Beta: more information

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A Beta particle is the same as an electron

It has a charge of -1, and a mass of around 1/2000th of a proton.

But wait a minute! If a nucleus contains protons and neutrons, what's an electron doing coming out of a nucleus?

To answer this, we need to know more about protons and neutrons:

Protons & neutrons are made of combinations of even smaller particles, called "quarks". Under certain conditions, a neutron can decay, to produce a proton plus an electron. The proton stays in the nucleus, whilst the electron flies off at high speed.


This means that when a nucleus emits a -particle:

  • the atomic mass is unchanged
  • the atomic number increases by 1.

This is because a neutron has changed into a proton (almost the same mass - we can ignore the tiny mass of the electron) and thus the number of protons has gone up.

Example: Strontium-90 undergoesdecay and forms Yttrium-90.
           a beta-deacy equation

This isn't the whole story - an almost massless particle called an "anti-neutrino" is also emitted. Furthermore, we are only considering "beta-minus" emission (negatively-charged electrons).

There is another type of beta decay, called "beta-plus", where a positively-charged electron (called a "positron") is emitted, along with a neutrino.

Don't worry about this right now, this is way beyond what's required for GCSE, and takes us into 'A' level physics concepts such as antimatter.


Beta decay occurs in very "neutron-rich" elements, for example, Strontium-90 and Iodine-130. These elements are typically created in nuclear reactors.

These elements have too few protons and too many neutrons to be stable. They can thus become more stable by emitting a beta particle.

Beta particles have a charge of -1, and weigh only a tiny fraction of a neutron or proton. As a result, particles interact less readily with other atoms than alpha particles.
Thus beta particles cause less ionisation than alphas, and have a longer range, typically a few metres in air.


Remember, in Beta decay :-

  • atomic number increases by one
  • atomic mass unchanged.


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