8.1.3

Test yourself

Unstable nuclei emit one of three types of radiation. alpha, beta and gamma.

• Does not change the proton or nucleon number of a nucleus.
• Makes the nucleus more stable by its emission.
• Is highly penetrative.
• Can be absorbed by several centimetres of lead, many metres of air and can travel through a vacuum indefinitely.

### Properties of γ radiation 2

• Gamma radiation is part of the electromagnetic spectrum.
• Gamma radiation follows an inverse square law:
• intensity at distance x from the source $= \frac{constant}{x^2}$

### Applications

• Gamma radiation is used extensively in medical imaging, curing cancer by destroying tumourous cells and for sterilising medical equipment.
• It can be used to irradiate food to stop food from going bad.

## Investigating the Inverse Square Law

Gamma radiation follows an inverse square law. We can perform an experiment to show that this is true.

• Do a background count with a Geiger-Muller (GM) tube and counter without a source present.
• Repeat this measurement and find an average background count per minute.

### 2) Experimental setup and method

• Position the source in a holder.
• Position the GM tube at different distances from the source.
• Measure the count rate at each distance several times and find an average for each distance.

### 3) Calculate corrected count rate

• Calculate the corrected count rate per min:
• Corrected count rate = average count rate at each distance − average background count

### 4) Plot graph

• Plot a graph of count rate against $\frac{1}{x^2}$ where x is the distance between the source and the GM tube.
• A straight line through the origin indicates that the inverse square law holds.
• The line might miss the origin if the distance to the source is uncertain.
• E.g. If the source is in the back of the holder, the distance 'x' may not be from the front of the tube but from somewhere inside it.
• Alternatively, plot log(count rate) against log(x) and the gradient should be −2.

• Background radiation can come from a large number of sources:
• Radon gas, emitted from the ground.
• Buildings (brick).
• Cosmic rays (from the sun and space).
• Medical procedures.
• Food and drink.

### Experimental error

• When measuring the radiation from a source, it is important to get accurate readings.
• The background radiation will skew readings if it is not accounted for.
• It is important to first measure the background radiation in an area and subtract it from the count found from a specific source.