Soft X-rays and photoelectrons from nickel at different temperatures

Abstract

The photoelectric method of investigating the intensity of soft X-rays was first employed independently and at about the same time by Richardson and Bazzoni (1921), Kurth (1921), Mohler and Foote (1922), Holweck (1922) and Hughes (1922). A large number of investigators (Rudberg 1929) have since adapted the method for the study of soft X-ray critical potentials on the one side (Thomas 1925; Compton and Thomas 1926; Richardson and Andrewes 1930; Richardson and Rao 1930; Richardson and Chalklin 1926, 1928 a , b ; Andrewes, Davies and Horton 1926, 1928; Horton, Andrewes and Davies 1923; Davies and Horton 1926), and the efficiency of soft X-ray excitation from metallic elements on the other (Richardson and Robertson 1927, 1929; Davies 1928; Nakaya 1929). The problem of the photoelectric efficiency of metals in the region of soft X-rays has also received some attention (Davies 1928; Bandopadhyaya 1928). Nakaya (1929) has studied the relation between the intensity of soft X-rays and the photoelectric current as a function of the nature of the photoelectric detector. Closely allied to the excitation of soft X-rays is the phenomenon of secondary electron emission from metal faces. The critical potentials in the secondary emission have been investigated by Petry (1925, 1926), Farns­ worth (1928, 1929), Krefft (1927, 1928), Ahearn (1931), Rao (1930) and others (cf. Hyatt and Smith 1928). Richardson (1930) has drawn attention to the similarity of the phenomena of the excitation of soft X-rays and the emission of secondary electrons and the approximate coincidence of the critical potentials in the two cases. To explain these and other properties, he has suggested that when a metal surface is bombarded with electrons, the first act is the excitation of a structure electron by the primary, which is returned as part of the high-energy group of secondaries. When the excited structure electrons return to the ground state, the low-energy secondary electrons and soft X-rays result as a consequence. This hypothesis accounts for the agreement between the soft X-ray and secondary electron breaks. In an earlier paper (1928) he has drawn attention to the difference in the order of efficiency of the two phenomena, the coefficient of the secondary emission being about a million times that of the soft X-ray excitation.