Proton pulses accelerate electrons to 2 GeV ???

Protons AWAKE: a 360° view of the AWAKE experiment at CERN. (Courtesy: Maximilien Brice/Julien Marius Ordan)

A beam of protons has been used for plasma wakefield acceleration for the first time, driving electrons to energies of 2 GeV over a distance of just 10 m. The technique was developed by CERN’s AWAKE collaboration and is still preliminary, but it could potentially accelerate fundamental particles to very high energies.

CERN’s Large Hadron Collider (LHC) accelerates protons to 6.5 TeV before smashing them together at a combined energy of 13 TeV. Protons are relatively heavy and comprise three quarks, which means that the collisions produce huge quantities of particles that must be detected and analysed.

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While sifting through the debris of these collisions has led to important discoveries – including the Higgs boson – it is a complicated and data intensive process. As a result, some particle physicists have proposed that the next big collider after the LHC should use lighter, fundamental particles such as electrons and positrons. This would result in much “cleaner” collisions that produce far fewer particles.

The problem is that circular accelerators like the LHC are ill-suited to colliding light fundamental particles. Accelerating charged particles in curved paths causes them to emit synchrotron radiation, and light particles lose much more energy in this process than heavier ones. Therefore, most designs for fundamental particle colliders are linear. The International Linear Collider – proposed for construction in Japan – would need to accelerate electrons for over 11 km to reach 0.25 TeV.

Plasma surfing

Plasma wakefield acceleration offers a very differ way of accelerating electrons over much shorter distances. An intense pulse of particles or laser light is fired into a plasma, separating electrons from ions to create a huge electric field that propagates like a ship’s wake (the wakefield) behind the pulse.  If electrons are injected at precisely the right time, they can surf this wave and be accelerated to very high energies over relatively short distances.

Much as larger wakes can be created by larger ships, larger wakefields can be created in plasmas by using more energetic pulses. Previous experimental demonstrations of plasma wakefield acceleration have used either laser pulses or electron bunches to create the necessary wakefields. Unfortunately, the maximum energy that can be packed into a single laser pulse, for example, is around 1 J, which means complex, multi-stage accelerators would be required to accelerate electrons to the highest energies.

Protons, however, are relatively easy to accelerate and in 2009, Allen Caldwell of the Max Planck Institute for Physics in Munich and colleagues proposed that a 100 micron-long proton bunch could accelerate electrons to over 0.5 TeV in less than 500 m. There was one problem with this scheme – 100 micron-long, ultradense proton bunches do not yet exist.

Self-modulating bunches

The bunches from CERN’s Super Proton Synchrotron used by AWAKE are around 10 cm long so the team first fire a bunch into a plasma, which causes it to “self-modulate” into a series of shorter bunches. “These small bunches are shorter and denser,” explains AWAKE member Matthew Wing of University College London. “Their electric fields are completely in phase, so they constructively interfere to drive stronger and stronger wakefields."

Plasma physicist Sébastian Corde of Laboratoire d’Optique Appliquée in France is impressed: “This solves a lot of potential issues that we have in plasma acceleration,” he says. He cautions, however, that much work remains: “For every 2600 electrons injected, only one gets trapped into the plasma wave and accelerated…Clearly that’s something they’ll have to work on.”By injecting electrons near the back of the bunch, the researchers accelerated them to 2 GeV in just 10 m of plasma. “Our theoretical colleagues have shown that, if you take the LHC bunch as it is right now, you could accelerate electrons to roughly 1 TeV in just over 1 km and get to 6 TeV in about 8-10 km,” says Wing. “We need to do further R&D to demonstrate that it’s possible to get up to those high energies with excellent beam qualities but hopefully this will kick start people to think how this could be incorporated into a design for a future collider.”

Wim Leemans, director of the Berkeley Lab Laser Accelerator Center in California, adds “Having been in [plasma wakefield acceleration] for over three decades, I find it very important that CERN – the major high-energy physics lab in the world – has started investing in this technology”.

The research is described in Nature.

Beating Braess’ paradox to prevent instability in electrical power grids

Extra capacity: secondary frequency control could be the key to avoiding Braess’ paradox in electrical power grids. (Courtesy: iStock/Chalabala)

Researchers have calculated that Braess’ paradox – whereby adding transmission capacity to a network can degrade the network’s performance – can be avoided in electrical power grids by implementing the appropriate secondary frequency control. If the result can be demonstrated in real networks, it could help engineers build resilient networks that are able to integrate new sources of energy.

It seems reasonable to expect that adding new transmission lines to a power grid will improve its performance. However, in the 1960s the German mathematician Dietrich Braess showed that adding roads into some traffic networks actually increases congestion – an effect dubbed Braess’ paradox. Since then scientists and engineers have shown that the paradox can also apply to similarly interconnected nonlinear, dynamical general electricity grids. These grids are essential for modern life – and are constantly evolving, particularly with increases in renewable energy generation – so understanding the implications of Braess’ paradox is essential

Now scientists in Spain and Germany have joined forces to gain a better understanding of how to mitigate the effects of Braess’ paradox in electricity networks. Benjamin Schäfer and colleagues at the Technical University Dresden, brought with them expertise in Braess’ paradox, whereas Eder Batista Tchawou Tchuisseu and colleagues at the Institute for Cross-Disciplinary Physics and Complex Systems in Mallorca contributed their expertise in controlling electric network failure.

Controlling frequency

Electrical power grids operate in alternating current (AC) mode and all generators in the grid operate at the same frequency (50 Hz in Europe) and are synchronized across the network.

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“Frequency stores information about the grid, telling you something about the balance of the grid,” explains Schäfer. “So, if frequency starts to drop this typically indicates that there is a shortage of supply,” he adds. “Think of a generator rotating at given frequency, if you draw energy out of the system it is taken from the rotating energy of this rotor, effectively slowing the generator down and causing grid frequency to drop.”

There are several mechanisms used in a grid to control frequency fluctuations caused by energy shortages. A few seconds after a frequency drop occurs, primary control kicks into action to stabilize the frequency. However, primary control is unable to restore frequencies to 50 Hz and this leaves the grid susceptible to another drop in frequency.

“In our current system, not all power plants contribute to all types of control, with only a few dedicated power plants having this very fast primary control response,” says Schäfer. He adds that the slower-responding secondary control, which integrates the stabilized low frequency to restore it back to 50 Hz, is rarely considered in dynamical modelling by either physicists or engineers.

Curing Braess’ paradox

Schäfer and colleagues, however, were keen to study secondary control because primary control fails to prevent Braess’ paradox affecting electricity grids. The team did an analytical investigation of the general stability of a secondary controller in a simple electric network model consisting of two connected nodes. Then they simulated the addition of lines to a more complex system, invoking Braess’ paradox.

“With no secondary control, adding a line causes a sudden blackout, a prototypical Braess’ paradox. But in the same network with secondary control, adding a line has no effect,” says Schäfer.

Schäfer admits that it had been a challenge to use analytical insight to explain how the secondary controller cured Braess’ paradox in all simulations, and so the team has also proposed additional intuitive explanations.

Ensuring future grid stability

The team describes its findings in the New Journal of Physics. Schäfer says that when the paper was being considered for publication a reviewer asked a very useful question: “How much control do you need?” For example, does secondary control need to be applied at every node, both supplier and consumer? The team tried simulations with varied levels of control and found that secondary control had to be implemented at all nodes for Braess’ paradox to be reliably cured in a network. 

“We firstly warn that new line installations should be double checked to ensure they don’t cause Braess’ paradox,” says Schäfer. “Our second recommendation is that to prevent Braess’ paradox it’s important to distribute secondary control.” The importance of secondary control distribution in local area nodes as well as in generators, led the team to encourage the involvement of energy consumers in future energy grid plans, for instance, implementing demand control schemes that incentivize households to use energy at times of low demand.

Looking towards the future, Schäfer and his team are keen demonstrate the prevention of Braess’ paradox experimentally. This, they hope, will go some way to demonstrate to engineers the importance of understanding Braess’paradox as a collective phenomenon. “Convincing engineers is important and then we can focus on counter-measures, experimentally figuring out which parts of the network we need to control to guarantee stability.”

Analogue computers could use sound to make rapid calculations

On the edge: calculations show how the acoustic system could be used to detect the edges of an image comprising the letters "EPFL"

A compact analogue computer based on an acoustic metamaterial has been proposed by Farzad Zangeneh-Nejad and Romain Fleury at the Federal Institute of Technology (EPFL) in Lausanne, Switzerland. They have shown that the system should be capable of rapid differentiation, integration, and instantaneous image processing, and the duo believe it could achieve yet more impressive feats in the future.

Analogue computers use interactions involving physical entities such as light, electrical current or a mechanical system to perform specific calculations. Some of the most sophisticated analogue computers were developed in the early to mid-20th century to help guide artillery and aerial bombing strikes.

While the advent of digital computers made these computers obsolete, they are now enjoying a resurgence thanks to ongoing research into artificial materials called metamaterials. These materials can be engineered to manipulate the light or sound waves passing through them in new ways – opening the door to new types of analogue computer.

 Subtle engineering

“Metamaterials are artificial structures composed of periodic subwavelength inclusions, which can be subtly engineered to provide the desired macroscopic characteristics of the overall material,” explains Zangeneh-Nejad.

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Metamaterials have already been used to create analogue computers that manipulate electromagnetic waves to perform mathematical operations. Zangeneh-Nejad and Fleury set out to design a device comparable to these optical computers, but using sound waves. However, the distinctive properties of sound waves meant that the researchers first needed to carefully consider how to design their metamaterial.

“Usually, when sound is incident on a hard wall, it reflects without being subject to any particular transformation, and the only thing that happens is the direction of propagation changes,” says Fleury. “Our metamaterial is capable of performing complex signal processing tasks on sound waves when they are reflected, directly on the fly and without delay. It can achieve this instantaneously without converting [sound] into electrical signals.”  Through their calculations, the physicists uncovered the physical properties required of their metamaterial. “It requires a very special acoustic property that does not exist in nature: an acoustic refractive index larger than that of air,” explains Fleury.

No transform required

An important feature of the proposed device is that it performs operations directly in the spatial domain. Previous metamaterial-based computers have worked in the frequency, or Fourier domain, requiring bulky Fourier transform sub-blocks to convert signals into the spatial domain. The new metamaterial has no need for these additional elements. “In our computing system, the mathematical operator of choice is directly performed in the spatial domain using a metamaterial known as a high-index acoustic slab waveguide,” Zangeneh-Nejad explains.

The duo have shown how their device could perform differentiation and integration, as well as instantaneous image detection. Writing in a preprint on arXiv, they explain how future generations of their design could be used to solve more complex differential equations, such as the Schrödinger equation. “We showed how more complex operators such as second order differentiator can be constructed simply by cascading more and more slab waveguides,” says Zangeneh-Nejad. Importantly, the researchers have worked-out that computing devices made from acoustic metamaterials could be entirely compatible with current computing infrastructure. “Our system is free of any Fourier bulk lens, highly miniaturized and potentially integratable in compact architectures, and can be implemented easily in practice.”

The physicists will now further explore the capability of their waveguide to perform calculations at faster rates than conventional computers. “We are investigating applications of our metamaterial in compressive sensing, ultrafast equation solving, neural networks, and a large variety of other applications necessitating real-time and continuous signal processing,” Fleury explains. Their device also has the potential for exploring the dynamics of complex biological systems, allowing for new advances in medicine. As Zangeneh-Nejad adds, “our system could explore the computation processes in human brains, and many other natural systems like DNA, membranes and protein-protein interactions”.

Quantum control of chemical reactions achieved with electrons

Directing chemical reactions by exploiting the quantum nature of electrons has been demonstrated for the first time by physicists at the Tata Institute of Fundamental Research, India, and the Open University in the UK. The technique could prove a cheaper alternative to lasers, which until now have been the main way researchers have sought to achieve coherent control of chemical reactions.

Tata’s E Krishnakumar and collaborators exposed molecular hydrogen and deuterium to a low-energy electron beam, and used a velocity map imaging (VMI) apparatus developed by Nigel Mason and colleagues at the Open University to observe the angular distribution of the reaction products (see video). Electrons interacting with hydrogen or deuterium molecules formed temporary negative ions in a process called resonant attachment. The molecular ions then dissociated to form neutral atoms and stable hydrogen or deuterium ions.

Unexpected asymmetry

Homonuclear diatomic molecules like hydrogen are inversion-symmetric, so such dissociative attachment involving just one electron should result in a symmetric distribution of ion products. Under the laser-induced scheme, the inversion symmetry is only broken when two coherent photons deliver odd and even angular momenta simultaneously. Yet the velocity slice images obtained by the team using VMI showed a distinct asymmetry along the inter-nuclear axis.

According to the researchers, a single electron can achieve the same symmetry breaking when it causes a superposition of two negative ion resonances with opposite parity. As the resulting quantum paths interfere, the relative phase between them determines the degree of asymmetry in the fragmentation pattern produced.

Understanding the dynamics of electron-induced dissociation will allow further insight into natural processes like radiation damage to DNA, and promises improved control over chemical reactions and nanofabrication techniques.

The research is reported in Nature Physics.

Quantum dot floating gates improve light-erasable memories

The OFET memory device

Photoresponsive flash memories made from organic field-effect transistors (OFETs) that can be quickly erased using just light might find use in a host of applications, including flexible imaging circuits, infra-red sensing memories and multibit-storage memory cells. Researchers from Hanyang University in Seoul and Pohang University of Science & Technology (POSTECH) have now found that they can significantly improve the performance of these devices by making use of floating gates based on cadmium selenide (CdSe) quantum dots whose surfaces have been modified.

“OFET-type light-erasable memories have recently emerged as promising elements for information delivery,” explain Yong Jin Jeong and Jaeyoung Jang, who led this research effort. “The erasing process in this type of device is usually controlled using only light following a photo-induced recovery mechanism that works thanks to photo-induced charge transfer across the interface between a semiconductor and floating-gate layers in the device.”

Photo-induced recovery after just one second

Jang and colleagues studied CdSe quantum dots whose surfaces they had covered with three different organic molecules that they used as photoactive floating-gate interlayers in light-erasable transistor memory architectures. They looked at how modifying the surfaces of the dots affected the performance of the memories and found that capping small ligands of octadecylphosphonic (ODPA) and fluorinated molecules improved the diffusion of holes between the dots and the conducting channels in the devices. This allowed for photo-induced recovery after just one second using low intensity light (with a power of 0.7 mW/cm2).

“We had made OFET memories before (containing a polymer/C60 composite floating-gate interlayer) that could also be erased using low-intensity green light (of 0.5 mW/cm2) and in which we could effectively remove trapped charge carriers (electrons and holes) and so recover the initial state,” says Jang. “These devices did suffer from several problems, however. The main one was that erasing stored information took as long as 30 seconds and read-out required a (destructive) applied gate voltage in addition to light.”

Fast erasing using low intensity light

“Employing tailor-made floating-gate materials for transistor memory devices overcomes these problems and allows for fast erasing using low intensity light (again, green) and a non-destructive read-out process. These gates perform many crucial tasks: they generate charge carriers (electrons and holes) when they absorb light and transfer these charges to the semiconducting layer in the transistor.”

Using quantum dots for the floating gates is much better than using C60 or other commonly employed materials, he adds. This is because the optical and electronic properties of these dots can be modified by engineering their particle size or adding surface ligands, as in this study.

The new devices boast high memory ratios of over 105 between OFF and ON bi-stable current states for over 10 000 seconds and good dynamic switching behaviour, he says.

Towards commercialization

The researchers believe that their light-erasable OFET memories could be commercialized in the near future and that their work could provide a “useful guideline” for designing photoactive floating-gate materials.

“To the best of our knowledge, we are the first to engineer the surface of quantum dot floating gates in such memories,” Jang says. “Our proof-of-concept memory devices also consume less power during operation and could be integrated with highly sensitive photodetectors for use in flexible imaging, integrated sensors and biomedical applications.”

So, where next? The Korea researchers say they are now busy trying to develop quantum dot-based light-erasable memories using all-solution-based processes. “In our present experiments, we used vacuum-based techniques to deposit the organic (pentacene) semiconductor and (gold) source/drain electrodes when making our devices,” says Jang. “These are not suitable for large-area, low-cost applications, however, because they are expensive, time-consuming and complicated, so an all-solution process would be better.”

fMRI reveals finger motion encoding in the brain

Topographic maps of flexion and extension across somatosensory and motor cortex. Finger transitions are better delineated in the flexion paradigm. (Courtesy: NeuroImage 10.1016/j.neuroimage.2018.06.062 under CC BY 4.0)

Researchers at the Brain Center Rudolf Magnusin the Netherlands have used population receptive field (pRF) modelling to confirm the ordered representation of human fingers in the motor cortex. While a topographic map (smooth transitions from the thumb to the little finger) of the fingers in the somatosensory cortex is well established, it was unknown whether this pattern was evident in neighbouring motor regions.

The team used computational pRF modelling to analyse functional MRI (fMRI) data of participants flexing and extending their fingers during a scan. In addition to finding finger organization, the group found a degree of interconnectedness between somatosensory and motor cortices, and showed differences in the neural population receptive field size across fingers (NeuroImage 10.1016/j.neuroimage.2018.06.062).

During a scan, the participant flexed each finger in turn (thumb to little finger, or vice versa) holding each position until all fingers were flexed. Then, each finger was extended in turn. The order and timings were cued by a visual stimulus; the subjects also wore an MR-compatible glove that tracked flexion, for quality control.

What is a pRF?

The receptive field (RF) of a single neuron is the region in sensory space to which that neuron responds most strongly/selectively. Since fMRI defines a volume element (voxel) in an image, each voxel therefore represents the sum of many neurons, i.e., a population receptive field.

Since the researchers knew the order and timings of flexion/extension, they could create a predicted model of the stimulation during the scan. This model is a voxel-wise time-series, which tracks the blood oxygenation level dependent (BOLD) response across a cortical region-of-interest (in this case, motor and somatosensory areas).

AdvertisementFor every voxel in the brain region, the researchers compared the predicted model time-series with the actual BOLD data acquired in the experiment, using a non-linear least squares minimization algorithm  to minimize the errors between predicted and real time-series. This gave a set of stimulus-relevant parameters – in this work, an explicit Gaussian model that possess a centre and a spread. For every voxel, the centre shows which finger moved, while a larger spread indicates that the voxel responds more to other fingers than its own preferred one.

The pRF method is a highly robust way of mapping topographic patterns in the brain, since it takes receptive field properties (such as pRF size and broad neural tunings) into account, explaining the fMRI signal to a larger extent. It is therefore better equipped to distinguish true signal from noise.

Flexion or extension?

The group found that ordered individual finger representations crossed both motor and somatosensory areas. They suggest that an interdependence of motor and somatosensory development explains the concurrent maps running across both regions. Overall, it seems that some neuronal populations in both sensory and motor areas respond to multiple fingers.

Smaller pRFs were found for the thumb and larger pRFs for the little finger, but this pattern wasn’t always perfectly linear. Finger organization was less clear with finger extension compared with finger flexion, and pRF size was larger for extension. This could be because letting go of something (extension) doesn’t require the same level of specificity as gripping something (flexion) in day-to-day usage.

The larger pRFs for the little finger and finger extension could imply that these neural populations are relatively unspecific (to the finger or to the motion).

One note of caution is that the pRF size estimate (spread of the Gaussian model) can be influenced by non-neural factors. For example, it also depends on the order of stimulation, which defines which finger moves first; or finger enslavement, where the ring and little fingers are difficult to move independently.

This work provides an interesting first look at applying pRF mapping to brain regions other than visual cortex. There are several challenges regarding the development of this method for somatosensory and motor domains, such as experimental design and pRF model choice, among others. This research is an exciting first glimpse of the rich information that can be revealed by this flexible method.

Meet the ‘angulon’, a new quasiparticle found in superfluid helium

Molecule traps: angulons spotted in helium droplets

The quasiparticle concept allows physicists to describe complex, many-body interactions in terms of the behaviour of a single particle-like entity. Usually these particles turn up in condensed-matter systems such as semiconductors, but a new type of quasiparticle known as an angulon has been proposed to describe the rotation of an atomic or molecular impurity within a solvent. First proposed theoretically two years ago, angulons have now been shown to explain the curious behaviour of a range of different molecules rotating within liquid helium. 

Click on the link to See Other QuasiParticles

Physicists have been studying quasiparticles since at least the 1940s, when Lev Landau and Solomon Pekar put forward the idea of the polaron to describe the behaviour of an electron travelling through a crystal lattice. As the electron moves forward it disturbs the surrounding atoms and so polarizes that region of the crystal. Describing the process completely would involve calculating the changing interaction between the electron and vast numbers of atoms, but Landau realized that it could be approximated by regarding the electron and the associated polarizations as a single particle that acts like a more massive electron travelling through free space.

In the latest work, Mikhail Lemeshko of the Institute of Science and Technology Austria just outside Vienna has looked at the collective motion of a rotating molecule interacting with the many atoms inside a drop of superfluid helium. Such drops allow scientists to hold single molecules at a fraction of a degree above absolute zero and record their spectra without distortions. In particular, it is useful for studying very reactive molecules such as free radicals.

Not enough atoms

The system can be analysed semi-classically by assuming that the trapped molecule creates a shell of non-superfluid helium around itself as it rotates, so slowing it down. But superfluid helium is a fundamentally quantum-mechanical material that is described by Bose–Einstein, as opposed to classical Boltzmann, statistics. Physicists have carried out brute-force numerical simulations of the system in recent years, but the complexity of the many-body interactions has limited the number of helium atoms in those simulations to around 100. The droplets used in experiments, in contrast, tend to contain more than 1000 atoms.

Lemeshko has found that he can simplify the problem enormously by using the concept of the angulon. Just as a polaron consists of an electron plus the deformations in the surrounding lattice, so an angulon is made up of the rotating molecule plus the disturbances it creates in the surrounding helium. And whereas a polaron is in effect a free-moving but more massive version of the electron, an angulon acts like an un-trapped version of the molecule in question but with a larger moment of inertia.

Having put forward the theory of angulons with Richard Schmidt of the Harvard-Smithsonian Center for Astrophysics in the US in 2015, Lemeshko has now compared that theory against 20 years of experimental results. For each of 25 different molecules, Lemeshko calculates the effect of the surrounding helium atoms on the molecule’s rotational constant – which is inversely proportional to its moment of inertia – and then compares the modified constant to the value obtained experimentally.

Two regimes

This was not a straightforward one-size-fits-all comparison, however. To obtain simple analytic expressions for molecular rotation, Lemeshko solved the angulon problem in two “regimes”. One regime, mainly applicable to heavy molecules such as those containing atoms of sulphur, involves molecules with significant coupling to the helium (a high potential energy) but with little kinetic energy. Conversely, the other regime, relevant to lighter molecules such as water, entails greater amounts of kinetic energy but weak coupling.

Although not all the predictions within the strong-coupling regime ended up within the experimental uncertainty, Lemeshko considers that for most heavy molecules he achieved “a good agreement with experiment”. He did even better in the weak-coupling regime, getting to within 2% of the experimental values for most light molecules. With some of the medium-sized molecules, however, he struggled, being unable to accurately predict their modified rotational constants within either the strong- or weak-coupling regimes. He says that an “intermediate-coupling” theory for angulons could in principle make accurate predictions here, but adds that rough estimates can be achieved in the meantime by splitting the difference between the strong- and weak-coupling predictions.

Despite the problems, Lemeshko concludes that the results of his study “provide strong evidence” that molecules rotating within superfluid helium do indeed form angulons. “An angulon is not a real physical entity in the sense that a fundamental particle such as an electron is,” he says. “But it is as real as any other quasiparticle.”

Electron angulons

Lemeshko is now looking to apply his theory beyond molecules within liquid helium. For example, he is investigating whether angulons could be used to represent electrons exchanging their orbital angular momentum with a crystal lattice. Doing so, he says, might aid the development of ultrafast switching and advanced data storage, but he cautions that this research is “very preliminary”.

The research is described in Physical Review Letters.

IBA launches SMARTSCAN beam commissioning system

IBA's SMARTSCAN beam commissioning system on show at the AAPM Annual Meeting.

Commissioning a new linear accelerator is an intensive process, requiring a thousand or so beam measurements on a water phantom. This can take days or even weeks to complete, and the repetitive nature of this task makes it prone to user error.

To ease this procedure, IBA Dosimetry launched its SMARTSCAN beam commissioning system. Announced at this week’s AAPM Annual Meeting, SMARTSCAN guides the user through the entire linac commissioning workflow – from system setup to scanning of the entire data set – and automates repetitive tasks. As a result, beam data commissioning and accelerator quality assurance (QA) is consistently executed with optimal quality. SMARTSCAN is connected to myQA, IBA Dosimetry’s Integrated QA platform

“IBA is launching a water phantom that automates the commissioning steps,” says IBA’s Ralf Schira. He notes that there are some crucial steps where the physicist still needs control, and that SMARTSCAN provides guidance through those procedures. “It’s not a black box, it’s more like a navigation system.”

AdvertisementSchira explains that the development was motivated by IBA’s discussions with physicists as to what they find to be the most problematic part of beam commissioning. As well as the process taking too long and being too user intensive, many interviewees pointed out that they were not 100% confident that all of the scans were 100% correct.

“SMARTSCAN addresses both problems, by making the process more efficient and providing the required beam quality data,” says Schira. “This gives the peace of mind that beam commissioning has been done perfectly.” This, in turn, provides the foundation for safe and accurate treatment of every patient.

To ensure optimal beam data quality, SMARTSCAN checks every single scan during the process, with suspicious measurements flagged immediately. SMARTSCAN also enables commissioning work to be completed efficiently in less time, allowing faster clinical implementation of new equipment. It does this by creating the most optimal scan sequence, as well as by adapting the speed of detector motion.

“SMARTSCAN provides a great deal of intelligence and guidance,” Schira tells Physics World.

Monte Carlo accuracy

The AAPM meeting also saw IBA launch SciMoCa, a Monte Carlo-powered secondary dose check and plan verification software. Monte Carlo is generally accepted as the gold standard for dose calculation accuracy in treatment planning. Now, SciMoCa makes Monte Carlo accuracy available for secondary independent dose calculation and verification, allowing users to verify their treatment plans with an equally robust QA system.

“Introducing Monte Carlo plan QA seamlessly for all major linacs, TPS systems and treatment modalities introduces an additional level of QA accuracy in our industry,” says Jean-Marc Bothy, president of IBA Dosimetry GmbH. “At this year’s AAPM in Nashville we received exceptionally positive feedback from the medical physics community about SciMoCa’s unprecedented Monte Carlo workflow automation, calculation speed and proven accuracy”.

The FDA-cleared SciMoCa software is developed by Radialogica and Scientific RT. IBA has entered into a global distribution agreement with Radialogica.

Organic ferroelectrics finally stick in the memory

Inorganic ferroelectrics have promised to change the face of semiconductor electronics for almost a century, but high processing costs have so far limited development. Now, researchers at Southeast University in Nanjing, China, have paved the way for progress by fabricating the first metal-free perovskite crystals. They present a set of materials that can achieve the performance of inorganic ferroelectrics but with the versatility, low-cost and low-toxicity inherent in organics.

To induce the directional switching of polarization characteristic of ferroelectricity, a material must contain a spontaneous dipole that can respond to an electric field. In other words, the centres of positive and negative charge within a crystal must be different. For metal-free perovskites, this should theoretically happen when a highly symmetric non-ferroelectric state is ‘frozen’ into a state with polar symmetry.

The MDABCO molecule (bottom-left) and the perovskite structure with the central “A” site highlighted. Credit: Yu-Meng You and Xiong Ren-Gen

From database to device

With this in mind, Ren-Gen Xiong and Yu-Meng You instructed their students to scour the hundreds of thousands of entries in the Cambridge Structural Database for molecules of suitable size and symmetry. Such candidates could then be incorporated into the traditionally metallic “A” site of the perovskite structure, yielding an all-organic perovskite ferroelectric.

The result of their efforts is the discovery of 23 metal-free perovskites including MDABCO-NH4I3(MDABCO is N-methyl-N’-diazabicyclo[2.2.2]octonium). This particular crystal displayed a spontaneous polarization of 22 microcoulombs per centimetre square, close to that of the state-of-the-art perovskite ferroelectric, BaTiO3 (BTO). In addition, crystals can be formed readily at room temperature, avoiding the excessive heat (>1000 oC) required to make inorganic ferroelectrics. This will lower fabrication costs and open the door for more delicate device applications such as flexible devices, soft robotics and biomedical devices.

The MDABCO molecule is crucial to the large spontaneous polarization that the researchers observed. At high temperatures, excessive thermal energy leaves the MDABCO molecule in a state of free rotation within the crystal. Here, the average centres of positive and negative charge at the molecule site are the same and ferroelectricity is forbidden. However, when cooled below the phase transition temperature of 448 K, the MDABCO molecule becomes locked in place revealing a significant dipole with eight possible polarization directions.

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Beyond binary

Ferroelectric random access memory (Fe-RAM) works on the principle that individual cells are charged to states “0” and “1”, represented by different polarization directions of the active material.  As ferroelectric crystals tend to have two polarization states, we obtain the well-known binary system. The eight possible polarization directions in MDABCO-NH4I3 then, will pique the interest of those looking to make next-generation memory devices.

“In principle, eight polarization directions could be used to make an octonary device with eight different logic states”, explains Yu-Meng You. “This is a potential strategy for increasing the density of future RAM devices”. While You expresses concern over increased architectural complexity in such a device, the potential for cramming eight bits into a single cell could add to the commercial prospects of this set of materials.

Prospects for perovskites

But the opportunities for advancement don’t stop at memory applications. “We have demonstrated a new system of perovskites with compositional flexibility, adjustable functionalization and low toxicity. We expect the metal-free perovskite system will attract great attention in near future”.

Full details are reported in Science.

Hundreds of new 'smart genes' discovered by scientists

Hundreds of new genes associated with intelligence have been discovered by brain scientists, says a new study published in the journal Nature Genetics. Hundreds of new genes associated with intelligence have been discovered .

Hundreds of new genes associated with intelligence have been discovered Sydney: Scientists have identified hundreds of new genes associated with intelligence. In a joint research project from the University of Queensland's Brain Institute and its partners in the Netherlands, the scientists identified 939 new "smart genes," and over 500 genes associated with neuroticism -- an important risk factor for depression and schizophrenia, Xinhua news agency reported. The findings suggest that our brains have distinct genetic gene clusters responsible for the effects of depression and worry. "These results are a major step forward in understanding the neurobiology of cognitive function as well as genetically related neurological and psychiatric disorders," the researchers said.

More than 250,000 individuals were tested for their genetic data and measurements of intelligence while the study into neuroticism took data from almost half a million respondents. You May Like Why this IIM Program is a must have for future entrepreneurs Talentedge Sponsored Links Together these studies provide new insights into the neurobiology and genetics of cognition. Scientists also believe the newly-found "smart genes" may help protect against Alzheimer's disease and conditions like Attention Deficit Hyperactivity Disorder (ADHD), the report said.

In a previous study, reported in the journal Nature Genetics, scientists announced the discovery of 52 genes linked to human intelligence. These "smart genes" accounted for 20 per cent of the discrepancies in IQ test results among tens of thousands of people examined, the researchers said.

Organic molecules found in ancient Martian rocks

Organic molecules have been found in ancient rocks under the surface of Mars. The discovery was made by NASA’s Curiosity Rover by drilling into mudstone that was laid down 3.5 bn years ago at the bottom of a Martian lake. The molecules found include sulphur-rich thiophenes, aromatic hydrocarbons, such as benzene, and aliphatic hydrocarbons such as propane.

While the presence of these molecules does not prove that life once existed on the red planet, the discovery suggests that conditions on Mars could have been like those here on Earth when life first emerged more than 3 bn years ago.

The discovery is reported in the journal Science by NASA’s Jennifer Eigenbrode and an international team of scientists. They used Discovery’s Sample Analysis at Mars (SAM) instrument to examine samples that had been gathered from Mars’ Gale crater using a drill that can probe 5 cm below the surface.

SAM works by heating rock samples to release any organic compounds that may be present. The emitted gases are then analysed using a gas chromatograph mass spectrometer and a laser spectrometer.

This is not the first time that Curiosity has detected organic molecules, but previous measurements were considered unreliable because of possible sample contamination and unwanted chemical reactions.

Curioser and curioser

While such organic compounds could have been produced by ancient life – or could have provided a food source for ancient organisms – it is also possible that the molecules were created in the complete absence of life. “Curiosity has not determined the source of the organic molecules,” explains Eigenbrode.

Apparently barren and devoid of life today, scientists believe that Mars may have once been a more hospitable environment. Data gathered by Curiosity in 2015 suggests that the Gale Crater was once home to streams and lakes of liquid water. Now, scientists know that some of this water contained molecules that could be associated with life.

NASA associate administrator Thomas Zurbuchen says the agency wants to keep searching for signs of life on Mars. “With these new findings, Mars is telling us to stay the course and keep searching for evidence of life”.

In a second paper in Science, NASA’s Christopher Webster and an international team describe how they have used instruments on-board Curiosity to measure a seasonal variation in methane levels in the Martian atmosphere. The study, which ran for three Martian years (about five Earth years), found that methane concentration in the summer was nearly three times higher than in the winter.  Webster and colleagues say that the variation cannot currently be explained by processes known to occur on Mars.

Spin-3/2 superconductor is a first, say physicists

High spin: artist's impression of spin-3/2 Cooper pairs (Courtesy: Emily Edwards/University of Maryland)

The first known superconductor in which spin-3/2 quasiparticles form Cooper pairs has been created by physicists in the US and New Zealand. The unconventional superconductor is an alloy of yttrium, platinum and bismuth, which is normally a topological semimetal.

The research was done by Johnpierre Paglione and colleagues at the University of Maryland, Iowa State’s Ames Laboratory, the Lawrence Berkeley National Laboratory and the Universities of Otago and Wisconsin.

Conventional superconductivity arises in a material when spin-1/2 electrons form “Cooper pairs” because of interactions between the electrons and vibrations of the material’s crystalline lattice. These pairs are bosons with integer (usually zero) spin, which means that at very low temperatures they can condense to form a state that conducts electrical current with no resistance.

Spin-orbit interaction

In the alloy studied by Paglione and colleagues, charge is carried by particle-like quasiparticles with spin-3/2. These quasiparticles arise from interactions between the spins of electrons and the positive charges of the atoms that make up the alloy. This effect is called spin-orbit coupling and is particularly strong in this material. The result is that the spin-3/2 state – which combines spin and orbital angular momentum – is the lowest energy state.

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When the team cooled the material, they found that it is a superconductor at temperatures below about 800 mK. This came as a surprise because this temperature is nearly 1000 times higher than expected if the superconductivity involved conventional Cooper pairs.

Paglione and colleagues also studied how magnetic fields penetrate the material. Superconductors can expel magnetic fields but the process is not perfect, with some magnetic field lines penetrating the surface of the material and persisting to small depths. Measuring this penetration effect gives important details about the nature of the pairing responsible for superconductivity.

Mind the gap

When the team measured the penetration depth as a function of temperature, they found that it increased linearly rather than exponentially – the latter being a characteristic of a conventional superconductor. This suggests that the energy gap between the superconducting and normal states of the material is not isotrophic in space, as is the case in conventional superconductors.

No one had really thought that this was possible in solid materials

Johnpierre Paglione, University of Maryland

This rules out spin-1/2 Cooper pairs so the team investigated other possibilities. They found that all possible pairings of spin-1/2 and spin-3/2 s in the alloy resulted in isotrophic gaps except the case where two spin-3/2 quasiparticles join to make a pair with a combined spin of 3.

“No one had really thought that this was possible in solid materials,” says Paglione, adding it “was quite a surprise given the simplicity of the electronic structure in this system”.

Non-trivial topology

What is particularly exciting about the material, say the researchers, is the topological nature of how the superconductivity arises. The spin-3/2 quasiparticles are a result of topology related to the strong spin-orbit coupling. Paglione also says, “the superconductivity that forms may itself have a non-trivial topology”. “This is a more subtle thing and harder to prove,” he adds, “but essentially the phase of the superconducting wave function may have a ‘twist’ in it that gives a non-trivial (chiral) topology. This has profound implications, such as possibility of Majorana fermion excitations from the superconducting condensate.”        Indeed, an important fundamental question, says Paglione, is how the spin-3/2 fermions pair up in the first place. “What’s the glue that holds these pairs together?” he asks. “There are some ideas of what might be happening, but fundamental questions remain – which makes it even more fascinating.”Paglione says that spin-3/2 superconductivity could exist in other materials and the phenomenon could have technological and fundamental applications. If such superconductors are indeed topological, he believes that they could form the basis for fault-tolerant quantum computers. On a fundamental level, he says that spin-3/2 fermions provide a very rich spectrum of possible pairing configurations for physicists to study – adding that their work has already garnered significant interest from other physicists.

The research is described in Science Advances.