The performance of a paraterphenyl-coated 1P21 multiplier has been studied in the extreme ultraviolet from 125 to 325 Å. The light source was a vacuum spark between aluminum electrodes with a peak current of 30 kA. A number of lines from the fourth, fifth, and sixth spectra of aluminum were observed and the results were compared with those of earlier work in this laboratory in which Kodak SWR photographic plates, a Bendix resistance-strip photomultiplier, and a sodium salicylate-coated photomultiplier were compared. The paraterphenyl, which was vacuum evaporated on a cover glass to a thickness of 2.74 mg/cm2, was found to be stable over a period of at least 10 days in the vacuum spectrograph. Its relative efficiency in the range from 312 to 200 Å is more nearly constant than that of the other three detectors. Below 161 Å the response of all the detectors decreases. Paraterphenyl appears to be a sensitive and highly satisfactory detector in the region below 325 Å.
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