natural radioactivity

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Natural Radioactivity. Some isotopes of elements have unstable nuclei---these are radioactive All isotopes of elements with Z > 83 are radioactive By releasing energy (radiation) the nuclei can become stable - PowerPoint PPT Presentation


  • Natural RadioactivitySome isotopes of elements have unstable nuclei---these are radioactiveAll isotopes of elements with Z > 83 are radioactiveBy releasing energy (radiation) the nuclei can become stableWe are exposed to radiation from many sources, including sunlight, x-rays and building materials

  • Some Common Forms of Radiation

  • Three Main Types of Radiation Emitted During Decay1.Alpha particles ( or )- Have two protons + two neutons- Have greatest mass of the three- Can travel 2-4 cm in air and 0.05 mm in tissue- Protection: lab coat and gloves, distance2.Beta particles ( or e-)- High energy electrons- Can travel 2-3 m in air and 4-8 cm in tissue- Protection: lab coat, gloves, plexiglass and distance3.Gamma rays ()- High energy rays (like x-rays)- Have no measurable mass- Can travel 500 m in air and > 50 cm in tissue- Protection: lead or thick concrete, distance

  • Alpha, Beta and Gamma Emission

  • Radiation and SafetyAlpha, beta and gamma are forms of ionizing radiationIonizing radiation causes electrons to be knocked out of atoms or molecules, creating unstable ions (will discuss ions, which are charged particles, later)Radioactive elements are used frequently in medical and research applicationsExposure to ionizing radiation should be minimized, especially with repeated exposureIntensity of radiation drops off as 1/D2, where D = ratio of new distance from source to old distanceSo, if you move twice as far from the source, you receive 1/4 the exposure

  • Nuclear Equations for Radioactive DecayRadioactive decay occurs by the following process:Radioactive nucleus New nucleus (more stable) + RadiationBalance nuclear equations so that the atomic numbers and the mass numbers add up the same on both sidesExamples:Alpha emitter: 238U 234Th + 4HeBeta emitter: 14C 14N + e-Gamma emitter: 99mTc 99Tc +

  • Producing Radioactive IsotopesStable isotopes can be converted to radioactive ones by bombardment with neutrons, protons or alpha particlesThis process is called transmutation (changing one element to another element)When the stable nucleus absorbs a high energy particle, it becomes unstable (or radioactive)Example: 66Zn + H 67Ga

  • Radiation Detection and MeasurementGeiger counter is used to measure beta or gamma radiationRadiation is measured in units of activityOne Curie (Ci) = 3.7 x 1010 disintegrations per second (often use micro or millicuries)One rad (radiation absorbed dose) = amount of radiation absorbed per 1 gram of tissueRem = rad x damage factor (measure of biological damage from radiation)The average exposure to radiation in the US is 0.17 rem/year (a small, but significant, amount)Larger doses can cause radiation sicknessThe LD50 = 500 rem for humans (means half of those exposed to that amount will die)Maximum permissible dose = 5 rem/year

  • Half-life of a RadioisotopeHalf-life = time needed for 1/2 of sample to decaySome radioisotopes have very short half-lives (very unstable, such as 15O = 2 min.)Some have very long half-lives (very stable, 238U = 4.5 x 109 years)A decay curve is a plot of the amount of radioactive isotope (activity) vs. time

  • Medical Applications of Radioactive IsotopesGamma rays are best for medical detection since they can travel far enough through tissue to be detectedSince they are damaging to tissues, the lowest possible dose is usedPET scans use positron emitters (C-11)Positrons are particles with the same mass electrons, but with a positive charge (when they collide with electrons, the mass is annihilated and gamma rays are produced)Ionizing radiation is most damaging to rapidly dividing cells (bone marrow, skin)Since cancer cells are rapidly dividing, radiation can be used to treat tumors, without damaging surrounding tissue with less rapidly diving cells (adult bone, nerves, muscle)Beta emitters are often used in cancer treatment because of their limited range of activity in tissue

  • Nuclear FissionFission = splitting of nucleus into smaller nucleiExample: 235U + n 91Kr + 142Ba + 3n + A lot of energy is released during fissionA very small amount of mass is lost upon splitting, and according to Einsteins equation E = mc2, where c is the speed of light (3 x 108 m/s), the energy produced by this loss of mass is very largeThis is how we get nuclear powerAlso, since 3 neutrons are produced upon splitting, a chain reaction occurs, accelerating the reactionWhat happens if you do this in a closed container?

  • Nuclear FusionFusion = combining two smaller nuclei to form one larger oneExample: 3H + 2H 4He + nAgain, mass is lost and a large amount of energy is produced (more than in fission)Very high temperature is required due to strong repulsion between H nuclei, so the method is currently impracticalCold fusion is the holy grail of nuclear chemistryFusion occurs in the sun, providing heat and light


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