Decay chains and the N-Z graph
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 3 practice questions, 5 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Following a nucleus downhill
- Moves on an N–Z graph
Read the transcript of the teaching part
Last time
Before we start: last time, in Potential dividers with thermistors and LDRs, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Following a nucleus downhill
A heavy nucleus like thorium does not decay once and stop. It decays, and its daughter decays, and so on, in a chain, until it reaches a stable nucleus like lead. A map of neutron number against proton number lets us follow every step as a simple move on a graph.
Moves on an N–Z graph
Plot the number of neutrons N on the vertical axis and the number of protons Z on the horizontal axis. Alpha decay: lose two protons and two neutrons. The point moves down two and left two. Beta-minus decay: a neutron becomes a proton. The point moves one right and one down. Beta-plus decay: a proton becomes a neutron. The point moves one left and one up.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Decay chains.
The whole chapter: Particle Physics, Particle Physics.
Chapter 11.2: Radioactive decay
- Isotopes, alpha and beta particles 6 min
- Alpha decay, energy and beam current 7 min
- Decay chains and the N-Z graph 7 min
Also for A Level Physics 9702
Which topics come up most in A Level Physics 9702 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.