J.V. Vallerga

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. Here, the necessary 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, are investigated.

High resolution cross strip anodes for photon counting detectors

A new photon counting, imaging readout for microchannel plate sensors, the cross strip (XS) anode, has been investigated. Charge centroiding of signals detected on two orthogonal layers of sense strip sets are used to derive photon locations. The XS anode spatial resolution (< 3 μm FWHM) exceeds the spatial resolution of most direct charge sensing anodes, and does so at low gain ( < 2 × 10 6 ). The image linearity and fidelity are high enough to resolve and map 7 μm MCP pores, offering new possibilities for astronomical and other applications.

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.

Cross delay line detectors for high time resolution astronomical polarimetry and biological fluorescence imaging

Ground based high time resolution astronomical polarimetry, imaging, and biological time-resolved molecular fluorescence lifetime imaging require specialized detectors. Photon counting detectors that combine high spatial resolution imaging with fast event timing capability for these uses have been a significant technical challenge. We have developed a high-temporal and spatial resolution, high-throughput sealed tube microchannel plate detector with electronic readout as tool for these applications. The design is based on a 25 mm diameter S20 photocathode followed by a microchannel plate stack, read out by a cross delay line anode with timing and imaging electronics. The detector supports 500 kHz global count rate, 10 kHz local count rate, 100 ps timing resolution and 40 /spl mu/m spatial resolution. We describe the performance of the detector, as well as imaging results obtained with quantum dots and live cells.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

The FUV detector for the cosmic origins spectrograph on the Hubble Space Telescope

Abstract The Cosmic Origins Spectrograph (COS) is a high throughput spectrometer that will be placed on the Hubble Space Telescope (HST) during the last servicing mission in the year 2003. COS will be the most sensitive UV spectrograph ever flown aboard HST and will investigate such fundamental issues as the ionization and baryon content of the intergalactic medium and the origin of large-scale structure of the Universe. The driving design goal for COS is to maximize throughput at a moderate spectral resolution of >20,000 using optics with very few reflections and detectors with high quantum efficiency in two bandpass channels: FUV (1150– 1775 A ) and NUV (1750– 3200 A ). The COS FUV detector, a windowless microchannel plate (MCP) detector, consists of two segments each 85 mm ×10 mm concatenated end to end with a 9 mm gap between them. The design is based on the Far Ultraviolet Spectroscopic Explorer detectors with identical format and front surface radius of curvature that matches the grating focal plane of the spectrograph. However, enhancements have been made in the design and fabrication of the MCPs, the photocathode, the delay line anode and the readout electronics. We discuss these design enhancements and their significance.

High Spatial and Temporal Resolution Neutron Imaging With Microchannel Plate Detectors

Special microchannel plates (MCPs) developed by Nova Scientific Inc. incorporate high efficiency neutron conversion materials into the MCP to provide a high neutron stopping power. 10B and natGd have high interaction cross sections for thermal neutrons and their incorporation into MCP glass is a convenient way to make efficient MCPs for neutron detection with high spatial resolution. We have evaluated neutron event counting 2D imaging detectors using these MCPs with a cross delay line readout, cross strip readout, and a Medipix2 readout. Tests at several reactors with the cross delay line and cross strip readouts have established spatial resolution with neutrons as good as ~30 microns FWHM over a 27 mm diameter detector, with event rates approaching 1 MHz, low fixed pattern noise, event time tagging of 25 ns and intrinsic background rates of < 0.05 events cm-2 sec-1. Evaluation of neutron sensing MCP detector with Medipix2 readout (14 mm2) has allowed operation at high counting rates (500 MHz) with the spatial resolution limited by the 55 micron pixel size of the Medipix2 readout. We have also used the Medipix2 for centroiding of neutron events to sub pixel resolution to obtain better spatial resolution (< 15 mum) for neutrons at reduced event rates (100 kHz). Initial measurements of thermal neutron detection efficiencies give values of 20% to 25% for thermal neutrons and 45% for cold neutrons without optimization of the detection geometry. Preliminary tests with shielding and a LaBr scintillator to gate neutron detections in coincidence with gamma rays produced by neutron interactions has enabled gamma ray rejection factors of 3 times 104 to be achieved. Further improvements in the neutron detection efficiency and gamma ray discrimination efficiency can be gained by optimization of the geometrical and electronic configurations.

Energy-Resolving Neutron Transmission Radiography at the ISIS Pulsed Spallation Source With a High-Resolution Neutron Counting Detector

Neutron transmission radiography can be strongly enhanced by adding spectroscopic data spatially correlated with the attenuation coefficient. This can now be achieved at pulsed neutron sources, utilizing a neutron detector with high spatial and temporal resolution. The energy of transmitted neutrons can be recovered from their time-of-flight, simultaneously with the acquisition of the transmission radiographic image by a pixelated detector. From this, the positions of Bragg edges can be obtained for each pixel of the radiographic image. The combination of both spectroscopic and transmission information enables high spatial resolution studies to be carried out on material composition, phase transitions, texture variations, as well as strain analysis, as long as the resolution and statistics are favorable. This paper presents initial results from proof-of-principle experiments on energy-resolved neutron transmission radiography, using a neutron counting detector consisting of neutron-sensitive microchannel plates (MCPs) and a Medipix2 electronic readout. These experiments demonstrate that the position of Bragg edges are measurable with a few mAring resolution in each 55-mum pixel of the detector, corresponding to DeltaE/E~0.1%. However, the limited intensity of most current neutron sources requires a compromise between the energy resolution and the area over which it was integrated. Still, the latter limitation can be overcome by combining energy information for several neighboring pixels, while transmission radiography can still be done at the limit of the detector spatial resolution.

Optimization of Timepix count rate capabilities for the applications with a periodic input signal

The capability of a Timepix readout to detect both position (with 55 ?m accuracy) and time (with up to 10 ns resolution) of multiple simultaneous events enables novel non-destructive testing methods. The first generation Timepix ASIC provides timing only for one event per pixel per frame. With fast parallel readouts capable of > 1 KHz frame rates up to ~ 3 ? 106 counts cm?2s?1 can be detected with 10% overlap probability. For some applications, where incoming fluxes exceed this rate the event overlaps distort the measured timing signal. In this case many particles arriving late in the frame are not registered as many pixels are already occupied by preceding counts. In this paper we demonstrate how the count rate capability of Timepix readout can be increased in the experiments where incoming flux has a periodic structure. It is shown how the timing characteristics of input flux can be accurately reconstructed even for ~ 80% pixel occupancy, where the probability of detection is reduced for the later events of the acquisition frame. The implementation of the correction algorithm is demonstrated for a UV flux varying with a 60 Hz frequency. The post-measurement reconstruction method is implemented for each individual pixel of the acquired images and thus can be applied even for the experiments where the incoming flux has a strong variation across the active area.

Large Area Microchannel Plate Imaging Event Counting Detectors With Sub-Nanosecond Timing

Progress towards the development of a 20 cm sealed tube optical detector with imaging and photon event time stamping is presented. Novel microchannel plates employing borosilicate micro-capillary arrays have been tested. These provide many performance characteristics typical of conventional MCPs, but have been made in sizes up to 20 cm, have low intrinsic background (0.08 events cm-2s-1) and very stable gain behavior for at least 7 C cm-2 of charge extraction. Bialkali (Na2KSb) photocathodes with >20% quantum efficiency have also been made on 20 cm borofloat-33 windows compatible with a large sealed tube device.