Chadwick and the Neutron (1932)
Alpha particles from polonium hit beryllium and produce a mystery radiation. Find out what it is, using the target that the radiation hits and an oscilloscope that shows the recoils.
Swipe the picture sideways to see all of it.
Show Chadwick’s argument and the calculations
1. The radiation has no charge. An electric field bends charged particles, but it does not bend this radiation, so it is neutral.
2. It is not a gamma ray. The Joliot-Curies thought the radiation was gamma rays, because it is so penetrating. But a photon would need an energy of about 55 MeV to knock a proton out of paraffin with 5.7 MeV. The reaction can supply only about 11 MeV (the 5.3 MeV of the alpha particle plus 5.7 MeV released by the reaction). So the radiation must be particles with a mass close to the proton’s.
⁹Be + α → ¹²C + n
3. Light nuclei recoil with the most energy. For a neutron of energy E hitting a nucleus of mass M head on, the nucleus recoils with a maximum energy of:
Emax = E × 4mM / (m + M)2
4. The mass. The recoil speeds of hydrogen and nitrogen nuclei depend on the neutron mass m: the ratio of their maximum speeds is r = (m + 14)/(m + 1). Solving gives m = (14 − r)/(r − 1) (with the masses in u). Chadwick’s own speeds gave about 1.15 u, then the energy of the reaction gave 1.0067 u. The modern value is 1.0087 u. A small error in the speeds makes a big error in the mass, so a careful measurement matters.
Only about one alpha particle in 10,000 makes a neutron. The simulation makes many more so that you can see what happens. The neutron cannot be seen by the chamber, because it has no charge: the chamber counts the charged recoils.
Learn more in the Chadwick lesson.