Section 1 of 6What 'random' means here
Unpredictable individual nuclei
- Unpredictable individual nuclei It is impossible to say when one particular unstable nucleus will decay.
- It is also impossible to say which nucleus in a sample will decay next.
Equal chance
- Equal chance Every unstable nucleus of the same isotope has the same probability of decaying in any given time interval.
Not affected by surroundings
- Not affected by surroundings Temperature, pressure and chemical reactions do not change the chance of a nucleus decaying.
Only probabilities
- Only probabilities What can be estimated is the probability that a nucleus decays in a given period of time.
Section 2 of 6Evidence from count readings
1
Place a Geiger-Muller tube near a source, or simply measure background radiation, and set the counter to zero.
2
Record the number of counts after a fixed time, for example one minute.
3
Reset the counter and repeat the measurement several times.
4
Look at the results: for example 32, 11, 25, 16 and 28 show no steady trend or pattern.
5
Conclude that each count comes from a decay, and since the counts fluctuate unpredictably, decay must be a random process.
Section 3 of 6Dice analogy
Rolling a dice for a six
Cannot predict one throw
- Cannot predict one throw On any single roll you cannot know whether a six will appear, just as you cannot tell when one nucleus will decay.
Known probability
- Known probability Each roll has a 1 in 6 chance of being a six, like a fixed probability of decay per unit time.
Large numbers behave predictably
- Large numbers behave predictably Over a thousand rolls you can predict roughly how many sixes will occur, just as a large sample of nuclei decays at a predictable overall rate.
Section 4 of 6Individual nuclei versus large samples
| Situation | Can it be predicted? | Explanation |
|---|---|---|
| When one named nucleus decays | No | It is a matter of chance |
| Which nucleus decays next | No | All have the same probability |
| Roughly how many decay in a time | Yes, approximately | Large numbers average out the randomness |
| Effect of heating the sample | No change in decay rate | Decay depends on the nucleus, not the conditions |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Random | Happening by chance, so that no individual event can be predicted. |
| Probability | A measure of how likely an event is to occur in a given time or trial. |
| Geiger-Muller tube | A detector that produces a count each time radiation enters it. |
| Background radiation | Low-level radiation that is always present from natural and man-made sources. |
| Count rate | The number of detections per unit time recorded by a detector. |
Section 6 of 6Exam technique
1
Saying a nucleus decays after a set time - it can decay at any moment, and only the probability can be given.
2
Believing that heating or chemically reacting a radioactive substance speeds up its decay; the rate is unaffected.
3
Describing varying counts as a faulty instrument rather than as evidence of random decay.
4
Thinking that the nucleus that has waited longest is 'due' to decay; each nucleus has the same chance at all times.