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Neutron scattering: strategic necessity or expensive luxury?

In 1994 the Nobel Prize for Physics was awarded to Cliff Shull and Bert Brockhouse for, in the words of the Nobel committee’s citation, showing “where atoms are” and “what atoms do”, respectively. Some 40 years ago Shull and Brockhouse developed and demonstrated a uniquely powerful and universally applicable non-destructive technique for probing the structures and dynamics of solids and liquids. Today the same technique underpins the whole of condensed-matter science, from physics to engineering, from chemistry to biology, and from materials science to the earth sciences. This remarkably ubiquitous tool is, of course, neutron scattering, and the award of the Nobel prize to Shull and Brockhouse provided an unequivocal but belated acknowledgement of the wide scientific and technological importance that neutron-beam research has assumed.

Over the last 40 years the available thermal-neutron fluxes have increased by three orders of magnitude, and the range of experimental neutron techniques and instrumentation has expanded enormously, allowing increasingly complex scientific and technological problems to be solved. In the last decade alone there has been tremendous progress in our understanding of the microscopic properties of polymers, proteins, plastics, zeolites, alloys, glasses, ionic conductors, liquid crystals, ceramics, surfactants, quantum fluids, magnets and superconductors arising directly, and often solely, from neutron-scattering studies. The high penetrating power of thermal neutrons has been exploited in studies of materials in complex environments at extreme temperatures and pressures, and in tomographic studies of internal strain fields in engine components and even in railway lines. More recently kinetic in situ studies of bulk-material processing under realistic conditions have become routine. The information provided by these neutron studies is crucial to the development and optimization of many structural and functional materials that contribute to our quality of life. The neutron-scattering community One measure of the success and increasing popularity of neutron scattering is the steady growth of an international multidisciplinary neutron-scattering community. The number of neutron-beam users in OECD countries has increased from some 4000 in 1994 to about 7000 today. According to a recent survey prepared by the European Neutron Scattering Association (ENSA) and published by the European Science Foundation (ESF), almost two-thirds of this community is based in Europe. Interestingly, the same ENSA survey indicates that more than half of all European neutron-beam users are chemists and materials scientists, while the majority use neutron scattering as only one component of much wider research programmes.

This dispels a common myth that neutron scattering is a specialist tool used mainly by physicists. Indeed, the ENSA survey reveals a vibrant and broad programme of research, in which European neutron scatterers are well served by no fewer than 13 neutron facilities. Two of these facilities – the high-flux research reactor at the Institut Laue Langevin (ILL) in Grenoble (the major partners in which are the UK, France and Germany) and the ISIS pulsed spallation neutron source at the Rutherford Appleton Laboratory in the UK – are by far the most intense neutron sources of their kind in the world. Together, the ILL and ISIS have allowed Europe to establish a clear world lead in neutron scattering, and hence enjoy a pre-eminent position in condensed-matter science. At first sight European neutron scattering thus appears to be enjoying a golden age. Sadly, this is not the case, and in many respects the prospects are bleak.

The impending neutron drought

The ENSA survey shows that the current demand for neutron beam time within Europe exceeds supply by almost a factor of two. It is generally considered that such an oversubscription engenders healthy competition and helps to maintain the high standard of European neutron science. However, a recent report written on behalf of the OECD Megascience Forum by Dieter Richter and Tasso Springer from the Institute for Solid-State Physics in Jülich, Germany, which presents a 20-year forward look at neutron-scattering facilities in OECD countries and Russia, indicates that at some time between 2010 and 2020 “the presently installed capacity of neutron sources for beam research will decrease to a level below one third that of today”.

This major shortfall in neutron supply is the widely discussed “neutron drought”, in which an oversubscription of at least one order of magnitude appears to be inevitable. The drought is a direct consequence of the rapidly expanding user base, the increasing demand for neutron beam time, and the dramatic fall in the number of neutron facilities. The US and Japan, however, are both planning third-generation neutron sources based on the pulsed spallation principle, and upgrading their existing facilities. Indeed, US’s ultra-high-intensity Spallation Neutron Source at Oak Ridge in Tennessee and Japan’s Hadron Facility are both scheduled to come on-line towards the end of the next decade. Europe, however, is lagging behind in the provision of such much-needed third-generation neutron sources. The detailed scientific case and feasibility study for a European Spallation Source (ESS) with a projected intensity some 30 times that of ISIS has only recently been completed (Physics World December 1997 pp27-32), as have plans for a proposed ISIS equivalent in Austria and a second target station for ISIS itself. Given that the gestation period for any new neutron project is at least ten years, the timing of the necessary political and financial decisions is therefore crucially important. As the Richter-Springer report emphasizes, some continuity in the supply of neutrons for condensed-matter research will be ensured only if decisions to launch these proposals as full-scale projects are taken now. Any reticence by governments or funding agencies to commit resources to a strategic third-generation European neutron source will undoubtedly undermine the long-term stability of neutron-beam research in Europe, eliminate our world lead in neutron science, and damage the European condensed-matter science, engineering and technology bases.

Access to existing neutron facilities

If we accept that neutron scattering has a strategic role to play in European condensed-matter research, it is essential that we ensure free access to existing neutron facilities while optimizing the use of available beam time. Historically, these twin aims have been achieved, at least at the major European facilities, via a stringent peer-review process. Each proposed experiment (generally for three to four days of beam time) is evaluated individually for originality, feasibility, timeliness and impact by international panels who are expert in both neutron scattering and the appropriate scientific disciplines. This system, which has evolved over three decades, is both effective and fair.

However, a radically new system of access has recently been devised by the UK’s Engineering and Physical Sciences Research Council (EPSRC) and implemented at ISIS, in the face of vociferous opposition from neutron-beam users. Now all but 10% of ISIS beam time is evaluated and allocated not through direct access on an experiment-by-experiment basis, but as “tickets” within an extended research programme that typically lasts for three years. It is clear, even from these early days of operation, that the new ticket system is destined to erode and narrow the UK user base by all but excluding the many scientists who need to use neutron beams only occasionally. It limits the flexibility and responsive character of neutron-beam research – features that have proved particular strengths in recent years – by tying up a large proportion of ISIS beam time for extended periods. Moreover, the quality of the neutron-scattering experiments will be affected: neutron measurements predicted three years in advance may well not be topical, relevant or necessary by the time they are finally scheduled. Peer review of these extended grant proposals is also fraught with problems. Requests for long periods of neutron beam time may now be assessed by only two or three referees who have little knowledge of neutron methods: as long as the underlying scientific case looks good, the beam time will be allocated. In addition, the notional cost of a ticket, valued at more than £9000 per day, includes additional expenses, such as infrastructural, staffing and overhead costs that would not normally be met by the EPSRC.

Research proposals seeking tickets for an average neutron programme therefore appear to be exceptionally expensive, with the tickets generally doubling or tripling the cost of otherwise modest research proposals. On the one hand, referees are unsure how to deal with such large requests for research funding. On the other hand, the absence of ring-fenced funds for ISIS allows resources that should be allocated to this major strategic facility to be bled away to more conventional university-based research programmes, which appear, because of the UK dual-funding system, to be much cheaper and hence more cost-effective.

Needless to say, the ticket system has little or no support within the UK neutron-scattering community. It is generally hoped that the ticket system will either be abolished entirely, or that a substantially greater percentage of ISIS beam time will revert to direct access. A further option might be for the EPSRC to cover only the infrastructural costs of ISIS, leaving scientists to bid for tickets priced at a level that reflects the cost of the neutron experiment itself (i.e. £2000-3000 per beam day). In any event, it is worrying to hear rumours that the same system is being viewed somewhat approvingly by other national funding agencies. European neutron scatterers beware!

Value for money?

Finally, there is a general misconception that neutron scattering is an intrinsically expensive technique. While it is true that the operation of an international neutron facility requires substantial funds, these funds are not excessive when considered against scientific output. A recent OECD survey of the scientific output of civil research in the “G7” countries places the UK well ahead of the field with an index of 17.2 research papers per million pounds of government spending. Interestingly, this figure is closely matched by the index evaluated for ILL and ISIS on the basis of statistics provided to me by these facilities. The cost-effectiveness of neutron-scattering research is therefore similar to that of UK civil research programmes on average, and is much greater than that of the national averages for other G7 countries.

I hope that the various research councils and their advisors in the UK will come to accept that the world-leading neutron sources at ISIS and ILL are a tremendous bonus rather than an expensive burden. The councils should be encouraged to make adequate financial provision for the running, maintenance and further development of these two flagship facilities, while also budgeting for a UK contribution to a third-generation European neutron source, such as the ESS. In the meantime, I also hope that the councils will recognize the strategic value of ISIS and ILL to the whole of the UK condensed-matter science, engineering and technology programmes, and therefore move to liberalize rather than inhibit access to these remarkable facilities.

Phosphors help switch on xenon

The glass tube in a conventional fluorescent light is coated on the inside with a phosphor layer and filled with a discharge material, usually mercury. Electrons excite the mercury atoms from the ground state into a short-lived excited state. Ultraviolet light is emitted as the atoms return to the ground state and this is absorbed by red, green and blue phosphors. The excited phosphor states subsequently emit the white light normally associated with fluorescent lighting. The most commonly used phosphors consist of luminescent centres – frequently lanthanide and/or transition-metal ions – incorporated into insulating inorganic oxide or halide host lattices.

The main drawback associated with fluorescent lights, however, is their reliance on mercury as a discharge material. Mercury is poisonous, making the disposal of fluorescent lights hazardous. In addition, the liquid mercury must evaporate each time the power is switched on. This warm-up period prevents the use of fluorescent lights in applications that require fast responses such as fax and photocopier machines.

These limitations have prompted a search to identify new, non-toxic discharge materials with short or no warm-up times. Among the acceptable alternatives to mercury, xenon gas is currently the most promising, as it is very efficient at producing ultraviolet light, has an immediate response and is safe. In order to create mercury-free fluorescent lights based on xenon, however, new phosphor materials must be developed. Today’s fluorescent lights are highly efficient because considerable research has been directed at identifying phosphor materials that efficiently absorb the ultraviolet light emitted by mercury. Since about 85% of the ultraviolet light produced by mercury has a wavelength of 254 nm, most of the phosphors that have been developed absorb strongly at this wavelength. White light is produced by the combined output of red, green and blue phosphors that couple efficiently to the ultraviolet output of mercury.

In contrast, most of the ultraviolet light emitted by xenon occurs at a wavelength of 172 nm, corresponding to a higher energy than that associated with mercury. As a result, new phosphors are needed that can efficiently absorb 172 nm ultraviolet light and convert it to red, green and blue wavelengths. It is relatively simple to find materials – based on the lanthanide elements, for example – that efficiently absorb the ultraviolet output of xenon. But the true challenge in working with xenon as a discharge medium is finding phosphors that efficiently convert the ultraviolet output of xenon into visible light. Two general considerations govern phosphor conversion efficiency: quantum efficiency and energy mismatch. Quantum efficiency measures the number of visible photons produced by the phosphor for every ultraviolet photon absorbed. High quantum efficiency means that once a phosphor is excited to its emitting state, it returns to its ground state primarily by emitting light. The efficiency is reduced if, instead, the excited state decays non-radiatively by dissipating heat via phonon emission. Highly efficient phosphors can be designed by varying the luminescent centre and the chemical composition of the host lattice. A quantum efficiency of close to 100% has been observed in many systems. Energy mismatch refers to the difference in energy between the photons that are absorbed and emitted by the phosphors. Since red, green and blue photons are much less energetic than ultraviolet photons, a considerable mismatch of energy is associated with most phosphors. In all phosphor materials known until now, this energy mismatch has led to non-radiative decays of the luminescent centre. Energy mismatch has therefore been an unavoidable, inefficient use of the electrical excitation energy in fluorescent lighting. The inefficiency due to energy mismatch is much more severe for xenon than mercury because of the much higher ultraviolet output energy of xenon (7.2 eV compared with 4.9 eV). The situation has changed dramatically following the Utrecht group’s discovery of the phosphor material europium-doped lithium gadolinium fluoride, LiGdF4:Eu3+. This phosphor is capable of producing red light efficiently when excited with ultraviolet light output from the xenon. The Utrecht team exploited the energy mismatch problem: since the energy of 172 nm xenon photons is more than double the energy of red, green or blue photons, it should be possible to obtain two visible photons for every ultraviolet photon absorbed by the phosphor. In other words, it should be possible to recover an extra visible photon from the energy mismatch instead of losing it non-radiatively as had always been observed previously. But the emission of two visible photons by sequential emission from a single luminescent centre had never been observed. The researchers designed a phosphor in which a luminescent centre of europium ions interacted with the lithium gadolinium fluoride host lattice. The idea was to use the gadolinium ions in the host lattice to absorb the energy from the ultraviolet light and then transfer it to europium ions to obtain visible light. If the transfer of energy occurred simply between one gadolinium ion and one europium ion, the phosphor would suffer from a significant energy-mismatch problem and would have low efficiency. However, the strategy used in the design of the LiGdF4:Eu3+ phosphor avoids this problem because two different mechanisms for transferring energy from the gadolinium ions to the europium ions are present.

The first stage involves the gadolinium ions absorbing the ultraviolet light, resulting in a highly excited state (step 1 in figure). The excited gadolinium ion next transfers energy to a europium ion (step 2). This excites the europium ion to a higher state, which subsequently emits a visible photon. During the energy transfer, the gadolinium ion decays from its original excited state to a lower-energy excited state. This excited state is still higher in energy than red, green or blue photons and the gadolinium ion continues to decay through a second energy-transfer process (step 3). This energy-transfer process returns the gadolinium ion to its ground state and excites a second europium ion to a high-energy state that decays non-radiatively to a lower-energy state, which then emits a second visible photon. Quantitative analysis of the results indicated that LiGdF4:Eu3+ has a quantum efficiency of 190%, a new record for visible light.

The Utrecht researchers have established for the first time that it is possible to obtain two visible photons from one ultraviolet photon. They used the term “quantum cutting” or “down-conversion” to describe the effect that they discovered. The process shows that fluorescent lights based on xenon discharges have the potential to be competitive with the current mercury-based lights.

Instead of being a disadvantage, the higher-energy ultraviolet output of xenon has actually proved to be advantageous. The photon produced in step 2 requires the higher-energy ultraviolet output of xenon in step 1 and would not be possible with a mercury discharge.

The discovery opens a wide range of new opportunities for xenon and other non-mercury discharges. However, the new phosphor emits primarily red light and equally efficient green and blue phosphors are still needed to make xenon-based fluorescent lighting a reality. We can expect work in this promising direction in the immediate future.

Science and societies

Two of the oldest and largest physical societies in the world celebrate anniversaries this year. The Institute of Physics, which publishes this magazine, can trace its roots back to the Physical Society that was formed in London 125 years ago. And the American Physical Society (APS) is currently celebrating its centennial. Other venerable societies of physicists include the Société Française de Physique (SFP) formed in 1873 and the Società Italiana di Fisica, which dates from 1897. Not surprisingly the roles of societies have changed considerably over the past century. The traditional activities of organizing meetings and publishing journals remain important – and remain big business – but the need for societies to act as advocates for their subject is increasing. This advocacy can take several forms: encouraging young people to study physics; supporting teachers in schools; lobbying on behalf of physics as a discipline in universities; and engaging in a host of activities related to innovation and technology transfer. Examples of specific activities include developing new curricula and syllabuses, and making the case for adequate research budgets to governments and funding bodies. Recent events in the US show that it is often in the interests of physics to present a united front with other scientific disciplines when lobbying the government for research funds.

However, physical societies also differ a great deal from nation to nation. The American Institute of Physics, for instance, is an umbrella organization of ten societies with a total membership in excess of 100 000, some 40 000 of whom are in the APS. Europe has two large societies – the Deutsche Physikalische Gesellschaft with over 30 000 members, and the Institute of Physics with over 20 000 – and a large number of much smaller societies. The SFP in France, for example, has around 3000 members. However, these figures do not reflect the relative level of activity in physics in these countries but other factors such as different criteria for membership. The European Physical Society, formed in 1968, has around 3000 individual members and a further 77 000 members in 36 national societies. Anniversaries are naturally a time for looking back and taking stock. The recently published history of the Physical Society and the Institute of Physics* is a highly readable volume that starts with the emergence of physics as a separate scientific discipline in the 1830s and 1840s, and goes on to cover Maxwell’s early resistance to the Physical Society, the propaganda committee of the Institute, thundering letters to The Times from Sir Lawrence Bragg and the Institute’s current work. The American Physical Society, meanwhile, has published an impressive 500-page special issue of Reviews of Modern Physics , with articles on all areas of physics, including applications and historical perspectives. It is hard to disagree with Hans Bethe’s comment in the introduction: “Looking at the predictions of 100 years ago, it would be foolish to make predictions for the next 100 years.”

*125 Years (ed) J Lewis (Institute of Physics Publishing)

Zero-sum games

When is something zero and when is it not? Recently we reported on a dispute between two particle-physics labs about an effect known as direct CP violation. In 1993 a team at CERN reported a value for this effect that was 3.5 standard deviations above zero: this means that the probability of the effect being zero (or less) was 0.05%. The same year a team at Fermilab reported a much lower value that was only 1.25 standard deviations above zero. So did direct CP violation really exist or not? An improved experiment at Fermilab has now reported a result that is an impressive 7.0 standard deviations above zero – and much closer to the old CERN result than the previous Fermilab one. As the two labs debate the difference between observing and establishing, the theorists who showed how to include CP violation in the Standard Model of particle physics – Makoto Kobayashi and Toshikide Maskawa – could well be in line for a Nobel prize.

Scientists illuminate gamma-ray burst

Although scientists have known about gamma-ray bursts since 1967, no one has come up with a feasible explanation of why they occur. The explosions may be caused by the collision of super-dense neutron stars; the sudden collapse of a massive star into a black hole; or other, exotic mechanisms. Many astronomers believe that the gamma rays occur when material from an explosion interacts with the interstellar medium. Another problem has been the difficulty in detecting and locating the GRBs. Some bursts last as short as 1 second, while others can last over an hour.

Since 1997 however, an Italian-Dutch X-ray satellite called BeppoSAX has been able to pinpoint the location of the explosions within seconds of their ignition. This information is then passed into a number of instruments on the ground. One such instrument is the Robotic Optical Transient Search Experiment (ROTSE-1) at Los Alamos National Laboratory in the US. ROTSE-1 looks specifically for the optical after-effects of a gamma- ray burst. Carl Akerlof of the University of Michigan and colleagues (Nature 398 400) found that ROTSE-1 saw an optical flare 22 seconds into the start of GRB990123. Over the next few seconds the event increased in optical brightness by a factor of 16 before rapidly declining by a factor of 100 over the next 12 minutes. “If you had been gazing at that spot with binoculars, you would have seen a ‘star’ appear, brighten, and fade within minutes” said Galen Gisler, one of the authors. They suggest that this is evidence that the emission of optical radiation came directly from the explosion.

On January 24, one day after the explosion, Andersen et al. used the Nordic Optical Telescope in the Canary Islands to measure a spectrum of elements that could have been absorbing light from the afterglow (Science 283 2075). This spectrum allowed the authors to calculate the energy and distance of the burst. According to their calculations, which were confirmed by Shri Kulkarni of the Palomar Observatory in California and colleagues (Nature 398 389), the afterglow of GRB990123 had a redshift of 1.6 indicating that it occurred outside our galaxy.

The standard “fireball” model of gamma-ray explosions says that the gamma-rays are produced in an explosion of material that is expanding at near to the speed of light. Using this model Andersen’s group showed that GRB 990123 had 45 times more energy than the total emitted by the most luminous “Type II” supernova – ruling out nearly all the astronomical candidates for gamma-ray explosions that astronomers can think of. According to the authors this suggests that the “fireball” model is incorrect. Instead, they propose that light from the explosion travelled in two jets, one of which pointed towards Earth, a result that would explain why the optical light diminished faster than expected.

Another team led by Alberto Castro-Tirad, also analyzed the optical afterglow of the explosion (Science 283 2069). They also found that after two days, the afterglow decreased rapidly in strength. They suggest that this is what would be expected if a relativistic jet pointing in your direction cooled down and suddenly started to expand sideways, increasing the cooling rate.

Jen Hjorth et. al. meanwhile discovered that light from GRB 990123 was hardly polarized at all – also confirming that the fireball model is incorrect – and indicating the existence of a relativistic jet (Science 283 2073). Finally, Chryssa Kouveliotou of the NASA Marshall Space Flight Center in Huntsville, Alabama, and colleagues (Nature 398 394) traced of the fading afterglow emissions in great detail, including the internal shocks that caused the explosion, the’reverse’ shocks responsible for the prompt optical flash, and the subsequent forward shock that caused the afterglow as the explosion ploughed into the surrounding medium at relativistic speeds.

Small melanin particles cause skin cancer

The radiation causes the eumelanin to generate large quantities of free oxygen radicals, which in turn damage cell DNA and hence may lead to cancer. His research was reported yesterday at the annual American Physical Society meeting in Atlanta, Georgia.

Spinach to fuel optoelectronics

Although they are only 5nm long, each complex can generate 1 V in picoseconds, making them suitable for ultrafast switching, logic devices, and solar-power generation. The work was presented this week at the American Physical Society’s centennial meeting in Atlanta, Georgia.

Gravity-wave detectors target quantum gravity

Both the French-Italian gravity-wave interferometer, called VIRGO, and the LIGO interferometers in the US are designed to detect very weak gravity waves by using lasers to monitor test masses placed at the ends of the arms of the interferometer. The arms in VIRGO are 3 km long, while those in LIGO are 4 km long. When a gravity wave passes through the detector, it causes the distance between the test masses to increase in one arm of the interferometer, and decrease in the other. The challenge is to detect this movement – which is only about 10-18 m, or one-hundred-millionth the diameter of a hydrogen atom – from the changes it causes in the interference patterns observed in the experiments.

According to Amelino-Camelia, the same type of distortion will be caused by fluctuations in space-time near the Planck length, 10-35 m, and he argues that these fluctuations should be observable as instrument noise on the 10-18 m scale. “We are finally at a point in quantum gravity research when we can set bounds on some candidate phenomena, ” he told PhysicsWeb. “One first rules out the most speculative proposals and then – as the phenomenological programme gets stronger – even the most conservative proposals can be significantly tested.”

Matter makes waves

Phillips and his colleagues produced a focused directional output from the Bose-Einstein condensate by firing two laser beams into the condensate. One beam adds energy to the atoms, while the second stimulates them to emit photons and drop down to a lower energy state. As the photons of the second laser beam have slightly less energy than photons from the first beam, the atoms gain a small amount of energy. This energy is converted into momentum, which pushes the atoms into a given in a direction.

To create the four-wave mixing, Phillips and his team again started with a Bose-Einstein condensate. This time, however, they pumped a series of Bragg laser pulses into the atom cloud. Each pulse pushed some of the atoms out of the condensate. The pulses created three wave packets, which immediately started to overlap and produced a new wave packet with its own unique momentum.

Solar system is chaotic

Murray and Holman ran numerical simulations of the planets positions over a 200 million year period using their new technique. They discovered that although the outer planets appear in stable orbits, over longer time scales their classical ‘predicted’ orbit diverges from the simulations due to chaotic effects. One resonance is caused by interactions between Jupiter, Saturn, and Uranus. The second, weaker effect originates from the gravity wells of Saturn, Uranus, and Neptune. According to their calculations there is a 20 percent chance that the outer planets are now in a chaotic state.

US dominates world patents

The National Science Foundation report also discovered that the US, Russia and Japan have the lowest number of patents awarded to foreign inventors. In all the other countries surveyed, over two-thirds of each countries patents went to foreign inventors. One reason for the low number of foreign-held Japanese patents is the high cost of translating international patents ($1.70 per Japanese character).

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