Decades-long changes of the interstellar wind through our solar system

The journey of the Sun through the dynamically active local interstellar medium creates an evolving heliosphere environment. This motion drives a wind of interstellar material through the heliosphere that has been measured with Earth-orbiting and interplanetary spacecraft for 40 years. Recent results obtained by NASA's Interstellar Boundary Explorer mission during 2009-2010 suggest that neutral interstellar atoms flow into the solar system from a different direction than found previously. These prior measurements represent data collected from Ulysses and other spacecraft during 1992-2002 and a variety of older measurements acquired during 1972-1978. Consideration of all data types and their published results and uncertainties, over the three epochs of observations, indicates that the trend for the interstellar flow ecliptic longitude to increase linearly with time is statistically significant.

Single-quantum dot imaging with a photon counting camera

The expanding spectrum of applications of single-molecule fluorescence imaging ranges from fundamental in vitro studies of biomolecular activity to tracking of receptors in live cells. The success of these assays has relied on progress in organic and non-organic fluorescent probe developments as well as improvements in the sensitivity of light detectors. We describe a new type of detector developed with the specific goal of ultra-sensitive single-molecule imaging. It is a wide-field, photon-counting detector providing high temporal and high spatial resolution information for each incoming photon. It can be used as a standard low-light level camera, but also allows access to a lot more information, such as fluorescence lifetime and spatio-temporal correlations. We illustrate the single-molecule imaging performance of our current prototype using quantum dots and discuss on-going and future developments of this detector.

Detectors for single-molecule fluorescence imaging and spectroscopy

Single-molecule observation, characterization and manipulation techniques have recently come to the forefront of several research domains spanning chemistry, biology and physics. Due to the exquisite sensitivity, specificity, and unmasking of ensemble averaging, single-molecule fluorescence imaging and spectroscopy have become, in a short period of time, important tools in cell biology, biochemistry and biophysics. These methods led to new ways of thinking about biological processes such as viral infection, receptor diffusion and oligomerization, cellular signaling, protein-protein or protein-nucleic acid interactions, and molecular machines. Such achievements require a combination of several factors to be met, among which detector sensitivity and bandwidth are crucial. We examine here the needed performance of photodetectors used in these types of experiments, the current state of the art for different categories of detectors, and actual and future developments of single-photon counting detectors for single-molecule imaging and spectroscopy.

Fluorescence lifetime microscopy with a time- and space-resolved single-photon counting detector

We have recently developed a wide-field photon-counting detector (the H33D detector) having high-temporal and high-spatial resolutions and capable of recording up to 500,000 photons per sec. Its temporal performance has been previously characterized using solutions of fluorescent materials with different lifetimes, and its spatial resolution using sub-diffraction objects (beads and quantum dots). Here we show its application to fluorescence lifetime imaging of live cells and compare its performance to a scanning confocal TCSPC approach. With the expected improvements in photocathode sensitivity and increase in detector throughput, this technology appears as a promising alternative to the current lifetime imaging solutions.

Extreme ultraviolet transmission of a synthetic diamond thin film

The transmission of a thin film of synthetic diamond was measured at various wavelengths in the extreme ultraviolet. The measurements agree with a prediction based on published carbon attenuation coefficients assuming the density of natural diamond. A betterfit to the data results when an additional approximately 200-A layer of silicon is included in the model. It is believed that this silicon layer exists as silicon carbide.

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium chloride photocathode layers on microchannel plates

We present measurements of the quantum detectio efficiency (QDE) of potassium chloride, applied directly to the surface of a microchannel plate (MCP), over the 44-1460-A wavelength range. The contributions of the photocathode material in the channels, and on the interchannel web, to the QDE have been determined. Two broad peaks in the QDE centered at lambda congruent with 500 A and lambda congruent with 900 A are apparent, the former with ~40% peak QDE and the latter with ~30% peak QDE. The photoelectric threshold is observed at lambda ~1400 A, and there is a narrow QDE minimum at lambda~ 670 A, which correlates with 2xthe band gap energy for KC1. The angular variation of the QDE from 0 to 35 degrees to the channel axis has also been examined. We describe a simple QDE model and show that its predictions are in accord with our QDE measurements. Assessment of the stability of KC1 shows that there was no significant degradation of the QDE at wavelengths of </=1066 A during a 4-month storage period in dry nitrogen with a limited exposure to air.

Soft x-ray and extreme ultraviolet quantum detection efficiency of potassium bromide photocathode layers on microchannel plates

The quantum detection efficiency (QDE) of potassium bromide, applied directly to the surface of a microchannel plate (MCP), has been measured over the wavelength range from 44 to 1216 A. We present the first measurements for the QDE of KBr between 44 and 256 A. These show that there is a high QDE peak (~70%) centered at ~70 A. The results at wavelengths above 256 A agree with our previous study. Investigation of the angular dependence of the QDE indicates that maximum efficiencies are achieved for graze angles </=10 degrees at wavelengths below 171 A. Examination of the photocathode surface with a scanning electron microscope shows that the KBr layer is rough, with granularity at the 0.5-microm level.

Ultraviolet quantum detection efficiency of potassium bromide as an opaque photocathode applied to microchannel plates

We have measured the quantum detection efficiency (QDE) of potassium bromide as a photocathode applied directly to the surface of a microchannel plate over the 250-1600A wavelength range. The contributions of the photocathode material in the channels and on the interchannel web to the QDE have been determined. Two broad peaks in the QDE centered at ~450 and ~1050 A are apparent, the former with ~50% peak QDE and the latter with ~40% peak QDE. The photoelectric threshold is observed at ~1600 A, and there is a narrow QDE minimum at ~750 A which correlates with 2x the band gap energy for KBr. The angular variation of the QDE from 0 to 40 degrees to the channel axis has also been examined. The stability of KBr with time is shown to be good with no significant degradation of QDE at wavelengths below 1216 A over a 15-day period in air.