THE NOBLE CLUE

A group of scientists at the University of Manchester run the world’s most sensitive instrument for analysing minute amounts of the gas Xenon, trapped in solid materials. What has motivated them to pursue this seemingly esoteric field, and how does their unusual instrument work?

Xenon is one of a group of gaseous elements so unreactive that their existence was not suspected until the end of the 19th century, even though one—argon—makes up nearly 1% of the Earth’s atmosphere. They are collectively known as the noble, inert or rare gases. ‘Rare’ is a bit misleading, since they are as abundant in the universe as other elements of similar mass, but it reflects the fact that they are rarely found in solid materials. Noble gases play a vital role in developing our understanding of the history of the solar system.

The atomic number of an atom tells us how many protons are in its nucleus. There is a surrounding cloud of negatively charged electrons, so that when the overall charge of the atoms is zero there is one electron in the cloud for every proton in the nucleus. Chemical reactions involve atoms swapping or sharing electrons. All atoms with the same atomic number have the same chemistry. This is how we come to recognise distinct chemical elements.

However, in some important ways, all atoms of a given element may not be quite the same. Alongside the protons in the nucleus, we find neutrons which have mass, but no charge, and so don’t much affect the chemical behaviour of the atom. Atoms of the same element with different numbers of neutrons are called different isotopes.

Different isotopes of the same element formed in different ways, and have different ultimate fates. Some are stable and survive indefinitely, but some are unstable or radioactive, and decay to form isotopes of a different element. Because nuclear processes affect isotopes in different ways, if we find out what isotopes are present in a sample, we can tell what nuclear processes have affected it.

Xenon has nine stable isotopes. Among them are some exclusively made by each of the two most significant processes that made the heavy chemical elements around us. The isotopic pattern of one of these processes – the ‘s-process’ – was noticed when samples of meteorites were analysed, and eventually traced to micron-sized grains of silicon carbide that had condensed around stars that died before our solar system formed.

Samples of meteorites tend to have unusual amounts of the isotope Xenon-129. This was produced in the early solar system when an isotope of iodine (Iodine-129) decayed. But Iodine-129 can only survive for about 100 million years. The universe as a whole is more than 13 billion years old. Therefore, the Iodine-Xenon evidence tells us that elements in our neighbourhood were still being made less than 100 million years before our solar system formed. Talk about fresh ingredients . . . We now think that a supernova exploded near our solar system just as it was coalescing. The shock and proximity of that explosion must have had far-reaching effects.

Using a unique instrument, RELAX (Refrigerator Enhanced Laser Analyser for Xenon), the Manchester group are currently studying samples from the Genesis mission, which brought back solar wind implanted into small Silicon wafers (about 1 million atoms of Xenon per square centimetre) and the grains of comet Wild-2 returned by the Stardust mission. What will they discover about our solar system’s history?

To find out more about this research, visit Manchester University 's Isotope Geochemistry and Cosmochemistry website by clicking on the link below!

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