Nuclear Reactions • Beta decay • Daughter element is different from parent • Transmutation: transformation from one element into another • Natural decay: reaction occurs on its own • Nuclear reaction: a nucleus is struck by another nucleus, or simpler particle, so that an interaction takes place • Rutherford (1919) • First nuclear reaction • 42He + 147N 178O + 11H
Nuclear Reactions • Conservation • Electric charge and number of nucleons • Energy and momentum • Used to determine if nuclear reaction is possible • Mass of products < initial mass of nucleus • Loss in mass = KE of outgoing particles • Mass of products > initial mass of nucleus • Incoming particle must have KE to start the process
Nuclear Reactions • a + X Y + b • X and Y are elements • a and b are small particles • Reaction energy, or Q-value • Q = (Ma + MX – Mb – MY)c2 • Q = KEtotal = KEb + KEY – KEa – KEX • For most reactions, KEX = 0 • If Q > 0: exothermic (energy is released) • If Q < 0: endothermic (energy is required to start the reaction)
Example • A neutron is observed to strike an 168O nucleus, and a deuteron is given off. (A deuteron, or deuterium, is the isotope of hydrogen containing one proton and one neutron, 21H). What is the nucleus that results?
Example • The nuclear reaction n + 105B 73Li + 42He is observed to occur even when very slow-moving neutrons (mass Mn = 1.0087 u) strike a boron atom at rest. For a particular reaction in which KEn≈ 0, the outgoing helium (MHe = 4.0026 u) is observed to have a speed of 9.3E6 m/s. Determine the following • The kinetic energy of the lithium (MLi = 7.0160 u) • The Q-value of the reaction
Example • Can the reaction p + 136C 137N +n occur when 136C is bombarded by 2 MeV protons?
Nuclear Reactions • Enrico Fermi (1930’s) • Neutrons were effective particles in inducing transmutations • Artificial transmutation became easier • No electrostatic repulsion • Works better if the energy of the neutron is low • Bombard heavier elements (i.e., uranium) • Created new elements (i.e., neptunium and plutonium)
Nuclear Fission • Otto Hahn and Fritz Strassmann (1938) • Bombarded uranium • Smaller nuclei were produced • Lise Meitner and Otto Frisch • Described the experiment in more detail • Until then, only small fragments were produced • Nuclear fission
Nuclear Fission • Nuclear fission • More common in 23592U than in 23892U • Liquid – drop model • n + 23592U 23692U X1 + X2 + neutrons • Internal energy given to 23692U (compound nucleus) elongates the nucleus • The strong nuclear force is weakened and the electrostatic repulsion separates the two ends • 23692U is very unstable and decays in less than 10-12 seconds • X1 and X2: fission fragments • n + 23592U 14156Ba + 9236Kr + 3n
Example • Identify element X in the fission reaction n + 23592U AZX + 9338Sr + 2n
Nuclear Fission • Fission fragments are usually 40% to 60% less than uranium’s mass • Enormous amount of energy is released • Binding energy for U ≈ 7.6 MeV/nucleon • Binding energy of average fission fragment ≈ 8.5 MeV/nucleon • Energy released ≈ 8.5 – 7.6 = 0.9 MeV/nucleon • Energy for each fission nucleus = (0.9 MeV/nucleon)(236 nucleons) ≈ 210 MeV
Nuclear Fission • Chain reaction • Neutron from original fission produces fission in subsequent nuclei • Self-sustaining chain reaction • Fermi (1942) • First nuclear reactor • University of Chicago
Nuclear Fission • Nuclear reactors • Moderator • Slows down neutrons • Most effective = same mass as neutrons • Deuterium in heavy water (21H) and graphite (126C) • Absorption of neutrons • 11H absorbs neutrons to produce 21H • 23992U absorbs neutrons to produce 23992U • 99.3% of natural uranium is 23892U • Enriched: increase the percentage of 23592U • Escaped neutrons • Leave reactor before causing fission • Critical mass: minimum mass necessary for a self-sustaining chain reaction • Depends on: type of fuel, moderator, and percent enrichment
Nuclear Fission • Multiplication factor (f) • Number of neutrons that cause subsequent fissions • If f ≥ 1: self-sustaining reaction takes place • If f < 1: reactor is subcritical • If f > 1: reaction is supercritical and the reaction must be slowed • Meltdown • Control rods absorb neutrons and slow the reaction • Cadmium or boron
Nuclear Fission • Types of reactors • Research • Produces neutrons to produce other nuclides not found in nature • Power reactors • Generate electrical power (i.e., TMI) • 23592U: enriched to about 2% to 4% • Heat from reactor boils water • Steam turns a turbine to produce electricity
Nuclear Fission • Power reactors • Problems • Thermal pollution • Increases temperature of environment • Common to all but hydroelectric power plants • Disposal of fission fragments • Fission fragments are unstable: radioactive decay • Accidental release causes health hazards • Reactors have limited lives due to structural weakening from fission process • High cost to decommision
Nuclear Fission • Breeder reactor • Converts 23892U to 23994Pu • 23994Pu is also highly fissionable • Low critical mass • Easy to separate • Good material for weapons
Example • Estimate the minimum amount of 23592U that needs to undergo fission in order to run a 1000 MW power reactor per year of continuous operation. Assume an efficiency of about 33%.
Nuclear Fission • Atomic bomb • German threat? • Controversy over development and employment • Uranium bomb: “Little Boy” dropped on Hiroshima • Plutonium bomb: “Fat Man” dropped on Nagasaki
Nuclear Fusion • Combine two lighter nuclei to make a heavier nuclei • Loss in mass is converted to energy • Binding energy (lighter elements) < binding energy (heavier elements) • Process inside stars is fusion • Produces elements up to iron • After iron, fusion requires additional energy • Not self-sustaining
Example • One of the simplest fusion reactions involves the production of deuterium, 21H, from a neutron and a proton: 11H + n 21H + γ How much energy is released in this reaction?
Nuclear Fusion • Three fusion reactions • Source of sun’s energy • 11H + 11H 21H + e+ + ν (0.42 MeV) • 11H + 21H 32He + γ (5.49 MeV) • 32He + 32He 42He + 11H + 11H (12.86 MeV) • Net effect: known as proton – proton cycle • 411H 42He +2e+ + 2ν + 2γ • Takes 2 of the 1st reaction to produce the two 32He for the 3rd reaction • Total energy • 24.7 MeV for the reactions • The 2 e+ annihilate with e- to produce 2mec2 = 1.02 MeV • Total = 24.7 MeV + 2(1.02 MeV) = 26.7 MeV
Nuclear Fusion • Hotter stars • Carbon cycle provides most of the energy • 126C + 11H 137N + γ • 137N 136C + e+ + ν • 136C + 11H 147N + γ • 147N + 11H 158O + γ • 158O 157N + e+ + ν • 157N + 11H 126C + 42He
Nuclear Fusion • Fusion reactors • Large amount of energy from small amount of fuel • Problems • Cannot develop intense pressure and temperatures from gravity of stars • Reaction of 2 hydrogen forming one deuterium has a low probability • Cannot gather enough material to make it happen • Use deuterium and tritium • 21H + 21H 31H + 11H (4.03 MeV) • 21H + 21H 32He + n (3.27 MeV) • 21H + 31H 42He + n (17.59 MeV)
Nuclear Fusion • Plenty of deuterium • 1 gram per 60 liters of salt water • Problems • Must get close enough so the strong nuclear force overcomes the electrostatic repulsion • Takes high amount of energy to get nuclei close • High temperatures: 2.4E8 Kelvin • Not easy to produce and control high temperatures • High temperatures found in fission explosion, but difficult to produce in a lab
Radiation Damage • Charged particles • Alpha particles, beta particles, and protons • Capable of ionization • Electric force removes electron from atom • Takes about 10 eV to remove an electron from outer shell • Alpha and beta particles • Contains around 1 MeV of energy • Can ionize thousands of particles
Radiation Damage • Neutral particles • Gamma rays and x-rays • Can ionize through photoelectric effect or Compton effect • Given enough energy, a gamma ray can undergo a pair production • Produces electron and positron which are free to damage more atoms • Neutron • Generally interact only with nuclei • Broken nuclei can become charged causing ionization
Radiation Damage • Biological tissue • Main damage is due to ionization • Ions produced in cells • Can be highly reactive • Interfere chemically with normal processes of cell • Ionizing radiation may remove a bonding electron • Break apart molecule or alter its structure • Molecule may not be able to perform normally • Proteins • Destruction or change in one molecule is not serious if there are other copies of the protein in the cell • Large doses of radiation may damage many molecules of the protein so additional copies cannot be made: the cell dies • Damage to DNA • Serious • May damage a gene so that needed proteins or other materials may not be made • Amount of damage • Damage to single cells are not as serious • If many cells die, the organism may not be able to recover
Radiation Damage • Two types of damage to biological tissues • Somatic • Any part of the organism other than reproductive organs • Cancers are common results • High doses result in radiation sickness or death • Nausea, fatigue, loss of body hair • Genetic • Affects the reproductive cells • Causes mutations • Transmitted to further generations
Measurement of Radiation: Dosimetry • Radiation is used in medicine • Need to measure potential damage • Dose: given amount of radiation • Source activity • Amount of disintegrations occurring per second • Curie (Ci) • 1 Ci = 3.70E10 disintegrations/second • Becquerel (Bq) • 1 Bq = 1 disintegration/second • Medical suppliers • Specify activity at a certain time • Activity decreases over time • ΔN/Δt = λN = (0.693/T½)N
Example • In a certain experiment, 0.016 μCi of 3215P is injected into a medium containing a culture of bacteria. After 1 hour the cells are washed and a detector that is 70% efficient (counts 70% of emitted beta rays) records 720 counts per minute from all the cells. What percentage of the original 3215P was taken up by the cells?
Measurement of Radiation: Dosimetry • Absorbed Dose • Measure of the effect radiation has on the absorbing material (tissue) • Roentgen (R) • Based on ionization produced by the radiation • 1 R = amount of gamma or x-ray radiation that deposits 0.878E-2 J of energy per kilogram of air • rad • Used for radiation of any type, not just air • 1 rad = amount of radiation that deposits 1E-2 J/kg in any medium (i.e., tissue) • Gray (Gy) • 1 Gy = 1 J/kg = 100 rad
Measurement of Radiation: Dosimetry • On what does the absorbed dose depend? • Strength of the radiation (number of particles per second) • Energy per particle • Material that is absorbing the radiation • Bone absorbs more radiation than skin because it is denser
Measurement of Radiation: Dosimetry • Alpha particles • Cause 10 to 20 times more damage than beta or gamma radiation • Move slower (heavier) • Ionization collisions are closer together • Relative Biological Effectiveness (RBE) • Also called Quality Factor (QF) • Number of rads of gamma or x-rays that produce the same damage as 1 rad of the source being studied • Effective Dose • Effective dose (in rem) = dose (in rad) * QF • rem = rad equivalent man • Effective dose (Sv) = dose (Gy) * QF • Sv = sievert
Measurement of Radiation: Dosimetry • Background radiation • Radiation present around us • Natural radioactivity in rocks, soil, and radioactive isotopes in our food • Radon gas is emitted from rock and concrete • Radon is inert chemically but not physically • Decays by alpha emission • Products after decay can attach to interior of lung • ≈ 0.30 rem/year • Depends on location and occupation
Measurement of Radiation: Dosimetry • Government limits • Suggested upper limit ≈ 0.1 rem/year (excluding background radiation) • People who work around radiation • Hospitals, power plants, research • Suggested upper limit ≈ 5 rem/year (below 2 rem/year averaged over 5 years) • Radiation film badges and thermoluminescent dosimeters (TLD) • Worn or carried by people working around radiation • Monitors the amount of radiation received
Measurement of Radiation: Dosimetry • Consequences • Low dose radiation has unknown consequences • 400 rem dose over several weeks is not usually fatal but will cause considerable damage • Short dose of 400 rem is fatal in 50% of cases • Short dose of 1000 rem is fatal • Ingesting or breathing in radiation causes greatest amount of harm
Example • What whole-body dose is received by a 70 kg laboratory worker exposed to a 40 mCi6027Co source, assuming the person’s body has a cross-sectional area of 1.5 m2 and is normally about 4 m from the source for 4 hours per day? 6027Co emits γ rays of energy 1.33 MeV and 1.17 MeV in quick succession. Approximately 50% of the γ rays interact in the body and deposit all their energy. (The rest pass through.)