Alpha, beta and gamma radiation

By the end of this lesson

By the end you can state what each of alpha, beta-minus, beta-plus and gamma radiation is made of, give its charge and its mass, and explain which particle is emitted in which kind of decay.

Four radiations, one table

Radioactive decay releases energy by emitting particles or photons from the nucleus. Each emission has a definite composition, and that composition fixes its mass and charge:

radiation	composition	notation	charge	mass
alpha, $\alpha$	2 protons + 2 neutrons (a helium nucleus)	$^{4}_{2}\text{He}$	$+2e$	about $4\ \text{u}$
beta-minus, $\beta^-$	an electron	$^{0}_{-1}\text{e}$	$-e$	about $\dfrac{1}{1840}\ \text{u}$
beta-plus, $\beta^+$	a positron	$^{0}_{+1}\text{e}$	$+e$	about $\dfrac{1}{1840}\ \text{u}$
gamma, $\gamma$	a high-energy photon (electromagnetic radiation)	$^{0}_{0}\gamma$	0	0

The nucleon numbers in the notation are the clue: an alpha particle carries four nucleons, so it is heavy; beta particles and photons carry none.

Where each one comes from

The composition is not arbitrary; each radiation comes from a different change in the nucleus.

Alpha emission happens in heavy nuclei that have too few neutrons for stability. The nucleus ejects two protons and two neutrons together, as one alpha particle. Losing two protons changes the element: $Z$ falls by 2 and $A$ falls by 4.
Beta-minus emission happens when a nucleus has too many neutrons. A neutron changes into a proton, and the nucleus emits an electron to conserve charge: $\text{n} \rightarrow \text{p} + \text{e}^- + \bar{\nu}_e$ . The electron is created in the nucleus, not one of the orbital electrons.
Beta-plus emission happens when a nucleus has too many protons. A proton changes into a neutron and a positron is emitted: $\text{p} \rightarrow \text{n} + \text{e}^+ + \nu_e$ . The proton number falls by 1.
Gamma emission follows either kind of decay. The daughter nucleus is often left in an excited state and drops to a lower energy state by emitting a photon. The proton and nucleon numbers do not change at all.

Why an alpha particle is a helium nucleus

An alpha particle is not "like" a helium nucleus — it is one, without the electrons. Two protons and two neutrons is exactly the nucleus of $^{4}_{2}\text{He}$ , which is why the symbol for an alpha particle is the same as the nuclide notation for helium-4.

That also explains its charge. Two protons give $+2e$ and the neutrons add nothing, so the charge is $+2e$ — twice the elementary charge. Its mass is about $4\ \text{u}$ because the four nucleons each contribute roughly $1\ \text{u}$ and the electrons are missing.

Beta particles confuse students because an electron is not a nucleon, so how can it leave the nucleus? The answer is that it is created there when a neutron changes into a proton. The nucleus contains no electrons beforehand.

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Apply

Fill in the blanksQuestion 3 · Apply

Answer the 3 questions above to continue.