Everything from the gold in your smart phone to the calcium in your bones was originally created in a star. But for a long time there has been a question about the levels of one particular element – strontium, which makes fireworks burn bright red and is used in glow-in-the-dark paint. A new paper published in Nature Communications Physics from a large group of researchers led by Caley M. Harris, a graduate student at Michigan State University, thinks they have found an answer for why that particular elemental level was skewed – and doing so required some very elaborate laboratory testing.
For decades, astrophysicists believed that there were two pathways to creating what they call “metals”, which means any element heavier than helium. One was the slow “s-process” that happens in dying stars, and one was the rapid “r-process” that happens in explosions like supernovae.
But as scientists started to look back at older stars – specifically those known as Carbon-Enhanced Metal-Poor (CEMP) stars – that formed only a little while after the universe was born, they found that the ratios for some of the heavier elements didn’t fit well with either known process. So they came up with a third process, known as the intermediate neutron-capture process, or “i-process”, which does a much better job of explaining the ratios in that particular group of stars. Except for one.
AstronomyCast episode where Fraser and Pamela discuss Nucleosynthesis.
Strontium appeared much more often in the actual observations of the stars than it does in simulation. And, according to the paper, there was one particular culprit – astronomers didn’t know the neutron capture rate of Krypton-88.
That might seem like a bit of a tangent, but actually the two are directly connected. Krypton-88 is a highly radioactive element that has a half-life of 2.8 hrs. During the time it does exist, it can take one of two paths – it can either absorb a neutron, becoming Krypton-89, and eventually decay into a heavier element like Yttrium-89. Or, if it doesn’t absorb a neutron, it could beta decay into Rubidium-88, which then quickly decays into – you guessed it – Strontium-88.
The problem was scientists had no idea how effectively Krypton-88 absorbed neutrons. They were simply guessing in their models – and, given the observational evidence, it seemed their models were wrong. Which gave rise to yet another problem – how do you actually measure the neutron absorption rate of an element that has a half-life of only a few hours?
Fraser talks about how stars form and what that formation process means for element creation.
You could, in theory at least, gather a bunch of Krypton-88 and bombard it with neutrons to see what happens. But the researchers came up with a much more elegant way to find the value. They turned to the Californium Rare Isotope Breeder Upgrade facility at Argonne National Laboratory, and, instead of firing a beam of neutrons at Krypton-88, they did so at Bromine-89, which has a half-life of – you probably didn’t guess it – 4.357 seconds.
The trick is, Bromine-89 decays into Krypton-89, and studying that nucleus can provide insights into what its slightly less heavy sibling isotope is capable of. They put the sample under test into a highly specialized instrument called a Summing NaI (SuN) detector. This detector watched the gamma-rays as the newly formed Krypton-89 cooled down and pulled two very specific values from it – the Nuclear Level Density and the gamma-ray Strength Function. With those numbers in hand, the researchers were able to mathematically reverse-engineer the neutron capture rate of Krypton-88 directly.
Plugging that newly found experimental value into the models used to calculate isotope ratios in CEMP stars aligned almost perfectly, with significantly higher amounts of strontium. In fact, its ratio with yttrium (the other long-lived product of this particular nucleosynthesis chain) lined up almost perfectly.
So, a fancy isotope beam and detector on Earth solved a mystery that had been bothering astronomers for decades. That kind of collaboration doesn’t happen every day, but when it does, and it works out as well as this one does, it’s a great thing to behold. And maybe, in this case, the research team could celebrate with some red fireworks.
Learn More:
Argonne National Laboratory – Unlocking the cosmic recipe for strontium
C.M. Harris et al – Influence of neutron-capture reactions on the nucleosynthesis of strontium
UT – Three of the Oldest Stars in the Universe Found Circling the Milky Way
