not just described, Explained


Contact us

Misconceptions quiz

Subscriptions FAQ




Physics subscription prices

subscribe log in



Work and Money

PSHE subscription prices

subscribe log in

Why beta particles have a range of energies

Two related problems

There were two things that were difficult to explain about beta particles.

The first was that they came with a range of energies.  How did this happen?  There must be some process which randomly selected energies.  Why did it choose one energy and not another?

The other was that when a nucleus emitted a beta particle it didn't recoil backwards in a straight line but normally at an angle.  This seemed to violate the principle of conservation of momentum.  Perhaps beta decay didn't obey momentum conservation?

Try Why Do Astronauts Float by Julian Hamm

Pauli makes a prediction

In 1930 the Austrian physicist Wolfgang Pauli predicted that there must be another particle given off when a beta particle is produced, which is very difficult to detect.

The Italian physicist Enrico Fermi named it the 'neutrino', which means little neutral one in Italian.

Pauli suggested that the energy liberated in beta decay was shared in a random way between the beta particle and the neutrino.  This would account for the range of observed energies of beta particles and the different angles at which a nucleus could recoil.

The power of science: finding something new that was predicted by theory

The neutrino’s existence, mass and probable behaviour had been worked out in the 1930s by Fermi and other scientists.  This is a good example of science’s unique ability to predict the existence of things that have never been seen.

And Hey Presto! in 1956 the gamma ray signature predicted for the neutrino was actually observed for the first time.

In fact, there are three ‘flavours’ of neutrino.  Simple beta decay produces a type of neutrino called an ‘electron antineutrino’.

back to Lesson 5: How Radiation Harms