Measurements of the Temperature Dependence of Radiation Induced Conductivity in Polymeric Dielectrics

This study measures Radiation Induced Conductivity (RIC) in five insulating polymeric materials over temperatures ranging from ~110 K to ~350 K: polyimide (PI or Kapton HNTM and Kapton ETM), polytetraflouroethylene (PTFE or TeflonTM), ethylene-tetraflouroethylene (ETFE or TefzelTM), and Low Density Polyethylene (LDPE). RIC occurs when incident ionizing radiation deposits energy and excites electrons into the conduction band of insulators. Conductivity was measured when a voltage was applied across vacuum-baked, thin film polymer samples in a parallel plate geometry. RIC was calculated as the difference in sample conductivity under no incident radiation and under an incident ~4 MeV electron beam at low incident dose rates of 0.01 rad/sec to 10 rad/sec. The steady-state RIC was found to agree well with the standard power law relation, σRIC(D) = kRIC(T) DÄ(T) between conductivity, óRIC and adsorbed dose rate, D. Both the proportionality constant, kRIC, and the power, Ä, were found to be temperature-dependent above ~250 K, with behavior consistent with photoconductivity models developed for localized trap states in disordered semiconductors. Below ~250 K, kRIC and Ä exhibited little change in any of the materials

Density of State Models and Temperature Dependence of Radiation Induced Conductivity

Expressions are developed for radiation induce conductivity (RIC) over an extended temperature range, based on density of states models for highly disordered insulating materials. A general discussion of the DOS of can be given using two simple types of DOS distributions of defect states within the bandgap for disordered materials are considered, one that monotonically decreases within the bandgap and one with a distribution peak within the band gap. Three monotonically decreasing models (exponential, power law, and linear), and two peaked models (Gaussian and delta function) are considered, plus limiting cases with a uniform DOS for each type. Variations using the peaked models are considered, with an effective Fermi level between the conduction mobility edge and the trap DOS, within the peaked trap DOS, and between the trap DOS and the valence band. The models are compared to measured RIC values over broad temperature ranges for two common materials, low density polyethylene (LDPE) and disordered silicon dioxide

Materials Characterization at Utah State University: Facilities and Knowledgebase of Electronic Properties of Materials Applicable to Spacecraft Charging

In an effort to improve the reliability and versatility of spacecraft charging models designed to assist spacecraft designers in accommodating and mitigating the harmful effects of charging on spacecraft, the NASA Space Environments and Effects (SEE) Program has funded development of facilities at Utah State University for the measurement of the electronic properties of both conducting and insulating spacecraft materials. We present here an overview of our instrumentation and capabilities, which are particularly well suited to study electron emission as related to spacecraft charging. These measurements include electron-induced secondary and backscattered yields, spectra, and angular resolved measurements as a function of incident energy, species and angle, plus investigations of ion-induced electron yields, photoelectron yields, sample charging and dielectric breakdown. Extensive surface science characterization capabilities are also available to fully characterize the samples in situ. Our measurements for a wide array of conducting and insulating spacecraft materials have been incorporated into the SEE Charge Collector Knowledgebase as a Database of Electronic Properties of Materials Applicable to Spacecraft Charging. This Database provides an extensive compilation of electronic properties, together with parameterization of these properties in a format that can be easily used with existing spacecraft charging engineering tools and with next generation plasma, charging, and radiation models. Tabulated properties in the Database include: electron-induced secondary electron yield, backscattered yield and emitted electron spectra; He, Ar and Xe ion-induced electron yields and emitted electron spectra; photoyield and solar emittance spectra; and materials characterization including reflectivity, dielectric constant, resistivity, arcing, optical microscopy images, scanning electron micrographs, scanning tunneling microscopy images, and Auger electron spectra. Further details of the instrumentation used for insulator measurements and representative measurements of insulating spacecraft materials are provided in other Spacecraft Charging Conference presentations. The NASA Space Environments and Effects Program, the Air Force Office of Scientific Research, the Boeing Corporation, NASA Graduate Research Fellowships, and the NASA Rocky Mountain Space Grant Consortium have provided support

Temperature Dependence of Radiation Induced Conductivity in Insulators

We report on measurements of Radiation Induced Conductivity (RIC) of thin film Low Density Polyethylene (LDPE) samples. RIC occurs when incident ionizing radiation deposits energy in a material and excites electrons into conduction states. RIC is calculated as the difference in sample conductivity under an incident flux and “dark current” conductivity under no incident radiation. The primary focus of this study is the temperature dependence of the steady state RIC over a wide range of absorbed dose rates, from cryogenic temperatures to well above room temperature. The measured RIC values are compared to theoretical predictions of dose rate and temperature dependence based on photoconductivity models developed for localized trap states in disordered semiconductors. We also investigated the variation of RIC as a function of material, applied electric field, and incident beam energy parameters

Secondary Electron Emission Study of Annealed Graphitic Amorphous Carbon

In the 1880s a curious phenomena was observed: when a ray of light, no matter how weak, hit certain metals; electrons were emitted from the surface. Called the “photoelectric effect”, this puzzle was never explained until much later. In 1905, Albert Einstein put forth one possible explanation, which is currently accepted as correct. Einstein proposed that light propagated in discrete energy packets rather than as a continuous wave. While most scientists disbelieved Einstein theory, it was later proved in detail by Robert Milikan.1 Rays of light traveling in discrete packets hit metal surfaces, depositing energy. If the energy is high enough, electrons will be emitted. Since emission is only dependant on incident energy, electrons and even ions may also be used to deposit energy. Electrons emitted as a result of this energy deposition are called secondary electrons (SE)

Secondary Electron Emission of Annealed Graphitic Amorphous Carbon

Abstract. We prove that, for every integer k ≥ 2, every graph has an edge-partition into 5k 2 log k sets, each of which is the edge-set of a graph with all degrees congruent to 1 mod k. This answers a question of Pyber. Pyber [8] proved that every graph G has an edge-partition into four sets, each of which is the edge set of a graph with all degrees odd; if every component of G has even order then three sets will do. This is best possible, as can be seen by considering K5 with two independent edges removed, which cannot be partitioned into fewer than four subgraphs with all degrees odd; and K4 with one edge removed, which requires three. Motivated by this result, Pyber [8] asked what happens when we consider residues mod k, rather than mod 2. In particular he asked whether for every integer k there is an integer c(k) such that every graph has an edge-partition into at most c(k) sets, each of which is the edge-set of a 1 graph with all degrees congruent to 1 mod k. The following theorem answers this question. Theorem 1. For every integer k ≥ 2 and every graph G there is a partition of E(G) into at most 5k 2 log k sets, each of which is the edge set of a graph with all degrees congruent to 1 modulo k. In order to prove this, we will need to show that if a graph has sufficiently large average degree then it must contain a nonempty subgraph with all degrees divisible by k. The simplest such subgraph would be a k-regular graph, but Pyber, Rödl and Szemerédi [9] have shown that a graph can have as many as n log log n edges without containing a k-regular subgraph. We will instead make use of a standard result about subgraphs with all degrees divisible by k (see [1]), which we prove for the sake of completeness. We use the following theorem of van Emde Boas and Kruyswijk ([4]; see also [6], [2], [1]) and Meshulam [5]. Theorem A. Let G be a finite abelian group and let m = m(G) be the maximal order of elements of G. Then for every sequence a1,..., as with s ≥ m 1 + log |G

Density of State Models of Steady-State Temperature Dependent Radiation Induced Conductivity

Radiation induced conductivity (RIC) occurs when incident radiation deposits energy and excites electrons into the conduction band of insulators. The magnitude of the enhanced conductivity is dependent on a number of factors including temperature and the spatial- and energy-dependence and occupation of the material’s distribution of localized trap states within the band gap—or density of states (DOS). Expressions are developed for steady-state RIC over an extended temperature range, based on DOS models for highly disordered insulating materials. A general discussion of the DOS of disordered materials can be given using two simple distributions: one that monotonically decreases below the band edge and one that shows a peak in the distribution within the band gap. Three monotonically decreasing models (exponential, power law, and linear), and two peaked models (Gaussian and delta function) are developed, plus limiting cases with a uniform DOS for each type. Variations using the peaked models are considered, with an effective Fermi level between the conduction mobility edge and the trap DOS, within the peaked trap DOS, and between the trap DOS and the valence band. Explicit solutions, limiting cases, and applications of the models to RIC measurements are presented