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A new type of cosmic element composition?  – The neutrino shower may explain the formation of rare types of neutron-poor atoms in the universe

A new type of cosmic element composition? – The neutrino shower may explain the formation of rare types of neutron-poor atoms in the universe

Ghost particles as atom creators: A previously unknown way of creating cosmic elements could explain how some types of neutron-poor atoms appear so unusual. Energetic but nearly massless neutrinos could play a key role in this virtual reality process. During some cosmic explosions, they cause the neutrons in these atomic nuclei to turn into protons. But where could this new form of nucleosynthesis occur?

Most elements in the periodic table are created either by nuclear fusion inside stars or by slow or fast neutron capture reactions in supernovas, neutron star collisions and other energy events in the universe. In these nucleosynthesis reactions, called the s and r processes, atomic nuclei grow through collisions with neutrons and the subsequent conversion of some of these neutrons into protons.

Stable isotopes from antimony to lanthanum and their synthesis method: s process = green, r process = red, neutron-poor p nucleus is colored yellow. © Bambot/CC-BY-SA 4.0

The mystery of the cores p

But there are atomic nuclei that could not have been formed by any of these known processes. They are about 35 isotopes of heavy elements that have unusually few neutrons in their nuclei compared to the number of protons in them. These proton-rich nuclei include molybdenum-92, ruthenium-96, 98, cadmium-106, and others. Although physicists have postulated several hypothetical paths to synthesis, they can only partially explain p-nuclei.

The same is true for some rare radionuclides: “In addition to p-nuclei, niobium-92 (92Nb) is another element of unknown origin,” reports Zewei Xiong from the GSI Helmholtz Center for Heavy Ion Research in Darmstadt. The long-lived but radioactive isotope niobium was present in the early solar system but has since decayed and thus no longer exists. But the composition of these atomic nuclei It has not been explained yet.

Neutrinos as catalysts?

Xiong and his team have now found a possible solution. For their study, they examined how neutrinos contribute to the formation of heavy atomic nuclei. “Previous studies of the R process in central collapse supernovas and neutron star collisions have shown that neutrinos can play a fundamental role,” they explain. “At high temperatures, the absorption of (anti)electron neutrinos and the resulting interactions determine the neutron abundance in the ejected material.”

Normally, high neutrino density is an obstacle to the r process: “It converts neutrons into protons, thus reducing the amount of particles available to capture neutrons,” physicists say. But as they have now discovered using theoretical modeling, this may be exactly what leads to the formation of neutron-poor nuclei. “Our discovery opens a new possibility to explain the formation of p-nuclei through neutrino absorption interactions,” says Xiong.

Neutrons become protons

Specifically, this new type of nucleosynthesis, called the VR process, works like this: During high-temperature cosmic events and intense neutrino release, atomic nuclei are initially created through the normal r process – and continue to grow By capturing free neutrons. When the temperature drops below a certain threshold, neutron trapping stops. Normally, the atomic nuclei formed continue to decay through beta decay until they reach a stable isotope.

But things look different for events with high neutrino density, Xiong and his team explain. The neutrinos are then absorbed by atomic nuclei and their energy is sufficient to catalyze the rapid conversion of neutrons bound to the nucleus into protons. “As a result, the nuclei cross the threshold of stable beta decay products and become neutron-poor nuclei,” Xiong and his colleagues explain. The VR process can then produce analogues for which there was no previously known synthetic route.

Where does virtual reality take place?

However, one question remains open: in what kind of cosmic event does the ν process occur. Although it requires a high neutrino density, the temperature must not be too high. “We believe that the VR process occurs in strong magnetic fluxes in which high neutrino fluxes occur,” the physicists wrote. Such conditions can exist in the polar regions of supernovae of rapidly rotating, highly magnetic stars or during some core collapse events.

The collision of neutron stars with strong magnetism, or so-called magnetars, can also provide a suitable environment for virtual reality operation. “A recently published study also showed that the right conditions can also exist in the winds of highly magnetized proto-neutron stars,” say Xiong and his colleagues.

Further investigations should now show whether and where the virtual reality process hypothesized by Xiong and his team actually occurs in the universe. The next step will be to narrow down the range of possible formation sites for p-nuclei through the VR process. It may then become possible to discover this new pathway for nucleosynthesis. (Physical Review Letters, 2024; doi: 10.1103/PhysRevLett.132.192701)

Source: GSI Helmholtz Center for Heavy Ion Research

May 21, 2024 – Nadia Podbrigar