Radiation from unstable nuclei
Definition
Unstable nucleus
A nucleus that breaks down by itself and emits nuclear radiation because the particles inside it are not held together permanently.
Definition
Nuclear radiation
The particles or electromagnetic waves given out by an unstable nucleus when it breaks down.
Definition
Radioactive decay
The breakdown of an unstable nucleus, in which the nucleus emits nuclear radiation and becomes more stable.
- Every atom has a nucleus made of protons and neutrons, with electrons arranged in shells around it.
- The protons and neutrons are held together by an attractive force that acts only over a very short range inside the nucleus.
- In some nuclei that attraction does not balance the electrostatic repulsion between the positive protons, so the nucleus is unstable.
- An unstable nucleus becomes more stable by emitting nuclear radiation, and this emission is called radioactive decay.
- The five emissions to recall are alpha, α\alphaα, beta minus, β−\beta^{-}β−, beta plus, β+\beta^{+}β+, gamma, γ\gammaγ, and neutron radiation.
- Alpha radiation is a particle of two protons and two neutrons thrown out of the nucleus.
- Beta minus radiation is a fast-moving electron that is created in the nucleus at the moment of decay and then emitted.
- Beta plus radiation is a positron, a particle with the same mass as an electron but a relative charge of +1+1+1.
- Gamma radiation is a high-energy electromagnetic wave emitted from the nucleus, so it has no mass and no charge.
- Neutron radiation is a neutron emitted from the nucleus, so it also carries no charge.
- All five emissions come out of the nucleus, never from the electron shells that surround it.
Decay is random and spontaneous
Definition
Random process
A process whose outcome cannot be predicted, so it is impossible to say which nucleus will decay next or exactly when it will decay.
Definition
Spontaneous decay
Decay that happens by itself, without being triggered by anything outside the nucleus.
- Radioactive decay is random, so you cannot pick out one nucleus in a sample and say that it will be the next to decay.
- You also cannot predict the exact moment at which a chosen nucleus will decay.
- Every unstable nucleus of the same type has the same chance of decaying in the next second, no matter how long it has already existed.
- Decay is spontaneous, so it is not started or speeded up by heating, cooling, crushing, lighting or reacting the material chemically.
- Because decay is random, a detector records a different count in each equal time interval even when the source and the detector have not been moved.
- A real sample contains an enormous number of nuclei, so the average rate of decay across the whole sample is steady and predictable even though single decays are not.
- Calling decay random describes the timing of each decay, not any uncertainty about which radiations can be emitted.
Example
Random counts from one source
- A student leaves a source and a detector clamped in place and records the count in four separate 10 s10\ \text{s}10 s intervals as 484848, 555555, 434343 and 505050.
- The four readings differ because decay is random, so the number of nuclei that happen to decay in each interval is not the same.
- The total count is 48+55+43+50=19648+55+43+50=19648+55+43+50=196 over a total time of 40 s40\ \text{s}40 s.
- The mean count rate is 19640=4.9\dfrac{196}{40}=4.940196=4.9 counts per second.
- Counting for longer, or taking more repeats, brings the mean closer to the true count rate because the random variations average out.
Ionising radiation
Definition
Ion
An atom that has gained or lost one or more electrons and therefore carries an overall electric charge.
Definition
Ionisation
The removal of an electron from an atom, which leaves the atom as a positively charged ion.
Definition
Ionising radiation
Radiation that carries enough energy to knock electrons out of atoms and turn those atoms into ions.
- An atom is electrically neutral because it contains equal numbers of positive protons and negative electrons.
- When ionising radiation passes close to an atom it can transfer enough energy to pull an outer electron away.
- The atom is then left with more protons than electrons, so it becomes a positive ion.
- The four ionising radiations to recall are α\alphaα, β−\beta^{-}β−, β+\beta^{+}β+ and γ\gammaγ.
- Any one of those four is a correct answer when a question asks you to name an ionising radiation.
- Neutron radiation is emitted by unstable nuclei but is not one of the four named ionising radiations.
- Ionisation is also what makes detection possible, because the ions and free electrons produced can be used to make an electrical pulse in a detector.
Exam technique
Naming radiations precisely
- Write β−\beta^{-}β− or β+\beta^{+}β+ rather than beta on its own, because the sign is part of the answer.
- Give positron radiation as the alternative name if a question asks what β+\beta^{+}β+ is also called.
- List all five emissions when asked what an unstable nucleus can emit, and only the four ionising ones when asked about ionising radiation.
- Define random as being unable to predict when a particular nucleus will decay, rather than saying that decay happens at any time.
- Use the word nucleus in your answer, because credit is regularly given for stating that the radiation comes from the nucleus.
Common Mistake
- Do not write that beta radiation comes from the electron shells, because a beta particle is created in the nucleus and emitted from it.
- Do not explain random by saying that scientists do not understand decay, because random describes the timing of each decay.
- Do not claim that heating a source makes it decay faster, because decay is spontaneous.
- Do not include neutron radiation in a list of the ionising radiations named in this topic.
- Do not drop the signs from β−\beta^{-}β− and β+\beta^{+}β+, because they are two different emissions.
Self review
- Name the five emissions that can come from an unstable nucleus and give the symbol for each.
- State the other name for β+\beta^{+}β+ radiation and give its relative charge.
- Explain what is meant by saying that radioactive decay is random.
- State two things that do not change the rate at which a source decays.
- Define ionisation and name the four ionising radiations.
