New silver mass brings us a step closer in our understanding of the antineutrino mass

The mass of neutrinos and their antineutrinos is one of the big unanswered questions in physics. Neutrinos are elementary particles in the Standard Model of particle physics and are very common in nature. They are produced, for example, by nuclear reactions in the Sun. Every second, trillions of solar neutrinos travel through us.

"Their mass determination would be of utmost importance," says Professor Anu Kankainen from the University of Jyväskylä. "Understanding them can give us a better picture of the evolution of the universe."

A path to understanding electron antineutrinos

One way of producing electron antineutrinos and determining their mass is via nuclear beta decay. It is a weak interaction process that produces a daughter nucleus, an electron and its antineutrino. The energy released in the process is known as the decay Q value. It is set by the masses of the parent nucleus and the decay products.

"Since the electron antineutrino mass is estimated to be at least five orders of magnitude smaller than the electron mass, it is very challenging to observe its contribution to the beta decay," says doctoral researcher Jouni Ruotsalainen from University of Jyväskylä, who is studying this issue as part of his doctoral thesis. "To make it more accountable, beta decays which release very little energy, the so-called low-Q-value beta decays, are of particular interest."

Beta decay of silver-110 isomer: a new and promising candidate for antineutrino mass measurements

Researchers at the University of Jyväskylä have discovered a potential nuclear beta decay that could be used for antineutrino mass determination.

"Previous searches have mainly focused on ground-state beta decays but also many long-lived excited states known as isomers decay via beta decay," says Ruotsalainen. "One such case is the isomer in the silver-110 isotope. It has a long half-life of around 250 days and decays primarily via beta decay to excited states in its daughter nucleus cadmium-110."

Researcher surprised by the ease of mass measurement and results

Based on the literature values, the beta-decay Q-value from the silver-110 isomer to an excited state at 3008.41 keV could be negative, meaning that the decay is not possible, or slightly positive. The main uncertainty comes from the parent and daughter nuclide ground states.

"We could considerably reduce the uncertainty of this Q value by measuring the mass difference between the stable silver-109 and cadmium-110 isotopes with the JYFLTRAP Penning trap mass spectrometer of the Accelerator Laboratory," explains Ruotsalainen. "It was quite easy to produce the stable silver and cadmium ions with our existing electric discharge ion sources and measure their mass difference using the phase-imaging ion cyclotron resonance technique. I was thrilled to see that the resulting Q value, 405(135) eV, is positive and actually the lowest for any allowed beta decay transition discovered so far."

Theoretical physicists confirmed experimental results

Not all the decays of the silver-110 isomer lead to the state at 3008.41 keV in cadmium-110. To estimate their fraction, shell-model calculations were performed.

"Our calculations show that about three out of every million decays from this isomer follow the fascinating, low-energy route. While that may sound tiny, it's actually quite significant for such a low-energy transition. Moreover, with a half-life of around 250 days, the isomer sticks around long enough for researchers to produce a meaningful sample and hopefully catch a good number of these rare decays in action," comments researcher Marlom Ramalho, who performed the theoretical work. Ramalho recently defended his PhD thesis at the University of Jyväskylä and is currently a postdoctoral fellow of the Oskar Huttunen Foundation at the University of York.

Measurements continue

The allowed character of the beta decay of the silver-110 isomer, the obtained very low Q value, and the fact that the isomer is easily produced in nuclear reactors via thermal neutrons, makes silver-110 a very attractive candidate for future antineutrino experiments.

"This is certainly a case to be studied in more detail," says Kankainen. "Our fruitful collaboration with the local theorists indicated also a couple of new isomeric beta decays that could be studied next for neutrino physics. It is nice to see that measurements of stable or near stable isotopes can still be very impactful."