535 research outputs found
Mean first passage time for fission potentials having structure
A schematic model of over-damped motion is presented which permits one to
calculate the mean first passage time for nuclear fission. Its asymptotic value
may exceed considerably the lifetime suggested by Kramers rate formula, which
applies only to very special, favorable potentials and temperatures. The
additional time obtained in the more general case is seen to allow for a
considerable increment in the emission of light particles.Comment: 7 pages, LaTex, 7 postscript figures; Keywords: Decay rate, mean
first passage tim
One-body energy dissipation in fusion reaction from mean-field theory
Information on dissipation in the entrance channel of heavy-ion collisions is
extracted by macroscopic reduction procedure of Time-Dependent Hartree-Fock
theory. The method gives access to a fully microscopic description of the
friction coefficient associated with transfer of energy from the relative
motion towards intrinsic degrees of freedom. The reduced friction coefficient
exhibits a universal behavior, i.e. almost independent of systems investigated,
whose order of magnitude is comparable with the calculations based on linear
response theory. Similarly to nucleus-nucleus potential, especially close to
the Coulomb barrier, there are sizable dynamical effects on the magnitude and
form factor of friction coefficient.Comment: 7 pages, 10 figure
Comment on ``Superconducting PrBa_2Cu_3O_x''
Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the
observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single
crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of
this Comment is to show the inconsistency of the value of effective magnetic
moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic
susceptibility data. The estimation made directly from their data points gives
a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the
values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single
crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This
suggests that Pr occupies only about a half of the RE sites in TSFZ crystal.
The other half of the RE sites is occupied most probably by the nonmagnetic Ba.
Noteworthy, SC with T_c=43 K was observed earlier for
Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius
than Pr^{3+} and so the substitution of Ba for Pr could give a natural
explanation not only for the SC in TSFZ Pr123 but also for the elongation of
the distance between the CuO_2 planes observed by Zou et al.Comment: Slightly extended version of Comment accepted to Phys. Rev. Lett.
(v.81, N24, 1998), tentatevely to be publ. 14Dec98. 1 page, REVTex; 1 EPS fi
Intermediate valence behavior in CeCo9Si4
The novel ternary compound CeCoSi has been studied by means of
specific heat, magnetisation, and transport measurements. Single crystal X-ray
Rietveld refinements reveal a fully ordered distribution of Ce, Co and Si atoms
with the tetragonal space group I4/mcm isostructural with other RCo9Si4. The
smaller lattice constants of CeCo9Si4 in comparison with the trend established
by other RCo9Si4 is indicative for intermediate valence of cerium. While
RCo9Si4 with R= Pr, .. Tb, and Y show ferromagnetism and LaCo9Si4 is nearly
ferromagnetic, CeCo9Si4 remains paramagnetic even in external fields as large
as 40 T, though its electronic specific heat coefficient (g~190 mJ/molK^2) is
of similar magnitude as that of metamagnetic LaCo9Si4 and weakly ferromagnetic
YCo9Si4.Comment: 2 pages, 3 figures, submitted to SCES 0
Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals: Evidence for magnetic coupling between the Cu and Pr sublattices
In Al-free PrBa_2Cu_3O_{7-y} single crystals the kink in the temperature
dependence of magnetic susceptibility chi_{ab}(T), connected with Pr
antiferromagnetic ordering, disappears after field cooling (FC) in a field H ||
ab-plane. The kink in chi_c(T) remains unchanged after FC in H || c-axis. As a
possible explanation, freezing of the Cu magnetic moments, lying in the
ab-plane, caused by FC in H || ab, hinders their reorientation and, due to
coupling between the Pr and Cu(2) sublattices, ordering of the Pr^{3+} moments.
A field induced phase transition and a field dependence of the Pr^{3+} ordering
temperature have been found for both H || c and H || ab.Comment: 11 pages (LaTex with elsart.sty), 5 EPS figs. Accepted to Physica
Continuum corrections to the level density and its dependence on excitation energy, n-p asymmetry, and deformation
In the independent-particle model, the nuclear level density is determined
from the neutron and proton single-particle level densities. The
single-particle level density for the positive-energy continuum levels is
important at high excitation energies for stable nuclei and at all excitation
energies for nuclei near the drip lines. This single-particle level density is
subdivided into compound-nucleus and gas components. Two methods were
considered for this subdivision. First in the subtraction method, the
single-particle level density is determined from the scattering phase shifts.
In the Gamov method, only the narrow Gamov states or resonances are included.
The level densities calculated with these two methods are similar, both can be
approximated by the backshifted Fermi-gas expression with level-density
parameters that are dependent on A, but with very little dependence on the
neutron or proton richness of the nucleus. However, a small decrease in the
level-density parameter was predicted for some nuclei very close to the drip
lines. The largest difference between the calculations using the two methods
was the deformation dependence on the level density. The Gamov method predicts
a very strong peaking of the level density at sphericity for high excitation
energies. This leads to a suppression of deformed configurations and,
consequently, the fission rate predicted by the statistical model is reduced in
the Gamov method.Comment: 18 pages 24 figure
Statistical fluctuations for the fission process on its decent from saddle to scission
We reconsider the importance of statistical fluctuations for fission dynamics
beyond the saddle in the light of recent evaluations of transport coefficients
for average motion. The size of these fluctuations are estimated by means of
the Kramers-Ingold solution for the inverted oscillator, which allows for an
inclusion of quantum effects.Comment: 12 pages, Latex, 5 Postscript figures; submitted to PRC e-mail:
[email protected] www home page:
http://www.physik.tu-muenchen.de/tumphy/e/T36/hofmann.htm
Improved modelling of helium and tritium production for spallation targets
Reliable predictions of light charged particle production in spallation
reactions are important to correctly assess gas production in spallation
targets. In particular, the helium production yield is important for assessing
damage in the window separating the accelerator vacuum from a spallation
target, and tritium is a major contributor to the target radioactivity. Up to
now, the models available in the MCNPX transport code, including the widely
used default option Bertini-Dresner and the INCL4.2-ABLA combination of models,
were not able to correctly predict light charged particle yields. The work done
recently on both the intranuclear cascade model INCL4, in which cluster
emission through a coalescence process has been introduced, and on the
de-excitation model ABLA allows correcting these deficiencies. This paper shows
that the coalescence emission plays an important role in the tritium and
production and that the combination of the newly developed versions of the
codes, INCL4.5-ABLA07, now lead to good predictions of both helium and tritium
cross sections over a wide incident energy range. Comparisons with other
available models are also presented.Comment: 6 pages, 9 figure
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