<P> Radiometric dating has been carried out since 1905 when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth . In the century since then the techniques have been greatly improved and expanded . Dating can now be performed on samples as small as a nanogram using a mass spectrometer . The mass spectrometer was invented in the 1940s and began to be used in radiometric dating in the 1950s . It operates by generating a beam of ionized atoms from the sample under test . The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as "Faraday cups", depending on their mass and level of ionization . On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams . </P> <P> Uranium--lead radiometric dating involves using uranium - 235 or uranium - 238 to date a substance's absolute age . This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two - and - a-half billion years . An error margin of 2--5% has been achieved on younger Mesozoic rocks . </P> <P> Uranium--lead dating is often performed on the mineral zircon (ZrSiO), though it can be used on other materials, such as baddeleyite, as well as monazite (see: monazite geochronology). Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead . Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert . Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event . In situ micro-beam analysis can be achieved via laser ICP - MS or SIMS techniques . </P> <P> One of its great advantages is that any sample provides two clocks, one based on uranium - 235's decay to lead - 207 with a half - life of about 700 million years, and one based on uranium - 238's decay to lead - 206 with a half - life of about 4.5 billion years, providing a built - in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost . This can be seen in the concordia diagram, where the samples plot along an errorchron (straight line) which intersects the concordia curve at the age of the sample . </P>

Which of the following techniques examines the decay of carbon-14 to date an object