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  • 1.
    Ashman, J
    et al.
    Department of Physics, University, Sheffield, United Kingdom.
    Lettenström, Frans
    Gustav Werners Institut, University, Uppsala, Sweden.
    Womersley, WJ
    Nuclear Physics Laboratory, University, Oxford, United Kingdom.
    Exclusive ρ0 and Φ production in deep inelastic muon scattering1988In: Zeitschrift für Physik C Particles and Fields, ISSN 0170-9739, E-ISSN 1431-5858, Vol. 39, no 2, p. 169-175Article in journal (Refereed)
    Abstract [en]

    Data are presented on exclusive ρ0 and φ{symbol} production in deep inelastic muon scattering from a target consisting mainly of nitrogen. The ratio of the total cross sections for ρ0 and φ{symbol} production is found to be 9:(1.6±0.4) at 〈Q2〉=7.5 GeV2, consistent with the SU(3) prediction of 9:2. The t dependence for exclusive ρ0 production is found to become shallover as Q2 increases and, for large Q2, the t dependence is typical of that for a hard scattering process. Furthermore, the ratio of the cross sections for coherent: incoherent production from nitrogen is found to decrease rapidly with Q2. Such behaviour indicates that even for exclusive vector meson production the virtual photon behaves predominantly as an electromagnetic probe.

  • 2.
    Birgersson, Evert
    et al.
    Örebro University, School of Science and Technology.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Oberstedt, Stephan
    Hambsch, Franz-Josef
    Properties of the reaction 238U(n, f) at the vibrational resonances2009In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 817, no 1-4, p. 1-34Article in journal (Refereed)
    Abstract [en]

    Recent fission cross-section calculations for the reaction 238U(n, f ), based on an extended statistical model, predict a significant change of fission fragment properties, such as the mean mass by A = 1.5 and a notable increase in total kinetic energy in the region of the vibrational resonance at an incident neutron energy En = 0.9 MeV. This model includes individual fission cross-sections by the asymmetric standard 1 (S1) and standard 2 (S2) as well as the symmetric super-long (SL) mode. In order to verify the model predictions, a dedicated experiment on 238U has been carried out to measure fission-fragment mass yield distributions for incident neutron energies from En = 2.0 MeV down to 0.9 MeV, where the fission characteristics at the vibrational resonance at En = 0.9 MeV were investigated for the first time. The previously reported distinct structure in the angular anisotropy around En = 1.2 and 1.6 MeV wasobserved at En = 0.9 MeV as well. The predicted large changes in fission fragment mass yield and total kinetic energy could not be confirmed. In the resonance the mean total kinetic energy is only about 0.5 MeV higher than at En = 1.8 MeV. At the same time, a slight decrease of the mean heavy fragment mass was observed, probably indicating a slightly increased contribution of the S1 fission mode.

  • 3.
    Birgersson, Evert
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Rochman, Dimitri
    Tsekhanovich, Igor
    Raman, Subramanian
    Light fission-fragment mass distribution from the reaction 251Cf(nth, f)2007In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 791, no 1-2, p. 1-23Article in journal (Refereed)
    Abstract [en]

    For mass numbers A = 80 to 124 the recoil mass spectrometer LOHENGRIN of the Institute Laue-Langevin in Grenoble was used to measure with high resolution the light fission-fragment mass yields and kinetic energy distributions from thermal-neutron induced fission of 252Cf* for the first time, using 251Cf as target material. The obtained mean light fragment mass AL = (107 ± 2) and the corresponding mean kinetic energy Ek,L = (103±2) MeV are within the expected trend. Emission yields around A = 115 are enhanced and the corresponding mean kinetic energy is higher compared to spontaneous fission of 252Cf. This could be explained by the existence of an additional super-deformed fission mode.

  • 4.
    Birgersson, Evert
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Rochman, Dimitri
    Tsekhanovitsch, Igor
    Binary fission-fragment yields from the reaction 251Cf(nth, f)2005In: Nuclear fission and fission-product spectroscopy: 3rd international workshop on nuclear fission and fission-product spectroscopy, American Institute of Physics , 2005, p. 349-352Conference paper (Refereed)
    Abstract [en]

    The recoil mass spectrometer LOHENGRIN of the Laue-Langevin Institute, Grenoble has been used to measure the light fission-fragment mass yield and kinetic energy distributions from neutron-induced 252Cf*, using 251Cf as target material. ©2005 American Institute of Physics

  • 5.
    Björnsson, Jonas
    et al.
    Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK.
    Fjelstad, Jens
    Department of Physics, Nanjing University, Nanjing, China.
    Modular invariant partition functions for noncompact G/Ad(H) models2011In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 83, no 8, article id 086007Article in journal (Refereed)
    Abstract [en]

    We propose a spectrum for a class of gauged non-compact G/Ad(H) WZNW models, including spectrally flowed images of highest, lowest, and mixed extremal weight modules. These are combined into blocks whose characters, due to the Lorentzian signature of the target space, are divergent and treated as formal expressions in need of regularisation. Assuming that this is possible, we show that these extended characters transform linearly under modular transformations, and can be used to write down modular invariant partition functions.

  • 6.
    Chen, Gang
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Cheung, Yeuk-Kwan E.
    Department of Physics, Nanjing University, Nanjing, China.
    Fan, Zheyong
    Department of Physics, Nanjing University, Nanjing, China.
    Fjelstad, Jens
    Faculty of Technology and Science, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Faculty of Technology and Science, Karlstad University, Karlstad, Sweden.
    No-ghost theorem for the bosonic Nappi-Witten string2009In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 80, no 8, article id 086003Article in journal (Refereed)
    Abstract [en]

    We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten spacetime. This is achieved in two steps. We first demonstrate unitarity for a class of NW/U(1) modules: the norm of any state which is primary with respect to a chosen timelike U(1) is non-negative. We then show that physical states - states satisfying the Virasoro constraints - in a class of modules of an affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules. Similar to the case of strings on AdS3, in order to saturate the spectrum obtained in light-cone quantization we are led to include modules with energy not bounded from below, which are related to modules with energy bounded from below by spectral flow automorphisms.

  • 7.
    Fjelstad, Jens
    Department of Physics, Karlstad University, Karlstad, Sweden.
    On 7-D TFT and 6-D chiral CFT2002Manuscript (preprint) (Other academic)
    Abstract [en]

    We perform a canonical and BRST analysis of a seven-dimensional Chern-Simons theory on a manifold with boundary. The main result is that the 7D theory induces for consistency a chiral two-form on the 6D boundary. We also comment on similar behaviour in a five-dimensional Chern-Simons theory relevant for $\N=4$ supersymmetric Yang-Mills theory in four dimensions.

  • 8.
    Fjelstad, Jens
    Department of Physics, Nanjing University, Nanjing, China.
    On duality and extended chiral symmetry in the SL(2,R) WZW model2011In: Journal of Physics A: Mathematical and General, ISSN 0305-4470, E-ISSN 1361-6447, Vol. 44, no 23, article id 235404Article in journal (Refereed)
    Abstract [en]

    Two chiral aspects of the SL(2,R) WZW model in an operator formalism are investigated. First, the meaning of duality, or conjugation, of primary fields is clarified. On a class of modules obtained from the discrete series it is shown, by looking at spaces of two-point conformal blocks, that a natural definition of contragredient module provides a suitable notion of conjugation of primary fields, consistent with known two-point functions. We find strong indications that an apparent contradiction with the Clebsch-Gordan series of SL(2,R), and proposed fusion rules, is explained by nonsemisimplicity of a certain category. Second, results indicating an infinite cyclic simple current group, corresponding to spectral flow automorphisms, are presented. In particular, the subgroup corresponding to even spectral flow provides part of a hypothetical extended chiral algebra resulting in proposed modular invariant bulk spectra.

  • 9.
    Fjelstad, Jens
    et al.
    Karlstad University, Karlstad, Swede.
    Fuchs, J.
    Karlstad University, Karlstad, Swede.
    Hwang, S.
    Karlstad University, Karlstad, Swede.
    Semikhatov, A. M.
    Lebedev Institute, Moscow, Russian Federation.
    Tipunin, I. Yu.
    Lebedev Institute, Moscow, Russian Federation.
    Logarithmic conformal field theories via logarithmic deformations2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 633, no 3, p. 379-413Article in journal (Refereed)
    Abstract [en]

    We construct logarithmic conformal field theories starting from an ordinary conformal field theory -- with a chiral algebra C and the corresponding space of states V -- via a two-step construction: i) deforming the chiral algebra representation on V\tensor End K[[z,1/z]], where K is an auxiliary finite-dimensional vector space, and ii) extending C by operators corresponding to the endomorphisms End K. For K=C^2, with End K being the two-dimensional Clifford algebra, our construction results in extending C by an operator that can be thought of as \partial^{-1}E, where \oint E is a fermionic screening. This covers the (2,p) Virasoro minimal models as well as the sl(2) WZW theory.

  • 10.
    Fjelstad, Jens
    et al.
    Institutionen för Fysik, Karlstads Universitet, Karlstad, Sweden.
    Fuchs, Jurgen
    Institutionen för Fysik, Karlstads Universitet, Karlstad, Sweden.
    Runkel, Ingo
    Max-Planck-Institut für Gravitationsphysik, Albert-Einstein-Institut, Golm, Germany.
    Schweigert, Christoph
    Fachbereich Mathematik, Universität Hamburg Schwerpunkt Algebra und Zahlentheorie, Zentrum für Mathematische Physik, Hamburg, Germany.
    TFT construction of RCFT correlators V: Proof of modular invariance and factorisation2006In: Theory and Applications of Categories, ISSN 1201-561X, E-ISSN 1201-561X, Vol. 16, no 16, p. 342-433Article in journal (Refereed)
    Abstract [en]

    The correlators of two-dimensional rational conformal field theories that are obtained in the TFT construction of [FRSI,FRSII,FRSIV] are shown to be invariant under the action of the relative modular group and to obey bulk and boundary factorisation constraints. We present results both for conformal field theories defined on oriented surfaces and for theories defined on unoriented surfaces. In the latter case, in particular the so-called cross cap constraint is included.

  • 11.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Fuchs, Jurgen
    Teoretisk Fysik, Karlstads Universitet, Karlstad, Sweden.
    Stigner, Carl
    Teoretisk Fysik, Karlstads Universitet, Karlstad, Sweden.
    RCFT with defects: Factorization and fundamental world sheets2012In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 863, no 1, p. 213-259Article in journal (Refereed)
    Abstract [en]

    It is known that for any full rational conformal field theory, the correlation functions that are obtained by the TFT construction satisfy all locality, modular invariance and factorization conditions, and that there is a small set of fundamental correlators to which all others are related via factorization - provided that the world sheets considered do not contain any non-trivial defect lines. In this paper we generalize both results to oriented world sheets with an arbitrary network of topological defect lines.

  • 12.
    Fjelstad, Jens
    et al.
    Cardiff School of Mathematics, Cardiff University, United Kingdom; Avdelning Fysik, Karlstads Universitet, Karlstad, Sweden.
    Fuchs, Jürgen
    Avdelning Fysik, Karlstads Universitet, Karlstad, Sweden.
    Runkel, Ingo
    Department of Mathematics, King's College London, United Kingdom.
    Schweigert, Christoph
    Organisationseinheit Mathematik, Universität Hamburg, Schwerpunkt Algebra und Zahlentheorie, Hamburg, Germany.
    Uniqueness of open/closed rational CFT with given algebra of open states2008In: Advances in Theoretical and Mathematical Physics, ISSN 1095-0761, E-ISSN 1095-0753, Vol. 12, no 6, p. 1283-1375Article in journal (Refereed)
    Abstract [en]

    We study the sewing constraints for rational two-dimensional conformal field theory on oriented surfaces with possibly non-empty boundary. The boundary condition is taken to be the same on all segments of the boundary. The following uniqueness result is established: For a solution to the sewing constraints with nondegenerate closed state vacuum and nondegenerate two-point correlators of boundary fields on the disk and of bulk fields on the sphere, up to equivalence all correlators are uniquely determined by the one-, two,- and three-point correlators on the disk.

    Thus for any such theory every consistent collection of correlators can be obtained by the TFT approach of hep-th/0204148, hep-th/0503194. As morphisms of the category of world sheets we include not only homeomorphisms, but also sewings; interpreting the correlators as a natural transformation then encodes covariance both under homeomorphisms and under sewings of world sheets.

  • 13.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Fuchs, Jürgen
    Teoretisk fysik, Karlstad University, Karlstad, Sweden.
    Stigner, Carl
    Teoretisk fysik, Karlstad University, Karlstad, Sweden.
    Schweigert, Christoph
    Algebra und Zahlentheorie, Hamburg University, Hamburg, Germany.
    Partition functions, mapping class groups and Drinfeld doubles2012In: Symmetries and Groups in Contemporary Physics: Proceedings of the XXIX International Colloquium on Group-Theoretical Methods in PhysicsTianjin, China, 20 – 26 August 2012, Shanghai: World Scientific, 2012, p. 405-410Conference paper (Refereed)
    Abstract [en]

    Higher genus partition functions of two-dimensional conformal field theories have to be invariants under linear actions of mapping class groups. We illustrate recent results [4,6] on the construction of such invariants by concrete expressions obtained for the case of Drinfeld doubles of finite groups. The results for doubles are independent of the characteristic of the underlying field, and the general results do not require any assumptions of semisimplicity.

  • 14.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Equivalence of Chern-Simons gauge theory and WZNW model using a BRST symmetry1999In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 466, no 2-4, p. 227-233Article in journal (Refereed)
    Abstract [en]

    The equivalence between the Chern-Simons gauge theory on a three-dimensional manifold with boundary and the WZNW model on the boundary is established in a simple and general way using the BRST symmetry. Our approach is based on restoring gauge invariance of the Chern-Simons theory in the presence of a boundary. This gives a correspondence to the WZNW model that does not require solving any constraints, fixing the gauge or specifying boundary conditions.

  • 15.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Sectors of solutions in three-dimensional gravity and black holes2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 628, no 1-2, p. 331-360Article in journal (Refereed)
    Abstract [en]

    We examine the connection between three dimensional gravity with negative cosmological constant and two-dimensional CFT via the Chern-Simons formulation. A set of generalized spectral flow transformations are shown to yield new sectors of solutions. One implication is that the microscopic calculation of the entropy of the Banados-Teitelboim-Zanelli (BTZ) black hole is corrected by a multiplicative factor with the result that it saturates the Bekenstein-Hawking expression.

  • 16.
    Fjelstad, Jens
    et al.
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Hwang, Stephen
    Department of Physics, Karlstad University, Karlstad, Sweden.
    Månsson, Teresia
    Institute of Theoretical Physics, Stockholm, Sweden.
    CFT description of three-dimensional Kerr-de Sitter space-time2002In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 641, no 1-2, p. 376-392Article in journal (Refereed)
    Abstract [en]

    We describe three-dimensional Kerr-de Sitter space using similar methods as recently applied to the BTZ black hole. A rigorous form of the classical connection between gravity in three dimensions and two-dimensional conformal field theory is employed, where the fundamental degrees of freedom are described in terms of two dependent SL(2,C) currents. In contrast to the BTZ case, however, quantization does not give the Bekenstein-Hawking entropy connected to the cosmological horizon of Kerr-de Sitter space.

  • 17.
    Fjelstad, Jens
    et al.
    Department of Physics, Nanjing University, Nanjing, China.
    Månsson, Teresia
    Department of Theoretical Physics, School of Engineering Sciences, Royal Institute of Technology (KTH), Stockholm, Sweden.
    New symmetries of the chiral Potts model2012In: Journal of Physics A: Mathematical and General, ISSN 0305-4470, E-ISSN 1361-6447, Vol. 45, no 15, article id 155208Article in journal (Refereed)
    Abstract [en]

    In this paper a hithertho unknown symmetry of the three-state chiral Potts model is found consisting of two coupled Temperley-Lieb algebras. From these we can construct new superintegrable models. One realisation is in terms of a staggered isotropic XY spin chain. Further we investigate the importance of the algebra for the existence of mutually commuting charges. This leads us to a natural generalisation of the boost-operator, which generates the charges.

  • 18.
    Oberstedt, Andreas
    et al.
    Örebro University, Department of Natural Sciences.
    Birgersson, Evert
    Örebro University, Department of Natural Sciences.
    Tovesson, Fredrik
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Oberstedt, Stephan
    Fritsch, Volker
    Vladuca, Gheorghita
    Tudora, Annabella
    Fogelberg, Birger
    Ramström, Elisabet
    Neutron reaction cross section data for advanced nuclear applications2005In: Proceedings of the 11th international topical meeting on nuclear reactor thermal-hydraulics (NURETH-11): 262, 2005, p. 1-9Conference paper (Refereed)
  • 19.
    Oberstedt, Andreas
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN2007In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 570, no 1, p. 51-54Article in journal (Refereed)
    Abstract [en]

    The recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments. Nevertheless, it was also used in the past for measuring light-charged particles from ternary fission. However, due to the electric field settings the energy distribution of the lightest particles was not completely accessible, which made the determination of mean kinetic energies, widths and, hence, emission yields difficult. In this paper we present an energy degrader technique that allows for the measurement of the entire energy spectrum of even the lightest ternary particles with LOHENGRIN.

  • 20.
    Oberstedt, Andreas
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Birgersson, Evert
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Fritsch, Volker
    Lövestam, Göran
    Vladuca, Gheorghita
    Tudora, Annabella
    Kornilov, Nikolai
    Recent results on the neutron-induced fission cross-section of 231Pa2005In: Proceedings of the 3rd international workshop on nuclear fission and fission-product spectroscopy, American Institute of Physics , 2005, p. 27-30Conference paper (Refereed)
    Abstract [en]

    The cross-section for the neutron-induced fission of 231Pa has recently been measured from the threshold to En = 3.5 MeV. The experimental results are described in terms of extended statistical model calculations.

  • 21.
    Oberstedt, Andreas
    et al.
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Gawrys, Michael
    Kornilov, Nikolai
    Identification of a Shape Isomer in 235U2007In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 99, no 4, p. 1-4Article in journal (Refereed)
    Abstract [en]

    The shape isomer in 235U has been searched for in a neutron-induced fission experiment on 234U, which was performed at the isomer spectrometer NEPTUNE of the EC-JRC IRMM. A neutron source, with a tunable pulse frequency in the Hz to kHz range and its individually adjustable neutron pulse width in connection with an appropriate detector system turned out to be the ideal instrument to perform an isomer search, when decay half-lives above 100 us are expected. From the delayed fission events observed for two different NEPTUNE settings and at mean incident neutron energies En = 0.95 and 1.27 MeV the isomeric fission half-life could be determined to be T_1/2 = (3.6 ± 1.8) ms. The corresponding cross section was determined to sigma_if = (10 ± 8) ub. With these results an experimental confirmation for the existence of a superdeformed shape isomer in odd-uranium isotopes is given for the first time.

  • 22.
    Oberstedt, Andreas
    et al.
    Örebro University, School of Science and Technology.
    Oberstedt, Stephan
    Rochman, Dimitri
    Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN2008In: Seminar on Fission VI, Corsendonk Priory, Belgium, September 18-21, 2007 / [ed] Cyriel Wagemans, Jan Wagemans, Pierre D'hondt, 2008, , p. 8p. 99-106Conference paper (Refereed)
    Abstract [en]

    Although the recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments, it was also used in the past for measuring light-charged particles from ternary fission. However, due to limited electric field settings the energy distribution of the lightest particles was not completely accessible. In this contribution we report on an energy degrader technique that allows the measurement of the entire energy spectra of ternary particles with LOHENGRIN. We demonstrate how the measured particle spectra are distorted by the energy degrader and present results from a Monte Carlo simulation that shows how the original energy distributions are reconstructed. Finally, we apply this procedure to experimental data of ternary particles from the reaction 235U(nth, f).

  • 23.
    Oberstedt, Andreas
    et al.
    Örebro University, School of Science and Technology.
    Oberstedt, Stephan
    Rochman, Dimitri
    Energy degrader technique for light-charged particle spectroscopy at LOHENGRIN2008In: ND-2007 International Conference on Nuclear Data for Science and Technology, Nice, France, 2008, , p. 4p. 379-382Conference paper (Refereed)
    Abstract [en]

    The recoil mass-separator LOHENGRIN at Institute Laue-Langevin was originally designed for the spectrometry of binary fission fragments. Nevertheless, it was also used in the past for measuring light-charged particles from ternary fission. However, due to the electric field settings the energy distribution of the lightest particles was not completely accessible, which made the determination of mean kinetic energies, widths and, hence, emission yields difficult. In this paper we present an energy degrader technique that allows for the measurement of the entire energy spectrum of even the lightest ternary particles with LOHENGRIN.

  • 24. Oberstedt, Stephan
    et al.
    Gawrys, M.
    Lövestam, Göran
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Plompen, Arjan
    Semkova, Valentina
    NEPTUNE: the new isomer spectrometer at IRMM2007In: NEMEA-3: neutron measurements, evaluations and applications : proceedings of the enlargement workshop / [ed] Arjan J. M. Plompen, 2007, , p. 4p. 93-97Conference paper (Refereed)
  • 25. Oberstedt, Stephan
    et al.
    Hambsch, Franz-Josef
    Kornilov, Nicolai
    Lövestam, Göran
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Gawrys, Mikael
    Shape isomers: a key to fission barriers2008In: Seminar on Fission VI, Corsendonk Priory, Belgium, September 18-21, 2007 / [ed] Cyriel Wagemans, Jan Wagemans, Pierre D'hondt, 2008, , p. 8p. 233-240Conference paper (Refereed)
    Abstract [en]

    Quantitative predictions of fission product yields are relevant for the reliable operation of different modern nuclear applications. This concerns the realistic characterizations of the radio-toxicity of the fuel elements after the envisaged extended irradiation, as well as sub-critical assemblies, where the number of delayed neutrons from minor actinides is determined by the characteristic emission yields of the corresponding so-called pre-cursor isotopes. However, to be able to make more reliable quantitative predictions of fission characteristics requires the better understanding of the fission process itself. For this purpose a better knowledge about the distinct structure of the nuclear energy landscape around the fission barrier is indispensable. In particular, the question should be answered, whether the fission barrier is either double- or triple-humped or even multi-humped as been proposed within the multi-modal neck rupture model. Despite quite some effort based on different experimental techniques and theoretical approaches, this question remains still unanswered. There is still no consistent picture of the fission barrier available and hence, different sets of barrier parameters are in use, unable to describe the different observed phenomena in a coherent way. With the systematic investigation of shape isomer population, its decay modes as well as the branching ratio, precise information can be obtained to resolve the puzzling situation. The experimental approach will be discussed and results from first experiments presented.

  • 26. Oberstedt, Stephan
    et al.
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Fritsch, Volker
    Lövestam, Göran
    Kornilov, Nikolai
    New results on the neutron-induced fission cross-section of 231Pa for incident neutron energies En = 0.8 to 3.5 MeV2005In: Annals of Nuclear Energy, ISSN 0306-4549, E-ISSN 1873-2100, Vol. 32, no 17, p. 1867-1874Article in journal (Refereed)
  • 27. Oberstedt, Stephan
    et al.
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Hambsch, Franz-Josef
    Fritsch, Volker
    Vladuca, Gheorghita
    Tudora, Annabella
    Neutron-induced fission cross-section of 231Pa2006In: Proceedings of NEMEA-2 Workshop on Neutron Measurements, Evaluations and Applications - 2, October 20-23, 2004, Bucharest, Rumania, EUR Report 22136 EN, 2006Conference paper (Refereed)
  • 28. Oberstedt, Stephan
    et al.
    Oberstedt, Andreas
    Örebro University, School of Science and Technology.
    Plompen, Arjan
    Semkova, Valentina
    Lövestam, Göran
    Gawrys, Mikael
    Research on isomer decay with the NEPTUNE spectrometer2008In: ND-2007 International Conference on Nuclear Data for Science and Technology, Nice, France, Les Ulis Cedex A, France: EDP Sciences , 2008, , p. 4p. 53-56Conference paper (Refereed)
    Abstract [en]

    A new spectrometer has been built at the EC-JRC IRMM to investigate isomer decay in the millisecond range and activation cross sections of isotopes, where isomeric states exist and are populated. The spectrometer is equipped with high-resolution gamma-ray detectors and an ionisation chamber for the measurement of charged particles, e.g., fission fragments. NEPTUNE provides pulsed quasi mono-energetic neutrons at pulse repetition frequencies up to 5 kHz and tuneable neutron pulse widths ranging from 10 µs into the ms-region. For this purpose a beam chopper based on a parallel-plate capacitor has been integrated into the accelerator beam line in order to deflect the charged-particle beam onto a tantalum beam dump. First research has been performed with the NEPTUNE spectrometer dedicated to the shape isomer search in the odd-A uranium isotopes 235U, 237U and 239U. In further experiments the population of long-lived spin isomers was investigated.

  • 29. Oberstedt, Stephan
    et al.
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Rochman, Dimitri
    Gönnenwein, Friedrich
    Tsekhanovich, Igor
    Becker, Julia
    Örebro University, Department of Natural Sciences.
    Sartz, Annika
    Örebro University, Department of Natural Sciences.
    Bax, Hugo
    Hambsch, Franz-Josef
    Raman, Subramanian
    Light charged particle emission in the reaction 251Cf(nth, f)2005In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 761, no 3-4, p. 173-189Article in journal (Refereed)
    Abstract [en]

    High resolution measurements of light charged particles (LCP) emitted in thermal neutron-induced fission of 252Cf ∗ (E=6.2 MeV) have been performed with the recoil mass-separator LOHENGRIN. For this compound nuclear system emission yields of LCPs, their mean kinetic energies and widths have been obtained for 8 isotopes with nuclear charges Z⩾2. For 13 further isotopes the emission yields were estimated on the basis of systematics on their kinetic energy distributions. 34Al and 36Si emission has been observed for the first time in thermal neutron-induced fission.

  • 30.
    Tovesson, Fredrik
    et al.
    Örebro University, Department of Natural Sciences.
    Birgersson, E.
    Örebro University, Department of Natural Sciences.
    Flenéus, M.
    Örebro University, Department of Natural Sciences.
    Fogelberg, B.
    Fritsch, V.
    Gustafsson, C.
    Örebro University, Department of Natural Sciences.
    Hambsch, F.-J.
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Oberstedt, Stephan
    Ramström, E.
    Tudora, A.
    Vladuca, G.
    233Pa(n, f) cross section up to En=8.5 MeV2004In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 733, no 1-2, p. 3-19Article in journal (Refereed)
    Abstract [en]

    The energy dependence of the neutron-induced fission cross section of 233Pa has been measured directly for the first time from the fission threshold up to En=8.5 MeV. This reaction plays an important role in the thorium-uranium fuel cycle, and is thus of interest for the design and modeling of advanced reactor and transmutation facilities. The existing information in the ENDF/B-VI and JENDL-3.3 evaluated nuclear data files differ by a factor of two for the 233Pa(n, f) cross section values and show different fission threshold energies. Our new experimental data give lower cross section values than both evaluations and resolves the question about the threshold energy. In addition to the experimental investigation, also a new theoretical calculation of the reaction cross section has been performed with the statistical model code STATIS, showing a good agreement with the experimental data.

  • 31. Vladuca, G.
    et al.
    Hambsch, F.-J.
    Tudora, A.
    Oberstedt, S.
    Oberstedt, Andreas
    Örebro University, Department of Natural Sciences.
    Tovesson, F.
    Örebro University, Department of Natural Sciences.
    Filipescu, D.
    Calculation of the neutron-induced fission cross section of 233Pa up to 20 MeV2004In: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 740, no 1-2, p. 3-19Article in journal (Refereed)
    Abstract [en]

    Since very recently, direct measurements of the 233Pa(n,f) cross-section are available in the energy range from 1.0 to 8.5 MeV. This has stimulated a new, self-consistent, neutroncross-section evaluation for the n+233Pa system, in the incident neutron energy range 0.01–20MeV. Since higher fission chances are involved also the lighter Pa-isotopes had to be re-evaluated in a consistent manner. The results are quite different compared to earlier evaluation attempts. Since 233Pa is a key isotope in the thorium based fuel cycle the quality of its reaction cross-sections is important for the modeling of future advanced fuel and reactor concepts. The present status of the evaluated libraries is that they differ by a factor of two in the absolute fissioncross-section and also in the threshold energy value.

1 - 31 of 31
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