Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Nuclear fission wikipedia , lookup
Radioactive waste wikipedia , lookup
Nuclear fission product wikipedia , lookup
Fallout shelter wikipedia , lookup
Nuclear and radiation accidents and incidents wikipedia , lookup
Nuclear fusion–fission hybrid wikipedia , lookup
Atomic nucleus wikipedia , lookup
Ionizing radiation wikipedia , lookup
Nuclear transmutation wikipedia , lookup
National 4 Unit 1: Section 5 • I can identify natural and artificial sources of nuclear radiation and associated medical and industrial applications. • I can explain some the pros and cons of generating electricity using nuclear fuel. • I can make comparisons of risk due to nuclear radiation and other environmental hazards. • I can describe how to manage the risks associated with radiation. 1 National 5 Unit 1: Section 5 • I can describe the nature of alpha, beta and gamma radiation in terms of relative effect of ionization, absorption, shielding. • I can identify sources of background radiation. • I can calculate absorbed dose, equivalent dose and make comparisons of equivalent dose due to a variety of natural and artificial sources. • I can describe some applications of nuclear radiation. • I can state that activity is measured in Becquerel’s. • I can give a definition of Half-life. • I can make use of graphical or numerical data to determine the half-life of a source. • I can give a qualitative description of fission and fusion, emphasising the importance of these processes in the generation of energy. 2 Section 5: Nuclear Radiation Alpha, Beta and Gamma Radiation The Atom Atoms are the smallest possible particles of the elements. Atoms make up everything around us. The three main particles which make up atoms are All atoms have a tiny central nucleus which has a _________ charge. We can imagine the ____________ charged electrons to be circling around this, rather like planets around the sun. The nucleus contains the ___________ protons and the neutrons, which are _______________. Ionisation Ionisation is the break up of a neutral atom into a positive ion and an electron. The electrons near the outside of the atoms are very light, and can easily be knocked away from the atom. This can happen if radiation from a radioactive substance passes nearby. Because radiations from radioactive substances make ions so easily, they are often called ____________ ____________. 3 Ionising Radiations When the alpha or beta or gamma radiation passes through a material they lose _________ by colliding with the atoms of the material. Eventually the radiations lose so much _________ that they cannot get through (____________) the material and so are ____________. Alpha particles: (α α) are the nuclei of helium atoms. They have 2 neutrons and 2 protons in the nucleus and are therefore ___________ charged. Alpha particles will travel about 5 cm through the air before they are fully absorbed. They will be stopped by a sheet of paper. Alpha particles produce the greatest ionisation. Beta particles: (β β) are fast moving electrons and so are ___________ charged. Beta particles can travel several metres through air and will be stopped by a few millimetres of aluminium. They cause less ionisation than alpha particles. Gamma rays: (γγ) have ____ mass or charge and carry energy from the nucleus leaving the nucleus in a more stable state. Gamma rays can only be stopped by a very thick piece of lead. They travel at the speed of light and very little ionisation. 4 Detecting Radiation Geiger-Muller Tube The Geiger-Muller tube is a detector which use the effects of ionisation to measure the amount of radiation present. The central wire inside the cylindrical tube is kept at a high voltage of about +400 V compared with the outer case. When radiation enters the tube and produces a few ions, these are accelerated towards the central wire. As they pass through the low-pressure gas, they bump into atoms at high speed and knock out many more electrons off. When they reach the central wire they send a pulse of current round the circuit. These pulses are counted electronically by the scalar or ratemeter, and so the amount of radiation being detected by the G-M tube is measured. 5 Measuring Background Radiation Experiment 8 What you need: A Geiger-Muller tube, a scalar meter What to do: Switch on the counter for one minute and measure the amount of radiation detected in the lab during this time. Reset the counter and repeat the experiment twice more. Complete the table using your own results and seven more obtained by other groups. Measurement number 1 2 3 4 5 6 7 8 9 Counts in 1 minute Are all the values the same? What is the average background count in counts per minute? 6 10 Background radiation Everyone is exposed to background radiation from natural and from man-made radioactive material. Background radiation is always present. Some of the factors affecting background radiation levels are: • Rocks which contain radioactive material, exposing us to ionising particles • Cosmic rays from the sun and outer space which emit lots of protons which cause ionisation in our atmosphere • Building materials containing radioactive particles and radioactive radon gas from the soil and which collects in buildings, mainly due to lack of ventilation. • The human body which contains radioactive potassium and carbon. • A persons chosen occupation. Radiographers exposed to X-rays used in hospitals and nuclear workers from the reactor. Natural radiation is by far the greatest influence on our exposure to background radiation. 7 Half-life In any radioactive source, the activity decreases with time because the number of unstable atoms gradually decreases leaving fewer atoms to decay. The half-life of a radioactive source is the time for the activity to fall to half its original value. To Find the Half Life of a Radioactive Source 1. Without the source being present measure the background count rate with the Geiger counter. 2. Place the radioactive source in front of the Geiger Muller tube and measure the total count rate (this is at t = 0). 3. Measure the count rate at regular intervals. 4. Correct all of the count rates for background radiation to find the source count rate. Source count rate = total count rate – background count rate 5. Draw a graph of source count rate against time. 6. Use the graph to find at least two values for the half-life of the source (the time it takes for the count rate to half). Find the average value for the half-life of the source. (Instead of using the count rate of the source, the activity of the source in Becquerels could be used) 8 Example 1. A Geiger-Muller tube and ratemeter were used to measure the half-life of radioactive caesium-140. The activity of the source was noted every 60 s. The results are shown in the table. By plotting a suitable graph, find the half-life of caesium-140. Time (s) 0 Count rate 70 (corrected) (count/s) 60 120 180 240 300 360 50 35 25 20 15 10 From the graph the time taken to fall from 70 counts/s to 35 counts/s = 120 s 35 counts/s to 17.5 counts/s = 120 s Average half life of caesium-140 = 120 s. 9 Dosimetry Activity The activity, A, of a radioactive source is the number of decays, N, per second. It is measured in Becquerel where: 1 Bq = 1 decay per second. Number of Decays Activity (Bq) = --------------------------Time (s) N A t Absorbed dose The greater the transfer of radiation energy to the body the greater the chance of damage to the body. The absorbed dose, D, is the energy absorbed per unit mass of the absorbing material and is measured in grays, Gy. 1 Gy = 1 Joule per kilogram Energy absorbed (J) Absorbed dose (Gy) = ------------------------Mass (kg) E D m 10 Questions 1. A radioactive tracer has an activity of 160 Bq. The tracer has a half life of 5 hours and decays for 15 hours. What is its final activity? 2. A radioactive source with a half life of 2.5 minutes decays for 10 minutes. The source has an initial activity of 64 kBq. Calculate the final activity of the source. 3. A sample of radioactive uranium has an initial activity of 600 kBq. After 10 days its activity has dropped to 150 kBq. Use this information to calculate the half life of the source. 11 4. The activity of an isotope varies with time as shown below. The count rate is uncorrected for background radiation. Count rate 230 190 160 130 110 95 80 70 0 1 2 3 4 5 6 7 (per minute) Time (hours) The background count is 30 counts per minute. a. Collect some graph paper from your teacher and lot a corrected graph of activity against time for the isotope. b. Calculate the half life of the isotope. 12 The biological effects of radiation All ionising radiation can cause damage to the body. The risk of biological harm from an exposure to radiation depends on: • the absorbed dose • the kind of radiation • the body organs or tissue exposed. The body tissue or organs may receive the same absorbed dose from alpha or gamma WR Type of Radiation 1 Beta particles/Gamma rays 10 Protons and Fast Neutrons 20 Alpha particles Effects of radiation on living things All living things are made of cells. Ionising radiation can kill or change the nature of healthy cells. This can lead to different types of cancer. 13 Equivalent Dose When scientists try to work out the effect on our bodies of a dose of radiation they prefer to talk in terms of equivalent dose. The equivalent dose H is the product of D and WR. equivalent dose = absorbed dose x radiation weighting factor H D WR Example A worker in the nuclear industry receives the following absorbed does in a year: 30mGy from Gamma radiation (WR =1) 300mGy from fast neutrons (WR = 10) Calculate the equivalent dose for the year. for Gamma H = 30 x 10-3 x 1 = 30 x 10-3 Sv for neutrons H = 300 x 10-6 x 10 = 3.0 x 10-3 Sv total H = 30 x 10-3 + 3.0 x 10-3 = 33 x 10-3 Sv 14 Question A technician in a nuclear power station is exposed to several types of radiation over a 150-hour working month. She receives a dose 0.2 mGy due to exposure to fast neutrons, 15 µGy due to α-particles, and an absorbed dose of 1mGy from gamma rays. Calculate the technician’s total equivalent dose. 15 Safety with Radioactivity • Always use forceps or a lifting tool to remove a source. Never use bare hands. • Arrange a source so that its radiation window points away from the body. • Never bring a source close to your eyes for examination. • After any experiment with radioactive materials, wash your hands thoroughly before you eat. Reducing the dose equivalent • Use shielding, by keeping all radioactive materials in sealed containers made of thick lead. Wear protective lead aprons to protect the trunk of the body. • Keep as far away from the radioactive materials as possible. • Keep the times for which you are exposed to the material as short and as few as possible Radioactive hazard warning sign The sign should be displayed on all doors where radioactive materials are stored. 16 Fission and fusion Advantages of using nuclear power to produce electricity • Fossil fuels are running out, so nuclear power provides a convenient way of producing electricity. • A nuclear power station needs very little fuel compared with a coal or oil-fired power station. A tonne of uranium gives as much energy as 25000 tonnes of coal. • Unlike fossil fuels, nuclear fuel does not release large quantities of carbon dioxide and sulphur dioxide into the atmosphere,which are a cause of acid rain. Disadvantages of using nuclear power to produce electricity • A serious accident in a nuclear power station is a major disaster. British nuclear reactors cannot blow up like a nuclear bomb but even a conventional explosion can possibly release tonnes of radioactive materials into the atmosphere. (The Chernobyl disaster was an example of a serious accident.) • Nuclear power stations produce radioactive waste, some of which is very difficult to deal with. • After a few decades nuclear power stations themselves will have to be disposed of. 17 Nuclear fission An atom of uranium can be split by a neutron. This can produce two new nuclei plus the emission of neutrons and the release of energy. Chain reaction Once a nucleus has divided by fission, the neutrons that are emitted can strike other neighbouring nuclei and cause them to split releasing energy each time. This results in what is called a chain reaction as shown below. In a controlled chain reaction, on average only one neutron from each fission will strike another nucleus and cause it to divide. This is what happens in a nuclear power station. In an uncontrolled chain reaction all the neutrons from each fission strike other nuclei producing a large surge of energy. This occurs in atomic bombs. 18 The nuclear reactor There are five main parts of a reactor as shown in the diagram below: 19 1. The fuel rods are made of uranium which produces energy by fission. 2. The moderator, normally made of graphite slows down neutrons that are produced in fission, since a nucleus is split more easily by slow moving neutrons. 3. The control rods are made of boron, and absorb neutrons when lowered into the reactor, so that the reaction can be slowed down. In the event of an emergency they are pushed right into the core of the reactor and the chain reaction stops completely. 4. A cooling system is needed to cool the reactor and to transfer heat to the boilers in order to generate electricity. British gascooled reactors use carbon dioxide gas as a coolant. 5. The containment vessel is made of thick concrete which acts as a shield to absorb neutrons and other radiations. Radioactive waste Nuclear power stations produce radioactive waste materials, some of which have half-lives of hundreds of years. These waste products are first set in concrete and steel containers then buried deep under ground or dropped to the bottom of the sea. These types of disposals are very controversial. Some scientists believe the containers will keep the radioactive material safe for a long time, other scientists are worried that the containers will not remain intact for a sufficient time. 20 Fusion Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release of large quantities of energy. Fusion is the process that powers active stars, the hydrogen bomb and some experimental devices examining fusion power for electrical generation. Many scientists are working to produce controlled nuclear fusion reactions, but have not yet been successful. If they do succeed the fuel and end products, hydrogen and helium, will not be dangerously radioactive. Nuclear fusion reactions require very high temperatures, such as found in the core of the Sun. A few scientists have claimed to have produced cold fusion reactions where they fused hydrogen into helium at approximately room temperature. If they were possible, such reactions would immensely help the world's energy problems. However other scientists have been unable to duplicate the work and the cold fusion claims have been discredited. 21 Unit 1: Section 5 - Additional notes _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ 22 Unit 1: Section 5 - Additional notes _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ 23 Unit 1: Section 5 - Additional notes _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ 24 Unit 1: Section 5 - Additional notes _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ 25 Unit 1: Section 5 - Additional notes _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ _____________________________________________ 26