Edexcel IGCSE Physics · Spec 7.6-7.8
Radiation Penetration and Nuclear Equations
Covers investigate penetration safely, Include neutron emission and Balance alpha, beta and neutron equations.
Physics revision video
Radiation Penetration and Nuclear Equations
Prefer to watch on YouTube? Open this video on YouTube.
Explained
How far each radiation travels, and balancing the equations
Alpha, beta and gamma differ in what they are, and everything else follows from that.
An alpha particle is two protons and two neutrons, so it is relatively large and carries a 2 plus charge. A beta particle is an electron emitted from the nucleus, far smaller, with a 1 minus charge. Gamma is an electromagnetic wave with no mass and no charge at all.
Penetration and ionisation are opposites
Alpha is stopped by a sheet of paper or a few centimetres of air. Beta passes through paper but is stopped by a few millimetres of aluminium. Gamma is never fully stopped, only reduced, by thick lead or concrete.
The ionising power runs the other way. Alpha is the most strongly ionising, beta less so, gamma least.
Those two facts are the same fact. Alpha is large and highly charged, so it interacts strongly with the atoms it passes, knocking electrons off them. Interacting strongly means losing energy quickly, which means not travelling far. Gamma interacts weakly, so it ionises little and travels a long way.
Investigating penetration safely
Measure the background count first, with no source present, and subtract it from every later reading.
Then keep the distance between source and detector fixed, and the counting time fixed, for every absorber. Place paper, then aluminium of increasing thickness, then lead, and record the count each time.
A drop when paper is added shows alpha. A further drop with a few millimetres of aluminium shows beta. A count that persists through lead shows gamma.
Handle the source with tongs, keep it at arm's length, point it away from people, and return it to its lead lined box as soon as the readings are taken. Distance, time and shielding are the three protections, and a safety question wants at least two.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme asks why an alpha source outside the body is less dangerous than one inside it, and credits three ideas: that the source is outside the body so the skin would absorb the alpha, that alpha has poor penetrating ability, and that alpha is absorbed by the glass of the apparatus. It condones highly ionising as part of that reasoning.
Notice how the danger reverses. Outside the body alpha is the least dangerous, because it cannot get through skin. Inside the body it becomes the most dangerous, because all that ionising power is deposited in a small volume of living tissue with no skin in the way.
A question about danger therefore has to say where the source is before it can be answered, and answers that rank the three radiations without reference to position are answering only half of it.
On the alpha particle itself, the same mark scheme wants what it contains: protons and neutrons, then two of each. Its notes say to ignore electrons and to ignore any reference to a helium nucleus. Calling it a helium nucleus is true but does not answer a question about what it contains.
Balancing the equations
Two totals must match on each side: the nucleon numbers on top, and the proton numbers below.
Alpha decay: nucleon number falls by 4, proton number falls by 2, because an alpha particle is helium with 4 and 2.
Beta decay: nucleon number is unchanged, proton number rises by 1. A neutron has become a proton and an electron, and the electron leaves. The beta particle is written with nucleon number 0 and proton number minus 1, and that minus is what makes the arithmetic work.
Gamma emission: both numbers unchanged, because gamma carries away energy but no nucleons and no charge.
Neutron emission: nucleon number falls by 1, proton number unchanged, since a neutron is 1 and 0. The element does not change, because the proton number decides the element.
Spec 7.6-7.8
What you need to know
- Investigate penetration safely
- Include neutron emission
- Balance alpha, beta and neutron equations
Active recall
Quick check
Answer each question before opening the answer.
Why subtract background count?
To isolate radiation from the source
How does neutron emission change the nucleus?
Mass number falls by one; atomic number is unchanged
What must balance in a nuclear equation?
Mass number and atomic number
Physics revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.