Figure 2. . Charcoal cannisters Figure 2. . Charcoal cannisters can be manufactured simply using a small can covered by a screen. The charcoal is contained in the space below the screen, which is held in place by a ring. A top is fitted over this arrangement until exposure, at which time it is removed. The top is replaced and sealed at the end of exposure, and the entire can placed on a NaI gamma counting system for analysis. ( From Cothern and Smith, 1987, p. 67).