A second-order focusing mass spectrograph and isotopic weights by the doublet method

The first survey of the masses of atoms, now called isotopic weights, made with an accuracy high enough to measure their deviations from whole numbers, was published ten years ago (Aston 1927). The instrument used, my second mass spectrograph, had a resolving power of about 1 in 600 and an accuracy approaching 1 in 10,000; with it work was further extended with results later reported in a series of communications to this journal. After the work described in the last of these (Aston 1935a.), methods of obtaining higher resolving power and accuracy were investigated, and the most hopeful appeared to be: first, stricter collimation of the rays by finer, and preferably adjustable, slits; and secondly, the use of second-order focusing. For the solution of the latter theoretical problem I have been indebted to Mr. W. W. Sawyer of St. John’s College, Cambridge, who analysed the conditions and determined the dimensions of the apparatus required to give the best conditions of focus (Sawyer 1936). Apparatus It was found that these conditions could be well satisfied by the use of pole pieces virtually identical with those in the existing magnet, so this was retained, and Messrs. W. G. Pye and Co., of Cambridge, carried out the necessary alterations and constructed the other parts of the apparatus here described. The appearance of the complete mass spectrograph is shown in the photograph, fig. 1, and the main details of its construction in the diagram, fig. 2.