Monday, 5 March 2012

Happy Birthday to Neu. Tron.

Okay, I'm a week late in remembering the birthday, as members of my family to whom I send birthday cards may not be surprised to hear, but 80 years ago last Monday a paper was published in the journal Nature entitled "Possible Existence of a Neutron."  It was penned by James Chadwick, and describes his work, which is regarded as the definitive proof that atomic nuclei contain neutrons, as well as protons, and that these neutrons can be observed as separate entities.  He won the Nobel prize for this discovery in 1935.

His experiment built, as most of science does, on the work of others, amongst whom were Irène and Pierre Joliot-Curie.  They had previously been investigating the radiation that is emitted when alpha particles from a radioactive source impinge upon the element beryllium.  This radiation was very highly penetrating - i.e. it could pass easily through matter.  At that time, only gamma rays were known to be so highly penetrating, and neither the Joliot-Curies, or others, such as the German physicist Walter Bothe, who also produced neutrons realised that the observed radiation was anything but gamma rays.

By some careful experimentation and deduction, Chadwick realised that rather than very high energy gamma rays (so high that they would seem to violate the conservation of energy and momentum), the radiation combing from the alpha-bombarded beryllium were in fact neutrons - neutral particles with about the same mass as protons.  The outreach people at Cambridge, where Chadwick performed these experiments, have produced a nice site about the work, going in to a little more detail about it.

I suppose the Joliot-Curies and Bothe were a little peeved that they missed this explanation, but all three went on to win Nobel prizes;  Irène and Pierre Joliot-Curie for the discovery of the transmutation of elements through nuclear reactions (ironically enough, Chadwick's experiment was transmuting elements, but he didn't observe or measure the direct proof of that), and Bothe for experimental techniques in nuclear physics.