Cosmic Rays - Detection By Particle Track-etch Technique

Detection By Particle Track-etch Technique

Cosmic rays can also be detected directly by particle detectors aboard satellites or high altitude balloons. In a pioneering technique developed by Robert Fleischer, P. Buford Price, and Robert M. Walker, sheets of clear plastic, like 1/4 mil Lexan polycarbonate, are stacked together and exposed directly to cosmic rays in space or high altitude. The nuclear charge causes chemical bond breaking or ionization in the plastic. At the top of the plastic stack the ionization is less, due to the high cosmic ray speed. As the cosmic ray speed decreases due to deceleration in the stack, the ionization increases along the path. The resulting plastic sheets are "etched" or slowly dissolved in warm caustic sodium hydroxide solution, that removes the surface material at a slow, known rate. The caustic sodium hydroxide dissolves at a faster rate along the path of the ionized plastic. The net result is a conical shaped pit or etch pit in the plastic. The etch pits are measured under a high power microscope (typically 1600X oil-immersion), and the etch rate is plotted as a function of the depth in the stacked plastic. This yields a unique curve for each atomic nucleus of Z from 1 to 92, allowing identification of both the charge and energy of the cosmic ray that traverses the plastic stack. The more extensive the ionization along the path; the higher the charge.

This technique has been used with great success for detecting not only cosmic rays, but fission product nuclei for neutron detectors.