Wednesday, February 9, 2011

NEUTRINO DETECTOR STARTS MEASUREMENT

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The Double Chooz collaboration recently completed its neutrino detector which will see anti-neutrinos coming from the Chooz nuclear power plant in the French Ardennes. The experiment is now ready to start collecting data in order to measure fundamental neutrino properties with important consequences for particle and astro-particle physics.

Neutrinos are electrically neutral elementary particles, three of a kind plus their antiparticles. Though already postulated in 1930 their first experimental observation was made in 1956. Because of their weak interaction with other particles, matter is almost completely transparent to neutrinos and large sensitive detectors are needed to capture them.

Neutrino oscillations were a major discovery in the late 1990s with the corresponding experiments being included in the 2002 Nobel Prize. Oscillations describe in-flight transformations of different neutrino species into each other and the observation of this effect implies that neutrinos do have mass. The oscillations depend on three mixing parameters, of which two are large and have already been measured. The third one is called theta13 and is known to be smaller with an upper limit coming from a previous experiment at Chooz. The new Double Chooz detector is the first of a new generation of reactor neutrino experiments with the aim of measuring this fundamental parameter in neutrino physics which is a key area of particle physics research. The results will also have important consequences for the feasibility of future neutrino facilities which will aim for even more precise measurements.

Double Chooz consists of two identical detectors. The first one, at a distance of about 1km from the reactor cores, has now been filled and started to collect data. The number of neutrinos measured compared to the expected flux from the reactors will allow considerably improvement in the sensitivity for theta13 already in 2011. The second detector, located at a distance of 400 metres, will start operating in 2012. At this distance no significant transformation into another neutrino species is expected. By comparing the results from both detectors, theta13 can be determined with even higher precision.

Both detectors use an organic liquid scintillator, which was developed specifically for this experiment. The neutrino target in the core of the detector consists of 10 cubic metres of Gadolinium doped scintillator which can be used to tag neutrons from inverse beta decays which are induced by anti-neutrinos emitted by the reactors. The target is surrounded by three layers of other liquids in order to protect against other particles and to dampen environmental radioactivity. These liquids are contained in very thin vessels so as to minimize inactive volumes inside the detector. The target is observed by 390 immersed photomultipliers which convert the interactions into electrical signals. These signals are processed in a data acquisition system which can collect data over the next five years. The new detectors will ensure that neutrino physics will stay one the most fruitful areas of particle physics, as it has been for the past 50 years.

An essential contribution to the project was the development of the gadolinium-doped liquid scintillator by the researchers at the Max Planck Institute for Nuclear Physics in Heidelberg. Their task was to find, test, produce and purify a gadolinium compound which is solvable in an organic liquid and chemically stable for many years. In collaboration with their colleagues from Japan they checked the photomultipliers in a specially built test-bed. These central contributions will also play a crucial role for the interpretation and data analysis. Universities and research institutes from Brazil, England, France, Germany, Japan, Russia, Spain and USA comprise the Double Chooz collaboration.

(Photo: © Double Chooz Collaboration)

Max Planck Institute

OUT OF MIND IN A MATTER OF SECONDS

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The dynamics behind signal transmission in the brain are extremely chaotic. This conclusion has been reached by scientists from the Max Planck Institute for Dynamics and Self-Organization at the University of Göttingen and the Bernstein Center for Computational Neuroscience Göttingen. In addition, the Göttingen-based researchers calculated, for the first time, how quickly information stored in the activity patterns of the cerebral cortex neurons is discarded. At one bit per active neuron per second, the speed at which this information is forgotten is surprisingly high.

The brain codes information in the form of electrical pulses, known as spikes. Each of the brain’s approximately 100 billion interconnected neurons acts as both a receiver and transmitter: these bundle all incoming electrical pulses and, under certain circumstances, forward a pulse of their own to their neighbours. In this way, each piece of information processed by the brain generates its own activity pattern. This indicates which neuron sent an impulse to its neighbours: in other words, which neuron was active, and when. Therefore, the activity pattern is a kind of communication protocol that records the exchange of information between neurons.

How reliable is such a pattern? Do even minor changes in the neuronal communication produce a completely different pattern in the same way that a modification to a single contribution in a conversation could alter the message completely? Such behaviour is defined by scientists as chaotic. In this case, the dynamic processes in the brain could not be predicted for long. In addition, the information stored in the activity pattern would be gradually lost as a result of small errors. As opposed to this, so-called stable, that is non-chaotic, dynamics would be far less error-prone. The behaviour of individual neurons would then have little or no influence on the overall picture.

The new results obtained by the scientists in Göttingen have revealed that the processes in the cerebral cortex, the brain’s main switching centre, are extremely chaotic. The fact that the researchers used a realistic model of the neurons in their calculations for the first time was crucial. When a spike enters a neuron, an additional electric potential forms on its cell membrane. The neuron only becomes active when this potential exceeds a critical value. "This process is very important", says Fred Wolf, head of the Theoretical Neurophysics research group at the Max Planck Institute for Dynamics and Self-Organization. "This is the only way that the uncertainty as to when a neuron becomes active can be taken into account precisely in the calculations".

Older models described the neurons in a very simplified form and did not take into account exactly how and under what conditions a spike arises. "This gave rise to stable dynamics in some cases but non-stable dynamics in others", explains Michael Monteforte from the Max Planck Institute for Dynamics and Self-Organization, who is also a doctoral student at the Göttingen Graduate School for Neurosciences and Molecular Biosciences (GGNB). It was thus impossible to resolve the long-established disagreement as to whether the processes in the cerebral cortex are chaotic or not, using these models.

Thanks to their more differentiated approach, the Göttingen-based researchers were able to calculate, for the first time, how quickly an activity pattern is lost through tiny changes; in other words, how it is forgotten. Approximately one bit of information disappears per active neuron per second. "This extraordinarily high deletion rate came as a huge surprise to us", says Wolf. It appears that information is lost in the brain as quickly as it can be "delivered" from the senses.

This has fundamental consequences for our understanding of the neural code of the cerebral cortex. Due to the high deletion rate, information about sensory input signals can only be maintained for a few spikes. These new findings therefore indicate that the dynamics of the cerebral cortex are specifically tailored to the processing of brief snapshots of the outside world.

(Photo: © MPI for Dynamics and Selforganisation)

Max Planck Institute

HIDDEN IN PLAIN SIGHT

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The idea of being able to become invisible, especially by simply covering up a person or an object with a special cloak, has a perennial appeal in science-fiction and fantasy literature. In recent years, researchers have found ways to make very exotic “metamaterials” that can perform a very crude version of this trick, keeping an object from being detected by a certain specific frequency of radiation, such as microwaves, and only working at microscopic scales. But a system that works in ordinary visible light and for objects big enough to be seen with the naked eye has remained elusive.

Now, a team of researchers in the Singapore-MIT Alliance for Research and Technology (SMART) Centre has found a relatively simple, inexpensive system that can hide an object as big as a peppercorn from view in ordinary visible light. The team’s discovery has been published online in Physical Review Letters and will appear soon in the print version of the journal.

Unlike the other attempts to produce invisibility by constructing synthetic layered materials, the new method uses an ordinary, common mineral called calcite — a crystalline form of calcium carbonate, the main ingredient in seashells. “Very often, the obvious solution is just sitting there,” says MIT mechanical-engineering professor George Barbastathis, one of the new report’s co-authors.

The paper was co-authored by SMART postdoctoral fellow Baile Zhang, MIT postdoctoral fellow Yuan Luo, and SMART researcher Xiaogang Liu, and the research was funded by Singapore’s National Research Foundation (NRF) and the U.S. National Institutes of Health (NIH).

In the experiment reported in this paper, the system works in a very carefully controlled setting: The object to be hidden (a metal wedge in the experiment, or anything smaller than it) is placed on a flat, horizontal mirror, and a layer of calcite crystal — made up of two pieces with opposite crystal orientations, glued together — is placed on top of it. When illuminated by visible light and viewed from a certain direction, the object under the calcite layer “disappears,” and the observer sees the scene as if there was nothing at all on top of the mirror.

For their demonstration, they placed the MIT logo upside-down on the vertical wall behind the apparatus, placed so that one of the letters could be viewed directly via the mirror, while the other two were behind the area with a 2-millimeter-high wedge (the height of a peppercorn) and its concealing layer of calcite. Then, the whole setup was submerged in liquid. They showed that the logo appeared normal, as though there was no wedge but a flat mirror piece, when illuminated with visible green light. Any imperfection in the cloaking effect would have shown up as a misalignment of the letters, but there was no such anomaly; thus, the cloaking operation was proven. With blue or red illumination, the cloaking was still effective but with some slight misalignment.

Calcite has long been known to have unique optical properties, including the ability to bend (or refract) a ray of light differently depending on the light’s polarization (the orientation of its electric field); these properties can cause the phenomenon of double refraction, or seeing “doubles” when looking through calcite with regular unpolarized light. In this research, the two pieces of calcite were oriented to bend the light in such a way that the emerging beam, after going through multiple reflections and refractions, appeared to be coming directly from the original mirror at the base of the setup, rather than from the actual higher point above the hidden object. The total optical path was also preserved, which means no scientific optical instrument can possibly uncover the cloaked wedge.

Submerging the apparatus in a liquid with a carefully chosen degree of refraction preserves the illusion. Barbastathis says the setup would work without the liquid, but the light’s transition into air would cause some blurring that would make the effect less convincing.

In principle, Barbastathis says, the same method could be used in real-life situations to conceal an object from view — and the only limitation on the size of the hidden object is the size of the calcite crystal that’s available. The team paid about $1,000 for the small crystal it used, he says, but much larger ones could be used to conceal much larger objects. (The largest known natural crystal of calcite measures 7 by 7 meters, or more than 21 feet across).

For now, the system is essentially two-dimensional, limiting the cloaking effect to a narrow range of angles; outside these angles, the cloaked object is quite visible. “We do have some ideas for how to make it fully three-dimensional,” says Barbastathis, the Singapore Research Professor of Optics and Professor of Mechanical Engineering. In addition, the team would like to eliminate the need for immersing the system in liquid and make it work in air.

Aside from its obvious potential applications in defense or law enforcement, the ability to render something invisible could have uses in research, Barbastathis suggests, such as providing a way to monitor animal behavior without any visible distraction. “The important thing is that now this is out in the open, people will start to think about” how it might be used.

Coincidentally, another independent research team, based at the University of Birmingham in the UK, has also published a paper this month describing a similar method for achieving a visible-light invisibility cloak using calcite.

The MIT and Birmingham results “are two beautiful experiments. I particularly like their simplicity,” says Ulf Leonhardt, chair in theoretical physics at Scotland’s University of St. Andrews, who was the author of one of the first papers that described a metamaterial-based cloaking system. “Cloaking has been inspired by research on metamaterials,” he adds, “but, ironically, these cloaking devices are almost ‘home-made.’ Instead of sophisticated optical metamaterials that are difficult to make and have many problems of their own, they use simple calcite crystals.”

Compared to the earlier versions of cloaking systems that only worked for microscopic objects, and only when viewed using radio or infrared wavelengths, the new approach is “closer to science fiction,” Barbastathis says. “Science is usually a bit disappointing when you compare it to sci-fi,” he says, but in this case “we came one step closer” to the imaginative vision.

(Photo: MIT)

Massachusetts Institute of Technology

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