# Nuclear physics

In terms of progressing through the material, the sequence below ensures that students starting from first principles will meet each item having the prerequisite knowledge. It is not a strict teaching order, as some items (for example, Rutherford scattering) can be taught in isolation and some depend only very lightly on previous material: for example, students don't need to know about the various *types *of radioactive decay in order to understand the exponential nature *of* decay.

- stability of the nucleus - the idea that some nuclei are unstable, and that they undergo radioactive decay (often in more than one step) to become stable
- nuclear changes in decay - the effect of alpha, beta and gamma decay on the number of protons and neutrons in the nucleus. For more able students, this could lead on to looking at balanced nuclear equations
- the range and penetrating power of alpha and beta particles and gamma rays
- the decay constant and exponential decay - the fact that radioactive decay is a random process (be extra aware of students' misconceptions of what random means!) and that this random process leads to exponential decay
- half-life - this follows on from the previous topic on exponential decay and involves working with the radioactive dice graphs produced
- fission and electricity generation - beginning with a brief look at the use of fission in electricity generation, and then moving on to a more detailed look at the fission process and the chain reaction
- fusion - this follows on from fission, and introduces students to the famous E=mc
^{2}equation. It is important to stress that the equation means that mass and energy are equivalent - the same thing - rather than it being an equation in the mould of speed = distance / time or voltage = current × voltage - Rutherford scattering - pupils do not
*need*to know what an alpha particle is before they begin this, but the concept will be easier to explain if they do

Points to consider:

- the way that students use the word random is very different to its meaning within physics. To many students, random means unlikely or strange, so it's important to point out this difference to them.
- students should have learnt it during GCSE, but some always forget that half-life is the time taken for half of the
*remaining*nuclei to decay - a sample isn't all gone after two half-lives - the liquid drop model may be useful in explaining to the more inquisitve pupils why larger nuclei are more prone to decay

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

## Links and Resources

### Teaching Radioactivity

Produced by the Institute of Physics, this general non-topic-specific resource (also available as a DVD), covers a wide range of radioactivity topics, and includes videos and animations. There are also ideas and instructions for teaching activities, including a sheet that allows students to calculate their individual background dose.

### Interactive Chart of the Nuclides

Clicking on one of the isotopes in the chart will display details about that isotope. The really important thing to point out to students is the line of stability (black squares) indicating stable isotopes, and the colouring of other isotopes according to their decay mode.

- isotopes that are diagonally right-and-down of the line of stability (coloured cyan) are neutron-rich, and therefore decay by beta-minus decay, turning a neutron into a proton and moving diagonally up-and-left towards the line of stability
- isotopes that are diagonally left-and-up of the line of stability (coloured magneta) are neutron-deficient, and therefore decay by beta-plus decay, turning a proton into a neutron and moving diagonally down-and-right towards the line of stability
- isotopes towards the top-right of the diagram (coloured yellow) are too heavy to be stable, and therefore decay by emitting alpha particles, moving down and left on the diagram, towards the tip of the line of stability (lead-208)
- excited nuclear states that decay by gamma emission do not move on the chart of the nuclides

A version of the chart suitable for printing as handouts, or using on PowerPoint presentations is available from Wikipedia (alternative version). A similar version of the chart, but coloured by half-life is also available. This is useful for drawing students' attention to the wide variety of half-lives and how they change as you move away from the line of stability.

### Penetrating Power of Alpha, Beta and Gamma

For schools without access to radioactive sources, this is a very useful video that demonstrates the different penetrating powers of alpha, beta and gamma radiation. There is no music or voice-over, and the readout from the GM tube is clearly visible, so it could be used without sound.

The absence of any narration or explanation means that teachers can show the video to students and have them draw conclusions about the penetrating power of the various forms of decay. Teachers should then compare students' conclusions and ensure that there are no misconceptions.

The general rule is this:

- alpha particles are stopped by a few centimetres of air
- beta particles are stopped by a few millimetres of aluminium
- gamma rays are "stopped" by a few centimetres of lead. It may be worth pointing out to students that gamma rays can never be completely stopped, but rather that their intensity decays exponentially as they pass through the lead, and after a few centimetres is so low as to be below the background level (see Nelson and Reilly [PDF] for detailed background).

The sources used are plutonium-239 (alpha), strontium-90 (beta) and radium-226 (gamma). The video of the Pu-239 alpha emitter is particularly useful, as it is not "contaminated" by gamma emission as the standard school americium-241 source is. The Ra-226 gamma emitter is also much more powerful than the standard school Co-60 source.

### Episode 515: The Radioactive Decay Formula

The important activity here is TAP 515-3, which uses dice to model radioactive decay. Students begin with a large number of dice (eg 50) and throw them all; they count the number that come up (eg 1), and remove these from the pile and then re-throw the remaining dice. They continue this process until there are no "un-decayed" dice remaining.

Plotting a graph of dice remaining against number of throws will yield an exponential decay curve. More able students can use Excel and it's curve-fitting trendline function to find the radioactive decay equation and therefore the time constant.

Analysis of exponential decay curves and the random decay of radioactive nuclei leads onto a discussion of half-life, which comes next.

### HyperPhysics Half-Life Section

This follows on from the random decay of radioactive isotopes. It contains some very useful materials that can be used to create resources (eg PowerPoint presentations). It also includes an interactive calculator that students and/or teachers can use.

Teachers could work through the website on an interactive whiteboard with students, or issue each student with a laptop or tablet and have them work through it at their own pace.

### Build Your Own Nuclear Reactor

This is a great fun video from the old Teachers' TV series *Inside Science*. It will be useful for teaching about the nuclear fission chain reaction.

The video looks at the mechanics of nuclear fission and the chain reaction, the safety of fission and the radioactive waste produced, and looks at the arguments for and against the use of nuclear power.

If teachers want to go into more detail about how nuclear reactors work then the TAP 528 episode on controlling nuclear fission is worth a look.

### Interactive Nuclear Fission Animation

An excellent interactive animation to use on an interactive whiteboard or with a data projector. This is a good opportunity to use PPPB questioning in class.

The first tab shows one nucleus of uranium-235, and how bombarding this nucleus with neutrons raises it into an excited state from which it then fissions. This is important for students' understanding of the very first stage of the nuclear chain reaction.

The second tab shows how the fission chain reaction propagates, and how various factors, including the ratio of U-235 to U-238 affect this chain reaction. Varying these factors systematically and re-running the simulation will help students to get a handle on how the chain reaction is maintained (note: the size of the containment case can be altered by clicking and dragging it).

The third tab shows the operation of a model nuclear reactor, and how the use of control rods alters the rate of reaction. Again, systematic variation of the position of the control rods should help students' to get an idea of their role in the chain reaction.

### Powering the Future

Dr Melanie Windridge's IOP Schools Lecture looks at the physics behind fusion reactions in the sun, and how JET is trying to use fusion for power on Earth. Highly recommended as an introduction to the topic of fusion.

### Rutherford Scattering Interactive Animation

This animation allows you to simulate the Rutherford-Geiger-Marsden alpha-particle scattering experiment. For a more quantitative approach, more able students can measure the ratio of deflection angles for particles on different approach trajectories and compare this with Rutherford's scattering formula.

Notes:

- make sure to use both tabs at the top of the screen. The second tab allows you to simulate the plum pudding model
- the animation is not to scale: the electron orbiting at the edge of the diagram would actually be much, much further away
- you may want to ask pupils what the effect of altering the number of neutrons in the nucleus will be - they may be surprised to discover that it has no effect
- you may wish to use this animation in tandem with the
*Models of the Hydrogen Atom*animation, to look at reasons for rejecting previous models of the atom

### Episode 521: Rutherford’s Experiment

This resource follows-on from the interactive Rutherford scattering animation, and contains discussion on the importance of Rutherford's experiment, and ideas for more practical investigations.

The follow-up to this episode, on the size of the nucleus, is located here.

Subject(s) | Science |
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Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

Rating | |

URL | https://www.stem.org.uk/lxpbr |