The hypothetical proton-electron combination, however, which was neither a wave form nor a charged particle was not expected to be able to ionize atoms. It would wander among the atoms without either attracting or repelling electrons and would therefore leave the atomic structure intact. Its pathway could not be followed. In short, then, the neutron was, so to speak, invisible, and the search for it seemed a lost cause. And until it was found, the proton-electron theory of nuclear structure, whatever its obvious deficiencies with respect to nuclear spin, remained the only one to work with. Then came 1930. The German physicist Walther Wilhelm Georg Bothe (1891-1957) and a co-worker, H. Becker, were bombarding the light metal, beryllium, with alpha particles. Ordinarily, they might expect protons to be knocked out of it, but in this case no protons appeared. They detected some sort of radiation because something was creating certain effects while the alpha particles were bombarding the beryllium but not after the bombardment ceased. [Illustration: _Walther W. G. Bothe_] To try to determine something about the properties of this radiation, Bothe and Becker tried putting objects in the way of the radiation. They found the radiation to be remarkably penetrating. It even passed through several centimeters of lead. The only form of radiation that was known at that time to come out of bombarded matter with the capacity of penetrating a thick layer of lead was gamma rays. Bothe and Becker, therefore, decided they had produced gamma rays and reported this. In 1932 the Joliot-Curies repeated the Bothe-Becker work and got the same results. However, among the objects they placed in the path of the new radiation, they included paraffin, which is made up of the light atoms of carbon and hydrogen. To their surprise, protons were knocked out of the paraffin. Gamma rays had never been observed to do this, but the Joliot-Curies could not think what else the radiation might be. They simply reported that they had discovered gamma rays to be capable of a new kind of action. [Illustration: _James Chadwick_] Not so the English physicist James Chadwick (1891- ). In that same year he maintained that a gamma ray, which possessed no mass, simply lacked the momentum to hurl a proton out of its place in the atom. Even an electron was too light to do so. (It would be like trying to knock a baseball off the ground and into the air by hitting it with a ping-pong ball.) Any radiation capable of knocking a proton out of an atom had to consist of particles that were themselves pretty massive. And if one argued like that, then it seemed that the radiation first observed by Bothe and Becker had to be the long-sought-for proton-electron combination. Chadwick used Harkins’ term, neutron, for it and made it official. He gets the credit for the discovery of the neutron.