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Astronomers propose telescope to monitor Betelgeuse dimming

An international team of astronomers has proposed a telescope to monitor the bright star Betelgeuse to provide clues about the cause of its sudden drop in brightness. The Betelgeuse Scope concept – which is anticipated to cost about $0.4m – would use twelve off-the-shelf 10 cm-aperture telescopes secured to a radio telescope dish to provide detailed, nightly observations of the supergiant star.

Betelgeuse’s “great dimming” began late last year and changed the naked-eye appearance of the constellation Orion. With it continuing to enthral astronomers, theories have emerged to explain why Betelgeuse’s glow has plummeted. A leading contender is that the star’s surface churned out an immense dust cloud that hid some of its famously ruddy light. To closely scrutinize this “mass-loss” activity, researchers will need frequent, high-resolution views of the roiling surface of the star, which are difficult to acquire with most telescopes right now, but possible via interferometry by connecting several telescopes as if they were one instrument.

The idea of a dedicated Betelgeuse Scope is a really nice one
Graham Harper

The team is building a prototype of the Betelgeuse Scope that would be placed on a University of Arizona 6.1 m radio antenna and are now seeking funding for the final telescope. “If successful, we will bring it to a larger antenna [of] 12 m or more to increase the interferometer array size,” says astronomer Narsireddy Anugu from the University of Arizona, who is leading the project. By using relatively inexpensive instruments affixed to the structure of an already-constructed radio telescope, the final Betelgeuse Scope system should be cheaper than a more complex, conventional arrangement. “It also saves money by using the pointing and tracking of the existing radio antenna,” adds Anugu. “So we don’t have to build it for all the individual amateur optical telescopes.”

A step forward

Graham Harper, an astrophysicist at the University of Colorado who has studied Betelgeuse and is not associated with the Betelgeuse Scope, says that if the proposal is successful then it would be “a major step forward” beyond existing observations. “Coordinating major observatories to look at a common object is difficult enough, even for one-off events, but for systematic monitoring of a couple of sources it is totally impractical,” he says. “The idea of a dedicated Betelgeuse Scope is a really nice one, because it attempts to address this problem.”

Astronomers plan huge neutrino observatory in the Pacific Ocean

According to Harper, the proposed telescope would also glean information about Betelgeuse’s surface temperature. “They should be able to determine how the surface temperatures change with time across the stellar surface,” he adds. “This would help tell us exactly where the interesting phenomena are occurring, for example shock waves, outbursts and ejections.”

Accretion, not colliding spaghetti, flares up as star is devoured by black hole

An extremely bright flare, originating from a star being devoured by a supermassive black hole, has been observed in a galaxy 215 million light-years away – making this the nearest tidal disruption event (TDE) ever seen. The event was spotted by an international team of astronomers, headed by Matt Nicholl at the University of Birmingham. They caught the event well before its climax using instruments at the European Southern Observatory (ESO) in Chile. For the first time, the observations allowed astronomers to connect the characteristic brightening of these events with rapid outflows of material from stars.

If a star wanders too close to the supermassive black hole at the centre of its galaxy, it can experience tidal forces that exceed the gravitational forces holding the star together. As a result, the star will be dramatically shredded into thin streams of debris, through the process of spaghettification. During these TDEs, stellar remnants will flare, emitting large amounts of light.

Colliding spaghetti

Currently, there are two competing theories for these surges in brightness: either they occur as material accretes onto the black hole; or they result from earlier collisions between spaghettified streams. Although astronomers are now discovering several TDEs every year, they have yet to determine which of these theories is best. The main problem is that as the stars disintegrate, their colliding streams, combined with strong inflows and outflows of gas, make for a messy debris geometry which is incredibly difficult to disentangle.

In September 2019 the ESO’s Very Large Telescope and New Technology Telescope each spotted a new surge in brightness in a spiral galaxy. Through further calculations, Nicholl’s team concluded that the flash originated from a supermassive black hole as large as 1 million solar masses, as it devoured a star with a similar mass to the Sun.

Over six months, the instruments recorded the event across multiple regions of the electromagnetic spectrum as its brightness grew and faded. Owing to its proximity, Nicholl and colleagues were able to identify the TDE well before its peak brightness. This allowed them to capture the whole process unfolding, well before the geometry of the debris became too convoluted to untangle.

Sudden transition

By tracking changes in the blueshift of key absorption lines in the stellar debris, Nicholl’s team showed that the origin of the TDE’s early optical emission was dominated by an outflow of bright material, with speeds reaching roughly 10,000 km/s. Then, after around 30 days, the outflow underwent a sudden transition: first cooling, and then contracting.

Overall, the size and mass of the outflow remained consistent with the high ratio between optical and -ray emissions observed by the team, as well as in many TDEs from previous studies. This suggested that the outflow was more likely to have been powered by black hole accretion, instead of collisions between spaghettified streams of debris.

With the upcoming launch of ESO’s Extremely Large Telescope, now scheduled for first light in 2025, the team’s findings will provide researchers with key guidance as they uncover increasingly fainter and more rapidly evolving TDEs.

The research is described in a paper in Monthly Notices of the Royal Astronomical Society.

Physics in the pandemic: ‘Our physics and dosimetry facilities were vacated for COVID-19 patients’

India reported its first case of COVID-19 infection on 30 January 2020, and currently has the largest number of confirmed cases in Asia. The first case in the state of Rajasthan was in Jaipur, reported on 3 March. Since then, the SMS Medical College and Hospitals in Jaipur has served as the main COVID-19 treatment centre in Rajasthan. The hospital established a fully equipped outpatient department and isolation wards, as well as procuring a number of new X-ray machines and mobile X-ray units for diagnosis and treatment evaluation of COVID-19 patients.

As part of this transition, with the exception of the radiation delivery facility and a small intensive care unit for cancer patients, all other radiotherapy and radiological physics facilities were utilized for COVID-19 patient management. When lockdown was initiated end-March, there were four medical physicists in the hospital (two of whom were in vulnerable groups). Having handed over department space for COVID-19 management, they had to evolve rapidly to cope with the ensuing professional and personal challenges.

The hospital’s radiological physics department, which provides medical physics services to all departments that utilize ionizing radiation, took the lead role in managing cancer patients. As well as maintaining equipment and infrastructure, the team needed to rearrange radiation physics equipment and facilities to ensure patient services continued unhindered. Challenges included establishing protocols for radiography and radiotherapy delivery, maintaining the quality of diagnostic and treatment systems, achieving high patient throughput in the minimum time possible and managing the workload with a reduced workforce.

Priya Saini is a medical physicist in the hospital’s radiological physics department. Here is her account of maintaining a radiotherapy service in the midst of a pandemic.

Optimizing efficiency

During lockdown, I worked on cobalt teletherapy (on the Bhabhatron-II system) and brachytherapy. My work included treatment planning, radiation safety monitoring, quality assurance (QA), routine equipment calibration and treatment plan reviews, as well as teaching and research.

In early March, at the start of the pandemic, COVID-19 isolation wards were created in one or two departments in our hospital. As the number of patients increased, our cancer wards and other departments’ wards were also converted into isolation wards. Our physics and dosimetry facilities were vacated for doctors and nurses treating COVID-19 patients. At that time, only one room was left for us to conduct our routine work.

In this early phase of the COVID-19 pandemic, I had many difficulties in managing patients because of lack of awareness and fear of this virus. Then I spent extra time reading the available information and followed the guidelines given by WHO, and slowly I overcame my fear.

In our department, we used to treat 100 to 120 patients each day on the Bhabhatron-II telecobalt machine. This number quickly halved, because it became harder for patients to travel to the hospital and some had already returned to their hometowns or villages. However, some patients recommended for surgery were transferred to radiation therapy, so workload in our department slowly increased.

For certain cases, including some patients with early-stage cancer, we delivered radiation therapy over a shorter period of time. The main reason for such hypofractionated treatments was to minimize viral exposure and the risk of contaminating patients, without reducing the effectiveness of the treatments. Our aim was to establish a better way to treat all patients who can benefit from radiotherapy; and not to delay the start of treatment of any patient whose deferral may worsen the prognosis of their disease.

After lockdown, as the necessity of providing regular medical services to the general public became essential, our hospital resumed normal activity by shifting the COVID-19 patients to the university hospital and radiation treatments continued normally again. Every day, healthcare workers screened patients with thermal scanning before registering them for treatment and providing them with hand sanitizer.

To help avoid cross contamination, I made a separate box in the manual treatment planning room for each patient to store their treatment documents. I also instructed the security guard to send only one patient at a time into the room, with every patient advised to maintain physical distancing of one metre minimum. Before starting treatment, every patient was verified to be COVID-19 negative.

At the brachytherapy treatment console, three people were present at the time of treatment: one technologist, one resident doctor and myself. We maintained physical distancing from each other. We did not allow patients or their companions to enter the treatment planning room or console and instead interacted with them while maintaining a one metre distance outside the minor operating theatre.

Additional challenges

Personal protective equipment (PPE) has become an emotive subject during the current COVID-19 epidemic, as an important component, though only one part, of a system to protect staff and patients from cross-infection. During the lockdown period, however, the temperature in Jaipur reached up to 46°C. As per guidelines at that time, we were not using centralized air conditioning, to prevent the spread of virus via air circulation. So it was very difficult, as well as unhealthy, for us to wear PPE kit while treating the patients.

In cases of intraluminal brachytherapy, there was direct interaction with patient while taking the measurements for planning. During such instances I wore PPE kit. After treatment completion, the PPE kits were discarded properly and I washed my hands and face with soap.

To increase awareness of COVID-19 to the patients and others, we pasted notices in the treatment planning room, the calculation room and on other rooms’ doors, with messages in local languages: “no entry without a mask”, “maintain social distancing” and “do not enter without permission”. We asked all ongoing patients about symptoms and other health-related issues; if found symptomatic, patients were sent for COVID-19 tests.

My other roles included QA of the machines in the department and teaching. We performed mechanical QA of both treatment machines weekly and performed dosimetry QA monthly. Before starting, we would sanitize the treatment room and control console to prevent contamination and spread of the virus.

During this period, a few resident doctors in my department became COVID-19 positive. I had interacted with one of them two days earlier, which made me scared. I did self-assessments for 3–4 days and also consulted with a general physician. Following that, two technologists posted on the Bhabhatron-II machine tested positive for COVID-19. Following institutional protocol, they were home quarantined for 14 days. After quarantine, they tested negative and re-joined the hospital.

As per local government instructions, undergraduate paramedical students were taught via online Zoom classes during lockdown. There are five postgraduate students and, after lockdown, they re-joined regular classes. I conducted many classes for students of radiotherapy technology and radiology technology, maintaining proper distance in a department seminar room. I also instructed them regarding hospital protocols, the necessity of proper hygiene, and not to sit together without masks and maintaining social distancing.

Recently, one of the students got severe symptoms similar to COVID. He was asked to take a COVID-19 test and not to return until he had a negative result, while the other four students were asked to self-quarantine. After 24 hours, he tested negative and rested for a few days to recover from weakness.

The COVID-19 pandemic has affected everyone globally, and our department of radiological physics is no exception. We are striving to keep ourselves and our near and dear safe, while continuing to provide medical physics services to all radiological facilities of our institute, without compromising international standards.

Software tool for MRI predicts motor development disorders in preterm babies

A new software quantification tool has been developed by researchers in the US for analyzing white-matter abnormalities in very preterm babies. This magnetic resonance imaging (MRI) biomarker can predict motor development risks, including cerebral palsy, in a much more objective way than existing diagnostic tools.

Today, most children born extremely prematurely (earlier than 30 weeks of gestation) survive. However, nearly 70% of these very preterm infants show abnormalities in the white matter of their brains. Furthermore, half of very prematurely born children develop minor motor abnormalities, and up to 10% develop cerebral palsy (CP).

CP is a group of permanent neurological disorders that affect movement, posture, and balance. It is the most common physical disability in children. Even though the last three decades have brought improved survival and care for extremely preterm babies, they are about 50 times more likely to develop CP than full-term babies. Improved care for these children has not translated into fewer disabilities, since most children with CP are diagnosed at 1–2 years of age and conventional neuroimaging is subjective and not sensitive on its own to accurately predict CP.

Early intervention

Yet, identifying biomarkers of CP at an early stage of life could significantly increase the quality of life for both affected patients and their families. Early intervention by physical and occupational therapy could take advantage of early neuroplasticity and mitigate symptoms such as tremors, stiff muscles, or difficulty performing precise movements.

To address these issues, a group of researchers led by Nehal Parikh, a neonatologist at the Perinatal Institute, Cincinnati Children’s Hospital Medical Center, has been developing better diagnostic tools using advanced imaging risk factors to improve outcome prediction. They have developed an algorithm that can analyze structural MRI scans of preterm infants to predict their risk of motor development abnormalities based on spotting and quantifying brain defects.

Fully automated detection

Parikh’s team targeted diffuse white matter abnormality (DWMA), the most common finding in premature infants. It appears on a brain MRI scan as areas with increased signal intensity. Early evidence suggests these abnormalities are associated with inflammation-initiating illnesses and may represent areas of increased fluid in the brain’s wiring.

Current tools to assess DWMA rely on visual analysis of the MRI data. “Manual tracing of DWMA regions within the brain is highly subjective and produces poor reliability and reproducibility,” explains Parikh. “It is clear that we need a better tool for diagnostics. I started working with computer scientists to come up with an algorithm that can segment these lesions.”

In the study, the team enrolled very prematurely born infants and performed brain structural MRI scans at the term-equivalent age. These images were fed into an algorithm created using a probabilistic brain atlas constructed specifically for a very preterm infant brain.

“Since it was not possible to create a gold standard manual brain atlas for DWMA, Lili He, a computer scientist in my lab, decided to use a synthetic one to train the algorithm. I drew out the respective brain regions manually,” explains Parikh. The resulting algorithm could detect signal intensity and location of normal white matter, gray matter, and cerebrospinal fluid and isolate DWMA regions based on their different signal intensity and spatial information with an accuracy greater than 95% when compared with the ground truth.

Towards clinical translation

Critics might say that the algorithm represents a computer simulation trained on a synthetic brain model that is not relevant to a real patient. However, it can already predict clinical outcomes relevant to patients and their families. Indeed, the team correlated their results with cognitive and language scores of their cohort at the age of two; and motor scores at three years of age. “The results correlated very nicely,” says Parikh. “It showed that DWMA volume was predictive of standardized developmental scores up to three years of age, independent of other conventional MRI and other known predictors.”

The team is now working on implementing a deep learning approach to improve the DMWA region identification. In addition, Parikh is planning to work with leading MRI vendors to implement the algorithm into their consoles to allow for the earliest diagnostics possible.

“Ideally, when the MRI scan is complete, our algorithm would do its job within seconds and provide a value for diffuse white matter abnormality,” he says. “If this value was abnormal, the child would be referred for aggressive early intervention therapies and/or research trials of novel interventions.”

Manon Benders, a neonatologist at the Wilhelmina Children’s hospital, UMC Utrecht, foresees a big future for this research as a decision-support tool. “It is very important since obtaining the most accurate prognosis as early as possible is essential in order to inform the child’s family adequately.”

“Moreover, beginning the intervention therapy even before the onset of clinical symptoms of cerebral palsy or adverse motor outcomes is also crucial, particularly given that the brain’s plasticity is highest in the first few months after birth,” she adds.

These research is described in Scientific Reports.

Celebrating Emmy Noether, Sameera Moussa, Caroline Bleeker, Toshiko Yuasa and other inspiring women in science

Tuesday was Ada Lovelace Day, which celebrates achievements of women in science, technology, engineering and maths (STEM). Named after the 19th-century polymath Ada Lovelace, the annual initiative also seeks to engage with the challenges of attracting more women into STEM careers and supporting career development.

Nature’s On Your Wavelength blog celebrated with a piece by Ankita Anirban about five inspiring female physicists. Perhaps the most intriguing of the biographies is that of the Egyptian nuclear physicist Sameera Moussa, who some believe was murdered in 1952 to prevent Egypt from developing nuclear weapons.

There are also tales of wartime daring. The Dutch physicist and entrepreneur Caroline Bleeker, for example hid Jewish people from Nazi occupiers in her factory. And when it was raided in 1944, she managed to usher them to safety. After the war, her factory produced the world’s first complete phase contrast microscopes.

Meanwhile in wartime Berlin, the Japanese physicist Toshiko Yuasa developed a double-focussing beta spectrometer that she carried on her back through Siberia to Japan when she was expelled from Germany by the Soviet army.

Emmy Noether: The most important mathematician you’ve never heard of is a new book from the Canadian children’s author Helaine Becker. Illustrated by Kari Rust, the book was commissioned by Lisa Lyons Johnston, president and publisher of Kids Can Press. Lyons Johnston became aware of the remarkable life and work of Noether when she joined the Emmy Noether Council at the Perimeter Institute for Theoretical Physics.

“Boys and girls, men and women, need to see more women in physics and science,” says Lyons Johnston. “It’s so frustrating that people haven’t heard of her. I wanted people to know more about Emmy, who is a remarkable person in her own right, and also an inspiration for girls.”

You can read more about the book here.

Sodium nanofluid helps extract heavy oil

Zhifeng Ren, director of the Texas Center for Superconductivity at UH. Credit: University of Houston

With the world’s reserves of light oil diminishing, oil companies are increasingly turning to heavier varieties – which make up 70% of global oil reserves – to meet rising energy demands. Existing extraction technologies for heavy oil are, however, inefficient, expensive and damaging to the environment in ways that extend beyond the CO₂emissions produced when the oil is eventually burned. A team of researchers from the University of Houston in the US have now developed an alternative method that uses an inexpensive, non-toxic sodium-based nanofluid to extract heavy oil from reservoirs, with an efficiency of 80% in laboratory tests.

Reserves of heavy oil – so-called because it has a viscosity of more than 100 centipoise, or cP – and extra-heavy oil, with a viscosity of more than 10 000 cP, are currently being exploited in several areas, notably Canada and Venezuela. The high viscosities of these grades of oils make them difficult to recover, and two main technologies are currently used to do it. The first relies on extracting oil from surface sands using hot water and air bubbles, then diluting it with solvents such as n-pentane or n-heptane. This method has been used for decades, but it requires large quantities of water and is thus not suitable for all locations.

Since most heavy oil resources are located below the surface, the second, in situ, recovery method is more common. In recent years, researchers have developed both thermal and non-thermal variants of these in situ techniques. The former, which include cold production with sands, vapour extraction, chemical injection and “flooding” the oil with steam, can be used for thin layers of oil formations. The problem is that these techniques are limited to shallow layers and relatively light (< 200 cP) viscous oils.

Alternative techniques sought

While thermal methods like in-situ combustion and steam flooding can recover oil with higher viscosities (potentially more than 70%), they are only economically viable when oil formations are thick. Producing the steam they require also consumes fuel such as natural gas, leading to additional CO₂emissions.

To overcome these challenges, researchers have begun to explore alternative techniques, including some that use nanomaterials. Such approaches are not yet mainstream, and so far the nanomaterials developed have only played a secondary role – for example, modifying the flow behaviour (rheology) of crude oil or being used as catalysts to upgrade crude oil during steaming processes.

80% recovery efficiency

A team led by Zhifeng Ren, who is director of the Texas Center for Superconductivity at the University of Houston (TcSUH), has now used a sodium (Na) nanofluid to recover over 80% of extra-heavy oil (with a viscosity of over 400 000 cP) from sand packs in a laboratory experiment. The researchers made this nanofluid by using a hand-held blender to mix commercially-available bulk sodium chunks with silicone oil. This non-reactive solvent is necessary because (as we all remember from chemistry classes at school) sodium reacts very strongly when it comes into contact with water.

When the researchers do add water to their sodium nanofluid, it reacts according to the formula 2Na + 2H₂0 –> 2NaOH + H₂. The substantial amount of heat produced in this reaction generates steam, which then reduces the viscosity of the heavy oil in a way that is similar to older steam flooding techniques – with the advantage that the sodium reaction is relatively easy to control and can be kick-started in situ simply by adding water. The sodium nanomaterials also dissipate once the reaction is complete, which means that the oil is not damaged by adsorbing the sodium. Finally, the technique does not use any fossil fuels to generate steam or hot water, thereby reducing the direct CO₂emissions from the oil-extraction process.

“Based on these advantages, we anticipate that the sodium nanofluid could become a game-changing technology for recovery of oil of any viscosity, as well as a milestone in using nanotechnology to solve oil-recovery problems in the petroleum industry,” Ren and colleagues say.

Side benefits

The researchers note that by mixing their sodium nanoparticles in silicone oil, the particles dispersed throughout the simulated reservoir before coming into contact with water. This dispersion has the effect of triggering smaller chemical reactions across a larger area. The team says it may also be possible to disperse the sodium nanoparticles in other solvents, such as pentane and kerosene, or even to mix them with polymers or surfactants for a higher oil recovery rate.

As a final benefit, the researchers note that one of their reaction’s products, NaOH (sodium hydroxide), is routinely employed in oil fields to spark a different reaction that also reduces oil viscosity – a technique known as alkaline flooding. The other reaction product, H₂, could be used for gas flooding – another common oil recovery technique – as well as for upgrading the heavy oil via a hydrogenation reaction.

The new technique is detailed in Materials Today Physics.

Very high-energy electrons could treat tumours deep within the body

Very high-energy electrons (VHEEs), typically defined as those above 40 MeV, provide a potential new radiotherapy modality with dosimetric advantages. Beams of such electrons penetrate deep into the patient, enabling treatment of deep-seated tumours that photon-based irradiation may not reach.

Speaking at the Medical Physics & Engineering Conference (MPEC), Louie Hancock from the University of Manchester described the recent resurgence in interest in VHEE radiotherapy. “Over the last couple of decades, new linac designs mean that it’s now possible to produce roughly 200 MeV electrons in about two to three metres, whereas before it might have taken 20 metres or so,” he explained. “This has spurred interest in using these VHEEs for treating deep-seated tumours.”

Hancock noted that a linac-based VHEE treatment system should be compact enough to fit into a hospital bunker. “It’s much cheaper to put a machine in an existing bunker than to build a new building,” he pointed out. “I expect that VHEEs will probably be more expensive than photons to produce, but cheaper than protons.”

While there are currently no clinical systems available, there are electron accelerators for research use, such as the high-gradient X-band linac at CERN’s CLEAR facility, for example, and the CLARA electron accelerator at Daresbury Laboratory. In the meantime, Monte Carlo simulations can provide insight into VHEE treatments without having to actually build a machine.

Depth–dose curves show that, as well as delivering dose deep inside a patient, VHEEs should also be extremely resilient to changes in patient geometry. To confirm this, Hancock performed a back-of-the-envelope calculation for a simple block of water containing a cylindrical 5 cm tumour in its centre. He simulated treatment of the tumour by rotating a beam around the target, and then introduced a cavity above the tumour (a 5 cm sphere of bone or air) to calculate its impact on the tumour dose.

For treatments simulated using 1.3 MeV X-rays, the introduction of an air bubble resulted in hot spots of about 15%, while an unexpected bony region led to cold spots of about 10%. For 250 MeV VHEEs, while hot and cold spots were seen, they were only of about 2%. “In this simple situation, VHEE appears to be nearly an order of magnitude more resilient to unexpected homogeneities when compared with X-rays,” said Hancock.

But will this advantage translate to patients? To find out, Hancock and colleagues examined a clinical case, comparing treatment plans for VHEE and volumetric-modulated arc therapy (VMAT).

VHEE treatment planning is a highly complex process, with millions of variables creating a huge optimization problem. As such, the team at Manchester has developed an open-source treatment planning system for VHEEs that incorporates various tools to create a full planning workflow. This includes organ identification using Slicer 3D software, Monte Carlo dose calculations using Geant4, and then optimization of the generated dose profiles with Python software written by Hancock.

Using their VHEE code, the researchers created a treatment plan for a patient with cervical cancer, which included two large target sites to irradiate and many nearby organs to avoid. They compared this with a VMAT plan creating using Monaco. The two plans delivered identical dose to the target, while nearby organs (the sigmoid colon, bowel and bladder) received slightly less dose from VHEE than VMAT. For organs further from the tumour, VHEE conferred significant dosimetric advantages, particularly to the femoral heads where it reduced the delivered dose from 35 to 15 Gy.

“We achieved coverage of all of the tumour with VHEEs, so can say for sure that these electrons are capable of treating a large tumour deep inside the patient,” said Hancock. “We could also see that the low-dose background was reduced compared with the VMAT plan.”

To examine the impact of geometric changes, the team re-simulated the treatment plans with the rectal cavity filled with air instead of water. The error between the two VHEE plans was roughly 0.15 Gy, while for the X-ray plans it was about 0.7 Gy. “In the simple water phantom, we saw a dose error nearly an order of magnitude lower with VHEEs, now we’ve seen this effect in silico in an actual patient case,” explained Hancock.

The next step, he said, will be to repeat the analysis with lung and brain cases, and “move towards putting real things in real beams”. The real thing is the MARVIN human head-and-neck phantom, while the real beams will be delivered by CLARA at Daresbury and CLEAR at CERN. These two facilities provide a wide span of electron energies and will enable VHEE measurements in realistic environments.

Preliminary simulations for planned experiments using a 45 MeV electron beam at CLARA to irradiate MARVIN demonstrated a near-uniform dose over a large area inside the phantom. Hancock noted that the study is currently on hold due to the pandemic — although there is a potential to perform these experiments in Q1 2021.

Hancock concluded that the clinical test case, using the team’s VHEE treatment planning software, demonstrates that VHEE radiotherapy has the ability to treat tumours that are both deep and large. “VHEE might well also be more insensitive to inhomogeneities and changes in patient geometry than photons, which I think is likely to be clinically beneficial,” he added.

Excitement grows over mysterious signal in dark-matter detector

In June this year, physicists working on the XENON1T dark-matter detector announced the measurement of a curious signal in their experiment – which comprises 2 tonne of ultrapure xenon. The signal had a statistical significance of 3.5σ or less, which is well below the 5σ level that is usually required for a discovery in particle physics.

In a preprint published at the time, the team suggested that the signal – an excess in low-energy electron recoil events – could have three explanations. The most mundane is that it was caused by contamination of the ultrapure xenon by radioactive tritium.

A more intriguing explanation, they said, is that the signal is the detection of hypothetical particles called axions that could be emitted the Sun. The third possibility is that the excess is caused by neutrinos interacting with the xenon in an unexpected way – which would also be very interesting.

Now, the original preprint has been published in Physical Review D and over in Physical Review Letters, five theoretical papers put forth a range of tantalizing explanations for the excess.

Axionlike particle

Working in Japan, Fuminobu Takahashi, Masaki Yamada and Wen Yin say that the signal could be related to a hypothetical axion-like particle (ALP) with a mass of a few keV/c² that interacts with electrons. As well as explaining the XENON1T signal, such an ALP could be a constituent of dark matter and its existence could explain an anomaly in the observed cooling of white dwarf and red giant stars.

Meanwhile, in Germany, Andreas Bally, Sudip Jana and Andreas Trautner reckon the mystery signal could be the work of a hypothetical gauge boson that mediates a new interaction between solar neutrinos and electrons.

A paper written by Nicole Bell et al. makes the case for a “relatively low-mass luminous dark-matter candidate” as the source of the excess. They suggest that this dark-matter particle could enter the detector in a “light state” and be scattered into a “heavy state” that would decay by emitting a photon. This photon would then interact with an electron in the detector to create the observed signal.

Galactic boost

Another dark-matter proposal comes from Bartosz Fornal and colleagues who suggest that otherwise sluggish cold-dark-matter particles could get a boost of energy from the galactic centre and collide with XENON1T electrons.

The fifth idea comes from Joseph Bramante and Ningqiang Song in Canada, who argue that the signal could come from the scattering of a type of dark matter that is a thermal relic from the early universe.

They can’t all be right, and it will be very interesting to see if a similar signal shows up in future dark-matter experiments.

Quantum spin liquid candidate becomes a superconductor under pressure

Researchers in China report that they have observed both superconductivity and an insulator-to-metal transition in sodium ytterbium (III) selenide (NaYbSe₂) simply by applying pressure to it. This inorganic substance, which is also a quantum spin liquid (QSL) candidate, could therefore become a new platform for investigating superconductivity in compounds that have electrons in their f orbitals, and for exploring the mechanisms of unconventional superconductivity in these materials.

Quantum spin liquids (QSLs) are solid magnetic materials that cannot arrange their magnetic moments (or spins) into a regular and stable pattern. This behaviour contrasts with that of ordinary ferromagnets, in which all the spins point in the same direction, or antiferromagnets, in which the spins point in alternating directions. Instead, the spins in a QSL constantly change direction in a fluid-like way – even at ultracold temperatures near absolute zero – and are thus said to be “frustrated”.

The late physicist and Nobel laureate Philip W Anderson proposed the existence of QSLs in the early 1970s, when he was studying the ground state of antiferromagnetically interacting spins on a triangular crystal lattice. Although Anderson did not follow up the idea at the time, he returned to it in 1986 after the discovery of high-temperature superconductivity in copper oxides (cuprates). A year later, his work bore fruit when he uncovered a potentially crucial link between QSL theory and “unconventional” high-temperature superconductivity. He described this link in the so-called resonant valence bond theory.

“Parent states”

Today, QSLs are thought to be the “parent state” for high-temperature unconventional superconductivity in cuprates. This class of superconductors could have applications in many areas, including energy grids, levitating transport and even quantum computing, but the physics underlying them is still not very well understood. Studying QSLs is thus important for condensed-matter physicists if these applications are to see the light of day.

A team of researchers jointly led by Run-Ze Yu and Chang-Qing Jin of the Institute of Physics at the Chinese Academy of Sciences, and He-chang Lei of Renmin University of China in Beijing, used a diamond anvil cell to measure the electronic conductivity of NaYbSe₂under varying high pressures (up to 126 GPa) and a high magnetic field. This crystalline material features a triangular lattice of 4f-orbital Yb³⁺ions bonded to six equivalent Se^2-atoms to form YbSe₆octahedra. These YbSe₆octrahedra share corners with six equivalent NaSe₆octahedra, while sharing edges with six equivalent YbSe₆octahedra.

The researchers found that their sample acts as a paramagnetic insulator (a material with a permanent magnetic dipole moment) at applied pressures below 8 GPa. As the pressure increases from 8 GPa to 50 GPa, the material remains an insulator, but its resistance decreases by nearly eight orders of magnitude. A metallic phase is eventually observed at about 60 GPa and further increases in pressure lead to the emergence of a superconducting state at 103 GPa, with a superconducting transition temperature of 8 K.

Mott transition

This transition from insulator to conductor is known as a Mott transition. Because the material contains exotic spin excitations carrying fractional quantum numbers at low energies, its metallic state behaves like a non-Fermi liquid – meaning that its electrical conductivity depends linearly on temperature. “The origin of the superconductivity should therefore be exotic,” Yu explains. “This implies that the relationship to the resonant valence bond theory proposed by Anderson for copper oxide superconductors needs to further investigated in experiments.”

The fact that the researchers observed such a transition by simply applying pressure means that it is an intrinsic, physical property of the material, free from chemical transformations that are usually introduced by doping. And since Yb³⁺ions contain f-electrons, Yu adds that the NaYbSe₂system provides a new platform to investigate unconventional superconductivity mechanisms in such compounds. “The work will certainly help researchers gain a deeper understanding of the interplay between the QSL and unconventional superconductivity,” he tells Physics World.

Members of the team, who report their work in Chinese Physics Letters, say they will now study the nature of the insulator-to-metal transition in NaYbSe₂in-depth, as well as the intrinsic physical properties of superconductivity in this QSL. “We will also be focusing on the possibility of ‘heavy fermion’ phenomena in this compound,” Yu adds. Such phenomena, he explains, arise when the conduction electrons, which are fermions, move as if they were hundreds of times more massive than electrons in conventional metals, like copper. This large “effective mass” comes about thanks to strong electron-electron interactions, which are also thought to play an important role in high-temperature superconductors.

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