Neptunium spikes for environmental research

While we are constantly surrounded by radioactive elements, they generally occur in such small quantities that they present no threat to us. Measuring them precisely can even help researchers to understand the pathways of water in the environment. In a research project funded by the Austrian Science Fund FWF, Karin Hain and her team at the University of Vienna are collaborating with colleagues in Japan to develop new methods to detect these radioactive clues – the element neptunium in particular – in the environment.

In addition to natural sources, there are also anthropogenic or man-made sources of radioactive elements. The best known among these are probably the nuclear disasters at Chernobyl and Fukushima. From a global perspective, however, nuclear weapons tests in the 1950s and 1960s and reprocessing plants for nuclear fuel rods continue to dominate the spread of man-made radioactive elements on Earth.

“We can use these radioactive elements as environmental tracers to identify their pathways in ocean or groundwater currents,” explains Hain. “If we know the exact amount and composition of the radioactive material in a water sample, we can determine where it comes from and it helps us to understand its pathway in water currents.” That is to say, once the researchers know, where the radioactive tracers come from, they could understand the pathways taken by water in the sea or underground, which constitutes an important aspect in environmental research.

This is more easily said than done, given that the quantities of these environmental tracers are in the range of less than one quadrillionth of the sample material, thereby requiring sophisticated measuring equipment such as the Vienna Environmental Research Accelerator (VERA),  which is able to detect even the most minuscule of traces.

Neptunium is a candidate

In their research project, Hain and her team are particularly interested in the radioactive element neptunium. “With an estimated 1.5 tons distributed all over the world, neptunium 237 is one of the most common elements in the environment that is heavier than uranium, which means it has been produced artificially,” says Hain. “It has a very long half-life of over two million years and as far as we know today it is also water-soluble in compounds. This makes it a highly interesting environmental tracer.” However, until now an important part of the analytical method was missing for the researchers to be at all able to use the neptunium which is already present in the environment as a tracer.

In principle, the researchers’ measuring equipment can only detect that neptunium is present in a sample, but not exactly how much. But in order to use it as an environmental tracer, it is vital to know the exact amount of neptunium. In order to identify the amount, they need a spike for neptunium 237. A spike is a precisely known quantity of an isotope of neptunium that is added to the sample material during analysis. Isotopes are different variants of an element with different numbers of neutrons but the same number of protons in the atom’s nucleus.