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Natural Radioactivity

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

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Natural Radioactivity

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  1. 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 • We are exposed to radiation from many sources, including sunlight, x-rays and building materials

  2. Some Common Forms of Radiation

  3. Three Main Types of Radiation Emitted During Decay 1. 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, distance 2. 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 distance 3. 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

  4. Alpha, Beta and Gamma Emission

  5. Radiation and Safety • Alpha, beta and gamma are forms of ionizing radiation • Ionizing 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 applications • Exposure to ionizing radiation should be minimized, especially with repeated exposure • Intensity of radiation drops off as 1/D2, where D = ratio of new distance from source to old distance • So, if you move twice as far from the source, you receive 1/4 the exposure

  6. Nuclear Equations for Radioactive Decay • Radioactive decay occurs by the following process: Radioactive nucleus  New nucleus (more stable) + Radiation • Balance nuclear equations so that the atomic numbers and the mass numbers add up the same on both sides • Examples: Alpha emitter: 238U  234Th + 4He Beta emitter: 14C 14N + e- Gamma emitter: 99mTc  99Tc + 

  7. Producing Radioactive Isotopes • Stable isotopes can be converted to radioactive ones by bombardment with neutrons, protons or alpha particles • This 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

  8. Radiation Detection and Measurement • Geiger counter is used to measure beta or gamma radiation • Radiation is measured in units of activity • One 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 tissue • Rem = 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 sickness • The LD50 = 500 rem for humans (means half of those exposed to that amount will die) • Maximum permissible dose = 5 rem/year

  9. Half-life of a Radioisotope • Half-life = time needed for 1/2 of sample to decay • Some 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

  10. Medical Applications of Radioactive Isotopes • Gamma rays are best for medical detection since they can travel far enough through tissue to be detected • Since they are damaging to tissues, the lowest possible dose is used • PET 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

  11. Nuclear Fission • Fission = splitting of nucleus into smaller nuclei • Example: 235U + n  91Kr + 142Ba + 3n +  • A lot of energy is released during fission • A very small amount of mass is lost upon splitting, and according to Einstein’s 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 large • This is how we get nuclear power • Also, since 3 neutrons are produced upon splitting, a chain reaction occurs, accelerating the reaction • What happens if you do this in a closed container?

  12. Nuclear Fusion • Fusion = combining two smaller nuclei to form one larger one • Example: 3H + 2H  4He + n • Again, 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 impractical • Cold fusion is the holy grail of nuclear chemistry • Fusion occurs in the sun, providing heat and light

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