Optical microscope for nuclear emulsion readout-system design and results in application

Experiments such as CHORUS at CERN require the inspection of a large amount of nuclear emulsion plates exposed to particle beams. Rare events need to be found, measured and analyzed. Their features are stored as grains in microscopic dimensions in a 3D stack of plates. A new, fully automatic immersion microscope system was developed. It features high resolution, small depth of focus, large working distance, large field of view and synchronization of illumination and detector. An additional requirement is given by variations in the refraction index and in the relative thickness of immersion oil and emulsion. The approach used is an imaging system based on a various objective lens with extreme numerical aperture, large working distance and wide field, combined with a matched high-aperture Koehler illuminator. The light source is a mercury arc lamp, combined with a filter package for the g-line. It includes liquid crystal elements for synchronized shuttering and variable attenuation. The theoretical resolution is less than 1 micron in x, y, z within a volume of 0.5 mm diameter times 1 mm scanning depth in all situations within a predefined index range. Three identical pieces of the system have been built. The experimentally measured resolution confirms the expectations and is better than 1 micron in all three dimensions. This is the result of a complex process of system design and manufacturing, unifying optical, opto-mechanical and opto- electronical contributions. This process spans from the early stages of feasibility and manufacturing up to the test and adjustment procedures. The three prototypes have been operational since the fall of 1998 in the frame of the CHORUS project. Practical experience and application results are presented. (0 refs).