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Three steps to safer stereotactic radiotherapy

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In recent years, stereotactic radiotherapy (SRT) has evolved into standard practice in radiation oncology. SRT of small targets using high dose per fraction with steep dose fall-off requires a comprehensive quality assurance programme to ensure that the prescribed dose is accurately delivered. However, dosimetry is still one of the major challenges faced by many clinical physicists when embarking on SRT.

In this educational webinar, Hui Khee Looe will touch on three important aspects of SRT:

  • Beam commissioning
  • Patient-specific plan QA
  • End-to-end tests

He will address the most frequently asked questions on each of these aspects and provide practical tips and step-by-step guides. Special focus will be placed on common pitfalls and how to avoid them. These will be illustrated by using real clinical cases to help understanding.

“Now is the time to understand more, so that we may fear less.” (Marie Curie)

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Hui Khee Looe received his Master’s degree and PhD in physics from the University of Oldenburg. He is currently deputy head of the Department of Medical Physics at Pius Hospital, where he is responsible for clinical duties in the Clinics for Radiotherapy, Nuclear Medicine and Radiology. In addition, he is a university lecturer and leader of the research group Computational Methods in Modern Dosimetry at Carl von Ossietzky University. Hui Khee has published more than 40 peer-reviewed papers. His research activities focus on dosimetry under non-equilibrium conditions, the development of mathematical models for modern dosimetry, dosimetry in magnetic fields, and multi-dimensional dose measurements.

Icequakes and rogue waves: geoscientists and musicians interpret the sounds of the sea

This episode of the Physics World Weekly podcast looks at how geoscientists and musicians interpret the soundscapes of the oceans in terms of both science and art.

Our first guest is geophysicist Rob Abbott of Sandia National Laboratory in the US. Earlier this year, he led an expedition to the arctic coast of Alaska’s North Slope where they used an undersea optical-fibre cable to listen to rumblings under the sea ice. He talks about detecting icequakes and possibly the icebreaking activities of a whale, as well the challenges of working in temperatures below -40 °C.

Next up is the geoscientist Rónadh Cox and the percussionist Cormac Byrne who share a love of Ireland’s rugged west coast and the bodhrán – a handheld drum associated with Irish folk music. Cox, who is at Williams College in Massachusetts, describes how huge Atlantic waves shape the Irish coastline – often shifting giant boulders. Byrne explains how he teamed up with Cox and musician  Rónán Ó Snodaigh to create music inspired by ocean waves – which he performs for us on the bodhrán.

Stormy rhythms: creating music from the giant waves that shape our coasts

From Debussy’s “La Mer” to the Beach Boys’ “Surfin’ USA”, the ocean has always been a vast font of musical inspiration. Now a new project will create music informed by the science of storm waves and the dramatic impacts they have on coasts.

A collaboration between the geoscientist Ronadh Cox and the musicians Cormac Byrne and Rónán Ó Snodaigh, “Drumming the Waves” is an outreach element of “Boulder Beaches: The Understudied Archive on High-Energy Coasts” – a project funded by the US National Science Foundation.

“I thought it would be nice to do a music project that uses rhythm and percussion to express the physics of waves to create a sense of periodicity, regularity and also chaos, disorder and suddenness,” says Cox, who is a based at Williams College in Massachusetts, US.

Focused on 22 sites in Ireland, the project seeks to understand how rocky beaches are shaped by factors such as wave energy, geology and coastal geometry.

Much of Cox’s fieldwork took place on the Aran Islands in the mouth of Galway Bay. Following the 2013/14 winter storms that battered the eastern side of the Atlantic, Cox and her team documented the movement of many large boulders, including one weighing 620 tonnes. As big as a small house, it was easily the largest boulder to have been unequivocally moved by storm waves.

It had been assumed these rocks were relics from ancient tsunamis. But recent research suggests they could be the result of “rogue waves” at least twice the height of the background waves, and “sneaker waves” that have an unexpectedly long run up on a shallow slope.

Poster for the drumming the waves project

As well as conveying the beauty of these events, the musical collaboration will also communicate their risks. “People are drawn to extreme environments in the Instagram era where people share information about where storms and big waves are happening. There is a lack of understanding of wave dynamics and the dangers they pose,” says Cox.

Both Byrne and Ó Snodaigh are specialists in the bodhrán, a traditional Irish drum. Songs will combine a range of percussion instruments with piano and synthesisers. “I’m not simply trying to map the data onto music. It’s an artistic process, we want to capture the visceral feeling of what it’s like to experience these coasts,” says Byrne who is a musician, composer and researcher at the University of Limerick.

Initially, the project is focused on studio music but once Covid restrictions ease it will probably include a live component too. The collaborators plan to create teaching packs that combine music with information about coastal erosion and climate change. Another idea is to design musical informational displays on clifftops.

• Find out more about Drumming the Waves in the Physics World Weekly podcast, where you can hear an early recording from the project as well as Byrne demonstrating how a bodhrán can be played to recreate the rhythms and textures of the sea.

Irregularly shaped Moon dust creates complex scattering effects

The Moon’s surface is covered with tiny rock grains that formed during eons of high velocity meteorite impacts. The shape of these grains affects how the lunar surface scatters light, and researchers in the US have now analysed these shapes in unprecedented detail. The results of their study – including the first computations of the optical scattering properties of nanosized Moon dust – should make it possible to create better models of the colour, brightness and polarization of particles on the Moon’s surface, and to understand how these quantities change as the Moon goes through its phases.

Researchers have been studying Moon dust ever since the first samples were brought back to Earth during the Apollo 11 mission in 1969. Initial reports found that the average size of particles in the lunar soil, or regolith, is around 50 μm, with only 14% of particles measuring less than 10 μm across. Newer measurements, however, indicate that submicron particles are also present, including many particles in the 100 nm to 1 μm range.

In either case, the dusty regolith is fundamentally different to soils found on Earth, says Jay Goguen, a senior researcher at the Space Science Institute in Boulder, Colorado and a co-author of the study. Lunar dust particles are also responsible for some unusual visual phenomena. When these particles become electrostatically levitated into the tenuous lunar exosphere, sunlight scatters off them, producing effects that the Apollo astronauts experienced as streamers, horizon glow, zodiacal light and crepuscular rays.

Measuring particle shape

In the new experiments, Goguen and colleagues at the US National Institute of Standards and Technology (NIST), the University of Missouri-Kansas City and the Air Force Research Laboratory measured the shape of particles between 400 nm and 1 μm in size using a method called X-ray nano computed tomography (XCT).

Like other forms of CT scanning, this non-destructive imaging technique uses X-rays to generate images of cross-sections (2D slices) of a 3D object. In-house software allowed the team first to build up a 3D image from such slices, and then to convert this data into a format where units of volume (or voxels) are classified as being either inside or outside the particles. From these segmented images, the researchers identified the 3D particle shapes and fed the voxels making up each particle into an open-source electromagnetic solver called Discrete Dipole Scattering (DDSCAT), which they used to compute the light scattered from each particle in the visible to infrared frequency range.

Infinite irregularities

The team focused on these small, wavelength-scale particles because of the crucial role they play in determining the intensity and polarization state of light scattered from the lunar surface (and indeed from the surface of other planetary bodies). Previous studies have shown that the way lunar dust particles scatter light depends on their size, shape, composition, surface roughness and how densely they are packed together. While these earlier studies tried to account for irregular particle shapes (using, for example Gaussian random shape generators for computational simulations), Goguen says the actual 3D morphologies of these particles were often overlooked.

“There are an infinite number of ways that a particle shape can be ‘irregular’”, he explains. “The goal of our new study was to use experimentally measured 3D shapes of Moon dust particles collected by Apollo 11 and computationally analyse how these specific measured particle shapes scatter light.”

Highly sensitive to shape

Thanks to this approach, the researchers were able to link the shape of the particles to their optical scattering characteristics with greater accuracy. Their results show that the frequency of light most efficiently scattered by a dust particle – the resonance wavelength – is highly sensitive to the particle’s shape, Goguen says. “For the lunar dust grain shapes, the resonance wavelength is 20% smaller than that for equivalent sized spheres,” he tells Physics World. “The lunar dust shapes also scatter light slightly more forward (towards the direction of propagation) than the spherical grains.”

The researchers, who detail their work in IEEE Geoscience and Remote Sensing Letters, now plan to study a wider range of shapes and sizes of lunar particles, including some that are more representative of the lunar highland visited during the Apollo 14 mission.

Arm Holdings faces uncertain future – and why it matters to you

Did you know that your smartphone has anything between 20 and 30 processors inside it designed by a British company? Arm Holdings doesn’t build the chips, but licenses its technology to virtually every semiconductor company on the planet. By the end of 2020, an estimated 160 billion processors had been built with its intellectual property (IP). They’re used in everything from your phone’s main processor to the WiFi chip, Bluetooth and even the battery charger.

To find out how this came to be and why it matters, let’s wind back to the late 1970s when computers were still big, expensive, remote devices. I was at school at the time and remember the faff of posting off punch cards to a distant computer for processing. They’d come back in the mail two weeks later – invariably with a syntax error on line one so you’d have to start again from scratch. You can imagine the magical feeling when my school bought its first computer – a Commodore PET – in 1979.

Arm Holdings’ chips are used in everything from your phone’s main processor to the WiFi chip, Bluetooth and even the battery charger

So when a device called the ZX80 came on the market in 1980 for just £99 – an order of magnitude less than the PET – I simply had to get my hands on one. Launched by the entrepreneur Clive Sinclair, it was the first of many home computers sold in Britain and was followed in 1981 by the BBC Micro. That machine was bought in huge numbers and even had an accompanying BBC TV show, which explained to an eager public how computers would change the way we live and work.

The tender to build the BBC Micro had been won by Acorn Computers, a company co-founded in Cambridge in 1978 by the inventor Chris Curry and the physicist Hermann Hauser, who had just completed his PhD at the Cavendish Laboratory. Acorn gave the BBC exactly what it wanted for the TV series, prototyping a machine in just five days. More than 1.5 million units were eventually sold, helping Acorn to reach a turnover of £100m by 1983. The company was floated on the UK stock exchange that year with a market capitalization of £135m, with Hauser’s and Curry’s stakes in it worth £64m and $51m respectively.

Going for growth

Acorn was, however, keen to build serious business machines that could take on the Apple II and the ubiquitous IBM PC, which had dominated the business market since its launch in 1981. Looking for a suitable microprocessor, Acorn approached Intel to use its 286 processor but didn’t like the package pin-out, so asked to buy the chip only. When Intel refused, Acorn decided to build its own chip.

Two Acorn engineers – Steve Furber and Sophie Wilson – decided to focus on “reduced instruction set computer” (RISC) processors, the ideas for which had been developed by John Hennessy at Stanford University and David Patterson at the University of California, Berkeley. Smaller and faster than conventional “complex instruction set computer” (CISC) processors developed by the likes of Intel, RISC processors needed more memory and a more complex compiler. But with memory costs falling quickly, Acorn decided to adopt the new approach and the Acorn RISC Machine (ARM) project began in October 1983.

Hermann Hauser has written an open letter to the British prime minister, strongly objecting to the acquisition

The first ARM microprocessor chip was tested in April 1985, requiring less than 5% of the power of a comparable CISC processor, which meant it could be put in a cheap plastic package without overheating a PC. Later, with Apple wanting to use ARM in its forthcoming Newton mobile device – but not wanting the chip produced by a competitor – Acorn decided to spin out ARM as a joint venture with Apple and another US firm (VLSI Technology) in 1990.

Renamed Advanced RISC Machines, it developed a business model in which other companies were allowed to build ARM chips by paying an upfront cost and a modest royalty fee per chip (usually a few per cent). Rivals could therefore manage their own supply chains, knowing they had access to advanced and widely used processor technology under reasonable terms. Attracted by the low power consumption, Nokia (then the market leader in mobile phones) used the ARM chip, as does Apple in its iPods, iPhones, iPads and, more recently, its computers too.

Where next?

In 1998 ARM floated on the London stock exchange and became the world’s most successful licensing firm, dealing even-handedly with every semiconductor manufacturer; it’s the Switzerland of semiconductors as Hauser puts it. Eventually, in 2016 Japanese telecoms firm SoftBank Group bought ARM for £23.4bn, seeking to become the leader in the “internet of things” and machine learning. There was some resistance to the sale from regulators, but ARM’s neutral “Switzerland” status was still retained, so the deal went ahead (with ARM rebranded Arm in 2017).

All seemed to be going well until the US chip maker Nvidia – one of Arm’s licensees – announced plans last year to buy Arm from SoftBank for $40bn. Most other licensees oppose the deal, not least because Nvidia is American, which means that Arm could be restricted from, say, Chinese companies if the US government so decreed. Hauser has already written an open letter to the British prime minister, strongly objecting to the acquisition.

The UK’s Competition and Markets Authority (CMA) is investigating the proposed takeover, focusing on the potential impact on competition in the UK and whether Arm, if sold, would have an incentive to “withdraw, raise prices or reduce the quality of its IP licensing services to Nvidia’s rivals”. I am expecting that the CMA will block the deal, given the huge opposition from almost all licensees and sovereign nations. But if the acquisition does go ahead, Arm’s business model may be damaged and its technology, I fear, may be turned into a potential political weapon. I hope they make the right decision.

Artificial intelligence technologies can reinforce inequalities

Computer technologies are often viewed as inanimate tools for improving our lives. Yes, there have always been issues around access, and there have always been some people who have used computers for harmful purposes. But the technology itself has always been considered to be free from human biases. This video explains why the concept of computer neutrality is no longer feasible.

We’re living in an age of digital information, where algorithms underpin many aspects of our lives. Increasingly, private companies and public authorities are using machine learning and artificial intelligence (AI) to make processes more effective. But in practice, the technologies are built by humans so their designs and functioning can reflect existing inequalities in society.

Physicists also use AI and machine learning in academic research, and many tech companies hire physicists. For these reasons, physicists can play a key role in better understanding the issues and regulating these emerging technologies. To find out more take a look at the article “Fighting algorithmic bias in artificial intelligence“, originally published in the May 2021 issue of Physics World.

Graphene oxide fibres fuse and fissure

Researchers in China have succeeded in assembling graphene oxide fibres using a process normally only seen in biological systems. The new process, which mimics cellular fusion and fission, could find use in applications such as the actuators or “artificial muscles” used in miniaturized medical devices, robotics and smart textiles.

Materials that respond to environmental changes in the same way that natural materials do are widely seen as ideal building blocks for emerging electronic devices. One natural mechanism that researchers are especially keen to mimic is biological self-assembly and, in particular, cellular fusion and fission. In fusion, two or more cells merge into one, while in fission they separate into two or more parts. Both processes are triggered by stimuli such as light, temperature or humidity.

Reversible solvent-triggered process

In the new work, Chao Gao of Zhejiang University and colleagues from Xi’an Jiaotong University began by assembling microfibres of graphene oxide (an oxidized version of carbon’s one-atom-thick form) using a technique called wet-spinning. The team chose this material because its super-flexible nature makes it relatively easy to wet-spin into fibres, while its oxygen functional groups make it chemically reactive. The resulting fibres have an outer “shell” that restricts the movement of the graphene oxide sheets.

When the researchers immersed the fibres in a suitable solvent, they found that the fibres self-assembled into a “hierarchical” yarn – that is, a yarn in which the same base structure repeats at different length scales – containing thousands of individual fibres. The team could also reverse this process by immersing the fibre assembly in water or polar organic solvents, thereby mimicking both parts of the biological fusion-fission cycle.

To understand what was happening at the scale of individual fibres, the researchers used optical and scanning electron microscopes to observe the fusion-fission processes. They found that when the fibres are placed in a water or polar organic solvents, they swell up, dramatically increasing in volume. The fibres’ elasticity is enhanced too, and the shape of the fibre shells reversibly switches between a wrinkled tube-like state and a flatter cylindrical state through swelling and deswelling. Gao and colleagues explain that this switching creates a transient fibre interface, leading to cyclic self-fusion and self-fission of an arbitrary number of graphene oxide fibres.

“A versatile strategy”

The researchers, who describe their work in Science, say that the fusion-fission behaviour they observed is a “versatile strategy” for designing functional responsive materials. Since graphene oxide fibres can easily be made to electrically conduct (via chemical reduction), the team argue that these fibres show promise for applications such as sensors, electronic components, smart textiles and actuators.

Rodolfo Cruz-Silva of Shinshu University in Japan and Laura Elias of Binghamton University in the US, who were not involved in the study, note that the new method is much less complex and involves fewer components than natural fusion-fission processes. Nevertheless, they argue in a related Perspectives article that the reversible assembly of graphene oxide fibres does indeed mimic nature, and thus “holds the refreshing potential to move the field forward”.

Gao and colleagues now plan to investigate the fusion-fission mechanism more carefully. “We also hope to explore applications in different areas,” Gao tells Physics World. The reversible fusion and fission property they discovered, he adds, “may help push forward the versatility of fibres like the ones we have studied”.

Lasers peer into a mysterious region of supercooled water

In an experimental first, scientists in the US have studied the dynamics of liquid water at temperatures below 230 K. Greg Kimmel and Loni Kringle of the Pacific Northwest National Laboratory in Richland, Washington used ultrafast laser pulses to “stop and start” the evolution of supercooled water in the nanoseconds before it froze, performing measurements in a temperature region that has been inaccessible to previous experiments. A paper describing their results is published in Proceedings of the National Academy of Sciences and suggests that the unusual properties of water might be attributed to the exchange of molecules between two coexisting liquid phases.

Water has more than 60 unusual properties that differentiate it from other liquids, including high heat capacity and a density that decreases upon freezing. There is evidence that these anomalies originate in the supercooled region, but despite decades of research, this remains unproven. Experiments on supercooled water are made almost impossible by a region between 160 and 230 K, which water researchers call “no man’s land”, where water crystallizes almost instantly.

Bridging the gap between theory and experiment

Kimmel has been studying supercooled water for more than two decades. Last year, Kimmel’s group showed, using an ultrafast laser heating technique, that water in the 160–230 K region always forms an equilibrium liquid before it crystallizes and that this liquid is a mixture of two structures, one high density and one low density.

Theorists have long predicted these two phases, but, as Kringle describes: “There is growing consensus that the anomalies of water, that are observable above 0 °C but become more pronounced upon supercooling, are related to the presence of these two structures but so little is known experimentally.”

The laser heating technique is fast enough that dynamics can be resolved as well as structure. As a supercooled water molecule moves through the liquid, it switches between high- and low-density motifs and the researchers wanted to know how this would affect the relaxation of the fluid.

Kringle and Kimmel entered this mysterious temperature region from below, heating amorphous (non-crystalline) ice at 70 K with laser pulses at rates of billions of degrees per second. The laser melted the water, but the heat dissipated after a few nanoseconds, cooling it so rapidly that the liquid structure was “locked in”. The fraction of low- and high-density water present after each laser pulse was measured with infrared spectroscopy, building up a series of “snapshots” of the liquid as it reached equilibrium.

Different densities, different dynamics

Like its liquid counterpart, amorphous ice can be high or low density. The researchers found that supercooled water relaxed to equilibrium more slowly if it started as low-density ice, though the final structures were similar. For both high- and low-density ice, they also found regions where the relaxation profile was a stretched exponential, which indicates molecules moving with lots of different speeds.

Whether a water molecule can switch between structures on its way to equilibrium depends on how easily it can navigate the potential energy landscape. The researchers developed a model of exchange between potential wells where they took the fractions of high- and low-density water at each stage of the experiments and calculated the switching rates that would keep the system at chemical equilibrium. They found that this model fitted the experimental data surprisingly well; reproducing the stretched exponential and predicting that a smaller number of deeper minima makes it more difficult for low density water to structurally evolve.

A new perspective on supercooled water

For their conclusions to be valid, the researchers need to show that the potential energy landscape of amorphous ice is equivalent to its liquid counterpart. New research on supercooled water is always contentious, but Kringle described her surprise at how well the potential energy landscape model fitted their data, saying “While it’s not perfect, it does provide a starting point for understanding how the transition between two structural motifs results in stretched exponential relaxation.” Certainty is hard to come by below 0 °C, but this research suggests a link between water’s dynamics and its unusual structure.

‘Keyhole surgery’ could reduce environmental burden of metal extraction

A new “keyhole surgery”-style mining technique could allow metals to be extracted from underground ore bodies without the need for vast physical excavations. The approach, which is based on electrokinetics and was developed by an international team of researchers, could reduce the environmental impact of mining while making deep ore deposits more accessible.

Industrial-scale mining is deeply damaging to the environment. Not only is the physical excavation of ore-bearing minerals highly energy-intensive, generating around 10% of all energy-related greenhouse gas emissions in 2018, it also produces incomparable quantities of waste. Globally, mining waste amounts to an estimated 100 gigatonnes per year, in the form of both overburden and the commercially useless “gangue” that surrounds valuable ore. This gangue is often highly toxic as well, meaning that disposing of it risks further environmental contamination.

“The current mining paradigm can be considered inherently unsustainable,” summarizes Rich Crane, a geochemist in the Cambourne School of Mines at the University of Exeter, UK, and an author of the new study. Demand for copper, for example, is expected to increase by 275–350% by the year 2050, yet freshly discovered deposits are increasingly of lower ore grade and found at greater depths. Mining these deposits in the traditional way would thus entail removing hundreds of metres of overburden.

Electrokinetic extraction

In their study, Crane and colleagues demonstrated an alternative approach based on electrokinetics (EK). This method, which is already used to extract metals from fly ash, soils and wastewater sludge, involves applying a direct current between two electrodes to drive the movement of dissolved charged species in the substance between, with metal ions flowing towards the cathode.

To adapt the method to work with intact, hard rock bodies, the researchers added an element of another technique, known as in situ leaching, which uses an acid to selectively dissolve the target metal from an ore deposit. In this way, metal might be recovered while bypassing the overburden and leaving most of the gangue in the ground. Then, once extraction is complete and the electric field is switched off, the acid is effectively “sealed” inside the rock – which remains essentially unchanged from a geotechnical perspective, minimizing the risk of subsidence.

“This new approach, analogous to ‘keyhole surgery’, has the potential to provide a more sustainable future for the mining industry,” Crane says. He adds that it could allow metal deposits to be recovered “while avoiding unwanted environmental disturbance and energy consumption”.

Proof-of-concept demonstration

In a laboratory-scale test of their approach, Crane and colleagues extracted 57 weight per cent of copper from a 4 cm-wide sample of low-permeability sulfidic porphyry ore. Though the full experiment took 94 days, 80% of the material recovery occurred in the first 50 days, at a relatively constant rate. The team’s numerical modelling suggests that in the field, metal could be recovered at comparable rate to that of traditional mining once the electrokinetic system was set up. The lead-up time would also be significantly reduced, as the need to remove substantial overburden would be replaced with simply drilling a grid of boreholes into which electrodes could be applied. According to Crane, the new process could be particularly cost-effective for ore deposits lying deep within the Earth’s crust or in areas where the storage of solid mine waste is, as he puts it, “problematic”.

“Application of EK to solid rock, rather than particulate soils or wastes, is certainly a novel approach,” says Mike Harbottle, a geoenvironmental engineer at Cardiff University, UK, who was not involved in the present study. However, Harbottle adds: “From experience in other applications there are plenty of challenges to come, not least the challenge of applying this sort of voltage gradient in the field and the resulting economic impact.”

Rodrigo Ortiz Soto, a chemical engineer from the Pontifical Catholic University of Valparaiso in Chile who was also not involved in the study, is more optimistic. “If this process is proven in field-scale experiments, it can shift the entire industry, and also can have applications in copper recovery from mine tailings and considerably extend copper availability,” he says.

The study is described in Science Advances.

Exploding stars alone cannot account for rapid heavy-element production, study reveals

Exploding stars alone cannot account for the abundance of heavy elements produced by the rapid neutron capture process, a new study has revealed. An international team of researchers, led by Anton Wallner at the Australian National University, came to this conclusion after analysing the abundances of plutonium and iron isotopes in a deep-sea crust sample. Their research suggests that other cataclysmic events, such as neutron star mergers, could be responsible for creating some heavy elements.

Elements heavier than iron form in astrophysical objects where nuclei are able capture neutrons in succession. For about half of the heavy nuclides, this neutron capture occurs slowly in stellar cores in what is called the “s-process” of nucleosynthesis. The other heavy nuclides – including actinides such as plutonium – are created rapidly in much more violent environments via the “r-process”.

Exactly where the r-process can occur is a subject of some debate. Some astronomers argue that it can only occur within certain types of supernovae (exploding stars), while others suggest that violent events such as merging neutron stars must be at least partially responsible for the heavy elements around us.

Wallner’s team has shed new light on this debate by analysing a core sample of Earth’s crust, taken from 1500 m below the surface of the Pacific Ocean. It contains a geological record spanning the past 10 million years and the researchers measured the abundance of two specific nuclides in the rock.

Iron and plutonium

One was iron-60, which is produced within the cores of massive stars, but is only ejected into space when the stars explode as supernovae. It has a half-life of 2.6 million years, so any iron-60 found in Earth’s crust must have been thrown out from supernovae relatively local to the solar system. The second nuclide the team looked for was plutonium-244, which can only be produced through the r-process. It has a half-life of 80.6 million years, so plutonium-244 in Earth’s crust can originate from far older, more distant events.

Within their sample, Wallner and colleagues detected two distinct influxes of iron-60, suggesting that two local supernovae occurred in the past 10 million years. Each of these events also deposited smaller amounts of plutonium-244, with similar ratios between nuclides for each event. Although the data show that both nuclides are associated with exploding stars, the ratios of plutonium-244 to iron-60 measured for both events are lower that would be expected if the nuclide were produced in supernovae alone.

This suggests that plutonium-244 and other r-process nuclides are made in astrophysical events additional to supernovae. Among the most popular ideas is that r-process nuclides are produced during neutron star mergers – such as the event detected in 2017 by gravitational-wave and conventional telescopes. Future multimessenger observations of such mergers could therefore provide crucial information about the origins of heavy elements.

The research is described in Science.

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