Ústav jaderné fyziky

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2018-03-09
13:20
Measurements and usage of cross sections of various (n,chi n) threshold reactions
Chudoba, Petr ; Vrzalová, Jitka ; Svoboda, Ondřej ; Krása, Antonín ; Kugler, Andrej ; Majerle, Mitja ; Suchopár, Martin ; Wagner, Vladimír
Current trend in nuclear reactor physics is a transition from technologies using thermal neutrons to technologies utilizing fast neutrons. Unfortunately focus was put mainly on the thermal neutrons for a long time and lead to very good knowledge about this low energy region, but very scarce coverage of the high energy region. This means that there is a gap in the knowledge of excitation functions for higher energies. This gap spreads from 20 MeV up to 1 GeV and higher. This is exactly the energy region needed for description of advanced nuclear systems such as accelerator driven systems (ADS). Our group from Nuclear Physics Institute (NPI) of the CAS is a member of an international collaboration Energy & Transmutation of Radioactive Waste (E&T RAW). This collaboration focuses on ADS for many years. In order to measure neutron field within ADS models it is necessary to know excitation functions of reactions used to monitor the neutron field. In many cases there are almost no experimental data for suitable reactions. Worse and quite common case is that there are no data at all. Therefore we are also focusing on measurements of these data in order to fill the databases as well as to allow further improvements of codes for nuclear data calculations.

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2018-03-09
13:20
Effect of the eta eta channel and interference phenomena in the two-pion transitions of charmonia and bottomonia
Surovtsev, Yu .S. ; Bydžovský, Petr ; Gutsche, T. ; Kaminski, R. ; Lyubovitskij, V. E. ; Nagy, M.
The basic shape of di-pion mass spectra in the two-pion transitions of both charmonia and bottomonia states is explained by a unified mechanism based on contributions of the pi pi, K (K) over bar, and eta eta coupled channels including their interference. The role of the individual f(0) resonances in shaping the di-pion mass distributions in the charmonia and bottomonia decays is considered.

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2018-03-09
13:20
Neutron Diffraction and Acoustic Emission Measurement During Loading and Unloading of Magnesium Aluminium Binary Alloys
Čapek, Jan ; Máthis, K.
The neutron diffraction and acoustic emission were measured during compression loading-unloading tests in randomly textured cast magnesium and magnesium 2 and 9 wt% aluminium binary alloys. The anelastic behavior and the change of twinned volume related to the detwinning phenomena can be observed during the unloading. The decrease of twinned volume fraction during unloading is most pronounced for Mg 9 wt% Al.

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2018-03-09
13:20
Top-Quark p(T)-Spectra at LHC and Flavor Independence of z-Scaling
Tokarev, M. V. ; Zborovský, Imrich
New results of analysis of top-quark differential cross sections obtained by the CMS and ATLAS Collaborations in pp collisions at the LHC in the framework of z-scaling approach are presented. The spectra are measured over a wide range of collision energy root s = 7, 8, 13 TeV and transverse momentum p(T) = 30-1000 GeV/c of top-quark using leptonic and jet decay modes. The LHC data on transverse momentum of the top-quark spectra are compared with the Tevatron data obtained by the Dempty set Collaboration in p (p) over bar collisions at root s = 1.96 GeV. Flavour independence of the scaling function psi(z) in pp and p (p) over bar interactions over a wide collision energy range root s = 19 - 13000 GeV is verified. This property of psi(z) was found for different hadrons from pi-meson up to. particle. The flavour independence of psi(z) is used as indication on self-similarity of to p-quark production. A tendency to saturation of psi(z) at low z for top-quark is demonstrated. We anticipate that data on low-and high-p(T) inclusive spectra of top-quark production at LHC energies could be of interest for verification of the self-similarity over a wide range of masses and different flavour content of produced particles.

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2018-03-09
13:20
Residual stress determination by neutron diffraction in low-carbon steel wires with accumulated shear deformation
Rogante, M. ; Mikula, Pavol ; Strunz, Pavel ; Zavdoveev, A.
Modern methods of severe plastic deformation (SPD) currently allow obtaining the ultrafine-grained (UFG) structure nearly in any material. In the manufacturing process of wire with UFG structure, a main restriction is the continuous drawing scheme in which this process occurs, thus it is important to consider the factors affecting the drawing efficiency. Despite the data on SPD steels such as strength, plasticity and conductivity, obtained by classical methodologies, further investigations are needed: a key information is the residual stresses (RS) status, and RS determination is an essential issue to improve knowledge of SPD effects. In this work, 15 wires samples made of low-alloyed quality structural steel Grade 08G2S GOST 1050 with accumulated shear deformation - as result of rolling with shear of the metal ingot and conventional wire drawing - have been investigated by neutron diffraction (ND). Results provide substantial data helping to evaluate the effect of shear deformation on RS of the considered steel, as well as additional support to complement the information already achieved by using the other characterization methodologies. Knowledge of the RS status can help developing a lowcarbon wire drawing technology with needed manufacturability and efficiency, playing a decisive role in the debugging of material selection and engineering design requirements.

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2018-03-09
13:20
On the scientific utilisation of low power research reactors
Mikula, Pavol ; Strunz, Pavel
In our contribution we focus our attention on the scientific utilisation of the beam tubes at the low power research reactor LVR-15. Namely, it will be reported about the neutron scattering instrumentation development and the educational possibilities at the low power neutron sources. The feasibility of carrying out the methodology and instrumental development research at the low power neutron sources will be demonstrated on designs of several high resolution and good luminosity neutron scattering instruments exploiting Bragg diffraction optics. Some of them have been already realized e.g. for small angle neutron scattering studies or residual strain/stress measurements. As the mentioned instrumental development and testing can be carried out at the low power neutron sources, due to the much lower safety requirements in comparison with the medium and high flux sources, they offer excellent educational and training programmes in neutron scattering or imaging for students.

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2018-03-09
13:20
Numerical modeling of surface elastic wave scattering in polycrystalline materials
Grabec, Tomáš ; Ryzy, M. ; Verres, I. A.
Grain-boundary scattering of acoustic waves in polycrystalline materials is studied with analytical and numerical methods in the presented paper. In particular, the attenuation of Surface Acoustic Waves (SAWs) propagating through a polycrystalline microstructure is investigated. A three dimensional timedomain finite-element method (FEM) is utilized for the numerical simulation, whereby the granular microstructure is modeled by Voronoi tessellation. The resulting frequency-dependent attenuation coefficients are compared to an explicit analytical model.

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2018-03-09
13:20
Position-Sensitive Coincidence Detection of Nuclear Reaction Products at the Prague Van-de-Graaff Accelerator
Granja, Carlos ; Kraus, V. ; Pugatch, V. ; Kohout, Z.
In low-energy nuclear reactions of astrophysical interest or fusion studies the spatial-and time-correlated detection of two and more reaction products can be a valuable tool in studies of reaction mechanisms, resolving reaction channels and measuring angular distributions of reaction products. For this purpose we constructed a configurable array of position-sensitive detectors based on the hybrid semiconductor pixel detector Timepix. Additional analog-signal electronics provide self-trigger together with extended multi-device control and synchronized readout electronics by a customized control and coincidence unit. The instrumentation, developed and used for detection of fission fragments in spontaneous and neutron induced fission as well as in charged particle detection in neutron induced reactions, is being implemented for low-energy light-ion induced nuclear reactions. Application and demonstration of the technique with two Timepix detectors on p+p elastic scattering at the Van-de-Graaff (VdG) accelerator in Prague is given.

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2018-03-09
13:20
Performance of self-developing radiography films in LVR-15's neutron beams
Šoltés, J. ; Viererbl, L. ; Klupák, V. ; Vinš, M. ; Michalcová, Božena
In the search for a suitable detector for demonstration neutron radiography measurements on the zero-power VR-1 training reactor at the Czech Technical University in Prague, some options were considered. Due to the reactor's low power and spatial limitations, an easy and practical solution had to be found. Self-developing films represent a flexible detection tool in x-ray imaging. Therefore, the goal of this study was to evaluate their potential for neutron detection. For this purpose, bare and converter covered films were studied in the thermal and epithermal neutron beams at the LVR-15 research reactor in Rez, Czech Republic.

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2018-03-09
13:20
Multi-(K)over-bar (hyper)Nuclei and Kaon Condensation
Gazda, Daniel ; Mareš, Jiří ; Friedman, E. ; Gal, A.
We report on recent relativistic mean-field calculations of multi-(K) over bar nuclei(1,2) which were performed fully and self-consistently across the periodic table. The (K) over bar separation energy B-(K) over bar as well as the nuclear and (K) over bar -meson densities were found to saturate with the number of antikaons in the nuclear medium. Saturation appears robust against a wide range of variations, including the nuclear model used and the type of boson fields mediating the strong interactions. In addition, we have explored properties of kaonic hypernuclei strange systems made of nucleons, hyperons and K- mesons. We observed saturation also in these objects. Since the (K) over bar separation energy B-(K) over bar does not exceed 200 MeV, multi-(K) over bar nuclei lie energetically well above multi-hyperonic nuclei and it is unlikely that kaon condensation could occur in strong-interaction self-bound hadronic matter.

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