Radioactivity. Ernest Rutherford, A New Zealand physicist proved in the early 1900s a new model of the atom.

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1 Radioactivity In 1896 Henri Becquerel on developing some photographic plates he found that the uranium emitted radiation. Becquerel had discovered radioactivity. Models of the Atom Ernest Rutherford, A New Zealand physicist proved in the early 1900s a new model of the atom. He discovered amazing facts about the nucleus: The nucleus is very small; Most of the atom is empty space; The repulsion of the positively charged alpha particle showed that the nucleus is positively charged. This discovery led to the idea of the nuclear atom. This was developed further by Neils Bohr, a Danish physicist. The neutron was discovered twenty years later by an English physicist, Chadwick. Since the nucleus is so small, the size of an atom is governed by the size of the electron shells. Therefore big atoms and small atoms are all roughly the same size, about m in diameter. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 1

2 The Atom The Basic Atom All matter is made up of atoms. The basic atom consists of a nucleus surrounded by electrons going round the nucleus in orbit. Electrons are negatively charged. Here is a Lithium atom: The nucleus consists of: Protons which are positively charged. Neutrons that have no charge. The protons and neutrons have very nearly the same relative mass. The mass of a proton or neutron in kilograms is about kg. The mass of an electron is about 1/1800 the mass of a proton. The mass of an electron is about kg. Particle Charge Proton + 1 Neutron 0 Electron - 1 The symbol e is often called the electronic charge. Its value is C. The protons and neutrons are the nucleons. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 2

3 Atoms and Ions Elements are often written like this: A is the total number of nucleons. This is called the mass number or the nucleon number. Z is the total number of protons. This is called the atomic number or the proton number. The number of protons determines the element. If we change the number of protons in the nucleus from 6 to 7, we change the element from carbon to nitrogen. This will change the chemistry radically. To work out the number of neutrons we take away the number of protons from the number of nucleons: No of neutrons = mass number - atomic number If the number of electrons is the same as the number of protons, the atom carries zero overall charge. It is described as neutral. The nucleus is very tiny, about 1/ the size of an atom. If we change the number of electrons, the atom is charged. It becomes an ion: Remove an electron, the overall charge is positive. We have a positive ion. Add an electron, we have a negative ion. Ions are NEVER made by adding or taking away protons. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 3

4 Isotopes Isotopes have the same number of protons, but different numbers of neutrons. If we change the number of protons, we change the element completely. Isotopes have the same chemical properties as the normal element. Examples of isotopes: (e.g. helium-3, carbon-12, iodine-131 and uranium-238). For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. Hydrogen Deuterium Tritium Ordinary hydrogen is written 1 H1, deuterium is 2 H1, and tritium is 3 H1. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable. Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 4

5 Radioactivity Some isotopes of atoms can be unstable. They may have: a) Too much energy or b) The wrong number of particles in the nucleus. We call these radioisotopes. To make themselves more stable, they throw out particles and/or energy from the nucleus. We call this process radioactive decay. The atom is also said to disintegrate. The atom left behind (the daughter) is different from the original atom (the parent). It is an atom of a new element. For example uranium breaks down to radon which in turn breaks down into other elements. The particles and energy given out are what we call radiation or radioactive emissions. Three types exist : Alpha decay; Beta decay ; Gamma radiation. Alpha and beta decays result in the emission of a particle. Gamma radiation is an electromagnetic wave of very short wavelength. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 5

6 Properties of Radiation The table shows some properties: Radiation Description Penetratio n Alpha ( ) Beta ( ) Helium nucleus 2p + 2n Q = + 2 e High speed electron Q = -1 e Few cm air Thin paper Few mm of aluminium Ionising Power Intensely ionising Less than alpha Effect of Electric or Magnetic field Deflection as a positive charge Deflection in opposite direction to alpha. Gamma ( ) Very short wavelength em radiation Several cm lead, couple of m of concrete Weakly ionising No effect. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 6

7 Alpha particle This consists of a helium nucleus. If we send alpha Alpha Particle particles through the poles of a magnet (a magnetic field), we find that they are deflected. This means that they are charged. If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the negatively charged plate. This means they are positively charged. Alpha particles are stopped by a few cm of air. This means that an alpha source can be used safely with minimal shielding. Your skin will stop alpha particles. Alpha particles are intensely ionising. Being quite big and moving fast, they collide frequently with other atoms, knocking off electrons, causing ionisation. They rapidly lose their energy. Eventually they stop and then pick up two stray electrons to become helium atoms. All the Earth's helium atoms are thought to come from alpha decay. Beta particle This consists of a fast Beta Particle moving electron. If we send a beta particles through the poles of a magnet (a magnetic field), we find that they are deflected in the opposite direction to alpha particles. This means that they are charged. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 7

8 If we pass them between a positively charged plate and a negatively charged plate (an electric field), we find that they are attracted to the positively charged plate. This means they are negatively charged. Gamma Radiation Gamma rays are very short wavelength and highly energetic electromagnetic radiation. They are given off by very energetic or excited nuclei when some other decay has occurred. Cobalt-60 is a common source of gamma rays. Gamma radiation does not in itself alter the nucleon and proton numbers. Gamma rays are not affected by electric or magnetic fields. Because alpha particles carry more electric charge, are more massive, and move slowly compared to beta and gamma particles, they interact much more easily with matter. Beta particles are much less massive and move faster, but are still electrically charged. A sheet of aluminum one millimeter thick or several meters of air will stop these electrons and positrons. Because gamma rays carry no electric charge, they can penetrate large distances through materials before interacting several centimeters of lead or a meter of concrete is needed to stop most gamma rays. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 8

9 Measuring Radiation In the old days, radiation was detected by exposing a sheet of photographic film to the radioactive source. Each decay caused the deposit of a grain of silver, and it was possible measure the density of the deposits when the film was developed. This method is still used today with film badges that people wear if they are working with radioactive materials. To get a real-time measurement, we measure the radiation from a radioactive sample using a radiation detector called a Geiger-Müller tube. This is connected to a counter. The radioactive decay is measured by the number of counts per second. When we take readings it is important that we measure the background count. There is radioactivity all around us; it's a natural part of the environment. So we find out what the background count is, then we take that away from the count we get with the source. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 9

10 cloud chamber, device used to detect elementary particles and other ionizing radiation. A cloud chamber consists essentially of a closed container filled with a supersaturated vapor, e.g., water in air. When ionizing radiation passes through the vapor, it leaves a trail of charged particles (ions) that serve as condensation centers for the vapor, which condenses around them. ALPHA PARTICLES PRODUCE STRAIGHT LONG LINES BETA PARTICLES PRODUCE STRAIGHT WEAK LINES GAMMA RAYS LOOK LIKE TINY CURLY STRANDS OF HAIR Half-Life Radioactive decay is a random process. If you look at a nucleus, it might decay within ten seconds, or twenty two million years. Since there are many billions of nuclei, a random decay pattern is seen. What is half-life? Radioactive substances will give out radiation all the time, regardless of what happens to them physically or chemically. As they decay the atoms change to daughter atoms, until eventually there won t be any of the original atoms left. Different substances decay at different rates and so will last for different lengths of time. We use the half-life of a substance to tell us which substances decay the quickest. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 10

11 Half-life is the time it takes for half of the radioactive particles to decay. It is also the time it takes for the count-rate of a substance to reduce to half of the original value. We cannot predict exactly which atom will decay at a certain time but we can estimate, using the half-life, how many will decay over a period of time. The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph. This would also work if you plotted the number of parent atoms against time. The longer the half-life of a substance the slower the substance will decay and the less radiation it will emit in a certain length of time. Each radioactive isotope decays in its own way and has its own half-life which is defined as: the time taken for half the original number of atoms to decay. This is shown on the graph: Form 5 Unit 3 Theme 7: Radiation and its Uses Page 11

12 If it takes 4 days for half the atoms to decay: after 4 days, 1/2 are left over; after 8 days, 1/4 are left over; after 12 days, 1/8 are left over. This is called exponential decay.. Some half lives are extremely short, much less than 1 second. Some are very long, about 4500 million years. Using radioactivity Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are the main uses you will be expected to know about: 1. Uses in medicine to kill cancer radiation damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body. Sources of radiation that are put in the body need to have a high count-rate and a short half life so that they are effective, but only stay in the body for a short period of time. If the radiation source is outside of the body it must be able to penetrate to the required depth in the body. (Alpha radiation can t travel through the skin remember!) Form 5 Unit 3 Theme 7: Radiation and its Uses Page 12

13 2. Uses in industry one of the main uses for radioactivity in industry is to detect the thickness of materials. The thicker a material is the less the amount of radiation that will be able to pass. Alpha particles would not be able to go through metal at all, gamma waves would go straight through regardless of the thickness. Beta particles should be used, as any change in thickness would change the amount of particles that could go through the metal. They can even use this idea to detect when toothpaste tubes are full of toothpaste! 3. Photographic radiation detectors these make use of the fact that radiation can change the colour of photographic film. The more radiation that is absorbed by the film the darker the colour it will go when it is developed. This is useful for people working with radiation, they wear radiation badges to show them how much radiation they are being exposed to. 4. Dating materials The older a radioactive substance is the less radiation it will release. This can be used to find out how old things are. The half-life of the radioactive substance can be used to find the age of an object containing that substance. There are three main examples of this: i) Carbon dating many natural substances contain two isotopes of Carbon. Carbon-12 is stable and doesn t disintegrate. Carbon-14 is radioactive. Over time Carbon- 14 will slowly decay. As the half-life is very long for Carbon-14, objects that are thousands of years old can be compared to new substances and the change in the amount of Carbon-14 can date the object. ii)uranium decays by a series of disintegrations that eventually produces a stable isotope of lead. Types of rock Form 5 Unit 3 Theme 7: Radiation and its Uses Page 13

14 (igneous) contain this type of uranium so can be dated, by comparing the amount of uranium and lead in the rock sample. iii) Igneous rocks also contain potassium-40, which decays to a stable form of Argon. Argon is a gas but if it can t escape from the rock then the amount of trapped argon can be used to date the rock. 5. Smoke Detectors and Americium Agricultural Applications - radioactive tracers Radioisotopes can be used to help understand chemical and biological processes in plants. 7. Food Irradiation Food irradiation is a method of treating food in order to make it safer to eat and have a longer shelf life. Uses and Hazard of Radiation Radiation Use Hazard Alpha ( ) Used in smoke detectors If taken in to the body (ingested), alpha emitters can do immense damage to living tissues Beta ( ) Gamma ( ) Checking the thickness of paper sheet in manufacture. Radioactive tracers in medical research and diagnosis Medical research. Non-destructive testing of castings. Some risk of tissue damage, although nowhere near as dangerous as alpha. Can cause genetic damage and cancer. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 14

15 Background radiation There is a certain amount of radiation around us (and even inside us) all the time. There always has been since the beginning of the Earth. It is called Background radiation. Background radiation comes from a huge number of sources. Cosmic radiation Radiation from rocks Radioactive waste In most areas, Background radiation is safe. It is at such a low level that it doesn t harm you. You need to be exposed to many times the normal background level before you notice any symptoms. Dangers of handling radioactive substances Each type of radiation that can be emitted can be absorbed by different materials and ionises different amounts. They are equally dangerous but for different reasons. Alpha particles: Although alpha particles cannot penetrate the skin, if it gets into the body it can ionise many atoms in a short distance. This makes it potentially extremely dangerous. A radioactive substance that emits just alpha particles can therefore be handled with rubber gloves, but it must not be inhaled, eaten, or allowed near open cuts or the eyes. Beta particles: Beta particles are much more penetrating and can travel easily through skin. Sources that emit beta particles must be held with long handled tongs and pointed away from the body. Inside of the body beta particles do not ionise Form 5 Unit 3 Theme 7: Radiation and its Uses Page 15

16 as much as alpha particles but it is much harder to prevent them entering the body. Gamma waves: These waves are very penetrating and it is almost impossible to absorb them completely. Sources of gamma waves must also be held with long handled tongs and pointed away from the body. Lead lined clothing can reduce the amount of waves reaching the body. Gamma waves are the least ionising of the three types of radiation but it is extremely difficult to prevent them entering the body. Units of Radioactivity The number of decays per second, or activity, from a sample of radioactive nuclei is measured in becquerel (Bq), after Henri Becquerel. One decay per second equals one becquerel. Nuclear energy gives off far more heat energy than chemical reactions. Reactors in nuclear power station do the same job as the boiler; they boil water to steam. They also can be used to make radioactive isotopes for medical purposes. Form 5 Unit 3 Theme 7: Radiation and its Uses Page 16