Joshua L. Hodges

Temperature Dependence of Radiation Induced Conductivity in Insulators

This study measures Radiation Induced Conductivity (RIC) of Low Density Polyethylene (LDPE) over temperatures ranging from ∼110 K to ∼350 K. 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 LDPE 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 fluxes of 10−4–10−1 Gr/sec. The steady‐state RIC was found to agree well with the standard power law relation, σRIC = kRIC⋅DΔ between conductivity, σ and adsorbed dose rate, D. Both the proportionality constant, kRIC, and the power, δ, were found to be temperature dependant above ∼250 K, with behavior consistent with photoconductivity models developed for localized trap states in disordered semiconductors. Below ∼250 K, kRIC and Δ exhibited little change. The observed ...

Charge-Enhanced Contamination and Environmental Degradation of MISSE-6 SUSpECS Materials

The effects of prolonged exposure to the low Earth orbit space environment and charge-enhanced contamination on optical, thermal, and electron emission and transport properties of common spacecraft materials have been investigated by comparing pre- and postflight characterization measurements. The State of Utah Space Environment and Contamination Study (SUSpECS), deployed in March 2008 on board the Materials International Space Station Experiment (MISSE-6) payload, was exposed for ~18 months on the exterior of the International Space Station (ISS) and was retrieved in September 2009. A total of 165 samples was mounted on three separate SUSpECS panels on the ram and wake sides on the ISS. Some samples, particularly those exposed to atomic oxygen in the ram direction, showed pronounced effects due to exposure. Biased samples for the charge-enhanced contamination study showed subtle variations in visible and infrared reflectivity.

In Situ Surface Voltage Measurements of Dielectrics Under Electron Beam Irradiation

New instrumentation has been developed for noncontact, in vacuo measurements of the electron beam-induced surface voltage as a function of time and position for nonconductive spacecraft materials in a simulated space environment. The novel compact system uses two movable capacitive sensor electrodes to measure surface charge distributions on samples, using a noncontact method that has little effect on charge dissipation from sample. Design details, calibration, and characterization measurements of the system are presented, with <;1 V to >30 kV surface voltage range, <;0.5 V voltage resolution, and <;1.5-mm spatial resolution. Used in conjunction with the capabilities of an existing ultrahigh vacuum electron emission test chamber, the new instrumentation facilitates measurements of charge accumulation, bulk resistivity, effects of charge depletion and accumulation on yield measurements, electron-induced electrostatic breakdown potentials, radiation-induced conductivity effects, and the radial dispersion of surface voltage. Three types of measurements of surface voltage for polyimide (Kapton HN serve to illustrate the research capabilities of the new system: 1) accumulation using a pulsed electron beam, while periodically measuring the surface voltage; 2) postcharging, as deposited charge dissipated to a grounded substrate; and 3) the evolution of spatial profile resulting from an incident Gaussian beam. Theoretical models for sample charging and discharge are outlined to predict the time, temperature, and electric field dependence of the samples net surface voltage.

Flight Experiments on the Effects of Contamination on Electron Emission of Materials

We report on a study of the effects of prolonged exposure to the space environment and of chargeenhanced contamination on the electron emission and resistivity of spacecraft materials. The State of Utah Space Environment & Contamination Study (SUSpECS) was deployed on the International Space Station (ISS) in March 2008 onboard the MISSE-6 payload during STS-123. The Materials International Space Station Experiment (MISSE-6) program is designed to characterize the performance of candidate new space materials over the course of its ~17 month exposure to the LEO environment, with a target return date of August 2009 on STS-127. Approximately 165 samples are mounted on three separate SUSpECS panels in the ram and wake sides on the ISS. They have been carefully chosen to provide needed information for different ongoing studies and a broad cross-section of prototypical materials used on the exteriors of spacecrafts. Design of the sample panels are detailed, including a three tiered configuration intended to provide variable atomic oxygen and ultraviolet radiation exposure. The methods used to simulate charge enhanced contamination by actively biasing samples to low positive and negative voltages are also described. A primary emphasis of SUSpECS is the study of modifications to the electron emission resulting from exposure to the space plasma environment and to environmental contamination. There is presently little available data related to the effects of sample deterioration and contamination on emission properties for materials actually flown in space. Electron emission and transport properties of materials are key in determining the amount of charge build-up and the time for the charge to dissipate, as well as the likelihood of deleterious spacecraft charging effects. Such materials properties are essential parameters in modeling spacecraft charging with engineering tools like NASCAP-2K code. SUSpECS studies will test the validity of our predictions from ground-based studies that very thin layers of contamination can lead to severe charging effects under certain circumstance. Electron-, ion-, and photon-induced electron emission yield curves, crossover energies and emission spectra, as well as resistivity and dielectric strength, have been tested for most SUSpECS samples in their pristine conditions before flight. These measurements will be compared with post-flight measurements. Additional pre- and post-flight characterization measurements include optical and electron microscopy, reflection spectroscopy, emissivity and Auger electron spectroscopy.