243 research outputs found
Probing ultrafast symmetry breaking in photo-stimulated matter
The nature of a phase transition is inherently connected to the changes in
the crystalline symmtry, which is typically probed by elastic or inelastic
scattering with neutrons, electrons or photons. When such a phase transition is
stimulated by light or other sudden perturbations the solid evolves along a
non-equilibrium pathway of which the underlying physics is poorly understood.
Here we use picosecond Raman scattering to study the photo-induced ultrafast
dynamics in Peierls distorted Antimony. We find evidence for an ultrafast
non-thermal reversible structural phase transition. Most surprisingly, we find
evidence that this transition evolves toward a lower symmetry, in contrast to
the commonly accepted rhombohedral-to-simple cubic transition path. Our study
demonstrates the feasibility of ultrafast Raman scattering symmetry analysis of
photo-induced non-thermal transient phases
Fluence Beam Monitor for High-Intensity Particle Beams Based on a Multi-Gap Ionization Chamber and a Method for Ion Recombination Correction
Tests of thin Low-Gain Avalanche Detectors for the characterization of therapeutic proton beams.
Single Event Upset tests and failure rate estimation for a front-end ASIC adopted in high-flux-particle therapy applications
none8A 64 channels Application Specific Integrated Circuit, named TERA09, designed in a 0.35 m technology for particle therapy applications, has been characterized for Single Event Upset probability. TERA09 is a current-to-frequency converter that offers a wide input range, extending from few nA to hundreds of A, with linearity deviations in the order of a few percent. This device operates as front-end readout electronics for parallel plate ionization chambers adopted in clinical applications. This chip is going to be located beside the monitor chamber, thus not directly exposed to the particle beam. For this reason, no radiation hardening techniques were adopted during the microelectronics design. The intent of the test reported in this paper is to predict the TERA09 upset rate probability in a real application scenario. Due to the fact that TERA09 has an extended digital area with registers and counters, it is interesting to estimate the effect of the secondary neutron field produced during the treatment. The radiation damage test took place at the SIRAD facility of the Italian National Institute for Nuclear Physics in Padova, Italy. The SIRAD facility allows to study the CMOS upset rate as a function of the energy deposited during irradiation. By irradiating the chip with ions of different Linear Energy Transfer, it is possible to calculate the single event effect cross-section as a function of the deposited energy. It resulted that the minimum deposited energy in a CMOS silicon sensitive volume of , responsible for a Single Event Upset probability higher than zero, is 690 keV. In the last part of the paper, we calculated the expected upset probability in a typical clinical environment, knowing the fluence of secondary, backward-emitted neutrons. Considering as an example a treatment room located at the CNAO particle therapy center in Pavia, the expected upset rate for TERA09 is events/year. Using a redundant and independent monitor chamber, the upset probability expected during one detector readout is lower than , as explained in the document.noneFausti, F.; Mazza, G.; Giordanengo, S.; Hammad Ali, O.; Manganaro, L.; Monaco, V.; Sacchi, R.; Cirio, R.Fausti, F.; Mazza, G.; Giordanengo, S.; Hammad Ali, O.; Manganaro, L.; Monaco, V.; Sacchi, R.; Cirio, R
'Survival': A simulation toolkit introducing a modular approach for radiobiological evaluations in ion beam therapy.
Preliminary test of innovative strip silicon detectors for therapeutic proton beam monitoring
Phase separation in the non-equilibrium Verwey transition in magnetite
We present equilibrium and out-of-equilibrium studies of the Verwey
transition in magnetite. In the equilibrium optical conductivity, we find a
step-like change at the phase transition for photon energies below about 2 eV.
The possibility of triggering a non-equilibrium transient metallic state in
insulating magnetite by photo excitation was recently demonstrated by an x-ray
study. Here we report a full characterization of the optical properties in the
visible frequency range across the non-equilibrium phase transition. Our
analysis of the spectral features is based on a detailed description of the
equilibrium properties. The out-of-equilibrium optical data bear the initial
electronic response associated to localized photo-excitation, the occurrence of
phase separation, and the transition to a transient metallic phase for
excitation density larger than a critical value. This allows us to identify the
electronic nature of the transient state, to unveil the phase transition
dynamics, and to study the consequences of phase separation on the
reflectivity, suggesting a spectroscopic feature that may be generally linked
to out-of-equilibrium phase separation
A new detector for the beam energy measurement in proton therapy: a feasibility study
Fast procedures for the beam quality assessment and for the monitoring of
beam energy modulations during the irradiation are among the most urgent
improvements in particle therapy. Indeed, the online measurement of the
particle beam energy could allow assessing the range of penetration during
treatments, encouraging the development of new dose delivery techniques for
moving targets. Towards this end, the proof of concept of a new device, able to
measure in a few seconds the energy of clinical proton beams (from 60 to 230
MeV) from the Time of Flight (ToF) of protons, is presented. The prototype
consists of two Ultra Fast Silicon Detector (UFSD) pads, featuring an active
thickness of 80 um and a sensitive area of 3 x 3 mm2, aligned along the beam
direction in a telescope configuration, connected to a broadband amplifier and
readout by a digitizer. Measurements were performed at the Centro Nazionale di
Adroterapia Oncologica (CNAO, Pavia, Italy), at five different clinical beam
energies and four distances between the sensors (from 7 to 97 cm) for each
energy. In order to derive the beam energy from the measured average ToF,
several systematic effects were considered, Monte Carlo simulations were
developed to validate the method and a global fit approach was adopted to
calibrate the system. The results were benchmarked against the energy values
obtained from the water equivalent depths provided by CNAO. Deviations of few
hundreds of keV have been achieved for all considered proton beam energies for
both 67 and 97 cm distances between the sensors and few seconds of irradiation
were necessary to collect the required statistics. These preliminary results
indicate that a telescope of UFSDs could achieve in a few seconds the accuracy
required for the clinical application and therefore encourage further
investigations towards the improvement and the optimization of the present
prototype
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