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Single Xe clusters are superheated using an intense optical laser pulse and the structural evolution is imaged with a single X-ray pulse. Ultrafast surface softening on the nanometre scale is resolved within 100 fs at the vacuum/sample interface. The ability to observe ultrafast structural changes in nanoscopic samples is essential for understanding non-equilibrium phenomena such as chemical reactions 1 , matter under extreme conditions 2 , ultrafast phase transitions 3 and intense light–matter interactions 4 . Established imaging techniques are limited either in time or spatial resolution and typically require samples to be deposited on a substrate, which interferes with the dynamics. Here, we show that coherent X-ray diffraction images from isolated single samples can be used to visualize femtosecond electron density dynamics. We recorded X-ray snapshot images from a nanoplasma expansion, a prototypical non-equilibrium phenomenon 4 , 5 . Single Xe clusters are superheated using an intense optical laser pulse and the structural evolution of the sample is imaged with a single X-ray pulse. We resolved ultrafast surface softening on the nanometre scale at the plasma/vacuum interface within 100 fs of the heating pulse. Our study is the first time-resolved visualization of irreversible femtosecond processes in free, individual nanometre-sized samples.

Biological imaging with the LCLS X-ray laser The start-up of the Linac Coherent Light Source (LCLS), the new femtosecond hard X-ray laser facility in Stanford, California, has brought high expectations of a new era for biological imaging. The intense, ultrashort X-ray pulses allow diffraction imaging of small structures before radiation damage occurs. Two papers in this issue of Nature present proof-of-concept experiments showing the LCLS in action. Chapman et al . tackle structure determination from nanocrystals of macromolecules that cannot be grown in large crystals. They obtain more than three million diffraction patterns from a stream of nanocrystals of the membrane protein photosystem I, and assemble a three-dimensional data set for this protein. Seibert et al . obtain images of a non-crystalline biological sample, mimivirus, by injecting a beam of cooled mimivirus particles into the X-ray beam. The start-up of the new femtosecond hard X-ray laser facility in Stanford, the Linac Coherent Light Source, has brought high expectations for a new era for biological imaging. The intense, ultrashort X-ray pulses allow diffraction imaging of small structures before radiation damage occurs. This new capability is tested for the problem of structure determination from nanocrystals of macromolecules that cannot be grown in large crystals. Over three million diffraction patterns were collected from a stream of nanocrystals of the membrane protein complex photosystem I, which allowed the assembly of a three-dimensional data set for this protein, and proves the concept of this imaging technique. X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded 1 , 2 , 3 . It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction ‘snapshots’ are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source 4 . We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes 5 . More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (∼200 nm to 2 μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes 6 . This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage.

Biological imaging with the LCLS X-ray laser The start-up of the Linac Coherent Light Source (LCLS), the new femtosecond hard X-ray laser facility in Stanford, California, has brought high expectations of a new era for biological imaging. The intense, ultrashort X-ray pulses allow diffraction imaging of small structures before radiation damage occurs. Two papers in this issue of Nature present proof-of-concept experiments showing the LCLS in action. Chapman et al . tackle structure determination from nanocrystals of macromolecules that cannot be grown in large crystals. They obtain more than three million diffraction patterns from a stream of nanocrystals of the membrane protein photosystem I, and assemble a three-dimensional data set for this protein. Seibert et al . obtain images of a non-crystalline biological sample, mimivirus, by injecting a beam of cooled mimivirus particles into the X-ray beam. The start-up of the new femtosecond hard X-ray laser facility in Stanford, the Linac Coherent Light Source, has brought high expectations for a new era for biological imaging. The intense, ultrashort X-ray pulses allow diffraction imaging of small structures before radiation damage occurs. This new capability is tested for the problem of imaging a non-crystalline biological sample. Images of mimivirus are obtained, the largest known virus with a total diameter of about 0.75 micrometres, by injecting a beam of cooled mimivirus particles into the X-ray beam. The measurements indicate no damage during imaging and prove the concept of this imaging technique. X-ray lasers offer new capabilities in understanding the structure of biological systems, complex materials and matter under extreme conditions 1 , 2 , 3 , 4 . Very short and extremely bright, coherent X-ray pulses can be used to outrun key damage processes and obtain a single diffraction pattern from a large macromolecule, a virus or a cell before the sample explodes and turns into plasma 1 . The continuous diffraction pattern of non-crystalline objects permits oversampling and direct phase retrieval 2 . Here we show that high-quality diffraction data can be obtained with a single X-ray pulse from a non-crystalline biological sample, a single mimivirus particle, which was injected into the pulsed beam of a hard-X-ray free-electron laser, the Linac Coherent Light Source 5 . Calculations indicate that the energy deposited into the virus by the pulse heated the particle to over 100,000 K after the pulse had left the sample. The reconstructed exit wavefront (image) yielded 32-nm full-period resolution in a single exposure and showed no measurable damage. The reconstruction indicates inhomogeneous arrangement of dense material inside the virion. We expect that significantly higher resolutions will be achieved in such experiments with shorter and brighter photon pulses focused to a smaller area. The resolution in such experiments can be further extended for samples available in multiple identical copies.

X-ray free-electron lasers provide unique opportunities for exploring ultrafast dynamics and for imaging the structures of complex systems. Understanding the response of individual atoms to intense X-rays is essential for most free-electron laser applications. First experiments have shown that, for light atoms, the dominant interaction mechanism is ionization by sequential electron ejection, where the highest charge state produced is defined by the last ionic state that can be ionized with one photon. Here, we report an unprecedentedly high degree of ionization of xenon atoms by 1.5 keV free-electron laser pulses to charge states with ionization energies far exceeding the photon energy. Comparing ion charge-state distributions and fluorescence spectra with state-of-the-art calculations, we find that these surprisingly high charge states are created via excitation of transient resonances in highly charged ions, and predict resonance enhanced absorption to be a general phenomenon in the interaction of intense X-rays with systems containing high- Z constituents. Researchers create high ionization states, up to Xe 36+ , using 1.5 keV free-electron laser pulses. The higher than expected ionization may be due to transient resonance-enhanced absorption and the effect may play an important role in interactions of intense X-rays with high- Z elements and radiation damage.

Local antimatter unveiled Antimatter is not an exotic rarity found only in the depths of the Universe: there are large quantities in our own Galaxy. We know this because we see the 511-keV γ-ray emission line, a signature of electron–positron annihilation, coming from the general direction of the Galactic Centre. The origin of the positrons has remained a mystery, but the distribution of the annihilation line radiation provides a clue. Astronomers now have the tools that can work out that distribution, and analysis of more than four years of spectroscopic data from the INTEGRAL satellite reveals an unexpected distribution of the 511-keV γ-ray emission from the inner Galactic disk, suggesting that the positrons originate in binary stars containing black holes or neutron stars. Gamma-ray line radiation at 511 keV is the signature of electron–positron annihilation, which comes from the general direction of the Galactic centre, but the origin of the positrons was a mystery. This paper reports a distinct asymmetry in the 511 keV line emission coming from the inner Galactic disk, which resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV, indicating that they may be the dominant origin of the positrons. Gamma-ray line radiation at 511 keV is the signature of electron–positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre 1 , but the origin of the positrons has remained a mystery. Stellar nucleosynthesis 2 , 3 , 4 , accreting compact objects 5 , 6 , 7 , 8 , and even the annihilation of exotic dark-matter particles 9 have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (∼10–50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV (‘hard’ LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron–positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10 41 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

Expression of a protein in Sf9 insect cells at high concentration triggers formation of in vivo crystals that can be analyzed by serial femtosecond X-ray crystallography. Protein crystallization in cells has been observed several times in nature. However, owing to their small size these crystals have not yet been used for X-ray crystallographic analysis. We prepared nano-sized in vivo –grown crystals of Trypanosoma brucei enzymes and applied the emerging method of free-electron laser-based serial femtosecond crystallography to record interpretable diffraction data. This combined approach will open new opportunities in structural systems biology.

Lipidic sponge phase crystallization yields membrane protein microcrystals that can be injected into an X-ray free electron laser beam, yielding diffraction patterns that can be processed to recover the crystal structure. X-ray free electron laser (X-FEL)-based serial femtosecond crystallography is an emerging method with potential to rapidly advance the challenging field of membrane protein structural biology. Here we recorded interpretable diffraction data from micrometer-sized lipidic sponge phase crystals of the Blastochloris viridis photosynthetic reaction center delivered into an X-FEL beam using a sponge phase micro-jet.

Galactic elements The radioactive isotope aluminium-26 has a short half-life of about 720,000 years, so the fact that we can detect γ-rays characteristic of 26 Al is a good indication that nucleosynthesis — the production of new atomic nuclei — is taking place in our Galaxy during the current epoch. Now a γ-ray survey by ESA's INTEGRAL space telescope has provided data of sufficiently high resolution to settle a long-running debate about where this nucleosynthesis takes place. The key finding is that the 26 Al sources co-rotate with the Galaxy, supporting an origin from massive stars scattered throughout the Galaxy, rather than in localized star-forming regions. Gamma-rays from radioactive 26 Al (half-life ∼7.2 × 10 5  years) provide a ‘snapshot’ view of continuing nucleosynthesis in the Galaxy 1 . The Galaxy is relatively transparent to such γ-rays, and emission has been found concentrated along its plane 2 . This led to the conclusion 1 that massive stars throughout the Galaxy dominate the production of 26 Al. On the other hand, meteoritic data show evidence for locally produced 26 Al, perhaps from spallation reactions in the protosolar disk 3 , 4 , 5 . Furthermore, prominent γ-ray emission from the Cygnus region suggests that a substantial fraction of Galactic 26 Al could originate in localized star-forming regions. Here we report high spectral resolution measurements of 26 Al emission at 1808.65 keV, which demonstrate that the 26 Al source regions corotate with the Galaxy, supporting its Galaxy-wide origin. We determine a present-day equilibrium mass of 2.8 (± 0.8) solar masses of 26 Al. We use this to determine that the frequency of core collapse (that is, type Ib/c and type II) supernovae is 1.9 (± 1.1) events per century.

The ability to test scientific software needs to be supported by adequate software design. Legacy software systems are often characterized by the difficulty to test parts of the software, mainly due to existing dependencies on other parts. Methods to improve the testability of physics software are discussed, along with open issues specific to physics software for Monte Carlo particle transport. The discussion is supported by examples drawn from the experience with validating Geant4 physics.