An Atomic Oxygen Beam System for the Investigation of Mass Spectrometer Response in the Upper Atmosphere

Abstract

An atomic oxygen beam system has been designed and tested for the laboratory evaluation of mass spectrometers used in upper atmospheric measurements. The atomic oxygen is generated by thermal dissociation of molecular oxygen on the surface of a tungsten filament heated to 2800 K. A symmetrical bidirectional beam is produced to permit simultaneous monitoring of the particle flux in the beam while target experiments are being conducted. Flux levels of 5×1014 particles cm−2 sec−1 over a cross-sectional area of 1 cm2 have been produced with a relative atomic oxygen concentration of 70%. At flux levels below 1013 particles cm−2 sec−1, relative atomic oxygen concentrations of more than 90% were obtained. The oxygen beam is of high purity and free from chemically active contaminants. Strong chemical and low temperature pumping are used to reduce background gas contributions to less than 1%. Measurements of the relative atomic concentration in the beam were made with a quadrupole spectrometer using an open flowthrough ion source. The absolute flux of molecular oxygen was determined with the aid of an enclosed omegatron mass spectrometer. The combined use of both instruments permitted a determination of the magnitude of the atomic oxygen flux in the beam.