P6 Radioactive MaterialsRadiationRadiation is ionising and could damage or kill living cells. Radioactive elements can be naturally occurring, or they can be man-made (produced because they are useful).

Radioactive decayThe spontaneous change in an unstable element, giving out alpha, beta, or gamma radiation. Alpha and beta emission result in a new element.

Atoms decay to become more stable. Radioactive decay happens in the nucleus of an atom. It is not affected by any physical or chemical changes that happen to the atom.

Rutherford’s Experiment This is what Geiger and Marsden observed:

Most of the alpha particles passed straight through the gold foil, deflected by no more than a few degrees.

A small fraction of the alpha particles were reflected back towards the direction from which they came from.

Rutherford realised that there must be something with positive charge that was repelling the alpha particles (al has a positive charge). And it must also have a lot of mass, or the alpha particles would just push it out of the way.

This was the nucleus of a gold atom. It contains all of the positive charge within the atom, and most of the mass. Rutherford’s data showed that the nucleus was

very tiny, because most alpha particles past without being affected.

IsotopesNucleus contains protons and neutrons. All atoms of any element have the same number of protons. E.g. carbon atoms always have 6 protons, but they can have different numbers of neutrons and still be carbon.

Isotope – different atoms of the same element.

Radiation What it is

Alpha particles (α) Small, high-speed particle with + charge

Beta particles (β) Smaller, higher-speed particle with - charge

Gamma radiation (γ)

High-energy electromagnetic radiation

Formula for describing an isotope:

Radioactive changesSome nuclei that are unstable can become more stable by emitting an alpha particle. An alpha particle is made of two protons and two neutrons – it is the same as a helium nucleus.

Alpha decay equation:

Other nuclei can become more stable when a neutron decays to form a proton. It does this by emitting a beta particle. A beta is the same as an electron, but I has come from a neutron in the nucleus. It is not one of the atom’s orbital electrons.

Beta decay equation:

The emission off either an alpha or a beta particle from an unstable nucleus produces an atom of a different element, called a daughter product or decay product. The daughter product may itself be unstable.

Alpha, beta or gamma radiation?

Ionising radiation can kill bacteria. Gamma radiation is used for sterilising surgical instruments and some hygiene products such as tampons. Irradiating food kills bacteria and prevents spoilage.

Irradiation – being exposed to radiation from an external source.

RadiationIf you switch on a Geiger counter, you will hear it click. It is picking up background radiation. Most background radiation comes from natural sources.

Radiation dose (Sv/mSv) - measures the possible harm the radiation could do to the body.

The dose measures the potential harm done by the radiation.

It depends on:

The amount of

radiation- number of alpha particles, beta particles, or gamma radiation photons reaching the body.

The type of radiation. Alpha is the most ionizing, so it can cause damage to a cell. The same amount of alpha radiation gives a bigger dose than beta or gamma radiation. It is only hazard if the source of radiation gets inside the body because alpha radiation has a short range in air.

The damage to the body will also depend on the type of tissue affected. E.g. lung tissue is easily damaged.

There is no such thing as a safe dose. The lower the dose, the lower the risk. But the risk is never zero.

Ionising radiation has the energy to break molecules in the cells in the body to ions. If the ionising radiation affects DNA molecules, this may cause the cell to be killed or to behave abnormally (can cause cancer).

Irradiation and ContaminationAlpha irradiation presents a very low risk because they only travel a few cm in air and are easily absorbed. Irradiation by beta particles is more risky as they penetrate a few cm into the body. Most gamma rays pass straight through the body. But they have high energy, so if they are absorbed they are dangerous.

Contamination – if radiation source enters the body, or gets on skin or clothes.

ProtectionTo reduce their dose, medical staff take a number of precautions:

Use protective clothing and screens

Radiation Dose Factor

Alpha 20Beta 1

Gamma 1

Wear gloves and aprons Wear special devices to monitor their dose Keeping radioactive materials in lead-lined containers, labelled with the appropriate hazard symbol Keeping your exposure time as short as possible and move far away as practicable

Employers must ensure that radiation workers receive a radiation dose a low as reasonable achievable. They keep a close eye on people who regularly wok with radioactive materials.

Half-lifeRadioactive decay is random. Scientists can’t predict whether a particular nucleus will decay today or in a thousand years’ time.

The amount of radiation from a radioactive material is called its activity.

Activity- the rate at which nuclei in a sample of radioactive material decay and give out alpha, beta, or gamma radiation.

The activity decreases with time.

At first there are a lot of radioactive atoms. Each atom gives out radiation as it decays to become more stable. The activity of the material falls because fewer and fewer radioactive atoms remain.

Half-Life – the time it takes for the activity to drop by life

All radioactive materials show the same pattern but they can have different half-lives. There is no way of slowing down or speeding up the rate at which radioactive materials decay. The shorter the half-life, the greater the activity for the same amount of material.

Half-life of Technetium-99m

Time Hours since injection

Number of half-lives

Fraction of original sample

remaining9:00 am 0 0 1

3:00 pm 6 1 ½

9:00 pm 12 2 ¼3:00 am 18 3 1/8

9:00 am 24 4 1/16

The decay curve for Technetium-

99m

Uses of RadiationUses of alpha radiation

Ionisation is useful for smoke detectors. Radioactive americium releases alpha radiation, which ionises the air inside the detector. Smoke from a fire absorbs alpha radiation, altering the ionisation and triggering the alarm.

Uses of beta radiation

Beta radiation is used for tracers and monitoring the thickness of materials.

Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body.

Radiation is used in industry in detectors that monitor and control the thickness of materials such as paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material. Check your understanding of this by watching the simulation.

Uses of gamma radiation

Gamma radiation is used in the treatment of cancer, testing equipment and sterilising medical instruments.

Nuclear FissionNuclear fission – a process where radioactive atoms have an unstable nucleus and some nuclei can be made so unstable that they split into two.

Example of nuclear fission equation:

For example, the nucleus of a uranium-235 atom breaks apart when it absorbs a neutron.When this happens a small amount of the mass of the nucleus is converted to a huge amount of energy. The energy released can be calculated from Einstein’s equation:

E = mc²

Where E = energy, m = change in mass and c = speed of light

The energy is transferred to the fission products, so they have a lot of kinetic energy. Each fission reaction produces roughly a million times more energy than when a molecule changes during a chemical reaction.

Chain reaction

The fission of one atom can set off several more, because each fission reaction releases a few neutrons. If there are enough U-235 atoms close together, there will be a chain reaction, involving more and more atoms.

Chain reactions are made use of in nuclear reactors and atomic bombs. In an atomic bomb Uranium is used above a critical size to get an uncontrolled chain reaction by ensuring all the available fissile material undergoes fission in the minimum time possible. In nuclear reactors the concentration of Uranium-235 is much less and the chain reaction is controlled in order to reduce it and more importantly stop it if required.

The fission takes place in fuel rods that contain uranium-235. This makes them extremely hot. Control rods, which contain the element boron, absorb neutrons. Moving control rods in or out of

the reactor decreases or increases the reaction rate.

Generating electricity

A fluid, called a coolant, is pumped through the reactor. The hot fuel rods heat the coolant to around 500ºC. It then flows through a heat exchanger in the boiler, turning water into steam. The steam s used in the same way as in a coal-fired power station. One reason for building nuclear power stations is to reduce the need for fossil fuels.

Nuclear wasteMost of radioactive waste comes from power stations. The rest comes from medical uses, industry and scientific research. The waste radioactive materials are separated out and taken away to be stored.

Radioactive waste has little average background radiation but it is hazardous because of contamination.

Category Examples DisposalLow level (LLW) Contaminated equipment,

materials and protective clothingThey are put in drums and surrounded by concrete, and put into clay lined landfill sites.

Intermediate level (ILW)

Components from nuclear reactors, radioactive sources used in medicine or research

They are mixed with concrete, then put in a stainless steel drum in a purpose-built store.

High level (HLW)

Used nuclear fuel and chemicals from reprocessing fuels

They are stored underwater in large pools for 20 years, then placed in storage casks in purpose-built underground store where air can circulate to remove the heat produced. High level waste decays into intermediate level waste over many thousands of years

Nuclear FusionStrong nuclear force- an attractive force that holds protons and neutrons together (despite the fact that protons repel each other because of their positive charges). The strong nuclear force has a short range. It only acts when two nucleons (protons and neutrons) are very close together. In a nucleus, the nucleus, the particles are separated by just the right distance so that the strong nuclear force is balanced by the electrical electrostatic force.

Nuclear fusion – the process in which two small nuclei combine to form a larger one, releasing energy. E.g. hydrogen combining to form helium. This happens in stars, like the Sun.

Bringing two atoms close together; their nucleus repel each other, because of the electrical electrostatic force. They will not fuse together. Push hard enough (under high temperatures, i.e. the Sun): they come close enough for the attractive force to take over, and the nuclei fuse. Energy is released.

Energy is released by a small amount of mass being converted to energy.

Hydrogen bombs, which fuse hydrogen, release hundreds of times more energy than atomic (fission) bombs. Hey are triggered using an atomic bomb to compress the hydrogen so that it fuses.

Ideas about Science Scientific or technological developments often introduce new risks. Background radiation is all around us, so there is always a risk of out cells being harmed by

ionising radiation. Low dose = small risk.

The development of radioactive materials in the early 20th century led to the deaths of many workers. As the materials were new, no one realised they could be dangerous.

Risk can sometimes be assessed by measuring its chance of occurring in a large sample. The safe dose that people may receive has been based on the rate of cancer in workers

exposed to radiation over many years. It is important to be able to assess the size of risks in any activity. No activity is completely

safe. The likelihood of dying from a nuclear accident has been calculated, and it is very low. Cycling

is much more dangerous. For most risky activities, there are benefits as well as risks. A gamma scan gives doctors valuable information to help cure a patient. This benefit

outweighs the slight risk from the gamma radiation itself.

People who are ill may choose to have treatment in the hope that they will be cured. Some

people may refuse treatment is doctors say they must have it, as they prefer to choose for

themselves.

Countries that have nuclear power stations keep account of all the radioactive waste, as this

could be processed to produce nuclear weapons.

Ideas about science - making decisions

Scientific applications give people things that they value, but may have undesirable impacts on the environment.

Our society uses more and more energy every year, but the carbon dioxide produced by most power stations is believed to be causing serious damage to the climate.

Natural resources should be used in a sustainable way. The use of renewable energy sources would guarantee energy for the future. At the moment,

renewable energy sources cannot provide enough energy. There are official regulations and laws which control scientific research and applications. The nuclear industry is regularly inspected to ensure that standards of safety are maintained. Some applications of science have ethical implications. One point of view is that the right decision is

the one which gives the best outcome for most people. Another point of view is that certain actions are never justified because they are wrong.

Disposal of nuclear waste raises ethical problems:

Some people say that we must have nuclear power, or we will not be able to combat global warming and still produce enough energy.

Others say that it is unethical to produce waste that will still be dangerous in many thousands of years’ time.

Weighing up the arguments - higher only

Before deciding on a course of action, it is important to ask if it is feasible. Can it be done? Then it is possible to consider if it ought to done.

If nuclear waste could be sent down into the Earth’s mantle, it would take millions of years to resurface. Unfortunately, there doesn’t seem to be any way to do this. It is not feasible.

Nuclear waste could be sent into space in rockets so that it falls harmlessly into the Sun. Unfortunately, this would be far too expensive, and any accident on take-off would spread dangerous waste over a large area. It ought not to be done.

In different social and economic contexts, different decisions might be taken.

Many developing countries insist that they need to burn fossil fuels in their power stations, even if it produces global warming. They need this to allow them to catch up with the standard of living that we enjoy.