During the first years Meitner worked with Hahn, they co-authored nine papers: three in 1908 and six in 1909. Together with Hahn, she discovered and developed a physical separation method known as radioactive recoil, in which a daughter nucleus is forcefully ejected as it recoils at the moment of decay. While Hahn was more concerned with discovering new elements (now known to be isotopes), Meitner was more interested in understanding their radiation. She observed that radioactive recoil, which had been discovered by Harriet Brooks in 1904, could be a new way of detecting radioactive substances. They soon discovered two more new isotopes, bismuth-211 and thallium-207. Meitner was particularly interested in beta particles. By this time, they were known to be electrons. Alpha particles were emitted with characteristic energy, and she expected that this would be true of beta particles too. Hahn and Meitner carefully measured the absorption of beta particles by aluminium, but the results were puzzling. In 1914, James Chadwick found that electrons emitted from the nucleus formed a continuous spectrum, but Meitner found this hard to believe, as it seemed to contradict quantum physics, which held that electrons in an atom can only occupy discrete energy states (quanta).