Suited radionuclides for the purpose of dating ivory
Like explained in the previous section even a very precise dating method like 14C age determination is prone to certain limitations. Therefore, combined analyses procedures have been developed to detect suited radionuclides for the purpose of unambiguous dating for the period from 1950 to 1990.
In addition to 14C, strontium 90Sr and the thorium isotopes 228Th and 232Th are well suited for the determination of age. 90Sr is produced at the nuclear fission and became accumulated in the food chain during the 1950s and 1960s as a result of the atmospheric nuclear weapon tests. As the alkaline earth element 90Sr reacts very similar to calcium (Ca), 90Sr is transferred to calcium containing tissues like ivory. 90Sr is present in detectable amounts until presence due to its long half life time of about 29 years. Because of the shape of the bomb curves of 90Sr and 14C, an unambiguous dating is possible only in the case of values 14C/C below about 100 pMC or above about 140 pMC. For the range between 100 pMC and 140 pMC, another radionuclide is necessary. The specific activity of such radionuclide must fulfil the criteria as follows:
- (i) independent from the atomic bombing fallout,
- (ii) dependent on the time of death, and
- (iii) detectable at very low amounts.
The naturally occurring isotopes of thorium, 228Th and 232Th meet these criteria. The activity ratios of 228Th to 232Th in elephant ivory have been found to be dependent of time. In addition, they can be determined very sensitive applying alpha-spectrometry.
Typical relations of values of all radionuclides mentioned above are called isotopic profile. It has been shown that isotopic profiles of the above mentioned radionuclides are able to determine the time of death unambiguously. The development of such an analytical tool which can be applied reliably in practice requires the combination of three separated analytical purification procedures and is further described in the methodology section.