5,179 research outputs found
Production cross sections from 82Se fragmentation as indications of shell effects in neutron-rich isotopes close to the drip-line
Production cross sections for neutron-rich nuclei from the fragmentation of a
82Se beam at 139 MeV/u were measured. The longitudinal momentum distributions
of 126 neutron-rich isotopes of elements 11 <= Z <= 32 were scanned using an
experimental approach of varying the target thickness. Production cross
sections with beryllium and tungsten targets were determined for a large number
of nuclei including several isotopes first observed in this work. These are the
most neutron-rich nuclides of the elements 22 <= Z <= 25 (64Ti, 67V, 69Cr,
72Mn). One event was registered consistent with 70Cr, and another one with
75Fe. The production cross sections are correlated with Qg systematics to
reveal trends in the data. The results presented here confirm our previous
result from a similar measurement using a 76Ge beam, and can be explained with
a shell model that predicts a subshell closure at N = 34 around Z = 20. This is
demonstrated by systematic trends and calculations with the Abrasion-Ablation
model that are sensitive to separation energies.Comment: 13 pages, 11 figures, accepted to Phys.Rev.
Recovering -invariant metrics on from the equivariant spectrum
We prove an inverse spectral result for -invariant metrics on
based on the so-called asymptotic equivariant spectrum. This is roughly the
spectrum together with large weights of the action on the eigenspaces.
Our result generalizes an inverse spectral result of the first and last named
authors, together with Victor Guillemin, concerning -invariant metrics on
which are invariant under the antipodal map. We use higher order terms in
the asymptotic expansion of a natural spectral measure associated with the
Laplacian and the action.Comment: 16 pages; minor revisions throughout following comments from referee
Distributions of Human Exposure to Ozone During Commuting Hours in Connecticut using the Cellular Device Network
Epidemiologic studies have established associations between various air
pollutants and adverse health outcomes for adults and children. Due to high
costs of monitoring air pollutant concentrations for subjects enrolled in a
study, statisticians predict exposure concentrations from spatial models that
are developed using concentrations monitored at a few sites. In the absence of
detailed information on when and where subjects move during the study window,
researchers typically assume that the subjects spend their entire day at home,
school or work. This assumption can potentially lead to large exposure
assignment bias. In this study, we aim to determine the distribution of the
exposure assignment bias for an air pollutant (ozone) when subjects are assumed
to be static as compared to accounting for individual mobility. To achieve this
goal, we use cell-phone mobility data on approximately 400,000 users in the
state of Connecticut during a week in July, 2016, in conjunction with an ozone
pollution model, and compare individual ozone exposure assuming static versus
mobile scenarios. Our results show that exposure models not taking mobility
into account often provide poor estimates of individuals commuting into and out
of urban areas: the average 8-hour maximum difference between these estimates
can exceed 80 parts per billion (ppb). However, for most of the population, the
difference in exposure assignment between the two models is small, thereby
validating many current epidemiologic studies focusing on exposure to ozone
Impulsive people have a compulsion for immediate gratification-certain or uncertain.
Impulsivity has been defined as choosing the smaller more immediate reward over a larger more delayed reward. The purpose of this research was to gain a deeper understanding of the mental processes involved in the decision making. We examined participants' rates of delay discounting and probability discounting to determine their correlation with time-probability trade-offs. To establish the time-probability trade-off rate, participants adjusted a risky, immediate payoff to a delayed, certain payoff. In effect, this yielded a probability equivalent of waiting time. We found a strong, positive correlation between delay discount rates and the time-probability trade-offs. This means that impulsive people have a compulsion for immediate gratification, independent of whether the immediate reward is certain or uncertain. Thus, they seem not to be concerned with risk but rather with time
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Microscopic picture of aging in SiO2
We investigate the aging dynamics of amorphous SiO2 via molecular dynamics
simulations of a quench from a high temperature T_i to a lower temperature T_f.
We obtain a microscopic picture of aging dynamics by analyzing single particle
trajectories, identifying jump events when a particle escapes the cage formed
by its neighbors, and by determining how these jumps depend on the waiting time
t_w, the time elapsed since the temperature quench to T_f. We find that the
only t_w-dependent microscopic quantity is the number of jumping particles per
unit time, which decreases with age. Similar to previous studies for fragile
glass formers, we show here for the strong glass former SiO2 that neither the
distribution of jump lengths nor the distribution of times spent in the cage
are t_w-dependent. We conclude that the microscopic aging dynamics is
surprisingly similar for fragile and strong glass formers.Comment: 4 pages, 7 figure
Anomalous velocity distributions in active Brownian suspensions
Large scale simulations and analytical theory have been combined to obtain
the non-equilibrium velocity distribution, , of randomly accelerated
particles in suspension. The simulations are based on an event-driven
algorithm, generalised to include friction. They reveal strongly anomalous but
largely universal distributions which are independent of volume fraction and
collision processes, which suggests a one-particle model should capture all the
essential features. We have formulated this one-particle model and solved it
analytically in the limit of strong damping, where we find that decays
as for multiple decades, eventually crossing over to a Gaussian decay for
the largest velocities. Many particle simulations and numerical solution of the
one-particle model agree for all values of the damping.Comment: 6 pages, 5 figure
Evaluating -functions with few known coefficients
We address the problem of evaluating an -function when only a small number
of its Dirichlet coefficients are known. We use the approximate functional
equation in a new way and find that is possible to evaluate the -function
more precisely than one would expect from the standard approach. The method,
however, requires considerably more computational effort to achieve a given
accuracy than would be needed if more Dirichlet coefficients were available.Comment: 14 pages; Added a new section where we evaluate L(1/2 + 100 i, Delta)
to 42 decimal places using no Dirichlet series coefficients at al
Infrared Parallaxes of Young Field Brown Dwarfs and Connections to Directly Imaged Gas-Giant Exoplanets
We have measured high-precision parallaxes for a large sample of candidate
young (~10-100 Myr) and intermediate-age (~100-600 Myr) ultracool dwarfs, with
spectral types ranging from M8 to T2.5. These objects are compelling benchmarks
for substellar evolution and ultracool atmospheres at lower surface gravities
(i.e., masses) than most of the field population. We find that the absolute
magnitudes of our young sample can be systematically offset from ordinary
(older) field dwarfs, with the young late-M objects being brighter and the
young/dusty mid-L (L3-L6.5) objects being fainter, especially at J band. Thus,
we conclude the "underluminosity" of the young planetary-mass companions HR
8799b and 2MASS J1207-39b compared to field dwarfs is also manifested in young
free-floating brown dwarfs, though the effect is not as extreme. At the same
time, some young objects over the full spectral type range of our sample are
similar to field objects, and thus a simple correspondence between youth and
magnitude offset relative to the field population appears to be lacking.
Comparing the kinematics of our sample to nearby stellar associations and
moving groups, we identify several new moving group members, including the
first free-floating L dwarf in the AB Dor moving group, 2MASS J0355+11.
Altogether, the effects of surface gravity (age) and dust content on the
magnitudes and colors of substellar objects appear to be degenerate.Comment: Proceedings of Cool Stars 1
Search for plant biomagnetism with a sensitive atomic magnetometer
We report what we believe is the first experimental limit placed on plant
biomagnetism. Measurements with a sensitive atomic magnetometer were performed
on the Titan arum (Amorphophallus titanum) inflorescence, known for its fast
bio-chemical processes while blooming. We find that the surface magnetic field
from these processes, projected along the Earth's magnetic field, and measured
at the surface of the plant, is less then ~0.6uG.Comment: 5 pages, 5 figures, to be published - modified one sentence in
abstract + reformatted fi
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