16,125 research outputs found
The B Neutrino Spectrum
Knowledge of the energy spectrum of B neutrinos is an important
ingredient for interpreting experiments that detect energetic neutrinos from
the Sun. The neutrino spectrum deviates from the allowed approximation because
of the broad alpha-unstable Be final state and recoil order corrections to
the beta decay. We have measured the total energy of the alpha particles
emitted following the beta decay of B. The measured spectrum is
inconsistent with some previous measurements, in particular with a recent
experiment of comparable precision. The beta decay strength function for the
transition from B to the accessible excitation energies in Be is fit to
the alpha energy spectrum using the R-matrix approach. Both the positron and
neutrino energy spectra, corrected for recoil order effects, are constructed
from the strength function. The positron spectrum is in good agreement with a
previous direct measurement. The neutrino spectrum disagrees with previous
experiments, particularly for neutrino energies above 12 MeV.Comment: 15 pages, 13 figures, 4 tables, submitted to Phys. Rev. C, typos
correcte
Structure Effects on Coulomb Dissociation of B
Coulomb Dissociation provides an alternative method for determining the
radiative capture cross sections at astrophysically relevant low relative
energies. For the breakup of B on Ni, we calculate the total Coulomb
Dissociation cross section and the angular distribution for E1, E2 and M1. Our
calculations are performed first within the standard first order semiclassical
theory of Coulomb Excitation, including the correct three body kinematics, and
later including the projectile-target nuclear interactions.Comment: 6 pages, proceedings from International Workshop on RNB, Puri, India,
January 1998 - to be published in J. Phys.
Low-energy quenching of positronium by helium
Very low-energy scattering of orthopositronium by helium has been
investigated for simultaneous study of elastic cross section and pick-off
quenching rate using a model exchange potential. The present calculational
scheme, while agrees with the measured cross section of Skalsey et al,
reproduces successfully the parameter ^ 1Z_{\makebox{eff}}, the effective
number of electrons per atom in a singlet state relative to the positron.
Together with the fact that this model potential also leads to an agreement
with measured medium energy cross sections of this system, this study seems to
resolve the long-standing discrepancy at low energies among different
theoretical calculations and experimental measurements.Comment: 4 latex pages, 3 postscript figure
Nature of the Darwin term and contribution to the Lamb shift for an arbitrary spin of the nucleus
The contact Darwin term is demonstrated to be of the same origin as the
spin-orbit interaction. The correction to the Lamb shift,
generated by the Darwin term, is found for an arbitrary nonvanishing spin of
the nucleus, both half-integer and integer. There is also a contribution of the
same nature to the nuclear quadrupole moment.Comment: 9 pages, latex, no figure
Determination of S17 from 8B breakup by means of the method of continuum-discretized coupled-channels
The astrophysical factor for 7Be(p,\gamma)8B at zero energy, S17(0), is
determined from an analysis of 208Pb(8B, p+7Be)208Pb at 52 MeV/nucleon by means
of the method of continuum-discretized coupled-channels (CDCC) taking account
of all nuclear and Coulomb breakup processes. The asymptotic normalization
coefficient (ANC) method is used to extract S17(0) from the calculated
breakup-cross-section. The main result of the present paper is S17(0)=20.9
+2.0/-1.9 eV b. The error consists of 8.4% experimental systematic error and
the error due to the ambiguity in the s-wave p-7Be scattering length. This
value of S17(0) differs from the one extracted with the first-order
perturbation theory including Coulomb breakup by dipole transitions: 18.9 +/-
1.8 eV b. It turns out that the difference is due to the inclusion of the
nuclear and Coulomb-quadrupole transitions and multi-step processes of
all-order in the present work. The p-7Be interaction potential used in the CDCC
calculation is also used in the ANC analysis of 7Be(p,\gamma)8B. The value of
S17(0)=21.7 +0.62/-0.55 eV b obtained is consistent with the previous one
obtained from a precise measurement of the p-capture reaction cross section
extrapolated to zero incident energy, S17(0)=22.1 +/- 0.6 (expt) +/- 0.6 (theo)
eV b, where (theo) stands for the error in the extrapolation. Thus, the
agreement between the values of S17(0) obtained from direct 7Be(p,\gamma)8B and
indirect 8B-breakup measurements is significantly improved.Comment: 13 pages, 9 figures, published in PR
Subharmonics and Aperiodicity in Hysteresis Loops
We show that it is possible to have hysteretic behavior for magnets that does
not form simple closed loops in steady state, but must cycle multiple times
before returning to its initial state. We show this by studying the
zero-temperature dynamics of the 3d Edwards Anderson spin glass. The specific
multiple varies from system to system and is often quite large and increases
with system size. The last result suggests that the magnetization could be
aperiodic in the large system limit for some realizations of randomness. It
should be possible to observe this phenomena in low-temperature experiments.Comment: 4 pages, 3 figure
The 13N(d,n)14O Reaction and the Astrophysical 13N(p,g)14O Reaction Rate
N()O is one of the key reactions in the hot CNO cycle
which occurs at stellar temperatures around 0.1. Up to now, some
uncertainties still exist for the direct capture component in this reaction,
thus an independent measurement is of importance. In present work, the angular
distribution of the N()O reaction at = 8.9
MeV has been measured in inverse kinematics, for the first time. Based on the
distorted wave Born approximation (DWBA) analysis, the nuclear asymptotic
normalization coefficient (ANC), , for the ground state of
O N + is derived to be fm. The
N()O reaction is analyzed with the R-matrix approach,
its astrophysical S-factors and reaction rates at energies of astrophysical
relevance are then determined with the ANC. The implications of the present
reaction rates on the evolution of novae are then discussed with the reaction
network calculations.Comment: 17 pages and 8 figure
The effect of interferon beta-1b treatment on MRI measures of cerebral atrophy in secondary progressive multiple sclerosis.
The recently completed European trial of interferon beta-1b (IFN beta -1b) in patients with secondary progressive multiple sclerosis (SP multiple sclerosis) has given an opportunity to assess the impact of treatment on cerebral atrophy using serial MRI. Unenhanced T-1-weighted brain imaging was acquired in a subgroup of 95 patients from five of the European centres; imaging was performed at 6-month intervals from month 0 to month 36. A blinded observer measured cerebral volume on four contiguous 5 mm cerebral hemisphere slices at each time point, using an algorithm with a high level of reproducibility and automation. There was a significant and progressive reduction in cerebral volume in both placebo and treated groups, with a mean reduction of 3.9 and 2.9%, respectively, by month 36 (P = 0.34 between groups). Exploratory subgroup analyses indicated that patients without gadolinium (Gd) enhancement at the baseline had a greater reduction of cerebral volume in the placebo group (mean reduction at month 36: placebo 5.1%, IFN beta -1b 1.8%, P < 0.05) whereas those with Gd-enhancing lesions showed a trend to greater reduction of cerebral volume if the patient was on IFN<beta>-1b (placebo 2.6%, IFN beta -1b, 3.7%; P > 0.05). These results are consistent with ongoing tissue loss in both arms of this study of secondary progressive multiple sclerosis. This finding is concordant with previous observations that disease progression, although delayed, is not halted by IFN beta. The different pattern seen in patients with and without baseline gadolinium enhancement suggests that part of the cerebral volume reduction observed in IFN beta -treated patients may be due to the anti-inflammatory/antioedematous effect of the drug. Longer periods of observation and larger groups of patients may be needed to detect the effects of treatment on cerebral atrophy in this population of patients with advanced disease
Asymptotic normalization coefficients (nuclear vertex constants) for and the direct astrophysical S-factors at solar energies
A new analysis of the precise experimental astrophysical S-factors for the
direct capture reaction [A.J.Junghans et al.Phys.Rev. C
68 (2003) 065803 and L.T. Baby et al. Phys.Rev. C 67 (2003) 065805] is carried
out based on the modified two - body potential approach in which the direct
astrophysical S-factor, , is expressed in terms of the
asymptotic normalization constants for and two additional
conditions are involved to verify the peripheral character of the reaction
under consideration. The Woods-Saxon potential form is used for the bound
()- state wave function and for the - scattering wave function.
New estimates are obtained for the ^{\glqq}indirectly measured\grqq values of
the asymptotic normalization constants (the nuclear vertex constants) for the
and at E 115 keV, including =0. These
values of and asymptotic normalization constants have been used for
getting information about the ^{\glqq}indirectly measured\grqq values of the
wave average scattering length and the wave effective range parameters
for - scattering.Comment: 27 pages, 6 figure
The Hilbert-Schmidt Theorem Formulation of the R-Matrix Theory
Using the Hilbert-Schmidt theorem, we reformulate the R-matrix theory in
terms of a uniformly and absolutely convergent expansion. Term by term
differentiation is possible with this expansion in the neighborhood of the
surface. Methods for improving the convergence are discussed when the
R-function series is truncated for practical applications.Comment: 16 pages, Late
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