Hans Geiger was awarded a PhD by the university of erlangen in 1906 and shortly after joined the staff of the University of Manchester, where he became one of the most valuable collaborators of Ernest Rutherford. At Manchester, Geiger built the first version of his particle counter and used it and other radiation detectors in experiments that led to the identification of the alpha particle as the nucleus of the helium atom and to Rutherfords correct proposal in 1912 that in any atom, the nuckeus occupies a ver small volume at the center.