Edexcel IGCSE Physics · Spec 7.2
Radioactivity: Alpha, Beta & Gamma
The three types of nuclear radiation — alpha, beta and gamma — and their properties.
Physics revision video
Radioactivity: Alpha, Beta & Gamma
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Explained
Why nuclei decay, and the three things they emit
Some nuclei are unstable, and an unstable nucleus emits radiation to become more stable. That is the whole cause, and it is worth stating carefully because two parts of it are commonly got wrong.
Decay is random. No one can predict which nucleus will decay next or when, and nothing you do to the sample changes it. Heating it, cooling it, dissolving it or compressing it all leave the rate of decay untouched.
Decay is spontaneous. It is not triggered by anything happening around the nucleus, so it is unlike a chemical reaction in every way that matters. The instability is in the nucleus itself, caused by an unhelpful ratio of neutrons to protons or simply by there being too many particles for the nuclear force to hold.
The three emissions
An alpha particle is two protons and two neutrons, identical to a helium nucleus. It has a charge of 2 plus and, on this scale, a large mass.
A beta particle is a fast moving electron emitted from the nucleus. It is created when a neutron turns into a proton and an electron, and the electron leaves. It has a charge of 1 minus and a very small mass.
A gamma ray is a high energy electromagnetic wave. It has no charge and no mass, and it is usually emitted alongside alpha or beta rather than on its own, as the nucleus sheds surplus energy after rearranging.
Everything else about them follows from those three descriptions. Charge and mass decide how strongly each interacts with matter, and how strongly it interacts decides how far it travels.
Penetration and ionisation
Alpha is stopped by paper or a few centimetres of air. Beta passes through paper but is stopped by a few millimetres of aluminium. Gamma is only reduced, never fully stopped, by thick lead or concrete.
Ionising power runs in the opposite order. Alpha is the most strongly ionising, beta less so, gamma least.
Those are two statements of one fact. Alpha is large and doubly charged, so it interacts strongly with the atoms it passes and knocks electrons off them. Interacting strongly means losing energy quickly, which means not getting far. Gamma has no charge at all, so it interacts weakly, ionises little and travels a long way.
In a magnetic or electric field the two charged types deflect in opposite directions, and beta deflects far more than alpha because it is so much lighter. Gamma is undeflected, having no charge.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks for the instrument used to detect radiation and credits the Geiger Muller tube, allowing GM tube, Geiger Muller detector, GM detector and Geiger counter.
It then awards two marks for safety precautions from five: increasing the distance from the source, allowing stay two metres away or do not stand too close; minimising the time of exposure, allowing keeping the source in a metal box when not in use; not pointing the source at people; handling the source with tongs or gloves, allowing not touching it with bare hands; and placing shielding between the person and the source.
Beside that list is one refusal: ignore lead vest or coat.
Time, distance and shielding are the three protections, and each of the credited answers is one of them made specific. The refused answer is specific too, but it is protective clothing for someone standing in a radiation field all day, not a precaution for a school demonstration where the source stays in its box except when in use.
On a separate paper the mark scheme asks for a source of background radiation, and credits cosmic rays or the Sun, rocks or soil, radon, weapons testing, food, nuclear disasters and medical equipment. Then it rejects one answer outright: the cosmic microwave background.
That rejection catches a plausible confusion between two topics. The cosmic microwave background is evidence for the Big Bang, a faint microwave signal filling the universe. Background radiation in this topic means the ionising radiation present around us all the time, most of it from radon gas seeping out of the ground. Two things called background, and only one of them is radioactive.
Background radiation and measuring it
Background radiation is always present, so any measurement of a source is really a measurement of the source plus the background.
Record the count rate with no source present, over a reasonable time, and subtract it from every later reading. That corrected figure is the count rate due to the source, and it is what any calculation should use.
Radon accounts for about half of the background in most places, and it varies from region to region depending on the rock underneath, which is why the correction has to be measured rather than looked up.
Spec 7.2
What you need to know
- Describe alpha, beta and gamma radiation
- Compare their penetrating power
- Explain why the nucleus decays
Active recall
Quick check
Answer each question before opening the answer.
What are alpha, beta and gamma radiation?
Alpha — a helium nucleus (2 protons + 2 neutrons); beta — a fast-moving electron; gamma — high-energy electromagnetic radiation.
Compare the penetrating power of alpha, beta and gamma.
Alpha is stopped by paper; beta by a few mm of aluminium; gamma is only reduced by thick lead or concrete.
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