Radioactivity PDF

Radioactivity History - Radioactivity was discovered by Henri Becquerel, a French Physicist. He was studying the effect of sunlight on crystals of uranium salts. He left some of the salts resting on a photographic plate, wrapped in black paper, in the darkness of a desk drawer. He found that the pla...

Radioactivity History - Radioactivity was discovered by Henri Becquerel, a French Physicist. He was studying the effect of sunlight on crystals of uranium salts. He left some of the salts resting on a photographic plate, wrapped in black paper, in the darkness of a desk drawer. He found that the plate became fogged in the area near the uranium salt. The uranium salt was giving off radiation which was penetrating the black paper and affecting the photographic plate. - Pierre and Marie Curie investigated the radioactivity of uranium salts in more detail. By carrying out thousands of recrystallisation’s on a uranium ore called pitchblende, they isolated the elements polonium and radium. The Nature of Radioactive Radiation Three types of radiation were discovered by the behaviour of the radiation in electric and magnetic fields. Alpha Particles - Groups of two protons and two neutrons stuck together. - They carry a double positive charge and may be represented as 24He - Alpha particles are emitted from the unstable nucleus of a radioactive element. These pieces of the nucleus are thrown out of the unstable nucleus as it changes to a more stable nucleus. 208 Po → 42He + 20482Pb 84 (1) - Since they have a relatively large mass, they travel slower than other types of radiation - They have a low penetrating power and are stopped by a few centimetres of air or paper - Americium 241 is element number 95 in the Periodic Table and is used in Smoke Detectors - Helium is the alpha particle 1 **Definition: Radioactivity is the spontaneous breaking up of unstable nuclei with the emission of one or more types of radiation.** Beta Particles - Beta particles are fast moving electrons - Formed when a neutron in an unstable nucleus is changed into a proton and an electron neutron → proton + electron The electron is then ejected from the nucleus 14 C → 147N + 0-1e 6 (2) - Lighter than alpha particles so travel faster and are more penetrating - Carbon-14 is an element that emits beta particles and is used in carbon dating Gamma Radiation - High energy electromagnetic radiation - It does not consist of charged particles so it is not deflected in electric or magnetic fields - An unstable nucleus emits gamma radiation in order to lose surplus energy (3) - Have a very high penetrating ability and can penetrate bricks and metal. They are only stopped by a thick slab of lead. - Most dangerous of the three types of radiation because of their penetrating ability - Cobalt- 60 gives off gamma rays and is used in radiotherapy to kill off cancer cells 2 Nuclear Reactions **Definition: A nuclear reaction is a process that alters the composition, structure or energy of an atomic nucleus.** Chemical reactions involve changes in the sharing and transfer of electrons, whereas nuclear reactions involve changes in the nucleus. The main differences between chemical and nuclear reaction are: Chemical Reaction Nuclear Reaction 1 Involves electrons rather Changes take place in than nucleus nucleus and electrons not involved 2 No new element formed A new element is formed 3 No release of nuclear Nuclear radiation is radiation released 4 Chemical bonds broken No chemical bond and formed breaking, or bond formation involved Half-Life **Definition: The half-life of an element is the time taken for half of the nuclei in any given sample to decay.** When radioactive isotopes give out radiation, scientists do not understand fully why a particular atom breaks up at any particular moment giving out radiation. It is found that each radioactive isotope decays at its own rate. Scientists use half- life to compare the rate of decay of radio-active isotopes. 3  - The half-life of Carbon-14 is 5730 years. - Beginning with 10g of Carbon 14, after 5730 years, half that amount will have decayed to nitrogen, leaving only 5g of Carbon 14. - After another 5730 years, half of the 5g sample will have decayed to nitrogen, leaving 2.5g of Carbon 14. - After another 5730 years, half of the 2.5g of Carbon 14 will have decayed to nitrogen, leaving 1.25g of Carbon 14. - Half of the sample decays with each half-life (4) Uses of Radioisotopes Medical Uses - Gamma rays can be used to penetrate the body and kill cancerous cells. - Used to sterilise medical instruments by destroying germs. - Iodine-131 is used to measure the uptake of iodine by the thyroid gland Archaeological Uses - By measuring the ratio of Carbon-12 to Carbon-14, scientists can work out the age of an item. **Definition: Radiocarbon dating is a technique used to determine the age of an object containing carbon. It is based on the ratio of Carbon-14 to Carbon-12in the object.** -Radiocarbon dating is used to determine the age of animal bones. Agricultural Research - Used to study the uptake of fertilisers by plants Food Irradiation - Treating food with gamma rays to kill disease carrying organisms Smoke Alarms - Amercium-241 is used to detect smoke in smoke alarms 4 Exam Questions 2013 – HL – Section B – Question 10 Caesium–137 is a radioactive isotope of the alkali metal caesium. Caesium–137 was released into the atmosphere when Japanese nuclear reactors were damaged by a tsunami in 2011. Caesium–137 decays by beta-particle emission with a half-life of 30 days. (i) Define radioactivity. Spontaneous random decay of a nucleus. (ii) Give two differences between chemical reactions and nuclear reactions. Chemical: Involves electrons. No change to nucleus. Nuclear: Electron cloud not involved. Nuclear change (iii) Give two properties of beta-particles. Negative charge and Negligible mass (iv) A certain mass of caesium–137 leaked on a particular day. What fraction of this mass remained as caesium–137 after 90 days? 1/8 2011 – HL -Section B – Question 10 What are isotopes? Atoms of same element having the same atomic number but different mass numbers due to the different numbers of neutrons in the nucleus Define (i) radioactivity, Spontaneous random decay of a nucleus to release α, β or γ radiation (ii) radioisotope Radioactive isotope Carbon–14 decays by beta particle emission. Write a balanced equation to describe beta-decay of the carbon–14 nucleus. 14 6C → 0 –1e + 14 7N The world’s oldest shoe, found in a cave in Armenia, is pictured on the right. In June 2010, having been radiocarbon dated, it was reported to be 5500 years old. Explain why the carbon–12 to carbon–14 isotope ratio in the shoe leather changed over the 5500 years since the shoe was made Carbon- 14 decayed 5  References 1. Shutterstock.com 2. Mirion.com 3. Researchgate.net 4. wsfcs.k12.nc.us 6