743 research outputs found
Monoenergetic proton beams accelerated by a radiation pressure driven shock
High energy ion beams (> MeV) generated by intense laser pulses promise to be
viable alternatives to conventional ion beam sources due to their unique
properties such as high charge, low emittance, compactness and ease of beam
delivery. Typically the acceleration is due to the rapid expansion of a laser
heated solid foil, but this usually leads to ion beams with large energy
spread. Until now, control of the energy spread has only been achieved at the
expense of reduced charge and increased complexity. Radiation pressure
acceleration (RPA) provides an alternative route to producing laser-driven
monoenergetic ion beams. In this paper, we show the interaction of an intense
infrared laser with a gaseous hydrogen target can produce proton spectra of
small energy spread (~ 4%), and low background. The scaling of proton energy
with the ratio of intensity over density (I/n) indicates that the acceleration
is due to the shock generated by radiation-pressure driven hole-boring of the
critical surface. These are the first high contrast mononenergetic beams that
have been theorised from RPA, and makes them highly desirable for numerous ion
beam applications
Exotic bulk viscosity and its influence on neutron star r-modes
We investigate the effect of exotic matter in particular, hyperon matter on
neutron star properties such as equation of state (EoS), mass-radius
relationship and bulk viscosity. Here we construct equations of state within
the framework of a relativistic field theoretical model. As hyperons are
produced abundantly in dense matter, hyperon-hyperon interaction becomes
important and is included in this model. Hyperon-hyperon interaction gives rise
to a softer EoS which results in a smaller maximum mass neutron star compared
with the case without the interaction. Next we compute the coefficient of bulk
viscosity and the corresponding damping time scale due to the non-leptonic weak
process including hyperons. Further, we investigate the role of the
bulk viscosity on gravitational radiation driven r-mode instability in a
neutron star of given mass and temperature and find that the instability is
effectively suppressed.Comment: 5 pages, 3 figure, presented in the Conference on Isolated Neutron
Stars: From the Interior to The Surface, London, UK, 24-28 April, 2006;
revised and final version to appear in Astrophys. Space Sc
Neutron Star Constraints on the H Dibaryon
We study the influence of a possible H dibaryon condensate on the equation of
state and the overall properties of neutron stars whose population otherwise
contains nucleons and hyperons. In particular, we are interested in the
question of whether neutron stars and their masses can be used to say anything
about the existence and properties of the H dibaryon. We find that the equation
of state is softened by the appearance of a dibaryon condensate and can result
in a mass plateau for neutron stars. If the limiting neutron star mass is about
that of the Hulse-Taylor pulsar a condensate of H dibaryons of vacuum mass 2.2
GeV and a moderately attractive potential in the medium could not be ruled out.
On the other hand, if the medium potential were even moderately repulsive, the
H, would not likely exist in neutron stars. If neutron stars of about 1.6 solar
mass were known to exist, attractive medium effects for the H could be ruled
out. Certain ranges of dibaryon mass and potential can be excluded by the mass
of the Hulse-Taylor pulsar which we illustrate graphically.Comment: Revised by the addition of a figure showing the region of dibaryon
mass and potential excluded by the Hulse-Taylor pulsar. 18 pages, 11 figures,
latex (submitted to Phys. Rev. C
Chiral Lagrangian for strange hadronic matter
A generalized Lagrangian for the description of hadronic matter based on the
linear -model is proposed. Besides the baryon
octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken
scale invariance is incorporated. The observed values for the vacuum masses of
the baryons and mesons are reproduced. In mean-field approximation, vector and
scalar interactions yield a saturating nuclear equation of state. We discuss
the difficulties and possibilities to construct a chiral invariant baryon-meson
interaction that leads to a realistic equation of state. It is found that a
coupling of the strange condensate to nucleons is needed to describe the
hyperon potentials correctly. The effective baryon masses and the appearance of
an abnormal phase of nearly massless nucleons at high densities are examined. A
nonlinear realization of chiral symmetry is considered, to retain a Yukawa-type
baryon-meson interaction and to establish a connection to the Walecka-model.Comment: Revtex, submitted to Phys. Rev.
Search for positively charged strangelets and other related results with E864 at the AGS
We report on the latest results in the search for positively charged
strangelets from E864's 96/97 run at the AGS with sensitivity of about per central collision. This contribution also contains new results of
a search for highly charged strangelets with . Production of light
nuclei, such as and , is presented as well. Measurements of yields
of these rarely produced isotopes near midrapidity will help constrain the
production levels of strangelets via coalescence. E864 also measures antiproton
production which includes decays from antihyperons. Comparisons with antiproton
yields measured by E878 as a function of centrality indicate a large
antihyperon-to-antiproton ratio in central collisions.Comment: 8 pages, 4 figures; Talk at SQM'98, Padova, Italy (July 20-24th,
1998
4f spin density in the reentrant ferromagnet SmMn2Ge2
The spin contribution to the magnetic moment in SmMn2Ge2 has been measured by
magnetic Compton scattering in both the low and high temperature ferromagnetic
phases. At low temperature, the Sm site is shown to possess a large 4f spin
moment of 3.4 +/- 0.1 Bohr magnetons, aligned antiparallel to the total
magnetic moment. At high temperature, the data show conclusively that ordered
magnetic moments are present on the samarium site.Comment: 5 pages, 2 figures, transferred from PRL to PRB (Rapid Comm.
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