A Forbidden Transition Allowed for Stars

The discovery of an exceptionally strong “forbidden” beta-decay involving fluorine and neon could change our understanding of the fate of intermediate-mass stars.

Researchers have measured the forbidden nuclear transition between 20F and 20Ne. This measurement allowed them to make a new calculation of the electron-capture rate of 20Ne, a rate that is important for predicting the evolution of intermediate-mass stars.

Every year roughly 100 billion stars are born and just as many die. To understand the life cycle of a star, nuclear physicists and astrophysicists collaborate to unravel the physical processes that take place in the star’s interior. Their aim is to determine how the star responds to these processes and from that response predict the star’s final fate. Intermediate-mass stars, whose masses lie somewhere between 7 and 11 times that of our Sun, are thought to die via one of two very different routes: thermonuclear explosion or gravitational collapse. Which one happens depends on the conditions within the star when oxygen nuclei begin to fuse, triggering the star’s demise. Researchers have now, for the first time, measured a rare nuclear decay of fluorine to neon that is key to understanding the fate of these “in between” stars . Their calculations indicate that thermonuclear explosion and not gravitational collapse is the more likely expiration route.

The evolution and fate of a star strongly depend on its mass at birth. Low-mass stars—such as the Sun—transition first into red giants and then into white dwarfs made of carbon and oxygen as they shed their outer layers. Massive stars—those whose mass is at least 11 times greater than the Sun’s—also transition to red giants, but in the cores of these giants, nuclear fusion continues until the core has turned completely to iron. Once that happens, the star stops generating energy and starts collapsing under the force of gravity. The star’s core then compresses into a neutron star, while its outer layers are ejected in a supernova explosion. The evolution of intermediate-mass stars is less clear. Predictions indicate that they can explode both via the gravitational collapse mechanism of massive stars and by a thermonuclear process . The key to finding out which happens lies in the properties of an isotope of neon and its ability to capture electrons.

The story of fluorine and neon is tied to what is known as a forbidden nuclear transition. Nuclei, like atoms, have distinct energy levels and thus can exist in different energy states. For a given radioactive nucleus, the conditions within a star, such as the temperature and density of its plasma, dictate its likely energy state. The quantum-mechanical properties of each energy state then determine the nucleus’ likely decay path. The decay is called allowed if, on Earth, the decay path has a high likelihood of occurring. If, instead, the likelihood is low, the transition is termed forbidden. But in the extreme conditions of a star’s interior, these forbidden transitions can occur much more frequently. Thus, when researchers measure a nuclear reaction in the laboratory, the very small contribution from a forbidden transition is often the most critical one to measure for the astrophysics applications….

Read more at https://physics.aps.org/articles/v12/151

 

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