Exercises: Detection of Ionising Radiation

1. a) Name two instruments used to detect ionising radiation.
2. b) What does a Geiger-Müller tube detect when radiation enters it?
3. c) Why are film badges worn by hospital radiographers?

1. a) What is background radiation?
2. b) Name two natural sources and one artificial source.
3. c) Which natural source gives the largest average dose to people in Ireland?

A Geiger counter records $240$ counts in $2.0$ minutes with no source present.

1. a) Calculate the background count rate in counts per second.
2. b) Why must this background be measured before experiments with a radioactive source?

A student measures a radioactive source with a Geiger counter. With the source present, $1,860$ counts are recorded in $5.0$ minutes. The background count in $5.0$ minutes is $300$.

1. a) Calculate the corrected count rate in counts per minute.
2. b) The source is at $5.0\ \text{cm}$ from the tube. Estimate the count rate if the distance is doubled, assuming the source behaves as a point source and obeys the inverse-square law.
3. c) State two reasons the measurement is subject to random fluctuations.

Explain how each of the following works in terms of ionisation:

1. a) A Geiger-Müller tube.
2. b) A cloud chamber (explain the visible tracks).
3. c) A gold-leaf electroscope used to detect radiation.

In some houses in Ireland radon gas accumulates in basements. The average annual dose from radon is about $2.0\ \text{mSv}$.

1. a) Explain why radon is a hazard when breathed in, relating it to alpha emission.
2. b) Calculate the dose over $70$ years at this average rate.
3. c) Describe one practical measure to reduce radon in a home.

An experiment is carried out to show the existence of natural background radiation, without any source.

1. a) Describe how to measure it with a Geiger-Müller tube and a counter, including how many readings to take and how to improve reliability.
2. b) A student records counts in successive $60\ \text{s}$ periods: $22, 18, 25, 20, 19, 24$. Calculate the mean count rate in counts per minute and the range.
3. c) Explain why the counts vary even though the source is constant, referring to the random nature of radioactive decay.
4. d) Explain why the percentage uncertainty falls when counting for a longer time.

A radioactive source emits alpha, beta and gamma radiation. A Geiger counter is placed in front, and absorbers are inserted one at a time. The corrected count rates are:

No absorber: $950\ \text{counts min}^{-1}$

Sheet of paper: $480\ \text{counts min}^{-1}$

$3\ \text{mm}$ aluminium: $50\ \text{counts min}^{-1}$

$5\ \text{cm}$ lead: $2\ \text{counts min}^{-1}$

1. a) Identify which radiation types are present and which are absorbed by each absorber.
2. b) Estimate the fraction of the original count due to each type.
3. c) Justify your conclusions, comparing ionising and penetrating ability.
4. d) State one safety precaution when handling the source.

A radiographer's annual occupational dose limit (excluding background) is $20\ \text{mSv}$ and the average background dose in Ireland is about $4\ \text{mSv}$ per year.

1. a) Calculate the maximum extra occupational dose, as a fraction of background.
2. b) A radiographer's film badge records $1.5\ \text{mSv}$ in a month. Calculate the annual dose if this rate continues, and state whether the limit would be exceeded.
3. c) Explain the three principles of radiation protection: time, distance, shielding, giving a practical example of each.
4. d) Discuss why background radiation must be taken into account when assessing the risk from a hospital source.