Neutrinos, including their antimatter counterparts, antineutrinos, are elementary particles within the Standard Model of particle physics, known for their extremely small but uncertain mass. Understanding their mass is a key objective in modern physics, as it provides crucial insights into the evolution of the universe. Trillions of neutrinos pass through the human body every second, generated by nuclear reactions in stars like the Sun.
"Their mass determination would be of utmost importance," explained Professor Anu Kankainen from the University of Jyvaskyla. "Understanding them can give us a better picture of the evolution of the universe."
Electron antineutrinos can be produced through nuclear beta decay, a weak interaction process where a neutron-rich nucleus transforms into a proton-rich one, emitting an electron and an antineutrino. The energy released in this process, known as the decay Q value, is determined by the mass difference between the parent nucleus and the resulting decay products. This Q value is critical for assessing the antineutrino mass.
"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 impact on beta decay," said doctoral researcher Jouni Ruotsalainen from the University of Jyvaskyla. "Low-Q-value beta decays, which release very little energy, are particularly promising for such measurements."
The researchers focused on the beta decay of the silver-110 isomer, a long-lived excited state of the silver-110 isotope with a half-life of approximately 250 days. This isomer decays primarily to the excited states of cadmium-110, offering a potentially promising candidate for precise antineutrino mass measurements.
By using the JYFLTRAP Penning trap mass spectrometer, the team precisely measured the mass difference between the stable silver-109 and cadmium-110 isotopes, reducing the uncertainty of the decay Q value. "It was quite easy to produce the stable silver and cadmium ions with our electric discharge ion sources and measure their mass difference using the phase-imaging ion cyclotron resonance technique," Ruotsalainen noted. "The resulting Q value, 405(135) eV, is positive and the lowest for any known allowed beta decay transition, making it a particularly exciting discovery."
Theoretical calculations supported the experimental findings, revealing that roughly three out of every million decays from the silver-110 isomer follow this rare, low-energy pathway. Despite the small fraction, this pathway is significant given the long half-life of the isomer, providing ample opportunity for detailed experimental study.
"This is certainly a case to be studied in more detail," added Kankainen. "Our collaboration with local theorists also highlighted a few additional isomeric beta decays worth investigating for neutrino physics. It is exciting to see that even near-stable isotopes can still provide impactful insights."
Research Report:Value for the Allowed Decay of 110 Ag Confirmed via Mass Measurements
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