50 research outputs found
Nuclear Mass Measurements With Radioactive Ion Beams
Nuclear masses are the most fundamental of all nuclear properties, yet they
can provide a wealth of knowledge, including information on astrophysical
sites, constraints on existing theory, and fundamental symmetries. In nearly
all applications, it is necessary to measure nuclear masses with very high
precision. As mass measurements push to more short-lived and more massive
nuclei, the practical constraints on mass measurement techniques become more
exacting. Various techniques used to measure nuclear masses, including their
advantages and disadvantages are described. Descriptions of some of the world
facilities at which the nuclear mass measurements are performed are given, and
brief summaries of planned facilities are presented. Future directions are
mentioned, and conclusions are presented which provide a possible outlook and
emphasis on upcoming plans for nuclear mass measurements at existing
facilities, those under construction, and those being planned.Comment: Accepted for publication in IJMP
Low-lying Resonances and Relativistic Screening in Big Bang Nucleosynthesis
We explore effects of the screening due to the relativistic electron-positron
plasma and presence of resonances in the secondary reactions leading to A=7
nuclei during the Big Bang Nucleosynthesis. In particular, we investigate and
examine possible low-lying resonances in the Be(He, )C
reaction and examine the resultant destruction of Be for various resonance
locations and strengths. While a resonance in the C compound nucleus is
thought to have negligible effects we explore the possibility of an enhancement
from plasma screening that may adjust the final Be abundance. We find the
effects of relativistic screening and possible low-lying resonances to be
relatively small in the standard Early Universe models.Comment: 8 pages, 13 figures, Physical Review C (2016) (in press
Explaining the Sr and Ba Scatter in Extremely Metal-Poor Stars
Compilations of abundances of Strontium and Barium in extremely metal-poor
stars show that an apparent cutoff is observed for [Sr/Ba] at [Fe/H]-3.6 and
large fluctuations for [Fe/H]-3.6 with a clear upper bound depending on
metallicity. We study the factors that place upper limits on the logarithmic
ratio [Sr/Ba]. A model is developed in which the collapses of type II
supernovae are found to reproduce many of the features seen in the data. This
model is consistent with galactic chemical evolution constraints of
light-element enrichment in metal-poor stars. Effects of turbulence in an
explosive site have also been simulated, and are found to be important in
explaining the large scatter observed in the [Sr/Ba] data
Promoting Instructional Change via Co-Teaching
Physics Education Research (PER) has made significant progress in developing
knowledge about teaching and learning as well as effective instructional
strategies based on this knowledge. Disseminating knowledge and strategies to
other faculty, however, has proven difficult. Coteaching is a promising and
cost-effective alternative to traditional professional development that may be
applicable in many situations. In this article, we discuss the theoretical
background of co-teaching and describe our initial experience with co-teaching.
A new instructor (MF) cotaught with an instructor experienced in PER-based
reforms (CH). The pair worked within the scaffolding of the course structure
typically used by the experienced instructor and met regularly to discuss
instructional decisions. An outsider (AB) conducted separate interviews with
each instructor at the beginning, middle, and end of the semester and observed
several class sessions. Classroom observations show an immediate use of
PER-based instructional practices by the new instructor. Interviews show a
significant shift in the new instructor's beliefs about teaching and intentions
towards future use of the PER-based instructional approaches.Comment: Manuscript submitted August 2006 to American Journal of Physics,
Physics Education Research Sectio
Impacts of the New Carbon Fusion Cross Sections on Type Ia Supernovae
Type Ia supernovae (SNe Ia) are thought to be thermonuclear explosion of
white dwarfs (WDs). Their progenitors are not well understood. One popular
scenario is the double-degenerate (DD) scenario, which attributes SNe Ia to
WD-WD binary mergers. The fates of the WD mergers depend on the rate of
C+C reaction. Recently, the C+C cross sections have
been measured and the analysis of the data using the Trojan Horse Method
suggested that the astrophysical reaction rate is larger than conventional
rates at astrophysical temperatures due to possible resonances. The resonance
contribution results in a decrease of the carbon burning ignition temperature.
Therefore accretion induced collapse occurs more easily and increases the
birthrate of Galactic neutron stars with the contribution of the DD scenario to
the SNe Ia rate becoming even smaller.Comment: Accepted for publication in MNRAS Letter
Chiral Selection, Isotopic Abundance Shifts, and Autocatalysis of Meteoritic Amino Acids
The discovery of amino acids in meteorites has presented two clues to the
origin of their processing subsequent to their formation: a slight preference
for left-handedness in some of them, and isotopic anomalies in some of their
constituent atoms. In this article we present theoretical results from the
Supernova Neutrino Amino Acid Processing (SNAAP) model, which uses electron
anti-neutrinos and the magnetic fields from source objects such as supernovae
or colliding neutron stars to selectively destroy one amino acid chirality and
to create isotopic abundance shifts. For plausible magnetic fields and electron
anti-neutrino fluxes, non-zero, positive enantiomeric excesses, s, defined
to be the relative left/right asymmetry in an amino acid population, are
reviewed for two amino acids, and conditions are suggested that would produce
for all of the -amino acids. The relatively high energy
anti-neutrinos that produce the s would inevitably also produce isotopic
anomalies. A nuclear reaction network was developed to describe the reactions
resulting from them and the nuclides in the meteorites. At similar
anti-neutrino fluxes, assumed recombination of the detritus from the
anti-neutrino interactions is shown to produce appreciable isotopic anomalies
in qualitative agreement with those observed for D/H and N/N.
The isotopic anomalies for C/C are predicted to be small, as are
also observed. Autocatalysis may be necessary for any model to produce the
largest s observed in meteorites. This allows the constraints of the
original SNAAP model to be relaxed, increasing the probability of meteoroid
survival in sites where amino acid processing is possible. These results have
obvious implications for the origin of life on Earth.Comment: 24 pages, 8 figures, to be published in Physical Review Researc
Sites That Can Produce Left-Handed Amino Acids in the Supernova Neutrino Amino Acid Processing Model
The Supernova Neutrino Amino Acid Processing model, which uses electron
anti-neutrinos and the magnetic field from a source object such as a supernova
to selectively destroy one amino acid chirality, is studied for possible sites
that would produce meteoroids having partially left-handed amino acids. Several
sites appear to provide the requisite magnetic field intensities and electron
anti-neutrino fluxes. These results have obvious implications for the origin of
life on Earth.Comment: Accepted to ApJ
Elimination of the Blue Loops in the Evolution of Intermediate-mass Stars by the Neutrino Magnetic Moment and Large Extra Dimensions
For searching beyond Standard Model physics, stars are laboratories which
complement terrestrial experiments. Massless neutrinos in the Standard Model of
particle physics cannot have a magnetic moment, but massive neutrinos have a
finite magnetic moment in the minimal extension of the Standard Model. Large
extra dimensions are a possible solution of the hierarchy problem. Both of
these provide additional energy loss channels in stellar interiors via the
electromagnetic interaction and radiation into extra dimensions, respectively,
and thus affect stellar evolution. We perform simulations of stellar evolution
with such additional energy losses and find that they eliminate the blue loops
in the evolution of intermediate-mass stars. The existence of Cepheid stars can
be used to constrain the neutrino magnetic moment and large extra dimensions.
In order for Cepheids to exist, the neutrino magnetic moment should be smaller
than the range ~2x10^{-10} to 4x10^{-11}mu_B, where mu_B is the Bohr magneton,
and the fundamental scale in the (4+2)-spacetime should be larger than ~2 to 5
TeV, depending on the rate of the ^{12}C(alpha, gamma)^{16}O reaction. The
fundamental scale also has strong dependence on the metallicity. This value of
the magnetic moment is in the range explored in the reactor experiments, but
higher than the limit inferred from globular clusters. Similarly the
fundamental scale value we constrain corresponds to a size of the compactified
dimensions comparable to those explored in the torsion balance experiments, but
is smaller than the limits inferred from collider experiments and low-mass
stars.Comment: 12 pages, 14 figures, accepted for publication in Ap
Nuclear Reaction Screening, Weak Interactions, and r-Process Nucleosynthesis in High Magnetic Fields
Coulomb screening and weak interactions in a hot, magnetized plasma are
investigated. Coulomb screening is evaluated in a relativistic thermal plasma
in which electrons and positrons are in equilibrium. In addition to temperature
effects, effects on weak screening from a strong external magnetic field are
evaluated. In high fields, the electron transverse momentum components are
quantized into Landau levels. The characteristic plasma screening length at
high temperatures and at high magnetic fields is explored. In addition to
changes to the screening length, changes in weak interaction rates are
estimated. It is found that high fields can result in increased -decay
rates as the electron and positron spectra are dominated by Landau levels.
Finally, the effects studied here are evaluated in a simple r-process model. It
is found that relativistic Coulomb screening has a small effect on the final
abundance distribution. While changes in weak interaction rates in strong
magnetic fields can have an effect on the r-process evolution and abundance
distribution, the field strength required to have a significant effect may be
larger than what is currently thought to be typical of the r-process
environment in collapsar jets or neutron star mergers. If r-process sites exist
in fields G effects from fields on weak decays could be
significant.Comment: 24 pages, 16 figure
Screening corrections to Electron Capture Rates and resulting constraints on Primordial Magnetic Fields
We explore screening effects arising from a relativistic magnetized plasma
with applications to Big Bang nucleosynthesis (BBN). %Specifically, due to
their small magnetic moments, energies of electrons and positrons can be easily
quantized via Landau quantization. The screening potential which depends on the
thermodynamics of charged particles in the plasma is altered by the magnetic
field. We focus on the impact of screening on the electron capture interaction.
Taking into account the correction in BBN arising from a homogeneous primordial
magnetic field (PMF), we constrain the epoch at which the PMF was generated and
its strength during BBN. Considering such screening corrections to the electron
capture rates and using up-to-date observations of primordial elemental
abundances, we also discuss the possibility of solving the problem of
under-estimation of the deuterium abundance. We find for certain values of the
PMF strength predicted D and He abundances are both consistent with the
observational constraints.Comment: 18 pages, 7 figures, submitted to Phys. ReV.