6.2.1 Contamination and irradiation

Irradiation

1. When radiation from a radioactive source reaches an object, the object is irradiated.
2. The source stays where it is, and only the radiation travels from the source to the object.
3. Alpha, beta and gamma radiation are all forms of ionising radiation, so they can knock electrons out of atoms in the object and turn those atoms into ions.
4. Irradiation stops as soon as the source is taken away, sealed in a lead-lined container or blocked by suitable shielding.
5. An irradiated object does not become radioactive, because no unstable nuclei have been added to it.
6. Hospitals use this idea to sterilise surgical instruments, by irradiating sealed packs with gamma rays so that any bacteria on them are killed.
7. The instruments are safe to handle straight afterwards, because the gamma rays have passed through them without leaving any radioactive material behind.

Contamination

1. An object is contaminated when radioactive material, made of atoms with unstable nuclei, ends up on it or inside it.
2. The contaminating atoms keep decaying, so the object keeps being exposed to radiation for as long as the material stays there.
3. External contamination happens when radioactive dust, liquid or gas settles on the skin, hair or clothing.
4. Internal contamination happens when radioactive material is breathed in, swallowed or enters the body through a cut.
5. Moving away from the hazard does not help, because the contaminated person carries the radioactive material with them.
6. External contamination can often be reduced by removing contaminated clothing and washing the skin thoroughly.
7. Internal contamination is much harder to deal with, because the radioactive material cannot be washed off and may stay in the body for a long time.
8. Radioactive material can be passed on by touch to other people, surfaces and objects, so contamination can spread.
9. A contaminated person is also being irradiated, because the material on or in their body emits radiation into their tissues.

Comparing the hazards

1. Irradiation and contamination are both hazardous because ionising radiation can damage the cells of living tissue.
2. Ionisation inside a cell can damage its DNA, which may cause a mutation that leads to cancer.
3. A very large dose can kill cells outright, causing radiation burns or radiation sickness.
4. The hazard from irradiation lasts only while you are exposed to the source, so it ends when you move away or the source is shielded.
5. The hazard from contamination lasts for as long as the radioactive material stays on or in the body, which may be weeks, years or a lifetime.
6. Internal contamination is usually the most hazardous situation, because the source sits right next to living cells with no air or skin in between.
7. The type of radiation changes the comparison, because alpha, beta and gamma radiation penetrate matter by different amounts.
   1. An alpha source outside the body is a low irradiation hazard, because alpha particles are absorbed by a few centimetres of air or the outer layer of dead skin.
   2. An alpha source inside the body is extremely hazardous, because all of its strongly ionising alpha particles are absorbed by the living cells around it.
   3. Beta and gamma sources outside the body are a greater irradiation hazard than alpha sources, because their radiation passes through the skin and reaches internal organs.
   4. A gamma source inside the body is less damaging than an alpha source, because gamma rays are weakly ionising and most of them pass straight out of the body.
8. Neither hazard is always the greater one, so a full comparison considers the type of radiation, where the source is and how long the exposure lasts.

Reducing the risks

1. People who work with radioactive sources reduce irradiation by keeping the exposure time short, keeping their distance from the source and placing shielding such as lead or concrete between themselves and the source.
2. Sources are handled with long-handled tongs, held at arm's length and never pointed at anyone.
3. Sources are kept in lead-lined boxes when they are not being used.
4. Workers reduce contamination by wearing gloves and protective clothing, and by using sealed sources from which radioactive material cannot escape.
5. Workers wear a dosimeter badge that records the total dose of radiation they receive over time.
6. Rooms where open radioactive material is used are checked regularly with detectors, so that any spill can be found and cleaned up.

6.2.2 Half-life and radiation hazards

Half-life and activity

1. Radioactive decay is random, so you cannot predict when any particular unstable nucleus will decay.
2. In a large sample, the same fraction of the remaining unstable nuclei decays in each half-life, so the activity falls as the number of undecayed nuclei falls.
3. The half-life tells you how quickly this happens, and each radioactive isotope has its own half-life.
4. Half-lives range from fractions of a second to billions of years, so the hazard from different radioactive materials varies enormously.
   1. Technetium-99m has a half-life of about 6 hours.
   2. Iodine-131 has a half-life of about 8 days.
   3. Caesium-137 has a half-life of about 30 years.
   4. Plutonium-239 has a half-life of about 24 000 years.
5. The hazard from a radioactive material depends on two things linked to half-life: how many decays happen each second, and how long the material keeps decaying.

Short half-life

1. A material with a short half-life has nuclei that decay quickly, so a large fraction of them decays every second.
2. For two samples with the same number of unstable nuclei, the one with the shorter half-life has the higher activity.
3. A high activity means many particles or rays are emitted each second, so anyone nearby receives a large dose of radiation in a short time.
4. The activity also falls quickly, so after a few half-lives only a small fraction of the original unstable nuclei is left.
5. After ten half-lives the activity has fallen to less than a thousandth of its starting value, because ${\left(\frac{1}{2}\right)}^{10}=\frac{1}{1024}$.
6. A material with a half-life of a few hours is therefore close to harmless within a few days.
7. The hazard from a short half-life material is intense but brief.

Long half-life

1. A material with a long half-life has nuclei that decay slowly, so only a small fraction decays each second.
2. Its activity is lower than that of a similar sample with a short half-life, so the dose received each second is usually smaller.
3. Its activity stays almost the same for years, so the material remains hazardous for a very long time.
4. Radioactive waste from nuclear power stations contains isotopes with half-lives of thousands of years, so it must be kept safe for many generations.
5. This waste is sealed in strong containers, kept away from water supplies and often stored deep underground, so that it cannot contaminate people or the environment while it slowly decays.
6. Land and buildings contaminated with long half-life material can stay unsafe for decades, which is why the area around the Chernobyl power station is still closed to the public.
7. The hazard from a long half-life material is weaker at any moment but lasts far longer.

Judging the overall hazard

1. To judge how hazardous a radioactive material is, you need to consider both how strongly it emits now and how long it will keep emitting.
2. A short half-life is the bigger concern for a brief, close exposure, such as handling a freshly prepared source.
3. A long half-life is the bigger concern when material might escape, be swallowed or be left in the environment, because it keeps emitting radiation for years.
4. Half-life combines with the type of radiation, so an alpha emitter with a long half-life is especially dangerous if it is breathed in, because it keeps irradiating nearby lung cells for many years.
5. The half-life of an isotope is therefore one of the main factors used to decide how it is stored, transported and disposed of.

6.2.3 Medical uses of nuclear radiation

Exploring internal organs

1. Doctors can examine organs inside the body without surgery by using a radioactive tracer.
2. The tracer is a small amount of a radioactive isotope, often attached to a chemical that the organ being investigated naturally takes up.
3. Depending on the organ, the tracer is injected into the bloodstream, swallowed or breathed in.
4. The tracer is carried around the body and collects in the target organ, such as iodine-123 collecting in the thyroid gland.
5. As the tracer nuclei decay, they emit gamma rays that pass out through the body.
6. A gamma camera outside the patient detects the gamma rays and records where they come from.
7. A computer uses this information to build up an image showing where the tracer has collected.
8. Areas that take up too much tracer show up as bright hot spots, and areas that take up too little show up as cold spots.
9. Hot and cold spots can reveal a tumour, a blocked blood vessel or part of an organ that is not working properly.
10. The image shows how well an organ is working, not only its shape, because healthy tissue takes up the tracer at a normal rate.

Choosing a tracer

1. The tracer must emit gamma radiation, because gamma rays penetrate tissue well enough to escape the body and reach the camera.
2. Gamma rays are also weakly ionising, so they cause relatively little damage to cells as they pass out of the body.
3. An alpha emitter would be useless for imaging, because its particles would be absorbed inside the body and would strongly ionise the cells around it.
4. A beta emitter is also unsuitable, because most beta particles are absorbed within a few millimetres of tissue and add to the dose without reaching the camera.
5. The tracer must have a short half-life, usually a few hours, so that it lasts long enough for the scan but then decays quickly to keep the patient's dose low.
6. A half-life of a few seconds would be too short, because the tracer would decay before it reached the organ.
7. A half-life of several years would be too long, because the tracer would keep irradiating the patient long after the scan had finished.
8. Technetium-99m is the most widely used tracer, because it emits only gamma rays and has a half-life of about 6 hours.
9. The tracer must also be non-toxic, because it is put into the body.

Destroying unwanted tissue

1. Cancer cells divide out of control and form a lump of unwanted tissue called a tumour.
2. A high dose of ionising radiation damages the DNA of cells so badly that they die or can no longer divide.
3. Radiotherapy uses this effect to control the growth of a tumour or to destroy it completely.
4. Radiation damages healthy cells in exactly the same way, so the treatment must give a high dose to the tumour and as small a dose as possible to the tissue around it.
5. External radiotherapy
   1. Beams of high-energy gamma rays or X-rays are aimed at the tumour from outside the body.
   2. The beams are directed at the tumour from many different angles, either by rotating the source around the patient or by using several beams at once.
   3. The tumour sits where all the beams cross, so it receives a large total dose, while each region of healthy tissue is crossed by only one beam and receives a much smaller dose.
   4. The treatment is usually split into many small doses over several weeks, which gives healthy cells time to repair themselves between sessions.
   5. The patient is irradiated but not contaminated, so they are not radioactive when they leave each session.
6. Internal radiotherapy
   1. A sealed radioactive source, such as a tiny pellet or wire, is placed inside the body next to or inside the tumour.
   2. The source is chosen so that its radiation travels only a short distance through tissue, so most of the energy is absorbed by the tumour rather than by healthy organs.
   3. Some treatments use a radioactive substance that the tumour takes up, such as iodine-131, which collects in the thyroid gland and destroys thyroid cancer cells with beta particles.
   4. The patient now has radioactive material inside them, so they are contaminated for a time and must follow precautions, such as keeping away from pregnant women and young children for a few days.

Benefits and risks

1. Every medical use of radiation carries some risk, because the radiation that diagnoses or treats can also damage healthy cells and slightly raise the chance of cancer later in life.
2. Doctors use radiation only when the benefit to the patient is judged to be greater than the risk.
3. For a scan, the benefit is an accurate diagnosis without surgery, and the dose is kept low by using a short half-life gamma emitter.
4. For radiotherapy, the benefit of destroying a tumour usually far outweighs side effects such as tiredness, sore skin and hair loss in the treated area.
5. Hospital staff protect themselves by using shielding, keeping their distance from patients who contain radioactive material and wearing a dosimeter badge.

6.2.4 Nuclear fission and chain reactions

Splitting large nuclei

1. Some very large nuclei, such as uranium-235 and plutonium-239, are unstable and can split into two smaller nuclei.
2. This splitting of a nucleus is called nuclear fission.
3. For fission to happen, the unstable nucleus usually has to absorb a neutron first.
4. A neutron has no charge, so it is not repelled by the positive nucleus and can get close enough to be absorbed.
5. When uranium-235 absorbs a slow-moving neutron it becomes uranium-236, which is so unstable that it splits almost immediately.
6. The two smaller nuclei produced are called daughter nuclei, and they are usually of unequal size, such as barium-141 and krypton-92.

Radiation from fission

1. Each fission event releases two or three neutrons as well as the two daughter nuclei.
2. Fission also releases gamma rays, because the newly formed nuclei are left with extra energy.
3. The daughter nuclei have too many neutrons to be stable, so they go on to decay and emit beta and gamma radiation.
4. This is why the spent fuel removed from a nuclear reactor is highly radioactive waste.
5. The total number of protons and the total number of neutrons are the same before and after a fission event, which you can check with a nuclear equation.

Energy transfer in fission

1. Each fission event releases a very large amount of energy, millions of times more than a chemical reaction involving a single atom.
2. Most of this energy is transferred to the kinetic energy stores of the daughter nuclei and the neutrons, which fly apart at very high speeds.
3. Some of the energy is carried away by the gamma rays.
4. The fast-moving particles collide with the surrounding atoms, so the energy is transferred to the thermal energy store of the material and it heats up.
5. In a nuclear power station, this thermal energy is used to boil water into steam, which drives turbines to generate electricity.

Chain reactions

1. The two or three neutrons released by one fission can each be absorbed by another uranium-235 nucleus.
2. Those nuclei then split and release more neutrons, which can cause still more nuclei to split.
3. This self-sustaining process is called a chain reaction.
4. If every fission causes more than one further fission, the number of fission events grows extremely quickly, from one to three to nine to twenty-seven and onwards.
5. In a nuclear reactor, the chain reaction is controlled, so that on average exactly one neutron from each fission goes on to cause another fission.
6. Control rods, made of a material such as boron that absorbs neutrons, are lowered into the reactor to slow the reaction and raised to speed it up.
7. In a nuclear weapon, the chain reaction is uncontrolled, so a huge amount of energy is released in a fraction of a second as an explosion.

6.2.5 Nuclear fusion

Joining light nuclei

1. Nuclear fusion is the joining of two light nuclei to make a single heavier nucleus.
2. The most common fusion reactions join nuclei of hydrogen isotopes to form helium.
3. Every nucleus contains protons, so every nucleus is positively charged.
4. Two nuclei therefore push each other apart by electrostatic repulsion, and this force gets stronger the closer they get.
5. Fusion only happens if the nuclei get close enough to join, so they must collide fast enough to overcome this repulsion.
6. At extremely high temperatures of millions of degrees Celsius, nuclei have enough kinetic energy to collide at the speeds needed.
7. A very high density is also needed, so that collisions between nuclei happen often enough for fusion to keep going.
8. At these temperatures atoms lose all of their electrons, so the material is a mixture of bare nuclei and free electrons.

Mass and energy in fusion

1. The mass of the particles produced in fusion is slightly less than the total mass of the nuclei that joined.
2. This missing mass has not disappeared, because it has been converted into energy.
3. The energy is released as radiation and as kinetic energy of the particles produced.
4. Only a tiny fraction of the mass is converted in each fusion, but a small amount of mass is equivalent to an enormous amount of energy.
5. Fusion releases more energy per kilogram of fuel than fission does.
6. This conversion of mass into the energy of radiation is the source of the light and heat given out by every star.

Fusion in stars

1. The Sun and other stars are powered by fusion in their cores, where the temperature and density are high enough.
2. In the Sun, hydrogen nuclei fuse in a series of steps, and the overall result is that four hydrogen nuclei become one helium nucleus.
3. The Sun converts around four million tonnes of mass into energy every second.
4. This energy travels outwards through the Sun and leaves its surface as electromagnetic radiation, which reaches the Earth as light and infrared.