Neutrinos produced by the elusive carbon–nitrogen–oxygen (CNO) cycle in the Sun have been observed for the first time – confirming a theory first proposed over 80 years ago. The observation was made by physicists working on Italy’s Borexino detector and provides an important insight into how stars power themselves by converting hydrogen into helium. Now that the CNO neutrinos have been detected, future studies could help resolve the mystery surrounding the “metallicity” of the Sun – the abundance of carbon, nitrogen and oxygen in the star.
Astrophysicists believe that stars convert hydrogen to helium via two processes of nuclear fusion. One is called the pp chain and accounts for 99% of fusion energy in the Sun. It involves a pair of protons fusing to create deuterium, which then fuses with a third proton to create helium-3. Finally, two helium-3 nuclei fuse to create a helium-4. There are two other branches of the pp chain that also produce helium-4 via the intermediary production of lithium, beryllium and boron.
The second process is the CNO cycle, which was proposed independently in 1938 by Hans Bethe and Carl Friedrich von Weizsacker. It is believed to account for about 1% of fusion energy in Sun-sized stars — but is thought to dominate the energy output of larger stars. The cycle is driven by the fusion of protons with carbon, nitrogen and oxygen nuclei in a six-step process that creates one helium-4 nucleus before repeating itself.
Now physicists working on the detector have measured the much weaker neutrino signal from the CNO cycle. To do so, the physicists had to overcome detection challenges posed by the relatively low energy and flux of the CNO neutrinos.
Borexino comprises 278 tonne of ultrapure liquid scintillator and detects solar neutrinos when they collide with electrons in the scintillator. As the electron recoils it produces light, which is captured by an array of photomultiplier tubes. Despite the huge flux of solar neutrinos that passes through Borexino, collisions rarely happen and only tens of neutrinos are detected daily. As a result, the detector is located deep under Gran Sasso mountain to shield it from cosmic rays, which would completely overwhelm the neutrino signal. Furthermore, the scintillator contains very low levels of radioactive impurities, which also contribute to the background signal.
The data in this study were acquired during phase-III of the Borexino experiment, which ran for over 1000 h in July 2016–February 2020. Because the CNO signal is very weak, the researchers had to account for background from two low-level impurities – bismuth-210 and carbon-11 – that can mimic the signal expected from CNO neutrinos. The team also had to account for neutrinos created by the proton–electron–proton process in the Sun, which can also be mistaken for CNO neutrinos.
Painstaking characterization
By painstakingly characterizing these background signals, the team was able detect neutrinos from the CNO process with a statistical significance of 5.1σ – above the 5σ level is considered a discovery in particle physics. As well as confirming the longstanding ideas of Bethe and von Weizsacker, the measurement also backs the current belief that about 1% of solar fusion energy is created by the CNO cycle.
While the result does provide a measure of the abundance of carbon, nitrogen and oxygen in the Sun, it is not precise enough to resolve the “metallicity puzzle” of the Sun. This mystery has emerged recently as spectrographic measurements of the opacity of the Sun and helioseismological measurements of the speed of sound in the Sun suggest conflicting values for metallicity. Following Borexino’s success, future improvements to neutrino detectors could address this mystery.
The result also provides an important confirmation of how the CNO cycle should dominate fusion within stars larger than the Sun.
Treatment plan dose distributions for a locally advanced nasopharyngeal carcinoma case with three targets, using MLCs with maximum leaf speeds of 1.0 cm/s (a, b) and 3.5 cm/s (c, d). The 3.5 cm/s MLC achieved better plan quality. (Courtesy: CC BY 4.0/J. Appl. Clin. Med. Phys. 10.1002/acm2.13020)
Increasing the multileaf collimator (MLC) leaf speed may help improve the quality of volumetric modulated arc therapy (VMAT) plans, according to a study published in the Journal of Applied Clinical Medical Physics. Researchers in Beijing investigated seven MLCs with different maximum leaf speeds to investigate their influence on plan quality. They determined that VMAT plans for locally advanced nasopharyngeal carcinoma (NPC) and rectal cancer improved significantly when maximum MLC leaf speeds were increased from 1 to 3.5 cm/s.
VMAT delivers highly conformal radiation dose to a targeted tumour, sparing surrounding healthy tissue. MLCs, which consist of several sets of metallic leaves, are used to shape the radiation beam as it exits the linear accelerator (linac). In addition to improving the dose distribution, MLCs are highly efficient and reduce treatment time. However, dose leakage and transmission through MLC leaves can occur, negatively impacting treatment plan quality.
To achieve highly modulated dose distribution, MLCs need to move at a high speed when the gantry rotates. But what is the optimum speed required to achieve the highest quality VMAT plan for specific types of cancer?
To answer this question, the research team – from the National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College – investigated six NPC cases, representing complex clinical treatments, and nine rectal cancer cases, representing simple treatments. All patients had previously received VMAT at the National Cancer Center.
Principal investigator Jianrong Dai and first author Jiayun Chen configured seven treatment plans for each patient, representing seven linacs configured with maximum MLC leaf speeds (MMLS) of 1.0, 1.5, 2.25, 3.5, 5.0, 7.5 and 10.0 cm/s. Automated VMAT plans with identical initial optimization parameters were designed based on the centre’s clinical protocols, thereby eliminating interoperator variability. The VMAT plans were calculated using 6 MV photons, with a maximum variable dose rate of 600 MU/min. Gantry angle spacing, arc and collimator rotation direction and degrees, and maximum rotation time of each arc were identical for all plans.
The researchers evaluated the plans and scored them using the Plan Quality Algorithm (PQA) tool and the Plan Quality Metric (PQM) of the ESTRO QUASIMODO project. The PQA provided an objective method by which to quantify plan quality, while the PQM provided a fair comparison of plan results.
Plan scores for rectal cancer increased dramatically when the MMLS increased from 1 to 3.5 cm/s, but only grew slowly beyond that speed. The researchers believe that high leaf speeds helped smooth out variations in dose distributions, either through small fields or by the leaves blocking high-dose regions for organs-at-risk. Findings were similar, but not as dramatic, for NPC cases.
“Plan quality is greatly improved as MMLS increases from 1 to 3.5 cm/s; above that, the quality change is marginal,” write the authors. “It demonstrated that the MMLS influences plan quality regardless of the tumour size.”
It should be noted that actual MLC performance depends on a number of other considerations, such as leakage and reliability, for example, and further research is needed to evaluate how speed affects clinically deployed models.
The authors tell Physics World that the team has now studied the effect on treatment plan quality of two major MLC characteristics: MLC leaf speed, as examined in this study; and MLC transmission, for patients with advanced lung cancer, published in Medical Dosimetry. The team’s future research will incorporate the interactions of dose rate, gantry speed and MLC speed, to obtain a more comprehensive representation of their dosimetric effects.
The annual meeting of the European Society for Radiotherapy and Oncology (ESTRO), originally due to take place in April in Vienna, was one of the early casualties of the Covid-19 pandemic. Postponed once to August, the event organizers decided to further delay the meeting to the end of November in the hope of convening a reduced live congress along with enhanced online participation.
However, the resurgence of coronavirus in Europe has forced another rethink, and ESTRO 2020 will now be staged as a fully digital event. The main online congress will take place from 28 November to 1 December, with attendees able to access the scientific programme via a digital platform that includes Q&A and polling functionality to enable interaction with the speaker. Several pre-congress sessions are also available online, including three short courses on a dedicated education platform, while all registered participants will be able to view all sessions on demand after the congress has finished.
The theme for this year’s event is “Translating research and partnership into optimal health”. Meeting chair Umberto Ricardi from the University of Turin, Italy, points out that technologies and methodologies in radiation oncology are evolving rapidly, but that these advances will “only truly provide impact when the new findings can be translated into clinical applications that will result in optimal benefit for each individual patient in daily practice”.
ESTRO 2020 will also host Europe’s largest industrial exhibition in radiation oncology. This year a virtual exhibition will enable attendees to interact with industry leaders and to find more about the latest innovations in technology, techniques and oncology products – a few of which are highlighted below.
Innovating and evolving radiotherapy quality assurance
Quality assurance (QA) plays a fundamental role in any radiotherapy procedure. IBA Dosimetry is working to shape QA to advance patient safety in radiation therapy, proton therapy and medical imaging. The company predicts that its latest innovations will bring the accuracy and efficiency of QA to a new level. Future solutions, meanwhile, will significantly reduce QA times and further streamline the medical physics workload.
Independent QA is essential to ensure reliable, trustworthy and accurate QA, and has been assumed as a given in the radiotherapy community. But as radiotherapy systems increasingly offer built-in “self-check” QA, the need for independent QA becomes imperative. To raise awareness of this topic, IBA Dosimetry has teamed with radiotherapy QA equipment vendors worldwide to launch an “Independent Quality Assurance” initiative.
Convergence of machine QA and patient QA will unlock the potential of real risk-based radiotherapy QA. (Courtesy: IBA Dosimetry)
IBA Dosimetry also highlights the need for convergence of machine and patient QA. Today, QA applications for validating the treatment machine and those for verifying the patient-specific plan generally have little or no connectivity. Combining data from patient QA and machine QA will provide more precise outcomes and faster results.
These QA innovations are based on four strategic pillars – implementation of measurements, integration, smart automation and prediction of QA results – that help save valuable time of the medical physicist and provide higher accuracy and increased confidence. IBA Dosimetry notes that while measurements will remain important for future QA, further integration, Monte Carlo-based predictive QA and automation will enable users to measure only where it really matters, leading to fewer measurements with better quality results.
IBA Dosimetry’s QA innovation strategy is based on four pillars: measure, integrate, automate and predict. (Courtesy: IBA Dosimetry)
Software platform standardizes treatment plan review
Peer review of treatment plans is a powerful strategy in radiation oncology clinics to ensure patient safety and plan quality, but time constraints in the daily routine can make it difficult to prepare and review patient cases in an optimal way. MIM Harmony is a dedicated software platform that provides rapid and reproducible treatment plan review, allowing medical teams to focus their efforts on a deeper, critical evaluation of patient cases and to improve the quality of their treatment plans.
MIM Harmony allows clinicians to prepare patient cases automatically, saving valuable time and ensuring that all the necessary information is available at the review meeting. Each review is guided by a disease site-specific worklist that can be fully customized to suit the needs of each clinical team, offering a flexible but standardized approach to patient care.
A dedicated platform from MIM Software is designed to improve both the efficiency and effectiveness of treatment plan reviews. (Courtesy: MIM Software)
MIM Harmony’s customizable worklist, along with flexible task automation, reporting tools, and advanced analytics, enhance multiple peer-review formats such chart rounds, contour rounds, and one-to-one consultations. Its robust data analytics platform moves beyond traditional peer review, compiling critical information after each meeting to help identify areas for improving quality or clinical practice.
Fast and filmless patient QA now extends to multitarget treatments
Quick and convenient patient-specific quality assurance (QA) is essential for delivering stereotactic radiotherapy safely and effectively. The SRS MapCHECK from Sun Nuclear is a patient QA solution that replaces conventional film with a high-density diode array, providing accurate dose measurements in minutes rather than hours. With nearly 400 devices in clinical use and a growing body of supporting literature, the SRS MapCHECK has been proven to efficiently detect the most common sources of errors in stereotactic treatments and has become the gold standard for time-sensitive patient QA.
The system comes with SNC Patient software, which corrects for angular dependence, field size and pulse rate to ensure accurate patient QA from any angle. A new update to the software now makes it possible to simplify QA workflows for single-isocentre multiple-target (SIMT) treatments, with a new QA Setup Tool providing guidance for the optimal set-up of SIMT plans and simplified shifts for larger field sizes.
The SRS MapCHECK from Sun Nuclear now provides enhanced support for single-isocentre multiple-target (SIMT) treatments. (Courtesy: Sun Nuclear)
This software update, described in more detail in this short video, complements Sun Nuclear’s MultiMet-WL Cube, designed for machine QA of SIMT treatments, and adds to a suite of StereoPHAN QA solutions for stereotactic radiotherapy. During ESTRO 2020, on 30 November at 13.00 CET, Sun Nuclear will host a satellite symposium entitled “Advancing QA: Latest Solutions from Sun Nuclear”. Presentations and demonstrations will showcase both SRS MapCHECK and the SunCHECK platform.
Tracking device promises more precision for prostate cancer treatment
New radiotherapy technologies such as image guidance, high-precision dose delivery and more accurate target definition are making it possible to administer higher radiation doses in fewer treatment sessions. For prostate cancer, an improved understanding of the biological mechanisms underlying the use of hypofractionation suggests that the delivery of fewer and larger fractions has the potential to improve the therapeutic ratio while also shortening the overall treatment time.
Hypofractionated radiotherapy delivered via stereotactic body radiation therapy (SBRT) offers a high dose per fraction within a small number of fractions, but the steep dose gradient demands a high level of reliability during the entire treatment delivery process. At even small distances from the target the radiation dose decreases rapidly, which means that prostate motion during treatment might result in spatial misses and the exposure of surrounding healthy tissues to radiation.
The RayPilot HypoCath system from Micropos Medical is a removable electromagnetic transponder that offers real-time tracking of the tumour throughout the treatment process. The electromagnetic tracking device is compatible with conventional linacs and requires no surgical intervention, with the transmitter integrated into a standard urinary catheter to enable localization of both the prostate and the urethra.
The RayPath HypoCath keep track of prostate motion during SBRT treatments. (Courtesy: Micropos Medical)
The system has already been trialled at the Ospedale San Gererdo in Monza, Italy, with the results reported in a recent white paper by Professor Stefano Arcangeli. Tests with four patients showed that the electromagnetic tracking device kept the average target motion within 2 mm throughout treatment delivery, with no impact on patient comfort. Arcangeli notes that further improvements in the accuracy could be achieved by fine tuning the workflow, which he believes would make SBRT well positioned to become the procedure of choice for patients with localized prostate cancer.
Upgraded software enhances daily testing regime
Quality assurance (QA) in radiation treatment planning is becoming ever more important as medical physicists seek to optimize image-guided radiotherapy (IGRT) protocols for the delivery of higher doses and emerging adaptive treatments. Imaging phantoms are essential to calibrate and commission IGRT systems on a daily basis, and the QUASAR Penta-Guide Phantom from Modus QA has become recognized globally as the preferred tool for efficient daily testing of key parameters such as system alignment and imaging performance.
Modus QA is now enhancing the capabilities of this phantom with the new Penta-Guide 2.0 software, a comprehensive daily QA solution that is free for all existing and new Penta-Guide users. Designed to improve the daily QA workflow, the software offers advanced features such as automated monitoring of system alignment, image quality analysis, and improved reporting and visualization tools.
Updated software for Modus QA’s Penta-Guide phantom is designed to improve the efficiency of daily testing protocols. (Courtesy: Modus QA)
An online presentation by product manager Rocco Flores will highlight the innovative features of the Penta-Guide phantom, review the daily QA user workflow, and explain the advanced utility of the Penta-Guide 2.0 software.
Researchers in the US and Israel have developed a way to make 3D superconducting nanostructures by combining DNA with niobium and silicon. This new technique might be used to make signal amplifiers that enhance the speed and accuracy of quantum computers as well as ultrasensitive magnetic field sensors for medical and geophysics applications.
While traditional nanofabrication techniques like electron-beam lithography can produce one-dimensional and two-dimensional superconducting nanostructures, their ability to produce three-dimensional structures is limited. For the past 15 years or so, researchers have instead turned to self-assembly techniques that use DNA to construct 3D nanoscale structures and integrate them with functional inorganic nano-components.
One such technique, known as “DNA origami”, uses the natural pairing of DNA’s four nucleotide bases – A, T, C and G – to produce a multitude of self-assembled engineered shapes. The process involves folding a long single strand of DNA with the help of shorter complementary strands at specific locations to make pre-defined nanoscale structures. These nanostructures can then be used as scaffolding for building 3D nanoscale architectures that can be “converted” into inorganic materials such as superconductors – as in this new work, which was led by Oleg Gang, a nanoengineer at Columbia University and the Brookhaven National Laboratory’s Center for Functional Nanomaterials.
DNA origami “frames”
In their experiments, Gang and colleagues at Columbia designed octahedral-shaped DNA origami “frames” using a computer software package called caDNAno. Each edge within the frames comprises a six-helix bundle of DNA that is 28.6 nm long and contains 84 base pairs. At each end of these bundles, the researchers added a single-stranded DNA chain that measured roughly 2 nm in length and was designed to complement the DNA chain of the opposing DNA origami.
To better visualize (and subsequently characterize) the structure, the researchers inserted 10-nm-diameter gold nanoparticles into each octahedral “cage”, where the particles were held in place by a structure made from DNA that is complementary to the inner strands of the frame. The researchers then assembled the cages into a simple cubic superlattice of octahedra made from two pairs of frames and designed to have specific DNA strands targeting four complementary counterparts in-plane and two counterparts out-of-plane. The resulting superlattice samples are flakes 5-10 microns long and 1-3 microns thick, and the researchers used small angle X-ray scattering at the Brookhaven National Synchrotron Light Source II to confirm their nanoscale structure.
Mechanically robust 3D architecture
The team then solidified their ensemble by using a wet chemistry technique to coat the DNA lattices with a layer of silicon dioxide. “In its original form, DNA is completely unusable for processing with conventional nanotechnology methods,” Gang explains. “But once we coat the DNA with silica, we have a mechanically robust 3D architecture that we can deposit inorganic materials on using these methods. This is analogous to traditional nanomanufacturing, in which valuable materials are deposited onto flat substrates, typically silicon, to add functionality.”
The next step was to use an evaporation technique to coat the silica-coated superlattices with a layer of niobium around 10 nm thick. This part of the work was done at the Institute of Superconductivity at Bar-Ilan University in Israel. There, researchers led by Yosi Yesurun carefully controlled both the temperature of the silicon substrate and the rate at which they deposited the niobium so that the niobium only coated the sample and did not penetrate all the way through it. They did this to prevent short circuiting between the electrodes used for later electronic transport measurements.
A periodic array of Josephson junctions
Once this step was complete, the researchers used scanning transmission microscopy with energy dispersive spectroscopy (STEM-EDS) to check the structure of their samples. This imaging technique revealed a porous superlattice structure made up of the silica-coated DNA and the gold nanoparticle “tracers”.
This structure forms a periodic array of Josephson junctions – thin non-superconducting barriers though which superconducting current tunnels – with the niobium in the superlattice being mainly confined to the top three pairs of octahedra layers (which have a total thickness of around 290 nm). As a final step, the researchers measured the current-voltage characteristics of their superlattices at temperatures between 1.9 and 3.7 K. The curves produced are typical of single Josephson junctions – that is, they show zero voltage for currents up to a certain temperature-dependent critical current (indicated by the appearance of resistance) and then a gradual voltage increase.
Gang and colleagues note that Josephson junctions are key components for leveraging quantum phenomena in practical technologies, with examples including the superconducting quantum interreference devices (SQUIDs) used to sense magnetic fields. The three-dimensional nature of the Josephson junctions created in this work means that more of them can be packed into the same small volume, which could increase the power of a device that uses them.
While DNA is not necessarily the most useful functional material for such work, the researchers say they have demonstrated that complex DNA organization can, in principle, be used to create highly nanostructured 3D superconducting materials. “This material conversion pathway gives us an ability to make a variety of systems with interesting properties – not only superconductivity but also other electronic, mechanical, optical, and catalytic properties,” they report. “We can envision it as a ‘molecular lithography’, where the power of DNA programmability is transferred to 3D inorganic nanofabrication.”
Spurred on by this possibility, the researchers, who report their work in Nature Communications, say they are now planning to apply the same strategy to create highly-structured 3D inorganic nanomaterials with a broad range of functions.
For nearly 50 years, researchers from around the world have converged in Boston for the Fall Meeting of the Materials Research Society. This year, however, as a result of the Covid-19 pandemic, the meeting will be convened as a fully digital event in combination with the Spring Meeting, which was due to take place in Pheonix, Arizona, back in April.
The joint 2020 MRS Spring/Fall Meeting & Exhibit, scheduled for 27 November ̶ 4 December, will include the scientific programmes from both meetings, with a combination of live and on-demand presentations. John Rogers of Northwestern University will give a plenary lecture on functional materials that enable small electronic devices to be integrated into the body, while a range of networking, Q&A sessions and professional development events are available to access before and after the meeting.
A virtual exhibit will enable attendees to interact directly with equipment vendors, with extended “booth hours” where delegates can chat with company representatives. It is also an ideal opportunity to up to date with the latest technology developments – a few of which are highlighted below.
Turnkey system delivers fast and precise Hall measurements
A fully integrated measurement platform from Lake Shore Cryotronics makes it quicker and more convenient to acquire high-precision Hall measurements. Ideal for anyone studying semiconductor materials, the MeasureReady FastHall Station incorporates Lake Shore’s M91 FastHall controller into a tabletop system that enables simplified Hall measurements and reduces the time needed for experimental setup.
Along with the M91 measurement controller, the all-in-one station includes a Windows-based computer, a 1 T permanent magnet, a high-precision sample holder, and all the necessary software and cabling to provide a comprehensive range of Hall measurements. The system can detect sample resistances up to 1 GΩ and mobilities as low as 0.01 cm2/V s –making it ideal for research into low-mobility materials.
The MeasureReady FastHall Station for Lake Shore Cryotronics provides an integrated solution for quick and easy Hall measurements. (Courtesy: Lake Shore Cryotronics)
Lake Shore’s patented FastHall method speeds up measurement times, while easy-to-use spring pin and solder sample holder cards accommodate up to 10 mm × 10 mm van der Pauw and Hall bar type samples. The station also features an electronically shielded, low-noise sample space with guarded contacts, making it quicker and easier to derive accurate measurements of the carrier type, carrier concentration, mobility, and Hall coefficient.
The system’s MeasureLINK-MCS software collects all the data, and provides standard sequences, charts, and test scripts that can be customized by the user. In addition to performing complete Hall analyses and outputting all measured and derived values, detailed reports can be generated that include all the supporting intermediate data, so a researcher can readily confirm the integrity of the results. Optional extras include a gate-bias instrument plus a liquid-nitrogen add-on to convert the standard room-temperature station to a cryogenically cooled sample space maintained at 77 K.
Large-sample AFM pinpoints polymer properties
Atomic force microscopy (AFM) has become a crucial tool in both research and industry for studying the properties and structure of many different materials and devices. In an online presentation that is now available to view on demand, Dr Vladimir Korolkov, a senior application scientist at AFM manufacturer Park Systems, highlights the use of AFMs to study the structure of polymer materials.
In the presentation, Korolkov explains how to exploit an AFM to resolve individual polymer chains in real space, which is particularly important because the properties of polymer materials are strongly influenced by the packing and conformation of individual macromolecules as well as their monomer composition. He also shows how an AFM can be used to acquire ultrahigh-resolution images of individual PTFE molecules on the semi-crystalline surface of a commercial Teflon tape.
The NX20 AFM from Park Systems is designed for failure analysis and large sample research. (Courtesy: Park Systems)
Using these real-world polymer samples, Korolkov highlights the capabilities of Park Systems’ NX20 AFM for high-speed acquisition of high-resolution images. The NX20 is designed to study large samples and conduct multi-sample analyses, and comes with an automated interface that makes it easy for non-experts to acquire high-quality data. The instrument supports single-click imaging through the auto mode of Park’s SmartScan software, while automatic operation of the cantilever brings it safely into contact with the surface within just 10 seconds.
The NX20 also controls the scan speed based on the peaks and valleys of the sample surface, minimizing the scanning time while also acquiring a high-quality image. When moving to a neighbouring location or zooming into a target, the system automatically applies a new optimal condition.
Single instrument combines AFM with scanning microscopy
A unique atomic force microscope (AFM) from Quantum Design can easily be integrated into the high-vacuum environment of almost any scanning electron or scanning ion microscope (SEM/FIB). The AFSEM® Nano enables simultaneous operation of SEM, FIB, and AFM inside the vacuum chamber, allowing these complementary techniques to be combined without needing to transfer the sample or break the vacuum.
The AFSEM Nano directly correlates AFM functionality to SEM or FIB information at the same location on the sample. This correlated functionality allows users to, for example, measure the real 3D-topography of their sample with (sub)-nanometer resolution inside their SEM or FIB system, or easily identify the region of interest with the SEM and then use the AFM to measure the physical, electrical, or magnetic properties at the same location.
Best of both worlds: the AFSEM Nano from Quantum Design can be integrated into the vacuum chamber of almost any scanning microscope. (Courtesy: Quantum Design)
Combining the different techniques makes it easy to analyse samples with very different shapes or sizes. Users can also obtain in-situ correlation of chemical (EDX) and crystallographic (EBSD) information, and achieve 3D subtractive tomography by combining FIB slicing with the mapping of nanomechanical properties using the AFM.
The AFSEM Nano is a laser-free AFM that exploits a self-sensing cantilever technology to avoid the need for optical alignment inside the vacuum chamber. Its open design allows for easy integration with other optional add-ons, such as tensile stages, nano-indentors or nano-manipulators.
Materials matter for 2D layered perovskite research
2D layered perovskites have attracted huge research interest for their unique optoelectronic properties, helped enormously by the discovery that the most promising structures – usually hybrid organic–inorganic lead halides – can be prepared easily and cheaply using solution processing techniques. Photoluminescent 2D perovskites have a direct bandgap with a narrow emission peak that can be tuned by changing the layer thickness and the material composition, and have been widely studied for applications in light-emitting diodes, phototransistors, lasers, and solar cells.
MilliporeSigma supplies the precursor materials needed to create 2D layered perovskites with different compositions and properties. (Courtesy: MilliporeSigma)
Materials company MilliporeSigma offers a range of fully synthesized layered 2D perovskites, including some of the most popular organolead halides. Also available is a comprehensive portfolio of precursor materials, including lead halides, organohalides, and halide acids, as well as other key materials needed to fabricate 2D layered perovskites.
China has successfully launched a mission to bring back rocks from the Moon– the first attempt to do so for nearly 45 years. Chang’e-5 was launched at 4:30 a.m. local time today by a Long March 5 rocket from Wenchang Satellite Launch Center. Once it lands on the Moon it is expected to grab up to two kilograms of soil from an area not previously sampled to better understand the evolution history of our closest neighbour.
Chang’e-5, weighing 8.2 tonnes, consists of four parts: an ascender, lander, returner and orbiter. Upon entering the Moon’s orbit, the ascender and lander will separate and touch down in the Mons Rümker region — a volcanic mound in the northwestern part of the Moon’s near side. The lander will use a panoramic camera, spectrometer and ground-penetrating radar among other payloads to document the landing site. It will also use a robotic arm to scoop up small rocks from the surface and drill up to 2 m into the ground.
Chang’e-5 is an important step in the plan. One that deserves close attention
John Logsdon
Once the sampling is done and before the lunar night falls, the ascender will lift off from the top of the lander and dock with the returner-orbiter in orbit. The sample container is then transferred to the returner, which will head back to the Earth. Using a technology called ballistic re-entry, the returner will safely travel through the Earth’s atmosphere towards a planned landing site in Inner Mongolia, north China. It is expected that most of the returned samples will be stored at the National Astronomical Observatories of China, Chinese Academy of Science, in Beijing with possible access by foreign scientists through collaboration with Chinese colleagues.
Rock collector
Scientists believe that part of Mons Rümker might have formed 1-2 billion years ago, being much younger than the sites visited by US and Soviet sample-return missions that were over three billion years old. Back in the 1960s and 1970s, six US Apollo crewed landings brought back 382 kg of rocks from the Moon while three Soviet Luna robotic missions returned 0.326 kg. The samples from the Chinese mission will help scientists improve their model to estimate the age of surfaces in the solar system, from rocky planets such as Mars and Mercury to asteroids. Surface ages are roughly defined by crater densities: more craters, older surfaces.
If the age of Chang’e-5 samples are confirmed to be 1-2 billion years, it may challenge our current theory on the formation of the Moon, which should have cooled off by that time due to its small size and limited “heat budget”. Scientists would need to find out what had fuelled those volcanic eruptions. “[Chang’e-5] can lead to a whole new understanding of recent volcanisms on the Moon,” says Clive Neal from the University of Notre Dame in the US. “The new samples from Chang’e-5 will give us a way to quantify the younger end of the crater-counting curve.”
As China’s most complex and ambitious lunar mission so far, Chang’e-5 could go wrong in many ways. “Safe return is the most important thing for a first attempt,” adds Neal. Brett Denevi from the Applied Physics Laboratory, Johns Hopkins University, who has analyzed Apollo lunar samples, notes that China has picked “one of the best places to go” for a lunar sample-return mission. “That’s why this mission has attracted broad, international interests,” she adds, cautioning that 2 kg is an ambitious target. “Two grams can already teach us a lot,” she says.
China is also working on multiple follow-up lunar missions that will eventually lead to a human mission in the 2030s. These include Chang’e-6, which will return samples from the south pole, as well as Chang’e-7 that will perform a detailed survey of the south polar region. With renewed interest in lunar exploration and the advances in sampling and analytic capabilities, space-policy expert John Logsdon from George Washington University says that Chang’e-5 could “set a new standard” for robotic lunar exploration. “Chang’e-5 is an important step in the plan,” says Logsdon. “One that deserves close attention.”
Organic–inorganic perovskite materials are usually studied in the context of making solar cells and other photovoltaic devices. Now researchers from the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing have shown that these hybrid halide materials could also be ideal platforms for realizing Bose–Einstein condensates of excitons (electron–hole pairs). Such condensates, which appear as vortex patterns, could be produced at liquid nitrogen temperatures – positively balmy, by the standard of the field – thanks to the long lifetimes of the excitons in the materials and their huge binding energies.
Bose–Einstein condensation (BEC) occurs when all the bosonic atoms or particles in a gas collapse into the same quantum ground state and can therefore be described by the same wavefunction. Such collapses are triggered by cooling the gas until the de Broglie wavelength of its constituent atoms or particles is comparable to the distance between them. Once in this state, the atoms or particles behave as a superfluid, flowing without friction.
Exciton BECs
Researchers made the first BEC in 1995 from rubidium atoms. Since then, condensates have been observed in various other types of particles, including polaritons, photons and magnons as well as other species of atoms and molecules. In all cases, however, the phenomenon has only appeared at ultralow temperatures of no more than a few Kelvin above absolute zero.
To make BECs easier to study – and perhaps also to put their amazing properties to practical use – researchers have long sought to increase the temperature at which they form. One way of doing this might be to make a BEC using excitons, which are bosons composed of the bound states of two fermions: a negatively charged electron and a positively charged hole, or electron vacancy. These fermions are bound together via weak Coulomb interactions that cause them to form dipoles. Since these bound states are much lighter than atoms, they can be packed together with higher density – meaning that they ought to Bose condense at much higher temperatures.
That, at least, is the theory. Unfortunately, previous attempts to make such excitonic BECs – for example in semiconductor wells and graphene – have succeeded only at disappointingly low temperatures of around 1 K, due to the small exciton binding energy in these material systems.
Calculating the exciton binding energy
In their new work, researchers led by Kai Chang studied a 2D hybrid perovskite with the chemical formula (PEA)2PbI4. Perovskites in general are promising thin-film solar-cell materials thanks to the fact that they can absorb light over a broad range of solar spectrum wavelengths. Electrons and holes have a long lifetime in these materials too (that is, they can diffuse through the material over long lengths) and this is the property that Chang and colleagues focused on.
The team’s chosen perovskite has a stable layered structure comprising layers of [PbI6]4− octahedra and long-chain organic molecules with the formula C6H5C2H4NH3+ (abbreviated PEA+). The inorganic PbI4 layers are sandwiched between two organic layers and have effective potential barriers with energies of 8.1 eV. These barriers make (PEA)2PbI4 behave like stacked quantum wells confined by “hard-wall” energy potentials, the researchers explain.
Using first-principles calculations and a theoretical framework known as the Keldysh model, the researchers calculated a binding energy as high as 238.5 meV for the excitons in monolayer (PEA)2PbI4, a value that agrees with that obtained in laboratory experiments. “The Keldysh model is a standard treatment for describing 2D excitons with ‘unscreened’ Coulomb interactions and the large exciton binding energy we calculated means that the critical temperature of the exciton BEC could approach the liquid nitrogen regime (77 K),” team member Dong Zhang tells Physics World.
Vortex patterns
The researchers studied their flakes of (PEA)2PbI4 further by applying an electric field perpendicular to them. From this, they found that the applied field slightly changed the material’s binding energy, while also causing all the electron–hole dipoles to line up in the same direction. In this situation, the interaction between the dipoles becomes repulsive.
When the researchers then “pumped” the flakes of the (PEA)2PbI4 using pulses from a laser, they found that the repulsive dipole–dipole interaction created by the perpendicular electric field can drive the laterally confined excitons into various vortex patterns. The time it takes for these vortices to evolve is on a par with the lifetime of the exciton itself and the result is a stable pattern with a certain number of vortices rotating at the centre.
Members of the team, who report their work in Chinese Physics Letters, say they now plan to study exciton BEC in few-layered hybrid perovskites as opposed to just monolayer ones. “We will also be looking at how to manipulate exciton vortices and make vortex-based information-storage porotype devices,” Zhang says.
Immunotherapy using checkpoint inhibitors is an emerging cancer treatment that has recently had great success in treating metastatic cancer. The technique works by blocking checkpoint proteins (such as CTLA-4 or PD-1) that stop the patient’s immune system from attacking cancer cells, and reactivating the immune system to fight cancer. Unfortunately, however, only a fraction of patients respond to such immunotherapy and only certain tumour types can be treated.
To increase its potential pool of patients and cancers, immunotherapy can be combined with radiation therapy, which under certain conditions also triggers an immune response. It has also been proposed that charged particles – such as the carbon-ion beams already used clinically to treat certain cancers – could prove more effective than X-rays in this combination.
“There are several explanations as to why carbon ions and immunotherapy are a good match,” explains lead author Alexander Helm. “The most prominent is actually the peculiar pattern of cell death that is induced by carbon ions compared with conventional radiotherapy. It is hypothesized that this very cell death is more immunogenic, which will eventually lead to a more efficient activation of the immune response and a better elimination of metastases. These effects, nonetheless, depend on the combination with immunotherapy to boost such immune responses.”
Radiation comparisons
The researchers, also from the Parthenope University of Naples and NIRS-QST in Japan, conducted their experiments at the accelerator in Chiba, Japan. They inoculated mice in both hind legs with osteosarcoma cells, a bone tumour that is generally considered radioresistant. They then treated each mouse with a 10 Gy dose of either carbon ions or X-rays, in combination with two immune checkpoint inhibitors: anti-PD-1 and anti-CTLA-4. Tumours on the animals’ left legs (representing primary tumours) were irradiated, while those on the right legs (abscopal tumours) were kept out of the radiation field.
Tumours that were directly irradiated, with either carbon ions or X-rays, demonstrated reduced growth compared with untreated mice. Tumour growth was better controlled in mice that received irradiation plus immunotherapy than in animals treated with radiotherapy alone, suggesting a synergistic effect likely based on the additional immune response boost by the checkpoint inhibitors.
In mice receiving the combination treatments, growth of the unirradiated abscopal tumours also decreased. This reduced growth was most pronounced when the animals received a combination of carbon ions plus checkpoint inhibitors.
The team also examined the effects of the various treatments on lung metastases, which form spontaneously from the bone tumours in the mice. When combined with immunotherapy, both radiation types essentially suppressed the metastases. As found previously, the combination of carbon ions plus checkpoint inhibitors had the greatest effect, resulting in the least number of metastases.
Carbon-ion irradiation alone also significantly reduced the number of lung metastases compared with the control group, comparable with results in animals that received only checkpoint inhibitors. This was not the case for mice treated with X-rays.
The team concludes that a combination of high-energy carbon-ion radiotherapy and checkpoint inhibitors has the highest potential to control distal metastases in this mouse model and could provide a potential clinical option for treatment of advanced tumours. Corresponding author Marco Durante, director of the department of biophysics at GSI, has previously demonstrated that carbon ions and protons have physical advantages over X-rays that enable drastically improved sparing of healthy tissue during radiotherapy. In fact, charged particles spare circulating immune cells in the blood much more than X-rays, which is necessary for an efficient immune response.
The researchers are now working to discover the mechanisms underlying these immune responses. Understanding these mechanisms should allow them to tailor radiation treatments to increase immune response activation.
“For example, the radiotherapy fractionation scheme has been shown to have a crucial role on the immunogenicity of the induced cell death,” explains Helm. “Hypofractionation, in which higher doses are applied in a shorter time span (than with conventional radiotherapy), has been reported as beneficial – and carbon ions are a perfect match for hypofractionation.”
The spin states of entangled erbium ions in a solid crystal can be controlled and read out individually using a new technique developed by Jeff Thompson and colleagues at Princeton University in the US. In doing so, the team has overcome the important challenge of making measurements on closely spaced ions in a crystal. Their technique could lead to the creation of new quantum devices that could be integrated with existing optical telecommunications technology.
Some atomic-scale impurities in solid-state crystals have spins states that endure for long periods of time and can therefore be used as quantum bits (qubits) of information. If the impurities are located close enough together in a crystal, their spins will become entangled with each other. This entanglement can be exploited to create quantum logic gates for quantum computers.
However, the nanoscale separation required for entanglement is typically well below the diffraction limit of visible light. This means that the optical lasers used to control and read out spin states cannot normally distinguish between the spins of individual impurities.
Random shift
One promising way around this problem is to use the rare-earth ions of erbium as impurities. Their spins can retain quantum information over long periods of time and they interact with light at wavelengths used for optical telecommunications. But most importantly, each erbium-ion impurity in a crystal experiences a random static shift in its optical transition energies. This means that even if the positions of multiple ions cannot be resolved spatially, their spin states can be controlled and read-out using the distinct wavelengths of the light they absorb and emit when illuminated by a laser.
To exploit this property, Thompson’s team doped a yttrium orthosilicate crystal with erbium ions. They then coupled the system to a photonic silicon cavity, which enhances the emission of light from the ions and makes it easier to read out the spins. Out of hundreds of erbium ions in the sample, the researchers focused on six ions in a sub-micron region, tuning the wavelength of a laser to match each of the ions. This approach allowed them to easily control and read out the spin states of individual ions with high fidelity.
Thompson and colleagues now hope that their approach can be scaled up to accommodate large numbers of rare earth ions with arbitrarily small separations – making them suitable for multiple-qubit systems. Crucially, they point out that the technology could be easily integrated into existing communications infrastructures: transmitting encoded signals in telecommunications frequency bands using present-day silicon devices and optical fibres. If achieved, this could soon allow erbium ion defects to provide a solid basis for future quantum computers, as well as ultra-secure quantum communication networks.
A new, slimline imaging spectrometer developed by researchers at the Massachusetts Institute of Technology (MIT) in the US boasts the same performance as the most advanced devices of its kind while being much more compact. Thanks to its reduced size, the instrument could perform remote terrestrial sensing from airborne vehicles such as drones or satellites and might even become a component of future space missions.
Imaging spectrometers that operate in the visible, near and short-wave infrared (VNIR/SWIR) regions of the electromagnetic spectrum are routinely deployed in studies of atmospheric science, ecology, geology, agriculture and forestry. They work by recording a series of monochromatic images, the spectrum of which is then analysed over a given imaging area. One of their advantages over traditional cameras is that they are very good at controlling the chromatic aberrations (blurring or distortion) that arise because of light being spread out over a region of space rather than focused to a point. They also have high signal-to-noise ratios. The snag, however, is that current devices are relatively bulky, which means they can’t be deployed on small satellites or drones.
11 times smaller in volume
Researchers led by Ronald Lockwood have now developed a miniaturized spectrometer with a volume of just 350 cm3 – less than one-tenth the size of standard instruments. One version of the new device, which is known as a “Chrisp” compact VNIR/SWIR imaging spectrometer (CCVIS), measures just 8.3 cm in diameter and 7 cm long.
To downsize their device, Lockwood and colleagues used an optical component called a catadioptric lens that consists of a concave meniscus with a reflective coating on the back. The lens acts as a concave mirror that corrects the spectrometer’s Petzval field curvature (that is, the optical aberration that prevents an object from being properly brought into focus). Because it combines reflective and refractive elements into a single component, the lens makes it possible for the spectrometer to control optical aberrations efficiently in a smaller volume.
In another miniaturizing move, the researchers used a special flat reflection grating that they immersed in a refractive medium, rather than in air as is usually the case. The flat grating takes up less space than a conventional convex or concave grating, but has the same spatial resolution, and Lockwood notes that it is also easier to make than a curved grating. While a flat grating can be made using a greyscale photolithographic microfabrication technique that only requires a one-time exposure, curved gratings require labour-intensive electron-beam processing, he explains.
New design fits on a smallsat
The researchers tested their new device on an aquatic remote sensing system made up of several CCVISs (each with a spectral range of 380 to 1050 nm) stacked behind a freeform telescope. The entire ensemble would easily fit onto a small satellite (smallsat) platform, and the miniaturized nature of the spectrometry modules means that they could be stacked in even greater numbers to further increase the field of view, Lockwood says. The spectrometers’ compactness also makes it easier to keep them at a stable temperature, thus ensuring consistent performance.
Lockwood tells Physics World that devices of this type could be deployed on a low Earth orbit satellite platform to map, monitor and track changes in coastal and inland aquatic systems ecosystems from an altitude of 500 km. Such measurements would come in useful for projects like the US National Research Council’s Decadal Survey, which aims to assess the characteristics and health of terrestrial vegetation and aquatic ecosystems, he explains. Surveys like these are important for understanding key consequences such as crop yields, carbon uptake and biodiversity.
“Laboratory and field experiments, even when coupled with numerical models, are unable to quantify ecosystem processes with sufficient temporal spatial and spectral resolution,” he says. “Measuring habitat extent and spatial distribution and observing changes in these quantities can be achieved, however, by detecting the spectral signatures of ‘foundation’ species (such as sea grass meadows, coral reefs, emergent and terrestrial vegetation), their structures and their environment using a compact spectrometer like ours.”
The researchers, who report their work in Applied Optics, are now planning to seek funding from NASA to develop a full prototype that they could test on a real airborne vehicle. “This is always a challenge since NASA receives many meritorious proposals, but our CCVIS is certainly applicable to both terrestrial and planetary missions,” Lockwood says.
