281 research outputs found
Enhanced low-energy -decay strength of Ni and its robustness within the shell model
Neutron-capture reactions on very neutron-rich nuclei are essential for
heavy-element nucleosynthesis through the rapid neutron-capture process, now
shown to take place in neutron-star merger events. For these exotic nuclei,
radiative neutron capture is extremely sensitive to their -emission
probability at very low energies. In this work, we present
measurements of the -decay strength of Ni over the wide range
MeV. A significant enhancement is found in the
-decay strength for transitions with MeV. At present,
this is the most neutron-rich nucleus displaying this feature, proving that
this phenomenon is not restricted to stable nuclei. We have performed
-strength calculations within the quasiparticle time-blocking
approximation, which describe our data above MeV very well.
Moreover, large-scale shell-model calculations indicate an nature of the
low-energy strength. This turns out to be remarkably robust with
respect to the choice of interaction, truncation and model space, and we
predict its presence in the whole isotopic chain, in particular the
neutron-rich .Comment: 9 pages, 9 figure
Level densities and thermodynamical properties of Pt and Au isotopes
The nuclear level densities of Pt and Au below the
neutron separation energy have been measured using transfer and scattering
reactions. All the level density distributions follow the constant-temperature
description. Each group of isotopes is characterized by the same temperature
above the energy threshold corresponding to the breaking of the first Cooper
pair. A constant entropy excess and is observed in
Pt and Au with respect to Pt and Au,
respectively, giving information on the available single-particle level space
for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is
revealed by sequential peaks in the microcanonical caloric curve
Low Energy Light Yield of Fast Plastic Scintillators
Compact neutron imagers using double-scatter kinematic reconstruction are
being designed for localization and characterization of special nuclear
material. These neutron imaging systems rely on scintillators with a rapid
prompt temporal response as the detection medium. As n-p elastic scattering is
the primary mechanism for light generation by fast neutron interactions in
organic scintillators, proton light yield data are needed for accurate
assessment of scintillator performance. The proton light yield of a series of
commercial fast plastic organic scintillators---EJ-200, EJ-204, and
EJ-208---was measured via a double time-of-flight technique at the 88-Inch
Cyclotron at Lawrence Berkeley National Laboratory. Using a tunable deuteron
breakup neutron source, target scintillators housed in a dual photomultiplier
tube configuration, and an array of pulse-shape-discriminating observation
scintillators, the fast plastic scintillator light yield was measured over a
broad and continuous energy range down to proton recoil energies of
approximately 50 keV. This work provides key input to event reconstruction
algorithms required for utilization of these materials in emerging neutron
imaging modalities.Comment: 15 pages, 6 figure
Structural Color Production in Melanin-based Disordered Colloidal Nanoparticle Assemblies in Spherical Confinement
Melanin is a ubiquitous natural pigment that exhibits broadband absorption
and high refractive index. Despite its widespread use in structural color
production, how the absorbing material, melanin, affects the generated color is
unknown. Using a combined molecular dynamics and finite-difference time-domain
computational approach, this paper investigates structural color generation in
one-component melanin nanoparticle-based supra-assemblies (called supraballs)
as well as binary mixtures of melanin and silica (non-absorbing)
nanoparticle-based supraballs. Experimentally produced one-component melanin
and one-component silica supraballs, with thoroughly characterized primary
particle characteristics using neutron scattering, produce reflectance profiles
similar to the computational analogues, confirming that the computational
approach correctly simulates both absorption and multiple scattering from the
self-assembled nanoparticles. These combined approaches demonstrate that
melanin's broadband absorption increases the primary reflectance peak
wavelength, increases saturation, and decreases lightness factor. In addition,
the dispersity of nanoparticle size more strongly influences the optical
properties of supraballs than packing fraction, as evidenced by production of a
larger range of colors when size dispersity is varied versus packing fraction.
For binary melanin and silica supraballs, the chemistry-based stratification
allows for more diverse color generation and finer saturation tuning than does
the degree of mixing/demixing between the two chemistries.Comment: 40 pages, Figure
Recommended from our members
Magnetization-driven Lifshitz transition and charge-spin coupling in the kagome metal YMn6Sn6
The Fermi surface (FS) is essential for understanding the properties of metals. It can change under both conventional symmetry-breaking phase transitions and Lifshitz transitions (LTs), where the FS, but not the crystal symmetry, changes abruptly. Magnetic phase transitions involving uniformly rotating spin textures are conventional in nature, requiring strong spin-orbit coupling (SOC) to influence the FS topology and generate measurable properties. LTs driven by a continuously varying magnetization are rarely discussed. Here we present two such manifestations in the magnetotransport of the kagome magnet YMn6Sn6: one caused by changes in the magnetic structure and another by a magnetization-driven LT. The former yields a 10% magnetoresistance enhancement without a strong SOC, while the latter a 45% reduction in the resistivity. These phenomena offer a unique view into the interplay of magnetism and electronic topology, and for understanding the rare-earth counterparts, such as TbMn6Sn6, recently shown to harbor correlated topological physics
Completing the nuclear reaction puzzle of the nucleosynthesis of 92Mo
One of the greatest questions for modern physics to address is how elements
heavier than iron are created in extreme, astrophysical environments. A
particularly challenging part of that question is the creation of the so-called
p-nuclei, which are believed to be mainly produced in some types of supernovae.
The lack of needed nuclear data presents an obstacle in nailing down the
precise site and astrophysical conditions. In this work, we present for the
first time measurements on the nuclear level density and average strength
function of Mo. State-of-the-art p-process calculations systematically
underestimate the observed solar abundance of this isotope. Our data provide
stringent constraints on the NbMo reaction rate,
which is the last unmeasured reaction in the nucleosynthesis puzzle of
Mo. Based on our results, we conclude that the Mo abundance
anomaly is not due to the nuclear physics input to astrophysical model
calculations.Comment: Submitted to PR
Mechanism of Structural Colors in Binary Mixtures of Nanoparticle-based Supraballs
Inspired by structural colors in avian species, various synthetic strategies
have been developed to produce non-iridescent, saturated colors using
nanoparticle assemblies. Mixtures of nanoparticles varying in particle
chemistry (or complex refractive indices) and particle size have additional
emergent properties that impact the color produced. For such complex
multi-component systems, an understanding of assembled structure along with a
robust optical modeling tool can empower scientists to perform intensive
structure-color relationship studies and fabricate designer materials with
tailored color. Here, we demonstrate how we can reconstruct the assembled
structure from small-angle scattering measurements using the computational
reverse-engineering analysis for scattering experiments (CREASE) method and
then use the reconstructed structure in finite-difference time-domain (FDTD)
calculations to predict color. We successfully, quantitatively predict
experimentally observed color in mixtures containing strongly absorbing melanin
nanoparticles and demonstrate the influence of a single layer of segregated
nanoparticles on color produced. The versatile computational approach presented
in this work is useful for engineering synthetic materials with desired colors
without laborious trial and error experiments.Comment: 23 Pages, 5 Figures, 1 ToC Figur
- …