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  • 3951.
    Zheng, Fengshan
    et al.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Rybakov, Filipp N.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Statistisk fysik. Russian Acad Sci, Ural Branch, MN Miheev Inst Met Phys, Ekaterinburg, Russia.;Ural Fed Univ, Ekaterinburg, Russia..
    Borisov, Aleksandr B.
    Russian Acad Sci, Ural Branch, MN Miheev Inst Met Phys, Ekaterinburg, Russia.;Ural Fed Univ, Ekaterinburg, Russia..
    Song, Dongsheng
    Tsinghua Univ, Natl Ctr Electron Microscopy Beijing, Sch Mat Sci & Engn, Beijing, Peoples R China..
    Wang, Shasha
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Li, Zi-An
    Chinese Acad Sci, Inst Phys, Beijing, Peoples R China..
    Du, Haifeng
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Kiselev, Nikolai S.
    Forschungszentrum Julich, Peter Grunberg Inst & Inst Adv Simulat, Julich, Germany.;JARA, Julich, Germany..
    Caron, Jan
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Kovacs, Andras
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Tian, Mingliang
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Zhang, Yuheng
    Chinese Acad Sci, Anhui Key Lab Condensed Matter Phys Extreme Condi, High Magnet Field Lab, Hefei, Anhui, Peoples R China.;Univ Sci & Technol China, Chinese Acad Sci, Hefei, Anhui, Peoples R China.;Nanjing Univ, Collaborat Innovat Ctr Adv Microstruct, Nanjing, Jiangsu, Peoples R China..
    Bluegel, Stefan
    Forschungszentrum Julich, Peter Grunberg Inst & Inst Adv Simulat, Julich, Germany.;JARA, Julich, Germany..
    Dunin-Borkowski, Rafal E.
    Forschungszentrum Julich, Ernst Ruska Ctr Microscopy & Spect Electrons, Julich, Germany.;Forschungszentrum Julich, Peter Grunberg Inst, Julich, Germany..
    Experimental observation of chiral magnetic bobbers in B20-type FeGe2018Ingår i: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 13, nr 6, s. 451-+Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Chiral magnetic skyrmions(1,2) are nanoscale vortex-like spin textures that form in the presence of an applied magnetic field in ferromagnets that support the Dzyaloshinskii-Moriya interaction (DMI) because of strong spin-orbit coupling and broken inversion symmetry of the crystal(3,4). In sharp contrast to other systems(5,6) that allow for the formation of a variety of two-dimensional (2D) skyrmions, in chiral magnets the presence of the DMI commonly prevents the stability and coexistence of topological excitations of different types(7). Recently, a new type of localized particle-like object-the chiral bobber (ChB)-was predicted theoretically in such materials(8). However, its existence has not yet been verified experimentally. Here, we report the direct observation of ChBs in thin films of B20-type FeGe by means of quantitative off-axis electron holography (EH). We identify the part of the temperature-magnetic field phase diagram in which ChBs exist and distinguish two mechanisms for their nucleation. Furthermore, we show that ChBs are able to coexist with skyrmions over a wide range of parameters, which suggests their possible practical applications in novel magnetic solid-state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers.

  • 3952. Zheng, S. J.
    et al.
    Xu, F. R.
    Shen, S. F.
    Liu, H. L.
    Wyss, Ramon
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Yan, Y. P.
    Shape coexistence and triaxiality in nuclei near Zr-802014Ingår i: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 90, nr 6, s. 064309-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Total-Routhian-surface calculations have been performed to investigate the shape evolutions of A similar to 80 nuclei: Zr80-84, Sr76-80, and Mo-84,Mo-86. Shape coexistences of spherical, prolate, and oblate deformations have been found in these nuclei. Particularly for the nuclei Sr-80 and Zr-82, the energy differences between two shape-coexisting states are less than 220 keV. At high spins, the g(9/2) shell plays an important role in shape evolutions. It has been found that the alignment of the g(9/2) quasiparticles drives nuclei to be triaxial.

  • 3953. Zheng, Y.
    et al.
    de France, G.
    Clement, E.
    Dijon, A.
    Cederwall, Bo
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Wadsworth, R.
    Bäck, Torbjörn
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Ghazi Moradi, Farnaz
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Jaworski, G.
    Nyako, B. M.
    Nyberg, J.
    Palacz, M.
    Al-Azri, H.
    de Angelis, G.
    Atac, A.
    Aktas, O.
    Bhattacharyya, S.
    Brock, T.
    Davies, P. J.
    Di Nitto, A.
    Dombradi, Zs.
    Gadea, A.
    Gal, J.
    Joshi, P.
    Juhasz, K.
    Julin, R.
    Jungclaus, A.
    Kalinka, G.
    Kownacki, J.
    La Rana, G.
    Lenzi, S. M.
    Molnar, J.
    Moro, R.
    Napoli, D. R.
    Singh, B. S. Nara
    Persson, Andreas
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Recchia, F.
    Sandzelius, Mikael
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Scheurer, J. -N
    Sletten, G.
    Sohler, D.
    Söderstrom, P. -A
    Taylor, M. J.
    Timar, J.
    Valiente-Dobon, J. J.
    Vardaci, E.
    gamma-ray linear polarization measurements and (g(9/2))(-3) neutron alignment in Ru-912013Ingår i: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 87, nr 4, s. 044328-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Linear polarization measurements have been performed for gamma rays in Ru-91 produced with the Ni-58(Ar-36,2p1n gamma)Ru-91 reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the gamma-gamma coincidences, gamma-ray linear polarization, and gamma-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3-1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in Ru-91. More specifically, calculations using a semiempirical shell model were performed to understand the structures of the first and second (21/2(+)) and (17/2(+)) levels. The results are in good agreement with the experimental data, supporting the interpretation of the nonyrast (21/2(+)) and (17/2(+)) states in terms of the J(max) and J(max) - 2 members of the seniority-three nu(g(9/2))(-3) multiplet.

  • 3954.
    Zheng, Y.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för bioteknologi (BIO), Centra, Albanova VinnExcellence Center for Protein Technology, ProNova. National Engineering Laboratory, Beijing 100084, China; Key Laboratory of Particle & Radiation Imaging of Ministry of Education (Tsinghua University), Beijing 100084, China; Department of Engineering Physics, Tsinghua University, Beijing 100084, China.
    Zeng, Z.
    Zeng, M.
    Wang, X.
    Zhao, Z.
    Discrimination of drugs and explosives in cargo inspections by applying machine learning in muon tomography2018Ingår i: Qiangjiguang Yu Lizishu/High Power Laser and Particle Beams, ISSN 1001-4322, Vol. 30, nr 8, artikel-id 086002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A previously under-explored difficulty in cargo inspections is how to efficiently detect drugs and explosives concealed in large dense metals. Cosmic ray muon tomography is a promising non-destructive imaging technique to solve the problem because muons are naturally generated in the atmosphere and have sufficient energy to completely penetrate large dense containers. In this work it is investigated that to what extent drugs and explosives of a certain size could be discriminated from air background and metals by muon tomography within acceptable measuring time. A Geant4 Monte Carlo simulation is built based on the Tsinghua University MUon Tomography facility (TUMUTY) and a support vector machine (SVM) classifier based on machine learning is trained to differentiate drugs and explosives from air background and metals automatically. For various 20 cm×20 cm×20 cm objects, with 10 min to 30 min measuring time, drugs and explosives could be discriminated from background and metals by muon tomography with an error rate of about 1%. With 1 min, the error rate deteriorates to 12.9%.

  • 3955.
    Zhong, Huaqiang
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftsäkerhet.
    A Study on the Coolability of Ex-vessel Corium by Late Top Water Flooding2011Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [en]

    The molten core-concrete interaction (MCCI) is treated as one of the important phenomena that may lead to the late containment failure by basemat penetration in a hypothetical severe accident of light water reactors (LWRs). The earlier research has showed that heat transfer limitation exists for the coolability of ex-vessel corium by atop water flooding due to crust formation on the melt/water interface that will isolate melt from water. However, several cooling mechanisms were identified in a series of intense investigations. A code (CORQUENCH) was developed and updated to incorporate the newly identified cooling mechanisms for the better predictions of cavity erosion and corium cooling behaviors. A description about such cooling mechanisms (i.e., bulking cooling, water ingression, eruption and crust breach) and the concrete ablation models implemented in the code is presented in this thesis.

    The technical work in the thesis includes two parts: first, the verification and validation of the code were performed against the CCI tests from the OECD/MCCI projects; and then a reactor-scale simulation was carried out for MCCI and ex-vessel corium coolability of a reference PWR with LCS concrete. The calculations of CCI tests have a plausible agreement with the experimental data.

    The calculation predicts an optimistic result for the reactor case, and a fast quenching achieved at about 145 minutes. In addition, a sensitivity study was also conducted on several important parameters, i.e., concrete type, corium composition, water flooding time, atmosphere pressure, concrete ablation temperature, initial temperature, decay power, cavity geometry, concrete decomposition model and melt upper heat transfer model. An attempt to explain the physics of the different predicted phenomena is presented as well.

    Finally, comparative calculations were performed by the other codes (ASTEC and FinCCI) for the same reactor-scale configuration. Discrepancies are found in the results. Some suggestions are proposed to improve the CORQUENCH code.

  • 3956.
    Zhou, Mingliang
    et al.
    SUNY Stony Brook, Dept Chem, Stony Brook, NY 11794 USA..
    Jensen, Bengt
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ohm, Christian
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ripellino, Giulia
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Sidebo, P. Edvin
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Strandberg, Jonas
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zwalinski, L.
    CERN, Geneva, Switzerland..
    Flow fluctuations in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector2019Ingår i: Nuclear Physics A, ISSN 0375-9474, E-ISSN 1873-1554, Vol. 982, s. 323-326Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Measurements of four-particle cumulants c(n){4} for n = 1, 2, 3, 4 are presented using 470 mu b(-1) of Pb+Pb collisions at root s(NN) = 5.02 TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of single harmonics p(v(n)). For the first time, a negative c(1){4} is observed. The c(4){4} is found to be negative in central collisions but changes sign around 20-25% centrality. This behavior is consistent with a nonlinear contribution to v(4) that is proportional to v(2)(2). c(2){4} and c(3){4} are calculated using two reference event classes in order to investigate the influence of volume fluctuations. Over most of the centrality range, c(2){4} and c(3){4} are found to be negative, while in the ultra-central collisions, c(2){4} changes sign and becomes positive, suggesting a deviation from Gaussian behavior in the event-by-event fluctuation of v(2). The magnitudes of the sign change are also found to be dependent of the event class definition .

  • 3957.
    Zhou, Tunhe
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Low-dose phase-contrast X-ray imaging: a comparison of two methods2012Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
  • 3958.
    Zhu, Kaixin
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Nuclear Reactor Seismic Analysis Considering Soil-Structure Interaction2018Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
  • 3959.
    Zukauskas, Andrius
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik.
    Liljestrand, Charlotte
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Viotti, Anne-Lise
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik.
    Pasiskevicius, Valdas
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik.
    Canalias, Carlota
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Highly-Efficient Cascaded Mirrorless OPO in Sub-mu m Periodically Poled RKTP Crystals2017Ingår i: 2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO), IEEE , 2017Konferensbidrag (Refereegranskat)
    Abstract [en]

    We demonstrate low threshold mirrorless optical parametric oscillation in sub-mu m periodically poled RKTP crystals reaching an efficiency of 43%. The generated signal serves as a pump in a cascaded MOPO process in the same crystal.

  • 3960.
    Zukauskas, Andrius
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik.
    Viotti, Anne-Lise
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik.
    Liljestrand, Charlotte
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Pasiskevicius, Valdas
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Canalias, Carlota
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Laserfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cascaded counter-propagating nonlinear interactions inhighly-efficient sub-μm periodically poled crystals2017Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, artikel-id 8037Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mirrorless optical parametric oscillators (MOPOs) are very attractive parametric devices that rely on the nonlinear interaction of counter-propagating photons to inherently establish distributed feedback, without the use of external mirrors or surface coatings. These devices offer unique spectral and coherence properties that will benefit a large variety of applications ranging from spectroscopy to quantum communications. The major obstacle in exploiting their full potential is ascribed to the difficulty in engineering a nonlinear material in which the generation of counter-propagating waves can be phase matched. Here we present a reliable and consistent technique for fabrication of highly-efficient sub-micrometer periodically poled Rb-doped KTiOPO4. We experimentally demonstrate the first cascaded counter-propagating interactions in which the generated forward signal serves as a pump for a secondary MOPO process, reaching pump depletion larger than 60%. The cascaded process exemplifies the high efficiency of our nonlinear photonic structures. Our domain-engineering technique paves the way to realize counter-propagating schemes and devices that have been deemed unfeasible until now.

  • 3961.
    Zyuzin, Alexander A.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Zyuzin, A. Yu.
    Flat band in disorder-driven non-Hermitian Weyl semimetals2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, nr 4, artikel-id 041203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the interplay of disorder and band-structure topology in a Weyl semimetal with a tilted conical spectrum around the Weyl points. The spectrum of particles is given by the eigenvalues of a non-Hermitian matrix, which contains contributions from a Weyl Hamiltonian and complex self-energy due to electron elastic scattering on disorder. We find that the tilt-induced matrix structure of the self-energy gives rise to either a flat band or a nodal line segment at the interface of the electron and hole pockets in the bulk band structure of type-II Weyl semimetals depending on the Weyl cone inclination. For the tilt in a single direction in momentum space, each Weyl point expands into a flat band lying on the plane, which is transverse to the direction of the tilt. The spectrum of the flat band is fully imaginary and is separated from the in-plane dispersive part of the spectrum by the "exceptional nodal ring" where the matrix of the Green's function in momentum-frequency space is defective. The tilt in two directions might shrink a flat band into a nodal line segment with "exceptional edge points." We discuss the connection to the non-Hermitian topological theory.

  • 3962.
    Zyuzin, Alexander
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Garaud, Julien
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Babaev, Egor
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Nematic Skyrmions in Odd-Parity Superconductors2017Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, nr 16, artikel-id 167001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study topological excitations in two-component nematic superconductors, with a particular focus on CuxBi2Se3 as a candidate material. We find that the lowest-energy topological excitations are coreless vortices: a bound state of two spatially separated half-quantum vortices. These objects are nematic Skyrmions, since they are characterized by an additional topological charge. The inter-Skyrmion forces are dipolar in this model, i.e., attractive for certain relative orientations of the Skyrmions, hence forming multi-Skyrmion bound states.

  • 3963.
    Zyuzin, Alexander
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Zyuzin, A. Y.
    Chiral anomaly and second-harmonic generation in Weyl semimetals2017Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 95, nr 8, artikel-id 085127Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study second-harmonic generation in centrosymmetric Weyl semimetal with broken time reversal symmetry. We calculate electric current density at the double frequency of the propagating electromagnetic field in the presence of an applied constant magnetic field, using the method of kinetic equation for electron distribution function. It is shown that the chiral anomaly contribution to second-harmonic generation in the lowest order is linearly proportional to the applied magnetic field. The limit when the chiral anomaly dominates over the Lorentz-type contribution to second-harmonic generation is discussed.

  • 3964. Ågren, O.
    et al.
    Moiseenko, V. E.
    Noack, K.
    Hagnestal, A.
    Kallne, J.
    Anglart, Henryk
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorteknologi.
    Hybrid Reactor Studies Based On The Straight Field Line Mirror2013Ingår i: Fusion science and technology, ISSN 1536-1055, E-ISSN 1943-7641, Vol. 63, nr 1T, s. 52-57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The straight field line mirror (SFLM) hybrid reactor studies aim to identify a concept where the safety of fission power production could be enhanced. A fusion neutron source could become a mean to achieve this. The SFLM studies address critical issues such as reactor safety, natural circulation of coolants, steady state operation for a year or more and means to avoid too strong material loads by a proper geometrical arrangement of the reactor components. A key result is that power production may be possible with a fusion Q factor as low as 0.15. This possibility arises from the high power amplification by fission, which within reactor safety margins may exceed a factor of 100. The requirements on electron temperature are dramatically lower for a fusion hybrid compared to a stand-alone fusion reactor. This and several other factors are important for our choice to select a mirror machine for the fusion hybrid reactor studies.

  • 3965. Ågren, O.
    et al.
    Moiseenko, V. E.
    Noack, K.
    Hagnestål, A.
    Källne, J.
    Anglart, Henryk
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorteknologi.
    The straight field line mirror concept aiming at a hybrid reactor2012Ingår i: 39th EPS Conference on Plasma Physics 2012, EPS 2012 and the 16th International Congress on Plasma Physics: Volume 1, 2012, 2012, s. 5-8Konferensbidrag (Refereegranskat)
  • 3966. Ågren, O.
    et al.
    Noack, K.
    Moiseenko, V. E.
    Hagnestål, A.
    Källne, J.
    Anglart, Henryk
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorteknologi.
    The hybrid reactor project based on the straight field line mirror concept2012Ingår i: Fusion For Neutrons And Subcritical Nuclear Fission / [ed] Kallne, J; Ryutov, D; Gorini, G; Sozzi, C; Tardocchi, M, American Institute of Physics (AIP), 2012, s. 173-185Konferensbidrag (Refereegranskat)
    Abstract [en]

    The straight field line mirror (SFLM) concept is aiming towards a steady-state compact fusion neutron source. Besides the possibility for steady state operation for a year or more, the geometry is chosen to avoid high loads on materials and plasma facing components. A comparatively small fusion hybrid device with "semi-poor" plasma confinement (with a low fusion Q factor) may be developed for industrial transmutation and energy production from spent nuclear fuel. This opportunity arises from a large fission to fusion energy multiplication ratio, Qr=Pfis/P fus>>1. The upper bound on Qr is primarily determined by geometry and reactor safety. For the SFLM, the upper bound is Qr≈150, corresponding to a neutron multiplicity of k eff=0.97. Power production in a mirror hybrid is predicted for a substantially lower electron temperature than the requirement T e≈10 keV for a fusion reactor. Power production in the SFLM seems possible with Q≈0.15, which is 10 times lower than typically anticipated for hybrids (and 100 times smaller than required for a fusion reactor). This relaxes plasma confinement demands, and broadens the range for use of plasmas with supra-thermal ions in hybrid reactors. The SFLM concept is based on a mirror machine stabilized by qudrupolar magnetic fields and large expander tanks beyond the confinement region. The purpose of the expander tanks is to distribute axial plasma loss flow over a sufficiently large area so that the receiving plates can withstand the heat. Plasma stability is not relying on a plasma flow into the expander regions. With a suppressed plasma flow into the expander tanks, a possibility arise for higher electron temperature. A brief presentation will be given on basic theory for the SFLM with plasma stability and electron temperature issues, RF heating computations with sloshing ion formation, neutron transport computations with reactor safety margins and material load estimates, magnetic coil designs as well as a discussion on the implications of the geometry for possible diagnostics. Reactor safety issues are addressed and a vertical orientation of the device could assist passive coolant circulation. Specific attention is put to a device with a 25 m long confinement region and 40 cm plasma radius in the mid-plane. In an optimal case (keff = 0.97) with a fusion power of only 10 MW, such a device may be capable of producing a power of 1.5 GWth.

  • 3967.
    Ågren, Peter
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Walter, Jochen
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Haviland, David B.
    KTH, Tidigare Institutioner                               , Fysik.
    Switching Current of a Cooper Pair Transistor with Tunable Josephson Junctions2002Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, nr 1, s. 14510-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the switching current of a Cooper pair transistor with tunable Josephson energy. The junctions are fabricated in a superconducting quantum interference device (SQUID) geometry which allows for an in situ tunable effective Josephson energy by application of a magnetic field. We find a 2e-periodic switching current versus gate charge. As the magnetic field is increased the switching current stays 2e-periodic but the magnitude is suppressed. At a magnetic field of half a flux quantum through the SQUID's the switching current is minimum. We can theoretically model the experimental data by assuming a switching current which is proportional to the ideal critical current squarred. We show that such a dependence is expected in the limit where the effect of thermal fluctuations on the system is strong.

  • 3968.
    Ågren, Peter
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Walter, Jochen
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Schöllmann, Volker
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Haviland, David B.
    KTH, Tidigare Institutioner                               , Fysik.
    Switching Currents and Quasi-Particle Poisoning in the Superconducting Single Electron Transistor2002Ingår i: INTERNATIONAL WORKSHOP ON SUPERCONDUCTING NANO-ELECTRONICS DEVICES, 2002, s. 25-31Konferensbidrag (Refereegranskat)
  • 3969.
    Ålander, Alexandra
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Dufek, Jan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Gudowski, Waclaw
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftsäkerhet.
    From once-through nuclear fuel cycle to accelerator-driven transmutation2006Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 562, nr 2, s. 630-633Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study simulation of different nuclear fuel cycle scenarios are performed. The reference scenario corresponds to a medium size nuclear power country, with 10 light water reactors (LWRs). The study addresses long-term, equilibrium fuel cycle scenarios, with and without plutonium recycling (MOX) in LWRs and transuranics (TRU) burning in accelerator-driven systems (ADS). However, also short-term phase-out scenarios, including TRU burning in ADS, are performed. The equilibrium simulation showed that four ADS units, each of 800 MWt, are sufficient to burn an amount of plutonium and americium corresponding to the build-up of those elements. The phase-out simulation of a country adopting an approach to reduce the spent nuclear fuel inventory, showed that complementary burning of TRU in three to four ADS units appear suitable. The fuel cycle simulations have been performed using the Nuclear Fuel Cycle Simulation (NFCSim) code [C.G. Bathke, E.A. Schneider, NFCSim User's Manual, Los Alamos National Laboratory Report LA-UR-04-8369, 2004.] and the Monteburns code [D.I. Poston, H.R. Trellue, User's Manual, Version 2.0 for Monteburns, Version 1.0, LA-UR-99-4999, 1999.].

  • 3970.
    Åslund, Magnus
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Digital Mammography with a Photon Counting Detector in a Scanned Multislit Geometry2007Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Mammography screening aims to reduce the number of breast cancer deaths by early detection of the disease, which is one of the leading causes of deaths for middle aged women in the western world. The risk from the x-ray radiation in mammography is relatively low but still a factor in the benefit-risk ratio of screening. The characterization and optimization of a digital mammography system is presented in this thesis. The investigated system is shown to be highly dose efficient by employing a photon counting detector in a scanning multislit geometry.

    A novel automatic exposure control (AEC) is proposed and validated in clinical practise. The AEC uses the leading detector edge to measure the transmission of the breast. The exposure is modulated by altering the scan velocity during the scan. A W-Al anode-filter combination is proposed.

    The characterization of the photon counting detector is performed using the detective quantum efficiency. The effect of the photon counting detector and the multislit geometry on the measurement method is studied in detail. It is shown that the detector has a zero-frequency DQE of over 70\% and that it is quantum limited even at very low exposures.

    Efficient rejection of image-degrading secondary radiation is fundamental for a dose efficient system. The efficiency of the scatter rejection techniques currently used are quantified and compared to the multislit geometry.

    A system performance metric with its foundation in statistical decision theory is discussed. It is argued that a photon counting multislit system can operate at approximately half the dose compared to several other digital mammography techniques.

  • 3971.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, M
    Danielsson, M
    Optimized AEC for scanning digital mammography based on local variation of scan velocity2005Ingår i: Medical Imaging 2005: Physics of Medical Imaging, Pts 1 and 2 / [ed] Flynn, MJ, BELLINGHAM: SPIE-INT SOC OPTICAL ENGINEERING , 2005, Vol. 5745, s. 468-477Konferensbidrag (Refereegranskat)
    Abstract [en]

    In mammography, there is an optimal photon energy and current time product that produce the required image quality at the minimal dose. The task of an automatic exposure control (AEC), in full field digital mammography (FFDM) is to minimize the dose by using optimized exposure settings. Each point in a mammogram has different radiological thickness. A conventional AEC samples the thickness in some regions to set the current time product and possibly also the beam quality. We define an ideal AEC as one that optimizes the beam quality and exposure in each point to produce a constant contrast-to-noise ratio (CNR) of structures of interest throughout the image. This paper presents the results from a theoretical evaluation of an AEC proposed for a scanning photon-counting FFDM system. The geometry enables the AEC to use information from the leading detector line to adjust the scan velocity during the scan. Thus, the irradiation can be better optimized in the scanning-direction as compared to a conventional AEC. The scan time is further reduced by increased velocity over sections that contain no tissue. The results are quantified in terms of reduction of entrance dose and scan time. The presented AEC is compared to an ideal AEC, a conventional AEC and is also benchmarked against an ideal regulator. The effect of the detector width is evaluated. Compared to a conventional AEC, both evaluated on a set of 266 mammograms, the ideal AEC would reduce the entrance dose by 39% on average while the proposed AEC for scanning systems reduces the entrance dose by 10-20% and scan-time by 25-32% on average, depending on detector width.

  • 3972.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    AEC for scanning digital mammography based on variation of scan velocity2005Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 32, nr 11, s. 3367-3374Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A theoretical evaluation of nonuniform x-ray field distributions in mammography was conducted. An automatic exposure control (AEC) is proposed for a scanning full field digital mammography system. It uses information from the leading part of the detector to vary the scan velocity dynamically, thus creating a nonuniform x-ray field in the scan direction. Nonuniform radiation fields were also created by numerically optimizing the scan velocity profile to each breast's transmission distribution, with constraints on velocity and acceleration. The goal of the proposed AEC is to produce constant pixel signal-to-noise ratio throughout the image. The target pixel SNR for each image could be set based on the breast thickness, breast composition, and the beam quality as to achieve the same contrast-to-noise ratio between images for structures of interest. The results are quantified in terms of reduction in entrance surface air kerma (ESAK) and scan time relative to a uniform x-ray field. The theoretical evaluation was performed on a set of 266 mammograms. The performance of the different methods to create nonuniform fields decreased with increased detector width, from 18% to 11% in terms of ESAK reduction and from 30% to 25% in terms of scan time reduction for the proposed AEC and detector widths from 10 to 60 mm. Some correlation was found between compressed breast thickness and the projected breast area onto the image field. This translated into an increase of the ESAK and decrease of the scan time reduction with breast thickness. Ideally a nonuniform field in two dimensions could reduce the entrance dose by 39% on average, whereas a field nonuniform in only the scanning dimension ideally yields a 20% reduction. A benefit with the proposed AEC is that the risk of underexposing the densest region of the breast can be virtually eliminated.

  • 3973.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Evaluation of an AEC system for scanning photon counting mammography based on variation of scan velocityIngår i: Medical physics (Lancaster), ISSN 0094-2405Artikel i tidskrift (Övrigt vetenskapligt)
  • 3974.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, Mats
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Optimization of operating conditions in photon counting multi-slit mammography based on Si-strip detectors - art. no. 61420A2006Ingår i: Medical Imaging 2006: Physics of Medical Imaging, Pts 1-3 / [ed] Flynn, MJ; Hsieh, J, 2006, Vol. 6142, s. A1420-A1420Konferensbidrag (Refereegranskat)
    Abstract [en]

    Measurements and simulations of the signal-difference-to-noise ratio (SDNR) and average glandular dose (AGD) have been performed on a photon counting full-field digital mammography system to determine the optimal operating conditions. Several beam qualities were experimentally evaluated by using different combinations of tube voltage, added filters and thickness of BR12 with a tungsten target x-ray tube. The SDNR and AGD were also calculated theoretically for an extended number of operating conditions and a more accurate breast model. As figure of merit for each operating condition, a spectral quantum efficiency (SQE) was calculated as the polychromatic SDNR squared over the optimal monochromatic SDNR squared at the same AGD. The theoretical model agreed within 4% relative the measured SDNR throughout the evaluated breast thickness (30-70 mm) and tube voltage range (26-38 kV). The optimization was performed with a constant SDNR-rate as compared to using a fixed filter thickness. The optimal combinations of tube voltage-filter material were: 32 kV-Ag, 34 kV-Cd, 36 kV-Sn for a breast thickness of 30, 50 and 70 mm respectively. These K-edge filter materials increased the SQE by less than 4% compared to the optimal Al filtration.

  • 3975.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Physical characterization of a scanning photon counting digital mammography system based on Si-strip detectors2007Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 34, nr 6, s. 1918-1925Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The physical performance of a scanning multislit full field digital mammography system was determined using basic image quality parameters. The system employs a direct detection detector comprised of linear silicon strip sensors in an edge-on geometry connected to photon counting electronics. The pixel size is 50 mu m and the field of view 24 x 26 cm(2). The performance was quantified using the presampled modulation transfer function, the normalized noise power spectrum and the detective quantum efficiency (DQE). Compared to conventional DQE methods, the scanning geometry with its intrinsic scatter rejection poses additional requirements on the measurement setup, which are investigated in this work. The DQE of the photon counting system was found to be independent of the dose level to the detector in the 7.6-206 mu Gy range. The peak DQE was 72% and 73% in the scan and slit direction, respectively, measured with a 28 kV W-0.5 mm Al anodefilter combination with an added 2 mm Al filtration.

  • 3976.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Lundqvist, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Medicinsk avbildning.
    Scatter rejection in multislit digital mammography2006Ingår i: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, nr 4, s. 933-940Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The scatter to primary ratio (SPR) was measured on a scanning multislit full-field digital mammography system for different thickness of breast equivalent material and different tube voltages. Scatter within the detector was measured separately and was found to be the major source of scatter in the assembly. Measured total SPRs below 6% are reported for breast range 3-7 cm. The performance of the multislit assembly is compared to other imaging geometries with different scatter rejection schemes by using the scatter detective quantum efficiency.

  • 3977.
    Åslund, Magnus
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Fredenberg, Erik
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Cederström, Björn
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Danielsson, Mats
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Spectral shaping for photon counting digital mammography2007Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 580, nr 2, s. 1046-1049Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spectral shaping properties of conventional filters have been evaluated for a photon counting digital mammography system, and the result has been compared with the theoretical spectrum from a multi-prism X-ray lens (MPL). The absorption filters and the MPL were evaluated using a theoretical model of the system which has been verified experimentally. The spectral shaping performance is quantified with the spectral quantum efficiency (SQE), calculated as the polychromatic signal-difference-to-noise ratio (SDNR) squared over the optimal monochromatic SDNR squared at the same average glandular dose. The MPL increases the SQE by 25% compared to the investigated absorption filter when compared with a Tungsten anode. This translates into a potential dose reduction of 20% at maintained SDNR.

  • 3978. Öjekull, J.
    et al.
    Andersson, P. U.
    Någård, M. B.
    Pettersson, J. B. C.
    Neau, A.
    Rosén, S.
    Thomas, R. D.
    Larsson, M.
    Semaniak, J.
    Österdahl, Fabian
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Atom- och molekylfysik.
    Danared, H.
    Källberg, A.
    af Ugglas, M.
    Dissociative recombination of ammonia clusters studied by storage ring experiments2006Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, nr 19, s. 194306-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dissociative recombination of ammonia cluster ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for dissociative recombination of H+(NH3)(2), H+(NH3)(3), D+(ND3)(2), and D+(ND3)(3) in the collision energy range of 0.001-27 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 1000 K are calculated from the experimental data and compared with earlier results. The fragmentation patterns for the two ions H+(NH3)(2) and D+(ND3)(2) show no clear isotope effect. Dissociative recombination of X+(NX3)(2) (X=H or D) is dominated by the product channels 2NX(3)+X [0.95 +/- 0.02 for H+(NH3)(2) and 1.00 +/- 0.02 for D+(ND3)(2)]. Dissociative recombination of D+(ND3)(3) is dominated by the channels yielding three N-containing fragments (0.95 +/- 0.05).

  • 3979.
    Örn, Rafaela
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Measurement and modeling of the Multileaf collimator MLCi22019Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
  • 3980.
    Österdahl, Fabian
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Ioniztion of molecules at the CRYRING facility2006Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 3981.
    Österdahl, Fabian
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Rosén, S
    Bednarska, V
    Petrignani, A
    Hellberg, F
    Larsson, M
    van der Zande, WJ
    Position- and time-sensitive coincident detection of fragments from the dissociative recombination of O-2(+) using a single hexanode delay-line detector2005Ingår i: DR2004: Sixth International Conference on Dissociative Recombination: Theory, Experiments and Applications / [ed] Wolf, A; Lammich, L; Schmelcher, P, BRISTOL: IOP PUBLISHING LTD , 2005, Vol. 4, s. 286-289Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    This contribution describes the use of the RoentDek hexanode delay line detector to detect fragments from the dissociative recombination of O-2(+) in an experiment at the ion storage ring CRYRING, Manne Siegbahn Laboratory, Stockholm. In this experiment, the fragments have a maximum time- and position-separation of 20ns and 40 mm, respectively. The position resolution obtained was 0.16mm (FWHM) on the 67 rum diameter detector. The time resolution obtained from the time-of-arrival difference between the product fragments was about 1 ns. The detector system handles event rates as large as 30 kHz. Techniques for the calibration of the absolute position of particles on the detector are discussed.

  • 3982.
    Östlund, Rasmus
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    PoGO+ Detector Cell Characterisation and Optimisation of Waveform Selection Parameters2016Självständigt arbete på avancerad nivå (masterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
  • 3983. Řípa, J.
    et al.
    Mészáros, A.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Cosmological effects on the observed flux and fluence distributions of gamma-ray bursts2011Konferensbidrag (Refereegranskat)
    Abstract [en]

    Several claims have been put forward that an essential fraction of long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5. This point-of-view follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift. The purpose of this article is to show that the most distant bursts in general need not be the faintest ones. We derive the cosmological relationships between the observed and emitted quantities, and arrive at a prediction that is tested on the ensembles of BATSE, Swift and Fermi bursts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gamma-ray burst model. We arrive to the conclusion that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can hold, where P(z) is either the peak-flux or the fluence. This also means that the hundreds of faint, long-duration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.

  • 3984.
    Žáková, Jitka
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Talamo, A.
    Neutronics of a liquid salt cooled - Very high temperature reactor2007Ingår i: 13th International Conference on Emerging Nuclear Energy Systems 2007, ICENES 2007, 2007, s. 423-430Konferensbidrag (Refereegranskat)
    Abstract [en]

    In the present studies, we have utilized the Monte Carlo MCNP code to investigate the neutronic and the safety parameters of the Liquid Salt cooled - Very High Temperature Reactor (LS-VHTR). More precisely, we focused on he void, fuel and moderator temperature reactivity coefficients for two fuel types, enriched uranium and spent LWR fuel, and five different molten salts, NaF, BeF2, LiF, ZrF4, Li2Be4F. The results show that the total void worth coefficient can be set negative provided that the fuel to moderator ratio is optimized and the moderating ratio of the coolant is sufficiently large; whereas the fuel and moderator temperature reactivity coefficients are negative at any fuel to moderator ratio.

  • 3985.
    Žáková, Jitka
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Talamo, Alberto
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Optimization of the neutronics of the advanced high temperature reactor2006Ingår i: Proceedings of the 2006 International Congress on Advances in Nuclear Power Plants, ICAPP'06, 2006, s. 294-301Konferensbidrag (Refereegranskat)
    Abstract [en]

    In these studies, we have investigated the neutronic and safety performance of the Advanced High Temperature Reactor (AHTR) for plutonium and uranium fuels and we extended the analysis to five different coolants. The AHTR is a graphite-mode rated and molten salt-cooled high temperature reactor, which takes advantage of the TRISO particles technology for the fuel utilization. The conceptual design of the core, proposed at the Oak Ridge National Laboratory, aims to provide an alternative to helium as coolant of high-temperature reactors for industrial applications like hydrogen production. We evaluated the influence of the radial reflector on the criticality of the core for the uranium and plutonium fuels and we focused on the void coefficient of 5 different molten salts; since the safety of the reactor is enhanced also by the large and negative coefficient of temperature, we completed our investigation by observing the keff changes when the graphite temperature varies from 300 to 1800 K.

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