I find the idea of binding energy very confusing. I thought it was energy that was stored in a nucleus.

Binding energy is energy that is lost when a nucleus is formed from protons and neutrons. A lot of binding energy means a stable nucleus. A similar idea exists in ordinary chemistry. When sodium reacts with chlorine, a lot of energy is released. The result is a very stable compound - sodium chloride.

Energy has mass. Losing energy means losing mass. In ordinary chemistry this change is too small to measure, but the amounts of energy involved in nuclear processes are so great that the change in mass is quite easily measurable.

To calculate binding energy we add together the masses of all the protons and neutrons that go to make up the nucleus. The total is greater than the actual mass of the nucleus. The decrease in mass when the nucleus is formed is the mass of the energy released. This energy can be calculated using Einstein's formula, E = mc², where is the speed of light

Remember that nuclear fission and nuclear fusion both depend on the changing of binding energy to heat. In nuclear fission a heavy nucleus splits into two smaller pieces. In nuclear fusion two light nuclei unite to make a heavier one. In each case binding energy is released. A more stable nucleus is formed. Fusion works for nuclei lighter than iron. Fission works for nuclei heavier than iron. Therefore iron is the most stable nucleus.

The binding energies holding nuclei together are roughly a million times greater than the binding energies holding molecules together. This is why nuclear explosions are vastly more powerful than chemical explosions.

Where do neutrons come from? Is it true that they are unstable?

In 1920, Rutherford suggested that an uncharged particle having about the same mass as a proton might exist. He thought of it as being a combination of a proton and an electron. Such a particle would explain why a nucleus having, say, 12 times the mass of a proton only weighed 6 times as much.

Rutherford had discovered in 1919 that very occasionally an alpha-particle would collide with a nitrogen nucleus and eject a proton from it. This also occurred with alpha-particles and other nuclei, but something strange happened with beryllium-17. Whatever was ejected, it was not a charged particle. Originally they thought it was a gamma-ray. Then, in 1932, Chadwick placed a slab of paraffin wax in front of the beryllium. Wax contains lots of protons (hydrogen nuclei). He found that protons were knocked forward out of the wax. Whatever was hitting them was giving all its energy to the proton. As it stopped dead after the collision, it must have the same mass as a proton. This was the neutron.

A convenient source of neutrons consists of beryllium mixed with a strong source of alpha-particles. This can be used to make other materials radioactive - it is more convenient than using a nuclear reactor.

Fast-moving neutrons are used to treat cancer. They are very penetrating - several feet of concrete are needed to stop them. In a nuclear reactor, fast neutrons are slowed down by bouncing them off light nuclei.

Unlike protons, solitary neutrons are unstable. They decay to give a proton, an electron and a neutrino. Their half-life is 13 minutes.

What is a chain reaction?

What happens if, on average, at least one of the neutrons produced by the fission of a uranium-235 nucleus goes on to be captured by another uranium-235 atom? The process will keep going and generating heat - we have a chain reaction.

What is needed to keep the reaction going? The problem is that the neutrons are travelling too fast to be captured easily. There are possible answers. One is to use pure uranium-235, but this very expensive to produce. The other is to slow down the neutrons so that they are more easily captured. This is done by bouncing them off the light atoms contained in a material known as a moderator. Graphite is often used, and heavy water is very effective. Ordinary water can be used as a moderator, but it absorbs a lot of neutrons. A layer of heavy atoms reflects escaping neutrons back into the reactor core.

How can we control the reaction? Rods made of boron are pushed into channels in the reactor. These rods absorb neutrons, so they damp down the reaction. The speed of the reaction is controlled by moving these control rods in and out. The heat produced is removed from the reactor by a flow of liquid or gas coolant. The heat is used to produce steam and drive turbines.

There is another metal that can be used instead of uranium-235: plutonium. In a bomb, pure uranium-235 or plutonium is used, and there is no moderator.

What do scientists mean by "splitting the atom"?

They mean nuclear fission. In biology a cell has a nucleus. The nucleus of a cell splits in two and one cell becomes two. This is called fission. Scientists studying atomic nuclei were reminded of what happened in biology, so they used the same words.

Normal uranium is nearly all uranium-238, but it does contain 0.72% uranium-235. Enriched uranium contains several percent uranium-235. A uranium-235 nucleus is special because it will undergo fission. This means that it will absorb a stray neutron and then become unstable and split into two roughly equal halves. Neutrons can enter the nucleus because they are not charged. They are not repelled by the positive charge on the nucleus

The reaction is not quite the same every time. In a typical case the uranium nucleus splits into a barium nucleus, a krypton nucleus and three neutrons. All these particles fly apart at high speed, banging into other atoms and making them vibrate more strongly. This increased vibration is increased heat energy - the temperature rises. The two halves of the uranium atom present a problem, because they are always extremely radioactive.

Fission also happens with plutonium-239 nuclei. Plutonium rather than uranium is usually used in bombs. It can also be used in reactors, but there are problems because it is so poisonous.

Why are hydrogen bombs so much more powerful than atom bombs?

An atom bomb is a fission bomb. It contains a sphere of plutonium-239 about the size of a grapefruit. Chain reactions start in this sphere whenever a stray neutron enters a plutonium nucleus, but they rapidly fizzle out; too few of the neutrons find other plutonium nuclei. The sphere is surrounded by shaped explosive charges. The sphere is suddenly compressed into a much smaller volume when these explode. The nuclei are now closer together and the chain reaction goes ahead. A neutron source ensures there is a neutron there to start the reaction. There is a terrific burst of heat energy. This is equivalent to about 20,000 tonnes of TNT. Many of the radioactive atoms produced are carried up into the stratosphere. Eventually they reach the ground, all over the world, as fallout.

Two heavy hydrogen nuclei can fuse together to produce a helium nucleus. This reaction can be triggered by a fission bomb. The trigger is surrounded by a jacket containing heavy hydrogen (deuterium). The resulting bang is equivalent to at least a million tonnes of TNT - the biggest was a hundred times more powerful.

Fission energy is released in a controlled way in a reactor. Nobody has yet managed to generate power by controlled fusion. The Sun and other stars, however, get their energy from nuclear fusion.

Why is plutonium so nasty?

Alchemists dreamed of turning lead into gold, but physicists regularly turn one element into another. They bombard uranium-238 with neutrons in a nuclear reactor. The uranium absorbs a neutron to become uranium-239. Beta decay changes this to neptunium-239. This emits another beta-particle to become plutonium-239 - a metal that does not occur naturally.

All uranium reactors produce plutonium in their fuel rods. Breeder reactors have a jacket of uranium-238 to increase the amount of plutonium produced. They produce more fuel than they consume.

The plutonium is mixed up with highly radioactive fission products. The spent fuel rods are stored under water for some time until the radioactivity has died down - at first it is powerful enough to give the water a blue glow. The rods are then dissolved in acid and the plutonium and unused uranium are recovered. All this has to be done by remote control because of the radiation and the dangerous chemicals involved.

Plutonium-239 has a half-life of 24 000 years, so it is not intensely radioactive. However, even a tiny amount inside someone's body can eventually cause cancer, because it emits alpha-particles. There is also the problem that terrorists might steal it, and use it to make weapons.

Plutonium-238 is strongly radioactive, with a half-life of 88 years. The heat that it produces can be used to warm a thermocouple and generate electricity. It can be used to power equipment such as heart pacemakers and space probes, where an ordinary battery might not last long enough.

Where are all the elements made?

The simplest atoms - hydrogen and helium - are by far the most abundant in the Universe. As nuclei become heavier and more complicated, there are fewer and fewer of them. Hydrogen and helium were formed during the early life of the Universe, when it was settling down after the Big Bang. Almost all other nuclei were made in the centres of stars. Protons repel one another. To make a nucleus, you have to push them together - enormous pressures are needed.

A star like the Sun gets its energy from the fusion of hydrogen atoms to produce helium. Once the hydrogen in the centre of the star runs out, it collapses under gravity. The helium nuclei now start to fuse, giving carbon and oxygen nuclei. Once the helium is used up, there is a further collapse. Then, if the star weighs more than 7.5 times as much as the Sun, there is a further series of fusion reactions involving carbon, oxygen and silicon nuclei. These produce elements up to iron. Nuclei heavier than iron cannot be produced by fusion. They build up very slowly by collecting neutrons, some of which will decay into protons.

The final stage in the life of a large star is a massive explosion - a supernova. This explosion scatters the elements around the galaxy, where they may eventually become part of planets. The atoms in your body were once part of a star.

I recently went round a plant making nuclear fuel rods. There were signs and arrows everywhere saying 'criticality run'. What did they mean?

At Los Alamos in May 1946, Louis Slotin was carrying out an experiment using two hemispheres of plutonium. They were mounted on a track and Slotin was edging them closer and closer together using a screwdriver. When the two came together they would exceed the critical mass and a chain reaction would begin. Slotin intended to stop just short of this point, but his hand slipped and the hemispheres came together. The room filled with blue light. An atom bomb was going off. He wrenched the two halves apart with his bare hands and the reaction ceased. There were other people in the room and Slotin drew a diagram of their positions on the blackboard to help work out their dose of radiation. Nine days later he died, but the others survived.

You do not actually need the innards of an atom bomb to produce a chain reaction. Merely stacking too much enriched uranium in one place will do. Of course there are stringent precautions to prevent this, but it could happen. If it did, people would need no encouragement to follow the arrows. This sort of chain reaction would lead to a massive burst of radiation, but not to a full-scale explosion. A nuclear bomb needs an inward pressure wave from conventional explosives to hold it together long enough for the chain reaction to develop fully.

When the Earth was young - 3.5 billion years ago - the rocks must have contained more than 30 times today's quantity of uranium-235. It is not too far-fetched to suggest that chain reactions must sometimes have begun spontaneously in rocks containing uranium.

Beta-rays and cathode rays sound the same to me - they're both electrons.

This is true - the difference lies in where they come from and how fast they're going.

Heat a metal wire in a vacuum. Electrons gain enough energy to leave the wire and form a cloud around it. These electrons can be attracted by a sheet of metal carrying a positive charge. They move towards it, gaining speed as they go. If these electrons hit a screen coated with a suitable chemical, they make it glow (fluoresce). This is what happens in a television. The voltage between the wire (cathode) and metal plate (anode) might be 3000 V. In this case the electrons will reach about one tenth of the speed of light.

A typical beta-particle, however, leaves a nucleus with an energy equivalent to being accelerated through 1 million volts. This means that it will be travelling at close to the speed of light. It will have so much kinetic energy that the mass of this energy will be twice the rest mass of the electron! Cathode rays are easily stopped by very thin sheets of material. Beta-rays can pass through a millimetre or more of aluminium.

Cathode rays are easily deflected by electric and magnetic fields - as in a TV tube. Beta-rays are also easily deflected by magnetic fields. Magnetic deflection is important, because it tells us if a particle is charged, and whether the charge is positive or negative. Gamma rays and neutrons are not deflected at all.

I read that the Shroud of Turin dates from the Middle Ages, not from the time of Jesus. How did they find out?

They used carbon dating. Carbon-14 is a radioactive isotope of carbon. When a neutron produced by a cosmic ray hits the nucleus of a nitrogen atom, the neutron turns into proton and the nitrogen-14 nucleus turns into a carbon-14 nucleus. ¹⁴C is radioactive. It decays by giving out a beta-particle. Its half-life is 5700 years. More ¹⁴C is constantly being produced to replace that which is decaying, so the proportion in the atmosphere stays constant.

Animals and plants contain carbon. While they are alive the ratio of ¹⁴C atoms to normal ¹²C atoms inside them is the same as in the atmosphere. After death, however, they no longer exchange atoms with their surroundings. The proportion of ¹⁴C begins to decrease as the atoms decay. When it has halved, for example, the specimen is 5700 years old.

This method of dating can be used for any organic material, such bone, wood or cloth. Originally it involved taking quite a large chunk of the material (about 50g of wood or 200g of bone) and burning it to separate the carbon. Modern techniques involve measuring the proportion of ¹⁴C atoms without waiting for them to decay. Much smaller samples are required - small enough for the Pope to give his consent for one to be taken from the Shroud.

One of the main problems with carbon dating is knowing the actual concentration of ¹⁴C in ancient times. Fortunately wood can also be dated by tree-ring measurements and this provides a check on radiocarbon dates.

Is it true that we are being bombarded by rays from outer space?

Cosmic rays are charged particles with very high energies that enter the Earth's atmosphere. They consist mainly of protons, together with electrons and nuclei. In outer space their paths are guided by the magnetic field lines of the Earth and Sun. They contribute to background radiation - particularly for people in high-flying aircraft.

Primary cosmic rays do not get closer to the ground than about 15 km. They interact with atoms high in the atmosphere to produce neutrons and mesons. The mesons can penetrate deep underground.

Mesons are charged particles - heavier than electrons, but lighter than protons. They have lives so short that they should not be able to travel as far as they do. However, time passes more slowly for them because they are travelling at almost the speed of light.

Astrophysicists want to know where cosmic rays come from. Nuclear physicists find them a cheap source of very high energy particles. They use rockets and balloons to carry blocks of photographic emulsion into the upper atmosphere, then study the tracks left when cosmic rays interact with atoms in the emulsion.

Primary cosmic rays sometimes give rise to a very high energy gamma-ray. This produces an electron-positron pair. The pair have such high energies that they produce more gamma-rays as they slow down. These gammas themselves produce pairs, which generate more pairs, and so on. The result is a cosmic ray shower.

Do cloud chambers really let you see alpha-particles?

Sometimes you see a vapour trail very high up in the sky. You cannot see what is on the end of it, but you assume it is a plane. The trail tells you where the plane has been. In just the same way, the vapour trail in a cloud chamber tells where a charged particle has been. In fact, vapour trail is a misnomer - we should say condensation trail. A cloud chamber is full of super-cooled vapour looking for somewhere to condense. Water droplets condense on the trail of ions left by a charged particle. Alpha-particles produce short, thick trails, because they produce lots of ions over a short distance. Beta-particles produce long straggly trails.

Nowadays, bubble chambers have largely replaced cloud chambers. If you remove the radiator cap on your car while the engine is very hot, the release of pressure causes the water to boil suddenly and violently. A bubble chamber is full of liquid hydrogen, under pressure, at a temperature above its normal boiling point (-253°C). A sudden drop in pressure makes the hydrogen want to boil. The bubbles of vapour, however, need centres round which to form. Ions provide these centres, and lines of bubbles mark the tracks of charged particles through the chamber.

Physicists hope that particles will react with hydrogen nuclei or electrons in the chamber, and something interesting will result. Flash photographs are taken each time there is an expansion. Thousands of these are analysed in the search for something significant. A magnetic field across the chamber curves the paths of the particles and gives away their charge.

What causes an atom to decay?

A nucleus will only decay if it can lose energy by doing so and become more stable. It is impossible, however, to predict when a particular nucleus will decay. We can predict that half of the atoms in a sample of radium-226 will decay in the next 1,620 years, but we cannot say which half. A particular atom might decay in one second or after 3,000 years. Even if we knew exactly what was going in inside a nucleus, we still could not predict when it was going to decay. The decay of an atom is completely random - it is an event without a cause.

Set up a Geiger counter and a radioactive source. Measure the counts over a period of one minute. Do this 100 times. Calculate the average. Repeat the experiment. The two averages will be very close to one another, but the individual counts will vary quite a lot. They cannot be predicted.

In the past some scientists had reasoned that the Universe was governed absolutely by the laws of science. Everything that was going to happen was already decided and we could not do anything about it. This must include our own behaviour. Radioactive decay and similar discoveries in atomic physics showed that this could not be so.

Albert Einstein could not come to terms with this idea. He said 'God does not play dice with the world'; as a result he became increasingly cut off from mainstream physics.

What is the point of neutrinos? They don't seem to do anything.

When a nucleus emits a particle it recoils, just like a gun does when you fire it. Physicists studying the recoil of a nucleus when it emitted a beta-particle (electron) got a nasty shock. Some energy and momentum seemed to have disappeared.

In 1933, Pauli suggested that another particle might be involved. He called it a neutrino. Like a gamma-ray it had momentum and energy, but no mass and no charge. Unlike a gamma-ray, however, it seemed to pass through matter without doing anything. If you stand near a nuclear reactor there is a tremendous flow of neutrinos through the shielding and straight through you.

The neutrino was finally detected in 1956. A hydrogen nucleus is a proton. A large tank of dry-cleaning fluid contains a lot of hydrogen atoms. Very occasionally a neutrino will join up with a proton to give a neutron and a positive electron or positron. Eventually a neutron and a positron were detected at the same instant, coming from somewhere in the tank. This confirmed the existence of neutrinos.

Today we have neutrino astronomy. Neutrinos were detected from the supernova that appeared in the Greater Magellanic Cloud in 1987. A supernova is the sudden spectacular collapse of a star that has finally run out of fusion fuel. Neutrinos can be detected from deep inside the Sun. Unfortunately the number does not agree with theories of how the Sun works.

What is it that comes out of radioactive materials?

Basically it is energy that comes out, but in 3 different forms - alpha, beta and gamma rays. When they were discovered, nobody knew what they were. They just knew there were three sorts, so they called them alpha, beta, gamma; (they could have just said a,b,c - but they wanted people to know that they had studied Greek at school).

Alpha rays are actually heavy, fast-moving particles with a positive charge. They only travel a few centimetres in air and are stopped by a sheet of paper. They are very good at making air conduct electricity. They turn out to be the nuclei of helium atoms.

Beta rays are also particles, but very much lighter and faster moving than alpha-paricles. They can travel through a metre or so of air, but are stopped by a few millimetres of aluminium. They have a negative charge and turn out to be electrons. They are not nearly as good as alpha particles at making air conduct electricity.

Gamma rays are waves - they are part of the electromagnetic spectrum like light waves and radio waves. They have a very short wavelength - smaller than an atom. They are similar to X rays, but with shorter wavelengths and more energy. They can pass through thick sheets of lead. They make air conduct electricity, but much less than alphas or betas do.

All three come from the nucleus of the atom. Some radioactive atoms give alphas and some betas. In some cases an atom will emit a gamma ray when it is settling down after emitting an alpha or a beta - a sort of nuclear burp.

Somebody said that everything is radioactive. Surely it can't be?

It is true that there is always very weak radiation around us. Scientists call it background radiation. It even makes a tiny number of people ill. Only a very small fraction of this radiation is due to the nuclear industry.

The main radioactive elements are uranium and thorium, but there are others. For example, potassium contains a trace of radioactive potassium-40. All living things contain a small proportion of radioactive carbon-14. In the 19th century, craftsmen used uranium to give glass a nice yellow colour. They didn't know about radioactivity. Luminous paint is radioactive. Workers at an American plant producing luminous dials put their brushes in their mouths. They developed jaw cancer.

When above-ground nuclear bomb tests were common, traces of radioactive fallout were everywhere. Strontium-90 was found in children's bones. The accident at Chernobyl has made parts of the surrounding area uninhabitable, but it also had an effect as far away as Wales. The mutton from sheep eating radioactive grass could not be sold.

Becquerel discovered radioactivity in 1896. An activity of 1 becquerel (Bq) means that one nucleus decays every second.

Gamma rays are supposed to be like X-rays, but are they as useful?

Gamma rays are like X-rays, but they come from the nuclei of atoms. X-rays are produced when fast-moving electrons hit heavy metal atoms. There is an overlap between the gamma-ray and X-ray regions of the spectrum, but some gamma rays are far more energetic than X-rays - they will pass through much thicker lead shields.

X-rays can be used to photograph the bones inside your body or the bomb in your suitcase because they are stopped by dense materials. Finding a dangerous hole or crack inside a metal casting, or a faulty weld, needs a more penetrating form of radiation. Gamma rays will do this job.

Gamma rays kill living cells, but they kill cancer cells more easily than they kill healthy cells. Therefore they can be used to treat tumours. Strong doses of gamma rays can be used to sterilise surgical dressings and instruments. They can be sealed in containers and lowered into the water tanks where used nuclear fuel rods are waiting to be processed. This gives them an ample dose of radiation. Food items - such as strawberries - can be sealed into airtight wrapping and then dosed with gamma rays. This kills most of the fungi and bacteria, so the fruit stays fresh for a long time. This does not make the strawberries radioactive, and they are quite safe to eat. People, however, are very suspicious of anything that has been treated in this way.

Someone told me that the helium in balloons was really alpha-particles. Is that true?

Helium is a very light gas, so it is soon lost into space from the Earth's atmosphere. The very tiny amount in the air has to come from underground. Experiments show that when alpha-particles finally come to a stop; they steal 2 electrons from other atoms and become atoms of helium. These atoms go around by themselves, so a molecule of helium is just one atom. When atoms in rocks emit alpha-particles, the gas gets trapped in the rocks. It can be released by grinding up and heating the rocks. A kilogram of uranium produces about a mugful of helium in 3 million years. This makes helium expensive, but it is used in balloons because it does not burn like hydrogen. Divers breathe a mixture of oxygen and helium to avoid getting the bends.

When a nucleus emits an alpha-particle, it loses 2 protons and 2 neutrons. Its mass number goes down by 4 and becomes an atom of the element two places down in the table - for example, radium-226 (element 88) becomes radon-222 (element 86).

When a beta-particle is emitted, a neutron becomes a proton and the atom moves one step down the table - for example carbon-14 (element 6) becomes nitrogen-14 (element-7).

I know that gamma-rays wouldn't turn me into the Incredible Hulk, but what would they do?

Radiation affects people's bodies in a number of ways. To begin with, a very heavy dose of radiation bronzes your skin like a suntan, and it can burn you like the sun. It also makes your hair fall out, and knocks out your body's immune system. You would be unlikely to live for long. Heavy doses have been used to try to stop people rejecting transplants, but this is a very extreme form of treatment. They have to be isolated from any possible infection for a long time afterwards.

Radiation may cause more damage if it is emitted from a source inside your body. This is particularly true of alpha-rays. If a particle strikes the nucleus of a cell, then it can break up the chains of DNA. These then come back together, but in a different arrangement. If it is an egg or sperm cell that is affected, then any baby will be formed according to a new set of instructions. Usually the result will be a failure, but very occasionally the change may be for the better. Mutations caused by background radiation have probably helped to speed up evolution.

Damage to other cells may lead to them multiplying uncontrollably and forming a tumour. This may not until years later, when it is too late to track down the cause. Gamma-rays, however, kill cancer cells more readily than they kill normal cells - so they can actually be used to treat cancer.

How does radon get into a house? Why is it dangerous?

Modern houses are snug and draughtproof, but in some parts of the country - such as Cornwall - this can actually make them dangerous to live in.

Ordinary uranium-238 decays through several stages to become radium-226. Radium decays to become a radioactive gas called radon-222, with a half-life of 4 days.Thorium also produces a form of radon.

In places where rocks like granite contain uranium, the radon produced seeps up through the ground and into houses through their foundations. If the house is draughtproof, the amount of radon in the air can be quite dangerous. The risk of dying from radon if you live in Cornwall is about 1 in 3,200.

When radon-222 decays, it emits an alpha-particle. It also continues to decay through a series of nuclides until it reaches lead-206, which is not radioactive. These decay products are all solids. The danger is that one of these atoms will lodge in someone's lung and emit an alpha-particle. Alphas cannot penetrate the skin, but if they are emitted inside the body they are very dangerous. They can eventually cause lung cancer. It is believed that quite a number of people die each year because of radon, though actual cases are difficult to pin down.

You can get rid of radon either by installing a sheet of material under the house to stop the radon getting through, or by installing fans to blow it away.

How could I protect myself from nuclear radiation?

Firstly remember that your eyes are particularly sensitive to radiation, and that your reproductive organs need protection for the sake of any children you may have.

Don't eat, drink or breathe in anything that might be radioactive - radiation does far more harm inside your body. Smoking makes things worse. People did not always realise the dangers - at one time radioactive drinks and radioactive toothpaste were advertised as being good for you!

If there is radioactive dust around, you must wear overalls, hat and overshoes and - if things are really bad - a special suit and breathing set.

There are three basic ways of protecting yourself from external radiation. They involve time, distance and shielding.

• Stay in the area for the shortest time possible.
• Keep as far away from radioactive sources as you can. Use long-handled tools to pick them up. The radiation is 100 times stronger 10 cm from a gamma-ray source than it is 1 metre away, and 10,000 times stronger at 1 cm from the source
• Put a barrier between yourself and the source. Barriers of the same size and mass offer about the same degree of protection. The denser the material, the thinner the barrier can be. Lead makes a thin barrier, but it is expensive. Steel barriers are thicker, but stronger and cheaper than lead. Thick layers of earth, concrete or even water are often used.

Where radiation is very strong, humans cannot function at all, and robot arms and TV cameras have to be used.

I read that they found what happened to mud dumped offshore by using radioactive tracers. What are tracers?

If you make some of the atoms in a material radioactive, you can find out what happens to the material even when it has become very spread out. In the case of the mud some powdered radioactive glass was mixed with it. Some radioactive solution might be mixed with the water flowing through a pipe. If there is a leak in the pipe some of the radioactivity will leak into the ground around the pipe. This can be detected by someone above ground following the line of the pipe.

A piston can be made radioactive by passing it through a nuclear reactor. It is then fitted into a car engine. After the car has run for some time, the engine oil is checked for radioactivity. This shows how much of the piston has worn away.

If a factory manager wants to know if materials are being mixed thoroughly, he can make one material radioactive. Then he checks if the activity is spread uniformly after mixing.

Radioactive chemicals can be injected into living things to see where they end up. For example, a plant can be watered with a solution containing radioactive phosphorus. When the plant has taken up the solution, a leaf can be pressed against X-ray film. A picture of the leaf is produced showing where the phosphorus is concentrated. They are also used in medicine. Iodine 131 is used in the detection of thyroid cancer.

The radioactive materials used as tracers need to have a short half-life, so that they soon disappear.

Is it really true that we are nearly all empty space?

All matter - and not just people - is mainly empty space. People began to realise this when they saw how easily particles like alphas and betas would pass through matter. They knew that there are no real gaps between atoms in solids, so the particles must actually be passing through the middle of the atoms. Very occasionally, however, a particle bounces back from something very small and very heavy in the centre of an atom. This is the nucleus.

To give you an idea of scale. If the atom were the size of a large room, its nucleus would be the size of a pinhead. The electrons round the nucleus would be like mosquitoes buzzing round the room. The rest would be empty space.

Solids exist because atoms attract one another and cling together. If one atom tries to get too close to another, however, attraction turns to increasingly strong repulsion. This repulsion is so strong that it is almost impossible to squash liquids or solids into a smaller volume. However, a metal ball can be squashed to about half its size for an instant, using specially shaped charges of high explosive. This is what happens to the sphere of plutonium at the centre of a nuclear bomb.

In stars that have collapsed at the end of their lives the pressures can be so high that the atoms collapse completely. The density becomes unbelievably high - a piece the size of a pinhead might weigh a million tonnes or more.

If an atom is the smallest piece of an element, how can there be things inside it?

The word atom comes from the Greek for 'not cut': originally the idea was that you could not split it: now we know better.

There is a tiny core at the centre of an atom, called the nucleus. It consists of particles called protons and neutrons. Protons and neutrons have about the same mass. Protons have a positive charge. Neutrons have no charge.

A hydrogen nucleus is just a single proton. All the other nuclei contain protons and neutrons. The nucleus gets bigger and heavier as we move down the list of elements, until we come to element 92 - uranium. This has 92 protons and 146 neutrons.

Surrounding the nucleus there is a cloud of electrons. Electrons have a negative charge. This charge will just cancel out the positive charge on a proton. The number of electrons in an atom is equal to the number of protons. This atomic number decides which element the atom belongs to, and how it will behave chemically.

It would take about 2,000 electrons to weigh as much as a proton, so nearly all the mass of an atom is in the nucleus.

How big are atoms? Look at 1 millimetre on a ruler. Imagine that millimetre magnified so that it was as big as the diameter of the Earth. The atoms in the ruler would then be about 1 millimetre across.

I read about a tribe of Native American wanting to make money by letting people store radioactive waste on their reservation. Why was this so unpopular?

Uranium fuel rods can be handled before they go into a reactor, but once in use they become so radioactive that nobody can go near them. After a few years each rod has to be removed from the reactor and stored under water while the radioactivity dies down enough for it to be processed to extract plutonium and unused uranium. The radiation is so strong that the water glows with blue light. This intense radioactivity is mainly due to the nuclei produced by fission, though other materials can be made radioactive by absorbing the neutrons that are flying round inside a reactor.

The shorter the half-life of a radioactive substance, the more intense the radiation from it. This is why the radioactivity dies down rapidly at first. The solution left after fuel rods have been processed is so intensely radioactive that it has to be refrigerated to prevent it from boiling. This makes it very difficult to store safely, particularly as the time involved is maybe 20,000 years. In the long-term the idea is to turn the liquid into a glass and bury it underground. The problem then is to make sure that the rocks are stable and not subject to earthquakes - no material can be allowed to get into the water that is underground.

Apart from the wate from reactors, there is also less radioactive waste from nuclear plants and hospitals. This is also a problem, because there is far more of it.

I saw a film about WW2 commandos attacking a heavy water factory. What is heavy water? Why was it so important?

A molecule of ordinary water (H₂O) contains two hydrogen atoms and one oxygen atom. In heavy water (D₂O) the hydrogen atoms are replaced by atoms of deuterium. Deuterium is an isotope of hydrogen.

An ordinary hydrogen atom has a nucleus consisting of a single proton. A deuterium atom has a nucleus consisting of a proton and a neutron, so it is twice as heavy as an ordinary hydrogen atom. This means that a molecule of heavy water has a mass of 20 units while an ordinary water molecule has a mass of 18 units. Pure heavy water is 10% denser than ordinary water; it freezes at 3.8°C and boils at 101.4°C. It is not radioactive.

Heavy water does not have to be produced: it is present as a tiny fraction of ordinary water. It can be separated by passing an electric current through water to split it into hydrogen and oxygen. The deuterium atoms move more slowly because they are heavier. They get left behind in the liquid. Nearly 30 000 litres of water have to be electrolysed to give one litre of almost pure heavy water. This takes a vast amount of electrical energy, which is why the plant being raided was in Norway. Hydroelectric power was plentiful there.

To produce a nuclear chain reaction, neutrons that are flying about at high speed have to be slowed down. They can then be captured by uranium-235 nuclei. This is done by bouncing them off light nuclei, such as deuterium. The nuclei recoil, taking away some of the neutron's kinetic energy. A substance used to slow down neutrons is called a moderator. Heavy water is a very good moderator - ordinary water absorbs too many neutrons.

At the beginning of the war it was thought that heavy water might be an important component of a nuclear bomb - hence the raids. This turned out not to be so, though the Germans did try unsuccessfully to build a nuclear reactor using heavy water as a moderator.

What does a nuclear reactor actually do?

Electricity is produced by dynamos. The generators in a power station are similar to bicycle dynamos, but they are about as big as a small bus and turn much faster. One dynamo will power millions of electric fires. These generators are turned by turbines, which are themselves powered by steam. The steam can be produced either by burning a
fuel (coal, oil or gas) or by a nuclear reactor.

The chain reaction in the uranium fuel rods inside a reactor creates heat. This heat is removed by passing a liquid or gas through the reactor. Carbon dioxide, water and liquid sodium are all used. This coolant is passed through heat exchangers where it boils water to create the steam to drive the turbines. In a nuclear submarine the turbines also drive the propeller.
 

Originally it was thought that nuclear reactors would be so efficient that everybody would have free electricity. Now nuclear reactors are becoming unpopular. People are worried about the small amounts of radioactive liquids and gases that they emit. They also create quantities of highly radioactive waste products that are expensive and dangerous to store. There are many problems with nuclear power, and there are many benefits, such as low carbon dioxide emissions. The debate for and against nuclear power is still very much in the headlines.

Why is Madame Curie so famous? What did she do?

Marie Curie is often portrayed as a sort of scientific saint - the Florence Nightingale of physics. In reality, like Florence, she was as tough as old boots and an able administrator. She was a left-wing atheist. At one point there was a terrific scandal when she was accused of breaking up another physicist's marriage. There was even a duel.

Marya Sklodowska was the daughter of a Polish science teacher. She came to Paris to study physics and chemistry, where she married a physics professor, Pierre Curie. In 1897, Marie decided to study radioactivity for her doctorate. To begin with, she worked through all the elements and found that uranium and thorium were radioactive. She then tried minerals and discovered that uranium ore was far more radioactive than it should be for the uranium that it contained.

Marie suggested that the ore might contain an unknown element in such a small concentration as to be invisible. This element would have to be extremely radioactive. They had little money. She and Pierre were forced to work in an old shed that had been a mortuary. It took them 4 years to extract a tiny quantity of radium from several tonnes of ore. Now they were famous, but Pierre was soon run over and killed when he slipped on a wet pavement. One of their daughters, Irene, and her husband Fred Joliot-Curie discovered that it was possible to make things radioactive using neutrons.

There is an idea out there that the Americans never got to the Moon because of the radiation in space that would fry astronauts. There are a number of reasons why this argument is flawed. Below is a basic outline:

The Van Allen Belts around the Earth trap particles from the Sun and concentrate them. This affords the Earth some protection from solar radiation, but the concentration of particles in the belts is much higher than that below or above the belts. Thus astronauts get a brief period of intense exposure as you pass through the belt. The radiation in question is particulate and occurs during bursts of intense solar activity such as flares. Solar flares liberate tremendous quantities of energy at many frequencies from X-rays and gamma rays to long-wavelength radio waves. They also emit high-energy particles called solar cosmic rays (protons, electrons and atomic nuclei).

Flare X-rays and ultraviolet radiation disrupt radio communication and the high-energy particle clouds, which are lethal to unprotected astronauts, reach the Earth in 30 minutes; clouds of low-energy particles and disturbances in the solar wind take 6 to 24 hours to reach to Earth.

Solar cosmic rays are formed when solar flares accelerate atomic particles leaving the Sun. A blast wave propagates through the solar wind at 1500 km/s. Protons, electrons and atomic nuclei are accelerated to high energies. Most of the particles are protons with alpha particles being second most abundant. Solar-particle energies range from keV to about 20 GeV. Solar flare particles are perhaps a million times more energetic than the ambient particles.

However, low energy particles (in the keV range) have little effect as they can be stopped easily by a spacecraft hull, space suit, camera case, etc. And high energy particles (in the GeV) range have so much energy they can actually pass straight through you without interacting with body cells at all. The most dangerous level of radiation is in the 1 MeV range. Someone in a low Earth orbit for 90 days would receive a radiation dose from the Van Allen belts twice that recommended for radiation workers. Astronauts on a mission to the Moon only pass through the high concentration of the belts briefly and then travel through space to the Moon - a total journey time of about one week. The radiation dose from a typical journey would thus be much less than that from a 90 day low-orbit mission.

The issue here, however, is the effects of a flare. In April 1981 the astronauts on the Space Shuttle Columbia would have been in serious danger had they been outside their craft as a large flare erupted while they were in orbit. They were also in a low-earth orbit so relatively safe.

However, flares don’t send particles out in all directions. The magnetic field from the flare determines the direction of the particle flow. So a flare may occur on one side of the Sun and thus not affect us.

Finally, although a high dose of any form of radiation is dangerous, being exposed to say one or two year’s worth of dose in a short period of time as some of the Apollo astronauts might have been during a flare, doesn’t necessarily imply that you’ll develop cancer. Being exposed to radiation increases the risk of developing cancer just as smoking does but it doesn’t necessarily mean that cancer is developed.

So in summary, high energy solar cosmic rays that occur during flares may well provide a high enough dose of radiation to be lethal. However, flares of this kind are few and far between and when they do occur, need to send out particles in the astronauts’ direction in order to affect them.

As for photographic film being fogged, again the same arguments apply. High energy particles will pass straight through the film without affecting it, while low energy ones will be stopped by the camera case.

NASA monitored solar flares during the Apollo missions using three satellites (whose names escape me!). Now they use earth based telescopes (radio and optical). The government doesn't really have a policy towards radiation in space as it's down to NASA to decide whether to launch or not. NASA are aware of the problem and have brought missions down early or delayed launches. However, because astronauts get some protection from their vehicles, they can launch most missions.

If the atoms in something are decaying, why doesn't it disappear?

When an atom decays, it does not disappear. It becomes an atom of another element. If an electron (beta-particle) is emitted, it eventually slows down and gets captured by an atom that is short of an electron. An alpha-particle will slow down, capture two electrons, and become a helium atom. The time taken for half the atoms to decay is called the half-life. The strength of the radioactivity also halves in one half-life. After two half-lives the activity will have dropped to one quarter, after three to one ninth and so on.

Substances with a very short half-life are very radioactive, but only for a short time. A longer half-life means that the activity is weaker, but it lasts a lot longer. In 10 half-lives the activity will drop to less than one thousandth of its initial value

In practice it is more complicated. Firstly a material like nuclear waste will be a mixture of substances with different half-lives. Secondly the new atoms produced are also likely to be unstable, and will decay in their turn.

Only half the atoms in a piece of uranium-238 will have decayed after 4.5 billion years. Just one gram of uranium, however, contains so many atoms that more than 12,000 will decay every second.

Are radioactive sources used in factories?

Radiation ionises air - it makes it conduct electricity. This can be useful in getting rid of static electricity. Static attracts dust, can cause dangerous sparks and makes some sheet materials difficult to handle. Antistatic cleaning brushes for vinyl LPs used to be made with radioactive bristles.

Beta and gamma rays are useful for measuring the thickness of materials such as tinplate and paper that are produced in a continuous sheet. A source is placed one side and a detector on the other. The thinner the sheet, the more radiation gets through. The measurements are then fed back to the system controlling the pressure on the rollers. Radiation can also be used to check whether sealed containers on a production line have been filled properly.

If beta-rays are directed at a sheet of something, some of them bounce back. This is called back-scattering. The number of betas bouncing back increases as the material gets thicker. Back-scattering is very useful as means of checking the thickness of paint . It can also be used to check if there is corrosion on the inside of a pipe or sealed container.

How do solar neutrino detectors work?
Solar neutrinos are very un-reactive particles made in the Sun. They can pass through the Earth without reacting with anything. In fact there are thousands of neutrinos passing through your body right now. Neutrino detectors are large containers filled with dry-cleaning fluid and buried underground. Dry-cleaning fluid is, strangely enough, one of the few things neutrinos actually react with. The chlorine atoms in the fluid occasionally react with the neutrinos causing a flash of light. This flash of light is then detected by cameras around the container.

Geiger counters are always clicking away in science fiction films. How do they work?

A Geiger counter depends on the fact that radiation knocks electrons out of the atoms in a gas and leaves them with an electric charge. These charged atoms (or ions) can then carry an electric current through the gas.

A Geiger-Müller (G-M) tube consists of a metal cylinder with a wire along its axis, sealed inside a glass envelope. At one end there is a very thin mica window, which allows radiation to enter the tube. The tube contains gas at low pressure. There is a high voltage between the wire and the cylinder. This produces a very strong electric field close to the wire. Normally no current can cross the gap. This means that there is no voltage across the 1 megohm resistor.

When an alpha- or beta-particle enters the tube, it produces some ions in the gas. These ions are then accelerated by the strong field close to the wire. They soon gain enough energy to ionise more atoms by bumping into them. There is an avalanche of ions which allows a current to flow through the gas. This current also flows through the resistor and produces a pulse of voltage across it. These pulses are counted by a special electronic circuit. Sometimes they give a click in a loudspeaker.

Geiger counters are best at counting beta-particles and those alpha-particles that have sufficient energy to pass through the window. Gamma-rays and X-rays will also be counted if they produce ions in the tube, but they often just go straight through.

 

Do cloud chambers really let you see alpha-particles?

Sometimes you see a vapour trail very high up in the sky. You cannot see what is on the end of it, but you assume it is a plane. The trail tells you where the plane has been. In just the same way, the vapour trail in a cloud chamber tells where a charged particle has been. In fact, vapour trail is a misnomer - we should say condensation trail. A cloud chamber is full of super-cooled vapour looking for somewhere to condense. Water droplets condense on the trail of ions left by a charged particle. Alpha-particles produce short, thick trails, because they produce lots of ions over a short distance. Beta-particles produce long straggly trails.

Nowadays, bubble chambers have largely replaced cloud chambers. If you remove the radiator cap on your car while the engine is very hot, the release of pressure causes the water to boil suddenly and violently. A bubble chamber is full of liquid hydrogen, under pressure, at a temperature above its normal boiling point (-253°C). A sudden drop in pressure makes the hydrogen want to boil. The bubbles of vapour, however, need centres round which to form. Ions provide these centres, and lines of bubbles mark the tracks of charged particles through the chamber.

Physicists hope that particles will react with hydrogen nuclei or electrons in the chamber, and something interesting will result. Flash photographs are taken each time there is an expansion. Thousands of these are analysed in the search for something significant. A magnetic field across the chamber curves the paths of the particles and gives away their charge.

"Quark" is a funny name. Where did it come from?

Murray Gell-Mann and George Zeweig first thought up the theory of these particles in 1964 to explain how protons and neutrons and other similar particles behaved.

Murray had just been reading Finnegan's Wake by James Joyce which contains the phrase "three quarks for Muster Mark". He decided it would be funny to name his particles after this phrase.

Murray Gell-Mann had a strange sense of humour!

Is antimatter really the opposite of ordinary matter?

Antimatter is a sort of mirror image of matter. A gamma ray is energy. Sometimes, however, a gamma-ray suddenly turns into an electron (matter) and a positron (antimatter). This can happen when it passes near a nucleus. When the positron meets an electron, however, they annihilate one another. Two identical gamma rays are produced, which shoot off in opposite directions. If there were only one gamma-ray, momentum could not be conserved. Energy has turned into matter plus antimatter; then matter plus antimatter has turned back into energy.

When a nucleus emits an electron, one of the neutrons actually becomes a proton. Some atoms, however, decay by emitting a positron. One of the protons becomes a neutron. It turns out that every particle of matter has a corresponding antiparticle. The antimatter version of hydrogen was recently produced, and actually existed for an instant. It consisted of an antiproton (negative) with a positron going round it. Then it was annihilated by contact with matter.

We would expect the Big Bang to have produced equal amounts of matter and antimatter, which would have rapidly annihilated one another. We still need to explain how enough matter got left over to produce the stars and galaxies.

Where do neutrons come from? Is it true that they are unstable?

In 1920, Rutherford suggested that an uncharged particle having about the same mass as a proton might exist. He thought of it as being a combination of a proton and an electron. Such a particle would explain why a nucleus having, say, 12 times the mass of a proton only weighed 6 times as much.

Rutherford had discovered in 1919 that very occasionally an alpha-particle would collide with a nitrogen nucleus and eject a proton from it. This also occurred with alpha-particles and other nuclei, but something strange happened with beryllium-17. Whatever was ejected, it was not a charged particle. Originally they thought it was a gamma-ray. Then, in 1932, Chadwick placed a slab of paraffin wax in front of the beryllium. Wax contains lots of protons (hydrogen nuclei). He found that protons were knocked forward out of the wax. Whatever was hitting them was giving all its energy to the proton. As it stopped dead after the collision, it must have the same mass as a proton. This was the neutron.

A convenient source of neutrons consists of beryllium mixed with a strong source of alpha-particles. This can be used to make other materials radioactive - it is more convenient than using a nuclear reactor.

Fast-moving neutrons are used to treat cancer. They are very penetrating - several feet of concrete are needed to stop them. In a nuclear reactor, fast neutrons are slowed down by bouncing them off light nuclei.

Unlike protons, solitary neutrons are unstable. They decay to give a proton, an electron and a neutrino. Their half-life is 13 minutes.

What are all the numbers that scientists use when they talk about atoms?

The first number is the atomic number (Z) (sometimes called the proton number). This is the number of protons in the nucleus. It is also the number of electrons going round the nucleus. Each value of the atomic number belongs to a particular element, from hydrogen at number 1 to uranium at number 92. The chemical behaviour of an element depends on the number of electrons, which is Z.

The second number is the neutron number (N). This is the number of neutrons in the nucleus. All atoms of the same element have the same Z, but they do not all have the same value of N. Atoms having the same number of protons, but different numbers of neutrons are called isotopes. They belong to the same element and behave the same in chemistry, but they do differ slightly in properties such as melting point. Isotopes can be separated, but it is not easy*.

Most elements have more than one isotope. You may have wondered why chlorine has an atomic mass of 35.5. This is because it is 75% chlorine-35 and 25% chlorine-37.

The mass number (N) is the total number of nucleons (protons and neutrons) in the nucleus. N = A - Z: for example, uranium-238 has Z = 92, A = 238 and N = 238 - 92 = 146.

A nuclide is a type of nucleus. The symbol for a nuclide is the symbol for the element with two numbers like this: ²³⁸₉₂U. The upper number is A and the lower number is Z.

*To separate uranium-235 from uranium-238, the uranium is turned into a gas - uranium hexafluoride. The gas molecules containing ²³⁵U will move slightly faster than those containing ²³⁸U. Allow the gas to pass down a long pipe with porous plugs in it: it will become steadily richer in ²³⁵U. There is a big uranium separation plant at Capenhurst, near Chester.

What is High Energy Physics about?

High energy physics involves using electric and magnetic fields to accelerate particles - such as protons - to enormously high energies, perhaps billions of electron volts. The particles soon reach a speed very close to that of light. Then, as their energy increases, they just become heavier and heavier. These very high energies are an attempt to get back to the sort of conditions that existed in the Universe shortly after the Big Bang and produce exotic particles - such as the Higgs Boson - that no longer exist in the modern universe

The high-energy particles are made to collide with other particles and nuclei, and the results examined using a bubble chamber or banks of detectors. The equipment involved is enormous - the accelerator at CERN is in a ring-shaped tunnel that crosses from Switzerland to France and back again - and costs billions of pounds. Countries are forced to co-operate because they cannot afford to go it alone.

High-energy physics has been described like this. You take a large watch. You then put a smaller watch in a gun and fire it so that hits the first watch at high speed. You take a photograph as all the bits fly apart. You then use the photograph to try and find out how a watch works.

What is a chain reaction?

What happens if, on average, at least one of the neutrons produced by the fission of a uranium-235 nucleus goes on to be captured by another uranium-235 atom? The process will keep going and generating heat - we have a chain reaction.

What is needed to keep the reaction going? The problem is that the neutrons are travelling too fast to be captured easily. There are possible answers. One is to use pure uranium-235, but this very expensive to produce. The other is to slow down the neutrons so that they are more easily captured. This is done by bouncing them off the light atoms contained in a material known as a moderator. Graphite is often used, and heavy water is very effective. Ordinary water can be used as a moderator, but it absorbs a lot of neutrons. A layer of heavy atoms reflects escaping neutrons back into the reactor core.

How can we control the reaction? Rods made of boron are pushed into channels in the reactor. These rods absorb neutrons, so they damp down the reaction. The speed of the reaction is controlled by moving these control rods in and out. The heat produced is removed from the reactor by a flow of liquid or gas coolant. The heat is used to produce steam and drive turbines.

There is another metal that can be used instead of uranium-235: plutonium. In a bomb, pure uranium-235 or plutonium is used, and there is no moderator.

Why can't antimatter be industrially produced yet? What are the difficulties?

This depends very much on what you mean by industrially and what you'd want to do with it when you'd produced it. Places like Cern that use antimatter in their experiments, produces approximately 10E14 (1 with 14 zeros after it) antiprotons a year. While this might seem like a lot, in terms of actual 'energy content' this number would only be enough to lift a person 10 metres into the air. In order to power a spacecraft and get it into orbit, Cern would need to produce antiprotons at this rate for 500,000 years. To produce enough energy for a medium sized bomb, Cern would have to run for 20,000 times the age of the Universe. So you can see that antiparticles don't exactly have many applications in industry.

There aren't really any problems in producing antiparticles except for the energy needed to produce them. Just as it takes energy to produce matter, it takes energy to produce antimatter and this energy is not recouped when the antimatter is annihilated. Storage isn't a problem. At the moment, antiparticles are stored in containers with strong magnetic or electric fields around them and a vacuum within. Kept in ideal conditions, antiparticles should survive forever, but obviously conditions aren't perfect and antiparticles will react with matter in the non-perfect vacuum and will sometimes collide with the sides of the container.