Sand that is deposited along the coast contains the mineral zircon (zirconium silicate). Zircon is often 'contaminated' with the trace elements uranium and thorium. The impurities emit alpha radiation and this damages the crystal structure of the zircon sand grains. The longer the irradiation period the greater the damage to the mineral. However, sunlight or heating to several hundred degrees Celsius repairs the damage. Beach sand which lies on the surface will therefore exhibit little damage, whereas sand which lies deeper will exhibit more.
In a Technology Foundation STW project under the leadership of Prof. H.W. den Hartog, researchers from the University of Groningen have developed a method to measure the damage in zircon and thus to determine the age of sand sediments. Carefully selected zircon sand grains are heated, thereby repairing the damage in the zircon. This is associated with the emission of visible light (thermoluminesence). The greater the amount of damage repaired, the greater the intensity of the light emitted. Zircon grains from old sentiments which were last exposed to sunlight a long time ago, will have been irradiated by alpha radiation for longer and will therefore exhibit much damage and thus produce much light.
Only zircon grains which are transparent are suitable for the dating method. These do not absorb the light from the thermoluminesence. The researchers developed a new selection installation to select these 'zircon jewels' from the zircon fraction in the sand, in which coloured and sometimes dark grains with much irreparable damage occur. An electrical technique separates ideal, clear, electrically non-conducting zircon grains from unsuitable, dark and coloured, electrically conducting zircon grains which disrupt the dating process.
Using zircon it might in future be possible to date sand grains with ages up to 100,000 years. The method can be used worldwide, because zircon occurs in almost all sediments. The researchers suspect that all sediments aged at least one year or more can be dated. In addition to this there are promising possibilities for forensic dating research. Using this technique the scene of a crime can be dated. For example, in the case of illegal dumping or if detectives are looking for information about the moment at which objects were buried.
The researchers have already tested the method. They took a 175-year-old sand sample from the Zwanenwater dunes on Ameland (The Netherlands). The zircon dating method gave an age of 177 years.
Further information can be obtained from Prof. H.W. den Hartog (Laboratory for Solid State Physics, University of Groningen), tel. +31(0) 50 3634789, fax +31 (0)50 3634879, e-mail h.w.den.hartog@phys.rug.nl.
The research was funded by the Netherlands Organisation for Scientific Research (NWO).