Quick questions on Rutherford's nuclear atom and Chadwick's neutron: HSC Physics Module 8

Both pointed to a neutral nuclear constituent.

Background. Thomson's plum-pudding model (1897) had positive charge smeared diffusely over the atom with embedded electrons. To test it, Hans Geiger and Ernest Marsden (under Rutherford, at Manchester) directed a beam of alpha particles (from radium decay) at a very thin gold foil and measured how many particles scattered into different angles using a movable scintillation detector.

1. Why don't the orbiting electrons radiate (since accelerating charges in classical electromagnetism should radiate and spiral in)? This was solved by Bohr's 1913 quantised-orbit model, see the related dot point. 2.

Rutherford had postulated as early as 1920 that the nucleus contained, in addition to protons, neutral particles of similar mass that he called "neutrons". The decisive evidence came from a chain of experiments.

For a head-on elastic collision of a particle of mass $m$, speed $v_0$, with a stationary target of mass $M$, the target recoils with speed:

This sets the stage for the rest of Module 8: nuclear stability (binding energy), radioactive decay (alpha, beta, gamma), and ultimately the quark structure of the nucleons.

Bothe and Becker (1930) observed that bombarding beryllium with alpha particles produced a highly penetrating neutral radiation that, until 1932, was assumed to be high-energy gamma rays. Curie and Joliot (1932) showed that this radiation could eject protons from paraffin wax with surprisingly high kinetic energies.

James Chadwick (1932) sent the neutral radiation onto various target nuclei (hydrogen, helium, lithium, nitrogen) and measured the recoil kinetic energies of each target. Using conservation of energy and momentum, he tested two hypotheses.

Most pass through. Only a small fraction scatter through large angles, and an even smaller fraction back-scatter. That fraction is small but non-zero, and that is what was unexpected.

The foil was as thin as could be made (a few hundred atoms thick), so that most alpha particles encountered at most one nucleus.

The whole point of his analysis was that gamma rays could not provide enough momentum given the energy available, while a massive neutral particle could.

Atomic radius $\sim 10^{-10}$ m; nuclear radius $\sim 10^{-15}$ m. The atom is mostly empty.

Before Chadwick, some models had nuclei containing protons and bound electrons to account for the extra mass and zero net charge of the neutron's role. The neutron made this unnecessary.