M. Moscovitch

Simulation of radiation effects on three-dimensional computer optical memories.

A model was developed to simulate the effects of heavy charged-particle (HCP) radiation on the information stored in three-dimensional computer optical memories. The model is based on (i) the HCP track radial dose distribution, (ii) the spatial and temporal distribution of temperature in the track, (iii) the matrix-specific radiation-induced changes that will affect the response, and (iv) the kinetics of transition of photochromic molecules from the colored to the colorless isomeric form (bit flip). It is shown that information stored in a volume of several nanometers radius around the particles track axis may be lost. The magnitude of the effect is dependent on the particles track structure.

Observation of radiation effects on three-dimensional optical random-access-memory materials for use in radiation dosimetry

Abstract The first experimental investigation has been performed of radiation effects on three-dimensional optical random-access-memory materials. Thin films of poly(methyl methacrylate) doped with spirobenzopyran were irradiated with uniform fluxes of protons, α-particles and 12 C +3 ions, at fixed energies per nucleon from 0.5 to 2.5 MeV and fluences from 10 10 to 10 14 cm −2 . The exposed films were examined under a confocal laser scanning microscopy system which is capable of optically sectioning the materials. The irradiation resulted in a permanent change in the materials from a nonfluorescent form to a form which is fluorescent under both 488 and 514 nm excitation. Profiles were measured of fluorescent intensity versus depth, and of intensity versus dose. It was found that both the particle energy and the dose can be obtained from measuring the width of the depth profile and the fluorescent intensity. These properties are very promising for dosimetry applications since they allow calculation of an accurate dose equivalent.

Inelastic cross-sections of energetic protons in liquid water calculated by model dielectric functions and optical data

Abstract Total and differential inelastic inverse-mean-free-paths for the energy loss of protons in liquid water are calculated on the basis of the dielectric approach. Energetic protons, much above the Bragg peak, are considered where the first-Born-approximation is justified (∼0.3–10 MeV). A model dielectric-response-function for the valence electrons of liquid water is constructed in accordance with the general properties of the Bethe-surface. A modified Drude-expansion model developed earlier provided the long-wavelength response, whereas several dispersion schemes were examined for introducing the momentum-transfer dependence. In particular, we have examined the δ-oscillator dispersion models of Ashley and Liljequist, and Ritchie’s extended-Drude models. A comparison with optical dielectric models will also be presented for assessing the influence of the dispersion. In view of the limited experimental information for the inelastic properties of liquid water, the present work provides a comparative study of various – computationally tractable – schemes that may be used in analog Monte Carlo transport codes for protons and heavier ions in liquid water.

Radiation dosimetry using three-dimensional optical random access memories.

Three-dimensional optical random access memories (3D ORAMs) are a new generation of high-density data storage devices. Binary information is stored and retrieved via a light induced reversible transformation of an ensemble of bistable photochromic molecules embedded in a polymer matrix. This paper describes the application of 3D ORAM materials to radiation dosimetry. It is shown both theoretically and experimentally, that ionizing radiation in the form of heavy charged particles is capable of changing the information originally stored on the ORAM material. The magnitude and spatial distribution of these changes are used as a measure of the absorbed dose, particle type and energy. The effects of exposure on 3D ORAM materials have been investigated for a variety of particle types and energies, including protons, alpha particles and 12C ions. The exposed materials are observed to fluoresce when exposed to laser light. The intensity and the depth of the fluorescence is dependent on the type and energy of the particle to which the materials were exposed. It is shown that these effects can be modeled using Monte Carlo calculations. The model provides a better understanding of the properties of these materials. which should prove useful for developing systems for charged particle and neutron dosimetry/detector applications.

Monte Carlo study of electron-beam penetration and backscattering in multi-walled carbon nanotube materials: The effect of different scattering models

A systematic study of electron-beam penetration and backscattering in multi-walled carbon nanotube (MWCNT) materials for beam energies of ∼0.3 to 30 keV is presented based on event-by-event Monte Carlo simulation of electron trajectories using state-of-the-art scattering cross sections. The importance of different analytic approximations for computing the elastic and inelastic electron-scattering cross sections for MWCNTs is emphasized. We offer a simple parameterization for the total and differential elastic-scattering Mott cross section, using appropriate modifications to the Browning formula and the Thomas-Fermi screening parameter. A discrete-energy-loss approach to inelastic scattering based on dielectric theory is adopted using different descriptions of the differential cross section. The sensitivity of electron penetration and backscattering parameters to the underlying scattering models is examined. Our simulations confirm the recent experimental backscattering data on MWCNT forests and, in partic...

Light-induced TL and light-induced fading of TL in α:Al2O3:C

Abstract Experiments were conducted to determine the effect of gamma radiation on the light-induced thermoluminescence (TL) and the light-induced fading of TL in α:Al 2 O 3 :C dosemeters. A high temperature peak, at approx. 330°C, was revealed even at the relatively low dose of 0.05 Gy, with greater resistance to light at certain doses. The light-induced fading of TL, the light-induced TL and the phototransferred TL of α:Al 2 O 3 :C depend greatly on the dose and the “history” of the dosemeters. For low doses, especially when performing environmental measurements, it is recommended to use new dosemeters with a special cover.

Shorter Exposures to Harder X-Rays Trigger Early Apoptotic Events in Xenopus laevis Embryos

Background A long-standing conventional view of radiation-induced apoptosis is that increased exposure results in augmented apoptosis in a biological system, with a threshold below which radiation doses do not cause any significant increase in cell death. The consequences of this belief impact the extent to which malignant diseases and non-malignant conditions are therapeutically treated and how radiation is used in combination with other therapies. Our research challenges the current dogma of dose-dependent induction of apoptosis and establishes a new parallel paradigm to the photoelectric effect in biological systems. Methodology/Principal Findings We explored how the energy of individual X-ray photons and exposure time, both factors that determine the total dose, influence the occurrence of cell death in early Xenopus embryo. Three different experimental scenarios were analyzed and morphological and biochemical hallmarks of apoptosis were evaluated. Initially, we examined cell death events in embryos exposed to increasing incident energies when the exposure time was preset. Then, we evaluated the embryos response when the exposure time was augmented while the energy value remained constant. Lastly, we studied the incidence of apoptosis in embryos exposed to an equal total dose of radiation that resulted from increasing the incoming energy while lowering the exposure time. Conclusions/Significance Overall, our data establish that the energy of the incident photon is a major contributor to the outcome of the biological system. In particular, for embryos exposed under identical conditions and delivered the same absorbed dose of radiation, the response is significantly increased when shorter bursts of more energetic photons are used. These results suggest that biological organisms display properties similar to the photoelectric effect in physical systems and provide new insights into how radiation-mediated apoptosis should be understood and utilized for therapeutic purposes.

Crossed-beam two-photon readout system for three-dimensional radiation dosimeters

Three-dimensional optical random access memory (3D ORAM) materials with enormous capacity and fast access speed have shown a great potential in overcoming limitations of access and storage capacity in current memory devices. As another useful development of this 3D ORAM, we have shown the application of 3D ORAM materials as a practical dosimeter. The local heating of the polymer matrix by the deposited energy of ionizing radiation is thought to contribute to the conversion of the fluorescent photochromic dye to a nonfluorescent form. The two-photon readout system is very useful in tracking the interactions of energy of ionizing radiation deposited in a polymer matrix. However, the polymer fracturing that has occurred during two-photon readout has been an obstacle in utilization of 3D ORAM materials as a dosimeter. In this work, we further evaluated the readout system using a high-energy variable attenuator in order to prevent polymer fracturing due to the strong absorption of the 1064 nm beam by the polym...

The applicability of the PTTL dose re-analysis method to the Harshaw LiF:Mg,Cu,P material

The phototransferred thermoluminescence (PTTL) technique is applied to the Harshaw LiF:Mg,Cu,P material. It is demonstrated that using 254-nm UV light, dose levels as low as 0.2 mGy can be re-estimated. The PTTL efficiency was found to be ∼ 6 % in the dose range of 0.2 mGy-1 Gy, and it appears to be dose-independent. This implies that a simple calibration factor could be applied to the PTTL data for the re-estimation of dose levels. It was demonstrated that with a proper choice of the TL readout parameters, and the UV-light irradiation conditions, dose levels that are relevant to personal or environmental dosimetry can be re-estimated.

The Principles of Phototransferred Thermoluminescence

The principles of phototransferred thermoluminescence (PTTL) are described, and some of the basic theoretical ideas underlying this technique are presented. It is demonstrated that the PTTL efficiency is dependent on the photon energy as well as on the activation energies of the various traps involved in the process. A simple two‐traps‐one‐recombination‐center model is capable of predicting a variety of different PTTL behaviors, some already were observed experimentally.