Harvel Wright

Calculated yields and fluctuations for electron degradation in liquid water and water vapor

Two sets of physical interaction cross sections for detailed electron track‐structure calculations in liquid water and water vapor have been used to investigate the possible magnitude of phase‐dependent differences in the primary yields of ionizations and their fluctuations produced during complete slowing down of electrons in the energy range from 10 eV to 10 keV. For fast electrons the calculated values of the mean energy absorbed per ion pair are 25.8 eV/ip for the liquid as compared to 30.0 eV/ip for the vapor; both results are consistent with experimental data. A similar phase effect is found in the ionization yields from each molecular subshell, since essentially the same partitioning of the total ionization cross section has been used in the calculations for the liquid and the vapor. The relative fluctuations of the ionization yields as described by the Fano factor are 0.15 for the liquid and 0.25 for the vapor; the Fano factors for each single molecular orbital are typically between 0.7 and 0.9 in...

Heavy-Ion Track Structure in Silicon

The energy deposition in the vicinity of a heavy ion path in silicon has been investigated by a Monte Carlo transport analysis of the delta rays produced along the track. The dose as a function of radial distance is presented for C, A1, and Fe ions with energies between 10 and 10,000 MeV. The average dose in cylinders of radii between 10-3 and 1 ¿m as a function of the distance of the cylinder from the path is also presented.

PHYSICAL ASPECTS OF CHARGED PARTICLE TRACK STRUCTURE

Abstract A plasmon generated by a swift charged particle constitutes a coherent excitation about the particle track. We discuss the physics of the plasmon in condensed matter and its effects on electron transport in liquid water. Criteria for the existence of the plasmon in a given substance, its characteristics when generated by a swift charged particle, its representation in impact parameter space, and its decay into localized excitations are described. We describe how plasmon excitation and decay are implemented in our Monte Carlo code, OREC, and how track structure calculations are affected by this mode.

Energy Spectra of Heavy Fragments from the Interaction of Protons with Communications Materials

A Monte Carlo computer code is used to calculate the energy spectra of recoil nuclei resulting from interactions of protons with communications materials. Results are presented for 130 MeV and 1 GeV protons incident on O, Si, Ga, As, and Au. The results for 130 MeV protons on Si are compared with previous calculations and measurements.

CALCULATION OF RADIATION DOSE FROM PROTONS AND NEUTRONS TO 400 Mev

1. THIS report is a part of a continuing reappraisal of the meaning and calculation of the dose due to high energy neutrons and protons in tissue. Estimates of such dose are given in the report of Committee IV (1953-1959) of the International Commission on Radiological Protection.(l) The estimates of Committee IV, based on the work of NEARY and MULVEY,@) utilized the best experimental and theoretical data which were available at that time. Within the last few years, however, a considerable amount of new information has been obtained which permits a more detailed study of interactions of high-energy neutrons or protons with tissue. Also, new techniques have been developed to describe typical histories of neutrons or protons within a tissue phantom. In this report, revised estimates of dose and dose equivalent in tissue are given for neutrons and protons with energies up to 400 MeV.

Aspects of degradation spectra

Abstract Monte Carlo calculations are presented of degradation spectra associated with 100 keV electrons slowing down in model physical systems. The resultant spectra are compared with those predicted by the continuous approximation, and the differences are discussed. Transitio yields in mixtures are obtained and found amenable to a linear representation.

Calculation of radiation dose due to protons and neutrons with energies from 0.4 to 2.4 GeV.

Estimates of the distribution of absorbed dose and dose equivalent with depth in a tissue slab have been made for irradiation by normally incident and also isotropically incident protons and neutrons with energies up to 2.0 GeV. The Monte Carlo calculations, based on a simplified model for nuclear interactions, take into account the production of pions as well as nucleons in nuclear interactions. The energy deposited due to ionization by charged particles is separated into LET ranges permitting different quality factors to be used. The total dose is broken down to show the contribution due to ionization by primary particles (in the case of incident protons), ionization by secondary particles, ionization by pions, excitation of residual nuclei following cascades, and the contribution from pions that stop within the tissue. Results are presented in the form of graphs showing the distribution of absorbed dose and dose equivalent with depth within the phantom for incident energies 0.6, 1.0, and 2.0 GeV. The results appear to be in reasonable agreement with experimental results. There is a rapid buildup of dose near the surface and then a more gradual buildup in the remainder of the 30 cm thick tissue slab. Pions are found to contribute less than 10% of the total dose. For normally incident neutrons, the quality factor decreases from near 10 at the surface from which the neutrons are incident to approximately 2.5 near the back of the slab.

Analysis of energy deposition in water around the site of capture of a negative pion by an oxygen or carbon nucleus

A study was made of the energy deposited in water spheres and cylinders of increasing size around the site at which an oxygen or carbon nucleus captures a stopped negative pion. No great differences were found either between results for O and C or between results for spheres and cylinders. As known from the literature, approximately 100 MeV is released as kinetic energy of secondary products after capture, about 60 MeV to neutrons and 40 MeV to charged particles. Most of the neutron energy is transported away from the immediate capture site. Calculations here show that 50% of the charged-particle energy is deposited within a distance of 0.1 cm from the site and 90% within 2 cm. Practically all charged particles with an LET ≥ 170 MeV cm-1 in water lose their energy within ~0.2 cm. The calculations also show that, on the average, 30 NeV of energy is deposited inside a sphere of 1 cm radius per π- capture by oxygen and 35 XeV per capture by carbon.

Monte Carlo calculations of initial energies of electrons in water irradiated by photons with energies up to 1GeV.

Previous calculations of the initial energies of electrons produced in water irradiated by photons are extended to 1 GeV by including pair and triplet production. Calculations were performed with the Monte Carlo computer code PHOEL-3, which replaces the earlier code, PHOEL-2. Tables of initial electron energies are presented for single interactions of monoenergetic photons at a number of energies from 10 keV to 1 GeV. These tables can be used to compute kerma in water irradiated by photons with arbitrary energy spectra to 1 GeV. In addition, separate tables of Compton-and pair-electron spectra are given over this energy range. The code PHOEL-3 is available from the Radiation Shielding Information Center, Oak Ridge National Laboratory, Oak Ridge, TN 37830.

Effects of tissue inhomogeneities on dose patterns in cylinders irradiated by negative-pion beams

A Monte Carlo computer program, PION-1, has been used to study the effects of bone and air inhomogeneities in cylindrical tissue targets irradiated by parallel and converging beams of negative pions.