The objective of this core TR&D project is to develop a system for the NCMIR IVEM with the necessary enhancements in resolution, speed and sensitivity to provide users with an electronic readout device comparable to or better than film using a next generation of 2k x 2k CCD imaging system . This subproject is very important to this research program. It will allow us to improve the image acquisition rate and precision for computerized 3-D reconstruction and visualization of thick biological specimens. We have recently completed the new lens coupled camera system. It has been installed on the IVEM and is undergoing testing. There are five parts to this project: 1) scintillating screen development: two US patents (#5,401,964 and #5,594,253) have been issued to us including one just issued in January, 1997. A thin foil-based P20 phosphor screen has been made for this system by Grant Scientific according to our patented design. The design was first described and characterized in 1994 (Fan and Ellisman, Ultramicroscopy 55:7-14, 1994) and optimized in 1996 (Fan et al, Ultramicroscopy 66:11-19, 1994) ; 2) Lens coupling system, designed by Optical Research Associates according to our specifications, and manufactured by Tinsley Laboratories, was delivered and has been installed on the IVEM. The design goal was challenging for both the optical designer and the manufacturer, and Tinsley had to make a modification after the lens was first delivered, as the lens failed to meet some of the designed goals in our optical bench test. The performance was significantly improved after the modification. The mod EMulation transfer function (MTF) is 55% at the Nyquist frequency, and is nearly flat across the entire field of view, which is an area over 10 cm in diameter. The overall light transmittance is 83%, exceeding the design goal of 80%. The resolution and relay efficiency of the optical system match well with that of the scintillating screen and the system delivers resolution exceeding that possible with a fiber-optically coupled system; 3) The CCD chip being employed is technologically more advanced than what is commercially available and was provided as part of a collaborative research effort with MIT's Lincoln Laboratory and the US Air Force. This device employs very advanced technology and has 8 high bandwidth ports which may be read out in parallel. Although only four ports are being used in our implementation, we will still achieve a substantial speedup as compared to our current 1k x 1k device, yet are able to image an array more than 2x the size of our current CCD imager; 4) The computer interface to the CCD camera controller has been designed and implemented. The interface employs a Unix-based workstation coupled to DataCube MV200 image processor to control the camera and to demultiplex and assemble the image. A new graphical interface has been designed for use of the camera. 5) Mechanical integration of the camera components was designed in house using a suite of 3D Solid Modeling/2D CADCAM software tools. The complete system was initially modeled in 3D to allow for visualization and validation of total integration prior to construction. From the final, optimized model, engineering schematics were generated for construction. The components of the system include: a vacuum compatible drop flange which supports and positions the scintillator screen and leaded glass window, adjustable lens support hardware, a mechanically isolated, gyroscopic camera support housing which allows for sub-micron centering and adjustment of the CCD chip, automated rotation, and precision focusing, and a camera housing adapter which allows quick swapping of the 2kx2k and 1k x1k camera heads. Preliminary tests indicate that the overall performance of this imaging system is considerably more sensitive than film and better than a fiber-optically coupled CCD system at 400 keV (Data from Arizona State University). A further quantitative evaluation is beginning conducted in collaboration with Drs. John Spence and Jian Ming Zuo at Arizona State University. We also plan to explore the use of an Application Specific Integrated Circuit (ASIC) detector as an alternative to a CCD for TEM imaging. The ASIC detector was developed by Dr. Xuong Nguyen-Huu of UCSD and co-workers for X-ray crystallography applications. We have recently tested the device for electron detection in the energy range of 80-400 keV (Fan et al, in review by Ultramicroscopy, 1997), and the results are very encouraging. An ASIC-based imaging system will possess many advantages over the CCD-based imaging systems (see Section 4A2.2). Dr. Xuong will collaborate with us on this project.
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