Influence of compacting pressure on the magnetic and structural properties in Gd5Ge2.15Si1.85 alloy obtained by powder metallurgy

The magnetocaloric effect (MCE) has been studied seeking their application in cooling systems near room temperature (domestic employ). The magnetic refrigerators work with nontoxic solids that substitute the refrigerant gases used in hermetic compressors, without relying on ozone-depleting coolants. Some alloys of Gd5(Ge xSi4-x) present the giant magnetocaloric effect (GMCE), and the Gd5Ge2Si2 alloy, announced in 1997 by Pecharsky e Gschneidner, have a first-order magneto-structural transition at the Curie Temperature (Tc = 273 K) and MCE around 30 J.(kg.K)-1, being probable their use in those cooling systems. However, when that material is sintered, it loses its initial characteristics during the powder metallurgy process. Therefore, this study will treat of the compacting pressure interferences on the structural and magnetic properties of the Gd5Ge2.15Si1.85 alloy.O denominado efeito magnetocalórico (EMC) vem sendo estudado visando sua aplicação em sistemas de refrigeração próximas à temperatura ambiente (uso doméstico). Os refrigeradores magnéticos operam com base em sólidos atóxicos que substituem os gases refrigerantes usados em compressores herméticos, como os CFC?s e HCFC?s, altamente poluentes. Algumas ligas da família Gd5(Ge xSi4-x) apresentam o efeito magnetocalórico gigante (EMCG), sendo que a liga Gd5Ge2Si2, apresentada em 1997 por Pecharsky e Gschneidner, possui transição magneto-estrutural de primeira ordem na temperatura de Curie (Tc = 273 K) e EMC por volta de 30 J.(kg.K)-1, sendo provável o seu uso nesses sistemas de refrigeração. Entretanto, quando submetida a uma pressão de compactação e posterior sinterização, o material perde suas características iniciais. Assim sendo, este estudo tratará da influência da pressão de compactação sobre as propriedades estruturais e magnéticas na liga de Gd5Ge2,15Si1,85.12512

Gamow-Teller Strength Distribution for 37-Cl(p,n)37-Ar

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Theory and computation of covariant Lyapunov vectors

Lyapunov exponents are well-known characteristic numbers that describe growth rates of perturbations applied to a trajectory of a dynamical system in different state space directions. Covariant (or characteristic) Lyapunov vectors indicate these directions. Though the concept of these vectors has been known for a long time, they became practically computable only recently due to algorithms suggested by Ginelli et al. [Phys. Rev. Lett. 99, 2007, 130601] and by Wolfe and Samelson [Tellus 59A, 2007, 355]. In view of the great interest in covariant Lyapunov vectors and their wide range of potential applications, in this article we summarize the available information related to Lyapunov vectors and provide a detailed explanation of both the theoretical basics and numerical algorithms. We introduce the notion of adjoint covariant Lyapunov vectors. The angles between these vectors and the original covariant vectors are norm-independent and can be considered as characteristic numbers. Moreover, we present and study in detail an improved approach for computing covariant Lyapunov vectors. Also we describe, how one can test for hyperbolicity of chaotic dynamics without explicitly computing covariant vectors.Comment: 21 pages, 5 figure

Progress Report on E356

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Search for the Kπ=1+ two-proton band in Er166

The Ho165 (α,t) and (3He,d) reactions have been measured at beam energies of 40 and 25 MeV to look for the Kπ=1+{9/2-[514]-7/2-[523]} two-proton band structure in Er166. Reaction products were analyzed using a quadrupole-three-dipole spectrometer. Distorted-wave Born approximation predictions were used to extract candidate states with angular momentum transfer l=5. The measured strengths of all the candidates for l=5 transitions were compared with the Nilsson fingerprint pattern of the Kπ=1+{9/2-[514]-7/2- [523]} band to identify the structure. No strong population of this configuration was found. At best two tentative candidates for the Iπ=3+ and 4+ members of this band could be identified, which would represent at most 38% of the expected strength for this configuration. The bandhead for this tentative Kπ=1+ excitation does not correspond to any Iπ=1+ state observed in the Er166(γ,γ') reaction

The Gamow-Teller Strength Function for 37-Cl → 37-Ar

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

A Comparison of 37-Ca(p,n) Cross Sections to 37-Ca β-Decay

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Alpha-decay branching ratios of near-threshold states in 19Ne and the astrophysical rate of 15O(alpha,gamma)19Ne

The 15O(alpha,gamma)19Ne reaction is one of two routes for breakout from the hot CNO cycles into the rp process in accreting neutron stars. Its astrophysical rate depends critically on the decay properties of excited states in 19Ne lying just above the 15O + alpha threshold. We have measured the alpha-decay branching ratios for these states using the p(21Ne,t)19Ne reaction at 43 MeV/u. Combining our measurements with previous determinations of the radiative widths of these states, we conclude that no significant breakout from the hot CNO cycle into the rp process in novae is possible via 15O(alpha,gamma)19Ne, assuming current models accurately represent their temperature and density conditions

Structure of 18Ne and the breakout from the hot CNO cycle

We used the 16O(3He,n)18Ne, 12C(12C,6He)18Ne, and 20Ne(p,t)18Ne reactions to study 18Ne states up to an excitation energy of 10 MeV, with emphasis on levels corresponding to 14O(α,p)17F and 17F(p,γ)18Ne resonances that could strongly affect these reaction rates in hot stellar environments. Excitation energies, widths, absolute cross sections, and angular distributions were measured. We found previously unidentified states at Ex=6.15±0.01 MeV, 7.12±0.02 MeV, 7.35±0.02 MeV, 7.62±0.02 MeV, 8.30±0.02 MeV, (8.45 ±0.03 MeV), 8.55±0.03 MeV, 8.94±0.02 MeV, and 9.58±0.02 MeV. We combined level width, cross section, and angular distribution data to infer Jπ values for a number of the new levels as well as for the previously known 5.1-MeV doublet. Using information from our experiments, we recalculated the 14O(α.p) 17F reaction rate, which constitutes a possible path out of the hot CNO cycle into the rp process and could play an important role in transforming nuclei involved in the hot CNO cycle into heavier nuclei with Z≥10