Track of a Heavy Nucleus

 

This track was found in a nuclear photographic emulsion exposed to primary cosmic radiation during a balloon flight at an altitude of 100,000 ft. over South Dakota. The assembled photomicrographs show the successive portions of the track. A nucleus, probably that of iron, with energy of 17 Bev and traveling with a speed of 0.66 c entered this picture from the left. Escaping catastrophic collision with nuclei of emulsion, and after traversing some 17 mm, it finally came to rest at the right. Near the entrance end, where the particle had a great energy, the "core" of the track is quite thin and the "delta rays" (tracks of struck electrons) can be observed to diffuse from the center of the track to considerable distances. At this point the nucleus is experiencing relatively few, high-energy transfer collisions. As the particle slows down, the number of collisions it experiences with the atomic electrons of the emulsion greatly increases, but the energy transfer per collision decreases. The decrease in the length of the individual "delta rays" is masked by the increase in their numbers, thus the track appears to widen. The further loss of velocity by the particle results in a further decrease of the length of the individual "delta rays" giving the effect of taper near the terminal end of the track. Thus the track is at first narrow then widens and finally tapers at the terminal end. The track width bears little resemblance to the total rate of energy loss which continuously increases with the decrease in particle speed toward almost the very end of the particle trajectory.

 

Photo by E.V. Benton