881 research outputs found
Functional Characterisation of Alpha-Galactosidase A Mutations as a Basis for a New Classification System in Fabry Disease
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.The study has been supported partially by an unrestricted scientific grant from Shire Human Genetic Therapies (Germany
Activation measurement of the 3He(alpha,gamma)7Be cross section at low energy
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a
major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted
by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis
calculations. The present work reports on a new precision experiment using the
activation technique at energies directly relevant to big-bang nucleosynthesis.
Previously such low energies had been reached experimentally only by the
prompt-gamma technique and with inferior precision. Using a windowless gas
target, high beam intensity and low background gamma-counting facilities, the
3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV
center-of-mass energy with a total uncertainty of 4%. The sources of systematic
uncertainty are discussed in detail. The present data can be used in big-bang
nucleosynthesis calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar energies
Luttinger-Liquid Behavior in the Alternating Spin-Chain System Copper Nitrate
We determine the phase diagram of copper nitrate Cu(NO)2.5DO
in the context of quantum phase transitions and novel states of matter. We
establish this compound as an ideal candidate to study quasi-1D Luttinger
liquids, 3D Bose-Einstein-Condensation of triplons, and the crossover between
1D and 3D physics. Magnetocaloric effect, magnetization, and neutron scattering
data provide clear evidence for transitions into a Luttinger liquid regime and
a 3D long-range ordered phase as function of field and temperature. Theoretical
simulations of this model material allow us to fully establish the phase
diagram and to discuss it in the context of dimerized spin systems.Comment: 5 pages, 4 figure
Thermal neutron induced (n,p) and (n,alpha) reactions on 37Ar
The 37Ar(n_th,alpha)34S and 37Ar(n_th,p)37Cl reactions were studied at the
high flux reactor of the ILL in Grenoble. For the 37Ar(n_th,alpha_0) and
37Ar(n_th,p) reaction cross sections, values of (1070+/-80)b and (37+/-4)b,
respectively, were obtained. Both values are about a factor 2 smaller than
results of older measurements. The observed suppression of the 37(n_th,alpha_1)
transition could be verified from theoretical considerations. Finally, evidence
was found for the two-step 37Ar(n_th,gamma-alpha) process.Comment: 11 pages, 5 figures, accepted for publication in Nuclear Physics
Thermonuclear Reaction Rate of 23Mg(p,gamma)24$Al
Updated stellar rates for the reaction 23Mg(p,gamma)24Al are calculated by
using all available experimental information on 24Al excitation energies.
Proton and gamma-ray partial widths for astrophysically important resonances
are derived from shell model calculations. Correspondences of experimentally
observed 24Al levels with shell model states are based on application of the
isobaric multiplet mass equation. Our new rates suggest that the
23Mg(p,gamma)24Al reaction influences the nucleosynthesis in the mass A>20
region during thermonuclear runaways on massive white dwarfs.Comment: 13 pages (uses Revtex) including 3 postscript figures (uses
epsfig.sty), accepted for publication in Phys. Rev.
Impact of a revised Mg(p,)Al reaction rate on the operation of the Mg-Al cycle
Proton captures on Mg isotopes play an important role in the Mg-Al cycle
active in stellar H-burning regions. In particular, low-energy nuclear
resonances in the Mg(p,)Al reaction affect the production
of radioactive Al as well as the resulting Mg/Al abundance ratio.
Reliable estimations of these quantities require precise measurements of the
strengths of low-energy resonances. Based on a new experimental study performed
at LUNA, we provide revised rates of the Mg(p,)Al
and the Mg(p,)Al reactions with corresponding
uncertainties. In the temperature range 50 to 150 MK, the new recommended rate
of the Al production is up to 5 times higher than previously
assumed. In addition, at T MK, the revised total reaction rate is a
factor of 2 higher. Note that this is the range of temperature at which the
Mg-Al cycle operates in an H-burning zone. The effects of this revision are
discussed. Due to the significantly larger Mg(p,)Al
rate, the estimated production of Al in H-burning regions is less
efficient than previously obtained. As a result, the new rates should imply a
smaller contribution from Wolf-Rayet stars to the galactic Al budget.
Similarly, we show that the AGB extra-mixing scenario does not appear able to
explain the most extreme values of Al/Al, i.e. , found
in some O-rich presolar grains. Finally, the substantial increase of the total
reaction rate makes the hypothesis of a self-pollution by massive AGBs a more
robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster
stars
The Bare Astrophysical S(E) Factor of the 7Li(p, α)α Reaction
The astrophysically important 7Li(p, α)α reaction has been studied via the Trojan horse method in the energy range E = 10-400 keV. A new theoretical description, based on the distorted-wave Born approximation approach, allows one to extract information on the bare astrophysical S-factor, Sb(E), with Sb(0) = 55 ± 3 keV barns. The results are compared with direct experimental data leading to a model-independent value of the electron screening potential energy, Ue = 330 ± 40 eV, much higher than the adiabatic limit Uad = 175 eV
Ba3 Mx Ti3−x O9 (M = Ir, Rh): A family of 5d/4d-based diluted quantum spin liquids
We report the structural and magnetic properties of the 4d (M = Rh) based and 5d (M = Ir) based systems Ba3Mx Ti3−x O9 (nominally x = 0.5, 1). The studied compositions were found to crystallize in a hexagonal structure with the centrosymmetric space group P 63/mmc. The structures comprise of A2O9 polyhedra [with the A site (possibly) statistically occupied by M and Ti] in which pairs of transition metal ions are stacked along the crystallographic c axis. These pairs form triangular bilayers in the ab plane. The magnetic Rh and Ir ions occupy these bilayers, diluted by Ti ions even for x = 1. These bilayers are separated by a triangular layer which is dominantly occupied by Ti ions. From magnetization measurements we infer strong antiferromagnetic couplings for all of the materials but the absence of any spin-freezing or spin-ordering down to 2 K. Further, specific heat measurements down to 0.35 K show no sign of a phase transition for any of the compounds. Based on these thermodynamic measurements we propose the emergence of a quantum spin liquid ground state for Ba3Rh0.5Ti2.5O9, and Ba3Ir0.5Ti2.5O9, in addition to the already reported Ba3IrTi2O9
Nuclear masses set bounds on quantum chaos
It has been suggested that chaotic motion inside the nucleus may
significantly limit the accuracy with which nuclear masses can be calculated.
Using a power spectrum analysis we show that the inclusion of additional
physical contributions in mass calculations, through many-body interactions or
local information, removes the chaotic signal in the discrepancies between
calculated and measured masses. Furthermore, a systematic application of global
mass formulas and of a set of relationships among neighboring nuclei to more
than 2000 nuclear masses allows to set an unambiguous upper bound for the
average errors in calculated masses which turn out to be almost an order of
magnitude smaller than estimated chaotic components.Comment: 4 pages, Accepted for publication in Physical Review Letter
- …