Strong evidence that positrons can exist in short-lived bound states within solid matter has been found by researchers in Japan. By analysing experiments using new theoretical models, Takayuki Tachibana and colleagues at the Tokyo University of Science showed how the exotic states are involved in the emission of molecular ions from ionic crystals that are being bombarded with low-energy positron beams.
Positrons are the antimatter counterpart to electrons and are produced by the radioactive decay of isotopes such as sodium-22. The antiparticles have a wide range of scientific and technological uses including studies of fundamental physics, materials engineering and medical imaging.
One especially intriguing area of research focuses on the exotic systems created when positrons briefly become integrated within normal matter. This has famously led to the creation of positronium, which is an unstable “atom” in which a single electron briefly orbits a single positron, before the two annihilate each other.
Other positronic compounds have also been observed in which positrons are bound to atoms, molecules, and ions.
Intriguing compounds
“Positronic compounds represent an intriguing aspect of positron-matter interactions and have been studied experimentally via observations of positron annihilation in the gas phase,” Tachibana describes. “However, the experimental production of these compounds in solid crystals has so far proven challenging.”
A possible route towards making and studying solid positronic compounds is the bombardment of ionic crystals with low-energy beams of positrons.
When this bombardment is done with electrons, rather than positrons, atoms on the crystal surfaces can be ejected as positive ions. Sometimes, negative ions are also produced by this process.
In their new study, Tachibana and colleagues bombarded a lithium fluoride (LiF) crystal with both low energy electrons and positrons and compared what happened.
Spectroscopic analysis
“We irradiated the surface of an ionic LiF crystal with a positron or electron beam, then detected the positive ions desorbed from the surface using spectroscopic analysis, allowing us to identify the composition of the ions,” Tachibana explains.
“We found that positron irradiation led to the desorption of molecular ions, such as F2+ ions, whereas electron irradiation enabled the desorption of monoatomic ions, such as Li+ and F+ ions.”
To explain the origins of these molecular ions, the team developed a new desorption model which included the interaction of lattice ions with both electrons and positrons. Their model predicts that when positron beams are injected into the LiF lattice, some of the antiparticles will return to the surface – losing energy through inelastic collisions with the ions.
Auger decay
These positrons can then attract pairs of negative fluorine ions in the surrounding lattice to form an unstable positronic compound. In certain cases, the positron will annihilate upon contact with an inner-shell electron in one of the fluorine atoms. That atom can then undergo a process called Auger decay, whereby the vacancy left by the annihilated electron is filled by an outer-shell electron. The energy released in this process can result in the ejection of an outer shell electron. This loss of an electron can then lead to the production of an F2+ molecular ion.
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The team’s model also showed how the desorption of molecular ions from LiF is more sensitive to temperature than the atomic ions produced through electron bombardment.
Overall, the result provides robust new evidence for the existence of positronic compounds. Tachibana and colleagues now hope it could provide new inspiration for future experiments. “Our study’s findings could further our understanding of matter–antimatter interactions,” he says. “Furthermore, the positron injection may pave the way for the future generation of novel molecular ions that cannot be achieved using other methods.”
The research is described in Physical Review Letters.
