Terahertz spectroscopy as a novel method for diagnosing the integrity of polymer insulated cables

Through examinations of the applicability of novel terahertz spectroscopy to the diagnosis of integrity of polymeric insulated cables, the following important examples have been obtained. First, the attenuation of the reflected intensity of THz light was scanned over a low density polyethylene sheet with water trees under a polyvinylchloride sheet. As a result, its intensity distribution agrees with the distribution of water trees in the sample. Therefore, water trees would be visualizable even if they are invisible to naked eyes. Secondly, chemically cross-linked polyethylene sheets were irradiated by gamma-rays to certain designated doses. A peak appears at around 44 cm^-1 (= 1.3 THz) when the dose is higher than 200 kGy. Combining the results obtained by instrumental analyses, the 44-cm^-1 peak is likely to be due to oxidation-related dipoles. Thirdly, the reflected THz light intensities were compared among an uncorroded copper plate and three corroded plates with copper oxide, basic copper carbonate, and copper chloride. The last two corroded plates were found to exhibit absorption strong enough for the THz light to give clear images of corrosion.