9,216 research outputs found
The use of in-situ cosmogenic 21Ne in studies on long-term landscape development
Cosmogenic Ne isotopes are stable and are routinely used for constraining the timing of events and the rate of surface change beyond the limit that can be studied with radionuclides 10Be, 26Al, and 36Cl. Cosmogenic Ne analysis can be used in quartz and in a range of other minerals. Analysis typically requires significantly less material than do cosmogenic 10Be and 26Al, opening up the technique for small samples—individual pebbles in river sediments, for example. Analysis is easier and faster than for radionuclides, not least because Ne measurements do not require significant chemical procedures. However, the presence of other sources of Ne in minerals tends to restrict the use of cosmogenic 21Ne to old landscapes and long exposure durations. In this review we briefly outline the background of cosmogenic Ne production in rocks and minerals at the Earth’s surface, then document the key uses of the technique by highlighting some earlier studies, and finish with a short perspective on the future of the technique
Impact of Simulated 1/f Noise for HI Intensity Mapping Experiments
Cosmology has entered an era where the experimental limitations are not due
to instrumental sensitivity but instead due to inherent systematic
uncertainties in the instrumentation and data analysis methods. The field of HI
intensity mapping (IM) is still maturing, however early attempts are already
systematics limited. One such systematic limitation is 1/f noise, which largely
originates within the instrumentation and manifests as multiplicative gain
fluctuations. To date there has been little discussion about the possible
impact of 1/f noise on upcoming single-dish HI IM experiments such as BINGO,
FAST or SKA. Presented in this work are Monte-Carlo end-to-end simulations of a
30 day HI IM survey using the SKA-MID array covering a bandwidth of 950 and
1410 MHz. These simulations extend 1/f noise models to include not just
temporal fluctuations but also correlated gain fluctuations across the receiver
bandpass. The power spectral density of the spectral gain fluctuations are
modelled as a power-law, and characterised by a parameter . It is found
that the degree of 1/f noise frequency correlation will be critical to the
success of HI IM experiments. Small values of ( < 0.25) or high
correlation is preferred as this is more easily removed using current component
separation techniques. The spectral index of temporal fluctuations ()
is also found to have a large impact on signal-to-noise. Telescope slew speed
has a smaller impact, and a scan speed of 1 deg s should be sufficient
for a HI IM survey with the SKA.Comment: 22 pages, 15 figures, 2 table
From random walk to single-file diffusion
We report an experimental study of diffusion in a quasi-one-dimensional (q1D)
colloid suspension which behaves like a Tonks gas. The mean squared
displacement as a function of time is described well with an ansatz
encompassing a time regime that is both shorter and longer than the mean time
between collisions. This ansatz asserts that the inverse mean squared
displacement is the sum of the inverse mean squared displacement for short time
normal diffusion (random walk) and the inverse mean squared displacement for
asymptotic single-file diffusion (SFD). The dependence of the single-file 1D
mobility on the concentration of the colloids agrees quantitatively with that
derived for a hard rod model, which confirms for the first time the validity of
the hard rod SFD theory. We also show that a recent SFD theory by Kollmann
leads to the hard rod SFD theory for a Tonks gas.Comment: 4 pages, 4 figure
Rotationally inelastic scattering of CD<sub>3</sub> and CH<sub>3</sub> with He:comparison of velocity map-imaging data with quantum scattering calculations
Indifferent Gender Preferences among Childless Beijing Citizens
Son preference, leading to skewed sex ratios at birth, is an important feature of contemporary Chinese demography, as well as being a critical policy issue. Using a 2006 representative survey, this article explores preferences for boys and girls among childless young adults in a district of Beijing who have a stated one child as their ideal number of children (though they may be eligible to bear more). The descriptive analysis finds no evidence of son-preference; rather an overall indifference to gender. A multivariate analysis provides some indicative evidence of indifference regarding predictors of desiring a girl, a boy or either. gs
Manipulating adenovirus hexon hypervariable loops dictates immune neutralisation and coagulation factor X-dependent cell interaction in vitro and in vivo
Adenoviruses are common pathogens, mostly targeting ocular, gastrointestinal and respiratory cells, but in some cases infection disseminates, presenting in severe clinical outcomes. Upon dissemination and contact with blood, coagulation factor X (FX) interacts directly with the adenovirus type 5 (Ad5) hexon. FX can act as a bridge to bind heparan sulphate proteoglycans, leading to substantial Ad5 hepatocyte uptake. FX “coating” also protects the virus from host IgM and complement-mediated neutralisation. However, the contribution of FX in determining Ad liver transduction whilst simultaneously shielding the virus from immune attack remains unclear. In this study, we demonstrate that the FX protection mechanism is not conserved amongst Ad types, and identify the hexon hypervariable regions (HVR) of Ad5 as the capsid proteins targeted by this host defense pathway. Using genetic and pharmacological approaches, we manipulate Ad5 HVR interactions to interrogate the interplay between viral cell transduction and immune neutralisation. We show that FX and inhibitory serum components can co-compete and virus neutralisation is influenced by both the location and extent of modifications to the Ad5 HVRs. We engineered Ad5-derived HVRs into the rare, native non FX-binding Ad26 to create Ad26.HVR5C. This enabled the virus to interact with FX at high affinity, as quantified by surface plasmon resonance, FX-mediated cell binding and transduction assays. Concomitantly, Ad26.HVR5C was also sensitised to immune attack in the absence of FX, a direct consequence of the engineered HVRs from Ad5. In both immune competent and deficient animals, Ad26.HVR5C hepatic gene transfer was mediated by FX following intravenous delivery. This study gives mechanistic insight into the pivotal role of the Ad5 HVRs in conferring sensitivity to virus neutralisation by IgM and classical complement-mediated attack. Furthermore, through this gain-of-function approach we demonstrate the dual functionality of FX in protecting Ad26.HVR5C against innate immune factors whilst determining liver targeting
Human exposure to halogenated and organophosphate flame retardants through informal e-waste handling activities - A critical review
Bridging Offline Reinforcement Learning and Imitation Learning: A Tale of Pessimism
Offline (or batch) reinforcement learning (RL) algorithms seek to learn an
optimal policy from a fixed dataset without active data collection. Based on
the composition of the offline dataset, two main categories of methods are
used: imitation learning which is suitable for expert datasets and vanilla
offline RL which often requires uniform coverage datasets. From a practical
standpoint, datasets often deviate from these two extremes and the exact data
composition is usually unknown a priori. To bridge this gap, we present a new
offline RL framework that smoothly interpolates between the two extremes of
data composition, hence unifying imitation learning and vanilla offline RL. The
new framework is centered around a weak version of the concentrability
coefficient that measures the deviation from the behavior policy to the expert
policy alone.
Under this new framework, we further investigate the question on algorithm
design: can one develop an algorithm that achieves a minimax optimal rate and
also adapts to unknown data composition? To address this question, we consider
a lower confidence bound (LCB) algorithm developed based on pessimism in the
face of uncertainty in offline RL. We study finite-sample properties of LCB as
well as information-theoretic limits in multi-armed bandits, contextual
bandits, and Markov decision processes (MDPs). Our analysis reveals surprising
facts about optimality rates. In particular, in all three settings, LCB
achieves a faster rate of for nearly-expert datasets compared to the
usual rate of in offline RL, where is the number of samples in
the batch dataset. In the case of contextual bandits with at least two
contexts, we prove that LCB is adaptively optimal for the entire data
composition range, achieving a smooth transition from imitation learning to
offline RL. We further show that LCB is almost adaptively optimal in MDPs.Comment: 84 pages, 6 figure
One-vortex moduli space and Ricci flow
The metric on the moduli space of one abelian Higgs vortex on a surface has a
natural geometrical evolution as the Bradlow parameter, which determines the
vortex size, varies. It is shown by various arguments, and by calculations in
special cases, that this geometrical flow has many similarities to Ricci flow.Comment: 20 page
- …