Silicon avalanche photodiodes (APDs) fabricated through a deep diffusion process underwent a modified surface treatment in an attempt to improve their response in the ultraviolet region of the optical spectrum. After adjusting the doping profile in the near-surface region of the detectors, APDs were fabricated and tested at several wavelengths from ultraviolet to the near-infrared. At the target wavelength of 355 nm, the detector bandwidth was increased by a factor of 20 over devices fabricated without the modified surface treatment. Modest improvements in the internal quantum efficiency were also measured. Most importantly, the modified detectors maintained the high gain and low noise performance specifications that are hallmarks of traditional deep diffusion APDs.
Photodynamic therapy (PDT) is becoming a treatment of choice for cancer because of
its low cost, high effectiveness and low damage to healthy tissue. Successful PDT outcome
depends on accurate dosimetry, which is currently lacking, leading to variable and/or
ineffective treatment outcome. We report on our research and developmental efforts
towards an implicit dosimetric method for PDT that will provide an accurate assessment of
treatment effectiveness by continuous monitoring of the in vivo drug concentration and the
oxygen concentration in tissue. This approach uses the same tools presently available for
PDT, making it attractive to the health professionals without increasing treatment cost.
In this paper we present the details of a Diffuse Optical Tomographic (DOT) prototype instrument developed and
characterized at RMD for concurrent operation with Magnetic Resonance Imaging (MRI) to obtain high resolution
spatial and functional images of hypoxic tumor tissue. We have developed a new system designed for in-vivo imaging
of luminescent agents that respond to tissue oxygenation to improve the contrast and spatial resolution of functional
optical images in deep tissue. High-resolution spatial and anatomical information obtained from MRI images is used to
improve the accuracy of the reconstructed optical images. The time domain lifetime imaging module has parallel
acquisition across a cooled 16-element avalanche photodiode (APD) array for high resolution and high throughput
imaging. The low-cost, compact lifetime imager is compatible with high magnetic and RF fields associated with MR
units in hybrid imaging systems. Using this APD module in a dual-modality imaging setup, phantom imaging was
performed to obtain oxygenation images with high resolution and contrast. Optical image reconstruction is aided by
spatial guidance obtained from the actual phantom dimensions to improve the accuracy of these images.
We will present research on the development of an optical receiver module with a wide frequency bandwidth and excellent response to near-infrared radiation. This module is being produced to promote new imaging modalities, allowing retinal specialist to utilize established diagnostic instruments, such as scanning laser ophthalmoscopes (SLO) in a unique or more effective manner. In particular, it can be applied towards more accurate visual threshold studies in both the healthy and diseased eye. With this goal in mind, measurements of the targeted receiver's performance with
and without additional amplification are presented, as is a survey of available APD detectors.
We report on the development of a practical, easy-to-use, multi-element, solid-state instrument for detecting and imaging
tritium contamination on surfaces. The innovation, which enables this instrumentation, relies on cutting-edge silicon
avalanche photodiode (APD) array detector technology to provide an effective coverage area without compromising the
overall sensitivity. We discuss the design and assembly of a prototype unit to monitor a surface area of over 900 mm2
while maintaining a spatial resolution of less than 4 mm. During tests at Los Alamos National Laboratories, we
demonstrated tritium counting efficiencies of over 40% and established that this unit can be used to expedite established
testing procedures by locating areas of potential activity or when combined with established swipe analysis.
A laser processing method was used to microstructure the surface of position-sensitive silicon avalanche photodiodes (PSAPDs) and enhance their near-infrared response. Following laser microstructuring and high-temperature annealing, experiments were performed on PSAPDs to determine their performance at 1064 nm. As a result of this processing
method, we fabricated APDs with quantum efficiencies as high as 58% at 1064 nm. The enhanced near-infrared response has now been realized in both lateral effect and quadrant-type PSAPDs without altering their electronic noise, avalanche gain or position resolution. A near-infrared-enhanced PSAPD module with temperature control and position output was assembled and tested.
A method for constructing an x-ray telescope with exceedingly hgh spatial resolution is to use a pair of coaxial, Fresnel zone plates aligned with an imaging x-ray detector. This combination allows the high sensitivity imaging of x-ray and gamma-ray sources ranging in energy from 1 keV to several hundred keV over a field of view of several degrees with spatial resolution of a fraction of an arc minute. We have implemented a version of such a telescope using several relatively new technologies. These include specialized techniques for constructing Fresnel zone plates from thin sheets of tungsten, a 64-element, avalanche photodiode (APD) array coupled to a matching, segmented, CsI(T1) scintillator, a new ASIC which provides 16-channels of low noise amplification, and image processing software that provides the user not only with localized intensity information, but also with localized spectral information.
The Avalanche Photodiode (APD) is a unique device that combines the advantages of solid state photodetectors with those of high gain devices such as photomultiplier tubes (PMTs). APDs have internal gain that provides a high signal-to-noise ratio. APDs have high quantum efficiency, are fast, compact, and rugged. These properties make them suitable detectors for important applications such as LADAR, detection and identification toxic chemicals and bio-warfare agents, LIDAR fluorescence detection, stand-off laser induced breakdown spectroscopy (LIBS), and nuclear detectors and imagers.
Recently there have been significant technical breakthroughs in fabricating very large APDs, APD arrays, and position sensitive APD arrays (PSAPD). Signal gain of over 10,000 has been achieved, single element APDs have been fabricated with active area greater than 40 cm2, monolithic pixelated arrays with up to 28 x 28 elements have been fabricated, and position sensitive APDs have been developed and tested. Additionally, significant progress has been made in improving the fabrication process to provide better uniformity and high yield, permitting cost effective manufacturing of APDs for reduced cost.
Avalanche Photodiode (APD) arrays are being applied to Laser-Induced Breakdown Spectroscopy (LIBS) for elemental analysis with standoff detection capability. This instrument, which represents a valuable addition to planetary rover missions as well as Earth-based applications, benefits from the advantages common to both Geiger-mode and proportional APDs, which are solid-state detectors with virtually single-photon sensitivity, higher quantum efficiency than photomultiplier tubes or intensified CCDs, and rapid sub-nanosecond response speed. We have demonstrated LIBS detectability better than 770 parts-per-billion of sodium utilizing the photon-counting Geiger-mode APD. In a LIBS system, an APD array offers the unparalleled prospect of selecting in each channel the most appropriate temporal window for detecting the target species. In real-time detection systems, such as microfluidics-based fluorescence detection of bacterial spores, these compact, robust APD arrays promise portable hand-held instruments that utilize tight optical coupling.
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