566 research outputs found
Measurement of the elastic scattering cross section of neutrons from argon and neon
Background: The most significant source of background in direct dark matter
searches are neutrons that scatter elastically from nuclei in the detector's
sensitive volume. Experimental data for the elastic scattering cross section of
neutrons from argon and neon, which are target materials of interest to the
dark matter community, were previously unavailable. Purpose: Measure the
differential cross section for elastic scattering of neutrons from argon and
neon in the energy range relevant to backgrounds from (alpha,n) reactions in
direct dark matter searches. Method: Cross-section data were taken at the
Triangle Universities Nuclear Laboratory (TUNL) using the neutron
time-of-flight technique. These data were fit using the spherical optical
model. Results: The differential cross section for elastic scatting of neutrons
from neon at 5.0 and 8.0 MeV and argon at 6.0 MeV was measured. Optical-model
parameters for the elastic scattering reactions were determined from the best
fit to these data. The total elastic scattering cross section for neon was
found to differ by 6% at 5.0 MeV and 13% at 8.0 MeV from global optical-model
predictions. Compared to a local optical-model for 40Ar, the elastic scattering
cross section was found to differ from the data by 8% at 6.0 MeV. Conclusions:
These new data are important for improving Monte-Carlo simulations and
background estimates for direct dark matter searches and for benchmarking
optical models of neutron elastic scattering from these nuclei
Measurements of the 40Ar(n, γ)41Ar radiative-capture cross section between 0.4 and 14.8 MeV
AbstractThe 40Ar(n, γ)41Ar neutron capture cross section has been measured between 0.4 and 14.8 MeV neutron energy using the activation technique. The data are important for estimating backgrounds in argon-based neutrino and dark-matter detectors and in the neutrino-less double-beta decay search GERDA, which uses liquid argon as cooling and shielding medium. For the first time the 40Ar(n, γ)41Ar cross section has been measured for neutron energies above 1 MeV. Our results are compared to the evaluation ENDF/B-VII.1 and the calculated prediction TENDL-2013. The latter agrees very well with the present results
Polarization observables in the semiexclusive photoinduced three-body breakup of 3He
The photon and 3He analyzing powers as well as spin correlation coefficients
in the semiexclusive three-body photodisintegration of 3He are investigated for
incoming photon laboratory energies E=12, 40 and 120 MeV. The nuclear states
are obtained by solving three-body Faddeev equations with the AV18
nucleon-nucleon potential alone or supplemented with the UrbanaIX three-nucleon
force. Explicit pi- and rho-meson exchange currents are taken into account, but
we also compare to other models of the electromagnetic current. In some
kinematical conditions we have found strong effects of the three-nucleon force
for the 3He analyzing power and spin correlation coefficients, as well strong
sensitivities to the choice of the currents. This set of predictions should be
a useful guidance for the planning of measurements. In addition, we compare our
results for two-body 3He breakup induced by polarized photons with a few
existing data.Comment: 23 pages, 16 figure
Effects of the magnetic moment interaction between nucleons on observables in the 3N continuum
The influence of the magnetic moment interaction of nucleons on
nucleon-deuteron elastic scattering and breakup cross sections and on elastic
scattering polarization observables has been studied. Among the numerous
elastic scattering observables only the vector analyzing powers were found to
show a significant effect, and of opposite sign for the proton-deuteron and
neutron-deuteron systems. This finding results in an even larger discrepancy
than the one previously established between neutron-deuteron data and
theoretical calculations. For the breakup reaction the largest effect was found
for the final-state-interaction cross sections. The consequences of this
observation on previous determinations of the ^1S_0 scattering lengths from
breakup data are discussed.Comment: 24 pages, 6 ps figures, 1 png figur
Measurements at low energies of the polarization-transfer coefficient Kyy' for the reaction 3H(p,n)3He at 0 degrees
Measurements of the transverse polarization coefficient Kyy' for the reaction
3H(p,n)3He are reported for outgoing neutron energies of 1.94, 5.21, and 5.81
MeV. This reaction is important both as a source of polarized neutrons for
nuclear physics experiments, and as a test of theoretical descriptions of the
nuclear four-body system. Comparison is made to previous measurements,
confirming the 3H(p,n)3He reaction can be used as a polarized neutron source
with the polarization known to an accuracy of approximately 5%. Comparison to
R-matrix theory suggests that the sign of the 3F3 phase-shift parameter is
incorrect. Changing the sign of this parameter dramatically improves the
agreement between theory and experiment.Comment: 12 pages, RevTeX, 5 eps figures, submitted to Phys. Rev.
Using two-stream theory to capture fluctuations of satellite-perceived TOA SW radiances reflected from clouds over ocean
Shortwave (SW) fluxes estimated from broadband radiometry rely on
empirically gathered and hemispherically resolved fields of outgoing
top-of-atmosphere (TOA) radiances. This study aims to provide more
accurate and precise fields of TOA SW radiances reflected from clouds
over ocean by introducing a novel semiphysical model predicting
radiances per narrow sun-observer geometry. This model was
statistically trained using CERES-measured radiances paired with
MODIS-retrieved cloud parameters as well as reanalysis-based
geophysical parameters. By using radiative transfer approximations as
a framework to ingest the above parameters, the new approach incorporates
cloud-top effective radius and above-cloud water vapor in addition to
traditionally used cloud optical depth, cloud fraction, cloud phase,
and surface wind speed. A two-stream cloud albedo – serving to
statistically incorporate cloud optical thickness and cloud-top
effective radius – and Cox–Munk ocean reflectance were used to
describe an albedo over each CERES footprint. Effective-radius-dependent asymmetry parameters were obtained empirically and
separately for each viewing-illumination geometry. AÂ simple equation
of radiative transfer, with this albedo and attenuating above-cloud
water vapor as inputs, was used in its log-linear form to allow for
statistical optimization. We identified the two-stream functional
form that minimized radiance residuals calculated against CERES
observations and outperformed the state-of-the-art approach for most
observer geometries outside the sun-glint and solar zenith angles
between 20 and 70∘, reducing the median SD
of radiance residuals per solar geometry by up to 13.2 % for
liquid clouds, 1.9 % for ice clouds, and 35.8 % for
footprints containing both cloud phases. Geometries affected by
sun glint (constituting between 10 % and 1 % of the
discretized upward hemisphere for solar zenith angles of 20 and
70∘, respectively), however, often showed weaker
performance when handled with the new approach and had increased
residuals by as much as 60 % compared to the state-of-the-art
approach. Overall, uncertainties were reduced for liquid-phase and
mixed-phase footprints by 5.76 % and 10.81 %,
respectively, while uncertainties for ice-phase footprints increased
by 0.34 %. Tested for a variety of scenes, we further
demonstrated the plausibility of scene-wise predicted radiance
fields. This new approach may prove useful when employed in angular
distribution models and may result in improved flux estimates, in
particular dealing with clouds characterized by small or large
droplet/crystal sizes
Neutron-proton analyzing power at 12 MeV and inconsistencies in parametrizations of nucleon-nucleon data
We present the most accurate and complete data set for the analyzing power
Ay(theta) in neutron-proton scattering. The experimental data were corrected
for the effects of multiple scattering, both in the center detector and in the
neutron detectors. The final data at En = 12.0 MeV deviate considerably from
the predictions of nucleon-nucleon phase-shift analyses and potential models.
The impact of the new data on the value of the charged pion-nucleon coupling
constant is discussed in a model study.Comment: Six pages, four figures, one table, to be published in Physics
Letters
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