approximately equal to the mass of a proton.h/Chadwick’s subatomic pool table. A disk of the naturally occur-
ring metal polonium provides a source of radiation capable of kicking
neutrons out of the beryllium nuclei. The type of radiation emitted by
the polonium is easily absorbed by a few mm of air, so the air has to be
pumped out of the left-hand chamber. The neutrons, Chadwick’s mystery
particles, penetrate matter far more readily, and fly out through the wall
and into the chamber on the right, which is filled with nitrogen or hydrogen
gas. When a neutron collides with a nitrogen or hydrogen nucleus, it
kicks it out of its atom at high speed, and this recoiling nucleus then rips
apart thousands of other atoms of the gas. The result is an electrical
pulse that can be detected in the wire on the right. Physicists had already
calibrated this type of apparatus so that they could translate the strength
of the electrical pulse into the velocity of the recoiling nucleus. The
whole apparatus shown in the figure would fit in the palm of your hand, in
dramatic contrast to today’s giant particle accelerators.Unfortunately a subatomic particle is not something you can
just put on a scale and weigh. Chadwick came up with an ingenious
solution. The masses of the nuclei of the various chemical elements
were already known, and techniques had already been developed for
measuring the speed of a rapidly moving nucleus. He therefore set
out to bombard samples of selected elements with the mysterious
new particles. When a direct, head-on collision occurred between
a mystery particle and the nucleus of one of the target atoms, the
nucleus would be knocked out of the atom, and he would measure
its velocity.
Suppose, for instance, that we bombard a sample of hydrogen
atoms with the mystery particles. Since the participants in the
collision are fundamental particles, there is no way for kinetic energy
to be converted into heat or any other form of energy, and Chadwick
thus had two equations in three unknowns:
equation #1: conservation of momentum
Section 3.1 Momentum in one dimension 141