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Could faster breast cancer radiotherapy replace longer treatments?

Accelerated partial breast irradiation (APBI) is non-inferior to conventional whole-breast irradiation in preventing breast tumour recurrence, according to long-term findings of a trial of over 2100 women. The study demonstrated comparable survival following either treatment. However, moderate late toxicity and adverse cosmetic results were greater for women who received the twice-daily APBI (Lancet 10.1016/S0140-6736(19)32515-2).

For women with early-stage breast cancer, whole-breast irradiation after breast conserving surgery reduces local recurrence, improves survival and provides good cosmetic results. However, the treatment usually takes three to five weeks. APBI provides a more convenient radiation treatment, as it is delivered over one week or less. By targeting radiation dose to only the area surrounding the tumour and not the entire breast, APBI techniques enable larger radiotherapy fractions to be delivered in a shorter period with similar toxicity to whole-breast irradiation.

The goal of the RAPID trial was to determine whether external-beam APBI was non-inferior to whole-breast irradiation with respect to preventing local recurrence. Led by Timothy Whelan, of McMaster University and the Juravinski Cancer Centre, the study was performed at 33 cancer centres in Canada, Australia and New Zealand.

The trial included 2135 patients aged between 54 and 68 years with ductal carcinoma in situ or node-negative invasive breast cancer. Patients were randomized to receive either APBI (1070 women) or whole-breast irradiation (1065) following surgery. Tumour size was smaller than 1.5 cm for around 70% of participants.

While APBI can also be delivered via brachytherapy or intraoperative therapy, the researchers chose external-beam radiotherapy for the APBI arm because it is non-invasive, uses modern CT planning systems and because linacs are widely available. Women in the APBI group received 38.5 Gy in 10 fractions administered twice daily, separated by 6–8 hr over five to eight days. Patients in the whole-breast irradiation group received 42.5 Gy in 16 daily fractions, or 50 Gy in 25 fractions if large-breasted. Moderate- to high-risk patients in this group also received boost radiation to the primary tumour site.

Sixty-five patients developed breast cancer recurrence, including 37 who received APBI and 28 who underwent whole-breast irradiation. The eight-year cumulative rates of ipsilateral breast tumour recurrence (IBTR) were 3.0% in the APBI group and 2.8% in the whole-breast irradiation group. In more than half of these, recurrences occurred at or near the primary site, though the researchers note that more women had recurrence in other areas of the breast in the APBI group than in the whole-breast irradiation group.

The trial found no differences in disease-free, event-free survival and overall survival between the two groups, and comparable levels of death caused by breast cancer.

Acute radiation toxicity (grade 2 or greater) occurring within 90 days of the start of treatment was much higher for the whole-breast irradiation group. However, the reverse was true for late toxicities, with 32% and 4.5% of the APBI group experiencing grade 2 and 3 toxicities, respectively, compared with 13% and 1.0% for the whole-breast irradiation group.

APBI patients experienced worse cosmetic outcomes three, five and seven years after treatment (evaluated by the patients, nurses and physicians) than those who received whole-breast irradiation. The researchers attributed this to an increase in late subcutaneous tissue fibrosis and skin telangiectasia, toxic effects that led to a deterioration breast appearance that worsened over time.

As a result of these findings, the authors do not recommend the twice-daily APBI protocol, referencing more recent clinical APBI trials that suggest a 6-hr interval between external-beam radiation treatments is insufficient to repair radiation injury to healthy tissues. They note that APBI given once per day might not be associated with increased toxicity and is under ongoing investigation.

The RAPID trial also revealed the unexpected finding that more recurrences occurred away from the primary treatment site after APBI. “The inference is that the part of the breast not radiated is at a higher risk of developing either recurrence or a new cancer,” the authors wrote. They recommend additional research, because partial breast irradiation is based on the observation that most recurrences occur at or near the site of the primary tumour, an assumption that the RAPID trial findings now challenge.

A similar randomized trial of 4216 women treated at 154 centres in the USA, Canada, Ireland and Israel concluded that APBI was not equivalent to whole-breast irradiation for in-breast tumour control. However, the absolute difference in the 10-year cumulative incidence of IBTR was less than 1%. More patients in the APBI group had recurrence-free interval events, but the absolute difference between 10-year recurrence-free estimates was also small. Distant distance-free interval, disease-free survival and overall survival did not differ between patient groups receiving APBI and whole-breast irradiation (Lancet 10.1016/S0140-6736(19)32514-0).

Led by Frank Vincini of NRG Oncology and the Radiation Oncology Institute of St. Joseph Mercy Hospital, this clinical trial had broader eligibility requirements, allowing younger participants, with larger tumours, and including lobular cancer. A total of 4% of the APBI-treated group and 3% of the whole-breast irradiation group developed recurrent breast cancer. The team did not see a similar pattern of late toxicities to the RAPID trial.

“The findings support whole-breast irradiation. However, the absolute differences in ipsilateral breast-tumour recurrence were small, such that APBI might be an acceptable treatment in some patients,” the researchers concluded.

Nanomaterial-coated fabric destroys chemical warfare agents

A textile coated with metal-organic frameworks (MOFs) could make an efficient anti-nerve agent material, according to experiments by researchers at Northwestern University in the US. The MOFs, which are based on zirconium, could act as catalysts to degrade chemical warfare agents such as VX and soman (GD) much faster than existing technologies, which are based on activated carbon and metal-oxide blends. The composite material, which might be used in protective suits and face masks for soldiers on the battlefield, does not require liquid water to work either, as previously thought.

MOFs are periodic nanoporous crystalline frameworks made from metal ions or clusters coordinated to organic ligands. The high surface area that comes from their porous structure means that they can be used in a host of applications, from sensing to gas separation and storage and catalysis.

Recent research has also revealed that the nanopores can effectively capture chemical warfare agents (CWAs) and then degrade them in a catalytic hydrolysis reaction. A team led by Omar Farha has now found that MOFs can continue to degrade these lethal chemicals, even when they are coated onto textile fibres.

MOFs can absorb enough water from the ambient humidity

Although the chemical degradation reaction requires water, the researchers say the nanopores in the MOFs can supply the needed water by absorbing it from ambient humidity. This will make it easier to deploy filters and other anti-nerve agent equipment based on this material in the field, Farha explains.

In their experiments, the team studied a Zr-based MOF with the chemical formula [Zr₆O₄(μ₃-OH)₄(OH)₆(H₂O)₆(BTC)₂]·nH₂O (more commonly known as MOF-808). They began by applying a solution containing this material and polyethylenimine (PEI) to strips of cotton fabric. After allowing the fabric to dry overnight, they exposed it to DMNP, a nerve agent simulant commonly used in academic research labs. These tests showed that the MOF/PEI composite degraded DMNP even in the absence of water.

Fabric is durable and remains active

Farha and colleagues also tested the material’s durability. They found that the MOF remains securely adhered to the cotton and retains its crystallinity even when immersed in water and agitated for 24 hours. The composite’s catalytic activity also remains high after being exposed to ambient air for 100 days. In a final series of tests, they found the material retains its catalytic activity when exposed to sweat, atmospheric carbon dioxide and pollutants such as octane – environmental and physiological conditions similar to what a soldier might face on the battlefield.

Designer metal-organic frameworks grow on graphene

The researchers, who report their work in the Journal of the American Chemical Society (JACS), hope their material will one day replace existing anti-CWA technology – namely activated carbon and metal-oxide blends, which react more slowly to nerve agents. Ultimately, they would like to create an improved MOF composite that instantly detoxifies these agents. “We are also interested in designing fabrics that can degrade multiple agents at the same time,” Farha tells Physics World.

The Euler spiral of rat whiskers, a colourful Inca statue, a “quantum bottleneck” in hiring

Like most humans, I’m not a big fan of rats. I do, however, have a grudging admiration for their cunning and endless adaptability, and it turns out that some of their keen rat-sense may be down to mathematics. In a study of 523 whiskers from 15 individual rats, researchers in London and Manchester, UK found that the variety of whiskers on a rat’s cheek can be described by a simple mathematical equation.

The whiskers all have different lengths and shapes, and their distribution is such that each whisker is represented as an interval on the Euler spiral. The researchers conjecture that this distribution is “a manifestation of linear laws underpinning rat vibrissae [whisker] growth”, similar to the logarithmic spirals that appear in seashells. Also like seashells, the pattern of a rat’s whiskers has function as well as form. “The size and natural shape of each whisker, including its taper and intrinsic curvature, strongly influence the manner in which it deforms, and therefore, the tactile signals in the follicle,” they conclude.

We at Physics World always like to see physics techniques applied to solve problems in other disciplines. This week brought news (via Physics World contributing editor Belle Dumé) that researchers in France have used X-ray fluorescence spectrometry to analyse the colours painted on a statue of the Inca god and oracle Pachacamac. This measurement, combined with the first carbon-14 dating of the statue, has shed light on how the Inca civilization and its predecessors used and valued coloured pigments.

Among other findings, the researchers learned that the statue’s red pigment is not derived from blood, as was previously thought, but from a mercury-bearing ore called cinnabar. This is interesting because cinnabar is uncommon in the Andes, and the nearest source to the Pachacamac site is a few hundred kilometres away. Meanwhile, carbon-dating revealed that the statue was fashioned around 731 AD, nearly 800 years before the Spanish conquest of the Inca Empire, and 700 years before the empire reached its apogee. This confirms that the Pachacamac site was already important for local people before the Incas adopted it as a centre of pilgrimage.

Finally, I was intrigued by an article in the Guardian newspaper about the so-called “quantum bottleneck”. It seems that the world at large, and the UK in particular, is not producing enough people with the skills required for the nascent quantum-computing industry. The article quotes Doug Finke, who manages a website called Quantum Computing Report, as saying that the expansion of commercial quantum computing “has encouraged a number of academics to leave academia and join a company”. Finke goes on to warn that this academic exodus may create a shortage of professors to teach the next generation of students.

I don’t doubt that this is a real problem, but even so, whenever anyone bemoans the fact that physicists are leaving academia for industry, what I hear is, “Talented people are getting hired into well-paid permanent jobs, rather than spending the next five or more years of their lives in itinerant, ill-paid and insecure postdocs.” In a week that also saw a damning report from the Wellcome Trust on academic research culture, I can’t help but wonder whether some of these departing physicists are being pushed away by poor working conditions in universities, as much as attracted by high salaries in companies.

Ultra-flat graphene goes wrinkle free

A new technique to make ultra-flat, wrinkle-free films of graphene could pave the way for a host of applications, including graphene-based flexible electronics and high-frequency transistors. The technique works by introducing protons into the film as graphene is synthesized using chemical vapour deposition (CVD), and its inventors say that it might be extended to other two-dimensional materials such hexagonal boron nitride (h-BN) and the transition-metal dichalcogenides (TMDCs). It could also aid the development of hydrogen storage devices made from layered 2D structures.

Graphene – a 2D honeycomb of carbon atoms just one atom thick – boasts several unique electronic properties. In contrast to conventional semiconductors, which have an energy gap between the electron valence and conduction bands, graphene is a “zero-gap” semiconductor. This means its electron valence and conduction bands just touch each other. At the point of contact, the electrons move at near-ballistic speeds, and their behaviour is governed by the Dirac equation for relativistic electrons – hence the name “Dirac point” for this section of graphene’s band structure.

Linear defects

So far, this electronic behaviour has only been observed in small flakes of graphene that have been shaved off, or exfoliated, from samples of bulk graphite. These flakes are not big enough to be practical for electronic circuits, and although larger, wafer-sized graphene films can easily be produced via CVD, their electronic performance is not as good. This is because CVD-grown graphene, unlike the exfoliated type, contains grain boundaries, atomic vacancies, impurities and wrinkles. These defects act as centres off which electrons can scatter as they travel, thus degrading the material’s electronic properties.

CVD-produced graphene is prone to wrinkling because the graphene must adhere to the surface of a substrate as it grows. If the thermal expansion coefficient of the substrate does not match that of the graphene itself, a change in temperature can lead to linear defects – wrinkles – forming as the ensemble strives to release compressive strain.

Researchers have attempted to reduce wrinkling by performing CVD at low temperatures, using substrates with a similar thermal coefficient to that of graphene, and developing single-crystalline substrates. A team of researchers led by Libo Gao at China’s Nanjing University has now shown that reducing the interaction between graphene and its substrate might be a good, alternative, strategy.

Intercalating hydrogen molecules

The Nanjing team began by introducing a plasma of protons – hydrogen ions – into the graphene’s growth chamber. During the CVD process, some of this hydrogen became intercalated between the graphene and its substrate, causing the two materials to decouple.

Gao and colleagues found that some of the wrinkles disappeared entirely from the graphene thanks to this proton penetration. They believe this is due to decreased van der Waals interactions between the carbon sheet and the substrate, as well as – possibly – an increase in the substrate’s distance from the growth surface thanks to the intercalation process.

The researchers also found that the electronic band structure of their graphene films shows a V-shaped “Dirac cone” (representing the density of states around the Dirac point) similar to the one observed in exfoliated graphene. They argue that this proves the proton-assisted CVD-grown graphene is indeed decoupled from its substrate.

The technique, which is detailed in Nature, could be extended to grow ultra-flat versions of other 2D materials, such as h-BN and the TMDCs, Gao says. It might also make it possible to develop hydrogen storage devices made from these layered materials.

“The physical and electronic properties of our ultra-flat graphene films are homogenous on the large scale, which means they might now be used in higher-performance electronic and photoelectronic devices,” he tells Physics World.

Solar cooking, an ultrasound success story, and why some business models just don’t work out

This episode of the Physics World Weekly podcast features an interview with Alan Bigelow, science director of Solar Cookers International, a non-profit organization that researches and promotes solar cooking. Bigelow explains how solar cookers work, why they are needed and the challenges of using such cookers in harsh environments. Caitlyn Hughes, the company’s executive director, also describes how the cookers can prove invaluable in refugee camps by removing the need for open-fire cooking.

We also hear from Dave Hughes, founding director of Novosound, which is developing novel thin-film ultrasound technology. The start-up company currently makes ultrasound systems for non-destructive testing, for customers in the aerospace or oil and gas inspection industries. Looking ahead, Novosound plans to move into the medical market, with products such as high-resolution ultrasound imaging systems. And a recent £3.3 million investment should certainly help them on their way.

And finally, we discuss why some new companies simply don’t manage to achieve such success and take a look at a couple of particularly bad business models.

Ancient rings surrounding the Sun could have divided the solar system

The clear divide between the inner and outer solar system is the legacy of a ring structure that existed a very long time ago in the planet-forming disc that surrounded the Sun. That is the conclusion of Ramon Brasser at Tokyo Institute of Technology and Stephen Mojzsis at the University of Colorado Boulder, who have combined computer simulations of Jupiter’s formation with observations of the discs surrounding young stars.

The solar system is clearly divided between the inner rocky planets and asteroids, and the outer gas giants – with the border between the two regions lying between Jupiter and the asteroid belt. This difference can be quantified in terms of carbon – with the element being much more abundant in the outer part of the solar system than it is in the inner rocky planets and asteroids. The difference is so stark that astronomers now widely believe that material in the newly-formed Sun’s planet-forming circumstellar disc was similarly divided in terms of its composition.

For some reason, carbon-rich material from the outer solar system has been prevented from migrating into the inner solar system. One explanation for this barrier is that it arose during formation of Jupiter. As the gas giant gathered mass, the theory proposes, it prevented carbon-rich dust and sub-metre “pebbles” from reaching the inner solar system.

Slowly accreting

However, Brasser and Mojzsis claim to disprove this hypothesis in their study. Through simulations recreating the conditions of the early solar system, they showed that Jupiter would not have accreted mass fast enough to create such a significant barrier.

For an alternative explanation, the duo looked to observations made by Chile’s ALMA telescope, which has observed a rich variety of ring structures in the circumstellar discs of many young stars. They propose that similar rings were likely to have existed around the Sun as the planets were forming. If so, they could have created regions of high-pressure gas and dust which would have been difficult, though not impossible, for carbon-rich objects to cross.

If these ring structures lasted long enough, Brasser and Mojzsis argue that they could have fundamentally altered the structure of the solar system, preventing today’s terrestrial planets from acquiring more matter to become giants. They also believe that other high-pressure rings are likely to exist further out in the solar system, and that the gas giants may have formed as they fell into the lower pressure sinks that lay between the rings.

Ultimately, their work suggests the need for a fundamental rethink of Jupiter’s role in the solar system’s characteristic distribution of carbon. With the diversity of observations of circumstellar discs gathered by ALMA, they could also help astronomers to learn more about the formation of star systems other than our own.

The research is described in Nature Astronomy.

Progress in physical chemistry may enhance light-based therapies

Researchers in the US have designed non-toxic silicon nanocrystals functionalized with specialized organic molecules and shown that these materials can readily combine low-energy photons into higher energy ones. This process, known as photon up-conversion, can address several key problems in biology and materials science (Nature Chemistry 10.1038/s41557-019-0385-8).

Recent advances in deep-tissue imaging and phototherapy for cancer treatment have been transformative. Such technologies mainly use near-infrared (NIR) light, which has a higher penetration depth through biological tissue than ultraviolet or visible light. However, as NIR light contains low-energy photons, it may not have enough energy to generate the free radicals needed to kill nearby abnormal cells.

As such, material scientists and chemists have been working to convert low-energy NIR photons to high-energy excited states, using functionalized inorganic nanocrystals (NCs) containing energy-accepting dyes. It is possible to achieve light up-conversion with efficiencies of more than 10% using such materials. However, the NCs employed contain toxic heavy elements, such as lead, which limits how they can be used.

Non-toxic design

To overcome these shortcomings, material scientists at the University of California, Riverside and the University of Texas at Austin – led by Sean Roberts, Lorenzo Mangolini and Ming Lee Tang – replaced the toxic NCs with non-toxic silicon infrared absorbers. Using silicon NCs to upconvert photons holds promise for their application in medicine, to generate light that can penetrate far enough into biological tissue and have high enough energy to generate the therapeutic radicals.

Making silicon nanocrystals — The reactor used to produce the silicon nanocrystals. (Courtesy: Lorenzo Mangolini/UC Riverside)

In the new design, the researchers employed nanocrystal-to-molecule triplet energy transfer to achieve photon up-conversion. The silicon NCs absorb 488–640 nm photons and produce excited electron–hole pairs (excitons). These excitons then transfer their energy to 9,10-diphenylanthracene (DPA) molecules in solution. As a result, the DPA molecules are excited to a spin-triplet exciton state (in which one electron is excited to a higher energy level than the ground state and its spin is no longer paired with the ground-state electron).

The newly synthesized silicon NCs are functionalized with organic molecules such as 1-octadecene, or a combination of 1-octadece and 9-vinylanthracene that becomes 9-ethylanthracene (9EA) upon attachment. The researchers chose DPA as it exhibits high (above 97%) fluorescence emission. By carefully studying the surface chemistry of the silicon NCs, the researchers learned how to attach surface ligands, enabling them to functionalize the silicon NCs with organic molecules such as 9EA.

To test their technique, the researchers shone laser light into a solution containing silicon NCs and DPA molecules. The silicon NCs absorbed laser light and transferred the energy through the 9EA molecules to DPA in solution in the form of high-energy photons: 425 nm (violet) light. “We got higher-energy light!” says co-first author Pan Xia.

The principle behind this higher-energy output relies on the use of quantum confined nanoparticles and the ability to hold the nanoparticles (silicon NCs) and organic molecules (9EA) close enough together to take advantage of the triplet state of the surrounding molecules.

“The challenge has been getting pairs of excited electrons to transfer between silicon and organic materials. It cannot be done just by depositing one on top of the other,” says Roberts. “It takes building a new type of chemical interface between the silicon and this material to allow them to electronically communicate.”

As well as medical applications, the authors believe that the new silicon NCs also hold promise for use in up-conversion photocatalytic systems and quantum information science. “Photocatalysts generally only work with ultraviolet or violet light, so this is a way to generate that from the rest of the solar spectrum,” Tang says.

Helicopter infrasound shakes historic rock formations

Rainbow Bridge, the largest known rock arch in the world, spans a tributary of Lake Powell in the western US state of Utah. As the main access is via a three-hour boat trip followed by a 1.5 km hike, some visitors opt for a quick helicopter tour to view the spectacular formation. Around 1500 such flights were recorded in 2018.

As early as 2015, a consortium of Native American tribes – Navajo, Hopi, Zuni and others – expressed concern that vibrations from rotor blades might be harmful to the bridge, which is sacred to all of them. Jeffrey Moore of the University of Utah conducted a study, which revealed that Rainbow Bridge has a natural resonating frequency of just 1 Hz and is therefore unlikely to be affected by helicopter flights, at least over human time scales.

But, what about other formations? Southern Utah is home to some 6000 rock arches of varying sizes, as well as towers, known locally as hoodoos, and many other formations that draw visitors from all over the world. How vulnerable are they to vibrations from the thousands of helicopter flights that approach them each year?

At the annual meeting of the American Geophysical Union (AGU), held in San Francisco, California, in December, University of Utah graduate student Riley Finnegan described a project she led to answer those questions. Under Moore’s supervision, she studied 11 sites on public lands, mostly in remote areas, but also in heavily visited Bryce Canyon National Park.

Little Egypt hoodoos — Riley Finnegan (atop formation) and colleagues place geophones to determine the effect of helicopter flights on the hoodoos known as Little Egypt in Utah. (Courtesy: Geohazards Research Group, University of Utah)

Finnegan and colleagues fitted rock formations with geophones and other instruments and then analysed the impact of helicopters, both regular tourist trips and specially chartered flights. She told reporters at AGU that twin rotor helicopters produce infrasound at 13 Hz, well below the threshold of human hearing, but at up to 100 dB, a deafening level if it were audible. Those powerful vibrations can cause some towers and arches to shake up to 100 times more strongly than they normally do, she found.

“You can consider any kind of arch like a guitar string. If you pluck the guitar strings, it will vibrate at certain frequencies,” Finnegan explained. “The Earth is constantly plucking the arch, so it is constantly vibrating.”

Relevant factors include distance, speed, number of blades and angle of the helicopter, as well as the size, shape and stiffness of the rock formation. One arch Finnegan described vibrates at 6 Hz, 16 Hz, 26 Hz and 36 Hz. “Twenty-six hertz happens to coincide with a sound frequency emitted by a helicopter,” she noted, which amplifies the arch’s natural vibration. She likened it to timing the pushes on a child’s swing to make it go higher and higher.

At another arch, visited by as many as 1000 helicopters per year, although no closer to it than 600 m, vibrations during flights increased by 100 times. However, “the vibrations don’t reach a level that we would consider instantaneously damaging,” Finnegan said.

Moore notes that the National Park Service (NPS) does not control the air space over its parks. At some facilities, like Rainbow Arch, voluntary agreements have been negotiated between NPS and tour operators to control the number, timing and location of flights. Still to be determined, he says, is the long-term influence of thousands of helicopter tours on rock formations that evolved for thousands of years in isolation from human energy sources.

Didier Queloz: the Nobel laureate searching for new worlds

How did you learn that you had won this year’s Nobel Prize for Physics, which you shared with Michel Mayor and James Peebles?

I was in a meeting when it was announced and so didn’t actually take a call from the Nobel Foundation. I first heard about it from the press office at the University of Cambridge who asked me whether I was aware that I had won. I simply replied, “You’re kidding me?” At first I didn’t accept it and had to double check.

Did you ever expect it would happen so soon?

Yes and no. We had been told for almost 20 years that our discovery would be worth a Nobel prize, but you can’t keep thinking about it otherwise you become obsessive. I was expecting it a little after the 20th anniversary of the first exoplanet discovery – which myself and Mayor made in 1995 – but after that I gave up. So the news caught me by surprise.

Has it had an impact on your life and research yet?

My research is a disaster at the moment. The number of e-mails I receive has exploded and I’m spending a lot of time answering calls. When you win a Nobel prize, you become an ambassador for science. Everybody in the field knows my work but other people will want to hear opinions about research. This is new to me and I don’t know exactly how I’m going to deal with it but I feel too young to give up on research. It’s in my blood, so I will find a compromise.

How did it feel to discover 51 Pegasi b – the first exoplanet?

Initially, bad. I was just a PhD student at the time and it is not easy to digest such a big discovery when so young. At 29, I had probably made the biggest impact I would possibly ever make. It changed my life.

What was the reaction to the discovery?

For years, most people didn’t believe that this kind of planet existed, but I always knew the field would advance. I started to enjoy the discovery about 10 years ago. But looking back I realise how stressful this has been to live with.

Did you ever imagine that so many exoplanets would be found following your discovery?

In my PhD defence, I noted that 51 Pegasi b is the tip of the iceberg. This was based on our then limited capacity to detect planets. But what is happening now is beyond my most optimistic expectations and we have since discovered over 4000 planets orbiting other stars.

How has it changed our view of the universe?

It has completely changed our understanding. We now know that our solar system is not unique – although we have not yet found a solar system-equivalent. We now need to better understand how solar systems form. It is a fascinating time.

What is the future of exoplanet research?

To understand the origins of solar systems, we need to really understand all the planets we are finding. This includes the structure of these planets as well as the size, mass and atmosphere. Does a planet have a primary atmosphere from when it formed or a secondary atmosphere which could show geophysical effects? Or perhaps there’s a third atmosphere from biology or chemistry activity at its surface? These are the questions that the next slew of experiments will address. Beyond this, is the question of life in the universe.

Do you think we will ever find evidence of life on an exoplanet?

I’m part of the Terra Hunting Experiment that will aim to detect an Earth-twin. Once we find it, we’ll need to design a mission to learn about this planet. Hopefully within 50 years we will find life.

What upcoming space-based exoplanet missions excite you?

Europe has a couple of major missions planned in the coming decade. In 2026 the European Space Agency will launch the Planetary Transits and Oscillations of stars to search for planets around a million stars, while two years later the Atmospheric Remote-sensing Infrared Exoplanet Large-survey will take-off to study and characterizing exoplanets’ chemical composition. Before then, NASA is expected to launch the James Webb Space Telescope in 2021.

What impact do you think the prize will have on new exoplanet missions?

As the field already has a lot, it may not get so much out of this prize in the short-term. But the impact of a Nobel prize is to go beyond a particular field of research. For example, I’m now talking with geophysicists and chemists to study the origin of life on Earth.

Were you surprised that the Nobel committee split the 2019 award between cosmology and exoplanet research?

There could have been a prize just for discovering exoplanets so why did the Nobel committee include cosmology as well? I don’t know, but I’m glad it did. Looking for planets or investigating the origin of the universe is the same topic. Some colleagues say that cosmology is different from planetary research, but I think this is nonsense. We’re all looking for the origin of life and the universe and we need to work together more.

Lithium-6 enriched semiconductor is efficient detector of thermal neutrons

A semiconductor material containing lithium-6 shows great promise for creating highly-efficient detectors of thermal neutrons. The 2D layered material was created and tested by scientists in the US and could someday be used in detectors at neutron scattering facilities – boosting the efficiency and spatial resolution of experiments. Devices based on the semiconductor material could also be developed as personal radiation monitors and have security applications such as detecting radioactive materials like plutonium.

Thermal neutrons are low-energy particles that are created in huge quantities in nuclear reactors, and more recently in accelerator facilities. They have a de Broglie wavelength that is on par with the spacing of atoms in solids and molecules and therefore can be used in diffraction experiments that probe the structure of materials.

Thermal neutrons are difficult to detect unless they happen to be absorbed by a nucleus, which then decays to charged particles that can be detected. The most common thermal neutron detector is a tube containing helium-3, which decays to two hydrogen ions when it absorbs a neutron. Although popular, these detectors have two downsides: they are relatively bulky; and helium-3 is a rare isotope that is becoming increasingly expensive.

Isotopic enrichment

The new neutron detector has been created by Mercouri Kanatzdis and colleagues at Northwestern University and Argonne National Laboratory. At its heart is a 2D semiconductor comprising lithium, indium, phosphorous and selenium (LiInP₂Se₆). The material is enriched to 95% with the isotope lithium-6, which is normally 8% of naturally-occurring lithium.

Lithium-6 nuclei like to absorb neutrons and decay to tritium and helium-4 ions, which share about 5 MeV of kinetic energy. The lithium-6 decay process delivers more energy than helium-3 or boron-10, which is also used in neutron detectors, and this means that it should create a stronger and less noisy detection signal.

Physicists have developed several schemes for detecting the lithium-6 decays, but all of these have their shortcomings. One design involves embedding lithium-6 in a scintillator in which the ions create flashes of light that can then be detected.

Direct versus indirect conversion

“Scintillation detectors are indirect conversion detectors and require photomultiplier tubes to collect the scintillation light generating by the nuclear reaction, which causes a large volume of the overall detection system, explains Kanatzdis. Instead, Kanatzdis and colleagues have taken a “direct-conversion” approach in which the absorption and detection processes occur in the same material – LiInP₂Se₆.

“A direct-conversion semiconductor-based thermal-neutron detector can directly convert the neutron events into detectable charges and collect those charges in the same semiconductor medium (without the scintillation process acting as ‘middleman’),” explains Kanatzdis. “This can increase spatial resolution. The detector volume could be greatly reduced and still exhibit high detection efficiency to thermal neutrons.”

Their detector works much like a conventional semiconductor detector — a biasing voltage is applied across a piece of LiInP₂Se₆ semiconductor. When a lithium-6 nuclei decays after absorbing a neutron, the helium and hydrogen ions plough through the semiconductor creating mobile electrons and holes. These charges are swept across the semiconductor by the bias voltage and detected as an electrical signal.

Easy integration

Because it is semiconductor based, Kanatzdis says the “detector could be readily integrated into the current semiconductor electronics system with fast processing characteristics and compact size”.

The team used a technique called chemical vapour transport (CVT) to create LiInP₂Se₆crystals with an area of about one square centimetre and thicknesses up to 1 mm (see figure). The response of the detector was assessed first using a source of alpha particles (helium-4 nuclei), which deliver approximately the same energy to the semiconductor as neutron-absorption decay events. The detector showed a very favourable response when compared to the performance of other neutron detectors that have been tested using alpha particles.

Then they studied the response of their detector to a flux of thermal neutrons that was created by moderating the output of a radioactive neutron source. Despite the flux being very low, thermal neutrons were detected. “LiInP₂Se₆ is the first semiconductor direct thermal neutron detector to demonstrate a resolved, full energy neutron peak above the background,” says Kanatzdis.

The team says that the performance of their detectors could be further boosted by growing larger crystals, which should be possible. Ultimately, they claim that “LiInP₂Se₆ has the potential to transform neutron detection technology”.

According to Kanatzdis, the detectors could be used to create “more compact neutron spectrometers at facilities like ILL or SPS where elastic and inelastic neutron scattering experiments are done”. Also possible are portable devices for homeland security applications such as the detection of materials such as uranium and plutonium. The compact nature of the detector means that it could be used in radiation safety applications such as personal radiation dosimeters.

“Our LiInP₂Se₆detector also demonstrates a negligible gamma-ray sensitivity, meaning it will not create a false alarm by confusing gamma-rays with neutrons,” he adds.

The new neutron detector is described in Nature.

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