131 research outputs found
Breakdown of the Isobaric Multiplet Mass Equation for the A = 20 and 21 Multiplets
Full text linkUsing the Penning trap mass spectrometer TITAN, we performed the first direct
mass measurements of 20,21Mg, isotopes that are the most proton-rich members of
the A = 20 and A = 21 isospin multiplets. These measurements were possible
through the use of a unique ion-guide laser ion source, a development that
suppressed isobaric contamination by six orders of magnitude. Compared to the
latest atomic mass evaluation, we find that the mass of 21Mg is in good
agreement but that the mass of 20Mg deviates by 3{\sigma}. These measurements
reduce the uncertainties in the masses of 20,21Mg by 15 and 22 times,
respectively, resulting in a significant departure from the expected behavior
of the isobaric multiplet mass equation in both the A = 20 and A = 21
multiplets. This presents a challenge to shell model calculations using either
the isospin non-conserving USDA/B Hamiltonians or isospin non-conserving
interactions based on chiral two- and three-nucleon forces.Comment: 5 pages, 2 figure
Stormy geomorphology: geomorphic contributions in an age of climate extremes
Get PDFThe increasing frequency and/or severity of extreme climate events are becoming increasingly apparent over multi‐decadal timescales at the global scale, albeit with relatively low scientific confidence. At the regional scale, scientific confidence in the future trends of extreme event likelihood is stronger, although the trends are spatially variable. Confidence in these extreme climate risks is muddied by the confounding effects of internal landscape system dynamics and external forcing factors such as changes in land use and river and coastal engineering. Geomorphology is a critical discipline in disentangling climate change impacts from other controlling factors, thereby contributing to debates over societal adaptation to extreme events. We review four main geomorphic contributions to flood and storm science. First, we show how palaeogeomorphological and current process studies can extend the historical flood record while also unraveling the complex interactions between internal geomorphic dynamics, human impacts and changes in climate regimes. A key outcome will be improved quantification of flood probabilities and the hazard dimension of flood risk. Second, we present evidence showing how antecedent geomorphological and climate parameters can alter the risk and magnitude of landscape change caused by extreme events. Third, we show that geomorphic processes can both mediate and increase the geomorphological impacts of extreme events, influencing societal risk. Fourthly, we show the potential of managing flood and storm risk through the geomorphic system, both near‐term (next 50 years) and longer‐term. We recommend that key methods of managing flooding and erosion will be more effective if risk assessments include palaeodata, if geomorphological science is used to underpin nature‐based management approaches, and if land‐use management addresses changes in geomorphic process regimes that extreme events can trigger. We argue that adopting geomorphologically‐grounded adaptation strategies will enable society to develop more resilient, less vulnerable socio‐geomorphological systems fit for an age of climate extremes
Two-neutron knockout as a probe of the composition of states in Mg, Al, and Si
Get PDFSimpson and Tostevin proposed that the width and shape of exclusive parallel
momentum distributions of the A-2 residue in direct two-nucleon knockout
reactions carry a measurable sensitivity to the nucleon single-particle
configurations and their couplings within the wave functions of exotic nuclei.
We report here on the first benchmarks and use of this new spectroscopic tool.
Exclusive parallel momentum distributions for states in the neutron-deficient
nuclei Mg, Al, and Si populated in such direct two-neutron
removal reactions were extracted and compared to predictions combining eikonal
reaction theory and shell-model calculations. For the well-known Mg and
Al nuclei, measurements and calculations were found to agree, supporting
the dependence of the parallel momentum distribution width on the angular
momentum composition of the shell-model two-neutron amplitudes. In Si, a
level at 3439(9) keV, of relevance for the important
Al(p,)Si astrophysical reaction rate, was confirmed to be
the state, while the state, expected to be strongly populated
in two-neutron knockout, was not observed. This puzzle is resolved by
theoretical considerations of the Thomas-Ehrman shift, which also suggest that
a previously reported 3471-keV state in Si is in fact the ()
level with one of the largest experimental mirror-energy shifts ever observed.Comment: Accepted for publication in Phys. Rev. C as a Rapid Communicatio
Isospin mixing and the cubic isobaric multiplet mass equation in the lowest <i>T</i>=2, <i>A</i>=32 quintet
Get PDFThe isobaric multiplet mass equation (IMME) is known to break down in the
first T = 2, A = 32 isospin quintet. In this work we combine high-resolution
experimental data with state-of-the-art shell-model calculations to investigate
isospin mixing as a possible cause for this violation. The experimental data
are used to validate isospin-mixing matrix elements calculated with newly
developed shell-model Hamiltonians. Our analysis shows that isospin mixing with
nonanalog T = 1 states contributes to the IMME breakdown, making the
requirement of an anomalous cubic term inevitable for the multiplet
Long-term survival in patients with non-small cell lung cancer and synchronous brain metastasis treated with whole-brain radiotherapy and thoracic chemoradiation
Get PDF<p>Abstract</p> <p>Background</p> <p>Brain metastases occur in 30-50% of Non-small cell lung cancer (NSCLC) patients and confer a worse prognosis and quality of life. These patients are usually treated with Whole-brain radiotherapy (WBRT) followed by systemic therapy. Few studies have evaluated the role of chemoradiotherapy to the primary tumor after WBRT as definitive treatment in the management of these patients.</p> <p>Methods</p> <p>We reviewed the outcome of 30 patients with primary NSCLC and brain metastasis at diagnosis without evidence of other metastatic sites. Patients were treated with WBRT and after induction chemotherapy with paclitaxel and cisplatin for two cycles. In the absence of progression, concurrent chemoradiotherapy for the primary tumor with weekly paclitaxel and carboplatin was indicated, with a total effective dose of 60 Gy. If disease progression was ruled out, four chemotherapy cycles followed.</p> <p>Results</p> <p>Median Progression-free survival (PFS) and Overall survival (OS) were 8.43 ± 1.5 and 31.8 ± 15.8 months, respectively. PFS was 39.5% at 1 year and 24.7% at 2 years. The 1- and 2-year OS rates were 71.1 and 60.2%, respectively. Three-year OS was significantly superior for patients with N0-N1 stage disease vs. N2-N3 (60 vs. 24%, respectively; Response rate [RR], 0.03; <it>p</it>= 0.038).</p> <p>Conclusions</p> <p>Patients with NSCLC and brain metastasis might benefit from treatment with WBRT and concurrent thoracic chemoradiotherapy. The subgroup of N0-N1 patients appears to achieve the greatest benefit. The result of this study warrants a prospective trial to confirm the benefit of this treatment.</p
Analyzing the Impacts of Dams on Riparian Ecosystems: A Review of Research Strategies and Their Relevance to the Snake River Through Hells Canyon
Get PDFRiver damming provides a dominant human impact on river environments worldwide, and while local impacts of reservoir flooding are immediate, subsequent ecological impacts downstream can be extensive. In this article, we assess seven research strategies for analyzing the impacts of dams and river flow regulation on riparian ecosystems. These include spatial comparisons of (1) upstream versus downstream reaches, (2) progressive downstream patterns, or (3) the dammed river versus an adjacent free-flowing or differently regulated river(s). Temporal comparisons consider (4) pre- versus post-dam, or (5) sequential post-dam conditions. However, spatial comparisons are complicated by the fact that dams are not randomly located, and temporal comparisons are commonly limited by sparse historic information. As a result, comparative approaches are often correlative and vulnerable to confounding factors. To complement these analyses, (6) flow or sediment modifications can be implemented to test causal associations. Finally, (7) process-based modeling represents a predictive approach incorporating hydrogeomorphic processes and their biological consequences. In a case study of Hells Canyon, the upstream versus downstream comparison is confounded by a dramatic geomorphic transition. Comparison of the multiple reaches below the dams should be useful, and the comparison of Snake River with the adjacent free-flowing Salmon River may provide the strongest spatial comparison. A pre- versus post-dam comparison would provide the most direct study approach, but pre-dam information is limited to historic reports and archival photographs. We conclude that multiple study approaches are essential to provide confident interpretations of ecological impacts downstream from dams, and propose a comprehensive study for Hells Canyon that integrates multiple research strategies
Constraining bedrock erosion during extreme flood events
Get PDFThe importance of high-magnitude, short-lived flood events in controlling the
evolution of bedrock landscapes is not well understood. During such events, erosion
processes can shift from one regime to another upon the passing of thresholds,
resulting in abrupt landscape changes that can have a long lasting legacy on
landscape morphology.
Geomorphological mapping and topographic analysis document the evidence for,
and impact of, extreme flood events within the Jökulsárgljúfur canyon (North-East
Iceland). Surface exposure dating using cosmogenic 3He of fluvially sculpted
bedrock surfaces determines the timing of the floods that eroded the canyon and
helps constrain the mechanisms of bedrock erosion during these events. Once a
threshold flow depth has been exceeded, the dominant erosion mechanism
becomes the toppling and transportation of basalt lava columns and erosion occurs
through the upstream migration of knickpoints. Surface exposure ages allow
identification of three periods of rapid canyon cutting during erosive flood events
about 9, 5 and 2 ka ago, when multiple active knickpoints retreated large distances
(> 2 km), each leading to catastrophic landscape change within the canyon. A single
flood event ~9 ka ago formed, and then abandoned, Ásbyrgi canyon, eroding 0.14
km3 of rock. Flood events ~5 and ~2 ka ago eroded the upper 5 km of the
Jökulsárgljúfur canyon through the upstream migration of vertical knickpoints such
as Selfoss, Dettifoss and Hafragilsfoss. Despite sustained high discharge of
sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1); there is no
evidence for a transition to an abrasion-dominated erosion regime since the last
erosive flood: the vertical knickpoints have not diffused over time and there is no
evidence of incision into the canyon floor. The erosive signature of the extreme
events is maintained in this landscape due to the nature of the bedrock, the
discharge of the river, large knickpoints and associated plunge pools. The influence
of these controls on the dynamics of knickpoint migration and morphology are
explored using an experimental study. The retreat rate of knickpoints is independent
of both mean discharge, and temporal variability in the hydrograph. The dominant
control on knickpoint retreat is the knickpoint form which is set by the ratio of
channel flow depth to knickpoint height. Where the knickpoint height is five times
greater than the flow depth, the knickpoints developed undercutting plunge pools,
accelerating the removal of material from the knickpoint base and the overall retreat
rate. Smaller knickpoints relative to the flow depth were more likely to diffuse from a
vertical step into a steepened reach or completely as the knickpoint retreated up the
channel. These experiments challenge the established assumption in models of
landscape evolution that a simple relationship exists between knickpoint retreat and
discharge/drainage area. In order to fully understand how bedrock channels, and
thus landscapes, respond and recover to transient forcing, further detailed study of
the mechanics of erosion processes at knickpoints is required
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