Is Helium 10 Xray Free

[Christian Swindler]

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Coherent diffractive imaging of individual free nanoparticles has opened routes for the in situ analysis of their transient structural, optical, and electronic properties. So far, single-shot single-particle diffraction was assumed to be feasible only at extreme ultraviolet and X-ray free-electron lasers, restricting this research field to large-scale facilities. Here we demonstrate single-shot imaging of isolated helium nanodroplets using extreme ultraviolet pulses from a femtosecond-laser-driven high harmonic source. We obtain bright wide-angle scattering patterns, that allow us to uniquely identify hitherto unresolved prolate shapes of superfluid helium droplets. Our results mark the advent of single-shot gas-phase nanoscopy with lab-based short-wavelength pulses and pave the way to ultrafast coherent diffractive imaging with phase-controlled multicolor fields and attosecond pulses.

Single-shot coherent diffractive imaging (CDI) with intense short-wavelength pulses became possible just recently with the advent of extreme ultraviolet (XUV) and X-ray free-electron lasers (FEL)1. This lensless imaging method has revolutionized the structural characterization of nanoscale samples, including biological specimens2, aerosols3, atomic clusters4,5,6, and nanocrystals7. By capturing high-quality diffraction patterns from a single nanoparticle in free flight using a single laser pulse, the sample morphology can be determined in situ and free from spurious interactions due to deposition on a substrate. For sufficiently regular structures the wide-angle scattering information even reveals the full three-dimensional particle shape and orientation6,7,8, as multiple projections of the same particle are encoded in a single diffraction image6. These unique capabilities enable the investigation of metastable or transient states that exist only in the gas phase. Pioneering FEL experiments have explored this frontier and demonstrated CDI of quantum vortices in helium droplets5, ultrafast nanoplasma formation9, and explosion of laser-heated clusters10. Using XUV and soft X-ray high harmonic generation (HHG) sources for single-shot nanoparticle CDI holds the promise to combine the nanoscale structural imaging capabilities of CDI with the exquisite temporal, spectral, and phase control inherent in the use of optical lasers, including the fascinating prospect of CDI with isolated attosecond pulses.

D.R. and Y.O. performed the feasibility studies in advance of the experiment. F.F., L.P., A.T., F.C., and M.N. developed the microfocusing optics setup and implemented it together with N.M. and A.R. The helium jet was set up by B.L., D.R. and M.S. set up the CDI detection system, and J.Z. developed the data acquisition system. N.M. and A.R. operated the HHG source, and D.R., M.S., B.L., N.M. and A.R. assembled and carried out the experiment. K.S., C.P., and T.F. developed and performed the scattering simulations. D.R., N.M., B.L., J.Z., K.S., C.P., and T.F. analyzed the data with input from all authors. The manuscript was discussed and written with input from all authors.

Single crystal X-ray analysis has been used as a powerful method to determine the structure of molecules. However, crystallographic data containing helium has not been reported, owing to the difficulty in embedding helium into crystalline materials. Here we report the X-ray diffraction study of He@C60 and the clear observation of a single helium atom inside C60. In addition, the close packing of a helium atom and a nitrogen atom inside fullerenes is realized using two stepwise insertion techniques, that is, molecular surgery to synthesize the fullerenes encapsulating a helium atom, followed by nitrogen radio-frequency plasma methods to generate the fullerenes encapsulating both helium and nitrogen atoms. Electron spin resonance analysis reveals that the encapsulated helium atom has a small but detectable influence on the electronic properties of the highly reactive nitrogen atom coexisting inside the fullerene, suggesting the potential usage of helium for controlling electronic properties of reactive species.

Here we report the single crystal X-ray observation of a helium atom inside C60. In addition, we also report the close contact of the helium atom with a nitrogen atom by placing a nitrogen atom inside He@C60 and He@C70 by the use of nitrogen radio-frequency (RF) plasma method17,18,19.

He@C60 and He@C70 were synthesized by following our previous report14. ESR spectra were measured by Bruker EMX and EMX plus spectrometers. The temperature was controlled with an Oxford ESR900 helium flow-type cryostat and an Oxford ITC503 temperature controller for the EMX plus. Simulation of the spectra was carried out on a WinSim program39. The atmospheric pressure chemical ionization mass spectrum was recorded on a Bruker micrOTOF-QII.

Single-crystal X-ray data were collected on a BL38B1 beamline in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute with a diffractometer equipped with an ADSC Quantum 315 CCD (charge-coupled device) detector. The collected diffraction data were processed with the HKL2000 software program. The structure solutions were obtained by direct method using the SHELXS-97 program40 and refined using the SHELXL-97 program40.

Accession codes: The X-ray crystallographic coordinates for the structure reported in this Article has been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC 921390. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Y.Mu. designed the total project. Y.Mu., T.K., S.N. and T.A. contributed equally to the study. Y.Mo. carried out most of the experimental work and AIM analysis and wrote the paper. F.T. synthesized He@C60, supervised by M.M. and K.K. A.W. conducted the X-ray measurement. S.S. and H.N. performed the RF plasma technique supervised by T.A. ESR spectra were measured by S.S., H.N., K.F. and T.K. N.M. performed theoretical calculations supervised by T.A. and S.N.

MRI magnets have superconducting coil windings, which require very low temperatures (4K) that are achieved by bathing the coils in liquid helium. Quenching is the process whereby there is a rise in temperature in the magnet coil windings. This introduces resistivity in the coil windings, which reduces the magnetic field and produces heat that rapidly converts liquid helium into its gaseous form . Quenching may happen accidentally or can be manually instigated in the case of an emergency.

Quenching may cause severe and irreparable damage to the super conducting coils, and so a manual quench should only be performed in extreme cases when the physician and service engineer are involved in the decision to quench. A fire in the scan room may also be a cause to quench the magnet, so the firefighting personnel can safely enter the room (see MRI Code Red Protocol). All systems should have helium-venting equipment, which removes the helium to the outside environment in the event of a quench. However if this fails, helium will vent into the room and replace the oxygen. For this reason all scan rooms should contain an oxygen monitor that sounds an alarm if the oxygen falls below a certain level. Under these circumstances immediate evacuation of the patient and personnel is necessary.

If the scan room door is closed when a quench occurs and helium escapes into the scan room, the depletion of oxygen causes a critical increase in pressure in the room compared with the control area. This produces high pressure in the scan room, which may prevent opening of the door. If this should happen, the glass partition between the scan and control rooms should be broken to release the pressure. The scan room door can then be opened as usual and the patient evacuated. In such a case the patient should be immediately evacuated and evaluated for asphyxia, hypothermia and ruptured eardrums.

We designed and built a dedicated system prototype for ion radiography using exclusively the pixelated semiconductor technology Timepix developed at CERN. Helium ions were chosen as imaging radiation due to their decreased scattering in comparison to protons, and lower damaging potential compared to carbon ions. The data acquisition procedure and a dedicated information processing algorithm were established. The performance of the method was evaluated at the ion beam therapy facility HIT in Germany with geometrical phantoms. The quality of the images was quantified by contrast-to-noise ratio (CNR) and spatial resolution (SR) considering the imaging dose.

We developed a dedicated method for helium ion radiography, based exclusively on pixelated semiconductor detectors. The achievement of a clinically desired image quality in simple phantoms at diagnostic dose levels was demonstrated experimentally.

The delivery of a sufficient dose to control the tumor growth can be challenging when an organ-at-risk (OAR) is close to the target, or for radioresistant tumors which require high tumor doses. The physical and radiobiological properties of the therapeutic ion beams allow for an increased dose conformation to the tumor in comparison to standard radiotherapy with photon beams (e.g. [55]). The superior dose distributions theoretically achievable with ions can be compromised by uncertainties from different sources in the clinical practice. From the geometrical point of view, interfractional changes along the course of the radiotherapy might arise due to patient positioning uncertainties, anatomical changes including weight gain or loss, or swelling of tissue. In addition, intrafractional changes might occur as a result of movement on different time scales: muscles (minutes to seconds), breathing (seconds), and heart beat (below seconds). Moreover, the conversion of the attenuation data acquired by a CT to the stopping power distribution, which is needed as an input to the treatment planning, is associated with range uncertainties of 2-3% and in some cases even more [29, 30, 34].

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