Edexcel IGCSE Physics · Spec 7.3
Nuclear Equations & Background Radiation
Writing nuclear equations and the sources of background radiation.
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Nuclear Equations & Background Radiation
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Explained
Balancing nuclear equations, and what is always around us
A nuclear equation is a bookkeeping exercise. Two numbers must be the same on both sides of the arrow: the total nucleon number on the top, and the total proton number on the bottom. Get those to balance and the equation is right.
Alpha decay
An alpha particle is two protons and two neutrons, which is a helium nucleus. It is written as helium with nucleon number 4 and proton number 2.
So alpha decay reduces the nucleon number of the original nucleus by 4 and its proton number by 2. Because the proton number has changed, the atom has become a different element, two places back in the periodic table.
Beta decay
A beta particle is an electron emitted from the nucleus. It is written with nucleon number 0 and proton number minus 1.
Where does it come from, when the nucleus contains no electrons? A neutron turns into a proton and an electron, and the electron is thrown out. So the nucleon number does not change, because a neutron has simply become a proton, but the proton number goes up by one. The atom moves forward one place in the periodic table.
The minus one on the bottom is what makes the arithmetic work, and it is the part most often written as plus one by mistake.
Gamma emission
Gamma is a wave rather than a particle, so it carries away energy but no nucleons and no charge. Both numbers stay the same, and the element does not change. Gamma is usually emitted alongside alpha or beta as the nucleus settles.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks what an alpha particle is made of and credits two things: that it contains protons and neutrons, and that it has two of each. Its notes then say to ignore electrons and to ignore any reference to a helium nucleus. Calling it a helium nucleus is true, but it does not answer a question about what it contains.
On the equation itself, the marks are separated into the proton number of the alpha, the nucleon number of the alpha, and the nucleon numbers balancing, with an error carried forward allowed from an incorrect alpha. Each number is earning its own credit, so filling in every box is worth doing even when you are unsure of one.
On background radiation, a different mark scheme awards one mark for the idea that it is always present, allowing phrases such as radiation all around us or in the environment, and a second mark for an appropriate named source. It lists cosmic rays or the Sun, rocks and soil, radon, weapons testing, food, nuclear disasters and medical equipment. Then it rejects the cosmic microwave background radiation, which is a different thing that happens to share an abbreviation.
The same paper accepts the unit becquerel written with any prefix and in almost any capitalisation, which is a reminder that examiners are strict about physics and generous about handwriting.
Background radiation and why it matters
Background radiation is the low level of ionising radiation that is always present. Most of it is natural. Radon gas seeping from rocks is the largest single contributor in many places, with cosmic rays, the ground itself, and even the potassium in food and drink adding to it.
A small proportion is artificial, mainly from medical procedures such as X-rays, with much smaller amounts from nuclear power and from weapons testing decades ago.
For any measurement, the background count must be subtracted before you use the reading, because the detector counts background whether your source is there or not. Forgetting to subtract makes every half-life calculation that follows wrong.
Spec 7.3
What you need to know
- Write nuclear decay equations
- Describe how alpha and beta decay change a nucleus
- Explain background radiation
Active recall
Quick check
Answer each question before opening the answer.
How do the mass and atomic numbers change in alpha decay?
The mass number decreases by 4 and the atomic number decreases by 2.
Give two sources of background radiation.
Any two of: radon gas, rocks and soil, cosmic rays, food and drink, medical sources.
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