M.J.G. Lee

Irradiation of gold nanoparticles by x‐rays: Monte Carlo simulation of dose enhancements and the spatial properties of the secondary electrons production

PURPOSE The aim of this study is to understand the characteristics of secondary electrons generated from the interaction of gold nanoparticles (GNPs) with x-rays as a function of nanoparticle size and beam energy and thereby further the understanding of GNP-enhanced radiotherapy. METHODS The effective range, deflection angle, dose deposition, energy, and interaction processes of electrons produced from the interaction of x-rays with a GNP were calculated by Monte Carlo simulations. The GEANT4 code was used to simulate and track electrons generated from a 2, 50, and 100 nm diameter GNP when it is irradiated with a 50 kVp, 250 kVp, cobalt-60, and 6 MV photon beam in water. RESULTS When a GNP was present, depending on the beam energies used, secondary electron production was increased by 10- to 2000-fold compared to an absence of a GNP. Low-energy photon beams were much more efficient at interacting with the GNP by two to three orders of magnitude compared to MV energies and increased the deflection angle. GNPs with larger diameters also contributed more dose. The majority of the energy deposition was outside the GNP, rather than self-absorbed by the nanoparticle. The mean effective range of electron tracks for the beams tested ranged from approximately 3 microm to 1 mm. CONCLUSIONS These simulated results yield important insights concerning the spatial distributions and elevated dose in GNP-enhanced radiotherapy. The authors conclude that the irradiation of GNP at lower photon energies will be more efficient for cell killing. This conclusion is consistent with published studies.

Periodic field-dependent photocurrent from a tungsten field emitter☆

Abstract We report the observation of a periodic electric-field-dependent component in the photo-current from a tungsten field emitter illuminated by the focused beam of a krypton ion laser tuned to a photon energy of ~3.51 eV. Over the field range we have investigated, the phase φ of the oscillations in the photocurrent can be related to the electric field strength F by the power law φ ∝ Fα, where α ≈ − 1 2 . In this preliminary study, we describe the frequency, amplitude, and harmonic structure of oscillations detected with the probe hole centred over the (510) region of the field emission pattern. A Fowler-Nordheim plot of the photocurrent is presented, and the nonoscillatory component is analysed on the basis of a simple theory of barrier penetration. The origin of the oscillatory component of the photocurrent is discussed.

Photofield emission spectroscopy of surface electronic states of tungsten

Abstract A novel photofield emission spectrometer has been used to observe departures from free electron behaviour in the total energy distributions of photofield emission from the (100), (110), (111), (211), and (510) facets of a tungsten field emitter. Measurements with p-polarized light at grazing incidence at seven photon energies in the visible and near ultraviolet show departures from free electron behaviour having the same form as those observed in field emission distributions, but shifted to higher energy by one photon energy. The shapes and polarization dependence of the distributions are precisely those expected for surface photoexcitation from electron states near the surface to vacuum tunnelling final states. These observations provide strong evidence that the surface photoeffect is the dominant mechanism of excitation by p-polarized light in photofield emission from tungsten.

Temperature dependence of the optical response: Application to bulk GaAs using first-principles molecular dynamics simulations

A novel approach has been developed to calculate the temperature dependence of the optical response of a semiconductor. The dielectric function is averaged over several thermally perturbed configurations that are extracted from molecular dynamic simulations. The calculated temperature dependence of the imaginary part of the dielectric function of GaAs is presented in the range from 0 to 700 K. This approach that explicitly takes into account lattice vibrations describes well the observed thermally-induced energy shifts and broadening of the dielectric function.

Measurement of the total energy distribution in photo-induced field emission☆

Abstract Accurate measurements of the total energy distribution of excited electrons in photoinduced field emission are needed to determine the mechanism of photo-excitation. Experimental measurements of the total energy distributions involve a correction for instrumental broadening introduced by the energy analyzer. The usual assumption of a symmetric resolution function of gaussian form is not always adequate for a retardation-type energy analyzer. We describe a method, which involves measurements of the total energy distribution in direct field emission, whereby the instrumental broadening can be deconvoluted from the total energy distribution observed in photo-induced field emission. The method also yields an accurate determination of the total energies of photoemitted electrons relative to the Fermi level. The method is illustrated by applying it to deconvolute total energy distributions observed in a preliminary study of the shape of the photopeak in photo-induced field emission.

Polarization dependence of photoexcitation in photofield emission

Abstract The mechanisms of one-photon photoexcitation in photofield emission have been studied experimentally by measuring the photocurrent from selected crystallographic planes of a tungsten field emitter as a function of the polarization of the incident light. Two distinct components of the photocurrent were identified. The corresponding excitation mechanisms were deduced from the results of the polarization measurements supplemented by band structure considerations. Surface effect emission dominates at large angles of light incidence, and is observed even from within the geometric shadow of the tip. It depends on the light polarization inside the metal in a characteristic way. Indirect bulk photoemission is prominent at small angles of light incidence and is independent of the polarization inside the metal. It is concluded that bulk and surface excitations in photofield emission can be distinguished by appropriately selecting the direction of polarization and the angle of incidence of the incident light.

Linear intensity dependence of photo-induced field emission from tungsten

Abstract As a first step in an investigation of the polarization dependence of photo-induced field emission from tungsten, the dependence of the photocurrent on the intensity of illumination has been studied. The beam from a CW krypton ion laser operating at selected wavelengths in the visible region of the spectrum was focussed onto the field emitter, and the emission current in a narrow energy range (chosen to minimize tip heating effects) was measured as a function of intensity. The apparatus was designed to ensure that the position of the focal spot and the intensity of illumination could be adjusted without mutual influence. The photocurrent from each crystal plane was found to be linearly proportional to the luminous intensity over the range of intensity investigated (0–35 MW m −2 ). It is concluded that in tungsten there exists a low intensity regime where the external current in photo-induced field emission is caused predominantly by one-photon excitations.

SU‐FF‐J‐152: Dosimetry On Gold Nanoparticle: A Microscopic and Macroscopic Study Using Monte Carlo Simulations

Purpose: This study investigated dosimetric characteristics of goldnanoparticle under a photon beam. A single goldnanoparticle (microscopic) and a mixture of gold and water (macroscopic) were considered using Monte Carlo simulations based on the Geant4‐ and EGSnrc‐based code, respectively. Methods and Materials: A single goldnanoparticle (diameter of 100 nm) was irradiated by photon beams with energies of 35.5keV, 73.3keV, 660keV, 1.2MeV and 6MeV in water. 250 million histories were used in Monte Carlo simulation to record different numbers of interactions (e.g. photoelectric and Compton) with and without the goldnanoparticle in water. Moreover, a mixture of gold and water was irradiated with photon beams. The dose enhancement ratios (dose of gold and water mixture/dose of water) were determined with different photon beam energies and concentrations of gold.Results: With a single goldnanoparticle, the number of photoelectric interaction was about 47 times larger than that of Compton for the 35.5keV photon beams. This was opposite to the 6MeV photon beams, where the number of Compton interaction was about 46.5 times larger than that of the photoelectric. Although the values of ratio were similar, the total number of interactions for the 35.5keV photon beams was in fact 348 times larger than that of the 6MeV. A larger energy deposition was therefore found when the photon beam energy was decreased from the MeV to keV range. This result agreed with that from a gold and water mixture. Moreover, increasing the concentration of gold increased the dose enhancement in water.Conclusions: Both microscopic and macroscopic study on goldnanoparticle agree that more energy deposition is found when the photon beam energy is decreased from MeV to keV range, due to the increase of photoelectric interaction. A higher concentration of gold can increase the dose enhancement in water.

Emission parameters of low-index surfaces by a combined field and photofield emission method

A new method is proposed to determine independently the work function and the field factor of selected low-index surface planes of a field emitter. The field dependences of the total emission current are measured both in field emission and in photofield emission, and the resulting Fowler-Nordheim plots are analyzed by means of an iterative procedure. Experimental measurements of the emission current from several low-index planes of tungsten show that reliable results are obtained so long as the surface is illuminated at a sufficiently large angle of incidence. Unexplained irreproducible behaviour is observed when the angle of incidence is small.

Thermal field emission and thermal photofield emission from tungsten (110)

Abstract The total energy distibution in field emission from the (110) surface of tungsten has been measured at temperatures in the range from 300 to 1400 K, which are sufficiently high to cause significant emission from thermally populated electronic states. Above the Fermi level, a striking departure from the prediction of free electron theory is observed. A similar anomaly is observed in the total energy distribution in photofield emission from thermally populated initial states on tungsten (110). In s-polarized light at normal incidence the anomaly vanishes when the polarization vector is parallel to the [001] direction. The anomalous behaviour is attributed to emission from the p-like energy band that is responsible for the ellipsoidal sheet of the Fermi surface centred at the symmetry point N. A comparison between the enhancement factors measured in sand p-polarized light yields no evidence for surface resonances close to the upper limit of the p-like energy band.