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Radioactivity Physics and Chemistry. 2 Radioactivity in Radium Killed Marie Curie Marie and Pierre Curie isolated 1/30 ounce of radium from one ton of.

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Presentation on theme: "Radioactivity Physics and Chemistry. 2 Radioactivity in Radium Killed Marie Curie Marie and Pierre Curie isolated 1/30 ounce of radium from one ton of."— Presentation transcript:

1 Radioactivity Physics and Chemistry

2 2 Radioactivity in Radium Killed Marie Curie Marie and Pierre Curie isolated 1/30 ounce of radium from one ton of uranium ore. Marie died from radiation-induced leukemia. The pages of her lab notebook were later found to be contaminated with radioactive fingerprints.

3 3 Radioactivity Radioactivity has become a matter of serious public concern. Ionising radiation emitted by radioactive matter cannot be detected by any of the senses but excess exposure to it can cause serious health problems. It can cause cancer which might only express itself in years later. It can produce defects in unborn children and can possibly lead to death.

4 4 Radioactivity Nevertheless, ionising radiation is used in the service of man in electric power generation, in medicine, in scientific research, in video display units and in industrial radiography.

5 5 What is radioactivity? Matter is made of elements and the smallest part of an element that can exist independently is the atom. Atoms of certain elements tend to be unstable; they tend to disintegrate spontaneously. These elements are said to be radioactive. Each disintegration is accompanied by the emission of high energy waves or particles.

6 6 What is radioactivity? These emissions are called ionising radiation. As the radiation is emitted, the atoms change their nature from one element to a daughter element. This may also be radioactive leading to a second generation radioactive daughter and so the process will continue until eventually a stable atom is reached.

7 7 Ionising Radiation A general word for any form of radiation that will knock off outer electrons of atoms, forming ions. Such radiation will cause ionisation when they are absorbed by the human body. – radiation, – radiation, – radiation, x – rays and neutrons are examples of ionising radiation. All ionising radiation are harmful to the human body. (There are also non – ionising radiations examples of these are uv, ir, radiowaves and microwaves) Ionising ability is the ability to knock electrons off atoms to create ions.

8 8 Ionising Radiation Can Cause: Skin burns similar to intense sunburn. Cataracts, leukaemia and other cancers. Genetic defects in children of parents exposed to the radiation. Death.

9 9 Background Radiation We are all exposed all the time to some radiation, called background radiation. Background radiation is natural radiation and comes from the following: Cosmic Radiation. Radiation coming from outer space. Rocks in the Earths crust. Rocks in the Earths crust contain traces of uranium and its decay products, one of which is radon gas. In Ireland, regions of granite rock release radon gas, which can accumulate in houses to levels that increase risk of lung cancer. Man-made radioactive materials.

10 10 Natural Radiation Natural sources account for about 87% of background radiation. Some natural radioactive substance are uranium, radon and thorium. They produce natural radioactivity. Relatively few naturally occurring atoms are radioactive. Many radioactive atoms that once existed have now emitted radiation and become non – reactive.

11 11 Experiment: To investigate the relative ionising power Method: 1.Charge the electroscope negatively. 2.Hold an alpha source near the cap. 3.Note the time taken for the electroscope to discharge. 4.Repeat the experiment using other sources Results: The alpha discharges the electroscope the quickest, then beta, then gamma.

12 12 Isotopes Same number of protons, but different numbers of neutrons. Electrical and chemical properties are the same, but nuclear properties are different.

13 13 Radioactivity is the spontaneous disintegration of unstable nuclei with the emission of one or more types of radiation Radioactivity

14 14 The three main types of radiation are: Alpha Radiation ( ) Beta Radiation( ) Gamma Radiation( )

15 15 Uranium Decays via Alpha-Particle Emission The first particle ejected from an unstable nucleus was called an alpha particle because alpha is the first letter of the Greek alphabet. It's now known to consist of two protons and two neutrons, which is the same as a helium nucleus. When an alpha particle is emitted from an unstable radioactive nucleus, the atom is transmuted (changed) into a different element.

16 16 Alpha Emission Note: A – stands for the atomic mass number. (No. of Protons & neutrons) Z – stands for the atomic number. (No. of Protons)

17 17 Carbon-14 Decays by Beta Emission The beta particle is now known to be just an electron, traveling at high speeds. They are emitted by atoms whose nuclei contain too many neutrons to be stable. A neutron is split into a proton (remains in nucleus) and an electron (which escapes)

18 18 Beta Emission Note: A – stands for the atomic mass number. (No. of Protons & neutrons) Z – stands for the atomic number. (No. of Protons)

19 19 Reaching Stability Through Gamma Ray Emission Nuclei with excess energy emit gamma-rays, which are extremely short- wavelength electro-magnetic waves, i.e. very high energy photons. The energy of the gamma ray accounts for the difference in energy between the original nucleus and the decay products. Gamma rays typically have about the same energy as a high – energy x- ray.

20 20 Nature of the Radiation – particles are identical to helium nuclei – particles are identical to an electron – radiation is electromagnetic radiation, which travels at the same speed as light (in a vacuum)

21 21 Penetrating Ability Alpha particles are 8,000 times as heavy as beta particles. Paper or clothing will block alpha particles, while beta particles require a few sheets of aluminum foil. Gamma radiation is extremely dangerous - a thousand times more potent than x-rays.

22 22 Experiment: To demonstrate the penetrating power Method: 1.Set up the apparatus and take a read on the ratemeter due to back ground radiation. 2.Place an alpha source in front of the G-M tube. Take the reading. 3.Slowly move the alpha source away from the G-M- tube until the reading is the same as the background count. Measure the distance. 4.Repeat the above steps for a beta source. 5.Repeat step 2 but place sheets of lead of varying thickness in front of a gamma source. Results: Beta more penetrating than alpha.

23 23 Summary Table PropertiesAlpha particle ( α) Beta particle ( β) Gamma Rays ( γ) Nature Helium nucleusElectronElectromagnetic radiation Deflection in electric & magnetic fields Yes (Slightly) Yes (strongly) No Penetrating Ability Poor (6 cm of air / Stopped by paper) Medium (5 m of air / Stopped by 3mm Al) Good (up to 10 cm of lead) Ionising Ability Good (Strong) Medium (Weak) Poor (Very weak) Speed 10 % speed of light95 % speed of lightSpeed of light Detectors Photographic film Cloud chamber G-M tube Photographic film Cloud chamber G-M tube Photographic film Cloud chamber G-M tube Charge +20 Mass (a.m.u.) 41/18400

24 24 Examining Reactions Note: When looking up isotopes in the PTE only go by the atomic number (smaller of the two) and NOT the atomic mass number as this can vary for each isotope.

25 25 Example 1: Solution:

26 26 Example 2: Solution:

27 27 Precautions When Using Radiations Minimise the time spend using sources of radiation. Use proper protective clothing, e.g. gloves, glasses, coat etc. Make sure sources are properly shielded from you. Keep as far away from the source as possible. Use tongs for handling sources. Store radioactive sources inside metal containers

28 28 Uses of Radioactive Isotopes Medicine. Image of an organ can be seen by radiation given off. Radiation can kill cancer cells. Smoke detectors. Food irradiation. Gamma rays can be used to sterilise food. Carbon dating. The age of archaeological specimens can be determined by the activity of the isotope C - 14 contained in them. In Industry. To check the fullness of containers, thickness of objects, to find leaks and to detect wear in components. Nuclear energy.

29 29 Radioactive Decay The half life T ½ is the time take for half the number of atoms of a radioactive isotope to decay. Half – lives vary over a very wide range, for example the half – life of polonium – 212 is 3 x seconds, and the half – live of uranium – 236 is 4.5 x 10 9 years.

30 30 Half life Calculations – Example 1 The half – life of a radioactive sample is 15 minutes. What fraction of the sample will remain after 1 hour. Soln:

31 31 Half life Calculations – Example 2 One – sixty fourth of the original quantity of a radioactive isotope was left after 1 year. Calculate the half – life of the radioactive isotope. Soln:

32 32 Nuclear Fission Nuclear fission is the splitting up of a large nucleus into two smaller nuclei of similar size with the release of energy. Fission is produced in a large nucleus by bombarding it with neutrons. During fission very large amounts of energy are given off. More neutrons are produced in the fission reaction. These can produce further fission.

33 33 Nuclear Fission U n Ba Kr n

34 34 A Fission Chain Reaction A fission chain reaction in U

35 35 Nuclear Fission 100,000,000 times more energy than is released when the same quantity of coal is burned. Slow neutrons are required. A chain reaction occurs if more than one neutron goes on to cause another fission. Neutrons can be slowed by bouncing them off of small objects, such as carbon nuclei.

36 36 Uses of Fission Nuclear Reactors produces energy by fission in uranium fuel rods. (controlled reactions) Nuclear Weapons. Atomic bombs – an uncontrolled chain reaction. (Using plutonium – 239 or uranium – 235) Hiroshima in Japan 1945, of the city was devastated. 75, 000 people killed.

37 37 Dangers of Fission Reactors. Mining Uranium ore. The mining of uranium ore releases radon gas, which can cause lung cancer in miners. The area around the mine may contain radioactive material. Containment of radioactive material within the reactor. Accidents have happened – Chernobyl Removal and treatment of spend fuel rods. Radioactive waste Remaining waste products must be stored securely for a very long time. This is likely to be a very big problem for future generations.

38 38 Nuclear Fusion Nuclear fusion is the joining together of two small nuclei to form one larger nucleus with the release of energy. Fusion can only occur if the two reacting nuclei are forced together with sufficient force to overcome the coulomb repulsion between them. This is done by heating them to extremely high temperatures, typically greater than 10 8 K. When fusion starts, energy is released which can help keep the reaction going. No one has yet managed to achieve a sustained controlled fusion reaction. A great deal of effort is currently being put into this project. The hydrogen bomb is an uncontrolled fusion reaction. The initial high temperatures are produced by a small fission bomb exploding in the deuterium. (One tested in whole island disappeared) Nuclear fusion is in the interior of the Sun is the principle source of the Suns energy. In a series of reactions hydrogen fuses to form helium, releasing energy in the process.

39 39 Nuclear Fusion Fusion is the opposite of fission. Deuterium must be moving extremely fast to fuse.

40 40 Examples of Fusion Reactions An important example is the fusion of two heavy hydrogen atoms (Deuterium) to form helium. Another is the fusion of deuterium and tritium. H H He 1010 n+ H H He 1010 n+

41 41 Advantages of Fusion over Fission There is less radioactive waste produced. The reaction produces far more energy from a given mass of material than any fission reaction. There is an abundance of deuterium in sea water, so the fuel is plentiful.

42 42 Albert Einstein and Mass-Energy Equivalence When a uranium nucleus splits, the mass of the remnants is less than the original mass. The difference appears as light, heat, and kinetic energy.

43 43 Mass – Energy Conservation In 1905 Einstein in his Special Theory of Relativity concluded that mass and energy are not independent. He stated that mass can be converted into energy and energy converted into mass. Principle of mass – energy conservation For any nuclear reaction, the mass – energy of the reactants equals the mass energy of the products. [Loss in mass = gain in energy] E = mc 2 is the equation that governs this. (where c = 3.0 x 10 8 ms -1 ). Because of the large value of c, the speed of light, a tiny decrease in mass can cause an enormous release of energy. It has been estimated that 1 gram of matter was converted into energy in the atomic bomb that was dropped on Hiroshima in 1945.

44 44 Problem based on E = mc 2 The difference between the masses of the reactants and products in a nuclear reaction is 1.2 x kg. How much energy is released in the reaction.

45 45 Cockcroft & Waltons Experiments

46 Q 1 (o) The atom was first split in 1932 by Cockcroft and Walton in the reaction: Explain why energy is released in this reaction.

47 47 Solution Loss in mass is converted to energy by the equation

48 48 Albert Einstein Albert Einstein 1879 – 1955 was probably the greatest theoretical physicist of the twentieth century. He developed mathematical models to explain physical phenomena. Einstein developed the mass – energy equation E = mc 2, which predicted the possibility of nuclear fission. He explained the nature of space and the time in the general theory of relativity, and predicted that light would bend near a large mass. This was later verified experimentally, and is the reason why light does not escape black holes. Einstein was awarded the Nobel Prize for physics in 1921.


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