News Article | May 19, 2017
The sessions on superconductivity at the IXth International Conference on High Energy Accelerators, held at the Stanford Linear Accelerator Centre from 2–7 May, were somehow rather frustrating. For many years, the potential of superconductivity, both in radio-frequency and magnet applications, has seemed on the brink of opening new doors. A lot has been achieved in practical realisation and in increasing basic understanding but, for many factors important for the future big projects, it is extremely difficult to get convincing answers. There was a woolliness about many of the discussions which needs to be cleared up. On the r.f. side, superconducting cavities could give high accelerating voltage gradients and low power absorption, allowing cavities to be operated for longer times resulting in high duty cycle linacs and separators. In r.f. conditions the losses do not disappear completely but fall exponentially with temperature near absolute zero. Hence there is interest in pushing temperatures lower than is adequate for superconducting magnets. The currents flow in the surface layer of superconductors and the major problems have been concerned with achieving good quality surfaces in large r.f. structures and retaining their properties in operation. The news of the work on superconducting magnets was equally frustrating. On the one hand, the past few years have seen d.c. superconducting magnets being brought into reliable operation at accelerators (for example, the Optique à Grande Acceptance OGA quadrupoles at Saclay and beam-line magnets at Berkeley and Brookhaven). These magnets have thousands of hours of physics use under their belts. Also many pulsed magnets have been through their paces in the laboratory with reasonable success. On the other hand, all the magnets have exhibited training to some degree. In other words, we still do not manage to avoid small mechanical movements of the superconductor and because of this, a multi-magnet project would need to design the machine for a field considerably lower than the optimum, since we would not be sure to what fields the magnets would “train”. Among other factors which could lead to accepting lower performance figures is the temperature sensitivity of the superconductor. Under usual operating conditions, with niobium-titanium superconductor at liquid helium temperature of 4.2 K giving fields of about 4.5 T, fluctuations of a few tenths of a degree can flip the magnets out of their superconducting state. There is a need to develop other superconducting materials, such as vanadium-gallium or niobium-tin. These materials have a much higher critical temperature (about 17 K) and could be operated at higher current densities to give fields of 6 T or above with comfortable temperature stability. The materials are however extremely brittle and the metallurgical problems of using them are not yet solved. The CERN rugby team won the final of the Swiss Cup on 26 May, having already taken the Swiss league championship for 1973–74. It was a strong year for CERN rugby – the reserve team also won their championship and the junior team won all the matches in its category. Professor Jentschke, Director-General of CERN Lab I, fires the starting pistol for the annual team race around the CERN site. Forty-five teams raced on 29 May 1974. A Theory Division team won in 12 minutes 5 seconds, comfortably beating the track record. Notwithstanding the somewhat pessimistic note struck at the Stanford Conference, in 1983 the first superconducting particle accelerator went into operation. The Tevatron at Fermilab was a 6.3 km circular synchrotron with 990 magnets at 4.4 K giving fields of 4.4 T. In 1995, the first large-scale superconducting r.f. accelerator came on air. The Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson Lab consisted of two linacs with 1.5 GHz Nb cavities operating at 2 K. And in 2008, CERN’s Large Hadron Collider (LHC) became the largest scientific instrument in the world. Around the 27 km circumference, 1706 main superconducting magnets cooled to 1.9 K provided fields of 8.3 T. The CERN Rugby Club – now the Rugby Club of CERN, Meyrin and St Genis (RC CMSG) – is one of the oldest in Switzerland and today it has male and female teams across all ages who take part in the Swiss leagues. Players span 23 nationalities, representing almost a quarter of the 100 or so countries that play the game worldwide. The CERN annual relay race also remains a popular social event, organised by the Running Club and Staff Association. In May 2016, 127 teams competed, with the winners scoring a record time of 10 minutes 19 seconds.
Zhang Z.,Shanghai JiaoTong University |
Chen L.-W.,Shanghai JiaoTong University |
Chen L.-W.,Accelerator Centre
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013
We show that the neutron skin thickness δrnp of heavy nuclei is uniquely fixed by the symmetry energy density slope L(ρ) at a subsaturation cross density ρc≈0.11 fm-3 rather than at saturation density ρ0, while the binding energy difference δE between a heavy isotope pair is essentially determined by the magnitude of the symmetry energy Esym(ρ) at the same ρc. Furthermore, we find a value of L(ρc) leads to a negative Esym(ρ0)-L(ρ0) correlation while a value of Esym(ρc) leads to a positive one. Using data on δrnp of Sn isotopes and δE of a number of heavy isotope pairs, we obtain simultaneously Esym(ρc)=26.65±0.20 MeV and L(ρc)=46.0±4.5 MeV at 95% confidence level, whose extrapolation gives Esym(ρ0)=32.3±1.0 MeV and L(ρ0)=45.2±10.0 MeV. The implication of these new constraints on the δrnp of 208Pb and the core-crust transition density in neutron stars is discussed. © 2013 Elsevier B.V.
Chen L.-W.,Shanghai JiaoTong University |
Chen L.-W.,Accelerator Centre
Physical Review C - Nuclear Physics | Year: 2011
Within the Skyrme-Hartree-Fock (SHF) approach, we show that for a fixed mass number A, both the symmetry energy coefficient asym(A) in the semiempirical mass formula and the nuclear matter symmetry energy E sym(ρA) at a subsaturation reference density ρA can be determined essentially by the symmetry energy E sym(ρ0) and its density slope L at saturation density ρ0. Meanwhile, we find the dependence of asym(A) on Esym(ρ0) or L is approximately linear and very similar to the corresponding linear dependence displayed by Esym(ρ A), providing an explanation for the relation Esym(ρ A)ρasym(A). Our results indicate that a value of Esym(ρA) leads to a linear correlation between E sym(ρ0) and L and thus can put important constraints on Esym(ρ0) and L. Particularly, the values of E sym(ρ0)=30.5±3 MeV and L= 52.5±20 MeV are simultaneously obtained by combining the constraints from recently extracted Esym(ρA=0.1 fm-3) with those from recent analyses of neutron skin thickness of Sn isotopes in the same SHF approach. © 2011 American Physical Society.
Funaki Y.,Accelerator Centre
Physical Review C - Nuclear Physics | Year: 2015
The excited states in C12 are investigated by using an extended version of the so-called Tohsaki-Horiuchi-Schuck-Röpke (THSR) wave function, where both the 3α condensate and Be8+α cluster asymptotic configurations are included. A new method is also used to resolve spurious continuum coupling with physical states. I focus on the structures of the "Hoyle band" states (02+,22+, and 42+), which were recently observed above the Hoyle state, and of the 03+ and 04+ states, which were also quite recently identified in experiment. Their resonance parameters and decay properties are reasonably reproduced. All these states have dilute density structure of the 3α or Be8+α clusters with larger root mean square radii than that of the Hoyle state. The Hoyle band is not simply considered to be the Be8(0+)+α rotation as suggested by previous cluster model calculations, nor to be a rotation of a rigid-body triangle-shaped object composed of the 3α particles. This is mainly due to the specificity of the Hoyle state, which has the 3α condensate structure and gives rise to the 03+ state with a prominent Be8(0+)+α structure as a result of very strong monopole excitation from the Hoyle state. © 2015 American Physical Society. ©2015 American Physical Society.
Yoshida K.,Accelerator Centre
Physical Review C - Nuclear Physics | Year: 2010
We investigate the roles of deformation on the giant monopole resonance (GMR), particularly the mixing of the giant quadrupole resonance (GQR) and the effects of the neutron excess in the well-deformed nuclei around Zr110 and in the drip-line nuclei around Zr140 by means of the deformed quasiparticle-random- phase approximation employing the Skyrme and the local-pairing energy-density functionals. It is found that the isoscalar (IS) GMR has a two-peak structure, the lower peak of which is associated with the mixing between the ISGMR and the Kπ=0+ component of the ISGQR. The transition strength of the lower peak of the ISGMR grows as the neutron number increases. In the drip-line nuclei, the neutron excitation is dominant over the proton excitation. We find that for an isovector (IV) excitation the GMR has a four-peak structure due to the mixing of the IS and IV modes as well as the mixing of the Kπ=0 + component of the IVGQR. In addition to the GMR, we find that the threshold strength is generated by neutrons only. © 2010 The American Physical Society.
Itahashi K.,Accelerator Centre
Acta Physica Polonica B | Year: 2014
Meson-nucleus bound systems have been providing precious information on the meson properties in nuclear medium, which is leading to understanding of non-trivial structure of the QCD vacuum. Two related experimental projects are discussed. One is a pionic atom factory project at RIBF and the other is a spectroscopy of η′-mesic nuclei in GSI/FAIR. The former is aiming at high precision systematic spectroscopy of pionic atoms followed by challenges for pionic atoms with unstable nuclei. The latter is aiming at spectroscopy of η′-mesic nuclei in (p; d) reactions by inclusive and semi-exclusive measurement.
Hiyama E.,Accelerator Centre
Nuclear Physics A | Year: 2013
The structure of the 10ΛBe and 10ΛB with an α + α + Λ + N four-body model cluster model is studied. The two-body ΛN interaction is adjusted so as to reproduce the 0 +-1 + splitting of 4ΛH. Also a phenomenological ΛN charge symmetry breaking (CSB) interaction is introduced. The ΛN CSB interaction works repulsively by +0.1MeV (attractively by -0.1MeV) in 10ΛBe (10ΛB). A neutron-rich Λ hypernucleus, 6ΛH is studied within a framework of t + Λ + n + n four-body model. As long as we reproduce the energy and width of 5H within the error bar, then ground state of 6ΛH is obtained as a resonant state. © 2013 Elsevier B.V.
Hiyama E.,Accelerator Centre
Few-Body Systems | Year: 2012
Recent development in the study of the structure of light Λ and double Λ hypernuclei is reviewed from the view point of few-body problems and interactions between the constituent particles. In the study the present author and collaborators employed Gaussian expansion method for few-body calculations; the method has been applied to many kinds of few-body systems in the fields of nuclear physics and exotic atomic/molecular physics. We reviewed the following subjects studied using the method: (1) Precise three- and four-body calculations of 7 Λ He, 7 Λ Li, 7 Λ Be, 8 Λ Li, 8 Λ Be, 9 Λ Be, 10 Λ Be, 10 Λ B and 13 Λ C provide important information on the spin structure of the underlying ΛN interaction by comparing the calculated results with the recent experimental data by γ-ray hypernuclear spectroscopy. (2) The Λ-Σ coupling effect was investigated in 4 Λ H and 4 Λ He on the basis of the N + N + N + Λ (Σ) four-body model. (3) A systematic study of double-Λ hypernuclei and the ΛΛ interaction, based on the NAGARA event data ( 6 ΛΛ He), was performed within the α + x + Λ + Λ cluster model (x = n, p, d, t, 3He and α) and α + α + n + Λ + Λ cluster model, (4) The Demachi-Yanagi event was interpreted as observation of the 2 + state of 10 ΛΛ Be, (5) The Hida event was interpreted as observation of the ground state of 11 ΛΛ Be. © 2012 Springer-Verlag.
Wada M.,Accelerator Centre
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2013
In order to overcome serious limitations in the universality of the traditional isotope separator on-line technique, various endeavors have been made on gas catcher cells for converting relativistic RI-beams from in-flight separators to low-energy RI-beams. The origin of the gas catcher is found in the IGISOL (Ion guide isotope separator on-line) technique. Many developments have been made over the years to overcome the various difficulties and drawbacks found in the IGISOL technique. © 2013 Elsevier B.V. All rights reserved.
Danielewicz P.,Michigan State University |
Lee J.,Accelerator Centre
Nuclear Physics A | Year: 2014
Using excitation energies to isobaric analog states (IAS) and charge invariance, we extract nuclear symmetry coefficients, representing a mass formula, on a nucleus-by-nucleus basis. Consistently with charge invariance, the coefficients vary weakly across an isobaric chain. However, they change strongly with nuclear mass and range from a a ~ 10MeV at mass A ~ 10 to a a ~ 22MeV at A ~ 240. Variation with mass can be understood in terms of dependence of nuclear symmetry energy on density and the rise in importance of low densities within nuclear surface in smaller systems. At A ≳ 30, the dependence of coefficients on mass can be well described in terms of a macroscopic volume-surface competition formula with aaV≃33.2MeV and aaS≃10.7MeV. Our further investigation shows, though, that the fitted surface symmetry coefficient likely significantly underestimates that for the limit of half-infinite matter. Following the considerations of a Hohenberg-Kohn functional for nuclear systems, we determine how to find in practice the symmetry coefficient using neutron and proton densities, even when those densities are simultaneously affected by significant symmetry-energy and Coulomb effects. These results facilitate extracting the symmetry coefficients from Skyrme-Hartree-Fock (SHF) calculations, that we carry out using a variety of Skyrme parametrizations in the literature. For the parametrizations, we catalog novel short-wavelength instabilities. In our further analysis, we retain only those parametrizations which yield systems that are adequately stable both in the long- and short-wavelength limits. In comparing the SHF and IAS results for the symmetry coefficients, we arrive at narrow (±2.4MeV) constraints on the symmetry-energy values S(ρ) at 0.04 ≲ ρ ≲ 0.13fm -3. Towards normal density the constraints significantly widen, but the normal value of energy aaV and the slope parameter L are found to be strongly correlated. To narrow the constraints, we reach for the measurements of asymmetry skins and arrive at aaV=30.2-33.7MeV and L = 35-70MeV, with those values being again strongly positively correlated along the diagonal of their combined region. Inclusion of the skin constraints allows to narrow the constraints on S(ρ), at 0.04 ≲ ρ ≲ 0.13fm -3, down to ±1.1MeV. Several microscopic calculations, including variational, Bruckner-Hartree-Fock and Dirac-Bruckner-Hartree-Fock, are consistent with our constraint region on S(ρ). © 2013 Elsevier B.V.