504 research outputs found
Theoretical study of elastic electron scattering off stable and exotic nuclei
Results for elastic electron scattering by nuclei, calculated with charge
densities of Skyrme forces and covariant effective Lagrangians that accurately
describe nuclear ground states, are compared against experiment in stable
isotopes. Dirac partial-wave calculations are performed with an adapted version
of the ELSEPA package. Motivated by the fact that studies of electron
scattering off exotic nuclei are intended in future facilities in the
commissioned GSI and RIKEN upgrades, we survey the theoretical predictions from
neutron-deficient to neutron-rich isotopes in the tin and calcium isotopic
chains. The charge densities of a covariant interaction that describes the
low-energy electromagnetic structure of the nucleon within the Lagrangian of
the theory are used to this end. The study is restricted to medium and heavy
mass nuclei because the charge densities are computed in mean field approach.
Since the experimental analysis of scattering data commonly involves
parameterized charge densities, as a surrogate exercise for the yet unexplored
exotic nuclei, we fit our calculated mean field densities with Helm model
distributions. This procedure turns out to be helpful to study the
neutron-number variation of the scattering observables and allows us to
identify correlations of potential interest among some of these observables
within the isotopic chains.Comment: 18 pages, 14 figures, revtex4; modifications in text and figure
Simulation of MeV electron energy deposition in CdS quantum dots absorbed in silicate glass for radiation dosimetry
Copyright @ 2010 IOP Publishing Ltd. The conference proceedings contain the written papers of the contributions presented at Quantum Dot 2010 (QD2010). The conference was held in Nottingham, UK, on 26th‐30th April, 2010.We are currently developing 2D dosimeters with optical readout based on CdS or CdS/CdSe core-shell quantum-dots using commercially available materials. In order to understand the limitations on the measurement of a 2D radiation profile the 3D deposited energy profile of MeV energy electrons in CdS quantum-dot-doped silica glass have been studied by Monte Carlo simulation using the CASINO and PENELOPE codes. Profiles for silica glass and CdS quantum-dot-doped silica glass were then compared
Electron scattering in isotonic chains as a probe of the proton shell structure of unstable nuclei
Electron scattering on unstable nuclei is planned in future facilities of the
GSI and RIKEN upgrades. Motivated by this fact, we study theoretical
predictions for elastic electron scattering in the N=82, N=50, and N=14
isotonic chains from very proton-deficient to very proton-rich isotones. We
compute the scattering observables by performing Dirac partial-wave
calculations. The charge density of the nucleus is obtained with a covariant
nuclear mean-field model that accounts for the low-energy electromagnetic
structure of the nucleon. For the discussion of the dependence of scattering
observables at low-momentum transfer on the gross properties of the charge
density, we fit Helm model distributions to the self-consistent mean-field
densities. We find that the changes shown by the electric charge form factor
along each isotonic chain are strongly correlated with the underlying proton
shell structure of the isotones. We conclude that elastic electron scattering
experiments in isotones can provide valuable information about the filling
order and occupation of the single-particle levels of protons.Comment: 13 pages; 19 figure
Monte Carlo simulation of kilovolt electron transport in solids
A Monte Carlo procedure to simulate the penetration and energy loss of low¿energy electron beams through solids is presented. Elastic collisions are described by using the method of partial waves for the screened Coulomb field of the nucleus. The atomic charge density is approximated by an analytical expression with parameters determined from the Dirac¿Hartree¿Fock¿Slater self¿consistent density obtained under Wigner¿Seitz boundary conditions in order to account for solid¿state effects; exchange effects are also accounted for by an energy¿dependent local correction. Elastic differential cross sections are then easily computed by combining the WKB and Born approximations to evaluate the phase shifts. Inelastic collisions are treated on the basis of a generalized oscillator strength model which gives inelastic mean free paths and stopping powers in good agreement with experimental data. This scattering model is accurate in the energy range from a few hundred eV up to about 50 keV. The reliability of the simulation method is analyzed by comparing simulation results and experimental data from backscattering and transmission measurements
A simplified model of the source channel of the Leksell Gamma Knife: testing multisource configurations with PENELOPE
A simplification of the source channel geometry of the Leksell Gamma
Knife, recently proposed by the authors and checked for a single
source configuration (Al-Dweri et al 2004), has been used to calculate the dose
distributions along the , and axes in a water phantom with a
diameter of 160~mm, for different configurations of the Gamma Knife including
201, 150 and 102 unplugged sources. The code PENELOPE (v. 2001) has been used
to perform the Monte Carlo simulations. In addition, the output factors for the
14, 8 and 4~mm helmets have been calculated. The results found for the dose
profiles show a qualitatively good agreement with previous ones obtained with
EGS4 and PENELOPE (v. 2000) codes and with the predictions of
GammaPlan. The output factors obtained with our model agree
within the statistical uncertainties with those calculated with the same Monte
Carlo codes and with those measured with different techniques. Owing to the
accuracy of the results obtained and to the reduction in the computational time
with respect to full geometry simulations (larger than a factor 15), this
simplified model opens the possibility to use Monte Carlo tools for planning
purposes in the Gamma Knife.Comment: 13 pages, 8 figures, 5 table
Deterministic Partial Differential Equation Model for Dose Calculation in Electron Radiotherapy
Treatment with high energy ionizing radiation is one of the main methods in
modern cancer therapy that is in clinical use. During the last decades, two
main approaches to dose calculation were used, Monte Carlo simulations and
semi-empirical models based on Fermi-Eyges theory. A third way to dose
calculation has only recently attracted attention in the medical physics
community. This approach is based on the deterministic kinetic equations of
radiative transfer. Starting from these, we derive a macroscopic partial
differential equation model for electron transport in tissue. This model
involves an angular closure in the phase space. It is exact for the
free-streaming and the isotropic regime. We solve it numerically by a newly
developed HLLC scheme based on [BerCharDub], that exactly preserves key
properties of the analytical solution on the discrete level. Several numerical
results for test cases from the medical physics literature are presented.Comment: 20 pages, 7 figure
Ultracold Neutron Production in a Pulsed Neutron Beam Line
We present the results of an Ultracold neutron (UCN) production experiment in
a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The
experimental apparatus allows for a comprehensive set of measurements of UCN
production as a function of target temperature, incident neutron energy, target
volume, and applied magnetic field. However, the low counting statistics of the
UCN signal expected can be overwhelmed by the large background associated with
the scattering of the primary cold neutron flux that is required for UCN
production. We have developed a background subtraction technique that takes
advantage of the very different time-of-flight profiles between the UCN and the
cold neutrons, in the pulsed beam. Using the unique timing structure, we can
reliably extract the UCN signal. Solid ortho-D is used to calibrate UCN
transmission through the apparatus, which is designed primarily for studies of
UCN production in solid O. In addition to setting the overall detection
efficiency in the apparatus, UCN production data using solid D suggest that
the UCN upscattering cross-section is smaller than previous estimates,
indicating the deficiency of the incoherent approximation widely used to
estimate inelastic cross-sections in the thermal and cold regimes
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