Development of tools for quality control on therapeutic carbon beams with a fast-MC code (FRED)

In the fight against tumors, different types of cancer require different ways of treatment: surgery, radiotherapy, chemotherapy, hormone therapy and immunotherapy often used in combination with each other. About 50% of cancer patients undergo radiotherapy treatment which exploits the ability of ionizing radiation to damage the genetic heritage of cancer cells, causing apoptosis and preventing their reproduction. The non-invasive nature of radiation represents a viable alternative for those tumors that are not surgically operable because they are localized in hardly reachable anatomical sites or on organs which removal would be too disabling for the patient. A new frontier of radiotherapy is represented by Particle Therapy (PT). It consists of the use of accelerated charged particle beams (in particular protons and carbon ions) to irradiate solid tumors. The main advantage of such a technique with respect to the standard radiotherapy using x-rays/electron beams is in the different longitudinal energy release profiles. While photons’ longitudinal dose release is characterized by a slow exponential decrease, for charged particles a sharp peak at the end of the path provides a more selective energy release. By conveniently controlling the peak position it is possible to concentrate the dose (expressed as the energy release per unit mass) to tumors and, at the same time, preserve surrounding healthy tissues. In particle therapy treatments, the achieved steep dose gradients demand highly accurate modelling of the interaction of beam particles with tissues. The high ballistic precision of hadrons may result in a superior delivered dose distribution compared to conventional radiotherapy only if accompanied by a precise patient positioning and highly accurate treatment planning. This second operation is performed by the Treatment Planning System (TPS), sophisticated software that provides position, intensity and direction of the beams to the accelerator control system. Nowadays one of the major issues related to the TPS based on Monte Carlo (MC) is the high computational time required to meet the demand for high accuracy. The code FRED (Fast paRticle thErapy Dose evaluator) has been developed to allow a fast optimization of treatment plans in proton therapy while profiting from the dose release accuracy of a MC tool. Within FRED, the proton interactions are described with the precision level available in leading-edge MC tools used for medical physics applications, with the advantage of reducing the simulation time up to a factor of 1000. In this way, it allows a MC plan recalculation in a few minutes on GPU (Graphics Processing Unit) cards, instead of several hours on CPU (Central Processing Unit) hardware. For the exceptional speed of the proton tracking algorithms implemented in FRED and for the excellent results achieved, the door to several applications within the particle therapy field has been opened. In particular, the success of FRED with protons determined the interest of CNAO (Centro Nazionale di Adroterapia Oncologica) center in Pavia to develop FRED also for carbon therapy applications, to recalculate treatment plans with carbon ions. Among the several differences between proton and carbon beams, the nuclear fragmentation of the projectile in a 12C treatment, which does not occur with protons, is certainly the most important. The simulation of the ion beam fragmentation gives an important contribution to the dose deposition. The total dose released is due not only to the primary beam but also to secondary and tertiary particles. Also for proton beams, there are secondary particles, mostly secondary protons from target fragmentation, which contribute on the level of some percent to the dose deposition for higher proton beam energies. However, fragments of the projectile, produced only by carbon beams, having on average the same energy per nucleon of the primary beam and a lower mass, can release dose after the peak causing the well-known fragmentation tail. This thesis is focused on the development of a fast-MC simulating the carbon treatment in particle therapy, with an entirely new nuclear interaction model of carbon on light target nuclei. The model has been developed to be implemented in the GPU based MC code, FRED. For this reason, in developing the algorithms the goal has been to balance accuracy, calculation time and GPU execution guidelines. In particular, maximum attention has been given to physical processes relevant for dose and RBE-weighted dose computation. Moreover, where possible, look-up tables have been implemented instead of performing an explicit calculation in view of the GPU implementation. Some aspects of the interaction of carbon ions with matter are analogous to the ones already used in FRED for proton beams. In particular, for ionization energy loss and multiple scattering, only a few adjustments were necessary. On the contrary, the nuclear model was built from scratch. The approach has been to develop the nuclear model parameterizing existent data and applying physical scaling in the energy range where the data are missing. The elastic cross-section has been obtained from ENDF/B-VII data while the calculation of the non-elastic cross-section was based on results reported on Tacheki, Zhang and Kox papers. Data used for the sampling of the combination of emitted fragments, energy and angle distributions, are relatives to the Dudouet and Divay experiments. To fill the gaps in the experimental data, an intercomparison between FRED and the full-MC FLUKA has been of help to check the adopted scaling. The model has been tested against the full-MC code FLUKA, commonly used in particle therapy, and then with two of the few experiments that it is possible to find in literature. The agreement with FLUKA is excellent, especially for lower energies

A superconducting absolute spin valve

A superconductor with a spin-split excitation spectrum behaves as an ideal ferromagnetic spin-injector in a tunneling junction. It was theoretical predicted that the combination of two such spin-split superconductors with independently tunable magnetizations, may be used as an ideal $absolute$ spin-valve. Here we report on the first switchable superconducting spin-valve based on two EuS/Al bilayers coupled through an aluminum oxide tunnel barrier. The spin-valve shows a relative resistance change between the parallel and antiparallel configuration of the EuS layers up to 900% that demonstrates a highly spin-polarized currents through the junction. Our device may be pivotal for realization of thermoelectric radiation detectors, logical element for a memory cell in cryogenics superconductor-based computers and superconducting spintronics in general.Comment: 6 pages, 4 color figures, 1 tabl

Revealing the magnetic proximity effect in EuS/Al bilayers through superconducting tunneling spectroscopy

A ferromagnetic insulator attached to a superconductor is known to induce an exchange splitting of the Bardeen-Cooper-Schrieffer (BCS) singularity by a magnitude proportional to the magnetization, and penetrating into the superconductor to a depth comparable with the superconducting coherence length. We study this long-range magnetic proximity effect in EuS/Al bilayers and find that the exchange splitting of the BCS peaks is present already in the unpolarized state of the ferromagnetic insulator (EuS), and is being further enhanced when magnetizing the sample by a magnetic field. The measurement data taken at the lowest temperatures feature a high contrast which has allowed us to relate the line shape of the split BCS conductance peaks to the characteristic magnetic domain structure of the EuS layer in the unpolarized state. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the hard gap and clear splitting observed in our tunneling spectroscopy measurements indicate that EuS/Al bilayers are excellent candidates for substituting strong magnetic fields in experiments studying Majorana bound states.Comment: 9 pages, 4 color figure

Surface-acoustic-wave driven planar light-emitting device

Electroluminescence emission controlled by means of surface acoustic waves (SAWs) in planar light-emitting diodes (pLEDs) is demonstrated. Interdigital transducers for SAW generation were integrated onto pLEDs fabricated following the scheme which we have recently developed. Current-voltage, light-voltage and photoluminescence characteristics are presented at cryogenic temperatures. We argue that this scheme represents a valuable building block for advanced optoelectronic architectures

An information-theoretic and dissipative systems approach to the study of knowledge diffusion and emerging complexity in innovation systems

The paper applies information theory and the theory of dissipative systems to discuss the emergence of complexity in an innovation system, as a result of its adaptation to an uneven distribution of the cognitive distance between its members. By modelling, on one hand, cognitive distance as noise, and, on the other hand, the inefficiencies linked to a bad flow of information as costs, we propose a model of the dynamics by which a horizontal network evolves into a hierarchical network, with some members emerging as intermediaries in the transfer of knowledge between seekers and problem-solvers. Our theoretical model contributes to the understanding of the evolution of an innovation system by explaining how the increased complexity of the system can be thermodynamically justified by purely internal factors. Complementing previous studies, we demonstrate mathematically that the complexity of an innovation system can increase not only to address the complexity of the problems that the system has to solve, but also to improve the performance of the system in transferring the knowledge needed to find a solution

Gate control of superconductivity in mesoscopic all-metallic devices

The possibility to tune, through the application of a control gate voltage, the supercon-ducting properties of mesoscopic devices based on Bardeen–Cooper–Schrieffer metals was recently demonstrated. Despite the extensive experimental evidence obtained on different materials and geometries, a description of the microscopic mechanism at the basis of such an unconventional effect has not been provided yet. This work discusses the technological potential of gate control of superconductivity in metallic superconductors and revises the experimental results, which provide information regarding a possible thermal origin of the effect: first, we review experiments performed on high-critical-temperature elemental superconductors (niobium and vanadium) and show how devices based on these materials can be exploited to realize basic electronic tools, such as a half-wave rectifier. Second, we discuss the origin of the gating effect by showing gate-driven suppression of the supercurrent in a suspended titanium wire and by providing a comparison between thermal and electric switching current probability distributions. Furthermore, we discuss the cold field-emission of electrons from the gate employing finite element simulations and compare the results with experimental data. In our view, the presented data provide a strong indication regarding the unlikelihood of the thermal origin of the gating effect