Physics · 5 Nuclear physics

5.2.5 Safety precautions

Before you start

Alpha particles, beta particles and gamma rays are all ionising, and alpha particles are the most strongly ionising of the three (from 5.2.2).
Penetrating abilities: alpha particles are stopped by a sheet of paper or a few centimetres of air, beta particles by a few millimetres of aluminium, and gamma rays are reduced by thick lead or concrete (from 5.2.2).
Emission from a nucleus is spontaneous and random in direction (from 5.2.2).
Corrected count rate = measured count rate − background count rate (from 5.2.1).
Half-life is the time for half the nuclei of an isotope in a sample to decay, so the count rate halves in each half-life (from 5.2.4).
X-rays are electromagnetic waves, and excessive exposure to them damages cells (from 3.3).

Syllabus objectives covered (3)

State the effects of ionising nuclear radiations on living things, including cell death, mutations and cancer
Describe how radioactive materials are moved, used and stored in a safe way
Explain safety precautions for all ionising radiation in terms of reducing exposure time, increasing distance between source and living tissue and using shielding to absorb radiation

Part 1 of 6

What ionising radiation does inside a living cell

Alpha particles, beta particles and gamma rays are ionising. As they pass through a material, they knock electrons out of its atoms and molecules and turn them into ions. Living tissue is a material like any other, so radiation absorbed by your body leaves ions inside your cells.
A cell depends on large, delicate molecules. The most important one is its genetic material, DNA. Ionisation can break or change these molecules. The objective names three things that can follow, and the difference between them is mainly how much damage is done.
Cell death. A large dose can damage a cell so badly that it dies. Radiotherapy uses this on purpose: gamma rays are aimed at a tumour to kill the cancer cells. When the same thing happens to healthy cells, it is harm.
Mutation. With less damage, the cell can survive, but its DNA may be changed. A change in the DNA is a mutation, and the cell copies the change each time it divides. A mutation in a sperm cell or an egg cell can be passed on to a child.
Cancer. Some mutations stop a cell controlling its own division. The cell divides again and again, out of control, and forms a tumour. That is cancer. So cancer is not a separate route from mutation. It is one result that a mutation can lead to.
Every precaution in this lesson rests on one rule: the more ionising radiation your body absorbs, the more ionisation there is in your cells, and the greater the chance of cell death, mutation and cancer. No precaution makes the chance zero. Each one reduces the amount absorbed.
The trap in a State question is vagueness. 'It is harmful' or 'it damages you' does not say what the damage is. Write the effects by name: cell death, mutation, cancer.

Part 2 of 6

Outside the body or inside it

Where the source is matters as much as what it emits. Use the penetrating abilities you already know.
From outside the body, alpha particles are the least dangerous of the three. A few centimetres of air stops them, and so does the dead outer layer of your skin, so they do not reach living cells. Beta particles pass through the skin into the living tissue under it, and gamma rays pass right through the body.
Inside the body, the order turns round. Suppose an alpha-emitting material is breathed in or swallowed. Now nothing stands between it and the living cells around it. Alpha particles are the most strongly ionising, so all their ionisation is packed into the cells next to the material. An alpha emitter inside the body is the most dangerous case.
So there are two jobs for safety precautions. One is to stop radiation from a source outside you reaching your cells. The other is to keep the radioactive material itself off your skin and out of your mouth and lungs. That second job is why nobody eats or drinks where radioactive materials are used, why gloves are worn to handle unsealed materials, and why hands are washed afterwards.

Part 3 of 6

Three ways to cut the radiation you absorb

Every precaution for ionising radiation, in a school laboratory, a hospital or a factory, uses one or more of three ideas: reduce the exposure time, increase the distance between the source and living tissue, and use shielding to absorb the radiation. The Extended objective asks you to explain precautions in exactly these terms.
Time. A source emits all the time you are near it, so the radiation that reaches you adds up second by second. If the count rate where you are stays steady, the total is the count rate multiplied by the time. Halve the time and you halve the radiation you receive.
$counts received = count rate \times time$
Distance. Emission is random in direction, so radiation spreads out from the source in all directions. The further away you are, the more it has spread out, and the less of it passes through any part of your body. Air also absorbs some radiation on the way, and a few centimetres of it stops alpha particles completely.
The objective does not ask for a formula that links count rate and distance. What you need is the idea: further away, less radiation reaches the tissue. If you are given measurements, subtract the background first and then compare the corrected count rates, because only the corrected rate comes from the source.
Shielding. A layer of material between the source and you absorbs radiation, so less of it gets through. The material must suit the radiation: a sheet of paper stops alpha particles, a few millimetres of aluminium stops beta particles, and thick lead or concrete reduces gamma rays.
apparatus diagram — not drawn yet
A source on the left sends three arrows to the right, labelled α, β and γ; in their path stand three vertical barriers in order: a thin sheet labelled 'paper', a thicker sheet labelled 'a few mm of aluminium' and a thick block labelled 'thick lead'; the α arrow ends at the paper, the β arrow ends at the aluminium, and the γ arrow continues through the lead as a thinner, fainter arrow.
Each kind of radiation needs its own shield, and even thick lead only reduces gamma rays: some still get through.
Gamma rays are the awkward case. Thick lead absorbs most of them, but never all of them. A gamma shield reduces the radiation; it does not stop it. That is why shielding is used together with time and distance, not instead of them.
To explain a precaution, build the chain: the precaution (for example, using tongs) → the idea it uses (greater distance between the source and the hand) → less radiation reaches and is absorbed by living tissue → less ionisation in the cells → a lower chance of cell death, mutation or cancer. An Explain needs both halves: which idea, and what that does to the radiation the body absorbs.

Part 4 of 6

The same three ideas for X-rays

The objective says all ionising radiation, not only radiation from nuclei. X-rays are ionising too, and the precautions in an X-ray room use the same three ideas.
Shielding: the radiographer stands behind a screen that contains lead, which absorbs X-rays. Distance: she operates the machine from behind that screen or from outside the room, away from the beam. Time: the machine is switched on only for the short moment that the image needs.
Why does she take these precautions when one image gives only a small amount of radiation? Because she works with X-rays every day. The radiation she absorbs adds up over every exposure, so each precaution reduces a total, not just one small amount.
One difference matters. An X-ray machine produces X-rays only while it is switched on. A radioactive source cannot be switched off: its nuclei decay spontaneously, whatever you do. So a radioactive source must be shielded all the time, including when nobody is using it. That is the reason for safe storage.

Part 5 of 6

Storing radioactive materials

In a school, a sealed source is kept in a lead-lined box when it is not in use. The lead is shielding. It absorbs radiation that would otherwise reach anyone near the store.
The box is labelled with the radiation hazard symbol, so that anyone who finds it knows what is inside. It is kept in a locked cupboard or store, so that only trained people can get to it.
The store is away from where people work and sit. That is distance doing its job even while the box is closed.
Radioactive waste is stored on the same principles: sealed in strong containers, shielded, labelled and kept in a secure place. Waste with a short half-life can be stored until its activity has fallen to a safe level. Waste with a long half-life stays radioactive for a very long time, so its store must stay secure for that long.

Part 6 of 6

Moving and using a source

Inside the laboratory, the source travels in its lead-lined box until the moment it is needed. Once it is out, it is handled with long tongs or forceps, never with fingers. The tongs put distance between the source and the hand.
apparatus diagram — not drawn yet
A person seen from the side, standing at a bench with one arm fully stretched out, holding a small cylindrical source in a pair of long tongs; the open end of the source points away from the person and away from a second person standing further back; on the bench sits an open box labelled 'lead-lined box' with the radiation hazard symbol on its side.
Tongs and an outstretched arm give distance, the source points away from people, and the lead-lined box waits to shield it again.
The source is held at arm's length and pointed away from the body and from other people. Both of these keep living tissue further from the radiation.
Over long distances, for example from a supplier to a hospital or a factory, radioactive materials travel in strong, sealed, shielded containers labelled with the hazard symbol. The seal keeps the material in, and the shielding absorbs the radiation, so people near the container on its journey are protected.
While the source is out, the work is done quickly, and the source goes back in its box as soon as the measurement is finished. That is the time idea. The source is never pointed at anyone and never left lying on the bench.
Where sources are much more active, as in industry and nuclear power, people do not hold them at all. They use remote-controlled handling equipment and work behind thick shielding, watching through a shielded window.
People who work with radiation every day wear a film badge or another kind of dosimeter. It records how much radiation the wearer has received, so the total can be checked and kept below the permitted limit. Notice what it does not do: it measures exposure, it does not protect against it.

Worked examples

Example 1. A teacher uses a sealed gamma source for a class experiment. She takes it from the store, places it in front of a detector for a 3-minute measurement and then puts it away. Describe how she should store, move and use the source safely, and explain each precaution in terms of time, distance or shielding.

Show the working

Follow the source through its journey in order: store, move, use, return. Taking the stages in order stops you from leaving one out.
Store: in a lead-lined box, because the lead is shielding that absorbs gamma rays. The box is labelled with the hazard symbol and locked in a cupboard away from where people work, which uses distance.
Move: carry the source to the bench inside its box, so it stays shielded. Lift it out with long tongs and hold it at arm's length, pointed away from the class. Both increase the distance between the source and living tissue.
Use: set up the detector before the source comes out, so the source is out only for the 3 minutes it is needed, then put it straight back in the box. That reduces the exposure time. The class stays back from the bench, which is distance again.
Check that each line has both halves: the action and the idea behind it. 'Use tongs' on its own is a description. 'Use tongs, which increase the distance between the source and her hand, so less radiation is absorbed by her hand' is an explanation.

Answer: Store it in a labelled, lead-lined box (shielding) in a locked cupboard away from people (distance). Move it in the box (shielding), and handle it with long tongs at arm's length, pointed away from people (distance). Have the apparatus ready, use it for the 3 minutes only and return it to the box at once (time). Each precaution reduces the radiation absorbed by living tissue, so it reduces the chance of cell death, mutation and cancer.

Example 2. A technician has to move a small gamma source. A detector placed where her hand would be records 2520 counts/min at 5 cm from the source, and 120 counts/min at 25 cm from the source (the length of a pair of tongs). The background count rate is 20 counts/min. (a) Calculate the corrected count rate at each distance. (b) Calculate how many times smaller the corrected count rate is at 25 cm. (c) Done carelessly, with her hand 5 cm from the source, the job takes 6 minutes. Planned in advance and done with the tongs, it takes 2 minutes. Calculate the corrected count that the detector would record at the hand position in each case, and compare them.

Show the working

(a) Background radiation is there whether the source is present or not, so subtract it to find the part that comes from the source.
$2520 - 20 = 2500 counts/min 120 - 20 = 100 counts/min$
(b) Divide the corrected rate close to the source by the corrected rate further away. Use corrected rates: the measured rates would give the wrong factor because the background is in both.
$\frac{2500}{100} = 25$
(c) The count rate stays steady during the job, so counts received = count rate × time. Carelessly, at 5 cm for 6 minutes:
$2500 \times 6 = 15 000 counts$
With tongs, at 25 cm for 2 minutes:
$100 \times 2 = 200 counts$
Compare: $15 000 \div 200 = 75$ . The distance reduced the rate by a factor of 25 and the shorter time reduced it by a factor of 3, and the two factors multiply: $25 \times 3 = 75$ . Using two of the three ideas together does far more than either one alone.

Answer: (a) 2500 counts/min at 5 cm and 100 counts/min at 25 cm. (b) 25 times smaller. (c) 15 000 counts done carelessly and 200 counts with the tongs: 75 times fewer counts at the hand position.

Example 3. A sealed source is tested with a detector at a fixed distance. The background count rate is 30 counts/min. The count rates are: no absorber, 830 counts/min; a sheet of paper, 830 counts/min; 5 mm of aluminium, 330 counts/min; 25 mm of lead, 90 counts/min. (a) Deduce which types of radiation the source emits. (b) Suggest what the source should be stored in, and explain your choice.

Show the working

Subtract the background from every reading first. The corrected count rates are 800, 800, 300 and 60 counts/min.
Paper: the corrected rate stays at 800 counts/min. Paper would stop alpha particles, so the source emits no alpha particles.
Aluminium: the rate falls from 800 to 300 counts/min, so 500 counts/min got through paper but not through 5 mm of aluminium. That is the behaviour of beta particles.
Lead: 300 counts/min got through the aluminium, and 25 mm of lead reduces it to 60 counts/min but not to zero. Radiation that passes through aluminium and is only reduced by thick lead is gamma radiation.
(b) The gamma rays pass through a few millimetres of aluminium, so an aluminium box is not enough. Lead absorbs most of the gamma rays (and all of the beta particles), so the source belongs in a lead-lined box. Some gamma rays still get through the lead, so the box should also be kept locked away from where people work.

Answer: (a) Beta particles and gamma rays, and no alpha particles. (b) A lead-lined box, labelled and locked away from people: lead absorbs the beta particles and most of the gamma rays, which aluminium would not do, and the distance reduces what gets through the lead.

Example 4. A hospital puts a bag of waste from a gamma-emitting substance into a shielded store. The substance has a half-life of 6.0 hours, and the corrected count rate from the bag is 1280 counts/min. The bag may leave the store when its corrected count rate is 80 counts/min or less. Calculate the shortest time the bag must be stored, and explain why the waste is kept in a shielded store while it waits.

Show the working

The count rate halves once in every half-life. Halve it until you reach 80 counts/min, and count the halvings: 1280 → 640 → 320 → 160 → 80. That is 4 half-lives.
Check: $1280 \div 80 = 16$ , and $16 = 2 \times 2 \times 2 \times 2$ , so the rate has halved 4 times.
$time = 4 \times 6.0 = 24 hours$
While it waits, the waste is still emitting gamma rays, and nothing can switch that off. The shielding absorbs the radiation, so less of it reaches the people who work nearby. Less radiation absorbed by their tissue means a lower chance of cell death, mutation or cancer.

Answer: 24 hours (4 half-lives of 6.0 hours). The waste keeps emitting gamma rays during storage, and the shielded store absorbs them so that less radiation reaches people nearby.

Where people go wrong

Mistake: Saying that a film badge or dosimeter protects the worker from radiation.

Instead: A film badge measures and records how much radiation the wearer has received. It gives no protection.

Its job is monitoring: the record lets the total be checked and kept below the permitted limit. The protection comes from time, distance and shielding.

Mistake: Writing that thick lead stops gamma rays completely.

Instead: Thick lead absorbs most gamma rays but not all of them, so it reduces the radiation that gets through.

Gamma rays are the most penetrating of the three. This is why a gamma source in a lead-lined box is still kept away from people, and handled for as short a time as possible.

Mistake: Saying that alpha sources are safe because alpha particles are stopped by paper.

Instead: An alpha source outside the body is the least dangerous of the three, but alpha-emitting material breathed in or swallowed is the most dangerous case.

Inside the body there is no skin or air to stop the alpha particles, and they are the most strongly ionising, so all their ionisation happens in the nearby cells.

Mistake: Comparing measured count rates at different distances without subtracting the background.

Instead: Subtract the background count rate from every reading first, then compare the corrected count rates.

Background radiation is the same at every distance. Left in, it hides how fast the radiation from the source falls, and any factor you calculate from it comes out wrong.

Mistake: Writing that a mutation always causes cancer.

Instead: A mutation can lead to cancer. Cancer happens when a mutation stops a cell controlling its division.

Not every change to the DNA affects how a cell divides, so a mutation raises the chance of cancer rather than making it certain.

Check yourself

State three effects that ionising radiation can have on living cells.
Answer
Cell death, mutation (a change in the DNA) and cancer.
Explain why an alpha source held a metre away from you is much less dangerous than the same material if you swallow it.
Answer
A metre of air stops the alpha particles, and so does the dead outer layer of the skin, so none reach living cells. Swallowed, nothing stops them, and because alpha particles are the most strongly ionising, all their ionisation happens in the cells next to the material.
Identify the idea (time, distance or shielding) behind each precaution: (a) handling a source with long tongs; (b) keeping a source in a lead-lined box; (c) putting a source away as soon as the measurement is done; (d) a radiographer standing behind a screen that contains lead.
Answer
(a) Distance. (b) Shielding. (c) Time. (d) Shielding, and distance as well, because the screen is away from the beam.
A worker at a nuclear power station wears a film badge. Does it protect her? What is it for?
Answer
No. It records how much radiation she has received, so the total can be checked and kept below the permitted limit.
The corrected count rate at a technician's work position is 450 counts/min. Calculate the corrected count she would receive in a task that takes 8 minutes, and in the same task done in 3 minutes.
Answer
$450 \times 8 = 3600$ counts, and $450 \times 3 = 1350$ counts.
Explain why a gamma source is stored in a lead-lined box and not in a box made of a few millimetres of aluminium.
Answer
Gamma rays pass through a few millimetres of aluminium. Thick lead absorbs most of the gamma rays, so much less radiation reaches people near the box.
An X-ray machine can be made safe by switching it off. Explain why a radioactive source cannot be made safe this way, and what is done instead.
Answer
X-rays are produced only while the machine is switched on. A radioactive source keeps emitting because its nuclei decay spontaneously, and nothing can stop that. So the source must be shielded at all times, by keeping it in a lead-lined box in a locked store when it is not in use.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A school keeps a small sealed radioactive source that emits beta particles and gamma radiation. A teacher uses it in a class experiment.

(a) State two effects that the radiation from this source can have on living cells. 2 marks No calculator

Mark scheme

B2 any two from: (cell) death / cells killed; mutation / change to the DNA or genetic material; cancer; one mark each. insufficient: 'harmful', 'damages the body'

(b) Describe how the source should be stored when it is not being used. 3 marks No calculator

Mark scheme

B3 any three from: in a lead(-lined) or shielded box or container; labelled with the radiation hazard symbol or a warning sign; in a locked cupboard or secure store; away from where people work or sit; one mark each. insufficient: 'in a box' with no mention of lead

(c) Describe two precautions the teacher should take while she moves the source from its box to the apparatus. 2 marks No calculator

Mark scheme

B2 any two from: handles it with (long) tongs or forceps; holds it at arm's length or away from the body; points it away from people; moves it quickly or keeps it out of the box for as short a time as possible; keeps the class or other people away from it; one mark each. ignore: wearing gloves

Question 2. In a factory, a technician works near a gamma source. She measures the count rate with a detector at different distances from the source. The background count rate is 30 counts/min. At 0.5 m the count rate is 1230 counts/min, at 1.0 m it is 330 counts/min and at 2.0 m it is 105 counts/min.

(a) Deduce, using the data, what happens to the count rate from the source each time the distance from the source is doubled. 2 marks

Mark scheme

B1 background subtracted to give corrected count rates of 1200, 300 and 75 counts/min
B1 the (corrected) count rate falls to a quarter (is divided by 4) each time; insufficient: 'it decreases' with no factor

(b) Her task takes 20 minutes at a distance of 1.0 m from the source. Calculate the corrected count that the detector would record at that position during the task. 2 marks

Mark scheme

C1 $300 \times 20$ , ecf from a corrected count rate in (a)
A1 6000 counts

(c) She changes how she works. She now stands 2.0 m from the source and finishes the task in 12 minutes. Calculate the corrected count that the detector would record at her new position during the task. 2 marks

Mark scheme

C1 $75 \times 12$ , ecf from (a)
A1 900 counts

(d) Explain why working in this new way reduces the risk to her health. 2 marks No calculator

Mark scheme

B1 less radiation reaches or is absorbed by her body (tissue), because she is further away for a shorter time
B1 so less ionisation in her cells / lower chance of cell death, mutation or cancer

Question 3. In a hospital, a radiographer takes X-ray images of many patients each day. In another department, waste from a gamma-emitting substance with a half-life of 6.0 hours is kept in a shielded store.

(a) While the X-ray machine is switched on, the radiographer stands behind a screen that contains lead. Explain why. 2 marks No calculator

Mark scheme

B1 the lead absorbs X-rays / acts as shielding; allow: keeps her away from the beam (distance)
B1 so less (ionising) radiation reaches or is absorbed by her body (over the many exposures each day) / lower chance of cell damage, mutation or cancer

(b) The corrected count rate from a bag of the waste is 2400 counts/min. The bag may leave the store when its corrected count rate has fallen to 150 counts/min. Calculate the shortest time the bag must stay in the store. 3 marks

Mark scheme

C1 2400 → 1200 → 600 → 300 → 150, or $2400 \div 150 = 16$
C1 4 half-lives
A1 24 hours

How this is examined

Explaining a precaution in terms of time, distance or shielding: a two- or three-mark Explain about one precaution, such as using tongs, a lead-lined box or a radiographer's screen Name the idea the precaution uses (greater distance between source and tissue / shorter exposure time / the shield absorbs the radiation), then state that less radiation reaches and is absorbed by the body, then link that to less ionisation and a lower chance of cell death, mutation or cancer. 'It is safer' on its own is insufficient.
How a source is stored, moved and used safely: a Describe of two or three marks, set in a school laboratory, a hospital or a factory Separate, concrete actions, one per mark: kept in a lead-lined box; labelled with the radiation hazard symbol; locked away from where people work; handled with long tongs; held at arm's length and pointed away from people; returned to its box as soon as possible; workers wear a film badge to monitor their exposure.
Choosing the right shield for the radiation: a Deduce or Suggest of two or three marks, which can come with a table of count rates through paper, aluminium and lead Background subtracted before readings are compared; no change with paper means no alpha; a drop with a few millimetres of aluminium means beta; radiation that is only reduced by thick lead is gamma; a gamma source needs a lead container, because aluminium does not absorb gamma rays well enough.
Counts received over a time, and how long waste must be stored: a two- or three-mark Calculate, using a corrected count rate or a half-life The background subtracted first; counts received = count rate × time; for storage, the number of half-lives counted down to the allowed count rate and then multiplied by the half-life; the unit on the final answer (counts, minutes, hours).
What ionising radiation does to living things: a one- or two-mark State The effects by name: cell death (cells are killed), mutation (a change in the DNA), cancer. 'It is harmful' or 'it damages the body' does not say which effect, so it is insufficient.

Words to know

Ionising radiation: Radiation that knocks electrons out of atoms or molecules and turns them into ions. Alpha particles, beta particles, gamma rays and X-rays are all ionising.
Cell death: Damage to a cell so severe that the cell is killed.
Mutation: A change in the DNA (genetic material) of a cell, which the cell copies each time it divides.
Cancer: A disease in which cells divide out of control and form a tumour. A mutation can lead to it.
Exposure time: How long living tissue is near a source of radiation. At a steady count rate, the radiation received is in proportion to the time.
Shielding: Material placed between a source and people to absorb the radiation, such as paper for alpha, a few millimetres of aluminium for beta and thick lead or concrete for gamma.
Lead-lined box: A storage container with lead in its walls, which absorbs the radiation from the source inside.
Tongs (forceps): A long tool for picking up a source, which keeps the hand at a distance from it.
Film badge (dosimeter): A badge worn by people who work with radiation. It records how much radiation the wearer has received. It does not protect the wearer.
Radiation hazard symbol: The warning sign on containers and stores of radioactive material, which tells people what is inside.

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