RADIOACTIVE waste has helped us peer inside a star explosion and solve a long-standing mystery about the cosmic origins of chemical elements.
Stars fuse hydrogen in their cores, creating helium and releasing energy. As they run out of hydrogen fuel, very massive stars start fusing heavier elements. Eventually, the star’s core becomes so massive that it collapses, triggering a brilliant explosion that tears the star apart and flings material into space. This process is what seeds the universe with heavy elements important for life.
“Core-collapse supernovae are truly the factories of element production and are largely responsible for producing many of the elements we take for granted in our everyday lives,” says Dan Bardayan at the University of Notre Dame in Indiana.
At first, the blast wave rips atoms apart, making a hot mix of fundamental particles. These particles then recombine to form a range of elements, including some radioactive versions that decay into more stable substances. Studying what emerges from a supernova and the rates at which different elements decay or get altered by high-speed collisions can help us understand the underlying physics of a star explosion.
One of the best clues is titanium-44, which is made naturally only by supernovae. When observed with space probes, titanium-44 produces a very clear signal in supernova remnants – but there is a catch. All observations to date have shown much more titanium-44 than our best models predict. This may be because reactions in the debris that destroy the element have been poorly understood.
To help solve the riddle, Alexander Murphy at the University of Edinburgh, UK, and his colleagues looked to a novel source of titanium: irradiated parts removed from a particle accelerator. The scrap metal, from the Paul Scherrer Institute in Villigen, Switzerland, contained enough titanium-44 for the team to run an experiment.
“We are the only group worldwide which can provide the needed amount of the material in the required chemical form,” says Maria Dorothea Schumann at the Paul Scherrer Institute. “Accelerator components represent a new and unique resource for harvesting such valuable material.”
Murphy’s team used the scrap metal to produce a beam of titanium-44 in an accelerator at CERN in Switzerland, and sent the beam into a helium-filled gas chamber. The experiment recreated the explosive energy of a supernova to see how quickly titanium-44 is destroyed by collisions with helium, a known process in supernova remnants.
“What we’ve found is that the rate of that reaction is less than half of what was previously thought,” says Murphy. Models predict at least 30 per cent less titanium-44 than a supernova actually makes, but the finding boosts the amount left behind (Physics Letters B, doi.org/r5c).
“This brings the observation and models into much closer agreement,” says Murphy.
This article appeared in print under the headline “Scrap metal solves cosmic conundrum”
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