General

Samples 1

If you want to measure the 14 C-concentration you need samples. There are some specific substances which are most qualified for 14 C-dating. For bones it is collagen and for wood it is cellulose. These substances need to be obtained from the material. Beside that, the different impurities need to be removed from the material as well. This is done by using an acid bath. For example collagen does not dissolute in the acid used in the acid bath, but the other substances in the sample do. After that the collagen that remains is dried and burned, the CO2 that was formed is collected with liquid nitrogen.

Radiometric dating

With radiometric dating you measure how much radioactive radiation is still being radiated. This happens with help of proportional counters. They register the little parts which are radiated when a radioisotope decays. Normally the radiation goes through the wall of the proportional counter and is measured in there . But the 14 C-isotope radiates a too small amount of radioactive radiation. The electron which is radiated can't pass the wall of the proportional counter. That is why the 14 C needs to be injected into the proportional counter. This will happen when 14 C is in the form of CO2 , because the proportional counter only works on gas.

A proportional counter is a cylinder with a wire at high tension in the middle. The proportional counter is filled with gas, for 14 C in this case CO2 (see paragraph samples). If a 14 C- nucleus declines, an electron is transmitted. This electron collides with other CO2 - molecules and releases electrons. With that a stream of negative particles is formed which is pulled to the high tension wire. At the moment when the electrons touch the wire, a electric pulse is created on the wire. Which eventually gets registered as radioactive decay.

There is always some radioactive radiation in our surroundings; also called background radiation. This radiation is much bigger than the radiation which is transmitted by a 14 C- nucleus. In a proportional counter only the radiation of 14 C is supposed to be measured, so the background radiation must be shut out. This can be done by placing the proportional counter in a lead castle. Not just lead is used, iron, paraffin and boric acid are used to keep the background radiation out of the proportional counter too. Paraffin and boric acid adsorb neutrons.

To register the background radiation that still penetrates the lead castle a ring of proportional couters is installed around the proportional counter that measures the radiation of 14 C. The amount of radiation which is registered in those proportional counters is deducted from the amount of radiation registered by the proportional counter that measures 14 C.

The last layer is another ring of lead. The lead used for this ring comes from the keel of a VOC ship. That's because lead contains a radioactive isotope too. The half-life of lead is pretty short (22 years), it's disappeared in about a 100 years. The keel of a VOC ship is certainly older that a 100 years, so it won't transmit any radiation.

To make the results even more precise, a measuring with fossil carbonate is preformed. This material is so old that all the 14 C-atoms are already decayed. The radiation that is still counted with this measuring is deducted from the 14 C measuring as well.

Accelerator mass spectrometry

With this method we don't look at the radioactivity but at the difference in mass of the C-isotopes. 14 C is the heaviest atom of the three. For AMS (Accelerator Mass Spectrometry) 14 C must be in a solid form.

It isn't always possible to make a solid form of the 14 C containing materials, so everything gets converted into CO2 first. After that the lab workers make the CO2 react with H2 . By doing that, graphite and water are formed. This is used for AMS

With AMS you use a series of machines, including magnets and a particle accelerator. First of all a ring including about 60 graphite samples is put through an ionizing source. These are ionized and turn into a negative ion (C - ), so that they can be bent off by a magnet in the machine later on. People choose to transform them into negative ions, because then the isobar 14 N isn't accelerated as well.

After that, the ions are led through a Particle accelerator, which is driven by a 2,5 MV generator. Because of this, the particles speed up and get stripped. They get rid of some electrons and turn into positive ions which can be accelerated even more. This happens with the help of argon gas. With this process the isobars 12 CH2 and 13 CH are destroyed , which leaves a clean C-bundle.

Next, the bundle of C-atoms is bent off trough an electromagnet. Because of their smaller mass the 12 C and 13 C get bended off first. The heavier 14 C-particles are registered by a detector. Which contains a gas in which the 14 C- particles cause ionization. With that electrons are released, which can be measured as a electric current.