A European-led mission to Mars is feared to have been lost after it failed to communicate that it had successfully landed on the red planet. The lander – called the Entry, Descent and Landing Demonstrator Module (EDM) – is part of the Trace Gas Orbiter (TGO) mission that arrived at Mars only a few days ago, following its launch earlier this year by the European Space Agency (ESA). Both missions are a collaboration between ESA and the Russian space agency Roscosmos. Given that the probe has only enough energy stored in its battery to last up to 10 days, scientists are now in a race against time to figure out what could have gone wrong as the probe descended through the martian atmosphere.
The EDM, which is also known as Schiaparelli, successfully separated from the TGO on 16 October and took around three days to reach the surface of Mars. Yesterday, it entered the planet’s atmosphere at about 21,000 km/h and was supposed to decelerate using aerobraking before deploying a parachute. However, at some point during descent, ESA lost the signal from Schiaparelli that it was monitoring via the Giant Metrewave Radio Telescope near Pune, India.
It is currently unknown whether Schiaparelli fired a thruster to brake just before landing on the surface. During landing, the 577 kg probe was supposed to have taken images of the surface of Mars as well as other data such as pressure and temperature. In the coming days, scientists will listen to possible signals from the lander through orbiting probes such as ESA’s Mars Express mission and NASA’s Mars Reconnaissance Orbiter. “It is clear that these are not good signs,” Paolo Ferri, ESA’s head of mission operations told reporters at a press briefing yesterday.
Towards ExoMars
Yet there was some good news for the ESA team. The TGO has successfully entered into a highly elliptical orbit around Mars. In January 2017 ESA scientists will manoeuvre the TGO into a more circular orbit with an altitude of 400 km. Its four instruments include spectrometers, high-resolution cameras and a neutron detector, which will map Mars for sources of methane and also chart hydrogen below Mars’s surface up to a depth of around 1 m. After a calibration period, the TGO is expected to start operations in December 2017, and will then operate for five years.
It is currently unclear what the possible failure of the EDM will mean for the next part of the joint mission between ESA and Roscosmos. The ExoMars rover, which is due to launch in 2020, will carry a drill and a suite of instruments dedicated to exobiology and geochemistry research, searching for possible signs of life, characterizing the water and geochemical distribution of the surface, and identifying any hazards for future manned missions to the planet. ExoMars is expected to use the same landing techniques as the EDM.
Zahid Hasan of Princeton University in the US is the leader of a group that recently found the first clear evidence for the existence of Weyl fermions – massless particles predicted in 1929 as a solution of the Dirac equation. In this video, Hasan explains why semimetals containing Weyl fermions could be ideal materials for examining a variety of physical theories. He explains that, for instance, these so-called “Weyl semimetals” could be used to recreate the environment of the early universe and explore how the Higgs boson can imbue particles with mass.
This video is part of our 100 Second Science series, in which researchers give concise presentations covering the spectrum of physics.
Two physicists are among the winners of the 2017 L’Oréal-UNESCO For Women in Science Award. The Australian quantum-physicist Michelle Simmons has won “for her pioneering contributions to quantum and atomic electronics, constructing atomic transistors en route to quantum computers”. An award has also been given to the Chilean astrophysicist María Teresa Ruiz, “for her discovery of the first brown dwarf and her seminal work on understanding the faintest stars, including stars at the final stages of their evolution (white dwarfs)”. The award is presented to five female researchers annually, with each winner receiving €100,000. Awards are made on a regional basis, with Ruiz being the laureate for Latin America and Simmons being the Asia-Pacific laureate. Simmons is director of the ARC Centre of Excellence for Quantum Computation and Communication Technology based at the University of New South Wales. Ruiz is director of the Center for Excellence in Astrophysics and Associated Technologies, which is based at the University of Chile.
Graphene protects glass from corrosion
Cloudy wine glasses could be a thing of the past, thanks to researchers at the Institute for Basic Science in Daejeon, Korea. Rodney Ruoff and colleagues have developed a graphene coating that protects glass from the corrosion and weakening that occurs when hydrogen ions from water penetrate the glass surface, causing its silicate structure to dissolve. Graphene could be the ideal coating to prevent glass corrosion because it is extremely thin – just one atom thick – chemically inert, transparent to light and also very tough. Ruoff and colleagues tested this hypothesis by growing sheets of graphene on a copper substrate and then transferring it to both sides of pieces of glass. After 120 days of immersion in hot water, the graphene-coated glass samples suffered no change in fracture strength and surface roughness. In comparison, uncoated samples underwent significant corrosion. “In the future, when it is possible to produce larger and yet higher-quality graphene sheets and to optimize the transfer on glass,” says Ruof, adding “it seems reasonably likely that graphene coating on glass will be used on an industrial scale.” The work is described in Nano Letters.
Humanitarian mapping programme celebrates 15 years at CERN
Disaster zone: members of UNOSAT work on satellite images of Haiti in October 2016 to assess the damage of Hurricane Matthew. (Courtesy: Maximilien Brice/CERN)
The United Nations satellite-mapping programme UNOSAT is celebrating its 15th anniversary at CERN in Switzerland. UNOSAT uses CERN’s powerful computer infrastructure to produce extremely precise maps of parts of the world that are experiencing humanitarian crises such as wars and natural disasters – including the recent Hurricane Matthew in Haiti. High-resolution satellite images from public and private sources are stored on CERN’s computer servers. The UNOSAT team then uses the Worldwide LHC Computing Grid to transform the data into maps that are relevant to governments and organizations providing assistance in affected areas. “CERN’s support is essential,” says UNOSAT manager Einar Bjørgo. “Without its powerful IT infrastructure, we wouldn’t be able to compile the satellite data we receive to make it usable.” UNOSAT has also “developed a smartphone application called UN-ASIGN, which allows people to take photos, geo-locate them and share them with UNOSAT”, says Bjørgo.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a mission to Mars
Consciousness appears to arise naturally as a result of a brain maximizing its information content. So says a group of scientists in Canada and France, which has studied how the electrical activity in people’s brains varies according to individuals’ conscious states. The researchers find that normal waking states are associated with maximum values of what they call a brain’s “entropy”.
Statistical mechanics is very good at explaining the macroscopic thermodynamic properties of physical systems in terms of the behaviour of those systems’ microscopic constituent particles. Emboldened by this success, physicists have increasingly been trying to do a similar thing with the brain: namely, using statistical mechanics to model networks of neurons. Key to this has been the study of synchronization – how the electrical activity of one set of neurons can oscillate in phase with that of another set. Synchronization in turn implies that those sets of neurons are physically tied to one another, just as oscillating physical systems, such as pendulums, become synchronized when they are connected together.
The latest work stems from the observation that consciousness, or at least the proper functioning of brains, is associated not with high or even low degrees of synchronicity between neurons but by middling amounts. Jose Luis Perez Velazquez, a biochemist at the University of Toronto, and colleagues hypothesized that what is maximized during consciousness is not connectivity itself but the number of different ways that a certain degree of connectivity can be achieved.
Many ways of connecting
Perez Velazquez’s colleague Ramon Guevarra Erra, a physicist at the Paris Descartes University, points out that there is only one way to connect each set of neurons in a network with every other set, just as there is only one way to have no connections at all. In contrast, he notes, there are many different ways that an intermediate medium-sized number of connections can be arranged.
To put their hypothesis to the test, the researchers used data previously collected by Perez Velazquez showing electric- and magnetic-field emissions from the brains of nine people, seven of whom suffered from epilepsy. With emissions recorded at dozens of places across the subjects’ scalps, the researchers analysed every possible pairing of these data “channels” to establish whether the emissions in each case were in phase with one another. They added up the number of synchronized pairs and plugged that figure along with the total number of all possible pairings into a fairly straightforward statistical formula to work out how many different brain configurations that level of synchronicity yields. They then took the logarithm of that number to establish the brain’s entropy.
The data were analysed in two parts. In one, they compared the emissions from four of the epileptic patients when undergoing a seizure and when in a normal “alert” state. In the second, they compared emissions from the other five individuals when sleeping and when awake. In both cases, the bottom line was the same: subjects’ brains display higher entropy, or a higher value of a similar quantity known as Lempel–Ziv (LZ) complexity, when in a fully conscious state.
Varying results
Guevarra Erra admits that the results are not watertight. Indeed, the LZ complexity of one of the four epileptic patients in the first analysis showed no change between seizure and alert states (although that person did remain conscious during part of the seizure). In another individual, LZ complexity actually increased in the second analysis while that person was asleep. Guevarra Erra says that he and his colleagues didn’t carry out a statistical analysis of their results in part because of the “very heterogeneous” nature of those results. But he nevertheless remains “highly confident” that the correlations they have identified are real, particularly, he argues, because they were seen in “two very different sets of data”.
Peter McClintock, a physicist who works on nonlinear dynamics at Lancaster University in the UK, describes the research as “intriguing” but says that the consciousness–entropy correlation should be confirmed using a larger number of subjects. He also suggests investigating “what happens in other brain states where consciousness is altered”, such as anaesthesia.
Emergent property
Perez Velazquez and colleagues argue that consciousness could simply be an “emergent property” of a system – the brain – that seeks to maximize information exchange and therefore entropy, since doing so aids the survival of the brain’s bearer by allowing them to better model their environment. On the question of entropy, however, Guevarra Erra is cautious. He says that personally he would like to have a better understanding of the physical processes taking place in the brain before employing the label “entropy”, explaining that Perez Velazquez was keen to use the term in their paper. One option, he says, would be to carry out fresh experiments that measure thermodynamic quantities in subjects’ brains. He notes, for example, that magnetic resonance imaging can be used to measure oxygenation, which is directly related to metabolism and therefore to the generation of heat.
Guevarra Erra adds that he would like to extend their investigations beyond the hospital to cover more subtle but general cognitive behaviour. The idea would be to monitor a person’s changing brain activity as they focus on carrying out a specific task, such as discriminating between musical tones or trying to find their way round a labyrinth. This, he says, should help to establish whether varying “entropy” correlates with degree of awareness as well as simply with the presence or absence of consciousness.
A paper describing the work will be published in Physical Review E and is also available on arXiv.
Artificial spheres that resemble living cells have been made from proteins by physicists at the University of Saarland in Germany. The structures could someday be used to encapsulate and transport drugs to targeted locations in the body and were created by Karin Jacobs and colleagues. The spheres are made from hydrophobin molecules, which are naturally occurring stringy proteins that are used by fungi to create waterproof coatings. After noticing that the molecules tend to form clumps when in solution, the team used criss-crossing streams of oil and water to push the clumps together to form tiny hollow spheres with walls made from a double-layer of the proteins (see figure). The team was able to “inflate” the spheres by boosting the water pressure inside. The researchers were also able to create ion channels in the walls of the spheres, which could be used to simulate how living cells exchange ions with their surroundings. The physicists are hopeful that they could even make artificial vesicles using their technique, which is described in Advanced Materials.
Shape-shifting seen in zirconium nuclei
One fascinating aspect of some atomic nuclei is that they adopt non-spherical shapes in their lowest energy states. This phenomenon is known as shape coexistence and has proven very difficult to describe theoretically. Now, Tomoaki Togashi and colleagues at the University of Tokyo have performed computer simulations of zirconium nuclei containing 50–70 neutrons. These suggest that the nucleus is a sphere when it has 52–56 neutrons and then undergoes a transition to a deformed shape for a higher number of neutrons – something that is backed up by experimental evidence. The simulations also suggest that the transition is a quantum phase transition, say the physicist writing in Physical Review Letters. The calculations also predict that zirconium nuclei in the transition region around 56 neutrons can coexist in spherical and deformed shapes that differ very slightly in terms of energy. To study this further, Togashi and colleagues joined forces with an international team working on the S-DALINAC accelerator in Darmstadt, Germany. There, they found evidence for this shape coexistence in zircon-96, which has 56 neutrons. The experimental work is also reported in Physical Review Letters.
Silicon-qubit lifetime boosted by factor of 10
Scanning electron microscope image of a spin qubit. Highlighted are the positions of the tuning gates (red), the microwave antenna (blue), and the single electron transistor used for spin readout (yellow). (Courtesy: Guilherme Tosi, Arne Laucht/UNSW)
The lifetime of a quantum bit (qubit) of information stored in the spin of an electron in silicon has been increased by a factor of 10 by Andrea Morello and colleagues at the University of New South Wales in Australia. Silicon could be useful for creating quantum computers because electronic devices based on the semiconductor can be made with great precision. Quantum information can be stored in the spins of electrons in silicon, but several challenges must first be overcome before practical quantum devices can be made. One problem is that the spins interact with their environment and lose their quantum information in a process called decoherence. In this latest work, Morello’s team worked with the spins of individual phosphorous atoms that were implanted in silicon. Previous studies have shown that these spins can store quantum information for about 200 μs in the presence of a magnetic field. Writing in Nature Nanotechnology, the team describes how it has used an applied microwave signal to boost the lifetime of the spin qubits to 2.4 ms. The microwaves cause the spins to oscillate at a specific frequency, making them much more robust to interference. An added benefit of the new spin qubit is that it can be controlled using microwave signals – unlike the team’s previous spin qubits.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on the entropy of consciousness.
A new spectrometer-on-a-chip that employs two frequency combs has been unveiled by physicists at Caltech in the US. The precision spectroscopy system is based on pulsed lasers and uses a technique known as dual-comb spectroscopy. This makes it one thousand times more precise and nearly one million times faster than the standard instruments used today. Based on a millimetre-sized silicon chip, the device is an important step towards the creation of portable devices capable of the real-time and precise characterization of the chemical composition of biological or environmental samples. Experts say such devices could have a number of medical and military applications.
Optical spectroscopy is a powerful tool that is used in a wide range of sciences – including astrophysics, biology and chemistry – to determine the chemical composition of unknown specimens. It uses the fact that the frequencies at which a substance will absorb light, known as its absorption spectrum, can serve as a “fingerprint” that reveals which atoms and molecules are in the substance. The accuracy of these chemical fingerprints depends on how precisely a spectrometer can resolve these absorption frequencies from another.
The new spectrometer-on-a-chip was created by Kerry Vahala and colleagues, and it offers a resolution that is about a thousand times better than a conventional grating spectrometer. “Spectroscopists, if you can give them more resolution – they will always take it,” Vahala says.
Wideband pulses
The dual-comb spectroscopy technique was first demonstrated about ten years ago and some implementations have already achieved resolutions 10 million times better than conventional spectrometers. It uses two lasers known as frequency combs that each emit femtosecond-long pulses. Unlike standard lasers that emit in a narrow frequency band, frequency combs have a wide frequency spectrum consisting of many hundreds or thousands of narrow, equally spaced peaks resembling the teeth of a comb. This means the comb is capable of investigating multiple absorption lines simultaneously.
One comb is tuned such that the spacing between its teeth is slightly greater than the spacing in the other comb. The light from the first comb illuminates the material, which absorbs specific wavelengths depending on its chemical composition. When the resulting light is mixed with the second comb’s light, the output includes a radio-frequency envelope signal equal to the frequency difference between the two combs. This envelope is a beat frequency similar to that produced by two guitar strings slightly out of tune with another.
One cycle of this microsecond-scale envelope contains information about the entire absorption spectrum of the sample. Thus, by electronically processing one cycle of this signal, the spectrum can be produced in just microseconds. In contrast, spectrometers that use a diffraction grating take about one minute to acquire the same information.
Race to miniaturize
Although table-sized frequency combs have existed for more than a decade, the effort to shrink these systems to chip-scale has intensified in the past two years. “There’s been a kind of a race to make these things,” says Vahala, whose group is one of a handful in the world that have successfully made millimetre-sized combs.
The chip they produced houses two frequency combs made from two glass rings 3 mm in diameter, known as microresonators. A different laser directs light into the cavity of each glass ring. Each ring amplifies its light to create pulses known as solitons that make up the frequency comb. The team verified the accuracy of the chip by measuring the spectrum of hydrogen cyanide, which has absorption lines that match the frequency range generated by the two combs. Vahala says that the group is working on expanding the number of frequencies their tiny combs can generate.
Research funding agencies like the Defense Advanced Research Projects Agency in the US have been “investing heavily” in these high-precision spectroscopy techniques, according to Peter Delfyett, a frequency comb expert at the University of Central Florida who was not involved in developing the dual-comb chip. Chip-based technologies are of particular interest because miniature systems could be useful in a variety of different tasks. A chip could be installed on a drone for remote environmental monitoring, for example, or used in a breathalyzer to diagnose illness. They could even lead to applications that “we don’t even know about yet,” Delfyett says.
While technical details of the spectrometer-on-a-chip need to be improved, Delfyett predicts that this technology will be ripe for commercialization in less than a decade. “I’m very encouraged by the tremendous amount of effort the scientific community is putting into miniaturizing these comb sources,” he says.
The basics of how a frequency comb works are explained in this video of Paul Williams of the National Institute of Standards and Technology: “What is a frequency comb?”.
Astrophysicists in the UK have created the largest ever map of voids and superclusters in the universe, which they say helps solve a long-standing cosmological mystery. The team, based at the University of Portsmouth, have mapped the positions of cosmic voids – large empty spaces that contain relatively few galaxies – and superclusters – huge regions with many more galaxies than normal. Lead author Seshadri Nadathur says their new technique allowed them to “make a very precise measurement of the effect that these structures have on photons from the cosmic microwave background (CMB)” as they pass through the structures. According to Nadathur, the photons are affected by the “stretching effect of dark energy”, which causes tiny changes in the temperature of CMB light depending on where it came from. Indeed, photons travelling through voids should appear slightly colder than normal and those navigating through superclusters should be hotter. This effect – known as the integrated Sachs–Wolfe (ISW) effect – has been previously studied, but early maps of the supercluster and void structure seemed to suggest that the effect was five times greater than the predicted value. The Portsmouth team’s new dataset – which used nearly a million galaxies from the Sloan Digital Sky Survey – is much larger. They also created a new statistical technique to be able to measure the ISW effect on the CMB data, as the effect was negligible. This allowed the team to make a very precise measurement of the ISW effect, and they found that the new result agreed extremely well with predictions using Einstein’s theory of general relativity. The work is published in The Astrophysical Journal Letters.
Plasma pressure record set for fusion
Plasma pressure: inside the Alcator C-Mod tokamak at MIT. (Courtesy: MIT)
A new world record for the highest plasma pressure created within a tokamak fusion reactor has been set by Earl Marmar and colleagues working on the Alcator C-Mod facility at the Massachusetts Institute of Technology (MIT). Achieving a high plasma pressure is crucial to the development of practical fusion reactors because the amount of power output by a reactor scales as the square of the plasma pressure. The MIT team achieved a plasma pressure of 2.05 atm within a space of 1 m3 – smashing the previous record of 1.77 atm, which was set by Alcator C-Mod in 2005. The plasma temperature reached more than 35 million Kelvin, which is twice as hot as the centre of the Sun. The plasma produced 300 trillion fusion reactions per second, but much more work must be done before fusion reactors become a viable source of clean energy.
Physicists predict rogue ocean waves
Sisters and rogues: numerical simulations of how waves can interfere in the ocean to produce rogues. The top left shows a normal sea state and the top right a rogue hole. Bottom left is a rogue wave and bottom right is a rogue wave group, also known as the ‘three sisters’. (Courtesy MBI)
Mariners have long known that certain parts of the sea are prone to extremely high rogue waves, which are more than twice the size of the surrounding waves. Now, physicists in Germany have made some progress towards predicting when these rare and dangerous wave will occur. Günter Steinmeyer at the Max Born Institute in Berlin and colleagues from the Leibniz University in Hannover and the Technical University in Dortmund have developed a new way of measuring the number of waves that interfere at specific locations in the ocean, and have shown that it can be used to alert mariners when sea conditions are right for the emergence of rogue waves. Measuring the “phase space dimension” metric can be done on board a ship, so it could provide an early warning of dangerous seas. In the future, the metric could be combined with meteorological data to provide forecasts of rogue conditions.
You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on a new frequency comb sensor.
It’s not often that a police officer greets you when you enter a physics laboratory. But then the Center for Neutron Research (CNR) at the National Institute of Standards and Technology (NIST) in the US is no ordinary lab. Nestled amid the green lawns of NIST’s Gaithersburg campus just north of Washington, DC, it houses a 20 MW nuclear reactor – powered by highly enriched uranium – that first went critical in 1967. For added safety, the centre is surrounded by large boulders to prevent unauthorized vehicles from driving up too close.
Once you are past security though, the CNR looks and feels like any ordinary neutron-physics facility. Neutrons from the reactor are used by more than 2000 researchers from university, business and government for almost 200 days each year. Seven of the 28 experimental stations are diffractometers and spectrometers that use “thermal” neutrons that have been slowed by the reactor’s heavy water to energies of 15–20 meV. Researchers scatter these neutrons off materials to reveal their properties, to develop radiation detectors and to maintain dosimetry standards.
The other 21 instruments at the CNR are different. Located in a separate experimental hall, they sit at the end of guide tubes some tens of metres long. The neutrons that enter these tubes from the reactor are “cold”, having been slowed to energies of 5 meV or less after passing through a liquid hydrogen moderator. The inside walls of the guide tubes are coated with nickel, which has the highest total external reflection of any element. Cold neutrons that graze the walls at shallow angles therefore do not get absorbed but instead bounce off the inner surfaces, like stones skipping over a lake. Researchers use the cold neutrons for studying everything from polymers and proteins to magnets and non-Newtonian fluids.
One fascinating new project using cold neutrons at NIST involves building what the lab claims is the world’s most advanced “neutron microscope”. Like a medical X-ray of the human body, the intensity of the neutron “shadow” of an object provides information about its internal make-up. Water, for example, blocks neutrons whereas lead, aluminium and other dense materials let them through. So by comparing, say, the neutron image of a fuel cell containing water with images of the same object as it dries, researchers could study how water diffuses as the cell’s electrodes degrade. Other applications might include imaging lithium batteries, studying fluid flow in rocks or even analysing works of art.
Beyond the pinhole
NIST is not the only lab creating a neutron microscope, but its approach has certain advantages. Researchers at the Paul Scherrer Institute in Switzerland, for example, are building a neutron microscope by creating a detector with very high resolution. But to reach the resolution, incoming neutrons have to be sent through a tiny pinhole aperture to form a collimated beam that shines on a sample. The detector records the transmitted neutrons, which can then be analysed to yield information on the properties of the sample.
The problem is that in creating a collimated beam, few neutrons pass through the aperture in a given period, meaning that it can take a long time to image a sample. To take images faster or to improve the spatial resolution, your options are to either improve the detector, place the sample up close to the detector or use very thin samples. “But if you’re forced to have thin samples, you lose all the advantages of neutrons over X-rays,” says Dan Hussey, who leads NIST’s neutron-microscope project. “Even with better detectors, in a 1 μm pixel with a typical flux of 106 cm–2 s–1, there’s only one neutron every 100 seconds.”
In focus Consisting of mirrors nested together like an onion, a Wolter lens (top right) is the key part of NIST’s planned neutron microscope. Neutrons that hit the lens at a shallow angle bounce off and can be focused onto a detector (blue lines), while neutrons that hit at a steep angle are lost or get stopped by an aperture block (dashed blue lines). (Courtesy: Sean Kelley/NIST PML)
The neutron microscope at NIST instead does away with pinholes and uses a neutron lens. That might seem an odd approach given that neutrons have no charge and interact weakly with matter, meaning they can’t easily be focused into beams. NIST, however, is using a technique first proposed by German physicist Hans Wolter in 1952 to focus X-rays, which – like neutrons – are also hard to shepherd with conventional optics. “Wolter lenses” have been successfully used on various space-based X-ray missions, including NASA’s Chandra telescope and the European Space Agency’s XMM-Newton telescope.
In the NIST neutron microscope, neutrons from the lab’s reactor shine directly on a sample before passing through a Wolter lens and onto a detector. The barrel-shaped lens, which is roughly 20 cm long and 13 cm in diameter, consists of 10 nested parabolic mirrors each made of a 0.5 mm layer of highly polished nickel (see figure, right). Neutrons that hit a nickel cone at a shallow enough angle bounce off the inside and can be focused onto the sample. The neutrons then pass to a detector containing a 20 μm thick layer of gadolinium oxysulphide. Light given off when a neutron hits this material is recorded by a CMOS camera and fed to a computer for analysis.
Our target is to increase the beam intensity by a factor of 100 and achieve a spatial resolution of 1 μm
Dan Hussey
“The Wolter lenses mean that many more neutrons strike the sample than if there was a pinhole because you no longer need to collimate the beam,” says Hussey, who is developing the lenses with teams led by Mikhail Gubarev from NASA’s Marshall Space Flight Centre and Boris Khaykovich at the Massachusetts Institute of Technology. “Our target is to increase the beam intensity by a factor of 100 and achieve a spatial resolution of 1 μm.” Currently NIST’s best effort using conventional pinhole optics is just 15 μm.
Forging ahead
The 10 mirrors, which range in radius from 55 to 68 mm, are being made at NASA’s Marshall lab by electroplating nickel onto a specially shaped and polished aluminium block, and then peeling the metal off after it’s been cooled in an ice bath. NASA is still fine-tuning the manufacture process, which is not easy. Gravity makes the foils sag by about 80 nm and so they have to be carefully adjusted in the Wolter lens to avoid losing resolution. “However, the beauty of the method is that the mirrors can be replicated from substrates so if any other neutron lab had similar needs they could use the same substrates to create their own neutron microscope,” says Hussey.
The other advantage of using Wolter lenses is that the sample can be placed a long way from the detector – 7 m in the case of NIST – allowing researchers to obtain magnified images of the sample, with Hussey hoping for magnifications of at least ×10. With a pinhole camera, in contrast, the sample has to be stationed right up close to the detector to maximize neutron flux, meaning it is hardly magnified at all. When the Wolter lens is complete in 2018, Hussey says that rather than taking 20 minutes to capture an image – as with pinhole optics – it will take just 20 seconds or less.
NIST’s plan is for the neutron microscope to become a fully functional user facility in 2018. The lab also wants to improve the system to allow neutron phase imaging, which uses variations in the phase of the signal – not its attenuation – to determine the details of a material’s structure. As Hussey and colleagues put it in a paper demonstrating the principle of the Wolter lens for neutron microscopy in 2013 (Appl. Phys. Lett.102 183508), their work could allow “game-changing improvements in the neutron-imaging technique”.
Sounds of Earth: An original golden record. (Courtesy: NASA)
By Hamish Johnston
An online initiative to reissue Carl Sagan’s golden record, which was attached to NASA’s Voyager 1 and 2 craft, has so far raised a whopping $1.1m, smashing its $198,000 goal. The campaign was created in September by David Pescovitz, editor and managing partner at the technology news site Boing Boing, after teaming up with Timothy Daly from Amoeba Music in the US, who was the original producer of the record, as well as US graphic designer Lawrence Azerrad. The original LP, which was created in 1977, contains sounds of the Earth along with recorded greetings and a mix of music, and has been unobtainable for decades, having been available only on CD-ROM in the early 1990s. Now that the cash has been raised, the golden record will be released next year as an LP to mark the 40th anniversary of the Voyager launches. So how much will it set you back? It’s yours for only $98, what a bargain.
“Movies” of electrons as they move across a semiconductor junction have been made by researchers at the Okinawa Institute of Science and Technology in Japan using a new imaging technique. Combining photoemission electron microscopy with femtosecond laser pump-probe methods, the technique tracks the motion of electrons on timescales shorter than 1 ps. The researchers say it could provide a better understanding of how semiconductor devices work and lead to more efficient solar cells.
Solar cells – along with diodes, transistors and other semiconductor devices – rely on the flow of electrons across “heterojunctions” between two different types of semiconductor. While this motion is crucial to just about every modern technology, it is not easy to image in real time. The problem is that pulsed laser techniques – which can measure the energy of electrons on very short time scales – do not offer high enough spatial resolution to track the electrons. On the other hand, electron microscopy techniques offer high spatial resolution, but cannot keep up with the fast-moving electrons.
Best of both
Now Keshav Dani, Michael Man and colleagues have unveiled a new technique that combines high spatial resolution with high temporal resolution. Their method uses the well-established “pump-probe” technique involving laser pulses that are only 200 fs in length. It involves first firing a relatively intense probe pulse at a sample in the form of an indium selenide/gallium arsenide heterojunction that functions as a solar cell. The probe pulse plays the role of sunlight, putting electrons into excited energy states from which electrical energy can be extracted.
About 1 ps after the pump pulse, an image of the excited electrons is taken by firing a less intense probe pulse at the heterojunction. This ejects some electrons from the semiconductor, carrying with them information about the excited electrons in the semiconductor. These ejected electrons are captured and studied using spectroscopic photoemission electron microscopy, which creates an image of the electrons in the semiconductor.
Variable delay
By varying the delay between the pump and probe pulses, the team acquired a series of images of the excited electrons with a spatial resolution of about 250 nm and a temporal resolution of less than 1 ps.
This is a new door to understanding the motion of electrons in semiconductor materials
Keshav Dani, Okinawa Institute of Science and Technology
“We have made a video of a very fundamental process: for the first time we are not imagining what is happening inside a solar cell, we are actually seeing it,” explains Dani. “This is a new door to understanding the motion of electrons in semiconductor materials”.
When creating a solar cell, it is important that the electron excited states endure for long enough for energy to be extracted from the semiconductor. The images revealed that the lifetimes of excited electrons varied throughout the sample, with thinner regions of the semiconductor sample having faster decay rates. The images also suggested that electrons get trapped in thicker regions of the sample – something that could affect the performance of solar cells.
