The puzzle of the origin of elements in the Universe

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 www.lngs.infn.it and journals.aps.org (Three New Low-Energy Resonances in the 22Ne(p,g)23Na Reaction)

Carbon nucleus seen spinning in triangular state

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. Continue reading Carbon nucleus seen spinning in triangular state

Fossil Galaxy May Be One of First Ever Formed

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?”
Read more at http://www.scientificamerican.com/article/fossil-galaxy-segue1/

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

Evidence for a new nuclear ‘magic number’

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…..
Read more at: http://phys.org/news/2013-10-evidence-nuclear-magic.html#jCp

This illustration shows how three helium-4 nuclei form a "bent arm" shape in a carbon-12 nucleus (Courtesy: North Carolina State University)

Carbon’s Hoyle state calculated at long last

This illustration shows how three helium-4 nuclei form a "bent arm" shape in a carbon-12 nucleus (Courtesy: North Carolina State University)

This illustration shows how three helium-4 nuclei form a “bent arm” shape in a carbon-12 nucleus (Courtesy: North Carolina State University)

By calculating the behaviour of protons and neutrons inside carbon nuclei from first principles, physicists in Germany and the US have identified the shape of carbon’s Hoyle state – which is an important step in the production of heavy elements inside stars. The researchers found the state to have an unusual bent structure, a finding that should help identify the forces at work in carbon production.

Carbon-12 comprises six protons and six neutrons and is a key step in nucleosynthesis – the process by which heavier elements are produced inside stars. Physicists studying stellar fusion in the 1940s and 1950s reckoned that carbon-12 forms when two helium-4 nuclei fuse to produce beryllium-8 – which then fuses with a third helium-4 nucleus.

There was a problem with this hypothesis, however. The energy of the fused particles is considerably higher than that of the ground state of carbon-12. This implies that the new particle is in fact extremely unlikely to form in this way – far too unlikely to account for the great abundance of carbon in the universe.

According to Hoyle
To overcome this apparent contradiction the British astronomer Fred Hoyle proposed in 1954 that carbon-12 has an excited state that had never been seen before. The idea is that carbon-12 would form readily in this state and then decay to its ground state, giving off a well defined amount of energy (7.6 MeV) in the process. This excited state was then observed three years later by researchers at the California Institute of Technology, when carrying out experiments involving beta decays of boron-12.

For the past 60 years nuclear physicists have been trying to understand the nature of this “Hoyle state”, which is not predicted by standard nuclear models. These models regard nuclei as being made up of individual protons and neutrons, and it was reckoned that the Hoyle state is better described as three helium-4 clusters.

Those clusters have now been identified by Ulf Meissner of the University of Bonn and colleagues, thanks to the number-crunching power of the JUGENE supercomputer in Jülich and a new form of Steven Weinberg’s “effective field theory”, which considers protons and nucleons as individual entities rather than as bound states of three quarks.

Space–time lattice
Weinberg’s theory reduces the number of particles that can be considered to make up a carbon-12 nucleus by a factor of three – from 36 to 12. Even 12, however, is too many for an analytical description of the nucleus. Instead, Meissner’s group combined the theory with numerical methods often used to describe the interaction of individual quarks via the strong force. This approach breaks down space and time into discrete chunks, constraining particles to exist only at the vertices of a space–time lattice and so radically simplifying the possible evolution of the particle system.

In a paper published in 2011, Meissner and co-workers described how they used this hybrid approach to identify the Hoyle state. To do this they first picked out carbon-12’s ground state, setting up vast numbers of configurations of the virtual protons and neutrons within JUGENE and then watching what happened as those configurations evolved in time.

The configuration that lasted the longest, being the most stable, was the ground state. Identifying the Hoyle state was a bit trickier since it involved stopping the simulation at some earlier point in time and then disentangling the various states that remained. Despite the challenges of calibrating their simulation using scattering and other data, their calculated values for the energy of the carbon-12 ground state and the Hoyle state agreed very well with experiment.

“Bent arm” shape
Now in this latest work, the team has calculated the structure of those states using a more sophisticated representation of the nuclear wavefunction. Likening the nucleons and groups of nucleons to LEGO bricks, Meissner says that “before we had bricks of just one size and now we have a whole series of different-sized bricks that we can use to construct more complex structures”.

Building up those structures, the group found that in the ground state, carbon-12 consists of three helium-4 clusters arranged in a compact equilateral-triangle formation, whereas in the Hoyle state the three clusters form an obtuse triangle or “bent arm” shape. This more open configuration, the researchers explain, results from the extra energy in the system.

One exciting aspect of the research, according to Morton Hjorth-Jensen of the University of Oslo in Norway, is that it should allow scientists to understand which part of the strong force dictates the carbon-12 decay. This is important because the force in fact consists of several elements, including some that deform the shape of nuclei. “Hoyle predicted his state on the basis of the anthropic principle, arguing that if the state didn’t exist we wouldn’t be here,” he says. “But we now want to understand the structure of this state in terms of its basic constituents and forces.”

Experimental tests
Meanwhile, David Jenkins of York University in the UK points out that the latest work makes a number of explicit predictions that could, in principle, be tested experimentally, including the existence of a number of electromagnetic transitions involving the Hoyle state. But he adds that these transitions are very weak and therefore hard to measure. “Such experiments will be no less challenging than the theoretical achievement,” he says, “but renewed effort is warranted given the strong topical interest.”

According to Meissner there is also more theoretical work to be done. One job, he says, is to reduce the spacings in their virtual lattice, in order to make more precise calculations. Another is to investigate larger nuclei, such as oxygen-16, as well as the reactions that give rise to these nuclei – in this case carbon-12 combining with a helium-4 nucleus. “This is a very important reaction in the sequence that generates life-giving molecules,” he adds.
The latest work is published in Physical Review Letters
Read more: http://physicsworld.com/cws/article/news/2013/jan/03/carbons-hoyle-state-calculated-at-long-last