The precise particle-identification and momentum-measurement capabilities of the ALICE experiment allow researchers to reconstruct a variety of short-lived particles or resonances in heavy-ion collisions. These serve as a probe for in-medium effects during the last stages of evolution of the quark–gluon plasma (QGP). Recently, the ALICE collaboration has made a precise measurement of the yields (number of particles per event) of two such resonances: K*(892)0 and φ(1020). Both have similar masses and the same spin, and both are neutral strange mesons, yet their lifetimes differ by a factor of 10 (4.16±0.05 fm/c for K*0, and 46.3±0.4 fm/c for φ).
The shorter lifetime of the K*0 means that it decays within the medium, enabling its decay products (π and K) to re-scatter with other hadrons. This would be expected to inhibit the reconstruction of the parent K∗0, but the π and K in the medium may also scatter into a K∗0 resonance state, and the interplay of these two competing re-scattering and regeneration processes becomes relevant for determining the K*0 yield. The processes depend on the time interval between chemical freeze-out (vanishing inelastic collisions) and kinetic freeze-out (vanishing elastic collisions), in addition to the source size and the interaction cross-sections of the daughter hadrons. In contrast, due to the longer lifetime of the φ meson, both the re-scattering and regeneration effects are expected to be negligible.
Using lead–lead collision data recorded at an energy of 2.76 TeV, ALICE observed that the ratio K*0/K– decreases as a function of system size (see figure). In small impact-parameter collisions, the ratio is significantly less than in proton–proton collisions and models without re-scattering effects. In contrast, no such suppression was observed in the φ /K– ratio. This measurement thus suggests the existence of re-scattering effects on resonances in the last stages of heavy-ion collisions at LHC energies. Furthermore, the suppression of K*0 yields can be used to obtain the time difference between the chemical and the kinetic freeze-out of the system.
On the other hand, at higher momenta (pT > 8 GeV/c), these resonances were suppressed with respect to proton–proton collisions by similar amounts. The magnitude of this suppression for K*0 and φ mesons was also found to be similar to the suppression for pions, kaons, protons and D mesons. The striking independence of this suppression on particle mass, baryon number and the quark-flavour content of the hadron puts a stringent constraint on models dealing with particle-production mechanisms, fragmentation processes and parton energy loss in the QGP medium.
In future, it will be important to perform such measurements for high-multiplicity events in pp collisions at the LHC.
Researchers from the XENON1T dark-matter experiment at Gran Sasso National Laboratory in Italy reported their first results at the 13th Patras Workshop on Axions, WIMPs and WISPs, held in Thessaloniki from 15–19 May (see “Exploring axions and WIMPs in Greece” in Faces & Places). XENON1T is the first tonne-scale detector of its kind and is designed to search for WIMP dark matter by measuring nuclear recoils from WIMP–nucleus scattering. Continuing the programme of the previous XENON10 and XENON100 detectors, the new apparatus contains 3200 kg of ultra-pure liquid xenon (LXe) – 20 times more than its predecessor – in a dual-phase xenon time projection chamber (TPC) to detect nuclear recoils. The TPC encloses about 2000 kg of LXe, while another 1200 kg provides additional shielding.
The experiment started collecting data in November 2016. A blind search based on 34.2 live days of data acquired until January 2017, when earthquakes in the region temporarily suspended the run, revealed the data to be consistent with the background-only hypothesis. This allowed the collaboration to derive the most stringent exclusion limits on the spin-independent WIMP–nucleon interaction cross-section for WIMP masses above 10 GeV/c2, with a minimum of 7.7 × 10–47 cm2 for 35 GeV/c2 WIMPs at 90% confidence level.
These first results demonstrate that XENON1T has the lowest low-energy background level ever achieved by a dark-matter experiment, with the intrinsic background from krypton and radon reduced to unprecedented low levels. The sensitivity of XENON1T will continue to improve as the experiment records data until the end of 2018, when the collaboration plans to upgrade to a larger TPC due to come online by 2019. Several other experiments, such as PANDA-X and LUX-ZEPLIN, are also competing for the first WIMP detection.
“With our experiment working so beautifully, even exceeding our expectations, it is really exciting to have data in hand to further explore one of the most exciting secrets we have in physics: the nature of dark matter,” says XENON spokesperson Elena Aprile of Columbia University in the US.
The Muon g-2 experiment at Fermilab has begun its three-year-long campaign to measure the magnetic moment of the muon with unprecedented precision. On 31 May, a beam of muons was fired into the experiment’s 14 m-diameter storage ring, where powerful electromagnetic fields cause the magnetic moment, or spin, of individual muons to precess. The last time this experiment was performed, using the same electromagnet at Brookhaven National Laboratory in the late 1990s and early 2000s, the result disagreed with predictions by more than three standard deviations. This hinted at the presence of previously unknown particles or forces affecting the muon’s properties, and motivated further measurements to check the result.
Sixteen years later, the reincarnated Muon g-2 experiment will make use of Fermilab’s intense muon beams to definitively answer the questions raised by the Brookhaven experiment. It turned out to be 10 times cheaper to move the apparatus to Fermilab than it would have cost to build a new machine at Brookhaven, and the large, fragile superconducting magnet was transported in one piece from Long Island to the suburbs of Chicago in the summer of 2013.
Since it arrived, the Fermilab team reassembled the magnet and spent a year adjusting or “shimming” the uniformity of its field. The field created by the g-2 magnet is now three times more uniform than the one it created at Brookhaven. In the past year, the team has worked around the clock to install detectors, build a control room and prepare for first beam. The work has included: the creation of a new beamline to deliver a pure beam of muons; instrumentation to measure the magnetic field; and entirely new instrumentation to measure the muonʼs spin-precession signal.
Over the next few weeks the Muon g-2 team will test the equipment, with science-quality data expected later in the year. The experiment aims to achieve a precision on the anomalous magnetic moment of the muon of 0.14 parts per million, compared to around 0.54 parts per million previously. If the inconsistency with theory remains, it could indicate that the Standard Model of particle physics is in need of revision.
On 20 June the European Space Agency (ESA) gave the official go-ahead for the Laser Interferometer Space Antenna (LISA), which will comprise a trio of satellites to detect gravitational waves in space. LISA is the third mission in ESA’s Cosmic Vision plan, set to last for the next two decades, and has been given a launch date of 2034.
Predicted a century ago by general relativity, gravitational waves are vibrations of space–time that were first detected by the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO) in September 2015. While upgrades to LIGO and other ground-based observatories are planned, LISA will access a much lower-frequency region of the gravitational-wave universe. Three craft, separated by 2.5M km in a triangular formation, will follow Earth in its orbit around the Sun, waiting to be distorted by a fractional amount by a passing gravitational wave.
Although highly challenging experimentally, a LISA test mission called Pathfinder has recently demonstrated key technologies needed to detect gravitational waves from space (CERN Courier January/February 2017 p34). These include free-falling test masses linked by lasers and isolated from all external and internal forces except gravity. LISA Pathfinder concluded its pioneering mission at the end of June, as LISA enters a more detailed phase of study. Following ESA’s selection, the design and costing of the LISA mission can be completed. The project will then be proposed for “adoption” before construction begins.
Following the first and second detections of gravitational waves by LIGO in September and December 2015, on 1 June the collaboration announced the detection of a third event (Phys. Rev. Lett.118 221101). Like the previous two, it is thought that “GW170104” – the signal for which arrived on Earth on 4 January – was produced when two black holes merged into a larger one billions of years ago.
CERN has recently implemented two important steps towards the High Luminosity LHC (HL-LHC) – an upgrade that will increase the intensity of the LHC’s collisions significantly from the early 2020s. Preparing CERN’s existing accelerator complex to cope with more intense proton beams presents several challenges, in particular concerning the system that injects protons into the LHC.
At a ceremony on 9 May, a major new linear accelerator, Linac 4, was inaugurated. Replacing Linac 2, which had been in service since 1978, it is CERN’s newest accelerator acquisition since the LHC and is due to feed the accelerator complex with higher-energy particle beams. After an extensive testing period, Linac 4 will be connected to the existing infrastructure during the long technical shutdown in 2019/2020.
To cope with the higher-intensity and higher-energy beams emerging from Linac 4, the Proton Synchrotron Booster (PSB), which is the second accelerator of the LHC injector chain, will be completely overhauled during that same period. At the beginning of June, the first radio-frequency cavity of the new PSB acceleration system was completed, with a further 27 under assembly. The new cavities are based on a composite magnetic material called FINEMET developed by Hitachi Metals, which allows them to operate with a large bandwidth and means that a single cavity can cover all necessary frequency bands. The PSB cavity project was launched in 2012 in collaboration with KEK in Japan, and involved intensive testing at CERN. KEK contributed a substantial fraction of the FINEMET cores and shared its experience with similar technology
On 12 June, two large detector modules for the ICARUS experiment were loaded onto trucks at CERN to begin a six-week journey to Fermilab in the US. ICARUS will form part of Fermilab’s short-baseline neutrino programme, which aims to make detailed measurements of neutrino interactions and search for eV-scale sterile neutrinos (CERN Courier June 2017 p25).
Based on advanced liquid-argon time projection technology, ICARUS began its life under a mountain at the Gran Sasso National Laboratory in Italy in 2010, recording data from neutrino beams sent from CERN. Since 2014, it has been at CERN undergoing an upgrade and refurbishment at the CERN Neutrino Platform (CERN Courier July/August 2016 p21). It left CERN in two parts by road and boarded a boat on the Rhine to a port in Antwerp, Belgium, where it was loaded onto a ship. As the Courier went to press, ICARUS was already heading across the Atlantic to Fermilab via the Great Lakes, equipped with a GPS unit that allows its progress to be tracked in real time (icarustrip.fnal.gov).
Just two days after ICARUS left CERN, another key component of the CERN Neutrino Platform was on the move, albeit on a smaller lorry. Baby MIND, a 75 tonne prototype for a magnetised iron neutrino detector that will precisely identify and track muons, was moved from its construction site in building 180 to the East Hall of the Proton Synchrotron. Following commissioning and full characterisation in the T9 test beam, at the end of July Baby MIND will be transported to Japan to be part of the WAGASCI experiment at JPARC, where it will contribute to a better understanding of neutrino interactions for the T2K experiment.
Massive stars are traditionally expected to end their life cycle by triggering a supernova, a violent event in which the stellar core collapses into a neutron star, potentially followed by a further collapse into a black hole. During this process, a shock wave ejects large amounts of material from the star into interstellar space with large velocities, producing heavy elements in the process, while the supernova outshines all the stars in its host galaxy combined.
In the past few years, however, there has been mounting evidence that not all massive-star deaths are accompanied by these catastrophic events. Instead, it seems that for some stars only a small part of their outer layers is ejected before the rest of the volume collapses into a massive black hole. For instance, there are hints that the birth rate and supernova rate of massive stars do not match. Furthermore, results from the LIGO gravitational-wave observatory in the US indicate the existence of black holes with masses more than 30 times that of the Sun, which is easier to explain if stars can collapse without a large explosion.
The results would explain why we observe less supernovae than expected
Motivated by this indirect evidence, researchers from Ohio State University began a search for stars that quietly form a black hole without triggering a supernova. Using the Large Binocular Telescope (LBT) in Arizona, in 2015 the team identified its first candidate. The star, called N6946-BH1, was approximately 25 times more massive than the Sun and lived in the Fireworks galaxy, which is known for hosting a large number of supernovae. Previously presenting a stable luminosity, the star was seen to become brighter, although not at the level expected for a supernova, during 2009, before completely disappearing in optical wavelengths in 2010 (see image).
The lack of emission observed by the LBT triggered follow-up searches for the star, both using the Hubble Space Telescope (HST) and the Spitzer Space Telescope (SST). While the HST did not find signs of the star in the optical wavelength, the SST did observe infrared emission. A careful analysis of the data disfavoured alternative explanations such as a large dust cloud obscuring the optical emission from the star, and the infrared data were also shown to be compatible with emission from remaining matter falling into a black hole.
If the star did indeed directly collapse into a black hole, as these findings suggest, the in-falling matter is expected to radiate in the X-ray region. The team is therefore waiting for observations from the space-based Chandra X-ray Observatory to search for this emission.
If confirmed in X-ray data, this result would be the first measurement of the birth of a black hole and the first measurement of a failed supernova. The results would explain why we observe less supernovae than expected and could reveal the origin of the massive black holes responsible for the gravitational waves seen by LIGO, in addition to having implications for the production of heavy elements in the universe.
The past few decades have witnessed an explosion in X-ray sources and techniques, impacting science and technology significantly. Large synchrotron X-ray facilities around the world based on advanced storage rings and X-ray optics are used daily by thousands of scientists across numerous disciplines. From the shelf life of washing detergents to the efficiency of fuel-injection systems, and from the latest pharmaceuticals to the chemical composition of archaeological remains, highly focused and brilliant beams of X-rays allow researchers to characterise materials over an enormous range of length and timescales, and therefore link the microscopic behaviour of a system with its bulk properties.
So-called third-generation light sources based on synchrotrons produce stable beams of X-rays over a wide range of photon energies and beam parameters. The availability of more intense, shorter and more coherent X-ray pulses opens even further scientific opportunities, such as making high-resolution movies of chemical reactions or providing industry with real-time nanoscale imaging of working devices. This boils down to maximising a parameter called peak brilliance. While accelerator physicists have made enormous strides in increasing the peak brilliance of synchrotrons, this quantity experienced a leap forward by many orders of magnitude when the first free-electron lasers (FELs) started operating in the X-ray range more than a decade ago.
FLASH, the soft-X-ray FEL at DESY in Hamburg, was inaugurated in 2005 and marked the beginning of this new epoch in X-ray science. Based on superconducting accelerating structures developed initially for a linear collider for particle physics (see “The world’s longest superconducting linac”), it provided flashes of VUV radiation with peak brilliances almost 10 orders of magnitude higher than any storage-ring-based source in the same wavelength range. The unprecedented peak power of the beam immediately led to groundbreaking new research in physics, chemistry and biology. But importantly, FLASH also demonstrated that the amplification scheme responsible for the huge gain of FELs – Self Amplified Spontaneous Emission (SASE) – was feasible at short wavelengths and could likely be extended to the hard-X-ray regime.
The first hard-X-ray FEL to enter operation based on the SASE principle was the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory in California, which obtained first light in 2009 using a modified version of the old SLAC linac and operates at X-ray energies up to around 11 keV. Since then, several facilities have been inaugurated or are close to start-up: SACLA in Japan, Pohang FEL in South Korea, and Swiss-FEL in Switzerland. The European X-ray Free-Electron Laser (European XFEL) in Schenefeld-Hamburg, Germany, marks a further step-change in X-ray science, promising to produce the brightest beams with the highest photon energies and the highest repetition rates. Construction of the €1.2 billion facility began in January 2009 funded by 11 countries: Denmark, France, Germany, Hungary, Italy, Poland, Russia, Slovakia, Spain, Sweden and Switzerland, with Germany (58%) and Russia (27%) as the largest contributors. It is expected that the UK will join the European XFEL in 2017.
The European XFEL extends over a distance of 3.4 km in underground tunnels (figure 1). It begins with the electron injector at DESY in Bahrenfeld-Hamburg, which produces and injects electrons into a 2 km-long superconducting linear accelerator where the desired electron energy (up to 17.5 GeV) is achieved. Exiting the linac, electrons are then rapidly deflected in an undulating left–right pattern by traversing a periodic array of magnets called an undulator (figure 1, bottom right), causing the electrons to emit intense beams of X-ray photons. X-rays emerging from the undulator, via 1 km-long photon-transport tunnels equipped with various X-ray optics elements, finally arrive at the European XFEL headquarters in Schenefeld where the experiments will take place.
In addition to the development of the electron linac, which was commissioned earlier this year and involved a major effort by DESY in collaboration with numerous other accelerator facilities over the past decade (see “The world’s longest superconducting linac”), the European XFEL has driven the development of both undulator technology and advanced X-ray optics. This multinational and multidisciplinary effort now opens perspectives for novel scientific experiments. When fully commissioned, towards the end of 2018, the facility will deliver 4000 hours of accelerator time per year for user experiments that are approved via external peer review.
Manipulating X-rays
Synchrotron radiation was first detected experimentally at Cornell in 1947, and the first generation of synchrotron-radiation users were termed “parasitic” because they made use of X-rays produced as a byproduct of particle-physics experiments. Dedicated “second-generation” X-ray sources were established in the early 1970s, while much more brilliant “third-generation” sources based on devices called undulators started to appear in the early 1990s (figure 2). The SASE technology underpinning XFELs, which followed from work undertaken in the mid-1960s, ensures that the produced X-rays are much more intense and more coherent that those emitted by storage rings (see SASE panel below). Like the light coming from an optical laser, the X-rays generated by SASE are almost 100% transversely coherent compared to less than one per cent for third-generation synchrotrons, indicating that the radiation is an almost perfect plane wave. Even though the longitudinal-coherence length is not comparable to that of a single-mode optical laser, the use of the term “X-ray laser” is clearly justified for facilities such as the European XFEL.
A major challenge with X-ray lasers is to develop the mirrors, monochromators and other optical components that enable high-energy X-rays to be manipulated and their coherence to be preserved. Compared with the visible light emerging from a standard red helium-neon laser, which has a wavelength of 632 nm, the typical wavelength of hard X-rays is around 0.1 nm. Consequently, X-ray laser light is up to 6000 times more sensitive to distortions in the optics. On the other hand, X-ray mirrors work at extremely small grazing incidence angles (typically around 0.1° for hard X-rays at the European XFEL) because the interaction between X-rays and matter is so weak. This reduces the sensitivity to profile distortions and makes errors of up to 2 nm tolerable on a 1 m-long X-ray mirror, before the reflected X-ray wavefront becomes noticeably affected. Still, these requirements on profile errors are extremely high – about 10 times more stringent than for the Hubble Space Telescope mirror, for example.
The technology to produce these ultra-flat X-ray mirrors was only developed in recent years in Japan and Europe. It is based on a process called deterministic polishing, in which material is removed atomic layer by atomic layer according to a very precisely measured map of the initial profile’s deviations from an ideal shape. After years of development and many months of deterministic polishing iterations, the first 95 cm-long silicon X-ray mirror fulfilling the tight specifications of the European XFEL was completed in March 2016, with 10 more mirrors of similar quality following shortly thereafter. In the final configuration, 27 of these extremely precise mirrors will be used to steer the X-ray laser beam along the photon-transport tunnels to all the scientific instruments.
Managing the large heat loads on the European XFEL mirrors is a major challenge. To remove the heat generated by the X-ray laser beam without distorting the highly sensitive mirrors, a liquid-metal film is used to couple the mirror to a water-cooling system in a tension- and vibration-free fashion. Another mirror system will be cooled to a temperature of around 100 K, at which the thermal-expansion coefficient of silicon is close to zero. This solution, which is vital to deal with the high repetition rate of the European XFEL, is often employed for smaller silicon crystals acting as crystal monochromators but is rarely necessary for large mirror bodies where the grazing-incidence geometry spreads the heat over a large area.
Indeed, the SASE pulses have potentially devastating power – especially close to the sample where the beam may be focused to small dimensions. A typical SASE X-ray pulse of 100 fs duration contains about 2 mJ of thermal X-ray energy (corresponding to 1012 photons at 12 keV photon energy), which means that a copper beam-stop placed close behind the sample would be heated to a temperature of several 100,000 °C and could therefore be evaporated (along with the sample) from just one pulse. While this is not necessarily a problem for samples that can be replaced via advanced injection schemes and where data can be collected before destruction takes place, it could shorten the lifetime of slits, attenuators, windows and other standard beamline components. The solution is to intersect the beam only where it has a larger size and to use only light elements that absorb less X-ray energy per atom. Still, stopping the X-ray laser beam remains a challenge at the European XFEL, with up to 2700 pulses in a 600 μs pulse train (figure 3). Indeed, the entire layout of the photon-distribution system was adapted to counteract this damaging effect of the X-ray laser beam, and a facility-wide machine-protection system limits the pulse-train length to a safe limit, depending on the optical configuration. Since a misguided X-ray laser beam can quickly drill through the stainless-steel pipes of the vacuum system, diamond plates are positioned around the beam trajectory and will light up if hit by X-rays, triggering a dump of the electron beam.
The business end of things
At the European XFEL, the generation of X-ray beams is largely “behind the scenes”. The scientific interest in XFEL experiments stems from the ability to deliver around 1012 X-ray photons in one ultrafast pulse (with a duration in the range 10–100 fs) and with a high degree of coherence. Performing experiments within such short pulses allows users to generate ultrafast snapshots of dynamics that would be smeared out with longer exposure times and give rise to diffuse scattering. Combined with spectroscopic information, a complete picture of atomic motion and molecular rearrangements, as well as the charge and spin states and their dynamics, can be built up. This leads to the notion of a “molecular movie”, in which the dynamics are triggered by an external optical laser excitation (acting as an optical pump) and the response of a molecule is monitored by ultrafast X-ray scattering and spectroscopy (X-ray probe). Pump-probe experiments are typically ensemble-averaged measurements of many molecules that are randomly aligned with respect to each other and not distinguishable within the scattering volume. The power and coherence of the European XFEL beams will allow such investigations with unprecedented resolution in time and space compared to today’s best synchrotrons.
In particular, the coherence of the European XFEL beam allows users to distinguish features beyond those arising from average properties. These features are encoded in the scattering images as grainy regions of varying intensity called speckle, which results from the self-interference of the scattered beam and can be exploited to obtain higher spatial resolution than is possible in “incoherent” X-ray scattering experiments (figure 4). Since the speckles reflect the exact real-space arrangement of the scattering volume, even subtle structural changes can alter the speckle pattern dramatically due to interference effects.
The combination of ultrafast pulses, huge peak intensity and a high degree of beam coherence is truly unique to FEL facilities and has already enabled experiments that otherwise were impossible. In addition, the European XFEL has a huge average intensity due to the many pulses delivered each second. This allows a larger number of experimental sessions per operation cycle and/or better signal-to-noise ratios within a given experimental time frame. The destructive power of the beam means that many experiments will be of the single-shot type, which requires a continuous injection scheme because the sample cannot be reused. Other experiments will operate with reduced peak flux, allowing multi-exposure schemes as also demonstrated in work at LCLS and FLASH.
Six experimental stations are planned for the European XFEL start-up, two per SASE beamline. The first, situated at the hard-X-ray undulator SASE-1, is devoted to the study of single-particles and biomolecules, serial femtosecond crystallography, and femtosecond X-ray experiments in biology and chemistry. SASE-2 caters to dynamics investigations in condensed-matter physics and material-science experiments, specialising in extreme states of matter and plasmas. At the soft-X-ray branch SASE-3, two instruments will allow investigations of electronic states of matter and atomic/cluster physics, among other studies. The three SASE undulators will deliver photons in parallel and the instruments will share their respective beams in 12 hour shifts, so that three instruments are always operating at any given time.
Eight years after the project officially began, the European XFEL finally achieved first light in 2017 and its commissioning is progressing according to schedule. The facility is the culmination of a worldwide effort lead by DESY concerning the electron linac and by European XFEL Gmbh for the development of X-ray photon transport and experimental stations. The facility is conveniently situated among other European light sources – synchrotrons that are also continuously evolving towards larger brilliance – and a handful of hard-X-ray FELs worldwide. The European XFEL is by far the most powerful hard-X-ray source in the world and will remain at the forefront for at least the next 20–30 years. Continuous investment in instrumentation and detectors will be required to capitalise fully on the impressive specifications, and the facility has the potential to construct about six additional instruments and possibly even a second experimental hall, all fed by X-rays generated by the existing superconducting electron linac. Without a doubt, Europe has now entered the extreme X-ray era.
Self-Amplified Spontaneous Emission (SASE), the underlying principle of X-ray free-electron lasers, is based on the interaction between a relativistic electron beam and the radiation emitted by the electrons as they are accelerated through a long alternating magnetic undulator array (see image). If the undulator is short, on the order of a few metres, and the undulating path is well defined with a small amplitude, the radiation emitted by one electron adds up coherently at one particular wavelength as it travels through the undulator. Hence, the intensity is proportional to N2p, where Np is the number of undulator periods (typically around 100). This is the regular undulator radiation generated at third-generation synchrotron sources such as the ESRF in France or APS in the US, and also at the next generation of diffraction-limited storage rings, such as MAX IV in Sweden. On the other hand, if the undulator is very long, the interactions between the electrons and the radiation field that builds up will eventually lead to micro-bunching of the electron beam into coherent packages that radiate in phase (see image). This results in a huge amplification (lasing) of emitted intensity as it becomes proportional to N2e, where Ne is the number of electrons emitting in phase within the co-operation length (typically 106, or more). The hard-X-ray undulators of the European XFEL have magnetic lengths of 175 m in order to ensure that SASE works over a wide range of photon energies and electron-beam parameters. High electron energy, small energy spread and a small emittance (the product of beam size and divergence) are crucial for SASE to work in the X-ray range. Together with the requirement of very long undulators, it favours the use of linac sources, instead of storage rings, for X-ray lasers.
The European X-ray Free Electron Laser (European XFEL) now entering operations at Hamburg in Germany will generate ultrashort X-ray flashes at a rate of 27,000 per second with a peak brilliance one billion times higher than the best conventional X-ray sources. The outstanding characteristics of the facility will open up completely new research opportunities for scientists and industrial users (see see “Europe enters the extreme X-ray era”). Involving close co-operation with nearby DESY and other organisations worldwide, the European XFEL is a joint effort between many countries. No fewer than 17 European institutes contributed to the accelerator complex, with the largest in-kind (> 70%) and other contributions coming from DESY.
The story of the European XFEL is a wonderful example of R&D synergy between the high-energy physics and light-source worlds. At the heart of the European XFEL are superconducting radio-frequency (SRF) cavities that allow the 1.4 km-long linac to accelerate electrons highly efficiently. Despite the clear benefits of using SRF cavities, before the mid-1990s the technology was not mature enough and too expensive to be practical for a large facility. Experience gained at DESY and other major accelerator facilities – including LEP at CERN and CEBAF at Jefferson Lab – changed that picture. It became clear that superconducting accelerating structures with reasonably large gradients can produce high-energy electron beams in long continuous linac sections.
Enter TESLA
A major character in the European XFEL story is the TESLA (TeV Energy Superconducting Linear Accelerator) collaboration, which was founded in 1990 by key players of the SRF community. Among its challenges was to make SRF cavities more affordable. DESY offered to host essential infrastructure and a test facility to operate newly designed accelerator modules housing eight standardised cavities. The first module was built in the mid-1990s in collaboration with many of the later contributors to the European XFEL, and the first electron beam was accelerated in 1997.
The enormous flexibility in how electron bunches can be structured has meant that there has long been a close connection between free-electron lasers and superconducting accelerator technology from the beginning: examples can be found at Stanford University, Darmstadt University and Dresden Rossendorf, Jefferson Lab, and DESY. From the start of the TESLA R&D, it was envisaged that SRF technology would drive a superconducting linear collider operating at a centre-of-mass energy of 500 GeV, with the possibility of extending this to 800 GeV. This facility would have had two linear accelerators pointing towards one another: one for electrons, which would also be used to drive an X-ray laser facility, and one for positrons. At the time, high-energy physicists were weighing up other linear-collider designs in the US and Japan, but TESLA was unique in its choice of superconducting accelerating cavities. In 1997, DESY and the TESLA collaboration published a Conceptual Design Report for a superconducting linear collider with an integrated X-ray laser facility.
Although DESY was preparing for a hard-X-ray FEL, first the goal was to build an intermediate facility operating at slightly lower X-ray energies (corresponding to an output in the VUV region). In 2005 the VUV-FEL at DESY (today known as FLASH) produced laser light at a wavelength of 30 nm based on the principle of self-amplified-spontaneous-emission (SASE), which allows the generation of coherent X-ray light. The project preparation phase for the European XFEL began in 2007, with the official start declared in 2009 after the foundation of the European XFEL company. Plans to build a linear collider at DESY were dropped, but in 2004 the TESLA design was chosen for a new International Linear Collider (ILC). This machine is now “shovel ready” and the Japanese government has expressed interest in hosting it, although a final decision is awaited. Since the European XFEL uses TESLA technology at a large scale, the now finished superconducting linac can be considered as a prototype for the linear collider. Moreover, the successful technology transfer with industry that underpinned the construction of the European XFEL serves as a model for a worldwide linear collider effort.
The European XFEL, measuring 3.4 km in length, begins with the injector, which comprises a normal-conducting RF electron gun with a high bunch charge and low emittance. This is followed by a standard superconducting eight-cavity XFEL accelerator module, which takes the electron bunch to an energy of around 130 MeV. A harmonic 3.9 GHz accelerator module (provided by INFN and DESY) further alters the longitudinal beam profile, while a laser heater provided by Uppsala University increases the uncorrelated energy spread. At the end of the injector, 600 μs-long electron-bunch trains of typically 500 pC bunches are available for acceleration.
Once in the main linac of the European XFEL, the electron beam is accelerated in three sections. The first consists of four superconducting XFEL modules and presents a fairly modest gradient (far below the XFEL design gradient of 23.6 MV/m). The second linac section consists of 12 accelerator modules, from which the beam emerges with a relative energy spread of 0.3% at 2.4 GeV. The third and last linac section consists of 80 accelerator modules with an installed length of just less than 1 km. Bunch-compressor sections between the three main linac sections include dipole-magnet chicanes, further focusing elements and beam diagnostics.
Taking into account all installed main-linac accelerator modules, the achievable electron beam energy of the European XFEL is above its design energy of 17.5 GeV, although the exact figure will depend on optimising the RF control. The complete linac is suspended from the ceiling to keep the tunnel floor free for transport and the installation of electronics. During accelerator operation the electrons are distributed via fast kicker magnets into one of the two electron beamlines that feed several photon beamlines. Here, undulators provide X-ray photon beams for various experiments (see “Europe enters the extreme X-ray era”).
Meeting the production challenge
The superconducting accelerator modules for the European XFEL linac were contributed by DESY, CEA Saclay and LAL Orsay in France, INFN Milano in Italy, IPJ Swierk and Soltan Institute in Poland, CIEMAT in Spain and BINP in Russia. More than 100 modules were needed, and although they were based on a prototype developed for the TESLA linear collider, they had to be modified for large-scale industrial production. DESY, which had responsibility for the construction and operation of the particle accelerator, developed a consortium scheme in which collaborators could contribute in-kind, either by producing sub-components or by assuming responsibility for module assembly or component testing. A sophisticated supply chain was established and the pioneering work at FLASH provided invaluable help in dealing with initial challenges.
A standard accelerator module contains eight superconducting cavities, each supplied by one RF power coupler, and a superconducting quadrupole package, which includes correction coils and a beam-position monitor. Each module also contains cold vacuum components such as bellows and valves, and frequency tuners. During the R&D and project preparation phases, less than one accelerator module per year was assembled, thus it took a factor 30 increase in production rate to build the European XFEL. Two European companies – Research Instruments in Germany and Zanon in Italy – shared the task of producing 800 superconducting cavities from solid niobium. Cavity string and module assembly took place at CEA Saclay/Irfu based on completely new infrastructure called the XFEL village. Assembly was directly impacted by the availability of all accelerator module sub-components, and any break in the supply chain was seen as a risk for the overall project schedule. In the end, a total of 96 successfully tested XFEL modules were made available for tunnel installation within a period of just two years.
The operation of the superconducting accelerator modules also requires extensive dedicated infrastructure. DESY provided the RF high-power system and developed the required 10 MW multi-beam klystrons with industrial partners. A total of 27 klystrons, each supplying RF power for 32 superconducting structures (four accelerator modules), were ordered from two vendors. Precision regulation of the RF fields inside the accelerating cavities, which is essential to provide a highly reproducible and stable electron beam, is achieved by a powerful control system developed at DESY. BINP Novosibirsk produced and delivered major cryogenic equipment for the linac, while the cryogenic plant itself (an in-kind contribution from DESY) guarantees pressure variations will stay below 1%. The largest visible contributions to the warm beamline sections are the more than 700 beam-transport magnets and the 3 km vacuum system in the different sections. While most of the magnets were delivered by the Efremov Institute in St Petersburg, a small fraction was built by BINP Novosibirsk and completed at Stockholm University. Many metres of beamline, be it simple straight chambers or the more sophisticated flat bunch-compressor chambers, were also fabricated by BINP Novosibirsk.
State-of-the-art electron-beam diagnostics is vital for the success of the European XFEL. Thus 64 screens and 12 wire scanner stations, 460 beam-position monitors of eight different types, 36 toroids and six dark-current monitors are distributed along the accelerator. Longitudinal bunch properties are measured by bunch-compression monitors, beam-arrival monitors, electro-optical devices and transverse deflecting systems. Major contributions to the electron-beam diagnostics came from DESY, PSI in Switzerland, CEA Saclay in France, and from INR Moscow in Russia.
Technology goes full circle
Commissioning for the European XFEL accelerator began in December 2016 with the cool-down of the complete cryogenic system. First beam was injected into the main linac in January 2017, and by March bunches with a sufficient beam quality to allow lasing were accelerated to 12 GeV and stopped in a beam dump. After passing this beam through the “SASE1” undulator, first lasing at a wavelength of 0.9 nm was observed on 2 May. Further improvements to the beam quality and alignment led to lasing at 0.2 nm on 24 May. More than 90% of the installed accelerator modules are now in RF operation, with effective accelerating gradients reaching the expected performance in fully commissioned stations.
The first hard-X-ray SASE free-electron laser, the Linac Coherent Light Source (LCLS) at SLAC in the US, was based on a normal-conducting accelerator. Upgrades to LCLS-II now aim for continuous wave operation using 280 superconducting cavities of essentially the same design as those of the European XFEL. Improvements to the superconducting technology were made to further reduce the cryogenic load of the accelerator structures. New techniques such as nitrogen doping and infusion, developed by Fermilab and other LCLS-II partners, are also essential, and established procedures and expertise with series production will benefit future FEL user operation. The now existing European SRF expertise and collaboration scheme also sketches out a mechanism for a European in-kind contribution to a Japan-hosted ILC.
The European XFEL is one of the largest accelerator-based research facilities in the world, and is driven by the longest and most advanced superconducting linac ever constructed. This was possible thanks to the great collaborative effort and team spirit of all partners involved in this project over the past 20 years or more.
Natural diamonds are old, almost as old as the planet itself. They mostly originated in the Earth’s mantle around 1 to 3.5 billion years ago and typically were brought to the surface during deep and violent volcanic eruptions some tens of millions of years ago. Diamonds have been sought after for millennia and still hold status. They are also one of our best windows into our planet’s dynamics and can, in what is essentially a galactic narrative, convey a rich story of planetary science. Each diamond is unique in its chemical and crystallographic detail, with micro-inclusions and impurities within them having been protected over vast timescales.
Diamonds are usually found in or near the volcanic pipe that brought them to the surface. It was at one of these, in 1871 near Kimberley, South Africa, where the diamond rush first began – and where the mineral that hosts most diamonds got its name: kimberlite. Many diamond sources have since been discovered and there are now more than 6000 known kimberlite pipes (figure 1 overleaf). However, with current mining extraction technology, which generally involves breaking up raw kimberlite to see what’s inside, diamonds are often damaged and are steadily becoming mined out. Today, a diamond mine typically lasts for a few decades, and it costs around $10–26 to process each tonne of rock. With the number of new, economically viable diamond sources declining – combined with high rates of diamonds being extracted, ageing mines and increasing costs – most forecasts predict a decline in rough diamond production compared to demand, starting as soon as 2020.
A new diamond-discovery technology called MinPET (mineral positron emission tomography) could help to ensure that precious sources of natural diamonds last for much longer. Inspired by the same principles adapted in modern, high-rate, high-granularity detectors commonly found in high-energy physics experiments, MinPET uses a high-energy photon beam and PET imaging to scan mined kimberlite for large diamonds, before the rocks are smashed to pieces.
From eagle eyes to camera vision
Over millennia, humans have invented numerous ways to look for diamonds. Early techniques to recover loose diamonds used the principle that diamonds are hydrophobic, so resist water but stick readily to grease or fat. Some stories even tell of eagles recovering diamonds from deep, inaccessible valleys, when fatty meat thrown onto a valley floor might stick to a gem: a bird would fly down, devour the meat, and return to its nest, where the diamond could be recovered from its droppings. Today, technology hasn’t evolved much. Grease tables are still used to sort diamond from rock, and the current most popular technique for recovering diamonds (a process called dense media separation) relies on the principle that kimberlite particles float in a special slurry while diamonds sink. The excessive processing required with these older technologies wastes water, takes up huge amounts of land, releases dust into the surrounding atmosphere, and also leads to severe diamond breakage.
Just 1% of the world’s diamond sources have economically viable grades of diamond and are worth mining. At most sites the gemstones are hidden within the kimberlite, so diamond-recovery techniques must first crush each rock into gravel. The more barren rock there is compared to diamonds, the more sorting has to be done. This varies from mine to mine, but typically is under one carat per tonne – more dilute than gold ores. Global production was around 127 million carats in 2015, meaning that mines are wasting millions of dollars crushing and processing about 100 million tonnes of kimberlite per year that contains no diamonds. We therefore have an extreme case of a very high value particle within a large amount of worthless material – making it an excellent candidate for sensor-based sorting.
Early forms of sensor-based sorting, which have only been in use since 2010, use a technique called X-ray stimulated optical fluorescence, which essentially targets the micro impurities and imperfections in each diamond (figure 2). Using this method, the mined rocks are dropped during the extraction process at the plant, and the curtain of falling rock is illuminated by X-rays, allowing a proportion of liberated or exposed diamonds to fluoresce and then be automatically extracted. The transparency of diamond makes this approach quite effective. When Petra Diamonds Ltd introduced this technique with several X-ray sorting machines costing around $6 million, the apparatus paid for itself in just a few months when the firm recovered four large diamonds worth around $43 million. These diamonds, presumed to be fragments of a larger single one, were 508, 168, 58 and 53 carats, in comparison to the average one-carat engagement ring.
Very pure diamonds that do not fluoresce, and gems completely surrounded by rock, can remain hidden to these sensors. As such, a newer sensor-based sorting technique that uses an enhanced form of dual-energy X-ray transmission (XRT), similar to the technology for screening baggage in airports, has been invented to get around this problem. It can recover liberated diamonds down to 5 mm diameter, where 1 mm is usually the smallest size recovered commercially, and, unlike the fluorescing technique, can detect some locked diamonds. These two techniques have brought the benefits of sensor-based sorting into sharp focus for more efficient, greener mines and for reducing breakage.
Recent innovations in particle-accelerator and particle-detector technology, in conjunction with high-throughput electronics, image-processing algorithms and high-performance computing, have greatly enhanced the economic viability of a new diamond-sensing technology using PET imaging. PET, which has strongly benefitted from many innovations in detector development at CERN, such as BGO scintillating crystals for the LEP experiments, has traditionally been used to observe processes inside the body. A patient must first absorb a small amount of a positron-emitting isotope; the ensuing annihilations produce patterns of gamma rays that can be reconstructed to build a 3D picture of metabolic activity. Since a rock cannot be injected with such a tracer, MinPET requires us to irradiate rocks with a high-energy photon beam and generate the positron emitter via transmutation.
The birth of MinPET
The idea to apply PET imaging to mining began in 1988, in Johannesburg, South Africa, where our small research group of physicists used PET emitters and positron spectroscopy to study the crystal lattice of diamonds. We learnt of the need for intelligent sensor-based sorting from colleagues in the diamond mining industry and naturally began discussing how to create an integrated positron-emitting source.
Advances in PET imaging over the next two decades led to increased interest from industry, and in 2007 MinPET achieved its first major success in an experiment at Karolinska hospital in Stockholm, Sweden. With a kimberlite rock playing the role of a patient, irradiation was performed at the hospital’s photon-based cancer therapy facility and the kimberlite was then imaged at the small-animal PET facility in the same hospital. The images clearly revealed the diamond within, with PET imaging of diamond in kimberlite reaching an activity contrast of more than 50 (figure 3). This result led to a working technology demonstrator involving a conveyor belt that presented phantoms (rocks doped with a sodium PET-emitter were used to represent the kimberlite, some of which contained a sodium hotspot to represent a hidden diamond) to a PET camera. These promising results attracted funding, staff and students, enabling the team to develop a MinPET research laboratory at iThemba LABS in Johannesburg. The work also provided an important early contribution to South Africa’s involvement in the ATLAS experiment at CERN’s Large Hadron Collider.
By 2015 the technology was ready to move out of the lab and into a diamond mine. The MinPET process (figure 4) involves using a high-energy photon beam of some tens of MeV to irradiate a kimberlite rock stream, turning some of the light stable isotopes within the kimberlite into transient positron emitters, or PET isotopes, which can be imaged in a similar way to PET imaging for medical diagnostics. The rock stream is buffered for a period of 20 minutes before imaging the rock, because by then carbon is the dominant PET isotope. Since non-diamond sources of carbon have a much lower carbon concentration than diamond, or are diluted and finely dispersed within the kimberlite, diamonds show up on the image as a carbon-concentration hotspot.
The speed of imaging is crucial to the viability of MinPET. The detector system must process up to 1000 tonnes of rock per hour to meet the rate of commercial rock processing, with PET images acquired in just two seconds and image processing taking just five seconds. This is far in excess of medical-imaging needs and required the development of a very high-rate PET camera, which was optimised, designed and manufactured in a joint collaboration between the present authors and a nuclear electronic technology start-up called NeT Instruments. MinPET must also take into account rate capacity, granularity, power consumption, thermal footprints and improvements in photon detectors. The technology demonstrator is therefore still used to continually improve MinPET’s performance, from the camera to raw data event building and fast-imaging algorithms.
An important consideration when dealing with PET technology is that radiation remains within safe limits. If diamonds are exposed to extremely high doses of radiation, their colour can change – something that can be done deliberately to alter the gems, but which reduces customer confidence in a gem’s history. Despite being irradiated, the dose exposure to the diamonds during the MinPET activation process is well below the level it would receive from nature’s own background. It has turned out, quite amazingly, that MinPET offers a uniquely radiologically clean scenario. The carbon PET activity and a small amount of sodium activity are the only significant activations, and these have relatively short half-lives of 20 minutes and 15 hours, respectively. The irradiated kimberlite stream soon becomes indistinguishable from non-irradiated kimberlite, and therefore has a low activity and allows normal mine operation.
Currently, XRT imaging techniques require each particle of kimberlite rock being processed to be isolated and smaller than 75 mm; within this stream only liberated diamonds that are at least 5 mm wide can be detected and XRT can only provide 2D images. MinPET is far more efficient because it is currently able to image locked diamonds with a width of 4 mm within a 100 mm particle of rock, with full 3D imaging. The size of diamonds MinPET detects means it is currently ideally suited for mines that make their revenue predominantly from large diamonds (in some mines breakage is thought to cause up to a 50% drop in revenue). There is no upper limit for finding a liberated diamond particle using MinPET, and it is expected that larger diamonds could be detected in up to 160 mm-diameter kimberlite particles.
To crumble or shine
MinPET has now evolved from a small-scale university experiment to a novel commercial technology, and negotiations with a major financial partner are currently at an advanced stage. Discussions are also under way with several accelerator manufacturers to produce a 40 MeV beam of electrons with a power of 40–200 kW, which is needed to produce the original photon beam that kick-starts the MinPET detection system.
Although the MinPET detection system costs slightly more than other sorting techniques, overall expenditure is less because processing costs are reduced. Envisaged MinPET improvements over the next year are expected to take the lower limit of discovery down to as little as 1.5 mm for locked diamonds. The ability to reveal entire diamonds in 3D, and locating them before the rocks are crushed, means that MinPET also eliminates much of the breakage and damage that occurs to large diamonds. The technique also requires less plant, energy and water – all without causing any impact on normal mine activity.
The world’s diamond mines are increasingly required to be greener and more efficient. But the industry is also under pressure to become safer, and the ethics of mining operations are a growing concern among consumers. In a world increasingly favouring transparency and disclosure, the future of diamond mining has to be in using intelligent, sensor-based sorting that can separate diamonds from rock. MinPET is the obvious solution – eventually allowing marginal mines to become profitable and the lifetime of existing mines to be extended. And although today’s synthetic diamonds offer serious competition, natural stones are unique, billions of years old, and came to the surface in a violent fiery eruption as part of a galactic narrative. They will always hold their romantic appeal, and so will always be sought after.
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