radioactivity, known as beta counters, and accelerator mass spectrometers. For beta counters, a sample weighing at least is typically required. Accelerator mass spectrometry is much more sensitive, and samples containing as little as 0.5 milligrams of carbon can be used. Measurement and results For decades after Libby performed the first radiocarbon dating experiments, the only way to measure the in a sample was to detect the radioactive decay of individual carbon atoms. In this approach, what is measured is the activity, in number of decay events per unit mass per time period, of the sample. This method is also known as "beta counting", because it is the beta particles emitted by the decaying atoms that are detected. In the late 1970s an alternative approach became available: directly counting the number of and atoms in a given sample, via accelerator mass spectrometry, usually referred to as AMS. AMS counts the / ratio directly, instead of the activity of the sample, but measurements of activity and / ratio can be converted into each other exactly. For some time, beta counting methods were more accurate than AMS, but AMS is now more accurate and has become the method of choice for radiocarbon measurements. In addition to improved accuracy, AMS has two further significant advantages over beta counting: it can perform accurate testing on samples much too small for beta counting, and it is much faster – an accuracy of 1% can be achieved in minutes with AMS, which is far quicker than would be achievable with the older technology. Beta counting Libby's first detector was a Geiger counter of his own design. He converted the carbon in his sample to lamp black (soot) and coated the inner surface of a cylinder with it. This cylinder was inserted into the counter in such a way that the counting wire was inside the sample cylinder, in order that there should be no material between the sample and the wire. Any interposing material would have interfered with the detection of radioactivity, since the beta particles emitted by decaying are so weak that half are stopped by a 0.01 mm thickness of aluminium. Libby's method was soon superseded by gas proportional counters, which were less affected by bomb carbon (the additional created by nuclear weapons testing). These counters record bursts of ionization caused by the beta particles emitted by the decaying atoms; the bursts are proportional to