5.2.1 Detection of radioactivity (3)
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1.
The following table shows the count rates recorded by a Geiger-Muller tube under different conditions. The table shows the count rate with the detector shielded and unshielded. Calculate the background radiation count rate. Explain your method.
| Condition | Count Rate (counts/minute) |
| Shielded | 10 |
| Unshielded | 40 |
To calculate the background radiation count rate, we subtract the unshielded count rate from the shielded count rate. This is because the shielded count rate represents the radiation from the source *plus* the background radiation. Therefore, subtracting the unshielded count rate isolates the background radiation.
Background Radiation Count Rate = Shielded Count Rate - Unshielded Count Rate
Background Radiation Count Rate = 10 counts/minute - 40 counts/minute = -30 counts/minute
There appears to be an error in the provided data, as the background radiation count rate cannot be negative. The unshielded count rate should be *lower* than the shielded count rate. Assuming the data is correct, the calculation would yield -30 counts/minute. However, in a real experiment, the unshielded count rate should be significantly higher than the shielded count rate. A more realistic scenario would be, for example, Shielded = 10 CPM and Unshielded = 50 CPM, resulting in a background rate of 40 CPM.
2.
A student is investigating the radioactivity of different brands of luminous watch dials. They measure the count rate of a Geiger-Muller tube over a period of 5 minutes for each brand. The results are shown in the table below:
Brand A: 250 counts/minute
Brand B: 310 counts/minute
Brand C: 280 counts/minute
Calculate the relative activity of Brand B compared to Brand A. Show your working.
Relative Activity = (Activity of Sample B) / (Activity of Sample A)
Activity of Brand B = 310 counts/minute
Activity of Brand A = 250 counts/minute
Relative Activity of Brand B = 310 / 250 = 1.24
Therefore, the relative activity of Brand B compared to Brand A is 1.24.
3.
Ionising nuclear radiation can be measured using a detector connected to a counter. Describe how a Geiger-Müller tube works and explain the principle behind its operation in detecting ionising radiation.
A Geiger-Müller tube is a type of radiation detector used to detect the presence of ionising radiation. It consists of a gas-filled tube with a central wire running through it, maintained at a high voltage. The tube has a metal casing which acts as the cathode and the central wire acts as the anode.
Principle of Operation:
- When ionising radiation (alpha, beta, or gamma) enters the tube, it ionises the gas atoms within the tube. This creates positive ions and free electrons.
- The high voltage potential difference between the anode and cathode creates a strong electric field.
- The free electrons are accelerated towards the anode, and the positive ions are accelerated towards the cathode.
- As the electrons accelerate, they collide with other gas atoms, causing further ionisation – this is a chain reaction.
- This chain reaction results in a sudden surge of electrons reaching the anode, producing a measurable electrical pulse.
- The size of the pulse is independent of the energy of the incoming radiation; it only indicates the presence of radiation.
- After the pulse, the gas is quenched to stop further discharge, usually by a halogen gas.
The counter then registers each pulse as a count, providing a measure of the radiation intensity.