZFS. It was found that long-range corrected functionals (in particular LC-BLYP) perform significantly better as compared to other functionals in predicting the magnitude of the static ZFS. We study hydrated Gd(III) and Eu(II) systems to compare and contrast these isoelectronic complexes (both contain 7 unpaired electrons in their valence shell) and through ab-initio molecular dynamics (AIMD) sampling followed by multi-reference quantum chemical calculations, it was established that inclusion of the first shell has a dominant influence (over 90%) on the ZFS. We also studied the complex [Gd(III)(HPDO3A)(H2O)], which is of clinical relevance as a contrast agent for MRI, through post-Hartree-Fock and DFT calculations by utilizing configurations derived from AIMD trajectories. From the fluctuations in the ZFS tensor, we extract a correlation time of the transient ZFS which is on the sub-picosecond time scale, showing a faster decay than experimental data.

  • 46.
    Kimiaei, Shahrokh
    Stockholm University.
    Improvements of SPECT by a new collimator design and simultaneous transmission-emission tomography1997Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    SPECT (Single Photon Emission Computed Tomography) and planar scintigraphy are the most established and world wide-spread nuclear medicine imaging techniques for clinical use today. SPECT is a tomographic technique that allows 3-D visualization of biochemical processes or physiological flow in the human body by external detection of photons from an administered radiopharmaceutical. However, its ability to depict a "true" activity map depends largely on the imaging properties of the SPECT-system as well as on the methods used for converting the acquired data into values reflecting the activity distribution. Photon attenuation, as well as the contribution of "false" (scattered) events are two of the most disturbing factors for achieving high quantitative accuracy in SPECT. Since these factors are related to the density and composition of the body tissues, it is crucial to have access to individual attenuation maps when high quantitative accuracy is desired.

    Two methods for simultaneous acquisition of emission and transmission data have been developed and analyzed. These methods may either be applied on single-head cameras or on opposed dual-head camera systems. Advantages and disadvantages of various radio-nuclides for use in transmission tomography have been analyzed. Their activity distribution or their scanning speed was of great importance for minimizing the image noise. By careful collimation of the external photon sources for the transmission measurements and by introducing accurate routines of corrections for camera non-uniformity and interferences of photons during simultaneous emission and transmission tomography, accurate attenuation maps can be obtained.

    In the second part of this work, a new collimator was developed in order to reduce the gap between a conventional camera and the convex shape of most body regions. These new collimators are designed planar in one direction and concave in the other which improves the radial spatial resolution and reduces the non-isotropic blur in SPECT. An improved lateral spatial resolution in planar scintigraphy was achieved as well. The impact on imaging quality was investigated by Monte Carlo simulations. The non-isotropic image blurring was reduced by up to 60% for a PC-collimator as compared to that for a conventional collimator. The image noise distribution in SPECT was more uniform but higher than for a planar collimator due to the reduced lateral sensitivity. Simulations of a Hoffman brain phantom showed that the rCBF values achieved with an optimized PC-collimator, were up to 10% higher in the lateral cortex (Brodmann areas 18,19), than those obtained with a planar collimator. Finally, a combination of a planar-concave collimator and a transmission tomography system described may give the same advantages of low noise and reduced requirements on high dynamic range of the camera, as was obtained for photon sources with non-linear activity distributions.

  • 47.
    Laura, Antonovic
    Stockholm University, Faculty of Science, Department of Physics.
    Radiobiological end-points for the theoretical evaluation of the effectiveness of carbon ions and photons in treating tumours with dynamic hypoxia2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Tumours are characterised by unorganised vasculature, which often results in hypoxic regions. Hypoxia is a common cause for photon radiotherapy (RT) treatment failure, as hypoxic cells require up to 2-3 times higher doses compared to well-oxygenated cells for the same effect in terms of cell kill. The increase in dose that would be required to treat the tumours of cancer patients is limited by the radiation sensitivity of surrounding normal tissues. Using carbon ions instead of photons, the radiation dose can be conformed to the tumour to a much higher degree, resulting in an improved sparing of normal tissues. In addition, carbon ions have a much higher radiobiological effectiveness near the end of their range, which is positioned in the tumour. Also, the radiation modes of action leading to cell death when carbon ions interact with living tissues, are less sensitive to the oxygen status compared with the action modes of photons.

    The focus of this thesis lies in the development of models for the computation of the cell surviving fraction and tumour control probability (TCP) in hypoxic tumours after photon and carbon ion RT. The impact of fractionation was evaluated with regard to possible spatial changes in oxygenation, both for stereotactic body RT and for carbon ion RT. The feasibility of a method to determine and deliver the optimal photon dose for achieving a high TCP according to spatial variations in radiation sensitivity was evaluated in a treatment planning study. The radiobiological models were finally used for the theoretical quantification of the gain in using carbon ions instead of photons.

    The results show that there are great possibilities to increase the number of positive outcomes of radiation treatment of tumours if the key influential factors are taken into account, such as level and distribution of hypoxia, radiation quality and choice of fractionation schedule.

  • 48. Lee, Sungmin
    et al.
    Holme, Petter
    Stockholm University, Faculty of Social Sciences, Department of Sociology.
    Wu, Zhi-Xi
    Cooperation, structure, and hierarchy in multiadaptive games2011In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 84, no 6, p. 61148-Article in journal (Refereed)
    Abstract [en]

    Game-theoretical models where the rules of the game and the interaction structure both coevolve with the game dynamics-multiadaptive games-capture very flexible situations where cooperation among selfish agents can emerge. In this work, we will discuss a multiadaptive model presented in a recent Letter [Phys. Rev. Lett. 106, 028702 (2011)] as well as generalizations of it. The model captures a nonequilibrium situation where social unrest increases the incentive to cooperate and, simultaneously, agents are partly free to influence with whom they interact. First, we investigate the details of how feedback from the behavior of agents determines the emergence of cooperation and hierarchical contact structures. We also study the stability of the system to different types of noise, and find that different regions of parameter space show very different response. Some types of noise can destroy an all-cooperator (C) state. If, on the other hand, hubs are stable, then so is the all-C state. Finally, we investigate the dependence of the ratio between the time scales of strategy updates and the evolution of the interaction structure. We find that a comparatively fast strategy dynamics is a prerequisite for the emergence of cooperation.

  • 49.
    Liljequist, David
    Stockholm University, Faculty of Science, Department of Physics.
    Limits of validity of trajectory simulation: correlation of the error with density of scatterers and particle wavelength2009In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 267, p. 3409-3419Article in journal (Refereed)
    Abstract [en]

    To a first approximation, the elastic scattering of long wavelength particles in amorphous matter may be modelled as scattering in a volume filled with a density n of N point scatterers in random positions. For not too large N (up to about 2×103), the error in trajectory simulation (classical transport theory) due to the neglect of interference effects can then be determined in detail by means of a comparison with an exact quantum calculation of the plural or multiple scattering process. A relative error RE is defined and calculated for the scattering in different directions as well as for the distribution of scattering events inside the volume. A very strong correlation is found between the relative error and the ratio λ/dnn, where λ is the wavelength of the incident particle and dnn=n-1/3 is an average distance between nearest neighbour scatterers. For scattering in a volume of dimensions large compared to the particle wavelength, present calculations suggest that the correlation can be described as REa·(λ/dnn)b, where the parameters a<0.05 and b2 depend on the s-wave phaseshift δ0 in the scattering process. The condition for validity of trajectory simulation, defined in terms of a limit of validity L (maximum acceptable relative error), may thus be written λ/dnn<ξ, where ξ=(L/a)1/b1. For λ/dnn<1, the relative error is generally less than 5%, and trajectory simulation may be regarded as valid with at least 95% accuracy. In the exact quantum calculation, two features of pronounced quantum character are observed in the distribution of scattering events: oscillations due to quantum interference in finite volumes, and, for small negative δ0, randomly localized peaks due to proximity resonance.

  • 50.
    Lindblom, Emely
    Stockholm University, Faculty of Science, Department of Physics.
    Searching for the optimal radiotherapy treatment time, dose and fractionation - the role of hypoxia and reoxygenation: A modelling study2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The search for the optimal choice of treatment time, dose and fractionation regimen is one of the major challenges in radiation therapy. Several aspects of the radiation response of tumours and normal tissues give different indications of how the parameters defining a fractionation schedule should be altered relative to each other which often results in contradictory conclusions. For example, the increased sensitivity to fractionation in late-reacting as opposed to early-reacting tissues indicates that a large number of fractions is beneficial, while the issue of accelerated repopulation of tumour cells starting at about three weeks into a radiotherapy treatment would suggest as short overall treatment time as possible. Another tumour-to-normal tissue differential relevant to the sensitivity as well as the fractionation and overall treatment time is the issue of tumour hypoxia and reoxygenation.

    The tumour oxygenation is one of the most influential factors impacting on the outcome of many types of treatment modalities. Hypoxic cells are up to three times as resistant to radiation as well oxygenated cells, presenting a significant obstacle to overcome in radiotherapy as solid tumours often contain hypoxic areas as a result of their poorly functioning vasculature. Furthermore, the oxygenation is highly dynamic, with changes being observed both from fraction to fraction and over a time period of weeks as a result of fast and slow reoxygenation of acute and chronic hypoxia. With an increasing number of patients treated with hypofractionated stereotactic body radiotherapy (SBRT), the clinical implications of a substantially reduced number of fractions and hence also treatment time thus have to be evaluated with respect to the oxygenation status of the tumour.

    The perhaps most promising tool available for the type of study aiming at determining the optimal SBRT approach with respect to fractionation is radiobiological modelling. With clinically-derived tissue-specific radiobiological parameters and well-established survival models, in silico modelling offers a wide range of opportunities to test various hypotheses with respect to time, dose, fractionation and details of the tumour microenvironment. Any type of radiobiological modelling study intended to provide a realistic representation of a clinical tumour should therefore take into account details of both the spatial and temporal tumour oxygenation.

    This thesis, consisting of papers I-III and a summary, presents the results of three-dimensional radiobiological modelling of the response of tumours with heterogeneous oxygenation to various radiation qualities, fractionation schemes, oxygenation levels and dynamics using different survival models. The results of this work indicate that hypoxia and its dynamics play a major role in the outcome of both photon and carbon ion radiotherapy, and that neglecting the oxygenation status of tumours treated with SBRT may compromise the treatment outcome substantially. Continued to include clinical studies on the impact of hypoxia on the treatment outcome in lung cancer patients treated with SBRT, this project will hopefully advance the evolution towards routinely incorporating functional imaging of hypoxia into treatment planning. This is ultimately expected to result in increased levels of local control with more patients being cured from their cancer.

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