A device to minimize errors that may occur when measurements are made to date radiocarbon samples. These measurements must be extremely accurate due to the very low level of activity (see radiocarbon dating). The error quoted on a radiocarbon age determination is solely an error in counting statistics. Such errors may rise from spurious counts generated by contamination of the sample, cosmic activity detected by the counter, and radioactive contaminants in the equipment being used. Initially the counter was shielded by surrounding it with large amounts of iron, lead, distilled mercury, or paraffin wax mixed with boric acid. An anticoincidence circuit is an alternative to material absorbers, and consists of a series of tangentially placed Geiger tubes operated in anticoincidence (i.e. they do not require input signals to arrive within specified intervals in order to be activated). These are positioned within an iron shield, and around the central counting chamber. Radiation from outside, or from within the shield, is detected by this ring of Geiger tubes and can be discounted. Special counters have now been developed in which the anticoincidence counters are built into the same tube as the main counter, so that the same gas is used in the whole system. The wall of such a counter usually consists of a polystyrene foil covered on both sides with aluminium. This is then surrounded by a ring of wires forming the anode for the anticoincidence circuit.