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Implementing a MRIdian program

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From Laura Bassi to Marie Curie, for centuries, women have been making important contributions to the world of physics. Now with ViewRay’s MRIdian system, women are leading the charge in bringing the latest advancement of MRI-guided radiation therapy to the forefront of radiation oncology and expanding the medical physics landscape.

MRIdian is the world’s first radiation therapy system to integrate a diagnostic-quality MRI with an advanced linear accelerator and the only system with MR-guided, real-time, 3D, multiplanar soft-tissue tracking and automated beam control. MRIdian offers precise and personalized care through on-table adaptive treatments without the need for fiducials. The technological foundations of MRIdian allows for the delivery of ablative dose with tighter margins in five or fewer fractions, all while maintaining low to no toxicity. With tens of thousands of patients treated, and an ever-growing body of clinical evidence, MRIdian is leading the MRI-guided revolution in radiation therapy.

This series of five webinars will specifically highlight women physicists across the globe that are using MRIdian to transform cancer care as we know it. Daphne Levin from Assuta Medical Center in Tel Aviv, Israel, will begin the series with “Implementing MRIdian into the Clinic”. Daphne will demonstrate what it takes to set up a successful MR-Linac program with efficient adaptive workflows.

This presentation is the first in a series of Women in Medical Physics, supported by ViewRay.

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Daphne Levin has been the chief of physics at the Department of Radiotherapy at Assuta Medical Centers since the department was established in 2007. Prior to that she was chief medical physicist at the Rabin Medical Center. She obtained her PhD in medical physics from the Tel-Aviv University, in co-operation with the University of Chicago, and was a postdoctoral fellow at the University of Chicago for two years. Daphne has been ABR certified since 2005. When she’s not hard at work treating patients on the MRIdian system, she enjoys surfing, running, trekking and kayaking.

Double dose of quantum weirdness pushes sensors past the limit

For most people, quantum mechanics seems pretty weird. Take the principle of delocalization, which states that a quantum particle can, in some sense, exist simultaneously in multiple locations. Then there’s entanglement: the invisible connection between particles that allows the state of one particle to determine that of another, even across vast distances. But as weird as delocalization and entanglement are, they can be very useful, and physicists at JILA in Boulder, Colorado, US have now incorporated both into a single quantum sensor for the first time. The new sensor can detect accelerations below the usual limit set by noise arising from quantum fluctuations, providing a sharper tool for exploring fundamental physics as well as for applications such as navigation and Earth monitoring.

The JILA team’s experimental setup uses a matter-wave interferometer, which interferes massive quantum particles in the same way as an ordinary interferometer interferes beams of light. While light-based interferometers can be very sensitive – they are used to detect gravitational waves – their matter-wave equivalents can in principle detect even smaller accelerations because the quantum wavelength of massive particles is so much shorter. These sensors therefore offer a way to search for phenomena such as dark matter and dark energy that currently cannot be detected directly, but that nevertheless make their presence known through gravitational effects.

Turn down the noise

In the experiment, atoms are first placed inside an optical cavity, which is a set of opposing mirrors with light trapped in between. The light that bounces between these mirrors interacts with the atoms, causing nearly 1000 of them to become entangled with each other.

The maximum sensitivity of quantum sensors is generally limited by noise produced by the random collapse of individual atoms’ quantum states whenever they are measured. Previous experiments sought to reduce this quantum noise by running the experiment in parallel a multitude of times with many atoms, then averaging out the quantum noise of each individual atom.

In the JILA team’s experiment, however, the researchers tested two alternatives in which the atoms actually conspire with each other to cancel each other’s quantum noise. The first approach involved so-called quantum non-demolition measurements, in which the researchers made a pre-measurement of the quantum noise associated with the atoms and then subtracted this quantum noise from the final measurement. In the second method, the researchers introduced light into the cavity that triggers an atomic process known as one-axis twisting, which causes the entangled atoms in different motional states (or momentum states) to have a lower uncertainty than they would if the atoms were not entangled.

The resulting quantum states are called squeezed spin states because they consist of two levels of momentum states that are essentially “squeezed” together to form an effective spin system. In the JILA team’s experiment, these squeezed spin states allow any quantum phase that accrues between the momentum states due to accelerations to be measured with a higher precision. In both approaches, due to the entanglement between the atoms, the quantum noise becomes correlated between atoms such that one atom’s quantum noise is cancelled by that of another, making the quantum sensor “quieter” and therefore more precise.

All over the place

In the experiment’s second step, the researchers introduced delocalization. Lasers separate the wave packets of the atoms, thereby bringing them into a superposition of different momentum states; as the two parts of the wave packets move apart, each atom is essentially in two locations at the same time. By cancelling this superposition with another laser, the atoms’ wave packets interfere with each other, and any influence on their location – for example due to falling under gravity – can then be detected with ultrahigh sensitivity. Combining this interference experiment with the entanglement approaches made it possible for the researchers to detect accelerations smaller than the standard quantum limit set by the quantum noise of the individual atoms.

James K Thompson, who led the team’s research together with PhD students Chengyi Luo and Graham Greve, says that using squeezed states for quantum sensing is often called “quantum 2.0”, a version of quantum sensing that moves beyond single-particle physics. He mentions that there is “a wonderful synergy” emerging between advances in quantum sensing and quantum simulation, and he is interested in exploiting this in two ways: by using momentum-state encoding of quantum states to perform quantum simulation, and by applying the tools of quantum sensing to measure the evolution of the quantum many-body system. “What we learn can then be used to enhance quantum sensors even further,” he says.

Trapped-ion crystal makes an ultra-precise quantum sensor

Diagram of a quantum sensor containing trapped beryllium ions with entangled spins

Timothy Kovachy, a physicist from Northwestern University in the US who was not involved in the research, says that the standard quantum limit can be a significant constraint on the precision of quantum sensors. He therefore describes the result as a major step forward for quantum sensing. He says that the incorporation of spin squeezing to surpass the standard quantum limit is essential for these interferometers to reach their full potential and that it is a significant achievement to realize cavity-based squeezing together with intracavity atom interferometry. He adds that cavities can also provide valuable features for atom interferometry, such as improved spatial mode quality and power build-up.

The research is published in Nature.

New technique boosts the performance of dual optical frequency combs

A new technique that could vastly improve the accuracy of the time and distance measurements made using dual optical frequency combs has been developed by researchers in the US and Canada. By the dynamic adjustment of one of the combs, Emily Caldwell and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado and Octosig Consulting in Quebec City have made the technique much more efficient.

First demonstrated at the turn of the millennium, the optical frequency comb has boosted the accuracy of time and distance measurements. A comb can be created using a laser that emits ultra-short pulses at regular intervals. The frequency spectrum of the pulses has sharp, evenly spaced peaks – giving it the appearance of the teeth of a comb.

To measure time and distance, comb pulses are reflected off a distant object. The reflected light is then combined with a second comb, which has pulses that are slightly delayed relative to the first comb. By measuring the relative alignment of the two combs, the return time of the first comb – and therefore the distance to the reflecting object – can be determined to very high accuracy.

Little overlap

However, an important shortcoming of this technique is that the length of the pulses is much shorter than the gaps between the pulses. Therefore, it is often the case that there is little overlap between the reflected pulse and the delayed pulse. This means that measurements sometimes rely on measuring very small numbers of photons – reducing accuracy and wasting a large portion of the reflected light. This is a particularly pressing problem for sensing applications outside the lab, where the light in the first comb is already attenuated as it travels long distances to and from the target object.

To overcome this problem, Caldwell’s team used a digital controller to track and control the timing of the pulse in the second comb to within an accuracy of 2 as. This allowed them to lock the second comb to the first, ensuring that the pulses arrive at the detector at the same time. As a result, all the photons in the first comb can potentially be used in a measurement.

This innovation allowed the team to take their measurements close to the quantum limit – a fundamental limit on the accuracy of the measurement that is imposed by quantum fluctuations. Another advantage of the system is that its efficient use of photons means that it can be run at much lower power – requiring only 0.02% of the photons used by previous systems for the same results.

As a result, the team’s approach could offer exciting new opportunities for sensing opportunities outside the lab. This includes measuring distances to faraway objects such as orbiting satellites to within nanometre precision.

The research is described in Nature.

NASA’s new rocket successfully fires Orion capsule towards the Moon

Update 12/12/2022: The Orion capsule, carrying a simulated crew of three mannequins wired with sensors, succefully landed in the Pacific ocean yesterday marking the end of the Artemis I mission.

Following months of delays, NASA’s new Moon rocket sucessfully lifted off from the Kennedy Space Center in Florida at 1:47 a.m. local time today.

The Space Launch System (SLS), which is now the most powerful rocket ever, launched an uncrewed Orion spacecraft into orbit. NASA announced that following launch the spacecraft is performing as expected as it now begins its journey to the Moon.

Orion is expected to fly-by the Moon on 21 November, performing a close approach of the lunar surface on its way to a distant “retrograde orbit”, a highly stable orbit that will see it travel approximately 64,000 kilometres beyond the Moon. It will then return to Earth where it will splash down in the Pacific Ocean, which is expected some 25 days from now.

The Artemis I mission is seen as a crucial test for NASA before the agency sends astronauts to the Moon on Artemis II, which is expected in 2024. Artemis III, currently ear-marked for launch in 2025, will be the first crewed lunar landing since the Apollo missions in the 1960s and 70s.

“What an incredible sight to see NASA’s Space Launch System rocket and Orion spacecraft launch together for the first time,” noted NASA administrator Bill Nelson. “This uncrewed flight test will push Orion to the limits in the rigours of deep space, helping us prepare for human exploration on the Moon and, ultimately, Mars.”

Why are external lasers essential at a bore-type linac?

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Linac’s latest development is the bore-type design that offers many advantages for the patient and the clinical user. So, the question arises whether and why external lasers should also be used at the bore-type linac.

In this webinar, we want to evaluate this question from the point of view of the therapist, the patient and the medical physicist while giving you an overview of laser solutions from LAP.

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Raphael Schmidt is responsible for the product management of laser systems for CT and MRI from LAP. During his studies at KIT (Karlsruhe Institute of Technology) he gained broad experience in different workflows in radiation therapy while analysing them. The topic of his final thesis dealt with the improvement of workflows in RT through to new information and assistance systems. Raphael holds a degree in industrial engineering and management.

Want to slash your energy bill? Try these record-breaking solar cells

Solar panels have been around for decades – in fact, they are now a common sight on houses even in the largely cloudy and dark nations of northern Europe. Cheaper and easier to install than ever before, the question is why aren’t solar panels fitted to every single house? Mostly, it’s down to cost. Consumers simply want a quick return on investment.

Having worked in the light-emitting diode (LED) sector for many years, I remember how hard it was to convince people to buy something – LED lighting – that cost more than an old-fashioned filament lamp. Why spend the money when you’d not get any payback for 10 years or longer? The new-fangled light bulbs were fine for green-minded eco-warriors but not for ordinary punters.

Fortunately, the efficiency, cost and quality of LEDs improved and consumers began to realize the LED lighting would survive long enough to recoup the initial outlay. Once payback times dropped below five years, things really started to motor. In fact, today’s LED lighting can provide more than 100 lumens per watt (twice that for high-end products) meaning they’re the only practical choice – with payback in months.

Selective bans of inefficient light sources helped, of course, but LEDs succeeded because of technical progress coupled with economies of scale. So is the same thing about to happen with solar panels? In the UK, the government kick-started things in 2010 with feed-in-tariffs (FiTs). Anyone who put electricity generated by solar panels into the grid was awarded payments of 41.3 p/kWh (pence per kilowatt hour) – at a time when electricity was about 10 p/kWh. The tariffs led to many people fitting solar photovoltaic (PV) panels, with installations rising from 140,000 in 2011 to 1.2 million by the end of last year.

Efficiency gains

Back in 2010, commercial solar PV panels were silicon based with efficiencies of typically 15%. If you bought them from a reputable manufacturer, some panels could last for tens of years, which was long enough to achieve a payback on investment. But as a 2020 report from the International Renewable Energy Agency found, the cost of solar PV globally dropped by 82% between 2010 and 2019, with panel efficiencies now close to 22%.

In fact all renewables have been dropping in price, including on-shore and off-shore wind as well as concentrated solar power, with solar falling by 13% during 2019 alone to just over 5 p/kWh. Despite the upheaval to the UK’s solar sector following the ending of FiTs in 2017-2018, the UK is likely to install more than 1 GW of solar PV by the end of 2022 according to the physicist Finlay Colville, head of market research at Solar Media.

In addition, indications suggest that the UK could have a total of 40 GW of solar PV installed by 2030 on the roofs of homes and offices and in ground-mounted solar arrays. Earlier this year as part of its Energy Security plan, the UK government will also look to increase the UK’s current 14 GW of solar capacity, which it estimates could grow up to five-fold by 2035.

Cost is key. The world record for solar-cell efficiency is currently 47.1%, which was achieved in 2019 by staff at the US National Renewable Energy Laboratory, who built six junction concentrator solar cells of various materials under the illumination of 143 Suns. No solar cell can ever be as cost effective as good old silicon, but a new “tandem” solar panel, unveiled at the recent World Conference on Photovoltaic Energy Conversion in Milan, could be a game-changer, having an efficiency of 30.1% under just one Sun’s illumination.

The new cell could be a game-changer, having an efficiency of 30.1% under just one Sun’s illumination

Developed by a team from various universities and institutes in the Netherlands, the cell combines a conventional silicon cell with one made from perovskite, allowing it to exploit a bigger fraction of the solar spectrum. That’s because the silicon part works well with visible and infrared light, while the perovskite is better with ultraviolet and visible light. The perovskite also allows more than 93% of near infrared light to reach the silicon cell.

Many teams around the world are working on tandem cells, including the UK firm Oxford PV. By coating an ordinary silicon cell with a thin perovskite film, it has achieved an efficiency of 29.52% in 2020, with the company saying its synthetic perovskite material is affordable and sustainable. With a factory already built, Oxford PV intends to be the first company to sell these next-generation solar cells. Initial products, designed for residential roofs, will generate 20% more power from the same number of cells.

Sunny future

Solar power has lots of potential, with 87% of the world’s nations able to power themselves using less than 5% of their land. As Elon Musk has pointed out, the US could do the job with just a few hundred kilometres of solar panels in a small corner of Utah. But the UK is not one of those lucky countries: 12.5% of the whole nation would have to be blanketed with solar panels to power itself. That’s a lot given that only 6% of the country is built on.

Solar PV will therefore be part of the mix but not the entire solution. We need to boost the efficiency of solar cells while keeping a lid on production costs, inverters and installation costs to really encourage take-up. The average four-person house in the UK currently needs about 16 solar panels operating at efficiencies of about 20% to power itself. Given the low levels of sunlight in the UK, the rule-of-thumb until recently was that it would take 11–14 years for solar panels to pay for themselves.

But according to some estimates, the pay-back on solar panels is now as little as four years and further improvements could shorten that time even more. With rocketing energy prices and the cost-of-living crisis, it seems that the time for solar PV has finally come.

Cell softening allows cancer cells in rigid tumours to spread

We typically think of a tumour as a rigid lump of cancerous cells; but how could such a rigid cluster invade its surrounding microenvironment? To answer this question, an international collaboration of researchers has combined computer simulations with mechanical measurements. Their findings, published in Nature Physics, demonstrate that a considerable percentage of cancer cells acquire a high degree of mechanical deformability to become more mobile, and consequently are able to enter dense surrounding tissue.

It is already recognized that cancer cells undergo dedifferentiation, a process in which they move towards a more disordered state with a softer cytoskeleton. However, cell aggregates are known to exhibit jamming, which prevents further spreading of cells. This highlights the mechanical impact of solid–fluid transitions on tissue bulk behaviour.

Furthermore, research has shown that the fluidity or rigidity of tumour cell clusters is regulated by cell unjamming. Cancer cells are also known to be highly mechanosensitive – they can mechanically adapt to their microenvironment.

“The paradox that in breast tumours, cells that become softer actually form a structure that is harder than the original tissue is only an apparent contradiction,” explains Joseph Käs from Leipzig University. “This effect is further enhanced because here, mainly very soft fat cells in the healthy breast are compared with cells that are softer than healthy epithelial cells, but still significantly harder than fat cells.”

Motivated by computer simulations performed by physicists at Northeastern University, the University of California, Santa Barbara and Syracuse University, Käs’s group investigated tissue explants from breast and cervical cancers using various techniques, including atomic-force-microscopy (AFM)-based bulk tissue rheology. Working in collaboration with a team of cancer researchers and pathologists at Leipzig University Hospital and Albert Einstein College of Medicine, they demonstrated the existence of a few solid islands of rigid cells, connected by mechanical stress bridges of soft, mobile cells.

Cell migration Simulations of an invading cell (green) moving through tissue containing both rigid (light blue) and soft (dark blue) cells. Top: the tissue is in a jammed, solid-like state and the invading cell is stuck and cannot move. Centre: in heterogeneous tissue the invading cell shows highly intermittent migration dynamics. Bottom: the tissue is in a fully unjammed, fluid-like state and the invading cell moves with relative ease. (Courtesy: Max Bi, Xinzhi Li)

AFM is a scanning probe-based microscopy technique with subnanometre resolution. In this study, the researchers used the technique to gain knowledge of mechanical parameters such as tumour cell elasticity across the live tumour explants. This enabled them to capture the local, heterogeneous distribution of tissue stiffness, as the AFM maps display both rigid (jammed) and soft (unjammed) regions.

This structure was further confirmed by tracking vital cells across cancer cell spheroids. The researchers elucidate that this heterogeneous state stabilizes the tissue sufficiently to allow tumour growth, while providing flexibility for soft, motile cells to escape the tumour and consequently form metastases.

Thomas Fuhs, one of the lead authors of this study, is optimistic that their latest results add new insight into the mechanics of cancer cells and tumour tissue. More explicitly, whether the cells in a tumour remain completely jammed – as in healthy tissue – or are able to unjam and soften can make all the difference as to whether a tumour metastasizes or not.

Qatar: the land where one sports pitch needs 50,000 litres of desalinated water a day

This weekend, the FIFA World Cup kicks off in Qatar, as 32 nations compete in the month-long tournament for national football’s biggest trophy. FIFA, football’s world governing body, estimates over one million spectators will attend the tournament’s 64 matches, with billions more watching on TV. But in the runup to Qatar 2022 concerns have been voiced – some on environmental grounds.

One eye-catching claim has been that Qatar 2022 will be carbon neutral, with organizers pledging to offset the tournament’s 3.63 million tonnes of carbon dioxide equivalent (CO₂e). Environmentalists, however, believe the carbon emissions are significantly underestimated and question the transparency of the offsetting process.

Stadiums controversy

Much discussion has centred around the eight tournament stadiums, seven of which were built from scratch after Qatar was awarded the tournament 12 years ago. Given Qatar’s hot and arid climate, pitches there require 10,000 litres of desalinated water daily in winter and 50,000 litres in summer, and grass seeds are flown in regularly from the US on climate-controlled aircraft. Yet, in its carbon accounting for the tournament, FIFA attributes a combined total of just 0.2 Mt of CO₂e to stadiums, on the basis they will be extensively used over many decades.

Carbon Market Watch questions whether Qatar – a relatively small country centred around one city – really needed lots of large stadiums, especially given the absence of large sports clubs prior to the World Cup bid. In a report, the not-for-profit organization suggests that the stadia’s real carbon impact could be eight times higher. On the plus side it welcomes the fact that a portion of seats are removeable and that one of the grounds – the so-called “Stadium 947” – is built from shipping containers and modular steel, making it fully demountable.

Going for green

But how can we minimize the environmental impacts of big sporting events more generally?

One recent event to win green plaudits was the 2022 Commonwealth Games in Birmingham, UK, where activities were hosted in revamped non-sporting sites as part of longer-term regeneration plans. It included temporary basketball and volleyball facilities on the Smithfield Market Site in the city centre, and the planting of urban forests across the West Midlands region. Organizers of the Paris 2024 Olympics are said to be following the lead from Birmingham and the Tokyo 2020 Olympics too.

Sport does seem to be waking up to its environmental responsibilities and there are an increasing number of green initiatives at national and local levels. In Germany, many football clubs offer a “KombiTicket” that includes free local and regional public transport to matches. In Spain, Real Betis offers its “Forever Green” platform, including discounts for fans to travel to the match using a local bike-sharing scheme.

Such initiatives are a good idea. A 2017 study across eight of English football’s lower tiers found that nearly seven out of 10 spectators travelled to matches by car. Individuals travelled an average distance of 41.5 km, with the total annual greenhouse gas emission from spectator transport estimated at 56.2 Kt of CO₂e per season.

So how is your local sports club cutting its carbon footprint and/or encouraging sustainable behaviours among its fanbase? Let us know on Twitter or e-mail us at pwld@ioppublishing.org

Exploring the mystery of neutrino mass using cryogenics deep under a mountain

Can you describe your dual role at CUORE?

Right now, I am run co-ordinator for this current experiment and site manager for CUORE. As run co-ordinator, I make sure that the experiment keeps running without stopping. This is important because we are looking for extremely rare events, so we want to take data for as long as possible without stopping. I work on both the cryogenic part of the experiment and the data collection part. I also work on minimizing the background noise level in the experiment – which is also important when looking for rare events.

My site manager role is a bit broader than run co-ordinator. I handle the interface between the experiment and Gran Sasso National Laboratory, coordinate onsite activities and organize the maintenance of all the systems and subsystems.

Can you describe CUORE and what it is seeking to measure?

CUORE looks for rare events in physics and it was specifically designed to search for neutrinoless double beta decay. This process is expected to occur if neutrinos are their own anti-particles – that is, if they are Majorana particles. Answering this question is important because if neutrinos are proven to be Majorana particles, the mystery of why neutrino masses are so small within the Standard Model of particle physics will be solved.

We search for neutrinoless double beta decay in the isotope tellurium-130 because it is known to undergo ordinary double beta decay and it has a high natural abundance. CUORE has 184 tellurium dioxide crystals that are kept near 10 mK inside a large cryostat. The cryostat does not use liquid helium but rather has five pulse tube cryocoolers.

The experiment must be kept at a very low temperature because we search for neutrinoless double beta decay by detecting the tiny increase in temperature within a crystal that occurs because of the decay. Before CUORE, only a small experimental volume and mass could be cooled but we have increased this tremendously by cooling up to 1.5 tonne of material at base temperature. Another advantage of CUORE is that the experiment has very good energy resolution and operates over a very broad energy range – which should help it identify decay events.

What is the significance of CUORE’s recent achievement of acquiring a “tonne–year” of data?

Tonne-year refers to the mass of the tellurium oxide being monitored multiplied by the length of time that the experiment collected data. The mass is 741 kg and data were acquired in runs that were done between 2017 and 2020. Not every run involved using the entire mass, but all together one tonne–year worth of data were collected

There are two significant aspects to this. First, this is the first time that such a large mass has been cooled in a cryostat. Second, because we were able run the experiment for such a long time, we have shown that cryogenic calorimeters are a viable way to search for neutrinoless double beta decay.

Part of the CUORE experiment — Chilling out: researchers work on heart of the CUORE experiment before it is place in its cryostat. (Courtesy: CUORE Collaboration)

What did this tonne-year of data tell you and your colleagues?

To be clear, we have not found Majorana particles. Instead, we have been able to set a lower limit on the half-life of neutrinoless double beta decay. We now know that the half-life is greater than 2.2×10²⁵ years. We can conclude this because if the half-life was shorter, we would have expected to see at least one or more events in CUORE.

Can CUORE be used to explore other areas of physics?

Yes. CUORE is designed to search for rare events and therefore it has the potential to look for dark matter. Dark matter particles are expected to interact with CUORE’s detector materials very rarely and this would involve the release of very small amounts of energy. So, the search for dark matter would benefit from the experiment’s large mass and long run time. A dark matter search would involve exploring another energy region in the detector and there are groups of physicists within the CUORE collaboration looking at that possibility.

Does CUORE’s cryogenic milestone have some bearing on quantum computing?

I am not an expert in quantum computing, but generally, solid state devices that process quantum information require long quantum coherence times. We know that heat and cosmogenic radiation both reduce quantum coherence times. Running experiments underground with advanced cryogenics offers protection from these negative effects. While CUORE’s tellurium dioxide crystals cannot be used for quantum computing, the fact that we have achieved such a long experimental run underground with a very large cryostat and with clean materials could be potentially very useful for the development of quantum technologies.

What will the future bring for the CUORE collaboration?

CUORE will run until 2024 and we are already working on the CUORE Upgrade with Particle Identification – or CUPID. We will replace CUORE’s current tellurium dioxide crystals with lithium molybdate crystals. When particles produced in neutrinoless double beta decay interact with lithium molybdate, they produce both heat and light. This light will be detected along with the heat, and the ratio of heat-to-light will allow us to reject background events involving particles that are not produced by neutrinoless double beta decay. The cryogenic structure of the experiment will also be upgraded.

Long-lived hot electrons spotted in ‘wonder’ semiconductor

By combining scanning electron microscopy with ultrashort laser pulses, researchers in the US have shown that cubic boron arsenide has an important property that could be used to create better solar cells and photodetectors. Usama Choudhry and colleagues at the University of California, Santa Barbara, and the University of Houston used scanning ultrafast electron microscopy (SUEM) to confirm that “hot” electrons in the semiconductor material have long lifetimes – something that could be useful in a wide range of applications in electronics.

Sometimes dubbed a “wonder material”, cubic boron arsenide is a semiconductor material with several promising properties that could lead to its widespread commercial use. It is a much better conductor of heat than silicon, so it could be used to create integrated circuits that are packed together at higher densities and run at higher frequencies. The material has an electron mobility that is on par with silicon, but it has a much higher hole mobility than silicon – a property that would be useful in designing electronic devices.

Now, Choudhry and colleagues have shown that cubic boron arsenide has another useful property: long-lived “hot” electrons. When light falls on a semiconductor it can cause the excitation of electrons with a range of energies. The lower energy electrons can persist for long enough so that they can be collected to create an electrical current – which is the basis for solar cells and light detectors. However, in most semiconductors the higher-energy hot electrons have very short lifetimes and are therefore lost before they can be collected.

Long lived hot electrons

Calculations done in 2017 suggested that hot electrons have relatively long lifetimes in cubic boron arsenide. However, limitations in fabricating and studying cubic boron arsenide crystals had made it difficult to confirm this prediction.

In their study Choudhry’s team used SUEM, which combines the temporal resolution of ultrashort laser pulses with the spatial resolution of scanning electron microscopy. The technique involves splitting the laser pulse into two parts. The first part of the pulse is used to excite hot electrons in a high-quality sample of cubic boron arsenide that was made by the Houston team. After a carefully controlled delay, the second part of the pulse is focused onto a photocathode. This generates an electron pulse that is just a few picoseconds long. This pulse is used by an electron microscope to characterize the electrons in the cubic boron arsenide.

By changing the delay, the team could measure the lifetime of the fast electrons in the sample, revealing that they persist for over 200 ps, which is far longer than the hot charge carriers in most semiconductors used in solar cells. The researchers say that the long lifetime suggests cubic boron arsenide could be used to make better solar cells, but much more work is needed to improve fabrication techniques.

The research is described in Matter.

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