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.

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.

State the conclusion drawn by Rutherford from the Geiger-Marsden gold-foil experiment. [2 marks]

Calculate the distance of closest approach of a $4.0 \text{ MeV}$ alpha particle to a gold nucleus ($Z = 79$). [3 marks]

Chadwick's 1932 discovery completed the proton-neutron-electron picture of the atom. (a) Write the nuclear reaction used by Chadwick. (b) Explain how he deduced the neutron's mass using momentum conservation in collisions with protons and nitrogen nuclei.