| 1. |
- Diakaki, M., et al.
(författare)
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Towards the high-accuracy determination of the 238U fission cross section at the threshold region at CERN -€“ n_TOF
- 2016
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X.
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Konferensbidrag (refereegranskat)abstract
- The U-238 fission cross section is an international standard beyond 2 MeV where the fission plateau starts. However, due to its importance in fission reactors, this cross-section should be very accurately known also in the threshold region below 2 MeV. The U-238 fission cross section has been measured relative to the U-235 fission cross section at CERN - n_TOF with different detection systems. These datasets have been collected and suitably combined to increase the counting statistics in the threshold region from about 300 keV up to 3 MeV. The results are compared with other experimental data, evaluated libraries, and the IAEA standards.
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| 2. |
- Leal-Cidoncha, E., et al.
(författare)
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High accuracy 234U(n,f) cross section in the resonance energy region
- 2017
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Ingår i: ND 2016. - Les Ulis : EDP Sciences. - 9782759890200
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Konferensbidrag (refereegranskat)abstract
- New results are presented of the 234U neutron-induced fission cross section, obtained with high accuracy in the resonance region by means of two methods using the 235U(n,f) as reference. The recent evaluation of the 235U(n,f) obtained with SAMMY by L. C. Leal et al. (these Proceedings), based on previous n_TOF data [1], has been used to calculate the 234U(n,f) cross section through the 234U/235U ratio, being here compared with the results obtained by using the n_TOF neutron flux.
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| 3. |
- Jentschel, M., et al.
(författare)
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EXILL - a high-efficiency, high-resolution setup for gamma-spectroscopy at an intense cold neutron beam facility
- 2017
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Ingår i: Journal of Instrumentation. - : IOP PUBLISHING LTD. - 1748-0221. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- In the EXILL campaign a highly efficient array of high purity germanium (HPGe) detectors was operated at the cold neutron beam facility PF1B of the Institut Laue-Langevin (ILL) to carry out nuclear structure studies, via measurements of gamma-rays following neutron-induced capture and fission reactions. The setup consisted of a collimation system producing a pencil beam with a thermal capture equivalent flux of about 10(8) ns(-1)cm(2) at the target position and negligible neutron halo. The targetwas surrounded by an array of eight to ten anti-Compton shielded EXOGAMClover detectors, four to six anti-Compton shielded large coaxial GASP detectors and two standard Clover detectors. For a part of the campaign the array was combined with 16 LaBr3:(Ce) detectors from the FATIMA collaboration. The detectorswere arranged in an array of rhombicuboctahedron geometry, providing the possibility to carry out very precise angular correlation and directional-polarization correlation measurements. The triggerless acquisition system allowed a signal collection rate of up to 6 x 10(5) Hz. The data allowed to set multi-fold coincidences to obtain decay schemes and in combination with the FATIMA array of LaBr3:(Ce) detectors to analyze half-lives of excited levels in the pico-to microsecond range. Precise energy and efficiency calibrations of EXILL were performed using standard calibration sources of Ba-133, Co-60 and Eu-152 as well as data from the reactions Al-27(n, gamma)Al-28 and Cl-35(n,gamma)Cl-36 in the energy range from 30 keV up to 10MeV.
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| 4. |
- Paradela, C., et al.
(författare)
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High accuracy 235U(n,f) data in the resonance energy region
- 2016
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X.
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Konferensbidrag (refereegranskat)abstract
- The U-235 neutron-induced cross section is widely used as reference cross section for measuring other fission cross sections, but in the resonance region it is not considered as an IAEA standard because of the scarce experimental data covering the full region. In this work, we deal with a new analysis of the experimental data obtained with a detection setup based on parallel plate ionization chambers (PPACs) at the CERN n_TOF facility in the range from 1 eV to 10 keV. The relative cross section has been normalised to the IAEA value in the region between 7.8 and 11 eV, which is claimed as well-known. Comparison with the ENDF/B-VII evaluation and the IAEA reference file from 100 eV to 10 keV are provided.
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| 5. |
- Krticka, J., et al.
(författare)
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The nature of light variations in magnetic hot stars
- 2018
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Ingår i: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842. ; 48:1, s. 170-174
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic stars show several types of light variability which is modulated by the stellar rotation. In chemically peculiar stars, the redistribution of the flux in the surface regions with peculiar chemical composition leads to the light variability with a typical amplitude of the order of hundredths of magnitude. The most efficient processes that cause the flux redistribution are bound-bound (line) transitions of iron and bound-free (ionization) transitions of silicon. This type of light variability typically leads to a complex dependence of the amplitude on the wavelength and shows antiphase light curves in the far ultraviolet and visual regions. In hot magnetic stars, the modulation of the stellar wind by the magnetic field and the wind blanketing cause the light variability with a typical amplitude of the order of millimagnitudes. We predict the light variations in selected magnetic hot stars and compare the simulated light curves with light variations derived from observations.
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| 6. |
- Mikulasek, Z., et al.
(författare)
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Differential rotation in magnetic chemically peculiar stars
- 2018
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Ingår i: CONTRIBUTIONS OF THE ASTRONOMICAL OBSERVATORY SKALNATE PLESO. - : SLOVAK ACADEMY SCIENCES ASTRONOMICAL INST. - 1335-1842 .- 1336-0337. ; 48:1, s. 203-207
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic chemically peculiar (mCP) stars constitute about 10% of upper-main-sequence stars and are characterized by strong magnetic fields and abnormal photospheric abundances of some chemical elements. Most of them exhibit strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by a rigidly rotating main-sequence star with persistent surface structures and a stable global magnetic field. Long-term observations of the phase curves of these variations enable us to investigate possible surface differential rotation with unprecedented accuracy and reliability. The analysis of the phase curves in the best-observed mCP stars indicates that the location and the contrast of photometric and spectroscopic spots as well as the geometry of the magnetic field remain constant for at least many decades. The strict periodicity of mCP variables supports the concept that the outer layers of upper-main-sequence stars do not rotate differentially. However, there is a small, inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades. The period oscillations may reflect real changes in the angular velocity of outer layers of the stars which are anchored by their global magnetic fields. In CU Vir, V901 On, and perhaps BS Cir, the rotational period variation indicates the presence of vertical differential rotation; however, its exact nature has remained elusive until now. The incidence of mCP stars with variable rotational periods is currently investigated using a sample of fifty newly identified Kepler mCP stars.
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| 7. |
- Mikulasek, Z., et al.
(författare)
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On the Nature of Rotation Period Variability of Magnetic Stars
- 2017
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Ingår i: Stars. - 9781583819043 ; , s. 220-223
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Konferensbidrag (refereegranskat)abstract
- The magnetic chemically peculiar (mCP) stars of the upper main sequence exhibit periodic light, magnetic, radio, and spectroscopic variations that can be adequately explained by a model of a rigidly rotating magnetized star with persistent surface structures. The majority of mCP stars rotate at strictly constant periods. However, there are a few mCP stars whose rotation periods vary on timescales of decades while the shape of their phase curves remains unchanged. In the case of CU Vir and V901 Ori, we have detected cyclic period variations. We demonstrate that the period oscillations of CU Vir may be a consequence of the interaction of the internal magnetic field and differential rotation.
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| 8. |
- Oksala, M. E., et al.
(författare)
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Revisiting the rigidly rotating magnetosphere model for sigma Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and light variability
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 451:2, s. 2015-2029
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Tidskriftsartikel (refereegranskat)abstract
- The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma Ori E by Townsend, Owocki & Groote triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength B-d = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic Ha emission and broad-band photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.
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| 9. |
- Wade, G. A., et al.
(författare)
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Magnetic B stars observed with BRITE : Spots, magnetospheres, binarity, and pulsations
- 2017
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Ingår i: Second brite-constellation science conference. - Warsaw : POLISH ASTRONOMICAL SOC. - 9788393827961 ; , s. 94-100
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Konferensbidrag (refereegranskat)abstract
- Magnetic B-type stars exhibit photometric variability due to diverse causes, and consequently on a variety of timescales. In this paper we describe interpretation of BRITE photometry and related ground-based observations of four magnetic B-type systems: epsilon Lupi, tau Sco, a Cen and epsilon CMa.
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| 10. |
- Krticka, J., et al.
(författare)
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HST/STIS analysis of the first main sequence pulsar CU Virginis
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 625
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Tidskriftsartikel (refereegranskat)abstract
- ContextCU Vir has been the first main sequence star that showed regular radio pulses that persist for decades, resembling the radio lighthouse of pulsars and interpreted as auroral radio emission similar to that found in planets. The star belongs to a rare group of magnetic chemically peculiar stars with variable rotational period.AimsWe study the ultraviolet (UV) spectrum of CU Vir obtained using STIS spectrograph onboard the Hubble Space Telescope (HST) to search for the source of radio emission and to test the model of the rotational period evolution. Methods. We used our own far-UV and visual photometric observations supplemented with the archival data to improve the parameters of the quasisinusoidal long-term variations of the rotational period. We predict the flux variations of CU Vir from surface abundance maps and compare these variations with UV flux distribution. We searched for wind, auroral, and interstellar lines in the spectra.ResultsThe UV and visual light curves display the same long-term period variations supporting their common origin. New updated abundance maps provide better agreement with the observed flux distribution. The upper limit of the wind mass-loss rate is about 10(-12) M-circle dot yr(-1). We do not find any auroral lines. We find rotationally modulated variability of interstellar lines, which is most likely of instrumental origin.ConclusionsOur analysis supports the flux redistribution from far-UV to near-UV and visual domains originating in surface abundance spots as the main cause of the flux variability in chemically peculiar stars. Therefore, UV and optical variations are related and the structures leading to these variations are rigidly confined to the stellar surface. The radio emission of CU Vir is most likely powered by a very weak presumably purely metallic wind, which leaves no imprint in spectra.
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| 11. |
- Krticka, J., et al.
(författare)
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Testing Opacities Using the SED Variability of Chemically Peculiar Stars
- 2018
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Ingår i: WORKSHOP ON ASTROPHYSICAL OPACITIES. - : ASTRONOMICAL SOC PACIFIC. - 9781583819142 ; , s. 195-200
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Konferensbidrag (refereegranskat)abstract
- Opacity variations across stellar surfaces are the key process producing the spectral energy distribution (SED) variability in chemically peculiar (CP) stars. The opacity variations are caused by the presence of surface spots with enhanced (or depleted) abundances of chemical elements. Simulations of the SED variability of chemically peculiar stars with abundances derived from Doppler mapping provide a detailed test of the continuum (bound free) and line opacities in the model atmospheres. The effect of opacities on the SED is most pronounced in the ultraviolet region. We simulate the ultraviolet and visual SED variability of selected chemically peculiar stars using model atmospheres calculated for actual surface abundances, and compare the predicted SEDs with observational results. We show that the simulations can reliably predict the observed SED and its variability as long as complete bound free and bound bound opacities are used. Therefore, the variability of chemically peculiar stars may serve as a test of opacities included in model atmospheres.
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