Archive for the ‘NUCLEAR ASTROPHYSICS’ Category

A Forbidden Transition Allowed for Stars

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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….



Written by physicsgg

January 1, 2020 at 12:47 pm

Introduction to neutrino astronomy

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neutrino flux

Energy dependence of the neutrino fluxes produced by the different nuclear processes in the Sun

Andrea Gallo Rosso, Carlo Mascaretti, Andrea Palladino, Francesco Vissani
This writeup is an introduction to neutrino astronomy, addressed to astronomers and written by astroparticle physicists. While the focus is on achievements and goals in neutrino astronomy, rather than on the aspects connected to particle physics, we will introduce the particle physics concepts needed to appreciate those aspects that depend on the peculiarity of the neutrinos. The detailed layout is as follows: In Sect.~1, we introduce the neutrinos, examine their interactions, and present neutrino detectors and telescopes. In Sect.~2, we discuss solar neutrinos, that have been detected and are matter of intense (theoretical and experimental) studies. In Sect.~3, we focus on supernova neutrinos, that inform us on a very dramatic astrophysical event and can tell us a lot on the phenomenon of gravitational collapse. In Sect.~4, we discuss the highest energy neutrinos, a very recent and lively research field. In Sect.~5, we review the phenomenon of neutrino oscillations and assess its relevance for neutrino astronomy. Finally, we offer a brief overall assessment and a summary in Sect.~6. The material is selected – i.e., not all achievements are reviewed – and furthermore it is kept to an introductory level, but efforts are made to highlight current research issues. In order to help the beginner, we prefer to limit the list of references, opting whenever possible for review works and books.


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July 21, 2018 at 9:00 pm

Neutrino Astrophysics

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Cristina Volpe
We summarize the progress in neutrino astrophysics and emphasize open issues in our understanding of neutrino flavor conversion in media. We discuss solar neutrinos, core-collapse supernova neutrinos and conclude with ultra-high energy neutrinos.

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September 23, 2016 at 5:36 pm

The puzzle of the origin of elements in the Universe

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A rare nuclear reaction that occurs in red giants has been observed for the first time at the Gran Sasso National Laboratory in Italy. This result was achieved by the LUNA experiment, the world’s only accelerator facility running deep underground.

The LUNA experiment at the INFN Gran Sasso National Laboratory in Italy has observed a rare nuclear reaction that occurs in giant red stars, a type of star in which our sun will also evolve. This is the first direct observation of sodium production in these stars, one of the nuclear reactions that is fundamental for the formation of the elements that make up the universe. The study has been published in Physical Review Letters.

LUNA (Laboratory for Underground Nuclear Astrophysics) is a compact linear accelerator. It is the only one in the world installed in an underground facility, shielded against cosmic rays. The experiment aims to study the nuclear reactions that take place inside stars where, like in an intriguing and amazing cosmic kitchen, the elements that make up matter are formed and then driven out by gigantic explosions and scattered as cosmic dust.

For the first time, this experiment has observed three “resonances” in the neon-sodium cycle responsible for sodium production in red giants and energy generation (the 22Ne(p,g)23Na. In the same way as in acoustics, a “resonance” is a particular condition that makes the reaction inside the star extremely likely. LUNA recreates the energy ranges of nuclear reactions and, with its accelerator, goes back in time to one hundred million years after the Big Bang, to the formation of the first stars and the start of those processes that gave rise to mysteries we still do not fully understand, such as the huge variety in the quantities of the elements in the universe.

“This result is an important piece in the puzzle of the origin of the elements in the universe, which the experiment has been studying for the last 25 years”, remarked Paolo Prati, spokesperson for the LUNA experiment. “Stars generate energy and at the same time assemble atoms through a complex system of nuclear reactions. A very small number of these reactions have been studied in the conditions under which they occur inside stars, and a large proportion of those few cases have been observed with this accelerator”.

LUNA uses a compact linear accelerator in which hydrogen and helium beams are accelerated and made to collide with a target (in this case, a neon isotope), to produce other particles. Special detectors obtain images of the products of the collisions and identify the reaction to be examined. These extremely rare processes can only be detected in conditions of cosmic silence. The rock surrounding the underground facility at the Gran Sasso National Laboratory shields the experiment against cosmic rays and protects its measurements.

LUNA is an international collaboration involving some 50 Italian, German, Scottish and Hungarian researchers from the National Institute for Nuclear Physics in Italy, the Helmholtz-Zentrum Dresden-Rossendorf in Germany, the MTA-ATOMKI in Hungary and the School of Physics and Astronomy of the University of Edinburgh in the UK.

Read more at and (Three New Low-Energy Resonances in the 22Ne(p,g)23Na Reaction)

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December 18, 2015 at 1:34 pm

Everything Under the Sun: A Review of Solar Neutrinos

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Spectra of neutrinos emitted by fusion reactions in the Sun. Solid lines represent neutrinos from the pp chain and dashed lines are neutrinos from the CNO cycle

Spectra of neutrinos emitted by fusion reactions in the Sun. Solid lines represent neutrinos from the pp chain and dashed lines are neutrinos from the CNO cycle


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April 16, 2015 at 9:31 pm

Carbon nucleus seen spinning in triangular state

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Physicists have obtained important new evidence showing that the structure of the carbon-12 nucleus – without which there would be no life here on Earth – resembles that of an equilateral triangle. The evidence was obtained by physicists in the UK, Mexico and the US by measuring a new rapidly spinning rotational state of the nucleus. The finding suggests that the “Hoyle state” of carbon-12, which plays an important role in the creation of carbon in red giant stars, has the same shape too. Recent theoretical predictions, in contrast, had suggested that the Hoyle state is more like an obtuse triangle or “bent arm”.

All the carbon in the universe is created in red giant stars by two alpha particles (helium-4 nuclei) fusing to create a short-lived beryllium-8 nucleus, which then captures a third alpha particle to form carbon-12. But exactly how this reaction occurs initially puzzled physicists, whose early understanding of carbon-12 suggested that it would proceed much too slowly to account for the known abundance of carbon in the universe. Then in 1954 the British astronomer Fred Hoyle predicted that carbon-12 had a hitherto unknown excited state – now dubbed the Hoyle state – which boosts the rate of carbon-12 production.

Three years later the Hoyle state was confirmed experimentally by physicists working at Caltech. However, the precise arrangement of the protons and neutrons in the carbon-12 nucleus remains a matter of much debate. While some physicists feel that carbon-12 is best thought of as 12 interacting nucleons, others believe that the nucleus can be modelled as three alpha particles that are bound together. The rational for the latter model is that alpha particles are extremely stable and so are likely to endure within the carbon-12 nucleus. Read the rest of this entry »

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July 9, 2014 at 1:19 pm


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Fossil Galaxy May Be One of First Ever Formed

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The stars in the nearby Segue 1 dwarf galaxy have fewer metals than any other galaxy known, suggesting the object is a relic from the baby universe

The Magellan Telescopes at Las Campanas Observatory in Chile targeted stars in the Segue 1 dwarf galaxy for between 6 and 15 hours each to measure their metal content.  Wikimedia Commons/Krzysztof Ulaczyk

The Magellan Telescopes at Las Campanas Observatory in Chile targeted stars in the Segue 1 dwarf galaxy for between 6 and 15 hours each to measure their metal content.
Wikimedia Commons/Krzysztof Ulaczyk

By Clara Moskowitz

A tiny galaxy circling the Milky Way may be a fossil left over from the early universe, astronomers say. A recent study found that the stars in the galaxy, called Segue 1, contain fewer heavy elements than those of any other galaxy known, implying that the object may have stopped evolving almost 13 billion years ago. If true, Segue 1 could offer a window into the conditions of the early universe and reveal how some of the first galaxies came to be.

Segue 1 is very, very tiny. It appears to contain only a few hundred stars, compared with the few hundred billion stars in the Milky Way Galaxy. Researchers led by Anna Frebel of the Massachusetts Institute of Technology collected detailed information on the elemental composition of six of the brightest of Segue 1’s stars using the Las Campanas Observatory’s Magellan Telescopes in Chile and the Keck Observatory in Hawaii. The measurements, reported in a paper accepted for The Astrophysical Journal, revealed that these stars are made almost entirely of hydrogen and helium, and contain just trace amounts of heavier elements such as iron. No other galaxy studied holds so few heavy elements, making Segue 1 the “least chemically evolved galaxy known.”

Complex elements are forged inside the cores of stars by the nuclear fusion of more basic elements such as hydrogen and helium atoms. When stars explode in supernovae, even heavier atoms are created. elements spew into space to infuse the gas that births the next generation of stars, so that each successive generation contains more and more heavy elements, known as metals. “Segue 1 is so ridiculously metal-poor that we suspect at least a couple of the stars are direct descendants of the first stars ever to blow up in the universe,” says study co-author Evan Kirby of the University of California, Irvine.

All supernovae are not created equal. When very massive stars blow up they form a mix of elements such as magnesium and calcium, whereas low-mass star explosions almost exclusively make iron. Frebel and her colleagues measured the content of each of these particular elements in Segue 1’s stars and found that they contained the products of high-mass stars but very few products of low-mass stars. Because high-mass stars die much younger than do low-mass ones, this evidence reveals how quickly star formation occurred in the dwarf galaxy. “Segue 1 is the only example that we know of now that was never enriched by these low-mass stars, meaning it formed stars really quickly, in the blink of an eye,” Kirby says. “If it had formed stars long enough those low-mass stars would have to contribute.”

The findings suggest Segue 1 went through one brief bout of star formation long ago, and then stopped forever. “The big question is, why did it stop?” says U.C. Irvine astrophysicist James Bullock, who was not involved in the study. “A galaxy like this should have been able to make a million more stars, but it didn’t.”

One possibility is the epoch of reionization. When the universe was born it was hot and dense, and all gas was ionized, meaning protons and electrons were isolated and could not band together to form atoms. Eventually the universe cooled enough to allow atoms to form in the gas and the first stars were born from this material. Those stars blasted out radiation, which energized the gas around them and reionized it sometime around 13.2 billion years ago. Because stars cannot form from ionized gas, reionization might have terminated star formation in the existing galaxies at the time. “Maybe Segue 1 was on its way to forming a bunch of stars but reionization turned on and killed all the star formation in the galaxy,” Kirby says. “That could also explain why the star formation lasted such a short time.”

The case is not closed, however. Bullock, one of the main authors of the reionization idea, says the latest theoretical simulations of galaxy formation suggest the shutdown caused by reionization looks to be less sudden than scientists previously thought. “It’s not obvious to me that reionization by itself could have done this,” he says. “Maybe, but I definitely think there are other possibilities.” For instance, perhaps some quirk has caused Segue 1 to be incredibly inefficient at forming stars compared with other galaxies.

Segue 1 may help reveal not just what halts galaxy evolution, but how it gets started as well. “This study is so interesting because I really want to know, can galaxies form this small?” says astronomer Beth Willman of Haverford College, who was not involved in the research. “Can galaxies form and look like Segue 1 when they form or do they have to form larger and then have some mass taken away?” It is possible, after all, that this dwarf was once a much larger galaxy and lost most of its stars, perhaps through disruptions from its close neighbor, the Milky Way. The extremely low metal counts in Segue 1’s stars, however, support the idea that it formed roughly the same size it is now, because disruptions would be unlikely to pull only the metal-rich stars from the galaxy, leaving behind the metal-poor.

If there is no barrier to such puny galaxies forming in the first place, then mini galaxies like Segue 1 could be plentiful, but unseen. Only Segue 1’s close proximity to the Milky Way makes such a small, dim galaxy detectable. “There could be 200 Segue 1-like galaxies around us,” Willman says. “My lifelong goal is trying to understand, are things like this the most abundant in the universe?”

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April 9, 2014 at 9:03 am

Evidence for a new nuclear ‘magic number’

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The Superconducting Ring Cyclotron at the Radioactive Isotope Beam Factory, Japan, which was used to accelerate the beam of zinc-70 nuclei reported in the present study. Credit: RIKEN Nishina Center for Accelerator-Based Science

The Superconducting Ring Cyclotron at the Radioactive Isotope Beam Factory, Japan, which was used to accelerate the beam of zinc-70 nuclei reported in the present study. Credit: RIKEN Nishina Center for Accelerator-Based Science

Researchers have come one step closer to understanding unstable atomic nuclei. A team of researchers from RIKEN, the University of Tokyo and other institutions in Japan and Italy has provided evidence for a new nuclear magic number in the unstable, radioactive calcium isotope 54Ca. In a study published today in the journal Nature, they show that 54Ca is the first known nucleus with 34 neutrons (N) where N = 34 is a magic number.

The protons and neutrons inside the atomic nucleus exhibit shell structures in a manner similar to electrons in an atom. For naturally stable nuclei, these nuclear shells fill completely when the number of protons or the number of neutrons is equal to the ‘magic’ numbers 2, 8, 20, 28, 50, 82 or 126.
However, it has recently been shown that the traditional magic numbers, which were once thought to be robust and common for all nuclei, can in fact change in unstable, radioactive nuclei that have a large imbalance of protons and neutrons.
In the current study led by David Steppenbeck of the Center for Nuclear Study, the University of Tokyo, the team of researchers focused on 54Ca, which has 20 protons and 34 neutrons in its nucleus. They were able to study this nucleus thanks to the Radioactive Isotope Beam Factory (RIBF) at RIKEN, which produces the highest intensity radioactive beams available in the world.
In their experiment, a radioactive beam composed of scandium-55 and titanium-56 nuclei travelling at around 60% of the speed of light, was selected and purified by the BigRIPS fragment separator, part of the RIBF. The radioactive beam was focused on a reaction target made of beryllium. Inside this target, projectile fragmentation of the 55Sc and 56Ti nuclei occurred, creating numerous new radioactive nuclei, some in excited states. The researchers measured the energy of the ? rays emitted from excited states of the radioactive nuclei using an array of 186 detectors surrounding the reaction target…..
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October 10, 2013 at 8:02 am