Episode 510: Properties of radiations
Lesson for 16-19
- Activity time 65 minutes
- Level Advanced
The focus of this episode is the properties of ionizing radiation. It is a good idea to introduce these through a consideration of safety.
- Discussion: Ionising radiation and health (10 minutes)
- Demonstration: Deflection of beta radiation (10 minutes)
- Student activity: Completing a summary table (10 minutes)
- Student experiment: Inverse square law for gamma radiation (30 minutes)
- Discussion: Safety revisited (5 minutes)
Discussion: Ionising radiation and health
Why are radioactive substances hazardous? It is the ionising property of the radiation that makes it dangerous to living things. Creating ions can stimulate unwanted chemical reactions. If the radiation transfers sufficient energy it can split molecules. Disrupting the function of cells may give rise to cancer. Absorption of radiation exposes us to the risk of developing cancer.
Thus it is prudent to avoid all unnecessary exposure to ionising radiation. All deliberate exposure must have a benefit that outweighs the risk.
Radioactive contamination is when you get a radioactive substance on, or inside, your body (by swallowing it or breathing it in or via a flesh wound). The contaminating material then irradiates you.
How can you handle sources safely in the lab? Point out that you will be safe if you follow your local rules which will incorporate the following:
- Always handle sources with tongs
- Point the sources away from your body (and not at any anybody else)
- Fix the source in a holder which is not adjacent to where your body will be when you take measurements
- Replace sources in lead-lined containers as soon as possible
- Wash hands when finished
Follow the local rules for using radioactive sources, in particular do not handle radioactive sources without a tool or place them in close proximity to your body.
Demonstration: Deflection of beta radiation
Show the deflection of β by a magnetic field. (Make sure you have a small compass to determine which are the N and S poles of the magnet.) Is the deflection consistent with the LH rule? (Yes; need to recall that electron flow is the opposite of conventional current.) This demonstration is no good with the α particles, as they are absorbed too quickly by the air.)
NB The diagram above is common in textbooks, but is only illustrative. For the curvature shown of beta particles, the curvature of the alpha tracks would be immeasurably small.
Student activity: Completing a summary table
Display the table, with only the headings and first column completed. Ask for contributions, or set as a task; compile results.
|name||symbol||nature||electric charge||stopped by||ionising |
|What is it?|
|Alpha||α||particle||+2||mm air; paper||very good||He nucleus|
|Beta||β||particle||-1||mm Al||medium||very fast electron|
|Gamma||γ||wave||0||cm Pb*||relatively poor||electromagnetic radiation|
Can you see any patterns in the table? (Most ionising – the largest electrical charge – is the least penetrating.) Can you explain this? (The most ionising transfer energy the quickest.) How can the electrical charge determined? Deflection in a magnetic field.)
*NB Gamma radiation is never completely absorbed (unlike alpha and beta) it just gets weaker and weaker until it cannot be distinguished form the background.
Student experiment: Inverse square law for gamma radiation
Note: since you are unlikely to have sufficient gamma sources for several groups to work simultaneously, this experiment can be part of a circus with others in the next episode. Alternatively, it could be a demonstration.
Gamma radiation obeys an inverse square law in air since absorption is negligible. (Radiation spreads out over an increasing sphere. Area of a sphere = 4 π r 2 , so as r gets larger, intensity will decrease as 1/ r 2 . The effect of absorption by the air will be relatively small.
(Some students could do an analogue experiment with light, with an LDR or solar cell as a detector.)
When detecting g radiation with a Geiger tube you may like to aim the source into the side of the tube rather than the window at the end. The metal wall gives rise to greater
secondary electron emission than the window and this increase the detection efficiency.
Correct readings for background.
How can we get a straight line graph? We expect
I ∝ r -2,
so a plot of I versus r –2 should be direct proportion (i.e. a straight line through the origin). It is much easier to see if a graph is a straight line, rather than a particular curve. Lift the graph and look along the line – it’s easy to spot a trend away from linear. However, two points are worth noting:
- Sealed γ sources do not radiate in all directions, so do not expect perfect 1r 2 behaviour
- You do not know exactly where in the Geiger tube the detection is taking place, so plotting I - ½ against r gives an intercept, the systematic error in the measurement.
Discussion: Safety revisited
Return briefly to the subject of safe working. Background radiation is, say, 30 counts per minute. How far from a gamma source do you have to be for the radiation level to be twice this? Would this be a safe working distance? (Probably.) How much has your lifetime dose of radiation been increased by an experiment like the above? (Perhaps one hour at double the background radiation level – a tiny increase. It will be safe enough to carry out a few more experiments.)