40-year cosmic theory confirmed
A stellar reaction long predicted but never seen has been demonstrated in the lab. Andrew Masterson reports.
An abundant new energy supply could be derived from controlling a quantum reaction that takes place in stars, according to research from the Australian National University (ANU).
The possibility arises because the ANU scientists plus others from institutions including the US Army Research laboratory and Poland's National Centre for Nuclear Research have succeeded in confirming the existence of a reaction first predicted four decades ago but unmeasured until now.
In a paper published in the journal Nature, ANU physicist Greg Lane and colleagues report the confirmation of a phenomenon known as Nuclear Excitation by Electron Capture (NEEC). Confirming that NEEC actually happens supplies a key mechanism for understanding how evolving stars produce elements such as gold and platinum.
NEEC can occur when an atom captures an electron. If the electron's kinetic energy and the energy required to capture it add up to just the right amount, the atom's nucleus is pushed to a higher state of excitation.
The energy increase, however, comes at the cost of a shorter life. What was a long-lived stable nucleus must now decay, either through an electromagnetic process known as internal conversion which spits out an electron, or by emitting a photon.
Although discussed since the 1970s, experimental proof for NEEC has remained elusive.
The new work, however, has now provided the necessary evidence. The researchers did so by creating an exotic isotope - molybdenum-93 - by firing a beam of zirconium atoms at lithium targets, using the ANU's Heavy Ion Accelerator and the ATLAS Accelerator at Argonne National Laboratory in the United States.
The resulting molybdenum atoms zipped around at as much as 10% of the speed of light, smashing into the remaining lithium, stripping off electrons and leaving highly charged ions behind.
As the interactions continued, the molybdenum ions lost kinetic energy until they reached a state where they could capture an electron with just the right energy to push the molybdenum nuclei from their long-duration "isomer" states into higher level but shorter-lived intermediate ones. These intermediate states decayed, giving off a unique gamma-ray signature that proved NEEC had occurred.
The research now provides a model against which other theoretical calculations for the NEEC effect in different elements can be tested, illuminating further the process by which nuclear interactions in stars produce certain metals.
"The abundance of the different elements in a star depends primarily on the structure and behaviour of atomic nuclei," says Lane.
"The NEEC phenomenon modifies the nucleus lifetime so that it survives for a shorter amount of time in a star."
As well cosmological implications, the confirmation of the NEEC effect opens the door to potentially accessing energy stored in longer-lived isomer nuclei. Lane suggests the technique could create energy sources 100,000 times more powerful than chemical batteries.
It is a possible outcome that has not gone unnoticed by at least one of the ANU's research partners.
"Our study demonstrated a new way to release the energy stored in a long-lived nuclear state, which the US Army Research Laboratory is interested to explore further," says Lane.