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Some molecular-beam measurements with a Pirani detector Jassby, Daniel Lewis

Abstract

A differential Pirani molecular-beam detector was developed with a sensitivity for noncondensable gases about five times greater than that of the previous most sensitive Pirani beam-detector. With a circular entrance channel of pressure-accumulation factor 28, at room temperature a one-microvolt signal was produced for a helium beam intensity of 5.4 x 10¹² atoms/sec/cm² . At liquid-nitrogen temperature, the sensitivity increased almost four times. This detector was used to investigate the directivity characteristics of molecular beams of helium, argon and nitrogen, produced with several circular, cylindrical tubes, mounted on a rotatable oven. In the molecular-flow region, the angular distributions were somewhat narrower than those reported by previous workers. With increasing source pressure, the full-width at half-maximum of the distribution for each tube with any gas became 75 to 78 degrees, but with further increase of source pressure, the full-width depended on the particular gas used. The electric circuit of the Pirani detector was easily modified so that the entrance channel and beam cavity could be used as a combination Pitot tube and pressure gauge for determining the Mach numbers of supersonic beams in vacuum. It was found that a 0.214-mm. conical converging nozzle produced a Mach 2.2 helium beam when the pressure ratio across the nozzle was about 120.

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