Lesson 6 of 7
Uses and hazards
I can discuss medical and industrial applications and hazards using both half-life and the penetration and ionisation of emissions.Syllabus
Syllabus K323, 20(i). Discuss medical and industrial applications and hazards using both half-life and the penetration and ionisation of emissions.
Make a guess. You are not marked.
A paper mill checks paper thickness with a source on one side of the sheet and a detector on the other. Which radiation should it use?
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Beta, because paper partly absorbs it, so the count changes with thickness.
Beta passes through paper but is partly absorbed, so a thicker sheet gives a lower count. The change in count can control the rollers.
Radioactivity is useful when its emissions can be detected or transfer energy where needed. Those same interactions can damage living tissue.
Alpha is strongly ionising with low penetration; gamma is highly penetrating. Beta lies between them in the usual comparison. Half-life describes how rapidly a source's activity falls, not the range of its radiation.
Follow the source, material and detector
- A gamma-emitting medical tracer
Radioactive material inside the body emits radiation that can reach a detector outside. The detected radiation gives information about the tracer's distribution.
The source is inside, and radiation travels outwards towards the detector. This differs from an external X-ray source sending radiation through the body. Suitable penetration is needed for the emitted signal to reach the detector.
- A beta thickness gauge
A source and detector sit on opposite sides of a moving sheet. Under matched material and geometry conditions, a thicker sheet absorbs more, giving a lower detected rate. A thinner sheet gives greater transmission.
The chosen radiation must be partly transmitted through the relevant sheet. Alpha may be absorbed too completely, while very penetrating radiation may show too little change. Beta is useful for suitable paper, plastic or light-metal sheets, but is not the correct choice for every material and thickness.
Keep the source output and detector conditions accounted for: a falling count rate caused by source decay or changed alignment would not, by itself, show that the sheet became thicker.
- Gamma radiation in cancer treatment
Ionisation can damage or kill tumour cells. It can also damage healthy cells, so the useful effect requires controlled exposure. Gamma rays do not automatically recognise and target cancer cells.
Follow the radiation and locate the radioactive material
Filled brown dots mark radioactive material. Teal arrows show selected radiation paths; blue arrows show sheet movement. These schematic paths and thicknesses are not numerical transmission measurements.
Thinner sheet: a larger detector rate
Thicker sheet: a smaller detector rate
Compare the same sheet material with the source, detector and other relevant conditions fixed. The useful comparison has partial transmission: some radiation is absorbed and some reaches the detector.
A tracer is an internal source
The source is inside and the detector is outside. Some emitted gamma radiation travels out to the detector.
Irradiation: radiation reaches the object
Contamination: radioactive material is present
Irradiation concerns radiation reaching an object. Contamination concerns radioactive material on or inside it. The two descriptions refer to different things and can occur together.
Distinguish irradiation from contamination
- Irradiation or exposure
- Radiation reaches an object or person. Exposure to an external source does not, by itself, mean radioactive source material has been transferred.
- Radioactive contamination
- Radioactive material is present where it is not wanted, such as on a surface or inside a body. It can continue emitting from that location.
Ionisation can damage cells or DNA, kill cells or increase cancer risk. The consequences depend on the radiation, amount and duration of exposure, and where its energy is absorbed.
Typical alpha particles have little penetration from outside, but alpha-emitting material inside the body can damage nearby cells. There may be no protective outer layer between that source and the tissue. Gamma can reach tissue from outside. Penetrating power alone does not rank every exposure's hazard.
A source's activity is its rate of decay. It does not directly give the energy absorbed by a particular person. Equal activities can produce different exposures because the emissions, shielding, separation and source locations can differ.
Match protection to the route of exposure
- Reduce exposure time: less time exposed can reduce the energy received under otherwise unchanged conditions.
- Increase separation: greater distance from an external source can reduce the radiation reaching a person.
- Use suitable shielding: the material and thickness must suit the emission and conditions. Reduced transmission does not mean the source has stopped being radioactive.
- Contain the source material: containment addresses transfer and spread of radioactive material, rather than only shielding radiation arriving from outside.
Check your understanding
Typical alpha particles are stopped by paper or the outer layer of intact skin. Why can an alpha-emitting material inside the body still be hazardous?
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It emits strongly ionising particles straight into nearby living cells.
Low penetration means a short range, not an absence of energy transfer. Internal alpha-emitting material can cause intense local ionisation. The source's location and the exposure conditions matter.
Choose the half-life and radiation for the task
A tracer must stay active long enough for the investigation, while a shorter subsequent persistence can limit continued activity afterwards. A continuously used industrial gauge needs suitably sustained output; a long-lived source also leaves storage and disposal considerations.
Penetration must suit the path. Radiation must reach a detector or target where required, while useful partial absorption makes a thickness gauge sensitive to changes. Ionisation explains both useful cell damage and the hazard. A suitable half-life cannot compensate for an emission that is unsuitable for the task.
Use the stated requirements
An invented industrial-tracer comparison
A detector at the far end of a pipe will follow the tracer. Each candidate source gives a corrected count rate of 1600 counts/min at the start and emits a suitable radiation. At least 400 counts/min must remain after a 2 h investigation. The source should not stay active for long afterwards.
- R: half-life 20 min
- Two hours is 120 min, containing six half-lives. The count rate falls to 1600/64 = 25 counts/min, below the required 400 counts/min.
- S: half-life 2 h
- One half-life leaves 800 counts/min. It meets the investigation requirement, then continues halving over the following hours.
- T: half-life 8 days
- It retains ample activity during the short investigation, but stays active much longer afterwards than S.
S best matches the supplied conditions. R decays too quickly for the task, while T remains active much longer. This conclusion depends on the sources starting at the same count rate with the same kind of radiation.
The shortest half-life is not automatically best, and half-life alone does not rank hazard. Initial activity, emission type and energy, exposure time and the location of the material also matter.
Check your understanding
A pipe tracer must still give at least 400 counts/min (corrected) after 2 h. Sources R, S and T each start at 1600 counts/min and emit a suitable radiation. Their half-lives are 20 min, 2 h and 8 days. Which source meets the task without staying active for long afterwards?
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S, with a half-life of 2 h.
One half-life leaves 800 counts/min after 2 h, which meets the requirement. It then keeps halving, so within a day it has fallen to a tiny fraction of its starting rate.
Practise exam questions
Write your answer first, then tick the mark points it makes.
Exam-style question
A factory uses a radioactive source and a detector to check the thickness of paper as it is made. Explain why a beta source is used, rather than an alpha or a gamma source. [3]
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Exam-style question
Data-based question: Finding a leak with a radioactive tracer [10]
An engineer wants to find a leak in a water pipe buried 1.0 m underground. She adds a small amount of a radioactive isotope to the water. A day later she walks along the ground above the pipe with a detector and records the count rate. The background count rate is 25 counts per minute.
| isotope | radiation emitted | half-life |
|---|---|---|
| A | alpha | 6 hours |
| B | beta | 15 hours |
| C | gamma | 6 hours |
| D | gamma | 30 years |
| distance along pipe / m | count rate / counts per min |
|---|---|
| 0 | 31 |
| 10 | 29 |
| 20 | 30 |
| 30 | 186 |
| 40 | 33 |
| 50 | 28 |
- (a) Choose the most suitable isotope from Table 1. Explain your choice using both the radiation and the half-life. [3]
- (b) Calculate the corrected count rate at 30 m. [1]
- (c) State where the leak is and explain why the count rate is high there. [2]
- (d) State one assumption you made in (c). [1]
- (e) A technician says the readings at the other positions show that the isotope is present there too. Evaluate this claim. [3]
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