HVEM MAINTENANCE, OPERATION & UPGRADES

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The high-voltage electron microscope (HVEM) is one of two such instruments in the US devoted to biological research. Although some of its capabilities can be taken over by more modern instruments, the HVEM still has unique capability for very thick sections, tomography at high tilt and crystallography. It is essential that the HVEM be maintained and improved so that it can continue to serve the biological community. The ambitious program begun last year has had to be curtailed, primarily because one of our staff is no longer employed here. We have concentrated our efforts on those aspects of development which have the greatest impact on the user. The HVEM continues to operate very well and routinely produces better than 1 nm resolution. The goals of this TRD are to ensure the long-term reliability of the HVEM, improve user functions, extend capability for imaging thick (several micrometer) sections, and support crystallography development. Last year several projects were undertaken. The Diffstak pump, which was obtained from the Wisconsin HVEM, was installed on the camera, and the camera box pumping connection was modified. These changes increased the camera pumping capacity by about a factor of ten. The resulting improvement in pump-down time allows the user to resume taking pictures more rapidly after a load of film has been replaced. This often allows two tomographic series to be obtained in one day; whereas, previously only one complete series could usually be obtained. In addition, the better ultimate vacuum allows operation in "window open" condition without degradation of the column vacuum. This shortens the camera cycle time by about 30%. These factors speed up tomographic data collection significantly, and they also provide a cleaner vacuum in the column, which will improve the operation and reliability of the instrument. One hundred new cassettes for the camera were fabricated, tested and put into service, and both our cameras--the one originally supplied with our instrument and that obtained from Wisconsin--were adjusted so that any of the cassettes would work well with either camera. The new cassettes are sufficient to provide sets for use with high-resolution film (Kodak 4489) and high-sensitivity film (Dumont LoDose). Users whose specimens are radiation-sensitive can now operate more easily in low-dose conditions. Tomographic projects in particular will benefit from having the new cassettes since, in addition to being able to take low-dose images more rapidly, larger groups of cassettes can be loaded into the camera reducing the number of change-over's required. Uranium liners, originally supplied with the HVEM, form vacuum seals at two places in the lens column. These liners are made from sintered uranium, and do not provide reliable seals. The shop has fabricated replacement parts from phosphor bronze, and these parts will improve the column vacuum by at least a factor of two when they are installed. Installation will be accomplished when the lens column is next disassembled for cleaning. Modifications to the high-voltage circuitry were made in consultation with an engineer from the Haefely company, the manufacturers of the high-voltage generator and electron accelerator for the HVEM. During the consultation, a circuit was discovered not to have been properly connected. This was corrected and several variable resistors were replaced by fixed resistors to improve the stability of the circuitry. Of at least equal importance, appropriate test points for the circuit modules were determined and baseline performance data collected. This has already paid off when an incident of high-voltage failure occurred, and examination of test-point voltages quickly uncovered the sources of the problem. Projects planned for this coming year are driven by improvements in reliability and user convenience. Development of the web site (see TRD "HVEM/IVEM websites for remote on-line image viewing") will be completed. A new computerized controller for stage tilt will be designed and fabricated. This will increase the reliability, accuracy and ease of use of tilting. A similar device for stage position control and readout will also be designed and fabricated. It will allow the user to reposition the specimen more quickly and accurately and will allow easy return to previously marked positions. A circuit to determine the proper exposure will be designed and integrated into the camera control so that the correct exposure time will automatically be achieved. The video camera will be protected by installing a shutter which automatically closes unless the user is actually viewing the specimen. This will prevent inadvertent damage which could occur, e.g., during use of the cut film camera.