Xray Helium 10

[Dimple Belousson]

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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.

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Type Ia supernovae are cosmic distance indicators1,2, and the main source of iron in the Universe3,4, but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed5 and suggested as Type Ia supernovae progenitors6-9. However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion10, is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs. 7,11,12), including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae13, within this scenario.

X-ray and extreme ultraviolet (XUV) coherent diffractive imaging (CDI) have the advantage of producing high resolution images with current spatial resolution of tens of nanometers and temporal resolution of tens of femtoseconds. Modern developments in the production of coherent, ultra-bright, and ultra-short X-ray and XUV pulses have even enabled lensless, single-shot imaging of individual, transient, non-periodic objects. The data collected in this technique are diffraction images, which are intensity distributions of the scattered photons from the object. Superfluid helium droplets are ideal systems to study with CDI, since each droplet is unique on its own. It is also not immediately apparent what shapes the droplets would take or what structures are formed by dopant particles inside the droplet. In this chapter, we review the current state of research on helium droplets using CDI, particularly, the study of droplet shape deformation, the in-situ configurations of dopant nanostructures, and their dynamics after being excited by an intense laser pulse. Since CDI is a rather new technique for helium nanodroplet research, we also give a short introduction on this method and on the different light sources available for X-ray and XUV experiments.

The CDI technique is not limited to X-ray FELs. It can also be applied to other light sources producing spatially coherent radiation including visible lasers, intense light pulses in the XUV radiation from FELs, such as FLASH in Germany and the seeded Free Electron Laser Radiation for Multidisciplinary Investigations (FERMI) in Trieste, Italy, and lab-based High Harmonic Generation (HHG) sources, which are becoming widely available in many laboratories [17]. Experiments performed in the XUV regime using either seeded FELs or HHG sources have used wide-angle scattering approach to determine the three-dimensional shape of the helium droplet [49, 50].

In this chapter, we review the current progress of research and discoveries in coherent X-ray and XUV imaging with helium droplets. Since the application of imaging is rather recent in the arsenal of techniques available for helium nanodroplet science, we begin with a short introduction in Sects. 7.2 and 7.3 on single-shot, lensless coherent diffractive imaging; on how the structure of the pure and doped droplets are determined from their corresponding diffraction image; and on the general experimental setup for imaging. In Sect. 7.4, we proceed in discussing the results on the sizes and shapes of helium droplets and what the shapes of the droplets tell us about their state of spin. In Sect. 7.5, we discuss results where numerical reconstructions show the positions of dopant clusters, which in some cases reflect the configuration and distribution of quantum vortices. We also consider the possibility of controlling the growth of dopant nanostructures by using different kinds of dopants, such as xenon, silver, acetonitrile, and iodomethane. In Sect. 7.6, we introduce experimental results on imaging doped helium droplets after excitation with an intense near infrared pulse. Finally, we present a brief outlook on further opportunities for studying helium droplets with CDI in Sect. 7.7.

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