The mechanical moment of the nucleus of cæsium

Abstract

In an earlier work, from measurements on the separations of the hyperfine structure doublets of the lines of the principal series, the author was able to find an approximate value for the magnetic moment of the nucleus of cæsium; but from these measurements it was not possible to determine experimentally the mechanical moment. In order to determine this, it is necessary to know accurately the intensity ratios of the components of the doublets. Accordingly accurate intensity measurements were made on the two most suitable doublets, 4555 and 4593. By working with extremely low vapour pressures of cæsium in the light source, the effect of self-absorption on the intensity ratios was almost entirely eliminated, being reduced to less than 2% in the stronger doublet and 1%, in the weaker doublet. When this small correction was applied, the intensity ratio was found to be 1.27±0.02 for both doublets. This establishes with certainty the value 7/2 for the mechanical moment of the cæsium nucleus. Excitation of the Spectrum.—The light source used was an external electrode discharge tube, the ends of which were 6 cm. long and 5 cm. in diameter; the capillary was 4 cm. long and 5 mm. bore. The tubs was made of pyrex glass, carefully "baked out" in a very high vacuum; after this a small quantity of pure cæsium was distilled into it. The metal was distilled three times before it entered the discharge tube in order to eliminate as far as possible traces of occluded hydrogen, which are always present in alkali metals, After the introduction of the metal, the tube was filled with neon, purified by charcoal cooled with liquid air, at a pressure of 2 mm. of mercury. External electrodes made of thin annealed sheet copper, and large enough to cover the whole of the ends of the tube, were fitted. In order to excite the tubs, one of the external electrodes was connected to one end of the plate coil of a high frequency oscillator of the type described by Gutton; the other electrode was earthed. The oscillator was capable of an output of about 1/2 kw., and was excited by a generator giving currents up to 5 amps, at a potential of 1750 volts.