A. Leyva

Monte Carlo assisted classical method for the calculation of dpa distributions in solids materials

In the present work the extended Monte Carlo assisted Classical Method (MCCM) is presented. The method consists on a calculation procedure for the determination of the displacements per atom (dpa) distribution in solid materials, which allows studying the gamma irradiation damage in different materials. The same one is based on the electrons elastic scattering classic theories and makes use of the Monte Carlo simulation of physical processes involved in the radiation interactions with substance. Recently, the contribution from positrons to dpa distributions has been also included. This method has been applied to different materials: metals (iron), semiconductors (Si and CZT) and high temperature superconductors like YBCO. Among other things, this procedure has allowed to study the dpa cross sections and the in-depth dpa distributions in a wide range of incident gamma energies. Also in compound materials, the contribution from each atomic species is possible to be evaluated.

MCSAD: Monte Carlo simulations of atom displacements induced by fast electrons in solids

Present contribution deals with Monte Carlo simulation of atom displacements rates resulting in solids based on a calculation algorithm supporting the description of the conditions favouring the occurrence of single fast electron elastic scattering in solids, leading to the displacement of atoms from their crystalline sites. Firstly, a McKinley-Feshbach differential cross-section renormalization is introduced by considering single elastic scattering events with scattering angles only within the interval [θl, π], where θl is the limiting angle, under which multiple scattering events with relative low scattering angles prevails over the single ones. On this basis, a Monte Carlo simulation code (MCSAD) of atom displacements induced by electrons and photons was implemented. In particular, total atom displacements produced along an electron travelling path were sampled in different solids matrix and compared with Oen-Holmes-Cahn theory predictions at different electron initial energies. As a result, it was concluded that Oen-Holmes-Cahn calculations overestimate normalized atom displacements rates in comparison with present MCSAD ones in a range up to between 10 to 73% rate, where maximum deviation were observed in YBCO for heaviest atom. It was also found that Oen-Holmes-Cahn absolute atom displacements distributions overestimated in about 30 to 40 times at the maximum of the AD rate simulated by MCSAD code.

Theoretical foundations of atom displacements induced by fast electron elastic scattering in solids

Present contribution deals with the theoretical description of the conditions favoring the occurrence of single fast electron elastic scattering in solids, leading to the displacement of atoms from their crystalline sites.

Radiation damage evaluation on LYSO and LuYAP materials through dpa calculation assisted by Monte Carlo method

The aim of the present work is to study the radiation damage induced in LYSO and LuYAP crystals by the gamma radiation and the secondary electrons/positrons generated. The displacements per atom (dpa) distributions inside each material were calculated following the Monte Carlo assisted Classical Method (MCCM) introduced by the authors. As gamma sources were used Sc-44, Na-22 and V-48. Also the energy of gammas from the annihilation processes (511 keV) was included in the study. This procedure allowed studying the in-depth dpa distributions inside each crystal for all four sources. It was also possible to obtain the separate contribution from each atom to the total dpa. The LYSO crystals were found to receive more damage, mainly provoked by the displacements of silicon and oxygen atoms.

Digital Radiography of Mammographic Phantoms and Biologic Samples Using a 64 Microstrips Crystalline Silicon Detector Coupled to the RX64 ASIC

The present paper synthesizes the results obtained in the evaluation of a 64 microstrips crystalline silicon detector coupled to RX64 ASIC, designed for high‐energy physics experiments, as a useful X‐ray detector in advanced medical radiography, specifically in digital mammography. Research includes the acquisition of two‐dimensional radiography of a mammography phantom using the scanning method, and the comparison of experimental profile with mathematically simulated one. The paper also shows the experimental images of three biological samples taken from breast biopsies, where it is possible to identify the presence of possible pathological tissues.

Simulation of a PET system and study of some geometry parameters

The Monte Carlo simulation of small animal conventional positron emission tomography (PET) is an important tool for geometry parameters optimization, image reconstruction algorithm tests, performance of different radioisotopic sources and some others. The present work deals with the Monte Carlo study of a small cylindrical PET system in the framework of the GEANT4 code. Two different accepted ring width values and spherical sources of 18F and 44Sc isotopes were considered. Then, the improvement of the detection efficiency and spatial resolution was analyzed in all the cases.

Multiscale modeling of radiation damage and annealing in Si samples implanted with 57-Mn radioactive ions

The radiation damage created in silicon materials by 57Mn→57Fe ion implantation has been studied and characterized by Mössbauer spectroscopy showing four main lines, assigned to: substitutional, interstitial and damaged configuration sites of the implanted ions. Nevertheless, the Mössbauer spectrum of 57Fe in this materials remains with some ambiguous identification regarding the implantation configurations before and after annealing, specially the damaged configurations and its evolution. In the present work some possible implantation configurations are suggested and evaluated using a multiscale approach by Monte Carlo ion transport and electronic structure calculations within DFT. The proposed implantation environments were evaluated in terms of stability and the 57Fe hyperfine parameters were calculated to establish the connections with the experimental observations. Good agreement was found between the experimental and the calculated hyperfine parameters for some configurations; suggesting which ones could be the implantation environments before and after sample annealing.

Monte Carlo Simulation in the Optimization of a Free-Air Ionization Chamber for Dosimetric Control in Medical Digital Radiography

During the earliest tests of a free‐air ionization chamber a poor response to the X‐rays emitted by several sources was observed. Then, the Monte Carlo simulation of X‐rays transport in matter was employed in order to evaluate chamber behavior as X‐rays detector. The photons energy deposition dependence with depth and its integral value in all active volume were calculated. The obtained results reveal that the designed device geometry is feasible to be optimized.

Digital Processing of Medical Images Obtained by a Si Microstrips Detector

We studied the capability of Matlab in digital processing of breast tissues images with microcalcifications. We obtained digital images of different byopsies through a Bede X‐ray tube, fixed at 20 kV and 1 mA. Radiation exposition time was varied. The byopsies were placed between a 120μm collimator and a 128 strips detector, which was used to measure the absorption of X rays in the tissue. Matlab allowed the manipulation of digital images, and this software was intended to improve the identification of microcalcifications in breast tissues.

Digital Images of Breast Biopsies using a Silicon Strip Detector

In our study we have used a silicon strip detector to obtain digital images of some breast tissues with micro calcifications. Some of those images will be shown and we will discuss the perspectives of using this technique as an improvement of breast cancer diagnostics.