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KS4 Physics. Radioactive Decay. Contents. Radioactive Decay. Rutherford's experiments. Notation. Radioactive decay & half-life. Nuclear power. Summary activities. Radioactive waste.
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KS4 Physics Radioactive Decay
Contents Radioactive Decay Rutherford's experiments Notation Radioactive decay & half-life Nuclear power Summary activities
Radioactive waste Radioactive waste from nuclear power stations is an environmental concern. The problem is the waste stays radioactive for thousands of years. • The current solutions are: • Store it at the nuclear power station until is full. • Dump it far out at sea. • Store it deep underground in non-permeable rock.
Radiation questions • What are the three types of radiation? • Which type of radiation is the most penetrating? • Why is radioactive waste not stored in permeable rock? • Why should nuclear power stations not be situated in geologically active regions? Alpha, beta and gamma Gamma It could contaminate water that seeps through the rock. Earthquakes could cause radioactive spills.
What we used to think Scientists once believed that atoms were spheres of positive charge with negative charges spread throughout. This resembled a plum-pudding, so it was called the ‘plum-pudding’ model. This was wrong! How did we discover current ideas about the structure of the atom?
Rutherford's team Ernest Rutherford and his team of scientists performed a famous experiment in Manchester. They fired alpha particles at a piece of very thin gold foil (only a few atoms thick). If the ‘plum-pudding’ model of the atom was correct, the alpha particles should pass straight through and only be slightly deflected. This did not happen.
What Rutherford’s team observed • Most of the alpha particles went straight through the foil. • Some alpha particles were deflected through large angles. 3. A very few alpha particles were reflected straight back.
Rutherford’s conclusions Atoms are mostly space. The nucleus is very small compared to the size of the atom and it contains most of the mass and all the positive charge.
Task Pretend you are Ernest Rutherford and you have just completed your investigation. Write a letter to a fellow scientist describing your observations and findings. Include the impact you think it will have on current thinking. Dear Dr Banner, I am writing to…
Contents Radioactive Decay Rutherford's experiments Notation Radioactive decay & half-life Nuclear power Summary activities
Notation Mass number (A) The number of protons plus the number of neutrons in a neutral atom. Element symbol Atomic number (Z) The number of protons (which is the same as number of electrons) in a neutral atom. A X Z
Notation exercise carbon P = 6N = 6E = 6 arsenic P = 33N = 42E = 33 iodine P = 53N = 74E = 53 Name the elements below and calculate how many protons (P), neutrons (N) and electrons (E) they have. 12 C 6 75 As 33 127 I 53
Notation exercise Calculate how many protons (P), neutrons (N) and electrons (E) these atoms have. 12 C 6 13 C 6 14 C 6 P = 6N = 6E = 6 P = 6N = 7E = 6 P = 6N = 8E = 6
Isotopes and radioisotopes These atoms are all carbon – what is the difference between them? They have different numbers of neutrons. 12 C 6 13 C 6 14 C 6 What are atoms of the same element with different numbers of neutrons called? Isotopes What are isotopes that are unstable and emit radiation to become more stable called? Radioisotopes
Contents Radioactive Decay Rutherford's experiments Notation Radioactive decay & half-life Nuclear power Summary activities
Radioactive decay Why is it that there are different types of radiation? What is going on inside the nucleus? The three types of decay are… gamma alpha beta
Alpha decay alpha particle (helium nuclei) What is emitted? Description of decay: Example of decay: Effect on A and Z: 2 neutrons and 2 protons are emitted from the nucleus. 238234 4 U Th + + energy 92 90 2 A decreases by 4 (A – 4)Z decreases by 2 (Z – 2)
Beta decay High-energy electron What is emitted? Description of decay: Example of decay: Effect on A and Z: 1 neutron in the nucleus decays into a proton and a high-energy electron, which is emitted. 1414 0 C N + + energy 6 7 -1 A stays the same (A)Z increases by 1 (Z + 1)
Gamma decay What is emitted? Description of decay: Effect on A and Z: High-energy electromagnetic radiation nucleus changes shape into a more stable shape, resulting in gamma radiation being emitted A stays the same (A)Z stays the same (Z)
Half-life There are two definitions of half-life: The time it takes the number of radioactive nuclei in a sample to decrease by 50%. The time it takes the count rate from a radioisotope to decrease by 50%. You must learn both of these definitions!
Graphing half-life How can you calculate the half-life of the radioisotope represented by this graph? Decay rate (counts/min) Calculate the time it takes the count rate to decrease from 80 per min to 40 per min. 80 60 40 20 2 mins Double-check that the time it takes the count rate to decrease from 40 per min to 20 per min is the same 2 mins 2 4 6 8 Time (min) The half-life of the radioisotope is 2 mins.
Contents Radioactive Decay Rutherford's experiments Notation Radioactive decay & half-life Nuclear power Summary activities
Carbon dating All living things absorb a little radioactive carbon-14 when they feed and breathe, as well as the normal carbon-12. When living things die, they stop taking in carbon-14 and so the carbon-14 present at death slowly decays to carbon-12 (half-life is 5,600 years). The amount of radioactivity from the decaying carbon-14 can be used to calculate the age of bones, wood, paper and cloth.
Carbon dating: example A fresh bone gives a radioactive count of 170 counts per minute. Another ancient bone of the same mass gives a count rate of 50 counts per minute. The background count is 10 counts per minute. How old is the bone? Counts due to bones are 170 – 10 = 160 (fresh) and 50 – 10 = 40 (ancient) The count rate of the carbon-14 has fallen to one quarter of its original value, i.e. 160/2 = 80, 80/2=40. This is two half lives. So the bone is 5,600 x 2 = 11,200 years old
Nuclear power When a nucleus decays it gives out heat energy. In a nuclear power station, the uranium-235 atoms decay and give out energy and neutrons. Each time a uranium atom splits it produces 2 or 3 neutrons (depending on the reaction). These go on to hit other uranium atoms, which causes them to decay. A chain reaction is set up where more and more energy is released. In a nuclear reactor the process is carefully controlled so that neutrons are absorbed harmlessly and the energy released is controlled. In a nuclear bomb the reaction is not controlled, and this causes the explosion!
Nuclear power – fission Fast neutron from previous decay cause the uranium nucleus to split.
Nuclear power moredecays Kr n fission n n uranium n In this reaction, a neutron from a previous decay can lead to more and more decays. Ba This is called a chain reaction.
Contents Radioactive Decay Rutherford's experiments Notation Radioactive decay & half-life Nuclear power Summary activities
Glossary • atomic number – The number of protons in the nucleus of an atom. • half-life – The time it takes the number of radioactive nuclei in a sample to decrease by 50%. This is also the time it takes the count rate from a radioisotope to decrease by 50%. • isotopes – Different versions of the same element, which have the same number of protons but different numbers of neutrons. • mass number – The total number of protons and neutrons in the nucleus of an atom. • radioisotope – An natural or artificially-created isotope of an element that is radioactive.
