Neutrinos could detect secret fission reactors

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Nov 24, 2010

Oil tankers fitted with neutrino detectors, hundreds of thousands of tonnes in mass, could be floated offshore to check for undeclared nuclear fission reactors. That's the idea of physicists in France, who have proposed the Secret Neutrino Interactions Finder (SNIF) as a way of enforcing the nuclear non-proliferation treaty – although some experts doubt its feasibility.

Currently, fission reactors over the world are monitored by the United Nations' International Atomic Energy Agency (IAEA), based in Vienna. The IAEA uses several "near-field" tools to make sure reactors are running legally, from CCTV-type cameras to metallic or fibre-optic networks that can detect when fuel is being loaded. In some cases, the agency installs thermal monitors to check that reactors are not being operated for too long, as might be required for the production of bomb-making plutonium.

Another, perhaps more fail-safe way to monitor reactors would be to detect the nearby levels of anti-neutrinos – light particles that are emitted copiously in nuclear-fission reactions. Because the flux of anti-neutrinos arriving at a certain area is proportional to the power of a reactor and its proximity, the anti-neutrino level at any point should be an indicator of what fission reactions are taking place nearby.

But, as researchers discovered almost a decade ago, the science is more complicated. Neutrinos have a small, finite mass – not zero, as was previously thought – and are able to oscillate from one type to another. This means that a detector looking for one type of anti-neutrino would always detect fewer than expected, because some of them oscillate into different types before arrival.

Thierry Lasserre at the French Alternative Energies and Atomic Energy Commission says that improvements in the understanding of neutrino oscillation have enabled his group to explore the use of anti-neutrino detectors for "far-field" reactor monitoring. Lasserre and his colleagues have calculated how anti-neutrino fluxes fall with distance from a reactor, taking into account oscillations. They have then analysed all the other sources of anti-neutrinos – 200 nuclear power stations over the globe – to produce a map of background anti-neutrino levels.

In a final calculation, Lasserre's group showed that a neutrino detector would need to be sunk just 500 m or more underwater to prevent catching any cosmic rays, which would confuse the signal. The researchers think that, for monitoring fission reactions in a radius of 100–500 km, a detector would need a scintillator mass of 1034 free protons – in the order of a hundred thousand tonnes.

John Learned, a physicist at the University of Hawaii, US, who first suggested using neutrino detectors for global fission-reactor monitoring, believes the group has performed some "excellent" calculations, but notes that the SNIF idea is not totally new. He adds, however, "With a network of monitors one can record the activity of a group of reactors, perhaps some friendly ones, and some clandestine reactors. With various methods under development we can do a better job, even than indicated in this paper."

Others are not so sure. Andrew Monteith of the IAEA's Novel Technologies Unit says that the IAEA is at present only interested in neutrino detectors for near-field detection, because only that is within its current remit. "The far-field approach that's discussed in the paper has never really been an official part of our thinking," Monteith explains. "We're taking it on a stage-by-stage basis, and the near-field one is certainly more realistic for us, in terms of cost and deployment."

Julian Whichello, head of the Novel Technologies Unit, believes Lasserre's SNIF detector could cost in the region of $100 million – almost the same as the IAEA's entire budget for global verification of fission reactors. "This is something that's well and truly outside of the current budget of the agency," he says.

Still, Lasserre explains that his group's goal was to explore the scientific possibilities rather than have political influence. "This is very futuristic," he says. "It's huge, it will cost a lot of money and it's a difficult effort. Technically it would be possible in the next 30 years, but I'm not aware of any programme in the world to build such devices."

The research is available at arXiv:1011.3850.

Jon Cartwright is a freelance journalist based in Bristol, UK

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Space–time invisibility cloak could 'edit history'

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Nov 16, 2010

Physicists in the UK have proposed a "space–time" invisibility cloak that, if built, could be used to prevent signal interference or give the illusion of a Star Trek teleportation device.

The idea comes after four years of research by different groups that are creating devices to make objects invisible. In 2006 researchers at Duke University in the US created the first device that could cloak a small object in two dimensions in the microwave region of the electromagnetic spectrum. Last year groups at Cornell University and the University of California at Berkeley, US, independently created 2D cloaks that operated at optical wavelengths. Then, earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

The latest development, by Martin McCall and colleagues of Imperial College, London, and the University of Salford, might see cloaks add yet another dimension to their capability: time. The idea is to create a tunnel through which an object could perform an action – move or change shape, for example – while appearing as though it is doing nothing at all.

"It means that you can allow an object to do something for a short period of time in such a way that it can't be detected," McCall told physicsworld.com. "A good way to think about it is a small piece of an object’s history just being cut out, so you would see the object suddenly jump from one place to another." In principle, says McCall, such a system would enable a thief to enter a room, steal the contents of a safe and leave the scene as it was before, while security personnel watching CCTV are none the wiser.

In practice, the device would need two transparent walls to act as the tunnel, or space–time cloak. As an object enters the cloak to perform its action, the rear wall would compress light waves passing through from a source behind. Once the object completes its action and leaves the cloak, however, the front wall would stretch the light waves passing through so that they would merge seamlessly with those outside, whose profile had not been altered.

An analogy, says McCall, is a chicken crossing a busy road. Once the chicken steps onto the road cars must stop to let it pass, but as soon as it leaves the other side the cars would accelerate to catch up with the traffic ahead. To an observer farther down the road, the stream of passing cars would display no evidence of having slowed down.

Although McCall gives safecracking as a potential for the space–time cloak, his group does have ideas for more savoury applications. In the basic set-up it might appear as similar to a transporter from Star Trek, with a person entering the cloak on one side appearing at the other side moments later, apparently having skipped the journey. But the cloak could also find uses in signal processing: a detector placed inside the cloak would be able to "pause" a signal travelling through the wall while it first deals with a signal passing through the tunnel.

All of this, however, relies on someone being able to make the device – and in particular the walls that compress or stretch light waves. McCall admits a perfect implementation "is certainly beyond current technology", but points to advances in so-called nonlinear systems: materials that change their refractive index – a property that governs light propagation speed – given illumination with strong lasers. One of the problems with this route is that changes in refractive index introduce reflections, which means the cloak, while hiding the object within, would nonetheless reveal its presence with a telling glow.

But, explains McCall, "Provided we're prepared to throw away some aspects of the cloak, we can point towards more practical, proof-of-concept experiments that are currently accessible with current technology."

The research is published today in the Journal of Optics.

Jon Cartwright is a freelance journalist based in Bristol, UK

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Space–time invisibility cloak could 'edit history'

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Nov 16, 2010

Physicists in the UK have proposed a "space–time" invisibility cloak that, if built, could be used to prevent signal interference or give the illusion of a Star Trek teleportation device.

The idea comes after four years of research by different groups that are creating devices to make objects invisible. In 2006 researchers at Duke University in the US created the first device that could cloak a small object in two dimensions in the microwave region of the electromagnetic spectrum. Last year groups at Cornell University and the University of California at Berkeley, US, independently created 2D cloaks that operated at optical wavelengths. Then, earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

The latest development, by Martin McCall and colleagues of Imperial College, London, and the University of Salford, might see cloaks add yet another dimension to their capability: time. The idea is to create a tunnel through which an object could perform an action – move or change shape, for example – while appearing as though it is doing nothing at all.

"It means that you can allow an object to do something for a short period of time in such a way that it can't be detected," McCall told physicsworld.com. "A good way to think about it is a small piece of an object’s history just being cut out, so you would see the object suddenly jump from one place to another." In principle, says McCall, such a system would enable a thief to enter a room, steal the contents of a safe and leave the scene as it was before, while security personnel watching CCTV are none the wiser.

In practice, the device would need two transparent walls to act as the tunnel, or space–time cloak. As an object enters the cloak to perform its action, the rear wall would compress light waves passing through from a source behind. Once the object completes its action and leaves the cloak, however, the front wall would stretch the light waves passing through so that they would merge seamlessly with those outside, whose profile had not been altered.

An analogy, says McCall, is a chicken crossing a busy road. Once the chicken steps onto the road cars must stop to let it pass, but as soon as it leaves the other side the cars would accelerate to catch up with the traffic ahead. To an observer farther down the road, the stream of passing cars would display no evidence of having slowed down.

Although McCall gives safecracking as a potential for the space–time cloak, his group does have ideas for more savoury applications. In the basic set-up it might appear as similar to a transporter from Star Trek, with a person entering the cloak on one side appearing at the other side moments later, apparently having skipped the journey. But the cloak could also find uses in signal processing: a detector placed inside the cloak would be able to "pause" a signal travelling through the wall while it first deals with a signal passing through the tunnel.

All of this, however, relies on someone being able to make the device – and in particular the walls that compress or stretch light waves. McCall admits a perfect implementation "is certainly beyond current technology", but points to advances in so-called nonlinear systems: materials that change their refractive index – a property that governs light propagation speed – given illumination with strong lasers. One of the problems with this route is that changes in refractive index introduce reflections, which means the cloak, while hiding the object within, would nonetheless reveal its presence with a telling glow.

But, explains McCall, "Provided we're prepared to throw away some aspects of the cloak, we can point towards more practical, proof-of-concept experiments that are currently accessible with current technology."

The research is published today in the Journal of Optics.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here

Honeycomb windows that could harvest the Sun

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Nov 11, 2010

A materials science breakthrough in the US and Taiwan could lead to a new type of window that can harness the power of the Sun. The newly created transparent material can efficiently capture photons to generate electricity thanks to its honeycomb structure, which blends the properties of a semiconductor polymer with those of a carbon-rich fullerene.

The chosen polymer, P1, is efficient at absorbing photons, which causes electrons and holes within the material to combine into bound states known as excitons. The role of the fullerene – which is a compound formed when a large number of carbon atoms form ball-shaped molecules – is to then undo this process by dissociating the electrons and holes. Suitably placed electrodes can then extract the charges to produce photocurrents.

Mircea Cotlet, one of the researchers based at Brookhaven National Laboratory near New York City, told physicsworld.com that the biggest challenge was finding a way to merge the polymer and fullerene into a honeycomb lattice. His team achieved this by creating a flow of micron-sized water droplets across a thin layer of the polymer/fullerene solution. Water droplets then self-assemble into large arrays within the solution. Once the newly formed solution has evaporated it leaves behind a hexagonal honeycomb pattern over a large area of the polymer, which the researchers observed using scanning probe and electron microscopy.

"Though such honeycomb-patterned thin films have previously been made using conventional polymers like polystyrene, this is the first report of such a material that blends semiconductors and fullerenes to absorb light and efficiently generate charge and charge separation," says Cotlet.

Cotlet is keen to stress that the idea behind the study was to explore the basic science and to develop self-assembly methods that do not require intense laboratory infrastructure. He reveals, however, that his team now intends to develop the work by implementing the honeycomb into devices and carrying out a number of tests. Among the applications that could spring from the work are optical displays and devices, including transparent solar cells.

Another possibility is to incorporate the honeycomb films into windows. As the polymer chains gather mostly at the edges of hexagons, the films would remain mostly transparent with remaining chains spread thinly across the hexagon centres. "Imagine a house with windows made of this kind of material, which, combined with a solar roof, would cut its electricity costs significantly. This is pretty exciting," says Cotlet.

It is not yet clear how much electricity these windows could generate but it will not be enough to keep a building self-sustained. "At the end, you have a window in any house, why not get some electricity out of it?" says Cotlet.

The research is described in a research paper in Chemistry of Materials.

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Topological insulators could help define fundamental constants

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Oct 25, 2010

A newly discovered class of materials known as "topological insulators" could help physicists to obtain new ways of defining the three basic physical constants – the speed of light (c); the charge of the proton (e); and Planck’s constant (h). That’s the claim of a team of physicists in the US, which has proposed a new experiment to measure the fine-structure constant (a), which is a function of h, c and e, by scattering light from such a material. Topological insulators are unusual in that electrical current flows well on their surface, but not through their bulk.

The new measurement has been proposed by Shou-Cheng Zhang and colleagues at Stanford University as well as researchers at the University of California at Santa Barbara and the University of Maryland. Although there are many other ways of determining a, their technique is the only method that involves measuring a phenomenon that is quantized in units of a. In principle, this means it could provide an extremely precise metrological definition of a.

A new way of defining h, c and e could then be obtained by combining the value of a with measurements of magnetic flux quanta and electrical conductance quanta in the materials, which both depend on h and e.

The strange properties of topological insulators arise from the fact that the shape – or topology – of the electron energy bands makes it impossible for a surface electron to backscatter. Zhang believes that, under certain conditions, this topology leads to an "exact quantization" of how a material responds to an external field. Physicists are already familiar with similar topological responses, which occur when a superconductor is exposed to a magnetic field – resulting in magnetic flux quanta. It is also seen in the quantum Hall effect, when a 2D conductor is exposed to a magnetic field resulting in quanta of electrical conductance.

The physicists argue that a "topological magneto-electric effect" – whereby an electric field can induce a magnetic polarization and a magnetic field can induce an electric polarization – can occur in some topological materials. Furthermore, the topology of the effect means that the response of the material to applied electromagnetic fields is quantized in units of a.

To measure the effect, Zhang and colleagues propose an experiment whereby light is shone at a thin film of topological insulator and the Kerr and Faraday rotations are measured. The Kerr rotation is the shift in the direction of the polarization of the reflected light relative to the polarization of the incident light. The Faraday rotation is the shift in the polarization of the transmitted light.

Both effects involve the interaction between light and matter in the presence of a magnetic field. The physicists have derived a formula that suggests that in some toplogical insulators a certain combination of the Kerr and Faraday angles is quantized in integer multiples of the fine structure constant.

The next step is to try to measure this effect – and Zhang says that three independent labs are currently trying to measure the effect in real materials.

The work is reported in Phys. Rev. Lett. 105 166803.

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Compact X-ray source could rival accelerators

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Oct 27, 2010

A compact source of high-quality X-ray pulses has been unveiled by an international team of researchers. The group claims that the source – which is contained in a vacuum chamber about 1 m3 – produces intense and highly coherent X-ray pulses that rival those produced by "wiggling" electrons in large and extremely expensive particle accelerator facilities.

The source was created by Zulfikar Najmudin and colleagues at Imperial College London along with researchers at the University of Michigan, Instituto Superior Técnico in Lisbon and Ecole Polytechnique Palaiseau in France.

The device creates X-rays using the "plasma wakefield" effect whereby an intense laser pulse is fired into gas to create a plasma. As the pulse travels through the gas, its electric field separates electrons from atoms. This creates an extremely large electric field in the wake of the pulse, which accelerates electrons. As the wake collapses, electrons are "wiggled" violently, causing them to radiate X-rays.

The team made the plasma at the University of Michigan using Hercules – a petawatt laser that creates some of the most intense laser pulses ever. The pulses are fired into a jet of helium gas and X-rays are created in a volume about 1 µm and propagate in the direction of the laser pulse. They are produced with a broad energy distribution and have an average energy of about 10 keV with some as energetic as 100 keV. According to the team, the X-ray source is 1000 times brighter than previous schemes for generating X-rays in "plasma wigglers".

The team evaluated the quality of the X-ray pulses by using them to image a number of microscopic test patterns. They concluded that pulses have a large degree of spatial coherence – which makes them well suited for studying the structural properties of materials on the nanometre scale. In addition, the pulses last only a few femtoseconds, which means that they can be used to study processes such as atomic and molecular interactions that occur on very short timescales.

"We think a system like ours could have many uses," said Najmudin. "For example, it could eventually increase dramatically the resolution of medical imaging systems using high energy X-rays."

However, the technique has one important shortcoming at the moment – it requires an extremely powerful and relatively large laser to work. While the Hercules laser is smaller than an accelerator facility, it still occupies several rooms at the University of Michigan.

"High-power lasers are currently quite difficult to use and expensive, which means we're not yet at a stage when we could make a cheap new X-ray system widely available," admitted Najmudin. "However, laser technology is advancing rapidly, so we are optimistic that in a few years there will be reliable and easy-to-use X-ray sources available that exploit our findings."

The work is described in Nature Physics DOI:1038/NPHYS1789.

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