100 years of ion beams: Willy Wien's canal rays

Abstract

When Goldstein's report on the "positive light" (or what is known as "Kanalstrahlen", canal rays) in gas discharge tubes first appeared in 1886, Willy Wien had just finished his thesis at the Helmholtz Institute in Berlin. Eleven years later he performed his first experiments on canal rays and found that they consisted of inert, charged and neutral particles. The charged component in canal rays could be deflected using electric and magnetic fields, enabling Wien to roughly determine their mass-to-charge ratio. Improving vacuum conditions and detection efficiency, Thomson finally resolved the lightest constituents of canal rays: the hydrogen ions H+ and H+2. This marked the beginning of mass spectrometry. The first mass spectrographs were parabola-image instruments being used by Thomson to discover isotopes. Until about 1923, canal rays became the most common ion source. Also Aston used canal rays as an ion source for the first double focussing mass spectrometer. -Wien continued his work on canal rays up to the end of his life (he died in 1928). He investigated their interaction with matter, i.e. the mean free path of canal rays in gases with respect to charge exchange and atomic excitation. His particular interest was addressed to the physics of light emission by canal rays, such as the line spectrum and the splitting of these lines in magnetic and electric fields, the Doppler effect and lifetimes.