12,885 research outputs found
Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses
We describe a setup to study ultrafast dynamics in gas-phase molecules using
time-resolved photoelectron and photoion spectroscopy. The vacuum ultraviolet
(VUV) probe pulses are generated via strong field high-order harmonic
generation from infrared femtosecond laser pulses. The band pass characteristic
in transmission of thin indium (In) metal foil is exploited to isolate the
harmonic of the 800 nm fundamental (H9, 14 eV, 89 nm) from all
other high harmonics. The harmonic is obtained with high
conversion efficiencies and has sufficient photon energy to access the complete
set of valence electron levels in most molecules. The setup also allows for
direct comparison of VUV single-photon probe with 800 nm multi-photon probe
without influencing the delay of excitation and probe pulse or the beam
geometry. We use a magnetic bottle spectrometer with high collection efficiency
for electrons, serving at the same time as a time of flight spectrometer for
ions. Characterization measurements on Xe reveal the spectral width of H9 to be
meV and a photon flux of photons/pulse after
spectral filtering. As a first application, we investigate the S excitation
of perylene using time-resolved ion spectra obtained with multi-photon probing
and time-resolved electron spectra from VUV single-photon probing. The time
resolution extracted from cross-correlation measurements is fs for
both probing schemes and the pulse duration of H9 is found to be fs
A new generation photodetector for astroparticle physics: the VSiPMT
The VSiPMT (Vacuum Silicon PhotoMultiplier Tube) is an innovative design we
proposed for a revolutionary photon detector. The main idea is to replace the
classical dynode chain of a PMT with a SiPM (G-APD), the latter acting as an
electron detector and amplifier. The aim is to match the large sensitive area
of a photocathode with the performance of the SiPM technology. The VSiPMT has
many attractive features. In particular, a low power consumption and an
excellent photon counting capability. To prove the feasibility of the idea we
first tested the performance of a special non-windowed SiPM by Hamamatsu (MPPC)
as electron detector and current amplifier. Thanks to this result Hamamatsu
realized two VSiPMT industrial prototypes. In this work, we present the results
of a full characterization of the VSiPMT prototype
The first version Buffered Large Analog Bandwidth (BLAB1) ASIC for high luminosity collider and extensive radio neutrino detectors
Future detectors for high luminosity particle identification and ultra high
energy neutrino observation would benefit from a digitizer capable of recording
sensor elements with high analog bandwidth and large record depth, in a
cost-effective, compact and low-power way. A first version of the Buffered
Large Analog Bandwidth (BLAB1) ASIC has been designed based upon the lessons
learned from the development of the Large Analog Bandwidth Recorder and
Digitizer with Ordered Readout (LABRADOR) ASIC. While this LABRADOR ASIC has
been very successful and forms the basis of a generation of new, large-scale
radio neutrino detectors, its limited sampling depth is a major drawback. A
prototype has been designed and fabricated with 65k deep sampling at
multi-GSa/s operation. We present test results and directions for future
evolution of this sampling technique.Comment: 15 pages, 26 figures; revised, accepted for publication in NIM
ポータビリティを意識したCMOSミックスドシグナルVLSI回路設計手法に関する研究
本研究は、半導体上に集積されたアナログ・ディジタル・メモリ回路から構成されるミクストシグナルシステムを別の製造プロセスへ移行することをポーティングとして定義し、効率的なポーティングを行うための設計方式と自動回路合成アルゴリズムを提案し、いくつかの典型的な回路に対する設計事例を示し、提案手法の妥当性を立証している。北九州市立大
The effect of heterogeneity on decorrelation mechanisms in spiking neural networks: a neuromorphic-hardware study
High-level brain function such as memory, classification or reasoning can be
realized by means of recurrent networks of simplified model neurons. Analog
neuromorphic hardware constitutes a fast and energy efficient substrate for the
implementation of such neural computing architectures in technical applications
and neuroscientific research. The functional performance of neural networks is
often critically dependent on the level of correlations in the neural activity.
In finite networks, correlations are typically inevitable due to shared
presynaptic input. Recent theoretical studies have shown that inhibitory
feedback, abundant in biological neural networks, can actively suppress these
shared-input correlations and thereby enable neurons to fire nearly
independently. For networks of spiking neurons, the decorrelating effect of
inhibitory feedback has so far been explicitly demonstrated only for
homogeneous networks of neurons with linear sub-threshold dynamics. Theory,
however, suggests that the effect is a general phenomenon, present in any
system with sufficient inhibitory feedback, irrespective of the details of the
network structure or the neuronal and synaptic properties. Here, we investigate
the effect of network heterogeneity on correlations in sparse, random networks
of inhibitory neurons with non-linear, conductance-based synapses. Emulations
of these networks on the analog neuromorphic hardware system Spikey allow us to
test the efficiency of decorrelation by inhibitory feedback in the presence of
hardware-specific heterogeneities. The configurability of the hardware
substrate enables us to modulate the extent of heterogeneity in a systematic
manner. We selectively study the effects of shared input and recurrent
connections on correlations in membrane potentials and spike trains. Our
results confirm ...Comment: 20 pages, 10 figures, supplement
Regulation of T cell expansion by antigen presentation dynamics
An essential feature of the adaptive immune system is the proliferation of
antigen-specific lymphocytes during an immune reaction to form a large pool of
effector cells. This proliferation must be regulated to ensure an effective
response to infection while avoiding immunopathology. Recent experiments in
mice have demonstrated that the expansion of a specific clone of T cells in
response to cognate antigen obeys a striking inverse power law with respect to
the initial number of T cells. Here, we show that such a relationship arises
naturally from a model in which T cell expansion is limited by decaying levels
of presented antigen. The same model also accounts for the observed dependence
of T cell expansion on affinity for antigen and on the kinetics of antigen
administration. Extending the model to address expansion of multiple T cell
clones competing for antigen, we find that higher affinity clones can suppress
the proliferation of lower affinity clones, thereby promoting the specificity
of the response. Employing the model to derive optimal vaccination protocols,
we find that exponentially increasing antigen doses can achieve a nearly
optimized response. We thus conclude that the dynamics of presented antigen is
a key regulator of both the size and specificity of the adaptive immune
response
High Repetition-Rate Wakefield Electron Source Generated by Few-millijoule, 30 femtosecond Laser Pulses on a Density Downramp
We report on an experimental demonstration of laser wakefield electron
acceleration using a sub-TW power laser by tightly focusing 30-fs laser pulses
with only 8 mJ pulse energy on a 100 \mu m scale gas target. The experiments
are carried out at an unprecedented 0.5 kHz repetition rate, allowing "real
time" optimization of accelerator parameters. Well-collimated and stable
electron beams with a quasi-monoenergetic peak in excess of 100 keV are
measured. Particle-in-cell simulations show excellent agreement with the
experimental results and suggest an acceleration mechanism based on electron
trapping on the density downramp, due to the time varying phase velocity of the
plasma waves.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
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