Radioactive fossils? New applications of ionising radiation

Even though radioactive isotopes have existed since the first second of the Big Bang, they are undergoing a second youth thanks to discoveries initiated in the 20th century. Although ionising radiation is usually associated with nuclear power, cancer diagnostics and therapy, as well as radiological exposure, it also has other, slightly less popular applications, such as among... stones!

Radioactive isotopes, which emit characteristic radiation, make it possible to mark materials. This is not artificial marking, but the use of isotopes commonly found in the environment. Every object that surrounds us contains a natural, typical composition of radioactive isotopes, which remain in certain relationships with each other, forming a characteristic combination for that object, a kind of fingerprint. The study of isotope ratios opens up a new spectrum of research, while the research methods are not trivial and commonly described in academic textbooks. Simply put, this is a completely new field for research and the problems that can be solved.

Let us move from words to practice. Let us consider where such methods, based on radioactive isotopes, could be useful?

First example - the atmosphere

Of course, even the purest air contains natural radioactive isotopes, which result, for example, from the evaporation of radon Rn-222 from the ground. Short-lived isotopes resulting from = radioactive decay of radon, e.g. Pb-210 and Po-210, attach to aerosols and can be transported with them even over long distances.

The degree of accumulation of Po-210 in relation to the Pb-210 isotope depends on the residence time of the aerosol in the atmosphere. The Po-210/Pb-210 isotope ratio is thus a clock, determining the lifetime of aerosols, i.e. their age. Fine fractions with a diameter of less than 1μm, usually more mobile, can stay in the troposphere for up to a month, while those with a diameter of 10 μm can only be there for a few hours.

If we add to this the fact that most often the air is polluted and contains additional components, the isotopic ratio can play a detective role to trace the source of pollution. Surely everyone has heard of the Fukushima accident in 2011. The study of that case provided a sizable set of isotopic data that acted as tracers of cloud propagation dynamics and data for predicting radiological exposure.

Second example - meteorites

Geological objects in space, like the Earth's crust, contain natural radioactive isotopes, although they are quite different. Their composition and mutual isotopic ratios, are also characteristic for each object. Meteorites, which are found on Earth, provide knowledge about the isotopic composition of objects in the solar system. Again, the isotopes can provide information about the age of the meteorite and can also indicate a common origin, e.g. from the same fall or from the same asteroid.

Third example - radioactive fossils

It turns out that traces of past life - fossils of animals or plants - fixed in the Earth's crust may be more radioactive than the sedimentary rock around them. The composition of radioactive isotopes in the structure of a fossil may be completely different from that present in the adjacent rock.

The decomposing organic matter created specific physico-chemical conditions that allowed the accumulation of certain elements, such as phosphorus or uranium. A higher concentration of uranium, including its radioactive isotopes, generates a higher concentration of natural series derivatives. These undergo radioactive transformations again into further isotopes, and all together consequently obtain higher radioactivity.

Interestingly, natural phosphatisation favoured the fixation of soft tissues, which under normal conditions never fossilise. Consequently, it happens that radioactive fossils turn out to be very well preserved including muscle tissues (bivalves) or siphon (ammonites). Such a phenomenon was considered a pioneering discovery.