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

Both pointed to a neutral nuclear constituent.

<!-- Diagram: Rutherford gold foil scattering | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 540 280" role="img" aria-labelledby="ruth-t ruth-d"> <title id="ruth-t">Geiger Marsden gold foil scattering</title> <desc id="ruth-d">A source of alpha particles on the left fires a beam toward a thin gold foil at the centre. Most alpha particles pass through with negligible deflection. A small fraction scatters at moderate angles. A few back scatter at angles greater than ninety degrees.

A head-on alpha particle slows as it climbs the nucleus's repulsive Coulomb potential, momentarily stopping at the distance of closest approach $r_{\min}$, where all of its kinetic energy has become electrostatic potential energy:

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:

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.