Radioactivity & Particles

Transcription

1 Radioactivity & Particles Introduction... 2 Atomic structure... 2 How are these particles arranged?... 2 Atomic notation... 4 Isotopes... 4 What is radioactivity?... 5 Types of Radiation: alpha, beta and gamma... 5 Ionisation... 6 Nuclear equations... 6 Activity... 9 Detecting radiation... 9 Background radiation... 9 Half-Life Using radioactivity Why is radiation harmful? Dangers of handling radioactive substances Disposal of Radioactive Waste Nuclear Power... Error! Bookmark not defined. Nuclear fission... Error! Bookmark not defined. Particles Fission... 18

2 Introduction Atomic structure You need to know about the structure of an atom and the particles that make it up. All atoms are made up of the same 3 basic particles: - protons - electrons - neutrons. The only difference between one atom and the next is the number of these particles in the atom. That is enough to make things as different as gold and oxygen. Neutrons and protons are heavy in comparison to electrons. In fact, a neutron or a proton weighs about 2000 times as much as an electron! The other thing to remember is that protons have a positive charge, electrons have a negative charge and neutrons have no charge at all. How are these particles arranged? An atom is not a solid thing. In fact, quite the opposite. Atoms are nearly completely empty. Protons and neutrons are tightly clumped together in the middle, in the nucleus, while electrons spin around them. This diagram gives you an idea of what they look like, but it is not to scale: 2

3 To give you an idea of the proportions, imagine a full size football stadium. The nucleus would be equivalent to the size of an ant in the middle, with the electrons whizzing around the outskirts. The central part of the atom is called the nucleus. That s where you find all the protons and neutrons. As we said above, protons and neutrons are heavy compared to electrons, so you can see that all the mass is concentrated in the middle of the atom. Also, as all the protons are in the nucleus of the atom, the nucleus has a positive charge. The electrons (negatively charged) orbit around the outside of the atom. So what have we learnt so far? An atom is made up of mostly empty space. Protons have a positive charge and a lot of mass. Neutrons are neutral but are as heavy as protons. Electrons are negative and are only about of the mass of the others. 3

4 Atomic notation To help us to describe atoms and nuclei, we use numbers and letters. Here is an example: The atom in the diagram is described by the numbers and letters shown next to it. All atoms of a certain element will have the same number of protons in the nucleus. The top number is called the mass number or the nucleon number. It tells you how many particles are in the nucleus, i.e. how many protons and neutrons. The bottom number is called the proton number or the atomic number. It tells you how many protons there are in the nucleus. The letters give you a clue as to the name of the atom. This is an atom of 'helium', He. The number of electrons in an atom is the same as the number of protons. That makes the atom neutral overall (neither negative nor positive). If the numbers are not equal, the atom becomes a charged particle. We call these charged particles ions. Isotopes The number of protons is the thing that decides how an atom is going to behave and therefore what element the atom belongs to. If you change the number of protons in an atom, you change the type of atom (it becomes an atom of another element.) However, you can change the number of neutrons in an atom without changing the type of atom. An isotope if an atom with the same number of protons (i.e. same element!), but a different number of neutrons. Examples: Oxygen usually has 16 nucleons (protons + neutrons). An isotope of oxygen-16 could be oxygen- 17 or oxygen-18, so it would have one or two more neutrons. 4

5 What is radioactivity? Some isotopes of atoms can be unstable. They may have: a) Too much energy or b) Too many or too few neutrons 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 nucleus) 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. Types of Radiation: alpha, beta and gamma There are three main types of radiation that can be emitted by radioactive particles. They are called alpha, beta and gamma. The Table below explains why different types of radiation are absorbed by different things: Alpha Beta These are large particles consisting of two protons and two neutrons, the same as the Helium nucleus. They have a positive charge and can ionise atoms easily so quickly lose their energy by ionising nearby atoms. This means they can be absorbed/stopped by just a few centimetres of air, a sheet of paper or by skin. These are high energy and high speed electrons. They can ionise fairly easily, although not as much as alpha. They can travel through thin materials before they are absorbed. Gamma This is a wave that carries a huge amount of energy, but waves are not as good at ionising atoms as particles are. It is therefore really difficult to absorb them. They can never be fully stopped, but several cm of lead of thick concrete will absorb some of the radiation. 5

6 The following diagram shows how the penetration ability differs between the different types of radiation. Ionisation The radiation emitted by radioactive substances has a huge amount of energy, which is why it is so dangerous. The energetic radiation causes ionisation; as this term has been used a few times in the last paragraph it needs some explanation. When radiation hits a neutral atom, some of the energy from the radiation is passed to the atom. This energy can cause an electron from the atom to escape, leaving the atom with a positive charge. This positively charged atom is called an ion, so the process is called ionisation. As the radiation travels along it ionises atoms that are close enough. The more atoms the radiation ionises the more energy the radiation gives away, until eventually there is no energy left. The radiation is then said to have been absorbed. Nuclear equations You can write equations for nuclear reactions. These equations show which radioisotope you have initially, which element it decays to and what type of radiation is emitted. In these equations you will need to make sure that: 6

7 1) The total number of protons is the same before and after the reaction. 2) The total number of nucleons is the same before and after the reaction. In this reaction, carbon and helium is combined to form oxygen. Notice that if you add up the numbers at the top of each atom (the nucleon numbers) on the left hand side you get the same as the total for the numbers on the right hand side. The same is true for the proton numbers. When atoms disintegrate by radioactive decay, new daughter atoms are produced. We can work out which elements will be produced using decay equations. These are like the equations you may have used for chemical reactions. Each type of radiation has a chemical symbol that is used in the equation. Note: the equations must always balance, so there are the same number of protons and neutrons on each side of the equation. In general: Alpha decay: the mass number decreases by 4, the atomic number decreases by 2. Beta decay: the mass number remains the same, the atomic number increases by 1. Gamma emission: neither the mass number nor the atomic number are changed. The alpha particle is often noted as the Helium (He nucleus). The beta particle is often written as e for electron. 7

8 Note: the atomic number of the β particle is -1. This is to balance the right and the left side of the arrow, i.e. showing that a proton was gained. Examples: Complete the following exercises using the periodic table of elements in your diary to find the new element produced. 8

9 Activity The activity of a radioactive substance is defined as the number of decays per unit time. It is measured in Becquerel (Bq) named after Henri Becquerel who won the Nobel Prize of Physics for the discovery of radioactivity together with Marie Curie and her husband Pierre. One decay per second is one Becquerel. In school experiments the count rate is measured, which is only the amount of radiation detected in one direction, not the whole activity. Detecting radiation It is hard to detect the actual particles or waves emitted by radioactive substances, but it is easy to detect the positive and negative ions produced by the ionisation they cause. In a device called the Geiger-Muller tube the radiation ionizes the gas inside. This leads to a small burst of current between the two electrodes and a count is recorded. The more radiation there is the higher the count rate. Another way of detecting radiation is photographic film. When exposed to radiation the film will become dark when developed. 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. The following diagram shows the different sources of background radiation. 9

10 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. The following diagram shows how background radiation differs in the various areas in Great Britain. However, some areas of the country have a higher level of background radiation than others because the rocks near the surface contain more radioactive isotopes (for example, Cornwall). Look at this example: You use a radiation detector to record that a sample of rock produces 100 decays per minute. You then remove the rock and record the background radiation in the room. It is 7 decays per minute. Note: when measuring radiation it is always necessary to subtract the background radiation from the results taken. This way one can obtain the count rate due to the activity of the substance alone. 10

11 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. Half-life is the time it takes for half of the unstable nuclei to decay. It is also the time it takes for the activity of a substance to reduce to half of the original value. The counter-intuitive fact about radioactivity is that the radiation decreases by 50% within a half-life. After two half-lives not all of the unstable nuclei have decayed, but first one half (in the first half-life) and then half of that half again (in the second half-life). So what remains of the unstable nuclei is ¼ of the original amount. After another half-life it is 1/8 and so on. While we cannot predict exactly which nucleus will decay at a certain time, we can estimate, using the half-life, how many nuclei 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. 11

12 This would also work if you plotted the number of parent atoms ( undecayed nuclei ) 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. Examples: 1.) The activity of a substance is 100Bq. It s half-life is 60s. What will the activity be after 4min? Answer: 4min = 320 s = 4 half-lives After 1 half-life: 100Bq 50 Bq After 2 half-lives: 50 Bq 25 Bq After 3 half-lives: Bq After 4 half-lives: 12.5 Bq 6.75 Bq 2.) How long does it take for the number of undecayed nuclei to decrease to 1/32 of the original number? One half-life is 10min long. Answer: Find the number of half-lives first: 1 ½ ¼ 1/8 1/16 1/32 You see that 5 half-lives have passed (each arrow represents one half-life). Time = 5 half-lives x 10 min = 50min 50 min have passed. 12

13 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!) 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 penetrate it. The image shows how this works in the thickness control of paper production. Alpha particles would not be able to go through the paper 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 paper. This principle can also be used for the production of aluminium foil or steel sheets. Gamma has to be used for steel plates as beta would not penetrate it. They can even use this idea to detect when toothpaste tubes are full of toothpaste! 13

14 3. Sterilising equipment or food strawberries last longer when the bacteria on them gets killed by radiation. As gamma penetrates the food completely, it kills the bacteria both on top, inside and underneath. The same principle is applied for sterilising surgical equipment. Again gamma radiation penetrates the equipment and kills bacteria on top and underneath it. 4. 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. 5. 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 (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. 14

15 6. Smoke detectors In smoke detectors there is an alpha source on one side of a chamber and a detector on the other side. When smoke particles enter the chamber they absorb some of the alpha particles and less reach the detector. This change triggers the alarm. 7. Finding leaks in pipes a radioactive substance is injected into a pipe. Where the leakage is, more radiation will be detected as the liquid with the tracer has accumulated in the ground. It is then only necessary to dig where the high count rate was measured. This method can only be used for pipes which do not contain drinking water. 8. Medical Tracers using a similar method a radioactive substance can be swallowed by a patient or injected. The gamma radiation emitted can then be recorded by a gamma camera and the doctors can conclude, e.g. how well the kidneys work. 9. Checking welds - If a gamma source is placed on one side of the welded metal, and a photographic film on the other side, weak points or air bubbles will show up on the film, like an X-ray. Why is radiation harmful? It is this process of ionisation that makes radioactive substances so dangerous. Living cells can be fatally damaged if molecules in the cell are ionised. This damage can kill cells and tissue or cause mutations or cancers to form. The greater the dose of radiation the higher the impact. 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. 15

16 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 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. Disposal of Radioactive Waste As some radioactive waste products have half-lives of millions of years they need to be stored in a safe way. Waste products from nuclear power plants are usually sealed in steel containers ( castors ) which are then placed underground. If there is sufficient soil above, a high level of gamma radiation will be absorbed by the ground. 16

17 Particles In this section we are looking at why the model of the atom that you learn about in Chemistry was developed. We are also looking at nuclear fission, a process that is used in nuclear power plants to produce heat and then electrical energy. Geiger Marsden Experiment The Greek Democritus came up with the idea that matter was made up of tiny, indivisible pieces. This was 2500 years ago. The Greek word for indivisible is atomos, hence the word atom, which we use today. Around 1800 John Dalton said that substances were made up of identical atoms, which he called elements. The atoms themselves in his mind were like snooker balls: solid spheres. In 1897 J.J. Thompson came up with the plum pudding model, a variation of Dalton s model, after discovering some very small, negative particles which he called electrons. He believed that Dalton s model had to be altered by adding electrons to it. The sphere as such was positive. The following experiment is the one that changed the model of the atom completely. Geiger and Marsden, working for Rutherford, set up this experiment: Alpha particles were fired at a very thin gold foil. A zinc-sulphide screen surrounded the gold foil. Where alpha particles hit the screen, tiny sparks of light called scintillations were given off. The experiment was carried out in vacuum so that no collisions with air particles could interfere with the results. The results were: Most of them the alpha particles went straight through. Some were deflected and a few were completely reflected. The diagram on the right shows a model of one gold atom. The arrows represent the paths of the alpha particles. The dot in the middle is the nucleus of the atom. Conclusion: o atoms consist mainly of empty space. o The nucleus is positive since it deflected/reflected the positive alpha particles o This model of the atom is called Rutherford s nuclear model, as it consists of a nucleus and surrounding atoms. 17

18 o Beyond this experiment it can be said that the degree of deflection generally depends on the charge (of both the nucleus and the passing particle) and the speed of the particle. A slow moving particle will be deflected more as it is exposed to the positive charge for longer. The greater the charge the greater the deflection. Fission The process of splitting big nuclei is called fission. In this process some of the mass is lost, i.e. it is converted into energy! This is where the energy of the atomic bomb and of nuclear power plants comes from! In a nuclear power plant (or an atomic bomb) the following process takes place: The big uranium 235 nuclei are split into smaller daughter nuclei and 2 3 neutrons are released. Two or three neutrons are released during the splitting of one nucleus. If there is sufficient mass (critical mass) available, a chain reaction starts, i.e. all the released neutrons split other nuclei and the reaction continues by itself (look at the diagram on the right). The energy released in this process is given off as kinetic energy of the fission products (daughter nuclei and neutrons). As you know an increase in kinetic energy of the particles means an increase in heat (e.g. hot gas molecules move faster than cool ones). 18

19 Nuclear Power Plants The following diagram shows how a nuclear power plant works. It works in a very similar way as a fossil fuel or geothermal power plant. In principle water is heated up and turned into steam. As the steam is under high pressure it drives a turbine. The turbine drives the generator. The water is then cooled down in the condenser to be turned back into water. There is another cycle which carries heated water from the actual nuclear reaction to the heat exchanger where it turns water into steam. This water is very hot, but kept under high pressure so it does not turn into steam. Now we come to the part with the actual nuclear reaction. In nuclear power plants the chain reaction needs to be controlled. This is done by taking some of the neutrons out of the chain reaction. Control rods made e.g. of boron are used for this. The neutrons also need to be slowed down so they can be absorbed by the Uranium-235 nuclei. The moderator (e.g. graphite) is used for this purpose. 19