Most of the mass in everyday matter around us is in protons and neutrons in the atomic nucleus. However, the lifetime of a free neutron — one not bound to a nucleus — is unstable and decays through a process called beta decay. For neutrons, beta decay involves the emission of a proton, an electron and an anti-neutrino. Beta decay is a common process.
However, scientists have some important uncertainties about the lifetime of the neutron and about the decay of the neutron in a nucleus leading to a proton emission. This is called beta-delayed proton emission. There are only a few neutron-rich nuclei for which beta-delayed proton emission is energetically allowed. The radioactive core beryllium-11 (11Be), an isotope consisting of 4 protons and 7 neutrons, the last neutron of which is very weakly bound, belongs to those rare cases. Scientists recently observed a surprisingly large beta-delayed proton decay rate for 11To be. Their work has been published in Physical Assessment Letters.
The discovery of an exotic near-threshold resonance that promotes proton decay is a key to explaining the beta-delayed proton decay of 11To be. The discovery is also a remarkable and not fully understood manifestation of many-body quantum physics. Many-body physics involves interacting subatomic particles. While scientists may know the physics that apply to each particle, the entire system may be too complex to understand.
The perception of a near-threshold resonance in 11B is key in explaining the large value of the beta-delayed proton decay of 11To be. The results point to a two-step process and away from more exotic explanations such as a dark matter decay channel. Understanding this condition helps scientists refine theories about unstable nuclear systems. It also raises questions about the nature of this decay process, including physics beyond the Standard Model.
Ever since 11Be is a radioactive neutron-rich nucleus, nuclear physicists did not expect it to decay via proton radioactivity. The large value observed for the beta-delayed proton decay in 11Be inspired to speculate about the nature of the decay, including exotic processes outside the Standard Model. An alternative explanation required the existence of an unobserved, very close resonance in 11b.
Physicists at the John D. Fox Accelerator Laboratory at Florida State University, using a radioactive 10Be radius in a measurement of the 10Be(d,n) reaction, observed a narrow proton-depleting resonance in 11B. This result supports the evidence that the beta-delayed proton decay of 11Be is actually a two-step sequential process where a near-threshold resonance in 11B is first populated in a beta decay with subsequent proton emission. The position of the resonance and its decay properties is a unique case highlighting the complex quantum physics of unstable systems.
Researchers observe exotic radioactive decay process
E. Lopez-Saavedra et al, Observation of a near-threshold proton resonance in 11b, Physical Assessment Letters (2022). DOI: 10.1103/PhysRevLett.129.012502
Quote: Near-threshold resonance helps explain a controversial measure of exotic decay in beryllium-11 (2022, September 28), retrieved September 28, 2022 from https://phys.org/news/2022-09-near-threshold-resonance- controversial-exotic-beryllium-.html
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