Nuclear Physics
Unit 5 of Cambridge IGCSE Physics 0625. The nuclear atom, isotopes, alpha, beta and gamma radiation, half life, detection and safety.
What you need to know
- Alpha is a helium nucleus, stopped by paper, strongly ionising. Beta is an electron, stopped by a few millimetres of aluminium. Gamma is an electromagnetic wave, reduced by thick lead.
- Isotopes are atoms of the same element with the same proton number but a different nucleon number.
- Half life is the time for half the undecayed nuclei to decay, or for the count rate to fall to half its value.
- Radioactive decay is random. You cannot predict which nucleus decays next, only the average behaviour.
- Background radiation must be subtracted from the measured count rate before a half life calculation.
- In alpha decay the nucleon number falls by 4 and the proton number by 2. In beta decay the nucleon number is unchanged and the proton number rises by 1.
Equations
- nucleon number A = protons + neutrons
- corrected count rate = measured count rate minus background
- after n half lives the activity is the original divided by 2^n
Nuclear Physics questions students ask
What are alpha, beta and gamma?
Alpha is a helium nucleus, two protons and two neutrons. Beta is a fast electron emitted from the nucleus. Gamma is a high energy electromagnetic wave.
What stops alpha, beta and gamma radiation?
Alpha is stopped by a sheet of paper or a few centimetres of air. Beta is stopped by a few millimetres of aluminium. Gamma is only reduced, by thick lead or concrete.
Which type of radiation is the most ionising?
Alpha, because it is large and highly charged. It is the most ionising and the least penetrating. Gamma is the opposite on both counts.
What is an isotope?
Atoms of the same element with the same number of protons but a different number of neutrons, so the same proton number and a different nucleon number.
What is half life?
The time taken for half the undecayed nuclei in a sample to decay, or for the count rate to fall to half its value.
Why must you subtract background radiation?
The detector picks up radiation from rocks, cosmic rays and the air as well as the source. Subtracting the background gives the true count rate from the sample, and examiners test this deliberately.
What happens to the nucleus in alpha decay?
The nucleon number falls by 4 and the proton number falls by 2, so the atom becomes a different element two places back in the periodic table.
What happens in beta decay?
A neutron turns into a proton and an electron. The electron is emitted, the nucleon number stays the same and the proton number rises by 1.
Is radioactive decay predictable?
No. It is random and spontaneous. You cannot say which nucleus will decay next, only what happens on average across a large number of nuclei.
Mistakes that cost marks
- Forgetting to subtract background count before working out half life.
- Saying radiation is "used up". The count rate falls because nuclei decay.
- Getting alpha and beta the wrong way round on penetrating power.
- Writing that beta particles come from the electron shells. They come from the nucleus.
- Treating decay as predictable rather than random.
Every Cambridge and Edexcel paper from 2016 to today, with its mark scheme, no login.
Open the paper libraryLive online classes across Egypt and the Gulf. No online payment.
Request accessOnline classes