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Home » Page 1089

X-rays yield ghost images

 

“Ghost imaging” generates images by analysing correlations between two light beams, the more powerful of which doesn’t bounce off the object in question. The technique has already been demonstrated at visible and infrared wavelengths, but now two groups of scientists – one in Australia and Europe, and the other in China – have extended it to the X-ray region. The new results could lead to new methods for medical diagnoses with lower doses of X-rays and for X-ray crystallography of non-crystalline materials, say the researchers.

Ghost imaging involves splitting a light beam into two beams. The “object beam” directed towards the object to be imaged, which has a single pixel “bucket” light detector behind. Meanwhile, the “reference beam” travels straight to a multi-pixel light detector. The idea is to build up a shadow image by incorporating the output from only some of the pixels in the reference detector: those for whom the corresponding parts of the bucket detector are not blocked by the object.

To do this, the detectors are exposed to a broad “speckled” beam whose cross-sectional intensity distribution varies with time. The changing correspondence between the total intensity recorded at the object detector and the pattern of intensities recorded at the reference detector then allows the image to be reconstructed.

Turbulent vision

Ghost imaging at visible wavelengths is already being studied to improve remote imaging of the Earth’s surface by satellites in turbulent conditions. Turbulence scatters light in random directions, which usually makes images noisier. But by measuring the intensity correlations of two correlated beams over an extended period of time, one of which is bounced off the object being observed, the effects of turbulence can be averaged out to almost zero.

There is another advantage of ghost imaging that also makes it attractive at X-ray wavelengths: a good image can still be generated even when the intensity of the object beam is low. This could potentially significantly reduce the size of X-ray doses administered to patients. However, the difficulties involved in building X-ray optics mean that it is far harder to split X-ray beams than it is beams of visible light.

Daniele Pelliccia of RMIT University in Victoria, Australia, and colleagues got round this problem by using X-rays from a synchrotron source – ESRF in Grenoble, France. They directed the synchrotron beam at a sliver of silicon, which left part of the beam undisturbed and diffracted the remainder through a small angle. By placing the object – a copper wire – in one of the beams and then using different portions of a single camera to serve as both object and reference detectors, the researchers were able to generate ghost images of the wire.

Shot noise

The speckle needed in the experiment was generated naturally by the very short pulses of electrons used to produce the X-rays; the distribution of electrons in each bunch being random and yielding what is known as shot noise. To make sure they really had generated ghost images, the team varied the frequency over which they analysed the data. They found, as expected, that they were only able to produce good images when the frequency matched the electron pulse rate. Failure to image at about one pulse per frame means losing the correlation between the two copies of the beam, explains Pelliccia.

Pelliccia says that he and his colleagues are now working out how to exploit this result in order to reduce the dose given in X-ray diagnoses “by at least an order of magnitude” over current levels. Doing so, he points out, will require quite a different technique to the one they used in their experiment, given that synchrotron sources are unsuited to routine medical imaging. In fact, he says, the other new study reporting X-ray ghost imaging, by Shensheng Han of the Shanghai Institute of Optics and Fine Mechanics and colleagues, may point to a more practical alternative.

That research also used X-rays generated by a synchrotron source – the Shanghai Synchrotron Radiation Facility – but did not involve beam splitting. Han and co-workers used a single beam directed at a solitary CCD detector, and instead used an actuator to move the object – in their case a thin gold film with five slits – into and out of the beam. To generate the speckle pattern, the researchers placed another gold film, much wider than the beam diameter and containing randomly distributed holes, between the X-ray source and the object. They found they could produce ghost images by moving the holey film across the beam so as to present a continually changing speckle pattern, and then moving the object into and out of the beam once for each different pattern.

Fourier technique

Pelliccia points out that the Chinese group didn’t generate images directly. Rather, they measured diffraction patterns from the object and then converted those patterns using a Fourier technique. But he says the fact that they created their speckle patterns simply by placing a film in the beam, rather than relying on electron shot noise from a synchrotron, makes it potentially better suited to medical imaging than his group’s technique. He notes that moving a patient in and out of an X-ray beam “wouldn’t be a practical option” but adds that in future it might be possible to move the detector instead of the patient.

In a paper describing their work, Han and colleagues emphasize the importance of their research to crystallography. They say that scientists in future could use their technique to create high-resolution images of non-crystalline samples using “widely accessible laboratory X-ray sources” rather than having to rely on synchrotron sources or free electron lasers. “The structure information of many important molecular materials, such as membrane proteins, is still out of reach because these materials are difficult to grow into macroscopic crystals,” they write.

Ivan Vartaniants of the DESY laboratory in Germany praises the two groups for their “excellent and challenging work”. However, he says that the approach put forward by Pelliccia and colleagues suffers from a number of shortcomings, including the current lack of suitably fast and high-resolution detectors as well as the fact that – unlike copper wire – biological samples will transmit most of the radiation that passes through them.

As regards the technique developed by the Chinese group, Vartaniants queries whether it could be used to image arbitrarily shaped objects and questions whether it really would be suited to conventional X-ray sources, given that the group tested it using a highly coherent, strong flux synchrotron source.

The work of both groups is reported in separate papers in Physical Review Letters.

Flash Physics: Superfluid helium dark-matter detector, Hinkley C will go ahead, why nanotubes are different

UK’s Hinkley Point reactors get the go-ahead

Britain’s prime minister Theresa May has approved the planned construction of the £18bn Hinkley Point C nuclear power station in south-west England. May had unexpectedly put the project under review in July, shortly after taking over as prime minister from David Cameron. Comprising two nuclear reactors, the station will be built by the French company EDF with a £6bn investment from China. According to the UK government, new safeguards have been put in place to ensure that Chinese participation in the project does not compromise the national security of the UK. The decision is expected to open the door to Chinese companies, which are keen to build new reactors elsewhere in the UK. There have been concerns about the high cost of electricity from the plant, which is pegged at £92.50 per megawatt hour and has not been changed by the review. Critics have pointed out that this is much more expensive than energy from a similar facility being built by EDF in France. Hinkley Point C will be based on new European Pressurized Reactor technology, which is being implemented in reactors under construction in Finland, France and China. There is more about Hinkley Point C in the recent Physics World Focus on Nuclear Energy.

Chirality explains why similar nanotubes behave differently

Computer model of a nanotube
Same but different: computer model of a nanotube. (Courtesy: SISSA-CNR-IOM)

Why is it that nanotubes fabricated from seemingly similar nanomaterials exhibit different properties? That is the question asked by researchers at the International School for Advanced Studies (SISSA) in Italy and Tel Aviv University in Israel, who have looked at why materials that have similar structures produce nanotubes that behave differently. For example, while both carbon nanotubes and boron nitride nanotubes are nearly indistinguishable in terms of their structures, they have different responses to frictional forces. The team created computer models of the nanomaterials and studied their characteristics in detail. Team-leader Roberto Guerra says the study showed differences in the chirality of the materials and that this may cause the differences in their properties.

How to detect light dark matter using superfluid helium

Superfluid liquid helium is an ideal medium for detecting low-mass dark-matter particles, according to Katelin Schutz and Kathryn Zurek at the Lawrence Berkeley National Laboratory in the US. While physicists have not been able to detect dark matter directly, several generations of experiments suggest that dark-matter particles have masses below about 10 GeV/c2. As a result, physicists are thinking about how to build detectors that are sensitive to light dark matter at masses as low as 1  keV/c2. This involves looking for extremely rare collisions between dark and ordinary matter in a large detector. The problem is that the dark matter that passes through the Earth is expected to be moving slowly and therefore such collisions will impart tiny amounts of kinetic energy to the detector – making interactions very difficult to see. Schutz and Zurek have calculated that all of the kinetic energy of a dark-matter particle could be absorbed in superfluid helium via the creation of two phonons – particle-like sound waves. These phonons could then be detected using existing technologies. A further benefit of the technique, which is described in Physical Review Letters, is that measurement of the momenta of the two phonons can help distinguish between real dark-matter collisions and background noise. Zurek has a separate paper in the same journal about using superconductors to detect light dark matter.

    • See our video below for more about the nature of dark matter.
    • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on ghost imaging.

The risks and rewards of radiation

One Sunday evening, as I finished reading Timothy Jorgensen’s Strange Glow, I closed the book and turned off the bedside lamps to prepare myself for the next day’s early start. Suddenly, a strong yellow glow caught my eyes. As a medical physicist who deals with X-rays and radioactive materials on a daily basis, I have seen the three-fan radiation symbol countless times, but there, printed on the book cover and shining brightly out of total darkness, it suddenly appeared awful and mysterious. It reminded me of the stories I had just read in Jorgensen’s book, from the very first X-ray image – which famously showed the skeletal ringed hand of Wilhelm Röntgen’s wife – to the innocent “radium girls” who were fatally exposed while painting watch dials with the radioactive paint, and the iconic photos of mushroom clouds rising in the skies of Hiroshima and Nagasaki in August 1945.

Humans have a long-term relationship with radiation. Many radioactive materials have existed for millennia, and some have probably been here ever since the birth of our planet. Yet, only since 1895 – the year Röntgen discovered X-rays in an early version of an X-ray tube invented by William Crookes – have we really started to comprehend the “strange glow” associated with these materials, and to appreciate the mighty power of radiation in treating cancers as well as in inducing them. In many cases, human beings have paid a stiff price for these hard-won lessons.

In Strange Glow, Jorgensen relates a brief history of our dealings with radiation, tracking the most important events and profiling the most pioneering researchers in the field. Through a clear timeline, he describes the discovery of this phenomenon, its health effects, and the risks and benefits of radiation in three distinct sections. All the great discoveries (which all physicist readers should know about and never forget) made by prominent early radiation scientists such as Röntgen, Marie Curie, William Bragg and Thomas Edison are introduced naturally, logically and chronologically. Jorgensen’s lucid writing and strong story-telling skills are demonstrated thoroughly in this book , making it a pleasure to read. Like Stephen Hawking’s A Brief History of Time (which I read a long time ago and still enjoy), Jorgensen’s book introduces only two equations, one on Bragg’s law and the other on Haber’s rule. In its simplicity and conciseness, it greatly contributes to removing some of the mystery and misunderstanding that surrounds radiation.

One real strength of this book is the way it dispels the myth that radiation risks are too complicated for ordinary people to grasp. Jorgensen does this by showing that radiation risks can be estimated effectively without resorting to some “expert” advice or relying on a lot of physics and mathematics background knowledge. This is particularly important in the modern era when radiation has become part of our daily lives, whether it stems from natural sources (such as radon gas in basements), consumer technology (mobile phones, microwaves, airport screening devices and so on) or from medical uses such as diagnostic X-rays, mammography, CT scans and radiation therapy.

Jorgensen, an associate professor of radiation medicine and director of the graduate programme in health physics and radiation protection at Georgetown University, US, has done an excellent job of elucidating many important medical-radiation concepts in layperson’s language. These include radiation-induced DNA damage, effective dose, number needed to treat (NNT) and number needed to harm (NNH). As such, readers can easily develop an objective mindset for assessing the risks of any radiation-related events they encounter. This is to be welcomed, as a more rational perception of radiation risks would help people make sensible decisions on their health – decisions that would reduce rather than increase their risks in the long term.

While a large portion of the book is dedicated to the risks and benefits of radiation, the application of these risks and benefits to the treatment of cancers could be further elaborated. As the author points out, this is a non-trivial task – not least because radiation therapy is primarily used to treat people who already have cancer; every patient and every cancer is different; and even the same cancer in the same patient can respond differently to radiation therapy over time. Hence, personalized radiation therapy with personalized cancer risk assessment would be highly desirable, and is becoming a very hot topic in medical physics research.

In fact, recent epidemiological studies carried out in the UK, France, the US and Australia have confirmed that there is a positive correlation between ionizing radiation and the risk of developing a second cancer in both children and radiation-monitored workers. Mean carcinogenic doses as low as 16 mGy – the sort of dose usually encountered in a CT scan – have been recorded when the targets were radiosensitive organs such as the brain, lungs and red bone marrow. A study carried out by my own group also indicated that the cumulative radiation doses from multiple imaging procedures can be comparable to the “scatter” and “leakage” doses from radiation treatment of cancers – thereby imposing extra risk of developing a second cancer on patients who have already had a primary cancer. This is particularly important for paediatric patients, who are both more vulnerable to the radiation damage and also have a longer life expectancy than the adults.

Mark Twain once described a “classic” as “a book which people praise and don’t read”. In that spirit, then, I will not describe this book as a classic, because I feel it will become a very useful resource to the general public as well as to radiation experts, thanks to its simplicity, conciseness and lucidity. I have certainly learned a lot from the book, as it has helped me project my own perspective on the uses and risks of radiation onto a broader spectrum of issues. I greatly appreciate the author making this book so accessible and readable, and making radiation less formidable than it first appears.

  • 2016 Princeton University Press £24.95/$35.00pb 512pp

Fighting science denial

Fidel Castro – that acerbic critic of anything American – once said that he liked the movie Jaws because it shows the inevitable consequences of the corruptions of capitalism. The former Cuban president was surely thinking of the scene in the film where oceanographer Matt Hooper, played by the nerdy Richard Dreyfus, realizes that a mangled woman’s body is evidence of a shark prowling the waters and tries to persuade the local mayor to close the beaches. The mayor, however, insists the beaches must stay open because shutting them will be expensive, and the mangled body is probably a boating accident. We know what happens next.

The scene is frightening – I find it more terrifying than the gory bits with sharks – because it shows that science denial is not the product of irrationality or scientific illiteracy. The mayor, a town native, knows full well what sharks do, but wants to protect the financial interests of the citizens who voted for him and so uses the boating-accident scenario as justification.

I am aware that “science denial” is a loaded and politicized term because it doesn’t refer to the outright rejection of all science, but only certain areas where political, economic and religious interests come into play, notably climate change, energy, food technology and health. But this is Castro’s point: when the going gets tough, capitalists turn into self-interested opportunists.

Until recently, most scientists I know viewed science denial like crime: it’s an unfortunate side of modern life, but one that’s tolerable at low levels. Things have changed though. It’s not just about disease-healing amulets and character-predicting zodiac signs any more. Here in the US, science denial has entered federal and state policy-making in ways that threaten public safety.

In 2012, for instance, the North Carolina legislature passed House Bill 819 – a law prohibiting the use of models of sea-level rise to protect people living near the coast from flooding. Formulated in response to a report by the science panel of the state’s coastal-resources commission, which predicted a substantial sea-level rise by the end of the century, the law reflected fears that the report would harm tourism and property values. Bills have also been introduced in the US Congress to stop politicians from using science produced by the Department of Energy in policies – evidently to avoid admitting the reality of climate change (so far these bills have failed).

In 2012, meanwhile, Congressman Paul Broun of Georgia, who is a medic by training, said that evolution, embryology and the Big Bang theory are “lies straight from the pit of hell”, adding that he believed the world was about 9000 years old. Broun was not only re-elected after making these remarks, but also retained on the House committee on science, space and technology, where he made decisions on non-defence R&D affecting his Georgia constituents as well as millions of other US citizens.

So is science denial really the inevitable by-product of capitalism? As the US gears up for the forthcoming presidential election, it seems that many US politicians from all sides of the political spectrum are determined to prove the former Cuban leader right. To deal with the problem of science denial, I believe that we need both long-term solutions and short-term strategies. And as science denial affects issues that are dire and immediate, I have drawn up five short-term strategies that should immediately be put into effect.

1. Force commitment

During the last US presidential election, I discussed the fashion for candidates to sign pledges to show their commitment to specific positions on abortion, taxes and gay marriage. My first anti-science-denial strategy is to adopt and extend that idea.

Take evolution denial. The president of my university, who is an epidemiologist, likes to say that microbes and viruses are “evolution in motion”. Outbreaks of new plagues and viruses mean that a legislator’s belief in evolution, and thus in the value of studying it, is a public-health issue. At debates and press conferences, evolution-denying politicians should therefore be asked to sign (or explain why they will not sign) an anti-evolution pledge: “I pledge that I will not use, nor let my constituents use, any medication whose development depended on evolution or evolutionary theory.”

Similar pledges can be crafted to test the sincerity of other science-denying politicians, including anti-vaccination activists and climate-change deniers. The latter should be required to sign (or explain why they will not sign) a pledge to take no action to protect their or their constituents’ properties against rising sea levels and other effects of climate change. Donald Trump, for instance, has said that climate change is “bullshit”, “pseudoscience” and “a total hoax”. Yet, as Politico reported, he has applied for permission to erect a sea wall to protect one of his golf courses in Ireland from rising seas due to “global warming and its effects”. Such a pledge would expose that action not as a mere business decision but as a betrayal of his would-be constituents.

2. Expose values

Civilizations have long used scientific methods to understand our world and discover tools to ward off threats, be they vaccinations to tackle disease or foodstuffs to prevent hunger. Whether and how to use these tools is a legitimate topic of political discussion, but politicians who try to stop ordinary citizens from having such tools at all are behaving, in a way, like people who don’t think citizens have the right to defend themselves. Many science deniers in the US also happen to believe that the right to use weapons in self-defence is a fundamental American value. So in seeking to prevent citizens from using scientific methods to protect themselves, many science deniers in the US are, perversely, betraying their own values.

Here’s an even more incendiary comparison: US politicians who attack science are like so-called Islamic State militants who bulldoze archaeological treasures and smash statues. I’m deliberately being over the top – but by how much? Science is a cornerstone of Western culture, not only to ward off threats but also to achieve social goals. In seeking to destroy those tools, science deniers are like ISIS militants in that they are motivated by higher authority, believe mainstream culture threatens their beliefs, and want to damage the means by which that mainstream culture survives and flourishes.

If anything, ISIS militants are more honest because they openly admit that their motive is faith and ideology, while Washington’s cultural vandals do not. It’s disingenuous, prevents honest discussion of the issues, and falsely discredits and damages American institutions. At debates and press conferences, I think such politicians should be asked: “Explain the moral difference between ISIS militants who attack cultural treasures and politicians who attack the scientific process.” How they respond will reveal much about their values and integrity.

3. Engage in comedy and ridicule

The magician James Randi once exposed a popular televangelist by playing recordings of secret transmissions between an audience plant and the televangelist; the televangelist declared bankruptcy the next year. The incriminating evidence against science denial is rarely as direct and dramatic because science deniers muddy the waters with cherry-picked data, fake experts and uncertainty. But comedy is often as effective in revealing the dynamics.

A Doonesbury cartoon strip, for instance, once featured an “honest” science denier interviewed on a radio talk show. “I don’t oppose sound climate policy because it’s flawed,” he says. “I oppose it because I care much more about my short-term economic interests than the future of the damn planet. Hello?” Comedy’s ability to be transparent and say unpleasant truths invites trust – one reason why a Pew Research poll of public trust of news sources ranked TV’s the Daily Show higher than the Economist. Comedy can also expose opportunism masked as sceptical science.

4. Proliferate parables

A fourth strategy is to tell parables involving science denial. A parable, like an Aesop’s fable, is a real or fictional story with a built-in moral that can easily be grasped. It is an effective teaching approach. After all, most people learn more easily through stories than data. Jaws is a famous modern example. Another is Henrik Ibsen’s play Enemy of the People, in which the doctor of a small town whose livelihood depends on its spa discovers that waste from a local tannery is injecting deadly bacteria into the spa’s waters. Yet the doctor can’t even make himself heard at a town meeting he arranges and is libelled, accused of conspiracy and fired. These powerful parables expose the all-too-rational calculus of science denial. We need 21st-century Aesops to tell more memorable stories of what happens when we wish away sharks.

5. Initiate prosecution

A final strategy is to prosecute science deniers. Last year, US senator Sheldon Whitehouse of Rhode Island proposed that organizations bankrolling campaigns of climate-science disinformation should be investigated for possible violation of federal law. The law in question prohibits “racketeering” – a type of fraudulent business activity that includes conspiracy to deceive the public about such things as risk. Such laws have, for example, been successfully used to prosecute tobacco companies for misleading the public about the hazards of smoking.

I think that the proposal is a great idea. What’s the difference between endangering the public by hiding evidence that smoking is hazardous and endangering the public by concealing evidence of climate change? The crime is like shouting “Stay put! Everything’s OK!” in a burning store so that people carry on shopping. Some might say that prosecuting science deniers is censorship and a denial of free speech, but if being misleading and deceptive about serious hazards isn’t a crime, it should be.

We should legally target those who seek to block scientific information from being used to protect life and property. With the displacement of people due to global warming already starting, we need to prosecute people who disrupt our ability to use the knowledge we have to develop solutions. They should be forced to pay for the damages, both personal and financial.

The critical point

Science denial, I think, is one of the most important issues of the current US presidential campaign. I rank it even higher than key social issues such as gay marriage and transgender bathrooms; anyway, the former is settled and the latter on the way. Science denial is more important even than energy and foreign policy, because poor choices will inevitably be made if scientific information is not incorporated into such decisions.

These five strategies involve taking more aggressive steps than scientists are used to. But explaining yet again the importance of science in addressing crises has not been sufficient. Fighting science denial is not just for scientists and educators, but for lawyers, comedians, storytellers and other citizens. We need to call people out – for irresponsibility and for betraying values, and even for the legality of their behaviour. These five strategies will not eradicate science denial. But doing all of them all of the time might help to prevent politicians who practise it from getting elected.

US facing medical-isotope shortage when production ceases in Canada

A report from the US National Academies of Sciences, Engineering and Medicine (NAS) warns that the US could be facing severe shortages of the vital medical isotope technetium-99m once the ageing NRU nuclear reactor in Chalk River, Canada, stops producing molybdenum-99 next month. Technetium-99m, which is derived from the molybdenum-99 isotope, is widely used for medical imaging.

Released this week, the report was commissioned by the US Congress and warns that there is a greater than 50% chance that severe shortages of molybdenum-99 and technetium-99m will occur in the US after the NRU stops production in October. Both isotopes have very short half-lives and cannot be stockpiled. While global supplies of molybdenum-99 are produced at six other reactors worldwide, most of these facilities are also very old and some are prone to unscheduled shutdowns.

Only North American source

Built in 1957, the NRU reactor has shut down unexpectedly twice in the past decade. It is the only source of molybdenum-99 in North America. Although both the US and Canada are developing accelerator-based facilities to produce the isotope, some of them are not expected to come online until 2018. The lack of molybdenum-99 production in the interim from the NRU reactor means that the supply of technetium-99m to the US could be vulnerable, warns the NAS report. NRU will, however, keep running until 2018 and could be called back into service if there is a shortage of technetium-99m.

About 75% of molybdenum-99 produced worldwide is made by irradiating targets that are highly enriched in uranium-235, which is a “weapons grade” material. Worries about potential terrorism and nuclear proliferation have led to a programme to convert production facilities so they can use targets with a much lower level of enrichment. Although all existing reactors could all be converted by 2019, the NAS report warns that delays or problems with the conversion process could also lead to shortages of technetium-99m.

Overseas production

“Current efforts to increase the supply of molybdenum-99 by expansion of existing overseas production and initiation of domestic production by methods not requiring highly enriched uranium are important to ensure future availability,” says medical biophysicist James Adelstein at Harvard Medical School, who chaired the 13-person committee that produced the report.

“Although there are plans from both existing international suppliers and potential domestic suppliers to fill the expected supply gap from Canada,” he adds, “the committee is concerned that any delays in bringing additional supplies of molybdenum-99 to the market would increase the risks of substantial shortages”.

Why should scientists communicate their work to the public?

Scientists have a responsibility to share the findings of their research with the general public. That is the message of Andrea Morello from the University of New South Wales, Australia, in this video from our 100 Second Science series. Morello, who is a quantum computing researcher, believes that is especially the case in situations where scientists are in receipt of public funds. Regarding his own field, Morello believes that quantum-based technologies are bringing a golden opportunity to debunk the idea that quantum mechanics is just a strange esoteric field with no real applications.

Flash Physics: HERA observatory bags $9.5m, rare-isotope decay eludes detection, putting pressure on iron

Cash boost for South African radio telescope

The Hydrogen Epoch of Reionization Array (HERA) observatory, located in Losberg near Carnarvon in South Africa, has received a $9.5m funding boost that will see it vastly increase its number of 14 m radio dishes. HERA currently has 19 dishes – a number that will soon increase to 37. Yet the new cash will increase that to 220 dishes by 2018. The rise in the number of dishes will allow astronomers to explore the large-scale structures that formed during and prior to the epoch of reionization – a billion-year period after hydrogen collapsed into the first galaxies, a few hundred million years after the Big Bang. The University of California, Berkeley, leads the experiment together with institutions in Italy, South Africa, the UK and the US. HERA is a precursor array to the upcoming Square Kilometre Array that will be built in southern Africa and Australasia in the coming decade.

Decay of nature’s rarest isotope eludes physicists

An attempt to measure the decay rate of tantalum-180m at the HADES underground lab in Belgium has come up empty handed. Tantalum-180m comprises 0.01% of naturally occurring tantalum, making it the rarest known natural isotope. The “m” denotes that the isotope is a metastable state, which means that its nucleons exist in an excited state. Tantalum-180m is also exceptional because it has the longest known lifetime of a metastable nucleus. Indeed, the lifetime is so long that physicists have yet to actually see it decay. In this latest attempt Björn Lehnert and colleagues at Technical University Dresden in Germany and the Joint Research Centre in Geel, Belgium placed samples of high-purity tantalum – with natural isotopic abundances – in the Sandwich detector system located 225 m underground where background radiation levels are very low. Sandwich comprises two high-purity germanium radiation detectors. After 176 days of measurement, the team saw no evidence for the decay of tantalum-180m. When combined with previous measurements, the team was able to place a new lower limit on the half-life of the isotope of 4.5 × 1016 years. The research is described in a preprint on arXiv.

Putting the pressure on iron

Illustrations showing ten layers of hexagonal iron (dark grey) above eight layers of cubic iron (pale blue-grey)
Under pressure: iron atoms spring back. Illustrations showing ten layers of hexagonal iron (dark grey) above eight layers of cubic iron (pale blue-grey). Each layer can transform from one structure to the other by pinching pairs of adjacent atoms toward or away from one another. (Courtesy: Lawrence Livermore National Laboratory)

Iron transforms in strange and unexpected ways when put under pressure, according to the latest work done by physicists at the Lawrence Livermore National Laboratory. The team studied iron under dynamic compression and found that iron crystals are deformed while compressed but then spring back into shape when the pressure is removed. In the work, described in Physical Review Letters, the team outlines first-principle calculations on two solid phases of iron, as well as on intermediate crystal structures along the transformation path from one phase to the other. “This reversible transformation is reminiscent of what happens in a shape-memory alloy. Such materials can change linear dimensions by almost 10% and then reversibly change back,” says team-leader Michael Surh. “It turns out that the different (fast versus slow, reversible versus irreversible) behaviour seen in iron depends on its changing magnetism.”

A third of humanity cannot see the Milky Way

Light pollution map of North America
City lights: 99% of Americans cannot see the Milky Way. Light pollution map of North America (Courtesy: Fabio Falchi et al./Science Advances)

Do you live in a city or any relatively large metropolitan area? If so, then can you remember the last time you have seen our Milky Way galaxy in the sky? If your answer is “never” then you can be sure to blame the growing number of artificial lights that have severely increased the amount of light in the night sky, creating an artificial skyglow. So says Fabio Falchi and a team of researchers at the Light Pollution Science and Technology Institute in Thiene, Italy, who have created detailed maps of the light pollution seen across the globe today. “Despite the increasing interest among scientists in fields such as ecology, astronomy, health care and land-use planning, light pollution lacks a current quantification of its magnitude on a global scale,” writes the team in a paper recently uploaded to the arXiv server. The researchers used their light-pollution propagation software to come up with a “world atlas of artificial sky luminance” – their atlas shows that more than 80% of the world and more than 99% of people in the US and Europe live under light-polluted skies.

 

  • You can find all our daily Flash Physics posts in the website’s news section, as well as on Twitter and Facebook using #FlashPhysics. Tune in to physicsworld.com later today to read today’s extensive news story on isotope production stopping at Canada’s NRU reactor.

Chirped laser pulses could deliver high-quality ion beams

 

Physicists in Sweden have come up with a new way of accelerating ions using intense laser pulses. The technique – which has not yet been tested in the lab – involves bouncing “chirped” pulses from a mirror and promises to deliver much more intense ions beams than existing laser acceleration schemes. With further development, the method could be used to provide high-energy ions for cancer treatment.

Beams of ions with energies in the 100–200 MeV range are ideal for treating some cancers because they can be fine-tuned to deposit most of their energy within a tumour, thereby minimizing the damage to nearby healthy tissue. Conventional ion therapy, however, requires big, expensive accelerators and beam-guiding systems, which means that relatively few hospitals have such facilities.

Ions can, however, also be accelerated to high energies by firing intense laser pulses at a target, which could lead to smaller and less expensive treatment facilities. The process involves a laser pulse creating a hot plasma in which the heated electrons expand rapidly away from the target, leaving the much more sluggish ions behind. Eventually, a huge electric field builds and this will accelerate ions in the plasma to very high energies.

This process is very inefficient and messy, however, and produces ions with a wide range of energies – which is not ideal for cancer therapy. And, because the process is essentially a thermal effect, a large increase in the power of the laser is required to achieve a modest increase in the average energy of the ions.

Sailing away

One way of improving the process is to use a “light sail” – a thin reflective metal foil that is very good at absorbing energy from the laser pulse. However, this technique suffers from instabilities arising from how the light sail is impacted by the laser pulse. What Felix Mackenroth, Arkady Gonoskov and Mattias Marklund from the Chalmers University of Technology have instead done is accelerate ions by placing a similar thin foil target next to a thick mirror.

Their technique involves firing a laser pulse through the foil target and onto the mirror. The light bounces off to create a standing wave in the region of the foil, with light pressure pushing on both sides of the foil, thereby stabilizing it as the plasma is created. Electrons in the plasma are attracted to the first minimum of the standing wave, which is located in front of the foil. As a result, the electrons are accelerated towards the source of the laser light. This creates a huge electric field that in turn accelerates the ions in the same direction.

Clever chirp

At this point, a clever trick is used to enhance the acceleration process. Instead of using a laser pulse with a fixed wavelength, the team proposes the use of a chirped pulse with a wavelength that changes continuously. Mackenroth, Gonoskov and Marklund have shown that by carefully selecting the chirp, the location of the first standing wave minimum will move away from the foil and mirror – dragging the electrons with it and further accelerating the ions.

Mackenroth told physicsworld.com that ion energies of about 100 MeV should be achievable using the technique. However, he admits that the acceleration technique will produce ions with an energy distribution that is too wide for medical applications, which need monoenergetic beams. To create such a beam, ions of the right energy would then have to be selected by bending the original ion beam with a magnet, which would increase the overall size of the system.

Higher energy

On the upside, the energy distribution of the ions created in the chirped technique is narrower than that of the light sail. It also produces as many as 1000 times more ions, which means that after energy selection is done, there should still be enough ions left over for medical applications. Another benefit of the technique is that it can produce more higher-energy ions than other methods. Furthermore, the energy of the ions can be adjusted by changing the chirp of the pulse, which can easily be done.

“With our method we can capture, stabilize and organize large numbers of ions with great precision without using a lot of energy,” says Mackenroth. “This is a small step towards the ultimate goal of treating cancer tumours in a way that provides enormous benefit to society. But we are still far from the ultimate goal.” Mackenroth adds that the technique will soon be tested in the lab.

The proposal is described in Physical Review Letters.

Flash Physics: No WIMPs for HESS, graphene production ramps up in China, double-helix semiconductor

HESS dark-matter search comes up empty after 10 years

The High Energy Stereoscopic System (HESS) observatory in Namibia has failed to find any evidence of dark matter in the inner halo of the Milky Way after ten years of observation. HESS uses an array of ground-based telescopes to observe the Cherenkov light that is created when a high-energy cosmic gamma ray interacts with the atmosphere. The Milky Way halo should harbour large amounts of dark matter, which, if described correctly by the weakly interacting massive particle (WIMP) model, should annihilate to create gamma rays. However, HESS has been unable to measure an excess of gamma rays coming from that region between 2004–2014, as described in Physical Review Letters. While no hint of dark matter was seen, the measurements allow physicists to fine-tune the WIMP model by putting a new limit on how likely the hypothetical particles are to annihilate.

Commercial graphene production begins in China

The first facility for the commercial production of graphene in China has started operations in Xiamen. The material is being made by the Hengli Shengtai company, which is controlled by the graphene scientist Bor Jang, who is co-founder of the US-based graphene maker Angstron Materials. Graphene production by Hengli Shengtai is expected to reach 5000 tonnes per year by 2020. Dubbed the “wonder material”, graphene is a sheet of carbon just one atom thick. It has a number of desirable properties such as high electron mobility and high mechanical strength, and is already used to make mobile-phone display screens. This video explains how graphene is made at a lab in Manchester UK.

Double-helix semiconductor is flexible and robust

Photograph of needles of the flexible semiconducting material
Fuzzy logic: filaments of the flexible semiconducting material SnIP. On the left of the photograph are residual black phosphorus and red tin iodide. (Courtesy: Andreas Battenberg/TUM)

An inorganic material with a double-helix structure has been discovered by Daniela Pfister and colleagues at the Technical University of Munich. Made up of tin, iodine and phosphorus, the material is a semiconductor, which the researchers say has extraordinary optical and electronic properties. The double-helix-like structure makes the material flexible, yet robust – which is unlike conventional inorganic semiconductors. Indeed the material, called SnIP, is so flexible that centimetre-long fibres can be bent without breaking. “This property of SnIP is clearly attributable to the double helix,” says Pfister, adding that it can be easily produced “on a gram scale and is, unlike gallium arsenide, which has similar electronic characteristics, far less toxic.” The material will be described in a paper in Advanced Materials and could be used in a range of applications including solar-cell energy, sensors and optoelectronic devices.

Federico Capasso wins 2016 Balzan Prize for Applied Photonics

Harvard University physicist Federico Capasso has won the 2016 Balzan Prize for Applied Photonics for his work on the development of quantum cascade lasers and “major contributions in plasmonics and metamaterials”. The prize, worth 750,000 Swiss francs (£580,000), will be presented during a ceremony in Rome on 17 November. In announcing the award, Carlo Wyss, a former director for accelerators at CERN, hailed Capasso’s “pioneering work in the quantum design of new materials with specific electronic and optical features”, which were crucial to the realization of quantum cascade lasers. Recently, Capasso has made significant progress in the development of flat lenses, which could find use in mobile phones and other consumer electronics.

 

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Squeezed light shatters previous record for manipulating quantum uncertainty

The quantum state of light has been squeezed more than ever before by physicists in Germany, who have developed a new low-loss technique. Squeezed light has been used to increase the sensitivity of gravitational wave detectors, and scientists are planning to deploy the new method on the GEO600 and LIGO gravitational wave detectors.

Detecting gravitational waves – the ripples in spacetime caused by energetic events in the Universe – relies on splitting a laser beam using an interferometer and sending the two halves back and forth along two orthogonal arms. When the two halves of the beam recombine, all the light normally comes out of one port of the interferometer. A passing gravitational wave will change the relative lengths of the two arms, creating an interference pattern and directing some of the light out of the “dark” port. However, by the time they reach Earth, gravitational waves from even the most dramatic events have tiny amplitudes, so sensitivity is crucial. The first confirmed discovery of a gravitational wave, announced by LIGO in February, was produced by the collision and merger of two black holes and changed the 4.2 km arm lengths by barely 10–19m (see “LIGO detects first ever gravitational waves – from two merging black holes“).

At such extreme sensitivity, one of the main noise sources in such detectors is uncorrelated photons emerging from the quantum vacuum as a result of its zero-point energy – the energy that Heisenberg’s uncertainty principle dictates can never be removed from a system. But, amazingly, even this source of noise can be minimized. The uncertainty principle puts a lower limit on the product of the variance in the amplitude (or number) of photons and the variance in the phase. Vacuum photons naturally have equal variance in both amplitude and phase. It is, however, possible to create a “squeezed state” of light, in which either one of these quantities is minimized (squeezed) and the other is allowed to increase (antisqueezed).

High and low frequencies

Both amplitude and phase variations cause noise problems, but amplitude variations cause more problems at low frequencies, whereas phase variations cause problems principally at high frequencies. The idea, therefore, is to squeeze either the amplitude or the phase of the vacuum photons, depending on the frequency one is scanning from gravitational waves. The result is that quantum vacuum noise causes fewer problems than it otherwise would across the entire spectrum of interest (around 10 Hz to 5 kHz).

The best way to squeeze vacuum photons is a technique called optical parametric amplification. This uses a laser to pump a nonlinear crystal inside an optical resonant cavity, with each laser photon producing two “daughter” photons. The vacuum photons interact with the laser photons and, as a result, the variance of choice is squeezed in the emergent photons while the other is antisqueezed. The amount of squeezing that is possible is limited by both optical loss and noise in the apparatus. In 2010 researchers at the Max Planck institute for Gravitational Physics in Hannover set a world record by squeezing the amplitude and phase variances (separately) by a factor of 19.

The Hannover team has now improved several aspects of its instrumentation. Most significantly, they have used a new, doubly resonant cavity: “You need two wavelengths to generate the squeezed light and we had a resonator that was resonant for both,” explains team member Moritz Mehmet. In addition, says his teammate Henning Vahlbruch, they upgraded several other features: “We used the best available materials, a different cavity topology and custom-made photodetectors.” The researchers broke their own record, squeezing vacuum photons by a factor of 32.

The observation of squeezing at the level they’re seeing it is very, very significant

James Hough, University of Glasgow

A version of the researchers’ squeezing scheme is planned for GEO600 – a gravitational wave detector near Hannover. This instrument has two 600 m arms and has used squeezed light since 2010. In addition, installation of an apparatus for creating squeezed vacuum states on LIGO is planned – “probably in the next year”, says Valbruch. By doing so, it should be possible to reduce the quantum noise in the interferometer readout, allowing fainter signals to be discerned from events occurring farther away in the universe.

The research could also be useful for calibrating the efficiency of photodetectors, says Mehmet. The efficiency of the detectors used is crucial to the amount of squeezing detected. The researchers used custom-made, ultra-efficient photodetectors, but one could also use the amount of squeezing detected as an absolute measurement of an arbitrary photodetector’s efficiency.

“It’s very, very high quality work,” says James Hough of the University of Glasgow, who specializes in gravitational wave detectors. “The observation of squeezing at the level they’re seeing it is very, very significant.” He says the next challenge is to extend the frequency range over which the squeezing is possible and adds, “I’m sure people in Hannover will soon be working on that.”

The research is described in Physical Review Letters.

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