Nuclear forces and Radioactivity Two forces are at work inside the nucleus of an atom
Forces act in opposing directions Electrostatic repulsion: pushes protons apart Strong nuclear force: pulls protons together Nuclear force is much shorter range: protons must be close together
Neutrons only experience the strong nuclear force Proton pair experiences both forces Neutrons experience only the strong nuclear force But: neutrons alone are unstable
Neutrons act like nuclear glue Helium nucleus contains 2 protons and 2 neutrons increase attractive forces Overall nucleus is stable
As nuclear size increases, electrostatic repulsion builds up There are electrostatic repulsions between protons that don t have attractive forces Long range repulsive force with no compensation from attraction More neutrons required
Neutron to proton ratio increases with atomic number Upper limit of stability
238 92 U 234 90 Th 4 2 He Upper limit to nuclear stability Beyond atomic number 83, all nuclei are unstable and decay via radioactivity Radioactive decay (Transmutation) formation of new element Mass number 238 U 234 Th 4 He 92 90 2 0 0 Atomic number Atomic number decreases Alpha particle emitted Gamma photon
Beta particle emission Neutron is converted into a proton + electron Proton stays in nucleus Electron is emitted (beta particle) 0 1 e
Atomic number increases with beta emission Here atomic number actually increases, but serves to reduce the neutron:proton ratio 234 Th 234 Pa 0 e 90 91 1 0 0 Beta particle emitted Beta particle emission occurs with neutron-excess nuclei Alpha particle emission occurs with proton-heavy nuclei
Positrons and antimatter Protons are converted to neutron and positively charged electron (positron) Neutron stays in nucleus Positron emitted 0 1 e Positron is antimatter and is annihilated by 0 0 electron: e e 1 1 0 0
Summary of nuclear processes Alpha emission: Mass number and atomic number decrease Beta emission: Mass number same, atomic number increases Positron emission: Mass number same, atomic number decreases Gamma ray emission: Mass number and atomic number same
Analyzing nuclei: filling in the blanks Mass number = protons + neutrons Atomic number = protons Element identity = atomic number
Writing nuclear equations Rules for balancing nuclear equations: 1. Conserve mass number (protons + neutrons) 2. Conserve atomic number (nuclear charge) 234 Th 234 Pa 90 91? 234 Th? 90 0 1 e
234 90 Th 234 91 Pa?? X Mass number sum: 234 = 234 +?? = 0 Atomic number sum: 90 = 91 +?? = -1 Particle is 0 1 e
234 90 Th?? X 0 1 e Mass number sum: Atomic number sum: Nucleus is 234 =? + 0? = 234 90 =? - 1 234 91? = 91 Pa
Worked examples 226 Ra? 88 4 2 He 60 Co? 27 0 1 e
Creation of radioisotopes Isotopes are created by bombarding nuclei with smaller particles Neutrons Protons Alpha particles Other nuclei 98 42 66 30 10 5 Mo Zn B 249 4 2 1 1 1 0 H He 15 n 99 42 67 31 13 7 Mo Ga N 260 1 0 n Cf N N 4 98 7 105 1 0 n
Radioactive decay occurs in series of steps The decay series from uranium-238 to lead-206. Each nuclide except for the last is radioactive and undergoes nuclear decay. The left-pointing, longer arrows (red) represent alpha emissions, and the right-pointing, shorter arrows (blue) represent beta emissions.
Half-life measures rate of decay Concentration of nuclide is halved after the same time interval regardless of the initial amount Half-life Can range from fractions of a second to millions of years
Half-life calculations 131 I decays to 131 Xe with a half-life of 8 days How much remains after 40 days if there are 10 grams initially?
The Dating Game Carbon-14 is produced in the upper atmosphere by the bombardment of nitrogen atoms with neutrons: 14 7 N + 1 0 n 14 6 C + 1 1 H Radioactive 14 CO 2 is produced, which mixes with ordinary 12 CO 2 and is taken up by plants during photosynthesis.
Carbon Dating During an organism s life, 14 CO 2 and 12 CO 2 are in equilibrium at a ratio of 1:10 12. When organism dies, 14 C/ 12 C ratio decreases as 14 C undergoes decay to 14 N. Measuring 14 C/ 12 C ratio determines age of sample with high degree of certainty. Ages of 1000 20,000 years are commonly determined. The half-life for 14 C is 5730 years.
The age of the earth U-238 decays eventually to Pb-206 Since half-life of U-238 is so long (4.5 billion years), the atom of Pb-206 appears almost instantly after its decay If the mineral was once pure U-238, after some billions of years it becomes a mixture of U and Pb Measuring the ratio of Pb:U gives us the age of the rock Note that the U-238 half-life is of the order of the age of the earth. If the earth was 6,000 years old or 50 billion years old it would not work
Other nuclear processes: Attempts to grow larger nuclei by bombardment with neutrons yielded smaller atoms instead. Distorting the nucleus causes the repulsive forces to overwhelm the attractive The foundation of nuclear energy and the atomic bomb fission and fusion
Inter-changeability of mass and energy When a radioactive nucleus divides to give two smaller ones, the combined mass of them is lower A B + C Loss in mass equals energy given out E = mc 2 M A > M B + M C Tiny amount of matter produces masses of energy: 1 gram 10 14 J In chemical process 1 gram may produce 10 3 J (10 11 less) Energy and mass are conserved, but can be inter-changed
Nuclear fission Nuclear fission produces nuclei with lower nucleon mass 1 235 91 142 n U Kr Ba 3 0 92 36 56 1 0 n One neutron produces three: the basis for a chain reaction explosive potential
Chain reactions require rapid multiplication of species
Nuclear fusion Small nuclei fuse to yield larger ones losing nucleon mass + E Example is the deuterium tritium reaction High energy output Clean products no long-lived radioactive waste or toxic heavy metals Problem is providing enough energy to initiate the process
Biological Effects of Radiation The penetrating power of radiation is a function of its mass: -rays > -particles >> -particles. When ionizing radiation passes through tissue it removes an electron from water to form H 2 O + ions. The H 2 O + ions react with another water molecule to produce H 3 O + and a highly reactive OH radical. Free radicals generally undergo chain reactions, producing many radicals in the biomolecules.
Biological Effects of Radiation 02 -rays are particularly harmful because they penetrate in the same way as X rays. -particles interact with the skin and -particles interact up to 1 cm into the tissue -particles are particularly dangerous when ingested or inhaled.
The Curie Different units for measuring Measure of amount of radioactivity The Roentgen (gamma and X- ray) Measure of interaction with air The Rad Radiation absorbed dose The Rem Measure of biological damage radiation Amount of material that produces 3.7x10 10 decays per second Amount of radiation needed to produce 2x10 10 ion pairs in air Dosage of radiation able to transfer 2.4x10-3 cal to one kg of matter Determined from rem and some factor which depends on the type radiation (1 for beta and 10 for alpha)
Biological Effects of Radiation Not all forms of radiation have the same efficiency for biological damage. To correct, the radiation dose is multiplied by the relative biological effectiveness (RBE), which gives the roentgen equivalent for man (rem). RBE is about 1 for - and - and 10 for radiation. SI unit for effective dosage is the Sievert (1 Sv = RBE x 1 Gy = 100 rem).
Biological Effects of Radiation
Sources of radiation