A radioactive ruler consists of a slender plastic tube containing alternate 1 cm sections of Co-57 and inactive material. On an imaging system, a radioactive ruler appears as a dashed line and are used to perform coarse measurement and reference. Well counter calibration standards are provided in two forms, Rod Standards and Tube Standards. Both contain activity at one end that is meant to be placed into the well portion of the well counter. Tube standards contain 0.75 ml of active epoxy and measure 75 mm x 12 mm or 55 mm x 12 mm.
Individual sources are also available.