I. Companis

Bio: I. Companis is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Neutron & Fission. The author has an hindex of 2, co-authored 9 publications receiving 52 citations.

The extrapolated-efficiency method, a new technique to determine the γ-cascade detection efficiency in experiments based on the surrogate-reaction method

Abstract: The surrogate-reaction method is an indirect technique to extract neutron-induced cross-sections of short-lived nuclei. In the last years several experiments have been performed to investigate whether this technique can be applied to infer radiative-capture cross-sections. A major difficulty in this type of measurements is the determination of the gamma-cascade detection efficiency. The pulse-height weighting technique (PHWT) has been previously used to determine this quantity in surrogate experiments. In this work, we present a new method to determine the gamma-cascade detection efficiency in the vicinity of the neutron-separation energy that is much simpler than the PHWT. We also investigate the possibility to apply this new technique in standard experiments using neutron beams.

Measurement of neutron capture and fission cross sections of 233 U in the resonance region

Abstract: In the framework of studies concerning new fuel cycles and nuclear wastes incineration experimental data of the α ratio between capture and fission cross sections of 233 U reactions play an important role in the Th/U cycle. The safety evaluation and the detailed performance assessment for the generation IV nuclear-energy system based on 232 Th cycle strongly depend on this ratio. Since the current data are scarce and sometimes contradictory, new experimental studies are required. The measurement will take place at the neutron time-of-flight facility GELINA at Geel, designed to perform neutron cross section measurements with high incident neutron-energy resolution. A dedicated high efficiency fission ionization chamber (IC) as fission fragment detector and six C6 D6 liquid scintilators sensitive to γ-rays and neutrons will be used. The method, based on the IC energy response study, allowing to distinguish between gammas originating from fission and capture, in the resonance region, will be presented.

Neutron-induced cross sections of short-lived nuclei via the surrogate reaction method

Abstract: The measurement of neutron-induced cross sections of short-lived nuclei is extremely difficult due to the radioactivity of the samples. The surrogate reaction method is an indirect way of determining cross sections for nuclear reactions that proceed through a compound nucleus. This method presents the advantage that the target material can be stable or less radioactive than the material required for a neutron-induced measurement. We have successfully used the surrogate reaction method to extract neutron-induced fission cross sections of various short-lived actinides. In this work, we investigate whether this technique can be used to determine neutron-induced capture cross sections in the rare-earth region.

Experimental Approaches to Studying the Fission Process Using the Surrogate Reaction Technique

Abstract: J. T. Burke, ∗ J. J. Ressler, J. E. Escher, N. D. Scielzo, I. J. Thompson, R. Henderson, J. Gostic, L. Bernstein, D. Bluel, M. Weideking, V. Meot, O. Roig, L. W. Phair, R. Hatarik, J. Munson, 4 C. Angell, B. Goldblum, C. W. Beausang, T. Ross, R. Hughes, M. Aiche, G. Barreau, N. Cappelan, S. Czajkowski, B. Hass, B. Jurado, L. Mathieu and I. Companis 8 Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA Service de Physique Nucleaire, Commissariat a l’Energie Atomique, Bruyeres-le-Chatel, Arpajon, France Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Department of Nuclear Engineering, University of California, Berkeley, California 94720, USA Nuclear Physics Department, University of Richmond, Richmond, Virginia 23173, USA Centre d’Etudes Nucleaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Centre d’Etudes Nuclaires Bordeaux Gradignan, CNRS/IN2P3, Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France Horia Hulubei National Institute for Physics and Nuclear Engineering, 077125 Bucharest-Magurele, Romania

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Nuclear fission: a review of experimental advances and phenomenology.

Abstract: In the last two decades, through technological, experimental and theoretical advances, the situation in experimental fission studies has changed dramatically. With the use of advanced production and detection techniques both much more detailed and precise information can now be obtained for the traditional regions of fission research and, crucially, new regions of nuclei have become routinely accessible for fission studies. This work first of all reviews the recent developments in experimental fission techniques, in particular the resurgence of transfer-induced fission reactions with light and heavy ions, the emerging use of inverse-kinematic approaches, both at Coulomb and relativistic energies, and of fission studies with radioactive beams. The emphasis on the fission-fragment mass and charge distributions will be made in this work, though some of the other fission observables, such as prompt neutron and γ-ray emission will also be reviewed. A particular attention will be given to the low-energy fission in the so far scarcely explored nuclei in the very neutron-deficient lead region. They recently became the focus for several complementary experimental studies, such as β-delayed fission with radioactive beams at ISOLDE(CERN), Coulex-induced fission of relativistic secondary beams at FRS(GSI), and several prompt fusion-fission studies. The synergy of these approaches allows a unique insight in the new region of asymmetric fission around 180Hg, recently discovered at ISOLDE. Recent extensive theoretical efforts in this region will also be outlined. The unprecedented high-quality data for fission fragments, completely identified in Z and A, by means of reactions in inverse kinematics at FRS(GSI) and VAMOS(GANIL) will be also reviewed. These experiments explored an extended range of mercury-to-californium elements, spanning from the neutron-deficient to neutron-rich nuclides, and covering both asymmetric, symmetric and transitional fission regions. Some aspects of heavy-ion induced fusion-fission and quasifission reactions will be also discussed, which reveal their dynamical features, such as the fission time scale. The crucial role of the multi-chance fission, probed by means of multinucleon-transfer induced fission reactions, will be highlighted. The review will conclude with the discussion of the new experimental fission facilities which are presently being brought into operation, along with promising 'next-generation' fission approaches, which might become available within the next decade.

Current Status of r-Process Nucleosynthesis

Abstract: The rapid neutron capture process (r process) is believed to be responsible for about half of the production of the elements heavier than iron and contributes to abundances of some lighter nuclides as well. A universal pattern of r-process element abundances is observed in some metal-poor stars of the Galactic halo. This suggests that a well-regulated combination of astrophysical conditions and nuclear physics conspires to produce such a universal abundance pattern. The search for the astrophysical site for r-process nucleosynthesis has stimulated interdisciplinary research for more than six decades. There is currently much enthusiasm surrounding evidence for r-process nucleosynthesis in binary neutron star mergers in the multi-wavelength follow-up observations of kilonova/gravitational-wave GRB170807A/GW170817. Nevertheless, there remain questions as to the contribution over the history of the Galaxy to the current solar-system r-process abundances from other sites such as neutrino-driven winds or magnetohydrodynamical ejection of material from core-collapse supernovae. In this review we highlight some current issues surrounding the nuclear physics input, astronomical observations, galactic chemical evolution, and theoretical simulations of r-process astrophysical environments with the goal of outlining a path toward resolving the remaining mysteries of the r process.

Constraining Neutron Capture Cross Sections for Unstable Nuclei with Surrogate Reaction Data and Theory

Abstract: Obtaining reliable data for nuclear reactions on unstable isotopes remains an extremely important task and a formidable challenge. Neutron capture cross sections---crucial ingredients for models of astrophysical processes, national security applications, and simulations of nuclear energy generation---are particularly elusive, as both projectile and target in the reaction are unstable. We demonstrate a new method for determining cross sections for neutron capture on unstable isotopes, using $^{87}\mathrm{Y}(n,\ensuremath{\gamma})$ as a prototype. To validate the method, a benchmark experiment is carried out to obtain the known $^{90}\mathrm{Zr}(n,\ensuremath{\gamma})$ cross section analogously. Our approach, which employs an indirect (``surrogate'') measurement combined with theory, can be generalized to a larger class of nuclear reactions. It can be used both with traditional stable-beam experiments and in inverse kinematics at rare-isotope facilities.