I didn't remember any of this so thanks for the
opportunity to catch up on some science history!
James Chadwick and the Beryllium Rays
Rutherford predicted the existence of the
neutron in 1920. Twelve years later, his assistant
James Chadwick found it. Chadwick had been a
student at Manchester University. After graduating
in 1911, he stayed at the laboratory doing
research for Rutherford.
In 1913, Chadwick went to Berlin, Germany, to
work with Hans Geiger. The war broke out the
following year. Because Chadwick was an
Englishman, he was detained as a civilian prisoner
of war. He was allowed to read books and talk to
other physicists, but he could not do experiments.
In 1918, when the war ended, Chadwick returned
to Manchester. He worked with Rutherford on the
transmutation of the elements. In 1919, Rutherford
went to Cambridge to become director of the
Cavendish Laboratory. Chadwick went with him.
At Cambridge, Chadwick searched for the
neutron. He tried in 1923, but did not find
it. He tried again in 1928, with no success. In
1930, the German physicists Walther Bothe and
Herbert Becker noticed something odd. When they
shot alpha rays at beryllium (atomic number 4) the
beryllium emitted a neutral radiation that could
penetrate 200 millimeters of lead. In contrast, it
takes less than one millimeter of lead to stop a
Bothe and Becker assumed the neutral radiation
was high-energy gamma rays.
Marie Curie's daughter, Irene Joliot-Curie, and
Irene's husband, Frederic, put a block of paraffin
wax in front of the beryllium rays. They observed
high-speed protons coming from the paraffin. They
knew that gamma rays could eject electrons from
metals. They thought the same thing was happening
to the protons in the paraffin. Chadwick said the
radiation could not be gamma rays. To eject
protons at such a high velocity, the rays must
have an energy of 50 million electron volts. An
electron volt is a tiny amount of energy, only
enough to keep a 75-watt light bulb burning for a
tenth of a trillionth of a second. The alpha
particles colliding with beryllium nuclei could
produce only 14 million electron volts.
The law of conservation of energy states that
energy can neither be created nor destroyed. It
certainly looked as if energy was being created
along with the neutral radiation.
Chadwick had another explanation for the beryllium
rays. He thought they were neutrons. He set up an
experiment to test his hypothesis.
Chadwick put a piece of beryllium in a vacuum
chamber with some polonium. The polonium emitted
alpha rays, which struck the beryllium. When
struck, the beryllium emitted the mysterious
In the path of the rays, Chadwick put a target.
When the rays hit the target, they knocked atoms
out of it. The atoms, which became electrically
charged in the collision, flew into a detector.
Chadwick's detector was a chamber filled with gas.
When a charged particle passed through the
chamber, it ionized the gas molecules. The ions
drifted toward an electrode. Chadwick measured the
current flowing through the electrode. Knowing the
current, he could count the atoms and estimate
their speed. Chadwick used targets of different
elements, measuring the energy needed to eject the
atoms of each. Gamma rays could not explain the
speed of the atoms. The only good explanation for
his result was a neutral particle.
To prove that
the particle was indeed the neutron, Chadwick
measured its mass. He could not weigh it directly.
Instead he measured everything else in the
collision and used that information to calculate
the mass. For his mass measurement, Chadwick
bombarded boron with alpha particles. Like
beryllium, boron emitted neutral rays. Chadwick
placed a hydrogen target in the path of the rays.
When the rays struck the target, protons flew out.
Chadwick measured the velocity of the
Using the laws of conservation of momentum
and energy, Chadwick calculated the mass of the
neutral particle. It was 1.0067 times the mass
of the proton. The neutral radiation was indeed
the long-sought neutron.
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