32 research outputs found
STARLIB: A Next-Generation Reaction-Rate Library for Nuclear Astrophysics
STARLIB is a next-generation, all-purpose nuclear reaction-rate library. For
the first time, this library provides the rate probability density at all
temperature grid points for convenient implementation in models of stellar
phenomena. The recommended rate and its associated uncertainties are also
included. Currently, uncertainties are absent from all other rate libraries,
and, although estimates have been attempted in previous evaluations and
compilations, these are generally not based on rigorous statistical
definitions. A common standard for deriving uncertainties is clearly warranted.
STARLIB represents a first step in addressing this deficiency by providing a
tabular, up-to-date database that supplies not only the rate and its
uncertainty but also its distribution. Because a majority of rates are
lognormally distributed, this allows the construction of rate probability
densities from the columns of STARLIB. This structure is based on a recently
suggested Monte Carlo method to calculate reaction rates, where uncertainties
are rigorously defined. In STARLIB, experimental rates are supplemented with:
(i) theoretical TALYS rates for reactions for which no experimental input is
available, and (ii) laboratory and theoretical weak rates. STARLIB includes all
types of reactions of astrophysical interest to Z = 83, such as (p,g), (p,a),
(a,n), and corresponding reverse rates. Strong rates account for thermal target
excitations. Here, we summarize our Monte Carlo formalism, introduce the
library, compare methods of correcting rates for stellar environments, and
discuss how to implement our library in Monte Carlo nucleosynthesis studies. We
also present a method for accessing STARLIB on the Internet and outline updated
Monte Carlo-based rates.Comment: Accepted for publication in the Astrophysical Journal Supplement
Series; 96 pages, 22 figure
Absolute Determination of the 22Na(p,g) Reaction Rate in Novae
Gamma-ray telescopes in orbit around the Earth are searching for evidence of
the elusive radionuclide 22Na produced in novae. Previously published
uncertainties in the dominant destructive reaction, 22Na(p,g)23Mg, indicated
new measurements in the proton energy range of 150 to 300 keV were needed to
constrain predictions. We have measured the resonance strengths, energies, and
branches directly and absolutely by using protons from the University of
Washington accelerator with a specially designed beamline, which included beam
rastering and cold vacuum protection of the 22Na implanted targets. The
targets, fabricated at TRIUMF-ISAC, displayed minimal degradation over a ~ 20 C
bombardment as a result of protective layers. We avoided the need to know the
stopping power, and hence the target composition, by extracting resonance
strengths from excitation functions integrated over proton energy. Our
measurements revealed that resonance strengths for E_p = 213, 288, 454, and 610
keV are stronger by factors of 2.4 to 3.2 than previously reported. Upper
limits have been placed on proposed resonances at 198-, 209-, and 232-keV. We
have re-evaluated the 22Na(p,g) reaction rate, and our measurements indicate
the resonance at 213 keV makes the most significant contribution to 22Na
destruction in novae. Hydrodynamic simulations including our rate indicate that
the expected abundance of 22Na ejecta from a classical nova is reduced by
factors between 1.5 and 2, depending on the mass of the white-dwarf star
hosting the nova explosion.Comment: 20 pages, 18 figures; shortened paper, accepted in Phys. Rev.
Revision of the derivation of stellar rates from experiment and impact on Eu s-process contributions
The final, definitive version of this paper has been published in Journal of Physics: Conference Series, 665(1), January 5, 2016, and is available on line at doi: 10.1088/1742-6596/665/1/012024 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing LtdA new, general formalism to include experimental data in revised stellar rates is discussed, containing revised uncertainties. Application to the s-process shows that the actual uncertainties in the neutron capture rates can be larger than would be expected from the experimental errors alone. As a specific example for how astrophysical conclusions can depend on the approach selected to derive stellar rates, the 151Eu/(151Eu+153 Eu) abundance ratio from AGB star models is presented. Finally, a recommended workflow for the derivation of stellar rates from experiment is laid out.Peer reviewe
First Measurement of the Neutron -Asymmetry with Ultracold Neutrons
We report the first measurement of angular correlation parameters in neutron
-decay using polarized ultracold neutrons (UCN). We utilize UCN with
energies below about 200 neV, which we guide and store for s in a Cu
decay volume. The potential of a static 7 T field
external to the decay volume provides a 420 neV potential energy barrier to the
spin state parallel to the field, polarizing the UCN before they pass through
an adiabatic fast passage (AFP) spin-flipper and enter a decay volume, situated
within a 1 T, superconducting solenoidal spectrometer. We
determine a value for the -asymmetry parameter , proportional to
the angular correlation between the neutron polarization and the electron
momentum, of .Comment: 4 pages, 2 figures, 1 table, submitted to Phys. Rev. Let
Final results for the neutron ÎČ-asymmetry parameter Aâ from the UCNA experiment
The UCNA experiment was designed to measure the neutron ÎČ-asymmetry parameter A0 using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7âT magnetic field, and then directed to a 1âT solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for A0 was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008â2009, 2010, and 2011â2013, which ultimately culminated in a 0.67% precision result for Aâ
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
STARLIB: A Next-Generation Reaction-Rate Library for Nuclear Astrophysics
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