Those of you who have been following this blog may have noticed that there are quite a number of posts about xenon. We even have a special mass spectrometer which can only measure xenon.
But why? Why are we so interested in xenon? Why do we spend so much time analysing xenon isotopes? What is so important about this particular element?
Xenon is the heaviest of the noble gases. It has 9 stable isotopes,124Xe, 126Xe, 128Xe, 129Xe, 130Xe, 131Xe, 132Xe, 134Xe and 136Xe, all of which can be considered to be special in some way.
A number of geo- and cosmochemically significant radioactive nuclei decay to produce distinct xenon isotopic signatures. 129I decayed in the early solar system to form 129Xe (as previously discussed) and “extra” 129Xe, relative to the other isotopes, is seen in many meteorites and can be used to date them. 244Pu and 238U decay via a process known as spontaneous fission to produce the heavier xenon isotopes (131Xe, 132Xe, 134Xe and 136Xe), each with a unique signature. So by looking for excesses of particular isotopes, we can calculate how old a meteorite is or determine what radioactive processes have occurred since the meteorite formed.
Some isotopes are uniquely produced by each of the known processes of stellar nucleosynthesis that produce the heavy elements. 124Xe and 126Xe are only produced in the p-process (proton capture). 130Xe is only produced by the s-process (slow neutron capture in AGB stars). 134Xe and 136Xe are overwhelmingly produced by the r-process (rapid neutron capture in supernovae). Looking at these isotopes can tell us something about the stellar environments in which particles formed.
Xenon in the Earth’s atmosphere puzzles scientists – it can not be directly derived from solar wind xenon. It shows excesses in 129Xe and 131-136Xe relative to solar xenon. The 129Xe excess is though to have derived from decay of 129I, but the 131-136Xe are more difficult to explain – the correct ratios can not be obtained from addition of fission products from 244Pu and 238U. So how did the xenon in our atmosphere get its unique isotopic composition?
Xenon makes a huge contribution to our understanding of our solar system. Measuring xenon isotope ratios can help us understand the environments in which particles formed, and the processes they have undergone in their history. And part of the intrigue of xenon is not what we do know about it, but rather what we still don’t know and what we still could learn from it.