Archive for the ‘NUCLEAR ASTROPHYSICS’ Category

Carbon’s Hoyle state calculated at long last

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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
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January 3, 2013 at 9:47 am

The Origin of the Elements

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The world around us is made of atoms. Did you ever wonder where these atoms came from? How was the gold in our jewelry, the carbon in our bodies, and the iron in our cars made? In this lecture, we will trace the origin of a gold atom from the Big Bang to the present day, and beyond. You will learn how the elements were forged in the nuclear furnaces inside stars, and how, when they die, these massive stars spread the elements into space. You will learn about the origin of the building blocks of matter in the Big Bang, and we will speculate on the future of the atoms around us today.

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November 22, 2012 at 12:05 pm

Tritium as an Anthropic Probe

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Andrew Gould (Ohio State)
I show that if tritium were just 20 keV lighter relative to helium-3, then the current deuterium burning phase of pre-main-sequence stellar evolution would be replaced by deuterium+tritium burning. This phase would take place at the same temperature but would last a minimum of 4 times longer and a maximum of 8 times longer than deuterium burning and so would yield total energies comparable to the binding energy of solar-type pre-main-sequence stars. Hence, it could in principle radically affect the proto-planetary disk, which forms at the same epoch. I suggest that this may be one of the most “finely-tuned” parameters required for intelligent life, with the mass range only a few percent of the neutron-proton mass difference, and 10-5 of their masses. I suggest that the lower limit of this range is set by the physics of disk formation and the upper limit by the statistical properties of fundamental physics. However, if this latter suggestion is correct, the statistical distribution of physical “constants” must be a power-law rather than an exponential. I also suggest a deep connection between fundamental physics and the search for extrasolar life/intelligence….
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July 11, 2012 at 8:00 am

Axion Dark Matter and Cosmological Parameters

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Blame dark matter underdog for mystery missing lithium

by David Shiga
AN UNDERDOG dark-matter particle could explain why the universe seems strangely low on lithium. If the idea holds up, it will be a boon in the hunt for dark matter, the stuff needed to account for 80 per cent of the universe’s matter.

In the universe’s first few fiery minutes, nuclear reactions forged a host of light elements, including helium, deuterium and lithium, in a process called big bang nucleosynthesis. The amounts of these elements present in the early universe, gleaned from ancient stars and primordial gas clouds, match theory, except in one respect: they contain much less of the dominant form of lithium, lithium-7, than expected. There has never been a satisfactory explanation for this.

Now help comes in the shape of hypothetical dark-matter particles called axions. These light particles were dreamed up in the 1970s as part of a theory to explain why the strong nuclear force, unlike the other forces, does not change if a particle is swapped for the antimatter counterpart of its mirror image. Axions are not the dominant theory for dark matter. That accolade goes to weakly interacting massive particles, or WIMPs. But as neither WIMPs nor axions have ever been observed, the jury is still out.

In the latest research, the underdog axions score a point. The rates of nuclear reactions that produced lithium-7 depend partly on the amount of energy that was present in the form of light. As we cannot tell how much light was there directly, we infer it from the cosmic microwave background (CMB), the echo of the big bang that emerged 380,000 years later. This is used to estimate how much lithium should be present: more light skews reaction rates and lowers expected levels of lithium.

Ozgur Erken of the University of Florida in Gainesville and colleagues suggest that something cooled photons between the synthesis of lithium and the emergence of the CMB, causing the photon energy to be underestimated, and inflating the expected amounts of lithium.

Born with very little kinetic energy, axions are a prime suspect. When their cooling power is accounted for, the predicted lithium abundance drops by half, the team calculate (Physical Review LettersDOI: 10.1103/PhysRevLett.108.061304). “We’re excited that it gives about the right correction,” says Pierre Sikivie, Erken’s colleague.

Adding in axions also creates a problem, however. Without them, CMB measurements are consistent with about four types of neutrino, close to the three types glimpsed in experiments. But if axions are present, they would skew this measurement and imply about seven neutrino types, Erken’s team calculate.  This makes Gary Steigman of Ohio State University in Columbus, who was not involved in the study, sceptical of the axion explanation for the lithium-7 anomaly
An answer should come in 2013 when much better measurements of the CMB are expected from the Planck satellite. Our best chance of glimpsing axions, meanwhile, lies in an upgraded version of an experiment called ADMX, due to start up towards the end of this year. It may also be possible to infer their existence from data from the Large Hadron Collider at CERN near Geneva in Switzerland, where they should boost the production of Higgs bosons…….

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February 23, 2012 at 5:30 pm

Solar Neutrinos in 2011

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The Borexino detector. It was designed to detect sub-MeV solar neutrinos. It features a high light-yield, ultra-pure liquid scintillator target. A non-scintillating buffer region serves as shielding for external γ -rays. Its location at a deep underground site and its muon veto suppress cosmic backgrounds

Alvaro Chavarria
I give an overview of the recent developments in the solar neutrino field.
I focus on the Borexino detector, which has uncovered the solar neutrino spectrum below 5 MeV, providing new tests and confirmation for solar neutrino oscillations. I report on the updated measurements of the 8B solar neutrino flux by water Cherenkov and organic scintillator detectors.
I review the precision measurement of the 7Be solar neutrino flux by Borexino and the search for its day-night asymmetry.
I present Borexino’s latest result on the study of pep and CNO neutrinos. Finally, I discuss the outstanding questions in the field and future solar neutrino experiments.
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January 31, 2012 at 7:05 am

Earthly machine recreates star’s sizzling-hot surface

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The Z Machine at Sandia National Labs generates bursts of x-rays that scientists have used to replicate the temperature and density conditions in white dwarf stars. Credit: Z Machine Collaboration, Sandia National Lab, Lockheed Martin, NNSA, DOE

Since we can’t go to the stars yet, let’s bring the stars to us. In a giant X-ray-producing facility, astronomers and plasma physicists have heated a cigar-sized sample of gas to over 17,000 degrees Fahrenheit in order to replicate the surface of stars called white dwarfs.
“As an astronomer, I am used to looking at these stars from light-years away,” says Don Winget of the University of Texas at Austin.
One of the primary methods astronomers use to study a distant object is to analyze its spectrum of light as it reaches Earth.

“So it was a remarkable moment the first time we took a spectrum from a distance of just 5 centimeters,” said Winget.
That first time was in April 2010. Since then, the team has been refining their experiment inside the Z Machine at Sandia National Laboratories in Albuquerque, N.M. The goal is to make precision measurements of a laboratory-recreated white dwarf surface in order to improve interpretations of data from spaced-based white dwarfs. Winget described the project today at an American Astronomical Society meeting in Austin.
White dwarfs currently occupy part of the limelight in astronomical circles, as researchers recently confirmed that one of them exploded in a nearby galaxy, producing a “Type IA supernova” that astronomers use to measure the size and acceleration of the universe. Winget said that his team’s work could eventually increase our understanding of these cosmic yardsticks, by providing details about what’s going on in the white dwarfs before they go boom.

Stellar Fossils
White dwarfs are the burnt embers of once bright-shining stars. Our sun is expected to “retire” to a white dwarf when it runs out of nuclear fuel in roughly 7 billion years. Without the energy from nuclear fusion, the sun will shrink down to about the size of Earth due to the inward pull of its gravity.
This has already been the fate of billions of stars in our galaxy. Although no longer burning fuel, white dwarfs contain a lot of heat. Relatively young ones can be more than 180,000 degrees F on their surface. But as the heat radiates away over time, the white dwarfs tend to cool down to less than 18,000 degrees.
Because these are some of the oldest stars around, these “graybeards” can often unravel the evolutionary history of our galaxy. To estimate the age of a white dwarf, astronomers analyze the spectra of light coming from the stars. Specifically, they look at how some of the light is absorbed in the outer surface of the white dwarf. In most white dwarfs, hydrogen, the simplest of elements, absorbs this light.
“Everyone assumes we know hydrogen so well,” Winget said. “As it turns out, that’s not the case.”
The surface of a white dwarf is largely a plasma, or electrically charged gas. In the midst of this dense plasma, hydrogen absorbs light in a slightly different way. Better understanding this behavior would help astronomers improve the accuracy of white dwarf age estimates. It would also help scientists studying exotic phenomena like the “freezing” of white dwarf cores as they cool.
“I would very much appreciate any experimental work that could help verify the models and determine the physical and chemical properties of the atmospheres of white dwarfs,” said astrophysicist Piotr Kowalski, who is not a part of the project, of the Helmholtz Centre Potsdam in Germany.
From Z to X-rays
Hot hydrogen plasmas have been made before in the lab, but only in small quantities.

“We need to have a large sample in order to observe how the plasma absorbs light,” said Ross Falcon, a graduate student at the University of Texas at Austin doing much of the legwork on the project.
To obtain enough plasma in the proper fashion requires an experimental facility that can generate a large amount of energy over a short time. Not many facilities can provide that, but the Z machine can. The Z was built to study nuclear weapons, but it now allots about 15 percent of its experimental time to academic research.
To replicate the conditions of a white dwarf, the team first prepares hydrogen in a gas cell that is several centimeters across. This sample is then placed a foot away from a coil of tungsten wires, which lies at the heart of the Z machine. When a switch is thrown, 26 million amps of current rush through the wires, causing them to implode. A burst of X-rays streams out, quickly ionizing the gas in the cell. Winget’s team collects spectra from this “little star” using fiber-optic cables…..
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January 13, 2012 at 2:09 pm


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The effect of 12C + 12C rate uncertainties on the evolution and nucleosynthesis of massive stars

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Chart of isotopes indicating the nuclear reaction networks used in this work: GENEC (blue squares) and MPPNP (pale red squares). The network used by MPPNP includes all stable isotopes, which are indicated by black squares. The outer boundary to each side of the valley of stability indicates the position of all currently known isotopes, including heavy transuranic isotopes. Parallel grid lines indicate values of Z or N that are magic as speci

M. E. Bennett, R. Hirschi, M. Pignatari, S. Diehl, C. Fryer, F. Herwig, A. Hungerford, K. Nomoto, G. Rockefeller, F. X. Timmes, M. Wiescher

The 12C + 12C fusion reaction has been the subject of considerable experimental efforts to constrain uncertainties at temperatures relevant for stellar nucleosynthesis.
In order to investigate the effect of an enhanced carbon burning rate on massive star structure and nucleosynthesis, new stellar evolution models and their yields are presented exploring the impact of three different 12C + 12C reaction rates.
Non-rotating stellar models were generated using the Geneva Stellar Evolution Code and were later post-processed with the NuGrid Multi-zone Post-Processing Network tool.
The enhanced rate causes core carbon burning to be ignited more promptly and at lower temperature. This reduces the neutrino losses, which increases the core carbon burning lifetime.
An increased carbon burning rate also increases the upper initial mass limit for which a star exhibits a convective carbon core. Carbon shell burning is also affected, with fewer convective-shell episodes and convection zones that tend to be larger in mass.
Consequently, the chance of an overlap between the ashes of carbon core burning and the following carbon shell convection zones is increased, which can cause a portion of the ashes of carbon core burning to be included in the carbon shell.
Therefore, during the supernova explosion, the ejecta will be enriched by s-process nuclides synthesized from the carbon core s process.
The yields were used to estimate the weak s-process component in order to compare with the solar system abundance distribution.
The enhanced rate models were found to produce a significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak component, which is primarily the signature of the carbon-core s process.
Consequently, it is shown that the production of isotopes in the Kr-Sr region can be used to constrain the 12C + 12C rate using the current branching ratio for a- and p-exit channels….
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January 6, 2012 at 8:55 am

The Supernova Triggered Formation and Enrichment of Our Solar System

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M. Gritschneder, D. N. C. Lin, S. D. Murray, Q.-Z. Yin, M.-N. Gong

We investigate the enrichment of the pre-solar cloud core with short lived radionuclides (SLRs), especially 26Al.
The homogeneity and the surprisingly small spread in the ratio 26Al/27Al observed in the overwhelming majority of calcium-aluminium-rich inclusions (CAIs) in a vast variety of primitive chondritic meteorites places strong constraints on the formation of the the solar system.
Freshly synthesized radioactive 26Al has to be included and well mixed within 20kyr.
After discussing various scenarios including X-winds, AGB stars and Wolf-Rayet stars, we come to the conclusion that triggering the collapse of a cold cloud core by a nearby supernova is the most promising scenario. We then narrow down the vast parameter space by considering the pre-explosion survivability of such a clump as well as the cross-section necessary for sufficient enrichment. We employ numerical simulations to address the mixing of the radioactively enriched SN gas with the pre-existing gas and the forced collapse within 20kyr.
We show that a cold clump of 10Msun at a distance of 5pc can be sufficiently enriched in 26Al and triggered into collapse fast enough – within 18kyr after encountering the supernova shock – for a range of different metallicities and progenitor masses, even if the enriched material is assumed to be distributed homogeneously in the entire supernova bubble.
In summary, we envision an environment for the birth place of the Solar System 4.567Gyr ago similar to the situation of the pillars in M16 nowadays, where molecular cloud cores adjacent to an HII region will be hit by a supernova explosion in the future. We show that the triggered collapse and formation of the Solar System as well as the required enrichment with radioactive 26Al are possible in this scenario….
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November 3, 2011 at 3:55 pm