Automated Reconstruction of CASCADE Paths in Emulsion Chambers

Digital image processing techniques are applied to high resolution images of emulsion chamber x-ray films to reconstruct the trajectories of incident particles. This technique produces equivalent results to the traditional manual approach. Advantages include speed, objectivity, and the transportability of data. The record of x-ray spots may further be used to prioritize successive stages of analysis. BACKGROUND Emulsion chamber data acquisition requires a great deal of labor by skilled workers (Strausz, 1983). Many DAQ tasks are ideal candidates for computer automation. We describe a system for the automated “mapping” of cascades in EC x-ray films, similar to trackers in accelerator experiments. In a cosmic ray experiment, incident particles come from all directions, and passive ECs provide no time-domain information. These complications make the problem unique. The product of mapping is a catalog of cosmic ray cascades in the detector satisfying a set of cut criteria, thus qualifying for further investment of analysis effort. It is analogous to the level-1 trigger in a counter experiment. METHOD In the conventional mapping method, a worker scans individual x-ray films and circles the 300–400 darkest spots, using pairs of films in coincidence. (Individual biases are eliminated by using only the darkest 150–250 of these in the final results, as measured by instrument). Once the spots have been identified in every layer, their locations on several consecutive films are transcribed onto a single plot. Spots from an event then fall in a straight line (the cascade’s projection on the film plane), while noise shows no correlation. The process is repeated for other layers of the chamber, and the results are combined into a single map (Olson, 1995). The automated system is made up of two programs which mimic the steps described, and a collection of smaller support programs. Spotfinder identifies spots in the digitized image of an x-ray film layer, using established image processing techniques. CTD (Connect-The-Dots) joins these spots into tracks through a tree-search algorithm. Spotfinder One JACEE emulsion chamber contains about 40 x-ray films, each measuring 40 50 cm (Burnett, 1986). We digitize these films as 340