Neutron Physics

Neutrons together with protons are the constituents of atomic nuclei. The neutron was discovered after more than two decades of speculation that electrically neutral particles exist in atoms (see Chapter 3). Because the neutron is electrically neutral, it easily interacts with nuclei and does not interact directly with electrons. Since the nucleus of an atom is about one ten-thousandth the size of the electron cloud, the chance of neutrons interacting with a nucleus is very small, allowing them to travel long distances through matter. As a free particle, the neutron is an important and yet unique tool used for various applications: in medicine to initiate powerful nuclear interactions whose products can directly destroy cancer cells (neutron capture therapy for example), for research on physical and biological materials, for imaging through easy allocation of light atoms especially hydrogen, to investigate properties of magnetic materials (neutrons possess a magnetic moment and thus act as small magnets), to track atomic movement (thermal neutron energies almost directly coincide with the energies of atoms in motion) and to maintain the fission chain reaction in nuclear reactors. Free neutrons are unstable (see Chapter 3) and decay in short time through β^– decay process into a proton, electron and neutrino. However, free neutrons will most likely interact with the surrounding matter and disappear through nuclear interactions long before they decay.

…I feel that I ought to let you know of a very sensational new development in nuclear physics. In a paper in the Naturwissenschaften Hahn reports that he finds when bombarding uranium with neutrons the uranium breaking up into two halves giving elements of about half the atomic weight of uranium. This is entirely unexpected and exciting news for the average physicist. The Department of Physics at Princeton, where I spent the last few days, was like a stirred-up ant heap. Apart from the purely scientific interest there may be another aspect of this discovery, which so far does not seem to have caught the attention of those to whom I spoke. First of all it is obvious that the energy released in this new reaction must be very much higher than in all previously known cases. It may be 200 million (electron-) volts instead of the usual 3–10 million volts. This in itself might make it possible to produce power by means of nuclear energy, but I do not think that this possibility is very exciting, for if the energy output is only two or three times the energy input, the cost of investment would probably be too high to make the process worthwhile. Unfortunately, most of the energy is released in the form of heat and not in the form of radioactivity.

I see, however, in connection with this new discovery potential possibilities in another direction. These might lead to a large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years… Leo Szilard (1898– 1964) in his letter to Luis Strauss on January 25, 1939.