R.N. Hamm

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...

Studies to link the basic radiation physics and chemistry of liquid water

Abstract Collaborative work between radiation physicists and chemists is described, leading to a Monte Carlo model of the detailed events that occur when ionizing radiation interacts with water in the liquid state. The model treats explicitly the initial physical changes produced by radiation, the formation of chemically active species, and the subsequent diffusion-controlled chemical reactions that take place along the path of a charged particle. Several examples of calculated tracks and chemical yields are presented. The calculated time decay of the hydrated electron and OH radical are in good agreement with measurements from electron pulse radiolysis. The effect of dissolved oxygen on the computed chemical yields is presented. While many uncertainties necessarily exist in such an endeavor, the basic model linking the physics and chemistry of irradiated water appears to be consistent with existing experimental and theoretical data.

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-induced DNA damage from photons and tritium beta-particles

Abstract Radiation-induced damage in nucleosomal DNA was modelled by Monte Carlo means. An atomistic representation of DNA with a first hydration shell was used. DNA single- and double-strand break (SSB and DSB) yields were calculated for 137Cs photons, x-rays and tritium beta-particles. Monte Carlo-generated electron tracks for liquid water were used to model energy deposition. Chemical evolution of a track and interactions between species and DNA following water radiolysis were modelled in an encounter-controlled manner. The effects of varying the scavenging capacity of the environment, the extent of DNA protection by histones and the setting of a threshold for direct energy depositions on DNA break yields were all systematically studied. DSB complexity was assessed in terms of DNA breaks and base damage accompanying a DSB. Model parameters were adjusted to make predictions consistent with experimental data on DSB yields and yield modification by a variety of factors including changing DNA conformation and incorporation of scavengers. An embedded model of nucleosomal DNA by histones was required to explain experimentally observed modification of DSB yield by removal of bound histones. Complex DSB, defined as DSB accompanied by an additional strand breakage, exhibited high association with base damage. It is shown that hydroxyl radical interactions with bases are a major contributor to DSB complexity. On average there were 1.15 and 2.69 OH-base interactions accompanying simple and complex DSB, respectively for 137Cs. Over 80% of complex DSB had at least one OH-base interaction associated with a DNA break.

Interactions of Low-Energy Electrons with Condensed Matter: Relevance for Track Structure

Here we review the state of knowledge about the transport of subexcitation electrons in water. Longitudinal optical phonon and acoustical phonon interactions with an electron added to the system can give rise to an increase in the effective mass of the electron and to its damping. We generalize the pioneering treatment of thermalization given by Frolich and Platzman to apply to both long-range, polarization-type interactions, and as well to short-range interactions of low-energy electrons in water. When an electron is ejected from a molecule in condensed matter quantum interference may take place between the various excitation processes occuring in the medium. We have estimated the magnitude of this interference effect, together with the effect of Coulombic interactions on the thermalization of subexcitation electrons generated in the vicinity of a track of positive ions in water.

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.

Dose Calculations for Si-SiO2Si Layered Structures Irradiated by X Rays and 60Co Gamma Rays

A Monte Carlo calculation of dose deposition in a layered Si-SiO2-Si structure is described. Results are given for dose distributions for 250-, 500-, and 5000-Å SiO2 thicknesses and for irradiation by 8-keV x rays and 60Co gamma rays. Spatial distributions of holes in the 500-Å SiO2 layer are presented.

Radiation damage to a biomolecule: New physical model successfully traces molecular events

For the first time, a complete computer simulation of physical and chemical reactions at the molecular level has been used to calculate the yield of a chemical species resulting from irradiation of a biological molecule in aqueous solution. Specifically, when a solution of glycylglycine is irradiated anaerobically, an ammonia molecule is released by the action of a hydrated electron, which is produced by irradiation of water. In the computations, Monte Carlo techniques are used to simulate the statistical progression of molecular events as they are assumed to occur. These include the initial physical ionization and excitation of water molecules along a particle track in the liquid; the subsequent formation of free radicals and other species: and the random diffusion and chemical reactions of the species with each other, the solvent, and solute molecules. We have calculated and measured the yield of ammonia from irradiation of glycylglycine with 250 kVp x-rays as a function of glycylglycine concentration between 0.01 and 1.2 M. Excellent agreement is obtained between predicted and measured results. The literal simulation of events, combined with specific experimental measurements, offers a powerful new tool for studying mechanisms of radiation action and damage at the molecular level.