X-Ray-Induced Photoconductivity in Dielectric Films

Abstract

LAWRENCE UVERMORE LABORATORY MS, tlaio; April 27, U"T2 -NOTICE-This, report n prepared as an accotrat of work spaasored fay the United States Gofcrnment. Neither tt~ United States nor the United States Atomic Energy CaamiB»n, nor any of t^-cir employees, nor any of thek contractors, fecoatneton. or their cmpSoyees. makes any warrant, express or implied, or essoincs any legal EacOity or responsibility for the teem icy. com* pUteness or cieiqtoess of any inforcutioa, sppvitos, product or process diseased, cr represents that its tac would not iafriaie priYaiely owned rights. We have measured the conductivity induced in films of polyethylene, epoxy, polytetrafluoroethylene, polyethylene terephthalate, polyimide, and glass by 9 x rays at dose rates between 10 and 10 rads/sec (dose in air). The films were 0.05 to 1.25 mm thick. The x-ray spectrum peaked in the vicinity of 10 keV, and the x-ray pulse width was about 40 nsec FWHRI. X-ray-induced photocurrents were found to obey Ohm's law at low bias voltages (less than I kVl. Above 1 kV, however, we observed that the peak photoconductivity signals from some of the 0.05-mm-thick materials tegan to When the electrical components and circuitry of a warhead are exposed to a nuclear radiation environment, induced voltages and currents are generated that may be large enough to damage critical components and disable the warhead. In our continuing program at LL.L to under stand the mechanisms by which these voltages and electrical currents are gen erated, we are making careful laboratory measurements of charge emission from material surfaces as well as the charge displacement effects that occur within the bulk of the insulating layer under increase at a slightly faster than linear rate with bias voltage. The glass samples exhibited no appr.r^nt delaved conductivity, while the other sample materials shewed various amounts. The magnitude of the delayed conductivity in polytetrafluoro ethylene, polyethylene terephthalate, and polyimide depended on the electric field, ec an effect that is consistent with PooleFrenkel fie'.d-assis :eo carrier generation. <A ContentsWe have qualitative!" described the magnitude and time dependence of the conductivity sig nals by a simple trapping model using reasonable values fcr mobility, trap density, capture cross sections, and trap depths.x-ray exposure. We are attempting to use this data in computer models to explain the complex signals observed from cables and other electrical com ponents in nuclear effects tests. The radiation induced conductivity in the dielectrics of these components is an essential parameter in the analysis of these signals.We have measured the radiation in duced conductivity of films of polyethylene terephthalate, polyimide, polytetrafluoro ethylene, polyethylene, epoxy, and glass at dose rates near 10 * rads/sec (air).

Effect of using polyimide capillaries during thermal experiments on the particle size distribution of supported Pt nanoparticles

“…In situ and ex situ transmission X-ray experiments at low temperatures commonly use Kapton HN-type polyimide capillaries as sample holders because of their optical properties (Arakawa, 1981), X-ray photoconductivity (Weingart et al, 1972;Pillai & Sharma, 1979), resilience and low cost (Chupas et al, 2008). This poly(oxydiphenylene-pyromellitimide) polyimide ( Fig.…”

Ultrafast Charge Separation and Recombination Dynamics in a Nanometer Thin Film of Polyimide Observed by Femtosecond Transient Absorption Spectroscopy

“…The mechanism of PI photoconductivity has also been of great interest. − Photogenerated intra- or intermolecular charge transfer by the absorption of radiation contributes to the photoconductivity of a polymer. − Using Kapton PI, the chromophores responsible for photoconduction were identified as radical anions. , The photoconduction is believed to follow the hopping of charge between the adjacent pyromellitic imide (PMDI) units by the assistance of an electric field. The initial photophysical process of the interaction of PI with light was studied mainly by Freilich and Lee. , Freilich showed that the addition of a small number of electron donors to PI results in an increase in the photocurrent .…”

“…The mechanism of PI photoconductivity has also been of great interest. [14][15][16][17][18][19][20] Photogenerated intra-or intermolecular charge transfer by the absorption of radiation contributes to the photoconductivity of a polymer. [21][22][23][24] Using Kapton PI, the chromophores responsible for photoconduction were identified as radical anions.…”

Observation of Radar Echoes from High-Energy Particle Cascades

“…For ice, τ ranges from O(1 − 10 ns) and is strongly dependent upon the temperature of the ice [42]. For HDPE, the lifetimes are comparable to those of cold polar ice [43].…”