Cryostatic Micro-CT Imaging of Transient Process System
A double walled copper vessel was fabricated for the micro-CT scanning of tissue specimens maintained at cryogenic temperature. The space between the two nested vessels was evacuated and in two opposing sides of the vessel the copper was replaced by beryllium foil. The vessel consists of two chambers, each of which is surrounded by vacuum for better thermal stability. The vacuum in this area is on the order of 1x10-3 torr. Nitrogen gas, boiling off liquid nitrogen, is injected continuously into the top of the chamber during the scanning process. Just prior to venting from the vessel the gas is heated and directed through a narrow gap over the outside of the beryllium windows to keep them frost-free. A temperature detector within the chamber is used to control the rate of inflow of the nitrogen gas. The frozen specimen is attached to a small horizontal platform on top of a vertical stainless steel pin which exits the base of the vessel through a closely fitting hole and is attached to the computer-controlled rotating stage under the vessel. The vessel and rotation-stage assembly is mounted on a chamber-controlled horizontal translation stage which can move the specimen out of the x-ray beam, from time to time, for x-ray beam calibration purposes.