CHEM 1120 1st Edition Lecture 10 Outline of Last Lecture 1 Chemical Reactions vs Nuclear 2 Radioactivity 3 Balancing Nuclear 4 Types of Emissions 5 Nuclear Stability Outline of Current Lecture 1 Nuclear stability continued 2 Nuclear transmutation 3 Rates of radioactive decay 4 Detection of radioactivity 5 Applications of radioactivity Current Lecture 21 2 Nuclear Stability continued from lecture 9 Type of radioactive decay predictable o Above the belt of stability Too many neutrons N Z 1 Loss of proton Z Beta emission o Below the belt of stability Too many protons N Z 1 o Positron emission or electron capture These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Nuclei with 83 protons always unstable Alpha emission o Radioactive Decay Series o Radioactive series produces isotopes o Each series fathered by a very long half life isotope o Reason why all 80 radioactive isotopes that occur naturally haven t decayed o See slides 21 23 provide graphs and pictures explaining half life decay and also have example 21 3 Nuclear Transmutations Nuclear transmutation o Induced conversion of one nucleus into another o Atomic structure research discovered neutron and production of artificial radioisotopes o Over 1000 radioisotopes produced o 1933 first artificial radioisotope produced by the Curies o Bombardment Reactions o Use of neutrons o Most synthetic isotopes used in medicine are prepared by bombarding neutrons at a particle which wont repel the neutral particle o Transuranium elements Elements immediately after uranium were discovered by bombarding isotopes with neutrons o Larger elements atomic number higher than 110 made by colliding large atoms with nuclei of light elements with high energy Particle accelerators o Invented in 1930s o Impact high kinetic energies to particles by placing them in an electric field usually in combination with a magnetic field o Linear accelerator Use alternating voltages to change the charges in a series of tubes o Cyclotron Uses electromagnets to give the particle a spiral path o Synchrotron Uses synchronously increasing magnetic field to make the path circular CERN France Switzerland worlds largest particle physics lab has a circular particle accelerator with a diameter of 5 3 miles o Applications of accelerators Producing radioisotopes used in medical applications Studying fundamental nature of matter Syntheses of transuranium elements Those elements with atomic numbers higher than uranium heaviest naturally occurring element 21 4 Rates of Radioactive Decay Radioactive nuclei decay at a characteristic rate independent of the chemical substance first order kinetic process k decay constant N number of radioactive nuclei Activity A a sample s rate of decay disintegration unit Specific activity decay rate g Becquerel Bq one nuclear disintegration per second SI unit Curie Ci 3 7x10 10 disintegrations per second d s Exercise half life see slide 31 for another exercise in disintegration per second Carbon 14 Dating o Atmospheric CO2 contains a fixed 14 C 12 C ratio o Photosynthesis results in biochemical in living plants and animals with this same 14 C 12 C ratio o Upon death decaying C 14 is not replenished o Dead objects have Carbon 14 12 ratio that is less than the atmospheric ratio o C 14 has a half life of 5730 years o Can be used to date objects up to 36 000 years old o See table 21 5 other nuclear isotope dating o For older materials U 238 Pb 208 and K 40 Ar 40 dating is used o Earth s oldest rocks have been dated at 3 8x10 9 yr o Earth s age is estimated to be 4 5x10 9 yr see slide 33 for radiodating exercise 21 5 Detection of radioactivity Oldest method of detection is photographic film Film strip white before exposure to radiation Film strip is darkened on exposure to radiation Scintillation counter o Two componets Scintillator A substance that emits light pulses when struck by radiation Photomultiplier Device that converts light pulses into electric current and amplifies this current Radiotracers o Ability to detect very small amounts of radioisotopes makes them powerful tools for application in medicine materials science environmental science o Radioactive tracers Small amount of a radioisotope mixed with a stable isotope can act as a tracer that emits nonionizing radiation that signals its presence radioactive labeling Reaction pathways material flow activation analysis medical diagnosis Largest use of radioisotopes is in medicine Tracers with half lives of a few minutes to a few days are to observe specific organs or body parts Positron Emission Tomography PET imaging o Powerful method for observing brain structure and function o Biological substance synthesized with one of tis atoms replaced by an isotope that emits positrons o The substance is injected into a patients bloodstream and is taken up into the brain o Photons emitted which are detected by array of detectors around the patient s head Cancer treatment with ionizing radiation o Some radioisotopes emit high energy ionizing radiation o Cancer cells divide more rapidly than normal cells but more susceptible to radiation that normal cells Molecular imaging o Radiolabeled therapeutics can be used to trace path of medications with unparalleled sensitivity Applications of ionizing radiation o Irradiation of food increases the shelf life by killing microorganisms that cause food to spoil Somewhat controversial 21 9 Radiation in the Environment and Living Systems Nuclear changes cause chemical changes in surrounding matter Emissions affect electrons of nearby atoms Radioactive emissions interact with matter in two ways o Excitation Nonionizing radiation of relatively low energy interacts with an atom of a substance which absorbs some of the energy and then re emits it o Ionization Ionizing radiation collides with an atom energetically enough to dislodge an electron Effects of ionizing radiation on living matter o Ionizing radiation Alpha beta gamma and x rays that can eject valence electrons and create ion in biological tissue o Free radicals Can initiate a chain of reactions producing damaged biomolecules o Dangers relatively Gamma and X rays can penetrate human tissue Beta rays can only about 1 cm Alpha rays are stopped by the skin but can be dangerous within the body Nuclear changes cause chemical changes in surrounding matter by excitation and ionization Radiation units and doses o o Radon Rn