Transcript Slide 1
Radioactivity and Nuclear Reactions Chapter 18 Section 1: Radioactivity Let’s Review… Atoms are made of three kinds of particles: electrons, protons, and neutrons. The 4 Forces The Strong Force The strong force causes protons and neutrons to be attracted to each other. The strong force is very short-ranged, so the protons and neutrons in the nucleus have to be VERY close together for it to work. Forces in a Large Nucleus Protons and neutrons are held together less tightly in a large nucleus. See pg. 538. Radioactivity When a nucleus breaks down and gives off matter and energy. All nuclei (plural of nucleus) that have more than 83 protons are radioactive. Many nuclei that have less than 83 protons are radioactive too! Isotopes and Stability The ratio of protons to neutrons has an effect on the stability of the nucleus. In elements with small nuclei, a ratio of 1:1 is stable. In larger nuclei, a ratio of 3:2 (neutrons to protons) is stable. Nuclei with too many or too few neutrons are radioactive and unstable. The Discovery of Radioactivity 1896 Henri Becquerel He left uranium salt in a drawer with a photographic plate. He later developed the plate and saw an outline of the uranium salt. He thought that the uranium salt had emitted some invisible rays that darkened the film. Marie Curie Discovered 2 new elements: polonium and radium. She named polonium after Poland, where she was born. She came up with the term “radioactivity” 2 Nobel Prizes: Physics and Chemistry Died of aplastic anemia- caused by radiation. Even today, her papers are considered too dangerous to handle, they are kept in lead-lined boxes, and anyone that reads them must wear protective clothing! Section 2: Nuclear Decay Nuclear Radiation When an unstable nucleus decays, particles and energy are emitted. 3 types of nuclear radiation: Alpha and Beta radiation are particles. Gamma radiation is a wave. Note: This is the atomic weight, which is the number of protons plus neutrons Alpha Particles (a) Radium Radon R226 Rn222 88 protons 138 neutrons + 86 protons 136 neutrons n p p n a (4He) 2 protons 2 neutrons The alpha-particle (a) is a Helium nucleus. It’s the same as the element Helium, with the electrons stripped off ! Damage from Alpha Particles Alpha particles are “large” respectively, and they lose energy quickly. Alpha particles can be stopped by a sheet of paper. Alpha particles are dangerous if they are released by radioactive atoms inside the human body. Damage from alpha particles can cause cells to not function properly, leading to illness and/or disease. Transmutation Transmutation-the process of changing one element to another through nuclear decay. Beta Particles (b) Carbon C14 6 protons 8 neutrons Nitrogen N14 7 protons 7 neutrons + eelectron (beta-particle) We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. Note: the mass # does not change!!! Damage from Beta Particles Beta particles travel much faster, and are more penetrating than alpha particles. They can pass through paper, but can be stopped by a sheet of aluminum foil. Beta particles can damage cells when they are emitted from radioactive nuclei inside the body. Gamma rays (g) Gamma rays are electromagnetic waves with the highest frequencies and shortest wavelengths. Neon Ne20 10 protons 10 neutrons (in excited state) Neon Ne20 + 10 protons 10 neutrons (lowest energy state) gamma A gamma is a high energy light particle that is emitted From a nucleus when alpha or beta decay occurs. It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum. Gamma Rays Neon Ne20 Neon Ne20 + The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!) Uses of Gamma Rays Sterilizing medical and dental instruments. Treatment of cancer Sterilizing food. Food Irradiation Food irradiation is the process of exposing food to ionizing radiation to destroy microorganisms, bacteria, viruses, or insects that might be present in the food. Logo (right): used to show a food has been treated with ionizing radiation Radioactive Half-Life The half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay. Radioactive Dating Used to determine ages of rocks, fossils, and remains. Section 3: Detecting Radioactivity Detecting Radioactivity Cloud Chambers- detects alpha and beta radiation. Bubble Chambers- holds a super heated liquid (doesn’t boil because of high pressure) Particles move and leave a trace of ions behind. Electroscopes- positive ions formed by alpha particles in the air cause the leaves of the electroscope to come together. Measuring Radiation Geiger counter- produces an electric current when it detects a charged particle (alpha or beta particle). Background Radiation Low levels of radiation are bombarding us all the time. Materials such as bricks, wood, and stones have low levels of radioactive materials. SMALL amounts of radioactive isotopes are found in the food, water, and air consumed by all animals and plants…so, animals, humans, and plants contain SMALL amounts of these isotopes too! Sources of Background Radiation Radon gas- produced in the earth’s crust when Uranium-238 decays. Radon gas can seep into houses and cause problems for the people and pets living inside. Sources of Background Radiation…Continued… Other sources of radiation come from cosmic rays that come in through the Earth’s atmosphere. Background radiation increases with altitude. Example: someone living in Denver, CO would have more background radiation yearly than someone living in Atlanta. Radiation in Your Body The amount of background radiation you receive each year depends on many factors, including: Type of rocks underground Type of materials used to build your home Elevation at which you live (many many more…) It is impossible to eliminate background radiation. Section 4: Nuclear Reactions Nuclear Fission History 1930- Enrico Fermi 1938- Otto Hahn and Fritz Strassmann Thought bombarding a nucleus with a neutron would create a bigger atom. Found that when a neutron strikes a nucleus, the nucleus splits into 2 smaller nuclei. 1939- Lise Meitner First to offer a theory to explain the above results. Nuclear Fission Definition Nuclear Fission- the process of splitting a nucleus into several smaller nuclei. FISSION- means “To Divide” How Does Fission Work? Only large nuclei like those of uranium and plutonium can undergo fission. Mass and Energy Chain Reactions Chain Reaction- the series of repeated fission reactions caused by the release of neutrons in each reaction. Nuclear Fusion Two nuclei with low masses are combined to form one nucleus of larger mass. FUSION- 2 nuclei come together to form a bigger nucleus FISSION- nuclei are broken apart to smaller particles. FUSION For FUSION to occur, EXTREMELY high temperatures must be achieved Fusion occurs in the center of stars and the sun when hydrogen nuclei fuse to form helium! When this process occurs heat and light are given off Nuclear Reactions in Medicine “Tracer” radioisotopes are used in the human body to help doctors study body functions. Example: PET (Positron Emission Tomography) Scan Radioactive isotope fluorine-18 injected into patient’s bloodstream and travels to the brain. There it begins radioactive decay and doctors can obtain a picture of how the brain is working. Treating Cancer with Radioactivity Radioactive isotopes like gold-198 or iridium192 can be implanted in or near the tumor Tumors can be treated from outside the body by focusing an intense beam of gamma rays on the tumor for a short time. Cancer cells grow faster than normal cells, so they take up more radiation, but normal cells can be affected too and patients can have severe side effects.