In the search to develop tools that are able to modify surfaces on the nanometre scale, the use of heavy ions with energies of several tens of MeV is becoming more attractive. Low-energy ions are mostly stopped by nuclei, which causes the energy to be dissipated over a large volume. In the high-energy regime, however, the ions are stopped by electronic excitations, and the extremely local (approximately 10 nm3) nature of the energy deposition leads to the creation of nanosized 'hillocks' or nanodots under normal incidence. Usually, each nanodot results from the impact of a single ion, and the dots are randomly distributed. Here we demonstrate that multiple, equally spaced dots, each separated by a few tens of nanometres, can be created if a single high-energy xenon ion strikes the surface at a grazing angle. By varying this angle, the number of dots, as well as their spacing, can be controlled.

The irradiation of SrTiO 3 single crystals with swift heavy ions leads to modifications of the surface. The details of the morphology of these modifications depends strongly on the angle of incidence and can be characterized by atomic force microscopy. At glancing angles, discontinuous chains of nanosized hillocks appear on the surface. The latent track radius can be determined from the variation of the length of the chains with the angle of incidence. This radius is material specific and allows the calculation of the electron-phonon-coupling constant for SrTiO 3 . We show that a theoretical description of the nanodot creation is possible within a twotemperature model if the spatial electron density is taken into account. The appearance of discontinuous features can be explained easily within this model, but it turns out that the electronic excitation dissipates on a femtosecond time scale and thus too rapidly to feed sufficient energy into the phonon system in order to induce a thermal melting process. We demonstrate that this can be solved if the temperature dependent diffusion coefficient is introduced into the model.

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Proton therapy is a rapidly increasing modality to treat cancerous tumors, but large-scale implementation, and therefore widespread availability for patients, is hindered by the size and upfront investment for treatment facilities. Superconducting technology can enable more compact, and therefore more affordable treatment systems, by increasing the magnetic field in the magnets for the proton accelerator (typically a cyclotron) and in the beam guidance up, over, and into the patient (the gantry). In this article, we discuss research at Varian Medical Systems Particle Therapy GmbH on various superconducting technologies for potential application in future, more compact cyclotrons and gantries. We discuss which technologies are feasible, and to what extent. We demonstrate why certain conductor choices are made, and show the development of novel new conductor and magnet technologies that will be required to enable the next generation of cryogen-free, conduction-cooled compact treatment systems. We conclude that superconductivity is certainly required for the next generation of proton treatment systems, but also that the amount of compactness that can eventually be achieved is not solely determined by the magnetic field strength that is generated in the magnets.

Articles you may be interested inA high resolution scanning electron microscope for in situ investigation of swift heavy ion induced modification of solid surfaces Rev. Sci. Instrum. 81, 033702 (2010); Development of an in situ ultra-high-vacuum scanning tunneling microscope in the beamline of the 15 MV tandem accelerator for studies of surface modification by a swift heavy ion beam Rev. Sci. Instrum. 72, 3884 (2001); 10.1063/1.1405791 Studies on surface damage induced by ion bombardmentSwift heavy ions can be used to modify material surfaces on the nanometer scale. In particular, the irradiation of a target surface under grazing angle of incidence offers new possibilities to create chains of individual nanodots with different lengths. The length of these chains can be controlled by the angle of incidence. So far, this method could be successfully applied for insulating materials. The present work dealt with nanosized tracks on the well-known highly oriented pyrolytic graphite surface. By using atomic force microscopy and scanning tunneling microscopy, comparative studies of two different ion beam energies and ion types have been performed. From the analysis of the scanning probe microscopy results, the same track length-angle relation was found, similar to earlier studies on other materials such as SrTiO 3 .