Los Alamos National Laboratory

Quantum Dot Technology Could Lead To Solar Panel Windows.

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Researchers from Los Alamos National Laboratory and the University of Milano-Bicocca have designed and synthesized a new generation of quantum dots for use in solar energy systems that overcome previous inefficiencies in harvesting sunlight. The study has been published in the journal Nature Photonics.

Quantum dots, which are nanocrystals made of semiconducting materials, appeal to scientists for use in solar photovoltaics (solar panel systems) because of their versatility and low-cost. In particular, they are desirable for use in luminescent solar concentrators (LSCs), which are photon-management devices that serve as alternatives to optics-based solar concentration systems.

LSCs are constructed from transparent materials containing emitters such as quantum dots. They concentrate solar radiation absorbed from a large area onto a significantly smaller solar cell, explains Victor Kilmov, one of the authors of the study. One exciting application of LSCs is the potential to develop photovoltaic windows, which could turn buildings into energy making factories.

Although quantum dots are highly efficient emitters, their small Stokes shift (the presence of an overlap between emission and absorption) means that some of the light produced is re-absorbed by the dots, resulting in losses of emission and therefore overall efficiency problems.

To resolve this issue, the team generated “Stokes-shift-engineered” giant quantum dots composed of a cadmium selenide (CdSe) shell which absorbs light, and a cadmium sulfide (CdS) core which is responsible for light emission. This separation of absorption and emission caused a significant reduction in re-absorption losses which previously caused inefficiencies. The dots were then incorporated into a high-quality polymethylmethacrylate (PMMA) matrix, and spectroscopic analysis revealed that re-absorption losses were minimal across distances of tens of centimeters.

The incorporation of the quantum dots into this PMMA matrix is not specific to a particular type of quantum dot; this means that it can be applied to different sized nanocrystals composed of various materials. This technology therefore represents a promising materials platform.

New neutrino cooling theory changes understanding of neutron stars’ surfaces.

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Massive X-ray superbursts near the surface of neutron stars are providing a unique window into the operation of fundamental forces of nature under extreme conditions.
X-ray nebula
A small, dense object only 12 miles in diameter is responsible for this beautiful X-ray nebula that spans 150 light years. At the center of this image made by NASA’s Chandra X-ray Observatory is a very young and powerful pulsar, known as PSR B1509-58 (B1509). The pulsar is a rapidly spinning neutron star which is spewing energy out into the space around it to create complex and intriguing structures, including one that resembles a large cosmic hand.
NASA
Massive X-ray superbursts near the surface of neutron stars are providing a unique window into the operation of fundamental forces of nature under extreme conditions.“Scientists are intrigued by what exactly powers these massive explosions, and understanding this would yield important insights about the fundamental forces in nature, especially on the astronomical–cosmological scale,” said Peter Moller of Los Alamos National Laboratory’s Theoretical Division.A neutron star is created during the death of a giant star more massive than the Sun, compressed to a tiny size but with gravitational fields exceeded only by those of black holes. And in the intense neutron-rich environment, nuclear reactions cause strong explosions that manifest themselves as X-ray bursts and the X-ray superbursts that are more rare and 1,000 times more powerful.The importance of discovering an unknown energy source of titanic magnitude in the outermost layers of accreting neutron star surfaces is heightened by the unresolved issue of neutrino masses, the recent discovery of the Higgs boson, and the fact that highly neutron-rich nuclei with low-lying states enable “weak interactions,” prominent in stellar explosions. The weak nuclear force is one of four fundamental sources, such as gravity, that interact with the neutrinos; it is responsible for some types of radioactive decay.These hitherto celestially operative nuclei are expected to be within the experimental reach of the Facility for Rare Isotope Beams (FRIB), a proposed user facility at Michigan State University (MSU) funded by the U.S. Department of Energy Office of Science.

Previously, a common assumption was that that the energy released in these radioactive decays would power the X-ray superburst explosions. This was based on simple models of nuclear beta decay, sometimes postulating the same decay properties for all nuclei. It turns out, however, that it is of crucial importance to develop computer models that realistically describe the shape of each individual nuclide since they are not all spherical.

At Los Alamos, scientists have carried out detailed calculations of the specific individual beta-decay properties of thousands of nuclides, all with different decay properties, and created databases with these calculated properties.

The databases are then used at MSU as input into models that trace the decay pathways with the passage of time in accreting neutron stars and compute the total energy that is released in these reactions.

The new unexpected result is that so much energy escapes by neutrino emission that the remaining energy released in the beta decays is not sufficient to ignite the X-ray superbursts that are observed. Thus the superbursts’ origin has now become a puzzle.

Solving the puzzle will require that scientists calculate in detail the consequences of shapes of neutron-rich nuclei, the authors said, and it requires that they simultaneously analyze the role played by neutrinos in neutron star X-ray bursts whose energetic magnitudes are exceeded only by explosions in the nova–supernova class.

The strong nuclear deformations that formed the basis for the neutrino cooling in neutron star crusts also play a role in a number of astrophysical settings and have been taken into account in studies of supernova explosions and subsequent collapses.

 

NASA Curiosity Team Pinpoints Site for First Drive On Mars.

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The scientists and engineers of NASA’s Curiosity rover mission have selected the first driving destination for their one-ton, six-wheeled mobile Mars laboratory. The target area, named Glenelg, is a natural intersection of three kinds of terrain. The choice was described by Curiosity Principal Investigator John Grotzinger of the California Institute of Technology during a media teleconference on Aug. 17.

“With such a great landing spot in Gale Crater, we literally had every degree of the compass to choose from for our first drive,” Grotzinger said. “We had a bunch of strong contenders. It is the kind of dilemma planetary scientists dream of, but you can only go one place for the first drilling for a rock sample on Mars. That first drilling will be a huge moment in the history of Mars exploration.”

The trek to Glenelg will send the rover 1,300 feet (400 meters) east-southeast of its landing site. One of the three types of terrain intersecting at Glenelg is layered bedrock, which is attractive as the first drilling target.

“We’re about ready to load our new destination into our GPS and head out onto the open road,” Grotzinger said. “Our challenge is there is no GPS on Mars, so we have a roomful of rover-driver engineers providing our turn-by-turn navigation for us.”

Prior to the rover’s trip to Glenelg, the team in charge of Curiosity’s Chemistry and Camera instrument, or ChemCam, is planning to give their mast-mounted, rock-zapping laser and telescope combination a thorough checkout. On Saturday night, Aug. 18, ChemCam is expected to “zap” its first rock in the name of planetary science. It will be the first time such a powerful laser has been used on the surface of another world.

“Rock N165 looks like your typical Mars rock, about three inches wide. It’s about 10 feet away,” said Roger Wiens, principal investigator of the ChemCam instrument from the Los Alamos National Laboratory in New Mexico. “We are going to hit it with 14 millijoules of energy 30 times in 10 seconds. It is not only going to be an excellent test of our system, it should be pretty cool too.”

Mission engineers are devoting more time to planning the first roll of Curiosity. In the coming days, the rover will exercise each of its four steerable (front and back) wheels, turning each of them side-to-side before ending up with each wheel pointing straight ahead. On a later day, the rover will drive forward about one rover-length (10 feet, or 3 meters), turn 90 degrees, and then kick into reverse for about 7 feet (2 meters).

“There will be a lot of important firsts that will be taking place for Curiosity over the next few weeks, but the first motion of its wheels, the first time our roving laboratory on Mars does some actual roving, that will be something special,” said Michael Watkins, mission manager for Curiosity from the Jet Propulsion Laboratory in Pasadena, Calif.

The Mars Science Laboratory spacecraft delivered Curiosity to its target area on Mars at 10:31:45 p.m. PDT on Aug. 5 (1:31:45 a.m. EDT on Aug. 6), which included the 13.8 minutes needed for confirmation of the touchdown to be radioed to Earth at the speed of light.

The audio and visuals of the teleconference are archived and available for viewing at: http://www.ustream.tv/nasajpl

The mission is managed by JPL for NASA’s Science Mission Directorate in Washington. The rover was designed, developed and assembled at JPL, a division of Caltech. ChemCam was provided by Los Alamos National Laboratory. France provided ChemCam’s laser and telescope.

For more information about NASA’s Curiosity mission, visit: http://www.jpl.nasa.gov/msl and http://www.nasa.gov/msl

Source: science daily