A spark chamber is an instrument created for detecting electrically charged particles. It consists of a stack of metal plates placed in a sealed box filled with a noble gas such as helium, neon or a mixture of the two. When a charged particle from a cosmic ray travels through the box, it ionises the gas between the plates. Ordinarily this ionisation would remain invisible. However, if a high enough voltage can be applied between each adjacent pair of plates before that ionisation disappears, then sparks can be made to form along the trajectory taken by the ray, and the cosmic ray in effect becomes visible as a line of sparks.
The high voltage cannot be connected to the plates permanently, as this would lead to arc formation and continuous discharging, so in order to control the moment when this voltage is applied, a separate detector (often containing a pair of scintillators placed above and below the box) is needed. When this trigger system senses that a cosmic ray has just passed through the detector, it fires a fast switch to connect the high voltage to the plates.
The instrument derives from scientific observations made in 1949 by Jack Warren Keuffel a physicist from the California Institute of Technology, who at the time was studying the discharge between parallel plates in Geiger Muller tubes. He reported that the discharge occurred along the physical path taken by the passage of a cosmic ray through the discharge. Based on this first experience, during the decade of the 50's several improvements were made to the chamber including several parallel plate counters, and enhanced trigger systems. Later developments added the possibility of observing more than one particle at once as well to increase the size and speed of response of the instrument to be able to observe spark trails of up to 20cm. The fully developed spark chamber became one of the principal detectors used in particle physics experiments in the 70's and with the adecquate combination of several auxiliary detectors, it was used to search for very rare cosmic ray events.
A lot of balloon-borne experiments were based on spark chambers specially adapted to stratospheric flight. They were used specially to search for high energy Gamma Ray particles. Some examples can be found in the instruments developed and flown by Glenn Frye's group at Case Western Reserve, Fazio's group at the Smithsonian Astrophysical Observatory, Greisen's group at Cornell University or Fitchell's group at Goddard Space Flight Center, to name a few.
At left we can see the scheme of the Goddard spark chamber, and below a picture of Case Western's digitized spark chamber being prepared for flight. Both instruments were flown on balloon missions from Australia in the 70's.
In current research the spark chamber has been replaced by detectors of higher sophistication which have improved time and spatial resolution, like drift chambers, or silicon detectors. However the spark chamber is still of great scientific value as it remains relatively simple and cheap to build as well is a very atractive tool for teaching physics, enabling an observer to view the paths of charged particles.