The nuclear atom
An atom is the smallest part of an element. Neutrons and protons form the nucleus of an atom. Electrons orbit the nucleus at high speed. Electrons are the smallest particle and are negatively charged. Protons have a mass 2000 time that of an electron and are positively charged. Neutrons have the same mass as protons and have no charge. An atom is neutral so has an equal number of electrons and protons. Nearly all the mass of the atom is concentrated in the nucleus.
Evidence about the structure of the atom from the 1909 Rutherford-Geiger-Marsden alpha scattering experiment during which alpha particles were fired at gold foil. The observations recorded were that most alpha particles passed striaght through the gold foil undeviated; a few particles were deflected through small angles and even fewer bounced straight back from the foil. The conclusions were that most of an atom is simply empty space since most particles passed straight through; the mass and charge of an atom is concentrated in a small area in the centre of the atom (the nucleus); the nucleus is positive since the positive alpha particles were repelled.
A strong nuclear force holds the protons and neutrons in the nucleus together. It has to balance the repulsive electrostatic force between the protons. The number of protons in a nucleus determines the element and its chemical properties. Isotopes are atoms of the same element with the same number of protons but differing numbers of neutrons in the nucleus.
Radioactive elements are unstable and constantly emit ionising radiation in order to make them more stable. Ionising radiation knocks out electrons from atoms, forming a positive ion. Ionising radiation can be either high-energy particles or high-energy electromagnetic waves.
Background radiation is low-level ionising radiation that is all around us. Some background radiation comes from outer space as cosmic rays but most comes from rocks and soil. Not all background radiation comes from natural sources- some comes from industry and medical uses. It can also come from food and drink and nuclear power and nuclear weapons testing.
Radioactivity is a random process. We never know when an unstable nucleus will decay and emit ionising radiation. The amount of radiation being emitted is dependant only on the amount of the radioactive element present. The behaviour of radioactive materials is not affected by physical or chemical processes.
Types of ionising radiation (1)
There are 3 types of ionising radiation emmited by radioactive materials.
Alpha is positive (2+) and is the heaviest of the three. It is deflected by magnetic and electric fields and has the lowest penetration. It is however, the most ionising.
Beta is negative (1-) and is very light. It is also deflected by magnetic and electric fields but in the opposite direction to alpha. It is not as ionising as alpha either but has medium penetration.
Gamma has a neutral charge and has no mass at all. It is not deflected by magnetic or electric fields and is the most highly penetrating. It has the lowest ionising power though.
An alpha particle is a helium nucleus. It consists of two protons and two neutrons, so has atomic mass of 4 and a charge of +2.
A beta particle is a fast moving electron so it has an atomic mass of zero and a charge of -1. It comes from the nucleus when a neutron changes into a proton and an electron.
Gamma rays are very high frequency electomagnetic waves.
Types of ionising radiation (2)
As alpha, beta and gamma radiation go through the air they ionise air molecules and lose energy. Alpha particles are massive and can easily knock off electrons, so lose energy quicker and can't travel as far (a few cm in air). Beta particles have a range in air of about a metre but can be stopped by about 3mm of aluminium. Gamma rays are not stopped by air, but just spread out and become less intense. Thick lead is used to absorb gamma rays.
Hazards of ionising radiation
Ionising radiation can damage living cells. The damage depends on the type and intensity of the radiation. Ionising radiation collides with living cells and knocks electrons out of the atoms, leaving positive ions. High intensity radiation may kill the living cells and cause tissue damage, and could lead to radiation sickness. Some high intensity radiation causes cells to become sterile (containing no living organisms). Low intenisty radiation can affect the cell's genetic makeup, causing mutations that could lead to cancer.
Alpha particles are large and highly ionising, but do not pass through the skin. Inside the body, alpha particles would be highly damaging, but are relatively safe outside the body. Beta and gamma are much more penetrating and will pass through skin, so are more dangerous outside the body, despite being poorer ionisers. The unit of radiation absorbed equivalent dose is the Sievert (Sv). One Sievert of alpha, beta or gamma produces the same biological effect, and is a measure of the possible harm done to your body. Oxygen, hydrogen, nitrogen and carbon are highly susceptible to ionisation and are abundant in the body. Ions can interfere with the structure of DNA, causing it to behave incorrectly and damage living cells.
Once atoms are ionised by radiation and form ions, these charged ions can break and make chemical bonds, therefore changing molecular structure.
When a radioactive nucleus emits an alpha or beta particle, the number of protons and neutrons changes and it becomes a new element.
The radioactivity of a material decreases over time because the amount of radiocative nuclei decreases. This is called radioactive decay. The actvity of a radioactive sample only depends on the number of unstable nuclei present. Many large nuclei are unstable and emit alpha particles. Carbon- 15 is an unstable form of carbon that emits a beta particle and becomes nitrogen. Emitting gamma rays does not change one element to another. The new element is known as the daughter product, and may or may not be radioactive. Many radioactive elements belong to a decay chain, in which the first radiocative element decays into a second element, which then decays into a third element, and so on. An unstable nucleus is in an energetic state and has too much energy. It needs to lose energy to become more stable. When nuclei emit alpha or beta particles they change into more stable nuclei, but may still be in an energetic state. For this reason, they often emit gamma rays as well, to reduce their energy.
The half life of a radioactive element is the time taken for half the nuclei in a sample to decay. This is specific to each radioactive element. Half lives can vary from fractions of a second to millions of years. The activity of a radioactive source (the amount of radiation emitted) is a measure of its rate of decay. When there is plenty of radioactive nuclei present at the beginning, the rate of decay is faster than when most of the nuclei have already decayed. The activity can never reach zero- it just continually decreases to a negligible value.
Scientists can find the half life of a sample by recording the radioactive count rate over time and plotting a graph. They read off the time it takes the count rate to halve and calculate the average.
Uses of ionising radiation
- Treating cancer: ionising radiation can kill cells, so can be used to kill cancerous cells. This is known as radiotherapy. Gamma radiation is usually used. Some healthy tissue around the tumour can be damaged so the radiation must be focused on the tumour.
- Sterilising medical instruments: these can be irradiated with gamma radiation to kill bacteria.
- Sterilising food: as soon as fresh food is picked and ready to transport, microorganisms will start the decay process. If the food is irradiated with gamma radiation the MOs will be killed. This makes the shelf life of the food much longer. Gamma rays are used as they are able to penetrate the food's packaging and are capable of killing the bacteria.
- Detecting tumours: brain and other tumours can be detected using a radioactive tracer. A gamma emitter with a half life of a few hours is injected; the radiation is detected from outside to build up a computer image of the tumour. Radioactive tracers are usually beta or gamma emitters, as they must be able to penetrate skin and tissue. The half life needs to be a few hours, so that it has time to reach the affected parts of the body in sufficient quantities, but not last so long that it damages the body.
Keeping people safe
Exposure to radiation is called irradiation. People are exposed to radiation all the time and the risk to health is usually insignificant. However, it does depend on the level of radiation and the length of exposure.
Contamination means that a surface or person is in contact with radioactive material. Radioactive waste can be contained to prevent contamination. If it cannot be contained, it must be diluted to safe concentrations. High level contamination, such as fallout from a nuclear explosion, will need more intervention.
People who work with radioactive sources (eg. radiographers and workers in nuclear power stations) regularly need to have their level of exposure monitored.
A film badge can monitor radiation. The level of exposure can be measured by assessing how black the film has become. Lead shielding in the form of aprons or protective walls is used to protect radiographers.
Energy from the nucleus
Nuclear fission releases energy by a heavy nucleus, e.g. uranium, splitting into two lighter nuclei. Nuclear fusion, on the other hand, realeases energy by two light nuclei, e.g. hydrogen, combining to create a larger nucleus.
Nuclear fusion and fission release much more energy than chemical reactions involving similar masses of materials. This is because the energy that holds nuclei together (binding energy) is much larger than the energy that holds electrons in place.
Materials that provide energy by changes in the nucleus are known as nuclear fuels.
In nuclear fission, a neutron is fired at a uranium or plutonium nucleus to make it unstable. The nucleus breaks down into two smaller nuclei of similar size, and releases some more neutrons. The neutrons released can go on to initiate more fission reactions. This is known as a chain reaction. More and more neutrons will be released in each subsequent reaction, and the chain reaction will get out of control unless the number of neutrons is controlled. Energy released in nuclear fission and fusion is calculated using E = mc 2, where E is the energy is joules, m is the mass in kg and c is a constant equal to the speed of light in a vacuum (3x10^8 m/s)
Nuclear power generation
About a sixth of the UK's electricity is generated in by nuclear fission. Nuclear wastes are catergorised according to their level of risk:
- Low level waste such as contaminated paper or clothing is not dangerous to handle but should be disposed of with care. It is burnt and sealed in containers before being buried in landfill.
- Intermediate level waste such as chemical sludges and reactor pparts are more radioactive and need shielding. Waste with a longer half life is buried undergroud.
- High level waste such as spent fuel rods are higly radioactive. Some of this waste is mixed with molten glass and contained in stainless steel drums before careful storage.
In a nuclear reactor, the fuel rods conatin pellets of uranium. Neutrons cause the fuel to undergo fission. The energy is released as kinetic energy of particles (heat). A coolant (gas or liquid) circulated around the reactor absorbs the heat and transfers it to a steam generator. Electricity is then generated in the same way as a conventional power station. Control rods (usually made of boron) absorb some of the neutrons. The control rods can be raised or lowered to control the fission rate.
Nuclear fusion as a power source
Despite nuclear fusion's great potential (the availability of hydrogen; more energy produced per kg than fossil fuels; no radioactive by-products; no CO2 released) more energy is consumed producing fusion reactions than is released by it currently. Scientists are working on this though.