Nuclear Reactions Chapter 19

Facts About the Nucleus Very small volume compared to volume of atom Essentially entire mass of atom Very dense Composed of protons and neutrons that are tightly held together Nucleons Every atom of an element has the same number of protons Atomic Number Isotopes are atoms of the same elements that have different masses Different numbers of neutrons Mass Number = number of protons + neutrons 2

Facts About the Nucleus The number of neutrons is calculated by subtracting the atomic number from the mass number The nucleus of an isotope is called a nuclide less than 10% of the known nuclides are nonradioactive, most are radionuclides Each nuclide is identified by a symbol Element -Mass Number = X-Z 3

Radioactivity Radioactive nuclei spontaneously decompose into smaller nuclei Radioactive decay We say that radioactive nuclei are unstable Decomposing involves the nuclide emitting a particle and/or energy During radioactive decay, atoms of one element are changed into atoms of a different element In order for one element to change into another, the number of protons in the nucleus must change All nuclides with 84 or more protons are radioactive We describe nuclear changes with using nuclear equations atomic numbers and mass numbers are conserved 4

alpha decay beta decay an  particle contains 2 protons and 2 neutrons helium nucleus loss of an alpha particle means atomic number decreases by 2 mass number decreases by 4 beta decay a  particle is like an electron moving much faster found in the nucleus when an atom loses a  particle its atomic number increases by 1 mass number remains the same in beta decay a neutron changes into a proton 5

gamma emission Gamma () rays are high energy photons Gamma emission occurs when the nucleus rearranges No loss of particles from the nucleus No change in the composition of the nucleus Same atomic number and mass number Generally occurs whenever the nucleus undergoes some other type of decay 6

positron emission electron capture positron has a charge of +1 c.u. and negligible mass anti-electron when an atom loses a positron from the nucleus, its mass number remains the same atomic number decreases by 1 positrons appear to result from a proton changing into a neutron electron capture occurs when an inner orbital electron is pulled into the nucleus no particle emission, but atom changes same result as positron emission proton combines with the electron to make a neutron mass number stays the same atomic number decreases by one 7

Artificial Nuclear Transformation Nuclear transformation involves changing one element into another by bombarding it with small nuclei, protons or neutrons reaction done in a particle accelerator linear cyclotron made-made transuranium elements 8

Detecting Radioactivity To detect something, you need to identify something it does radioactive rays cause air to become ionized Geiger-Müller Counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical a scintillation counter is able to count the number of flashes per minute 9

Half-Life Not all radionuclides in a sample decay at once The length of time it takes one-half the radionuclides to decay is called the half-life Even though the number of radionuclides changes, the length of time it takes for half of them to decay does not the half-life of a radionuclide is constant Each radionuclide has its own, unique half-life The radionuclide with the shortest half-life will have the greater number of decays per minute For samples of equal numbers of radioactive atoms 10

Half-Life half of the radioactive atoms decay each half-life 11

Half-Life First order rate reaction as we’ve done before k = 0.693/t1/2 or ln Xo/X = kt Activity measured in Bequerels (Bq), number atoms decaying per second Also measured in curies (Ci) 3.700 x1010 atoms per second

Object Dating mineral (geological) compare the amount of U-238 to Pb-206 compare amount of K-40 to Ar-40 archeological (once living materials) compare the amount of C-14 to C-12 C-14 radioactive with half-life = 5730 yrs. while living, C-14/C-12 fairly constant CO2 in air ultimate source of all C in body atmospheric chemistry keeps producing C-14 at the same rate it decays once dies, C-14/C-12 ratio decreases limit up to 50,000 years 12

Medical Uses of Radioisotopes, Diagnosis radiotracers certain organs absorb most or all of a particular element can measure the amount absorbed by using tagged isotopes of the element and a Geiger counter use radioisotope with short half-life use radioisotope low ionizing beta or gamma 13

Mass energy relations The energy change in a nuclear reaction can be determined by finding the change in mass and multiplying by the speed of light squared DE = Dmc2 To find the change in mass use products – reactants. Use the masses in amu from Table 19.3 (can’t use periodic table because these are average atomic masses) Nuclear binding energy A nucleus weighs less than the protons and neutrons it is composed of. The difference is called mass defect and the energy change is called binding energy. Use E=mc2 to find

Other Nuclear Changes a few nuclei are so unstable, that if their nucleus is hit just right by a neutron, the large nucleus splits into two smaller nuclei - this is called fission small nuclei can be accelerated to such a degree that they overcome their charge repulsion and are smashed together to make a larger nucleus - this is called fusion both fission and fusion release enormous amounts of energy 14

Fissionable Material fissionable isotopes include U-235, Pu-239, and Pu-240 natural uranium is less than 1% U-235 rest mostly U-238 not enough U-235 to sustain chain reaction fission produces about 2.1 x 1013 J/mol of U-235 26 million times the energy of burning 1 mole CH4 to produce fissionable uranium the natural uranium must be enriched in U-235 15

Fission Chain Reaction a chain reaction occurs when a reactant is also a product in the fission process it is the neutrons only need a small amount of neutrons to keep the chain going many of the neutrons produced in the fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238 minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass 16

Nuclear Power Plants use fission of U-235 or Pu-240 to make heat heat picked up by coolant and transferred to the boiler in the boiler the heat boils water, changes it to steam, which turns a turbine, which generates electricity the fission reaction takes place in the reactor core 17

Nuclear Power Plants - Core the fissionable material is stored in long tubes arranged in a matrix called fuel rods subcritical between the fuel rods are control rods made of neutron absorbing material B and/or Cd neutrons needed to sustain the chain reaction the rods are placed in a material used to slow down the ejected neutrons called a moderator allows chain reaction to occur below critical mass 18

Breeder Reactor Design common in Europe Makes its own fuel by converting U-238 to Pu-239 Use liquid sodium as a moderator Use water filled radiator to transfer heat to boiler Plutonium highly toxic and spontaneously combusts in air 19

Nuclear Fusion Fusion is the process of combining two light nuclei to form a heavier nucleus The sun’s energy comes from fusion of hydrogen to produce helium Releases more energy per gram than fission Requires high temperatures and large amounts of energy to initiate, but should continue if you can get it started 20

Factors that Determine Biological Effects of Radiation The more energy the radiation has the larger its effect can be The better the ionizing radiation penetrates human tissue, the deeper effect it can have Gamma >> Beta > Alpha The more ionizing the radiation, the more effect the radiation has Alpha > Beta > Gamma The radioactive half-life of the radionuclide The biological half-life of the element The physical state of the radioactive material The amount of danger to humans of radiation is measured in the unit rems 21

Somatic Damage Somatic Damage is damage which has an impact on the organism Sickness or Death May be seen immediately or in the future Depends on the amount of exposure Future effects include cancer 22

Genetic Damage Genetic Damage occurs when the radiation causes damage to reproductive cells or organs resulting in damage to future offspring 23