850 research outputs found
Coulomb Excitation of 68,70Cu: First Use of Postaccelerated Isomeric Beams
We report on the first low-energy Coulomb excitation measurements with radioactive Iπ=6- beams of odd-odd nuclei 68,70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2.83 MeV/nucleon. γ rays were detected with the MINIBALL spectrometer. The 6- beam was used to study the multiplet of states (3-, 4-, 5-, 6-) arising from the π2p3/2ν1g9/2 configuration. The 4- state of the multiplet was populated via Coulomb excitation and the B(E2;6-→4-) value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z=28 shell and N=40 subshell closures. A comparison with large-scale shell-model calculations using the 56Ni core shows the importance of the proton excitations across the Z=28 shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with N≈40
Computational Modeling of Channelrhodopsin-2 Photocurrent Characteristics in Relation to Neural Signaling
Channelrhodopsins-2 (ChR2) are a class of light sensitive proteins that offer
the ability to use light stimulation to regulate neural activity with
millisecond precision. In order to address the limitations in the efficacy of
the wild-type ChR2 (ChRwt) to achieve this objective, new variants of ChR2 that
exhibit fast mono-exponential photocurrent decay characteristics have been
recently developed and validated. In this paper, we investigate whether the
framework of transition rate model with 4 states, primarily developed to mimic
the bi-exponential photocurrent decay kinetics of ChRwt, as opposed to the low
complexity 3 state model, is warranted to mimic the mono-exponential
photocurrent decay kinetics of the newly developed fast ChR2 variants: ChETA
(Gunaydin et al., Nature Neurosci, 13:387-392, 2010) and ChRET/TC (Berndt et
al., PNAS, 108:7595-7600, 2011). We begin by estimating the parameters for the
3-state and 4-state models from experimental data on the photocurrent kinetics
of ChRwt, ChETA and ChRET/TC. We then incorporate these models into a
fast-spiking interneuron model (Wang and Buzsaki., J Neurosci,
16:6402-6413,1996) and a hippocampal pyramidal cell model (Golomb et al., J
Neurophysiol, 96:1912-1926, 2006) and investigate the extent to which the
experimentally observed neural response to various optostimulation protocols
can be captured by these models. We demonstrate that for all ChR2 variants
investigated, the 4 state model implementation is better able to capture neural
response consistent with experiments across wide range of optostimulation
protocol. We conclude by analytically investigating the conditions under which
the characteristic specific to the 3-state model, namely the mono-exponential
photocurrent decay of the newly developed variants of ChR2, can occurs in the
framework of the 4-state model.Comment: 10 figure
Experimental Performance of a Micromachined Heat Flux Sensor
Steady-state and frequency response calibration of a microfabricated heat-flux sensor have been completed. This sensor is batch fabricated using standard, micromachining techniques, allowing both miniaturization and the ability to create arrays of sensors and their corresponding interconnects. Both high-frequency and spatial response is desired, so the sensors are both thin and of small cross-sectional area. Thin-film, temperature-sensitive resistors are used as the active gauge elements. Two sensor configurations are investigated: (1) a Wheatstone-bridge using four resistors; and (2) a simple, two-resistor design. In each design, one resistor (or pair) is covered by a thin layer (5000 A) thermal barrier; the other resistor (or pair) is covered by a thick (5 microns) thermal barrier. The active area of a single resistor is 360 microns by 360 microns; the total gauge area is 1.5 mm square. The resistors are made of 2000 A-thick metal; and the entire gauge is fabricated on a 25 microns-thick flexible, polyimide substrate. Heat flux through the surface changes the temperature of the resistors and produces a corresponding change in resistance. Sensors were calibrated using two radiation heat sources: (1) a furnace for steady-state, and (2) a light and chopper for frequency response
Very fast optical flaring from a possible new Galactic magnetar
Highly luminous rapid flares are characteristic of processes around compact
objects like white dwarfs, neutron stars or black holes. In the high energy
regime of X- and gamma-rays, outbursts with variability time-scales of seconds
and faster are routinely observed, e.g. in gamma-ray bursts or Soft Gamma
Repeaters. In the optical, flaring activity on such time-scales has never been
observed outside the prompt phase of GRBs. This is mostly due to the fact that
outbursts with strong, fast flaring usually are discovered in the high-energy
regime. Most optical follow-up observations of such transients employ
instruments with integration times exceeding tens of seconds, which are
therefore unable to resolve fast variability. Here we show the observation of
extremely bright and rapid optical flaring in the galactic transient SWIFT
J195509.6+261406. Flaring of this kind has never previously been reported. Our
optical light-curves are phenomenologically similar to high energy light-curves
of Soft Gamma Repeaters and Anomalous X-ray Pulsars, which are thought to be
neutron stars with extremely high magnetic fields (magnetars). This suggests
similar emission processes may be at work, but in contrast to the other known
magnetars with strong emission in the optical.Comment: 8 pages, 3 figures. A substantially revised version of this
manuscript was published in Nature. Due to license issues, the accepted
manuscript will only be put on astro-ph as v2 6 months after this versio
Updated dispersion-theoretical analysis of the nucleon electromagnetic form factors
In the light of the new data on the various neutron and proton
electromagnetic form factors taken in recent years, we update the
dispersion-theoretical analysis of the nucleon electromagnetic form factors
from the mid-nineties. The parametrization of the spectral functions includes
constraints from unitarity, perturbative QCD, and recent measurements of the
neutron charge radius. We obtain a good description of most modern form factor
data, with the exception of the Jefferson Lab data on G_E^p/G_M^p in the
four-momentum transfer range Q^2=3...6 GeV^2. For the magnetic radii of the
proton and the neutron we find r_M^p = 0.857 fm and r_M^n = 0.879 fm, which is
consistent with the recent determinations using continued fraction expansions.Comment: 5 pages, 3 ps figures, final version, exp. errors in Figs. 1 and 3
correcte
Evolutionary dynamics of cancer cell populations under immune selection pressure and optimal control of chemotherapy
Increasing experimental evidence suggests that epigenetic and microenvironmental factors play a key role in cancer progression. In this respect, it is now generally recognized that the immune system can act as an additional selective pressure, which modulates tumor development and leads, through cancer immunoediting, to the selection for resistance to immune effector mechanisms. This may have serious implications for the design of effective anti-cancer protocols. Motivated by these considerations, we present a mathematical model for the dynamics of cancer and immune cells under the effects of chemotherapy and immunity-boosters. Tumor cells are modeled as a population structured by a continuous phenotypic trait, that is related to the level of resistance to receptor-induced cell death triggered by effector lymphocytes. The level of resistance can vary over time due to the effects of epigenetic modifications. In the asymptotic regime of small epimutations, we highlight the ability of the model to reproduce cancer immunoediting. In an optimal control framework, we tackle the problem of designing effective anti-cancer protocols. The results obtained suggest that chemotherapeutic drugs characterized by high cytotoxic effects can be useful for treating tumors of large size. On the other hand, less cytotoxic chemotherapy in combination with immunity-boosters can be effective against tumors of smaller size. Taken together, these results support the development of therapeutic protocols relying on combinations of less cytotoxic agents and immune-boosters to fight cancer in the early stages. © EDP Sciences, 2014
Ambiguities in the partial-wave analysis of pseudoscalar-meson photoproduction
Ambiguities in pseudoscalar-meson photoproduction, arising from incomplete
experimental data, have analogs in pion-nucleon scattering. Amplitude
ambiguities have important implications for the problems of amplitude
extraction and resonance identification in partial-wave analysis. The effect of
these ambiguities on observables is described. We compare our results with
those found in earlier studies.Comment: 12 pages of text. No figure
A strong optical flare before the rising afterglow of GRB 080129
We report on GROND observations of a 40 sec duration (rest-frame) optical
flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a
power law with indices +12 and -8, respectively, inconsistent with a reverse
shock and a factor 10 faster than variability caused by ISM interaction.
While optical flares have been seen in the past (e.g. GRB 990123, 041219B,
060111B and 080319B), for the first time, our observations not only resolve the
optical flare into sub-components, but also provide a spectral energy
distribution from the optical to the near-infrared once every minute. The delay
of the flare relative to the GRB, its spectral energy distribution as well as
the ratio of pulse widths suggest it to arise from residual collisions in GRB
outflows \cite{liw08}.If this interpretation is correct and can be supported by
more detailed modelling or observation in further GRBs, the delay measurement
provides an independent, determination of the Lorentz factor of the outflow.Comment: accepted for publ. in ApJ, 5 Fig
The size of the proton - closing in on the radius puzzle
We analyze the recent electron-proton scattering data from Mainz using a
dispersive framework that respects the constraints from analyticity and
unitarity on the nucleon structure. We also perform a continued fraction
analysis of these data. We find a small electric proton charge radius, r_E^p =
0.84_{-0.01}^{+0.01} fm, consistent with the recent determination from muonic
hydrogen measurements and earlier dispersive analyses. We also extract the
proton magnetic radius, r_M^p = 0.86_{-0.03}^{+0.02} fm, consistent with
earlier determinations based on dispersion relations.Comment: 4 pages, 2 figures, fit improved, small modifications, section on
continued fractions modified, conclusions on the proton charge radius
unchanged, version accepted for publication in European Physical Journal
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