Molecular reorientation in pyrene hexafluoroarsenate salts

The quasi-one dimensional pyrene (PY) organic conductor (PY)7(PY)4(AsF6)4 ${\cdot}$4CH2Cl2  shows parallel as well as 60$^{\circ}$ rotated PY radical cations in its stacks, but crystallizes in two different modifications I and II. One of the seven intra-stack pyrene molecules is susceptible to a reorientation by 60$^{\circ}$, that is stable already at room temperature for modification II, but occurs at a hysteretic first order transition between 170 K and 240 K for modification I. Crystal structure, microwave conductivity and static magnetic susceptibility are typical for a quasi-one dimensional organic conductor with Peierls transition at TP = 73 K (mod. I) or TP = 105 K (mod. II). The pyrene radical packing is analysed by continuous wave and pulsed electron spin resonance measurements, using 9.45 GHz as well as 425 MHz as measuring frequency. Anisotropy of the conduction electron diffusion constant exceeds 1000 in the metallic phase

Clinical microbeam radiation therapy with a compact source: Specifications of the line-focus X-ray tube.

Background and purpose: Microbeam radiotherapy (MRT) is a preclinical concept in radiation oncology with arrays of alternating micrometer-wide high-dose peaks and low-dose valleys. Experiments demonstrated a superior normal tissue sparing at similar tumor control rates with MRT compared to conventional radiotherapy. Possible clinical applications are currently limited to large third-generation synchrotrons. Here, we investigated the line-focus X-ray tube as an alternative microbeam source. Materials and methods: We developed a concept for a high-voltage supply and an electron source. In Monte Carlo simulations, we assessed the influence of X-ray spectrum, focal spot size, electron incidence angle, and photon emission angle on the microbeam dose distribution. We further assessed the dose distribution of microbeam arc therapy and suggested to interpret this complex dose distribution by equivalent uniform dose. Results: An adapted modular multi-level converter can supply high-voltage powers in the megawatt range for a few seconds. The electron source with a thermionic cathode and a quadrupole can generate an eccentric, high-power electron beam of several 100 keV energy. Highest dose rates and peak-to-valley dose ratios (PVDRs) were achieved for an electron beam impinging perpendicular onto the target surface and a focal spot smaller than the microbeam cross-section. The line-focus X-ray tube simulations demonstrated PVDRs above 20. Conclusion: The line-focus X-ray tube is a suitable compact source for clinical MRT. We demonstrated its technical feasibility based on state-of-the-art high-voltage and electron-beam technology. Microbeam arc therapy is an effective concept to increase the target-to-entrance dose ratio of orthovoltage microbeams

THE LINE FOCUS X-RAY TUBE: AN X-RAY SOURCE FOR FLASH AND SPATIALLY FRACTIONATED RADIATION THERAPY

Background and Aims: FLASH and spatially fractionated radiationtherapy (SFRT) demonstrated reduced side effects at equal tumourcontrol compared to conventional radiotherapy. Currently onlylarge synchrotrons may facilitate clinical x-ray FLASH or SFRTtreatments. We are constructing a prototype of an innovative, table-top x-ray source that will allow FLASH and SFRT treatments. Thesource is based on the line focus x-ray tube (LFxT) concept and willeventually deliver dose rates of up to 200 Gy/s.Methods: We designed a thermionic electron gun that generatesa low-emittance, high-current electron beam at 300 keV. Twoquadrupole magnets focus the electrons onto a 50 micrometer wide focalspot on a tungsten-molybdenum target that spins at 250 Hz. Weassessed the radiation field, temperature and mechanical stressconditions with finite-element and Monte Carlo simulations. Theseparation tube equipped motor drive and liquid metal bearings fitfor operation in ultra-high vacuum. We developed a high-voltagesupply based on modular multi-level converter (MMC) technologyfor increased power in a future clinical source.Results: Finite element simulations showed an operation of theLFxT in the heat capacity limit permitting substantially enhanceddose rates at small focal spot sizes. Thermal and mechanical stressesare tolerated by the target design. An MMC based DCDC converterwith 320 battery-powered units can store enough energy to supplythe source with 2 MW electrical power in a duty cycle of 2%