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Beta decay and quarks

Quarks have bizarre fractional electric charge

An up quark has an electric charge of +2/3.  A down quark has an electric charge of -1/3.

A proton is made of two ups and a down, giving it a charge of +1.  A neutron is made from two downs and an up giving it a charge of 0.

Beta minus decay

With beta minus decay one of the down quarks changes into an up quark.  It emits an intermediate particle called a W-minus boson.  The W-minus boson decays almost immediately into an electron (the beta particle) and a difficult-to-detect anti-electron-neutrino.

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Changing the flavour of a quark is called a ‘weak force interaction’.  It is the only one of the four fundamental interactions that can do this.

Feynman diagrams

In the 1950s Richard Feynman invented a way of expressing the very complex mathematics involved in electromagnetic interactions in a simple diagram.  Feynman diagrams can be adapted to describe the interactions of the weak force.

‘Adding up’ all the possible diagrams for a particular decay (e.g. beta minus) tells us about the probability of the decay.

Beta plus decay

It’s also possible for a proton to change into a neutron.  This involves an up quark changing into a down quark.  This time a positron is given off rather than an electron so it’s called beta plus decay.  The positron is the electron’s antiparticle.

Most beta plus emitters are artificially produced in particle accelerators.  Positrons are used in medical imaging.

back to Lesson 10: Changes to the Nucleus