In 1921, Meitner accepted an invitation from Manne Siegbahn to come to Sweden and give a series of lectures on radioactivity as a visiting professor at Lund University. She found that very little research had been done on radioactivity in Sweden, but she was eager to learn about X-ray spectroscopy, which was Siegbahns specialty. At his laboratory, she met a Dutch doctoral candidate, Dirk Coster, who was studying X-ray spectroscopy, and his wife Miep, who was working on her doctorate in Indonesian language and culture. Armed with her newly acquired knowledge of X-ray spectroscopy, Meitner took a fresh look at the beta-ray spectra when she returned to Berlin. It was known that some beta emission was primary, with electrons being ejected directly from the nucleus, and some was secondary, in which alpha particles from the nucleus knocked electrons out of orbit. Meitner was sceptical of Chadwicks claim that the spectral lines were entirely due to secondary electrons, while the primary ones formed a continuous spectrum. Using techniques developed by Jean Danysz, she examined the spectra of lead-210, radium-226 and thorium-238. Meitner discovered the cause of the emission of electrons from surfaces of atoms with "signature" energies, now known as the Auger effect. The effect is named for Pierre Victor Auger, who independently discovered it in 1923.
Women were granted the right of habilitation in Prussia in 1920, and in 1922 Meitner was granted her habilitation and became a Privatdozentin. She was the first woman to receive her habilitation in physics in Prussia, and only the second in Germany after Hedwig Kohn. Since Meitner had already published over 40 papers, she was not required to submit a thesis, but Max von Laue recommended that the requirement for an inaugural lecture not be waived, since he was interested in what she had to say. She therefore gave an inaugural lecture on "Problems of Cosmic Physics". From 1923 to 1933, she taught a colloquium or tutorial at Friedrich Wilhelm University each semester, and supervised doctoral students at the KWI for Chemistry. These included Arnold Flammersfeld, Kan-Chang Wang and Nikolaus Riehl. In 1926, she became an außerordentlicher Professor (extraordinary professor), the first woman university physics professor in Germany. Her physics section became larger, and she acquired a permanent assistant. Scientists from Germany and around the world came to the KWI for Chemistry to conduct research under her supervision. In 1930, Meitner taught a seminar on "Questions of Atomic Physics and Atomic Chemistry" with Leó Szilárd.
Meitner had a Wilson cloud chamber constructed at the KWI for Chemistry, the first one in Berlin, and with her student Kurt Freitag studied the tracks of alpha particles that did not collide with a nucleus. With her assistant Kurt Philipp she later used it to take the first images of positron traces from gamma radiation. She proved Chadwicks assertion that the discrete spectral lines were entirely the result of secondary electrons, and the continuous spectra were therefore indeed entirely caused by the primary ones. In 1927, Charles Drummond Ellis and William Alfred Wooster measured the energy of the continuous spectrum produced by the beta decay of bismuth-210 at 0.34 MeV where the energy of each disintegration was 0.35 MeV. Thus, the spectrum accounted for nearly all of the energy. Meitner was so stunned by this result that she repeated the experiment with Wilhelm Orthmann using an improved method, and verified Ellis and Woosters results. It appeared that the law of conservation of energy did not hold for beta decay, something Meitner regarded as unacceptable. In 1930, Wolfgang Pauli wrote an open letter to Meitner and Hans Geiger in which he proposed that the continuous spectrum was caused by the emission of a second particle during beta decay, one that had no electric charge and little or no rest mass. The idea was taken up by Enrico Fermi in his 1934 theory of beta decay, and he gave the name "neutrino" to the hypothetical neutral particle. At the time there was scant hope of detecting neutrinos, but in 1956 Clyde Cowan and Frederick Reines did just that.