6.5.1 Uses of radioactivity

Matching radiation to the job

1. Every use of a radioactive source is chosen by matching two properties of the radiation to the job: how penetrating it is and how strongly ionising it is.
2. Alpha radiation ($\alpha$) is a helium nucleus, ${}_2^{4}He$, so it is the most strongly ionising type and the least penetrating; it travels a few centimetres in air and is absorbed by a sheet of paper.
3. Beta radiation (${\beta }^{-}$) is a fast-moving electron, ${}_{-1}^{0}e$, so it is moderately ionising and moderately penetrating; it passes through paper but is absorbed by about $3mm$ of aluminium.
4. Gamma radiation ($\gamma$) is a high-energy electromagnetic wave, so it is the least ionising and by far the most penetrating; it is only reduced, never fully stopped, by thick lead or concrete.
5. Penetration decides whether the radiation can reach the thing it must affect or the detector that must measure it.
6. Ionising power decides how much change the radiation makes when it arrives, whether that change is ionising air, damaging the DNA of a microorganism or damaging a tumour cell.
7. The activity of the source, measured in becquerel ($Bq$), is chosen alongside the type: a smoke alarm needs a tiny activity, whereas a sterilisation plant needs a very large one.

Smoke alarms

1. An ionisation smoke alarm contains a very small alpha source, usually americium-241, sealed inside an ionisation chamber.
2. Two metal electrodes in the chamber are connected across a potential difference, with a gap of air between them.
3. Alpha particles ionise the air in the gap, knocking electrons off air molecules to create positive ions and free electrons.
4. These charged particles are attracted to the electrodes, so a small steady current flows across the gap even though air is normally an insulator.
5. When smoke enters the chamber, smoke particles absorb alpha particles before they can ionise much of the air.
6. Fewer ions are produced each second, so fewer charge carriers cross the gap and the current falls.
7. An electronic circuit monitors the current and sounds the alarm as soon as it drops below a set value.
8. Alpha is the right choice because it is strongly ionising, so a tiny activity still produces enough ions to give a measurable current, and because its range in air is only a few centimetres, so the radiation cannot escape the casing into the room.
9. Gamma would fail on both counts: it is weakly ionising, it would pass through the smoke without being absorbed, and it would escape from the alarm.

Irradiating food

1. Food travels on a conveyor into a heavily shielded chamber containing a gamma source, typically cobalt-60.
2. Gamma rays pass through the packaging and through the full depth of the food, ionising molecules inside any bacteria, fungi, moulds and insect eggs present.
3. Ionisation damages the DNA of those organisms, so they are killed or can no longer reproduce.
4. With far fewer microorganisms alive, the food spoils more slowly and its shelf life increases, which cuts waste and lowers the risk of food poisoning.
5. Gamma is essential because alpha and beta would be absorbed by the packaging and would never reach the food inside.
6. The food is exposed to radiation but no radioactive material is put into it, so it does not become radioactive and is safe to eat.
7. Irradiation can slightly reduce the vitamin content of some foods, and irradiated food must be labelled so shoppers can still choose whether to buy it.

Sterilising equipment

1. Syringes, scalpels, dressings and surgical instruments are sealed inside their packaging first and then passed through a gamma irradiation plant.
2. Gamma rays penetrate the packaging and the instrument, ionising molecules inside any microorganisms on the surfaces and killing them.
3. Because the item is sealed before it is irradiated, nothing can land on it afterwards, so it stays sterile until the pack is opened.
4. Gamma sterilises without heating the object, so plastic syringes and delicate instruments are not melted or damaged, which is the main advantage over steam sterilisation in an autoclave.
5. The source is kept in a thick concrete cell and loading is done remotely, because the activity needed to sterilise in a reasonable time is very large.

Industrial tracers

1. To find a leak in an underground pipe, a gamma-emitting tracer is added to the liquid flowing through it.
2. A Geiger-Muller tube connected to a counter is moved along the ground above the pipe and the count rate is recorded at each position.
3. Along an intact section the count rate stays roughly steady, because the tracer inside the pipe is the same distance from the detector at every point.
4. At a leak, tracer escapes into the soil and collects there, so the detector records a sharp rise in count rate at that position.
5. The engineers then dig at that one position instead of excavating the whole length of the pipe.
6. Gamma is the only sensible choice because the radiation must pass through the pipe wall and the soil to reach a detector at the surface, and alpha and beta would both be absorbed long before that.
7. The tracer is chosen with a half-life long enough to complete the survey but short enough that its activity soon falls to a safe level.
8. The same method tracks the movement of sewage and silt at sea and finds blockages inside industrial pipework.

Thickness gauging

1. In a paper or aluminium foil mill a beta source such as strontium-90 sits above the moving sheet and a detector sits directly below it.
2. Some beta particles are absorbed by the sheet and the rest reach the detector, so the count rate depends on the thickness of material between them.
3. If the sheet becomes too thick, more beta particles are absorbed and the detector records a lower count rate.
4. If the sheet becomes too thin, fewer beta particles are absorbed and the detector records a higher count rate.
5. A control system compares the count rate with the target value and moves the rollers closer together or further apart until the reading returns to that value.
6. Beta is used because it is partly absorbed, which is exactly what makes the count rate sensitive to small changes in thickness.
7. Alpha would be absorbed completely, so the detector would read only background whatever the thickness, and gamma would pass almost entirely through, so the reading would barely change.
8. A source with a long half-life is chosen so that the activity, and therefore the target count rate, does not drift during the life of the machine.
9. Thicker or denser products such as rolled steel use a gamma source instead, because beta would be completely absorbed by steel.

Finding and treating cancer

1. A gamma-emitting tracer can be swallowed or injected, and it collects in the organ the doctor wants to examine.
2. Gamma is used because it is penetrating enough to leave the body and reach a detector outside it, whereas alpha and beta would be absorbed by the surrounding tissue.
3. A tumour is treated by aiming high-energy gamma radiation at it, so that ionisation damages the DNA of the tumour cells and stops them dividing.
4. Cancer cells divide more often than most healthy cells, which is why they are more strongly affected by the same dose.
5. Healthy cells in the path of the beam are damaged as well, which causes side effects, so the beam must be aimed accurately at the tumour and no wider.

6.5.2 Dangers of ionising radiation and precautions

How radiation damages cells

1. Alpha, beta and gamma radiation are all ionising, which means each one carries enough energy to knock an electron out of an atom it passes.
2. The atom left behind has lost a negative charge, so it becomes a positive ion.
3. When this happens inside a living cell, the atom that is ionised is part of a molecule, so ionisation breaks or alters that molecule.
4. Cells are mostly water, so most ionisation happens in water molecules and produces highly reactive fragments that then attack proteins, cell membranes and DNA nearby.
5. A single ionisation is harmless because a cell can repair it, so the danger comes from the very large number of ionisations produced along the path of the radiation.
6. Alpha radiation produces the most ionisations per centimetre of its path, so it does the most damage in the small volume it reaches, while gamma radiation produces far fewer ionisations but spreads them right through the body.
7. From a source outside the body, beta and gamma radiation are the greater hazard, because alpha radiation is absorbed by the dead outer layer of skin and never reaches living tissue.

Tissue damage and mutation

1. There are two separate outcomes to learn: the cell is killed, or the cell survives with damaged DNA.
2. If enough molecules inside one cell are ionised, the cell can no longer function and it dies.
3. When large numbers of cells in the same place are killed, the organ or the skin stops working properly, and this is called tissue damage.
4. A very large exposure in a short time causes radiation sickness, with reddened skin like a burn, hair loss, vomiting and damage to the bone marrow that makes blood cells.
5. If instead the cell survives but its DNA has been altered, the instructions the cell copies when it divides are no longer the original ones, and this change is a mutation.
6. Most mutations have no noticeable effect, because they fall in a part of the DNA that the cell does not use or because the cell repairs them.
7. Some mutations affect the genes that control how often a cell divides, so the cell begins to divide uncontrollably.
8. The mass of cells that builds up is a tumour, and a tumour that spreads and invades other tissue is cancer.
9. A mutation in a sex cell can be passed on to a child, whereas a mutation in any other cell affects only the person exposed.
10. Tissues whose cells divide most often are the most sensitive, which is why bone marrow, the lining of the gut and a developing foetus are damaged by exposures that leave muscle or bone unharmed.

Why exposure changes risk

1. Radioactive decay is a random process, and so is the path each emitted particle takes through the body, so nobody can say which cell will be hit or which mutation will occur.
2. This is why exposure is described using risk, the chance that harm occurs, rather than as a certain outcome.
3. A greater exposure means more ionising particles pass through the body, which means more ionisations, which means a greater chance that a mutation happens in a gene that matters.
4. So the risk of cancer increases with exposure, but a person who is exposed will not necessarily develop cancer, and a person who is not exposed still can.
5. Everyone is exposed to background radiation all the time, so precautions are about keeping any extra exposure as small as is practical rather than reducing it to zero.

Precautions when using sources

1. Every precaution works by cutting the number of ionising particles that reach living tissue, and there are three ways of doing it.
2. Reduce the time spent near the source, because the total exposure is the rate at which radiation arrives multiplied by how long you stand there.
3. Increase the distance from the source, because the radiation spreads out in all directions, so the further away you stand the smaller the fraction of it that passes through you.
4. Use shielding between the source and the person, choosing a material that absorbs that type of radiation: a few millimetres of aluminium for beta, and thick lead or concrete for gamma.
5. In practice these three principles become a set of working rules that a technician or a teacher follows every time a source is used.
   1. Lift the source with long handling tongs rather than fingers, which puts the whole length of the tongs between the source and the hand.
   2. Point the open face of the source away from people and never look directly into it.
   3. Take the source out only for the reading itself and return it to its lead-lined, clearly labelled container immediately afterwards.
   4. Store that container in a locked cupboard and keep a record of when each source is signed out.
   5. Keep everyone else at the far side of the room while the source is out of its container.
   6. Never eat, drink or apply cosmetics in the area, and wash hands afterwards, so that no radioactive material can be swallowed.
6. Industrial sources that cannot be shielded by hand are handled by remote manipulators behind thick concrete walls, which is the same three principles applied on a larger scale.

6.5.3 Half-life and radiation dangers

Linking half-life to activity

1. The half-life of an isotope tells you what fraction of the undecayed nuclei in a sample decay in a given time, and it is fixed for that isotope.
2. Take two samples containing the same number of undecayed nuclei: the one with the shorter half-life has a larger fraction of its nuclei decaying each second, so it has the greater activity.
3. A greater activity means more ionising particles are emitted each second, so more of them pass through anyone standing nearby and more ionisation happens in their tissue.
4. Activity halves every half-life, so after $n$ half-lives the activity is $A=A_0{\left(\frac{1}{2}\right)}^n$, where $A_0$ is the starting activity.
5. The number of half-lives that have passed in a time $t$ is $n=\frac{t}{T_{1/2}}$, where $T_{1/2}$ is the half-life.
6. Half-life therefore controls two different things at once: how intense the hazard is at the moment, and how long the hazard lasts.

Sources with a short half-life

1. A short half-life source emits a large number of ionising particles each second while it is fresh, so anyone close to it receives a large exposure in a short time.
2. The immediate hazard is therefore high, and a few minutes of careless handling can matter more than a whole day near a weaker source.
3. The activity then falls quickly, because each half-life removes half of the nuclei that are left.
4. This means the hazard has a short lifetime, so the material can simply be locked away and left to decay until its activity is close to background.
5. A short half-life is an advantage wherever the source has to stop emitting soon after it has done its job, and it is a disadvantage wherever a steady output is needed for years.
6. It also means the source has to be made shortly before it is used, because a great deal of it decays away while it is being transported and stored.

Sources with a long half-life

1. A long half-life source has only a small fraction of its nuclei decaying each second, so for a given number of nuclei its activity, and the immediate hazard, is lower.
2. That lower activity is spread out over a very long time, so the material is still emitting ionising radiation decades, centuries or thousands of years later.
3. The hazard cannot be waited out, so it has to be contained instead.
4. If long half-life material escapes into soil, water or the food chain, that land or water stays hazardous long after the accident that released it.
5. The responsibility for that containment also outlasts the people who created the material, so records and warnings have to be kept for future generations.
6. A long half-life is an advantage where a source must give an almost unchanging output for years, such as an industrial gauge that would otherwise need constant recalibration.

Matching precautions to half-life

1. Because half-life sets both the intensity and the duration of the hazard, the precautions that matter most are different for the two kinds of source.
2. For a short half-life source, the priority is protecting people during the few hours or days when the activity is high.
   1. Plan the work in advance so the source is out of its container for the shortest possible time.
   2. Handle it remotely with tongs and keep the shielding in place while it is fresh.
   3. Store it in a shielded container and measure its activity before deciding it is safe to dispose of as ordinary waste.
3. For a long half-life source, the priority is stopping the material escaping at any point over a very long period.
   1. Seal it inside a corrosion-resistant container that will not fail as it ages.
   2. Label the container clearly and keep written records of what it holds and where it is.
   3. Store it in a secure site with restricted access, often deep underground in stable rock.
   4. Monitor the site and the groundwater around it for as long as the material remains active.
4. Shielding is chosen by the type of radiation rather than by the half-life, so a long-lived gamma emitter still needs lead or concrete and a short-lived beta emitter still needs only a few millimetres of aluminium.

6.5.4 Safety precautions for radiation exposure

Measuring exposure as dose

1. Dose is measured in sievert ($Sv$), and because a sievert is a very large dose, medical exposures are usually quoted in millisievert ($mSv$) or microsievert ($\mu Sv$).
2. Two things fix the dose: how much energy the tissue absorbs from the radiation, and how damaging that particular type of radiation is for the same absorbed energy.
3. Alpha radiation is counted as far more damaging than gamma radiation for the same absorbed energy, because it produces its ionisations packed closely together in a small volume of tissue.
4. The dose a person receives builds up with time, so $dose=dose\; rate\times time$.
5. Doses add up over a lifetime, which is why records of past scans are kept and why a worker's exposure is totalled over months and years rather than judged one shift at a time.
6. For scale, the average person in the United Kingdom receives roughly $2.7mSv$ each year from background radiation, a chest X-ray adds about $0.02mSv$, and a whole-body computed tomography scan can add several $mSv$.
7. Because the harm is a matter of chance, there is no dose small enough to carry no risk at all, so the working rule everywhere is to keep every dose as low as is reasonably possible while still doing the job properly.

Justifying the exposure

1. Before any patient is exposed, a doctor has to judge that the expected benefit of the scan or the treatment outweighs the risk the radiation carries.
2. A scan that finds a tumour early can save a life, so a small increase in the lifetime risk of cancer is worth accepting; a scan that would tell the doctor nothing new is not.
3. Methods that use no ionising radiation, such as ultrasound or magnetic resonance imaging, are used instead wherever they would answer the same question.
4. The patient's record of previous exposures is checked, so that repeated scans of the same person are not ordered without good reason.
5. Extra care is taken with children and with pregnant patients, because dividing cells are the most easily damaged and a child has more years ahead in which a cancer could develop.

Limiting the dose to patients

1. Use the lowest intensity or activity that still produces a usable image, because the dose rate rises with the intensity of the beam.
2. Use the shortest exposure time, since the dose is the dose rate multiplied by the time for which the beam is on.
3. Narrow the beam with a collimator so that it covers only the part of the body being examined, which keeps the rest of the body out of the beam entirely.
4. Cover the organs just outside the beam with a lead apron or thyroid collar, because lead absorbs much of the scattered radiation that would otherwise reach them.
5. Check the settings and the patient's position before switching the beam on, because a poor image has to be retaken and a repeated scan doubles the dose.
6. Use sensitive digital detectors rather than old photographic film, since a detector that responds to fewer arriving photons needs a smaller dose to form the same image.
7. Where a radioactive substance is given to the patient, choose one whose activity falls away soon after the procedure, so the patient is not still being exposed days later.
8. Encourage the patient to drink plenty of fluid afterwards, so any such substance is removed from the body more quickly.
9. In treatment rather than imaging, a large dose is deliberately given to the tumour, so the precautions there are about protecting the healthy tissue around it rather than about reducing the dose to the target.

Protecting medical personnel

1. A patient may be scanned once or twice in a lifetime, but a radiographer works beside the same machine every day, so it is the total dose over a career that has to be controlled.
2. The strongest protection is to be somewhere else, so staff leave the room and operate the machine remotely from a control area.
3. Where they must stay in the room, they stand behind a lead-lined screen or a lead-glass window, which absorbs most of the radiation heading towards them.
4. The walls of an X-ray room are lined with lead or made of thick concrete, so that people in the corridor and in neighbouring rooms are shielded as well.
5. Staff who have to be at the bedside wear lead aprons, thyroid collars and lead gloves, and stand as far from the beam as the procedure allows.
6. Radioactive sources for treatment are moved with long handling tools and kept in shielded containers, so that no one holds a source in their fingers.
7. Work is shared out between staff, so no single person is repeatedly the one standing closest to the machine.
8. Every worker wears a dosimeter badge, which contains a film or a crystal that changes measurably in proportion to the radiation that has reached it.
9. The badges are collected and read at regular intervals, and each worker's running total is compared with the legal dose limit for the year.
10. If someone's total is climbing towards that limit, they are moved to work that involves no exposure, and the reason for the high reading is investigated.
11. A badge gives no protection of its own; it monitors the dose so that action can be taken before the exposure becomes unacceptable.

6.5.5 Contamination and irradiation

Two different things that can happen

1. The whole distinction rests on one question: has any radioactive material been moved onto or into the object?
2. In irradiation the answer is no: alpha particles, beta particles or gamma rays travel across from the source, deposit energy and are absorbed, but the unstable nuclei stay where they were.
3. In contamination the answer is yes: atoms with unstable nuclei end up on the skin, on a surface, or inside the body, and they carry on decaying there.
4. An object can be irradiated without being contaminated, contaminated without anyone realising, or both at the same time.

The effects of irradiation

1. While the radiation is arriving, it ionises molecules in whatever it passes through, so living cells can be killed and DNA can be altered.
2. The moment the source is taken away, shielded or switched off, the exposure stops immediately.
3. The irradiated object does not become radioactive, because nothing has been added to it that has an unstable nucleus.
4. A patient who has had an X-ray, a syringe that has been sterilised and a strawberry that has been irradiated are all safe to handle straight afterwards for exactly this reason.
5. The size of the hazard depends on the type of radiation, the activity of the source, the time spent near it, the distance from it and the shielding used, so all of those can be controlled.
6. From outside the body, gamma radiation is the greatest irradiation hazard because it penetrates deeply, while alpha radiation is the least because the dead outer layer of skin absorbs it.

The effects of contamination

1. Contamination happens when radioactive dust settles on skin or clothing, when a radioactive liquid is spilled, when radioactive gas is breathed in, or when radioactive material is swallowed with food or water or enters through a cut.
2. Once the material is there, the object or person carries the source with them, so moving away from the original spill changes nothing.
3. The exposure continues until the material is washed or cleaned away, is excreted by the body, or has decayed to a low activity, and none of those can be done instantly.
4. Contamination also spreads, because contaminated hands touch door handles and contaminated boots walk material out of the laboratory.
5. Contamination inside the body is the most serious case, because there is no layer of skin between the source and living cells.
6. This reverses the usual ranking: an alpha emitter is the most dangerous contaminant, because it is strongly ionising and deposits all of its energy in a very small volume of living tissue.
7. Some contaminants are worse still because the body concentrates them: radioactive iodine collects in the thyroid gland and strontium behaves chemically like calcium, so it is built into bone.
8. Contamination is controlled by keeping sources sealed, working over trays, wearing gloves and laboratory coats, banning food and drink, and monitoring hands and surfaces with a detector before leaving.

Comparing the hazards

1. Both hazards do their damage the same way, by ionising molecules in cells, so the difference is not in the kind of harm but in how long it goes on and how easily it can be stopped.
2. Duration: irradiation is over the instant the source is gone, whereas contamination keeps exposing the person for as long as the material stays put.
3. Control: irradiation is reduced by time, distance and shielding, while contamination is dealt with by removing the material and by stopping it spreading.
4. Reach: irradiation affects only what is within range of the source, while contamination travels wherever the contaminated person, object, air or water goes.
5. Radiation type: gamma is the worst for irradiation from outside, and alpha is the worst for contamination inside the body.
6. Neither is always the more dangerous, and the honest comparison depends on the numbers in the situation.
7. A few minutes beside a high-activity gamma source can give a larger exposure than a trace of contamination ever would.
8. Contamination of a large area with a long-lived isotope is the harder problem, because the exposure cannot be switched off and the land may stay unusable for decades.