Unit 31 Radioactivity

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Unit 31

Radioactivity

That there are three forms of radiation: alpha, beta and gamma.

That radioactivity can be detected using different methods.

That alpha, beta and gamma radiation have different properties.

That their penetrating ability through materials depends on their properties.

How Rutherford used radioactivity to determine the atomic structure of elements.

That elements have isotopes, many of which are unstable emitting alpha or beta particles or gamma rays when they break apart.

How isotopes decay releasing either alpha or beta particles or gamma radiation.

That radioactive decay follows a half life.

Some of the uses of radioactivity.

Radiation, Beta, Alpha Gamma, Rays, Particles, Cloud, Chamber, Geiger, Muller, Tube, Penetrating, Rutherford, Atomic, Structure, Proton, Electron, Neutron, Isotopes, Unstable, Emission, Decay, Half life, Sources & Radioactivity. Unit 31: Radioactivity web: www.science-interactive.co.uk email: sales@science-interactive.co.uk Science Interactive LTD PO BOX 50764 LONDON NW6 9AT Click mouse to begin Science Interactive LTD Copyright 2005 Understand: Keywords:

Discovering radioactivity In the late 1800s, a French scientist called Becquerel discovered that some substances like Uranium 235 altered p__________ film, even when the film was wrapped in black paper and placed in complete darkness. Becquerel called substances that affected film in this way radioactive. He also discovered that Uranium gives out r_________ at the same rate when hot or cold. Detecting radiation in nuclear power stations or in hospitals is very important so that workers are protected. There are four main ways in which we can detect radioactive emissions. Detecting radioactivity: Science Interactive LTD Copyright 2005 Word bank: photographic radiation In a cloud chamber, a ray from a radioactive source causes a line of ions on which a thin vapour trail forms. This is shown as white lines in the picture. In an electroscope, the air surrounding the gold leaf is ionised by the radioactive source. The gold leaves becomes charged and moves away. The Geiger-Muller tube is sensitive to radiation. A counter shows us how many radioactive emissions are given out per second by radioactive sources. The photographic film shows how some of the film has been darkened by emissions from radioactive substances except in the shadow of the key. Notes Electroscope Cloud chamber Diagram Method Photographic film GM Tube

Sources of radioactivity Radioactive substance and radiation is all around us and is called background radiation . Every single second of the day, our bodies are exposed to low levels of r___________. Sources of radiation can include the food and n_________ in our diet, the rocks we walk over, the air that we breath, the building materials that surround us and from cosmic rays emitted by sources far out in space. In Cornwall, radioactive Radon gas can seep into houses from deep underground. In high risk areas, residents have a higher risk of developing cancer. What are the other causes of cancer in humans e.g. smoking ? Sources of radioactivity: Science Interactive LTD Copyright 2005 Word bank: radiation nutrients Cosmic rays are absorbed by our atmosphere, although it still accounts for about 10% of annual exposure. Radiation from food and drink account for about 15% of your annual exposure to background radiation. How does this enter the food chain ? Radiation from medical X rays and nuclear medicine accounts for about 15% of your annual exposure to background radiation. The air we breath adds about 60% to our annual radiation dose. Why can we not protect ourselves from this radiation ? Notes Diet Cosmic rays Diagram Air Radon gas

The Geiger Muller tube The Geiger-Muller tube is the most commonly used detector of radioactivity from gamma, alpha and beta emitters. Inside the GM tube is a thin wire and a gas at very low pressure. Radiation from the radioactive source ionises this gas producing a pulse of electrical c________. This is then amplified to give an audible clicking s_______, each time a ray passes through the tube. It is also counted by a rate-meter which shows the radioactivity in ‘counts per second.’ Inside the Geiger Muller tube: Meter Path of particle Ionised tube Science Interactive LTD Copyright 2005 Word bank: current sound The GM tube with all its components including the ionising tube, circuitry, rate-meter and loudspeaker. Radiation from a radioactive source causes a current pulse to flow from the wire which is surrounded by a gas at very low pressure. This is amplified and passed to a rate-meter. A rate-meter allows us to compare how strong various radioactive sources are. Notes Rate - meter Diagram GM tube Inside the GM tube

Types of radioactivity The nature of all three types of radioactivity differs greatly. Alpha particles, for example are positively charged, containing four nucleons ; two p_______ and two neutrons. Alpha radiation is easily stopped by paper. Gamma radiation is not affected by a magnetic field and is part of the electromagnetic spectrum. Gamma rays can even penetrate lead metal and thick concrete. Alpha, Beta and Gamma Radioactivity : Science Interactive LTD Copyright 2005 Word bank: protons Alpha Beta Gamma Stopped Stopped Weakened Paper 3mm Aluminium Thick lead Penetrating ability

Film

Cloud chamber

GM Tube

Film

Cloud chamber

GM Tube

Film

Cloud chamber

Gold leaf electroscope

Detectors No Yes Yes Electric fields 100% speed of light 50% speed of light 10% speed of light Speed Yes Yes Yes Dangerous Electromagnetic wave Negative electron Helium nucleus Nature Gamma Diagram Radiation Alpha Beta

Rutherford’s atomic structure Rutherford performed an experiment where he fired alpha particles at thin gold foil. Detectors were positioned at every angle to find how the a_______ particles were scattered or reflected by the gold atoms. It was discovered that some alpha particles were reflected back to the source – rather like firing a gun at tissue paper and finding some of the bullets bouncing back . He proposed that each atom had a tiny core or n_________ with a mass and a positive charge. This charge repelled the alpha particles. Rutherford's experiment: Most particles are undeflected Alpha particles Gold atoms Science Interactive LTD Copyright 2005 Word bank: alpha nucleus The atom’s ability to scatter alpha particles demonstrated the presence of a nucleus with a positive charge. The nucleus contains 99.99% of the atom’s mass. Before Rutherford’s experiment the nature of the atom was unknown. It was thought that the mass and charge was spread throughout the atom and there was no nucleus. Notes Diagram Rutherford’s experiment Rutherford’s experiment in detail

Atomic structure All atoms consist of three sub-atomic particles, neutrons, p________ and electrons. The simplest atom is hydrogen. Its nucleus contains a single proton with a single positive charge. There is a single electron in orbit of the positive nucleus. Because the charges are equal, but opposite, the atom is n________. It is the attraction between the electron and proton that holds the electron in it’s orbit. Helium has two protons and two uncharged particles called n_______ in its nucleus with two electrons held in orbit. Elements and sub atomic particles: He Li O 4 2 7 3 16 8 Science Interactive LTD Copyright 2005 Word bank: protons nucleus neutrons + + 2+ 3+ 8+ - - - - - - - - - - - - - - H 1 1 _____________ ? _____________ ? Two neutrons _____________ ? Neutrons _____________ ? _____________ ? Two electrons _____________ ? Electrons _____________ ? _____________ ? Two protons _____________ ? Proton Lithium Oxygen Diagram Hydrogen Helium

Understanding isotopes of atoms Some elements like carbon and hydrogen have naturally occurring isotopes. Atoms of isotopes like hydrogen are made up of atoms of different m_______. The number of protons in isotopes of the same element is always identical, but the number of n________ is different. Isotopes of the same element have identical chemical properties, because their e__________ are arranged in the same way. Some isotopes are unstable emitting either alpha, beta or gamma radiation. How could we separate different isotopes of the same element ? Isotopes of atoms: Pu 240 94 Isotope Symbol Radiation type Plutonium 240 Technetium 98 Uranium 238 Radium 228 Te 98 52 U 238 92 Ra 228 88 Alpha Gamma Alpha Beta Science Interactive LTD Copyright 2005 Word bank: masses neutrons electrons Hydrogen Deuterium Tritium 1 _____ ? 1 Electron configuration 1 _____ ? 1 Electrons 1 _____ ? 1 Protons 2 _____ ? 0 Neutrons Diagram Common radioactive isotopes Tritium Deuterium Hydrogen Isotope

Alpha decay An alpha particle is a helium nucleus with four nucleons: two protons & two neutrons . Pu 240 decays by Alpha emission . When it loses an alpha particle its mass number of 240 must decrease by four to 236 . Radium’s atomic number of 94 also decreases by two to become 92 . Radium has changed by losing an alpha particle to form the new element: Uranium 236 . Alpha decay: Science Interactive LTD Copyright 2005 Yes, if swallowed Dangerous 10% speed of light. Speed Blocked by paper. Penetration _______________? Affected by electric fields _______________? Nature Alpha radiation Property Plutonium 240 loses a helium nucleus, 2 neutrons and 2 protons emitting an alpha particle. Notes Diagram Alpha decay

Beta Decay A beta particle is an electron with almost zero mass and a negative charge. Radium 228 decays by Beta emission . This is an emission of an electron from the nucleus . One of the neutrons in the nucleus changes into a proton and an electron is emitted. This means that there is an additional p_______ in the nucleus increasing the atomic number by one to 89 . The mass number remains unchanged at 240 . Radium has changed by one of its neutrons changing to form a proton and by emitting an electron to form the new element: Actinium 228 . Beta decay: Science Interactive LTD Copyright 2005 Word bank: proton Yes Dangerous 50% speed of light. Speed Stopped by 3mm aluminium Penetration _______________ ? Affected by electric fields _______________ ? Nature Beta radiation Property Radium 228 decays by losing an electron by a neutron changing into a proton. Notes Diagram Beta decay

Gamma emission Gamma rays are waves - they are part of the electromagnetic spectrum like light and radio waves. They have a very short wavelength - smaller than an atom. They can pass through thick sheets of lead. They are very difficult to shield. When a radioactive isotope loses an alpha or beta particle it is left in an excited state . It loses this surplus energy by emitting a gamma ray. Gamma emission: Science Interactive LTD Copyright 2005 Yes Dangerous 100% speed of light. Speed Reduced but not stopped by lead or concrete. Penetration No, uncharged Affected by electric fields _______________? Nature Gamma radiation Property Plutonium 240 * is an isotope in an excited state that emits excess energy by emitting gamma rays. Notes Diagram Gamma emission

Uses of alpha radiation At home, in the kitchen or landing you might have a smoke detector. Inside the smoke detector is a very weak radioactive s_______ containing Americum 241 which is an alpha emitter. Like all alpha radiation it causes the ionisation of air inside a detection chamber which in turn generates a small current flow. If smoke particles from a fire enters the air space between the radioactive source and the chamber it causes a reduction in the current. This then triggers an a_______. Why is a gamma source not used in smoke detectors ? Smoke detectors and alpha radiation: Scan of a human thyroid following a radioactive dose of an alpha emitter (Iodine 131 ) Science Interactive LTD Copyright 2005 Word bank: source alarm If you have thyroid cancer, radioactive iodine is injected into the blood and is absorbed by the thyroid. The alpha radiation then destroys the cancer causing cells. How does alpha radiation only destroy local cells in the thyroid ? Smoke caused by fires alters the current flow inside a smoke detector, therefore triggering an alarm. This can provide an essential early warning system for those who live in the house, therefore saving many lives. Notes Diagram Smoke detector Treating thyroid cancer

Uses of Beta radiation During the manufacture of aluminium foil and sheet p_____, the thickness of the material can be controlled by using beta radiation . The amount of beta-radiation passing through the thin material is monitored by a Geiger-muller tube. The GM tube is connected to computer circuitry, which controls the amount of p________ exerted by rollers before and after. With thicker materials what kind of source should be used ? (Alpha, Beta or Gamma) Thickness control: Science Interactive LTD Copyright 2005 Word bank: paper pressure radiation Foil or paper is of desired thickness. 4 The rollers responds and reduce or increase the thickness of the foil or paper. 3 Changes in paper or foil thickness triggers a change in the amount of r________ detected by th GM tube. 2 Beta particles penetrate and are detected by the Geiger-Muller tube. 1 Notes Step Radioactive source ( beta ) GM tube Machinery to control rollers Rollers Paper Diagram Thickness control of aluminium foil or paper

Use of Gamma radiation one Gamma radiation is the most useful type of radiation and is used in many applications. It is also the most dangerous form of radiation to work with, because of its ability to penetrate materials including thick l_____ and concrete. The normal source for gamma radiation is Uranium 235 a radioactive i_______. How would you monitor the radiation received by workers using gamma radiation ? Uses of gamma radiation : Science Interactive LTD Copyright 2005 Word bank: lead isotope Fatigue in metals and their welds can be imaged using gamma radiation and photographic film. Explain how the image is formed and why is light, beta or alpha radiation not used ? The quality of metal welds can be constantly checked using gamma radiation. Film is used to image the weld. If the weld is poor, what will the film show along the weld line ? In some cancers like breast cancer, cancerous cell are irradiated using a gamma source. The cells are killed by the destructive nature of the gamma radiation. List the possible side effects ? Notes Checking cracks Diagram Use Radiotherapy Checking welds

Use of Gamma radiation two Gamma rays from excited radioactive isotopes can be used to destroy m_______ including bacteria, present on the surfaces of foods like fruits and medical equipment. Food can be packaged and then irradiated, prolonging the shelf life of the food. This is completely safe as gamma rays cannot be absorbed into the fruit, but it can alter the taste of the food. Gamma rays are used by hospitals to s_________ heat sensitive products like blood and plastics. Why is the irradiation of fruit like strawberries currently not popular with the public ? Gamma rays and sterilisation: Science Interactive LTD Copyright 2005 Word bank: microbes sterilise Plastics and surgical tools are sterilised before use during operations. They cannot be steam sterilised because they are heat sensitive. Untreated fruit have a short shelf life because their surface is home to many surface microbes particularly bacteria which spoil the fruit. Fruit like strawberries are irradiated using gamma rays. This gives them a longer shelf life, lasting up to seven days on supermarket shelves. Human blood products are irradiated using gamma rays before transfusion into another patient. This destroys any viruses. Notes Non-irradiated fruit Medical equipment Diagram Blood Irradiated fruit

Use of Gamma radiation three In some cancers like breast cancer, cancerous cell are irradiated using a gamma source. The cells are killed by the destructive nature of the g_______ radiation. Furthermore, where surgery is impossible, radiotherapy can be used, although the success rate is not high. Killing all the cancerous c_____ whilst leaving healthy ones intact is very difficult, even for the most skilled surgeons. The survival rate for brain cancer remains low despite using radiotherapy . Explain why cancers are difficult to treat ? Irradiation using gamma rays: Science Interactive LTD Copyright 2005 Word bank: gamma cells Where surgery is impossible for example many brain tumours cannot be removed because of the surrounding tissue, radiotherapy is the only option. The pictures above show how using gamma rays, the cancer shrinks and then completely disappears. What are the side effects of radiotherapy ? Here the brain cancer is located using 3D imaging techniques like MRI, CAT scans or more traditional X-rays. Notes Diagram Locating the cancer Using radiotherapy to kill tumours

Radioactive Half-lives one Unstable isotopes like Uranium 235 and Plutonium 240 decay by emitting either alpha, beta or gamma radiation, forming new e__________ over time. The time taken for half the atoms to decay is called the half life of the substance. Half-lives of different isotopes vary, from fractions of a single second to many millions of years. Plutonium 240 has a relatively short half life of 6500 years. Uranium 235 has a relatively long half life of approximately 700 million years. Half-lives of radioactive isotopes: 1 st half life 2 nd half life 3 rd half life Science Interactive LTD Copyright 2005 Word bank: elements Some radioisotopes have relatively short half lives. Sodium 24 has a half life of around fifteen hours Determine from the graph ? After a second half life, the activity has halved again, so that only 1/4 of the atoms survive. After 3 half lives, the number of radioactive atoms has halved again and only 1/8 th survive. What fraction survives after 4 & 5 half lives ? All atoms present are of the original isotope e.g PU 240 . By the end of the first half life only 50% of all atoms are the original isotope. By the end of the second half life only 25% of the atoms are the original isotope. Notes Sodium 24 decay pattern Sodium 24 half lives Diagram Half lives

Inside a nuclear power station Electrical energy can be produced using nuclear energy. Increasingly, nuclear energy is used as an alternative to the combustion of fossil fuels including c______ oil and natural gas. This is partly because they are a finite resource and currently contribute to global warming because of c___________ dioxide emissions. A nuclear reactor uses Uraniun 235 as a fuel source. These Uranium rods glow white hot, in turn heating water, producing s_______, which drives a turbine and generates e__________. Name three renewable ways of generating electricity ? Nuclear energy: Science Interactive LTD Copyright 2005 Word bank: crude carbon steam electricity One problem is the radioactive waste and its safe storage. It has to be stored for up to 10,000 years safely away from humans. About 8% of our electrical energy is produced using nuclear fuels. In future years this will rise as fossil fuels decrease. Uranium 235 rods glow white hot and heat water turning it into steam. This drive the turbine generators producing electricity. A nuclear power station with a characteristic dome housing the radioactive core. They are very expensive to build and decommission. Notes Electricity Radioactive waste Diagram Power station Radioactive core

Generating electricity using nuclear power Using fossil fuels to produce electricity adds to acid rain and global warming. They are also non – renewable f_______ making it necessary to find alternative energy sources. A nuclear power station uses radioactive substances like Uranium as a fuel. As Uranium decays, huge amounts of energy is released which is then used to heat steam and drive t________ generators. Nuclear fuels are a highly concentrated energy source, with large nuclear power stations only requiring about 10 kg of nuclear fuel per day. The only problem is that radioactive waste materials need to be safely stored for many thousands of years. Generating and supplying electricity: Science Interactive LTD Copyright 2005 Word bank: fuels turbine Steam produced from the energy given out during the radioactive decay of Uranium 235 drives the turbine and generator. A current is induced in the cables. Using nuclear fuels, although expensive does not produce any greenhouse gases like carbon dioxide or acid rain producing gases like sulphur dioxide. Radioactive waste and high decommissioning costs still limit their use here. Notes Diagram

Nuclear energy Generating electricity using nuclear energy raises a number of issues. Furthermore using isotopes like Uranium 235 for nuclear w_______ and dealing with radioactive waste enjoys little support in Britain. There are many reasons ‘for and against’ using nuclear energy and Uranium 235 . The biggest argument for, includes reducing our use of f_______ fuels and that only a small percentage of background radiation comes from the nuclear power industry. List three reasons against using nuclear fuels for generating electricity ? Nuclear energy: Science Interactive LTD Copyright 2005 Word bank: weapons fossil Three-mile island in the US was a nuclear power station. A coolant failure led to a ‘meltdown’ and an explosion in the reactor. The radioactive cloud affected a huge area. Receiving high doses from gamma radiation can lead to death and radiation sickness. After the initial death toll in Japan up to 500,000 people died within 10 years. In Japan, during WWII, America dropped two ‘Atom bombs’ on Japan. The photograph shows Nagasaki in complete ruins just hours after the bomb dropped. During testing of nuclear weapons and warheads, US soldiers were asked to ‘witness’ the bombs close to the testing site. Many have since died prematurely from various cancers. Notes Radiation sickness Three-mile island Diagram Nuclear weaponry Nagasaki

Radioactive dating one As time moves forward, the amount of radioactive Uranium 235 has gradually decreased in the Earth’s rocks, changing slowly into stable lead . Because we know the h_____ life of Uranium 235 ( 700 million years ), by measuring the ratio of Uranium 235 and lead in a rock sample, we can determine its a_____. Geologists use these methods to age rock samples from Earth, as well as meteorites from other planets including Mars. Using this method, scientists have dated the age of Earth and Mars at about 4.5 billion years. Radioactive dating: Uranium 235 Lead 206 700 million years alpha beta gamma Science Interactive LTD Copyright 2005 Word bank: half age The age of the Earth is around 4.5 billion years. We know this because of radio dating of rocks. Created at the same times as Mars and probably the other seven planets. Uranium 235 transmutes or changes through 14 different isotopes finally producing stable lead, with a half life of 700 million years. Decay sequence: Uranium 236 Thorium 234 Protactinium 234 Uranium 234 Thorium 230 Radium 226 Radon 222 Polonium 218 Lead 214 Bismuth 210 Polonium 210 Lead 206 . Notes Diagram Uranium 235 decay to lead 206 Earth’s age

All living organism contain carbon. Most of the c________ in your body and tissues is stable carbon 12 . Another form or isotope of carbon is carbon 14 with a half life of 5700 years. While the amount of carbon 14 in the living human body remains constant, after your death, the amount of carbon 1 4 decreases by half every 5700 years. Using this method we can find the time of d_______ for organisms up to 60,000 years ago. Why is the carbon 14 method not suitable for dating rocks ? Radiocarbon dating: Radioactive dating two Science Interactive LTD Copyright 2005 Word bank: carbon death - - - - - - Carbon 14 Other organic materials like cloth and remains of dead organisms can be dated using this method. Using the Carbon- 14 method, the age of the ‘Shroud of Turin’ a cloth believed to have wrapped the body of Jesus around 33 AD was proved to be a fake, dated at only 600 years old. Carbon 14 is an unstable form of carbon with a half life of 5700 years. It is present in the atmosphere (in CO 2 ) Notes Dating cloth Diagram Carbon 14 The Shroud of Turin

Radioactive dating three All living organisms contain carbon. Very small amount of the carbon found in plant and animal tissues is the unstable isotope, carbon 14 . The levels of carbon 14 remain constant during an organism’s life. When an o_________ dies, the unstable carbon 14 begins to decrease with half the unstable atoms remaining after 5700 years. Comparing the ratio of carbon 14 to carbon 12 can give scientists an accurate age of a fossil. Another advantage is that you only require a very small sample for t_________. Why is this important when you are dating valuable artefacts like the ‘Shroud of Turin.’ Carbon dating: Carbon 14 Carbon 12 Science Interactive LTD Copyright 2005 Word bank: organism testing The amount of carbon 14 begins to reduce from death onwards. Comparing the ratio of unstable carbon 14 to stable carbon 12 gives us an accurate age of fossil or material. Notes: Carbon 14 : Carbon 12 Diagram Key Fossil Living organism

1: Define the following terms: Radiation, Beta, Alpha, Gamma, Proton, Electron, Neutron, Isotopes, Unstable, Emission, Decay, Half-life, Radioactivity . 2: Look at the three pictures. Complete the table. 3: Answer the following: a) Most of the carbon in your body and tissue is carbon 12 (C 12 ) Draw a dot and cross of this atom. b) Another form or isotope of carbon is carbon 14 (C 14 ) with a half life of 5700 years. What fraction of carbon 14 is left after 2825 years. c) While the amount of carbon 14 in the human body when you are alive remains constant, after your death, the carbon 14 decays with a half life of 5700 years. Imagine your bones gives a reading of 70 mSV -1 using a Geiger-muller tube and an fossilised bone gives a reading of 20 mSV -1 . What is the age of the old bone. Extension questions and homework Science Interactive LTD Copyright 2005 Type of radioactivity used Smoke alarm Three Sterilisation Two Radiotherapy One Explanation Application Picture

4: Look at the information in the table below. It shows the decay of radium over time. a) Complete the table for mass of radium. b) Plot time (x) against mass left on graph paper. c) Work out when you would have 30 g and 10 g of radioactive radium in the lab. d) Draw a decay scheme for radium. e) Mark each half life on the x axis. 5: a) Give two different uses of alpha, beta and gamma radiation. b) Strontium 90 has a half life of 28 years. It is a beta emitter and may be absorbed by human bone tissue. How much time would pass before the activity falls to 1/32 nd of its original value. c) Explain why workers at a nuclear factory wear badges containing unexposed photographic film that is checked every week. 6: Complete the following table: Internet: Go to google.co.uk and find out about carbon dating and the Shroud of Turin. Explain how scientists disproved the theory that this was the piece of cloth that wrapped Jesus' body after his death. Science Interactive LTD Copyright 2005 12.5 4400 ? 5200 ? 6000 ? 3600 ? 6800 ? ? 50 100 Mass (g) 8400 7600 2800 2000 Time (year) Gamma ray Beta particle Alpha particle Sources Charge Nature Item