Counting the atoms

The title of this article was inspired by a similar one used by Ernest Rutherford for an unpublished lecture note now in the University of Cambridge library. Rutherford's title was “The Counting of Atoms,” by which he could have meant the counting of individual ionized helium atoms (alpha particles), or, alternatively, he may have had in mind “decay counting” as an indirect indication that a parent atom had transmuted. With the modern pulsed laser, Rutherford's idea becomes entirely practicable, and individual atoms can now be counted by several different methods, the subject of the present article. Our group at Oak Ridge has demonstrated these methods with apparatus like that shown in figure 1.