Radiation-induced Conductivity Research Articles

Radiation induced changes in electrical conductivity of chemical vapor deposited silicon carbides under fast neutron and gamma-ray irradiations

The radiation-induced changes in the volume electrical conductivities of chemical vapor deposited silicon carbides (CVD-SiCs) were in-site investigated by performing irradiation using 1.17 and 1.33-MeV gamma-ray and 14-MeV fast neutron beams in air and vacuum. Under gamma-ray irradiation at ionization dose rates of 3.6 and 5.9 Gy/s and irradiation temperature of approximately 300 K, the initial rapid increase in electrical conductivity; this is indicative of radiation-induced conductivity (RIC), occurred due to electronic excitation, and a more gradual increase followed up to a dose of approximately 10–50 kGy corresponding to the results in base conductivity without radiation; this is indicative of radiation-induced electrical degradation (RIED). However, the radiation-induced phenomena were not observed at irradiation temperatures above 373 K. Under neutron irradiation at a further low dose rate below approximately 2.1 Gy/s, a fast neutron flux of 9.2 × 10 14 n/m 2 s, and 300 K, the RIED-like behavior according to radiation-induced modification of the electrical property occurred with essentially no displacement damage, but ionizing effects (radiolysis).

The role of stable free radicals in the radiation-induced conductivity of low-density polyethylene

The radiation-induced conductivity (RIC) of low-density polyethylene (LDPE) under continuous irradiation with fast electrons (50 keV) was experimentally studied. The dose dependence of the concentration of stable paramagnetic centers was determined. The kinetics of RIC in LDPE was calculated on the basis of the Rose-Fowler-Vaisberg (RFV) model taking into account the buildup of radiation-induced traps. Good correlation between the experimental results and calculated data was found.

Terahertz radiation-induced conductivity, Kerr and Faraday angles, and spin textures in a two-dimensional electron gas with spin-orbit coupling subjected to a high magnetic field and periodic potential

The terahertz radiation-induced conductivity and dielectric polarization tensors as well as the Faraday and Kerr rotation angles and the non-equilibrium spin textures are studied for two-dimensional electron gas with strong spin-orbit coupling subjected to high magnetic field and to tunable periodic potential of a two-dimensional gated superlattice. It is found that both real and imaginary parts of the frequency-dependent induced conductivity approach maximum values with sharp and detectable peaks at frequencies corresponding to the inter-subband transitions between spin-split magnetic subbands. The observed properties of the conductivity tensor frequency dependence are applied for the description of the Kerr and Faraday rotation angles which can be used as another experimental tool for describing the electron gas in periodic structures with significant spin-orbit coupling. The formation of radiation-induced spin textures is predicted having both in-plane and out-of-plane components with space distribution scale comparable to the superlattice cell size which can be observed experimentally.

Radiation-induced changes in electrical and proton conductivities of BaCe 0.9Y 0.1O 3 − δ upon irradiation by electron beams

The radiation-induced changes in the electrical and proton conductivities of hydrogen (H or D)-doped BaCe 0.9Y 0.1O 3 − δ were investigated by performing irradiation using 1.8-MeV electron beams at ionization dose rates of 10–1000 Gy/s and an irradiation temperature of 473 K. For dose rates below 300 Gy/s, radiation-induced conductivity (RIC) occurred due not only to electronic excitation but also to the increased hydrogen diffusion caused by the ionizing effects of radiation. For dose rates above 300 Gy/s, the base conductivity and RIC were gradually reduced as the irradiation dose increased up to a dose of approximately 8.0 MGy; this is indicative of radiation-induced electrical degradation (RIED). The dependence of RIC and RIED on the dose is not affected by the presence of doped hydrogen. Radiation-enhanced diffusion of the constituent oxygen in BaCe 0.9Y 0.1O 3 − δ not only leads to the occurrence of RIC and RIED but also suppresses the migration of protons activated by the ionizing effects of radiation. This suppression occurs because of the formation of Ba and Ce hydroxides and/or hydrides on the surface of the irradiated samples; the formation of these compounds is revealed by XPS and optical absorption spectra.

Radiation effects in single-walled carbon nanotube papers

The effects of ionizing radiation on the temperature-dependent conductivity of single-walled carbon nanotube (SWCNT) papers have been investigated in situ in a high vacuum environment. Irradiation of the SWCNT papers with 4.2MeV alpha particles results in a steady decrease in the SWCNT paper conductivity, resulting in a 25% reduction in room temperature conductivity after a fluence of 3×1012 alpha particles/cm2. The radiation-induced temperature-dependent conductivity modification indicates that radiation damage causes an increase in the effective activation barrier for tunneling-like conductivity and a concomitant increase in wavefunction localization of charge carriers within individual SWCNTs. The spatial defect generation within the SWCNT paper was modeled and confirms that a uniform displacement damage dose was imparted to the paper. This allows the damage coefficient (i.e., differential change in conductivity with fluence) for alpha particles, carbon ions, and protons to be compared with the corresponding nonionizing energy loss (NIEL) of the incident particle. The resulting nonlinear relationship with NIEL between these parameters is distinct from the more typical linear response observed in many bulk semiconductors and superconductors and indicates that localized radiation damage in the SWCNT papers has a greater impact than distributed damage. Although SWCNT papers behave largely as a bulk material with properties that are a convolution of the underlying SWCNT distribution, the radiation response appears to be largely dominated by degradation in the preferred one-dimensional conduction within these two-dimensionally confined nanostructures.

Effects of ionization on migration of hydrogen doped into BaCe 0.9Y 0.1O 3 − δ

The effects of radiation on the electrical and proton conductivities of hydrogen (H)-doped BaCe 0.9Y 0.1O 3 − δ were studied by performing irradiation using 1.8-MeV electron beams at ionization dose rates of either 10 or 1000 Gy/s. The base conductivity ( σ BC) of BaCe 0.9Y 0.1O 3 − δ became higher under electron irradiation that is indicative of radiation-induced conductivity (RIC; σ RIC). The difference between RIC and the base conductivity, ΔRIC ( σ RIC − σ BC), at an ionization dose rate of 10 Gy/s increased as the irradiation temperature increased from 298 to 623 K. It was found that the ΔRIC value for H-doped BaCe 0.9Y 0.1O 3 − δ only at 10 Gy/s and 473 K was approximately one order of magnitude higher than that of nondoped BaCe 0.9Y 0.1O 3 − δ . The results indicate that the radiation-induced conduction occurs not only due to electronic excitation but also due to hydrogen diffusion increased by the ionizing effects of radiation. The RIC, ΔRIC, and base conductivity of samples irradiated at 10 Gy/s and 623 K and at 1000 Gy/s and 473 K gradually reduced as the irradiation time increased; this is indicative of radiation-induced electrical degradation (RIED). The dependence of the conductivities on the irradiation time is not affected by the presence of doped hydrogen. Radiation-enhanced diffusion of the constituent oxygen not only leads to the occurrence of RIED but also suppresses the migration of protons activated by the ionizing effects of radiation.

Geant 4-based calculation of electric field in deep dielectric charging

Based on Geant4 particle transfer simulation, the internal current density, dose rate and charge deposition profiles of Teflon are acquired, and then the electric field under high energy and low density e-beam radiation is calculated with radiation-induced conductivity model of charging dynamics, which includes the continuity equation, Poisson equation and deep trap equation. According to Geant4 package, the charging model of dielectric is simplified as a compound structure consisting of shielding aluminum and multiple-layer plates radiated by 1.0MeV energy and 0.1 pA/cm2 flux density surface source. With statistical method, the distributions of internal current density, dose rate and charge deposition are obtained and normalized with the above flux density. Substituting these results into RIC model, the distributions of electric field in Teflon are calculated under various grounding conditions. It shows that the charge density, electric field and potential for the back side grounding are higher than these for the front side grounding, which tends to induce internal discharging. The electric field calculated for the critical internal discharging conditions in space is also consistent with the threshold field of dielectric Teflon(1e7V/m).

Dose rate dependence of radiation induced conductivity for hydrogen-doped perovskite ceramics at 473 K

Dose rate dependence of radiation induced conductivity (RIC) of hydrogen doped yttrium-doped perovskite-type barium-cerium oxide ceramics (BaCe 0.9Y 0.1O 3−δ) was investigated under 1.8 MeV electron irradiation within dose rate of 10–1000 Gy/s at temperature of 473 K. The RIC in the initial dose for the H-doped BaCe 0.9Y 0.1O 3−δ exponentially increased as a function of dose rate, and eventually became about three orders of magnitude higher than the base conductivity without radiation. For the dose rates below 200 Gy/s, the RIC of the H-doped BaCe 0.9Y 0.1O 3−δ was higher by one order of magnitude than that of the H-undoped one. The results may show that the RIC takes place due to hydrogen diffusion as well as electronic excitation, enhanced by ionizing effects. For the dose rate range 300–1000 Gy/s, the RIC of H-doped BaCe 0.9Y 0.1O 3−δ is similar to that of H-undoped one. The radiation enhanced diffusion of constitutive oxygen dominates for the RIC.

Time-dependent charge distributions in polymer films under electron beam irradiation

The time-dependent charge distribution in polymer film under electron beam irradiation is studied by both experiment and numerical simulation. In the experiment, the distribution is measured with the piezoinduced pressure wave propagation method. In the simulation, the initial charge distribution is obtained by the Monte Carlo method of electron scattering, and the charge drift in the specimen is simulated by taking into account the Poisson equation, the charge continuity equation, Ohm’s law, and the radiation-induced conductivity. The results obtained show that the negative charge deposited in the polymer film, whose top and bottom surfaces are grounded, drifts toward both grounded electrodes and that twin peaks appear in the charge distribution. The radiation-induced conductivity plays an important role in determining the charge distribution in the polymer films under electron beam irradiation.

Electron-beam charging of polytetrafluoroethylene

The charging of polytetrafluoroethylene films by irradiation with fast electrons having an energy insufficient for the complete penetration through the entire thickness of the polymer was theoretically analyzed. The calculation results take into account the distribution of both the dose rate and the bulk electron thermalization rate over the film thickness in the approximation of prescribed radiation-induced conductivity of polytetrafluoroethylene. The influence of the parameters of radiation-induced conductivity on the charging behavior and on the time dependence of the transient current detected with the use of the split Faraday cup was examined. It was found that the observed transient current inversion is adequately explained with allowance for the specifics of radiation-induced conductivity in polytetrafluoroethylene.

Analysis of internal charge distribution in electron irradiated polyethylene and polyimide films using a new experimental method

The charge distribution in low density pyethylene and polyimide films under a quasi-mono-energetic electron beam irradiation in the range [5–400 keV] is analysed using a new experimental set-up based on the pulsed electro-acoustic (PEA) method. The device allows measurements without any physical contact between the excitation electrode used in the PEA method and the irradiated surface which remains at a floating potential during the entire duration of the experiment. This special configuration makes it possible to study bulk as well as interface phenomena. In particular, emission of charges from the irradiated surface to the vacuum is in evidence as well as charge injection from the ground electrode to the bulk. The time-dependent charge distribution during irradiation is approached by combining a Monte Carlo toolkit for modelling radiation transport in matter in conjunction with a macroscopic description of charge transport based on the concept of a radiation-induced conductivity generated by the injected electrons. Although the macroscopic description yields a fair agreement with the experimental data, it fails to depict the detailed shape of the charge distribution, thereby calling for a microscopic, bi-polar model.

Deep Dielectric Charging of Spacecraft Polymers by Energetic Protons

The majority of research in the field of spacecraft charging concentrates on electron charging effects with little discussion of charging by protons. For spacecraft orbiting in the traditional LEO and GEO environments, this emphasis on electrons is appropriate since energetic electrons are the dominant species. However, for spacecraft in orbits within the inner radiation belts, or for interplanetary and lunar space probes, proton charging effects may also be of concern. To examine bulk spacecraft charging effects in these environments, several typical highly insulating spacecraft polymers were exposed to energetic protons with energies from 1 to 10 MeV to simulate protons from the solar wind and from solar energetic proton events. Results indicate that effects in proton-charged dielectrics are distinctly different than those observed due to electron charging. In most cases, the positive surface potential continued to increase for periods on the order of minutes to a day, followed by long-time-scale decay at rates similar to those observed for electron charging. All samples charged to positive potentials, with substantially lower magnitudes than for equivalent electron fluence. Possible explanations for the different behavior of the measured surface potentials from proton irradiation are discussed; these are related to the evolving internal charge distribution from energy-dependent electron and proton transport, electron emission, charge migration due to dark current and radiation-induced conductivity, and electron capture by embedded protons.

Theoretical description of the prompt component of pulsed radiation-induced conductivity in polymers in terms of the ion pair mechanism of radiolysis

The properties of the prompt component of radiation-induced conductivity in polymers was theoretically analyzed on the basis of numerical solution of the modified Smoluchowski equation. The time dependences for the concentration of trapped electrons and the geminate-pair current after δ-pulse irradiation were calculated. In addition, calculation for a pulse of a finite duration was performed. The field dependence of the prompt conductivity was determined: deviations from the Ohmis law begin at field strengths on the order of 107 V/m, the character of this dependence strongly varying with a decrease in the electron thermalization length.

Reduction of radiation-induced conductivity in poly(ethylene terephthalate): Effect of dopant structure

This paper focuses on developing radiation tolerant polymeric films by incorporating small molecule dopants into the material formulation. The radiation tolerance is imparted to the polymer by dopants capturing the mobile electrons generated upon radiation exposure. A wide range of conjugated small molecules was utilized for the reduction of radiation-induced conductivity (RIC) in semi-crystalline poly(ethylene terephthalate). Using these results, the process of electron and hole trapping by π-conjugated molecules may be generalized to many organic systems. All conjugated ring cores examined, when substituted with an appropriate electron-withdrawing species, led to excellent RIC reduction. The nitro group is the strongest electron-withdrawing substituent, and therefore leads to the lowest RIC, but other substituents also reduce RIC, and the interactions may be quantified by the Hammett parameter. It was also found that the dopant must be present in a certain range of concentrations, between 10 and 200mol/m3 (6×1018 and 1×1020molecules/cm3), for proper trapping of the photo-induced electron-hole pairs. This understanding of the interactions between conjugated dopants, photons, and electrons will benefit other applications, such as controlled conductivity in coatings and organic electronics.

Dose effects in radiation-induced conductivity of polypyromellitimide

Dose effects in radiation-induced conductivity of polypyromellitimide were numerically simulated in terms of the Rose-Fowler-Vaisberg model with allowance for bipolar carrier transport and the generation of radiation traps during irradiation. The reasons for the lack of a noticeable dose effect in this polymer upon pulse irradiation in light of its presence in the case of continuous irradiation are discussed.

10.1007/s11498-008-2017-8

Numerical simulation of dose effects in radiation-induced conductivity in polymers and of roomtemperature annealing of these effects was performed in the formalism of the generalized Rose-Fowler-Vaisberg model, which allows for bipolar electron transport of charge carriers and for generation of radiationinduced traps. It was shown that neither the distinct dose effect nor its long annealing time can be explained unless the radiation-chemical aspect of irradiation is taken into account. It was found that satisfactory fit of theoretical prediction to published data for a number of polymers (PS, PET, LDPE, polyvinylcarbazole) can be achieved in the case of the reasonable choice of the parameter for the injection of radiation-induced traps (free radicals), although certain difficulties are met. This coincidence is attainable if the other parameters of the model are fixed and their values have been determined in independent experiments.

Radiation tolerant polymeric films through the incorporation of small molecule dopants in the polymer matrix

Radiation induced conductivity (RIC) in semicrystalline polyethylene terephthalate (PET) films can be reduced by incorporating small molecule electron traps into the polymer. The electron traps contained an aromatic core with strong electron withdrawing functionality pendant to the core and were incorporated into the PET film by immersing the polymer in a solution of dopant and solvent at elevated temperatures. The chemical functionality of the electron trapping molecule and the number of pendant functional groups had a strong impact on the equilibrium doping level and the most effective doping solvent. In addition, all of the electron traps exhibited effectiveness at reducing the RIC. The technique of incorporating small molecule dopants into the polymer matrix in order to reduce the RIC can potentially be exploited with other polymers films and coatings utilized in electronics devices such as encapsulants, conformal coatings, and polymeric underfills.

Research on Simulation of Deep Charging for Spacecraft Dielectrics

针对航天器介质深层充电问题,提出了一种基于蒙特卡罗模拟和充电动力学RIC模型的介质电荷分布及电场预估新方法,利用地面试验验证了其正确性.航天器介质平板充电过程被简化为屏蔽铝板与分层介质组成的Geant 4模型,通过统计方法计算出了实际入射束流下Teflon介质内的注入电流密度和剂量率分布曲线,利用RIC模型获得了背面接地时介质中的电荷密度和电场分布,利用脉冲电声法(PEA)对不同束流密度辐照下的Teflon内部空间电荷密度进行了测量.数值模拟和地面试验结果表明,Teflon在100 keV能量电子辐照下,电荷密度和电场随着束流密度的增加而不断增大,其电荷密度峰值位置约为0.042 mm,且背面接地时接地侧电场最大.由于Geant 4粒子输运模拟和RIC模型具有通用性,因此该方法适用于各种航天器介质材料.

A theoretical description of geminal recombination and radiation-pulse-induced electric conduction in polymers in the approximation of probability of survival of ionic pairs

An approximate analytical theory of geminal recombination and geminal radiation-induced electric conduction of polymers is developed. The theory is based on an ion-pair mechanism of radiolysis and the concept of dispersion of electron transport and is valid at fairly strong fields and not overly low temperatures. The proposed approach was used to perform a preliminary analysis of the published data on pulsed-radiation-induced electroconductivity of low-density polyethylene.

Effect of dose on radiation-induced conductivity in polymers

Numerical simulation of radiation-induced conductivity in polymers upon long-term irradiation on the basis of the generalized Rose-Fowler-Vaisberg model, which allows for both dipolar carrier transport and generation of radiation traps during irradiation, was performed. The unusual properties of radiation-induced conductivity, such as the appearance of a maximum on current transients, the absence of a steady state, and a substantial difference between these curves for the first and subsequent irradiation, are rationalized in terms of the formation of free radicals, the major feature of radiolysis in the chemical aspect. This interpretation does not require the involvement of degradation or crosslinking processes, unlike other interpretations that appear in the literature. With the use of low-density polyethylene as an example, it was shown that radiation-induced conductivity both upon pulse and continuous irradiation can satisfactorily be described with the unified set of parameters of the generalized Rose-Fowler-Vaisberg model.