In 1919 Rutherford ivestigated collisions between alpha particles and light nuclei including hydrogen. Rutherford had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially all the mass of the atom is concentrated in the relatively tiny nucleus. As of about 1930 it was presumed that the fundamental particles were protons and electrons, but that required that somehow a number of electrons were bound in the nucleus to partially cancel the charge of A protons.it was known from the “uncertainty principle – confinement calculations that there just wasn’t enough energy available to contain electrons in the nucleus.An experimental breakthrough came in 1930 with the observation by Bothe and Becker that bombardment of beryllium with alpha particles from a radioactive source produced neutral radiation which was penetrating but non-ionizing. They presumed it was gamma rays, but Curie and Joliot showed that when you bombarded a paraffin target with this radiation, it ejected protons with energy about 5.3 MeV.Gamma rays are high frequency photons.They are emitted after an alpha or beta decay when the newly formed nucleus is an excited state. They take away energy but dont affect A or Z, they simply allow the nucleus to the de-excite.The existence of discrete gamma ray spectra led to the idea that nuclei, like atoms, can exist in a set of quantised energy levels. The necessary energy for the gamma ray explanation was much greater than any energy observed to be available from the nucleus, so the neutral radiation must be some kind of neutral particle.
Chadwick was able to prove that the neutral particle could not be a photon by bombarding targets other than hydrogen, including nitrogen, oxygen, helium and argon. Not only were these inconsistent with photon emission on energy grounds, the cross-section for the interactions was orders of magnitude greater than that for Compton scattering by photons.The task which remained for Chadwick was that of determining the mass of the neutral particle. He chose to bombard boron with alpha particles and analyze the interaction of the neutral particles with nitrogen. These particlular targets were chosen partly because the masses of boron and nitrogen were well known.
For this discovery (1932.) he was awarded the Nobel Prize for Physics in 1935. After Chadwick’s discovery of the neutron many physicists continued to think of it as a composite particle, a kind of collapsed hydrogen atom containing an electron and proton in tightly bound state. The idea that nucleons are bound by a new strong nuclear force developed through several distinct theoretical stages.
– In 1932 Heisenberg developed a theory of exchange forces. The essential idea in this theory is that neutrons and protons, neutrons and neutrons and protons and protons bind to one another by exchanging properties such as charge and position.
– In 1933 Fermi constructed a theory for beta decay using Pauli’s neutrino hypothesis and variation on Dirac’s quantum field theories.
– In 1934 Irene Curie and Frederic Joliot discovered beta plus decay, the emission of a positron from proton-rich nucleus. They were awarded the Nobel Prize for Chemistry in 1935.
-In 1937 – “heavy electron” now called a muon. Yukawa’s theory of the strong nuclear force. Nucleons exchange mesons. This binds them together. He was awarded the Nobel Prize for Physics in 1949. Yukawa’s work changed the way about forces…..
Hardly any mass at all
Always teased for being small
Whizzing at the speed of light
Never stopping day or night.
That’s not fair
I do the work
They just sit there.
Crammed up in a ball
Weighing down the atom
hoping to change the end
im not very happy with it