* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 30
Introduction to quantum mechanics wikipedia , lookup
Mathematical formulation of the Standard Model wikipedia , lookup
Relativistic quantum mechanics wikipedia , lookup
Electric charge wikipedia , lookup
Double-slit experiment wikipedia , lookup
Quantum chromodynamics wikipedia , lookup
Future Circular Collider wikipedia , lookup
Weakly-interacting massive particles wikipedia , lookup
Grand Unified Theory wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
ALICE experiment wikipedia , lookup
Strangeness production wikipedia , lookup
Identical particles wikipedia , lookup
Nuclear force wikipedia , lookup
ATLAS experiment wikipedia , lookup
Nuclear structure wikipedia , lookup
Electron scattering wikipedia , lookup
Standard Model wikipedia , lookup
Compact Muon Solenoid wikipedia , lookup
Chapter 30: Radioactivity By: PF Wesley Herbie Affel Summers History of Radioactivity • Antoine Henri Becquerel, a French physicist, discovered radioactivity in 1896. • Canadians Ernest Rutherford and Frederick Soddy studied uranium atoms and found that they changed to other atoms. • As a result of this observation, polonium and radium were discovered as new elements by Marie and Pierre Curie. • The study of radioactivity allowed for a better understanding of the nucleus in an atom. The Nucleus • Rutherford determined through his experiments that the number of deflected α particles is proportional to the square of the charge. • Based on this, he further discovered the numbers of electrons that different types of atoms contained. The Nucleus (Cont.) • Since the atom is neutral, the nucleus must also posses positive charge to balance the negative electrons. • The charge of a proton is equal to that of an electron (an elementary charge), but it is a positive charge. • A proton’s mass is approximately equal to one atomic mass unit (u). The number of protons is the atomic number of the atom (Z). The Nucleus (Cont.) • Not all of the mass of an atom was compensated for, so Rutherford determined that there must be another component. • Through this idea the neutron was discovered, which has no charge, but makes up the rest of the mass. • The mass number (A), therefore is the sum of the protons and neutrons in an atom. • Rutherford understood that the nucleus was very small. For example, the diameter for a hydrogen atom is now accepted to be about 2.6 x 10-15 m. Isotopes • The puzzle of atomic masses that were not integral numbers of atomic mass units was solved with the mass spectrometer. • Mass spectrometer demonstrated that an element could have atoms with different masses. • Example: when analyzing a pure sample of neon, two spots appeared on the film of the spectrometer, which were produced by neon atoms with different masses. • Different forms of an atom are called isotopes. • The nucleus of an isotope is called a nuclide. • All isotopes of an atom have the same number of protons and electrons, but different numbers of neutrons. • Many isotopes form naturally • There is special notation used to describe an isotope. • A subscript representing the atomic number, Z, is written in the lower left of the symbol for the element. • A superscript written to the upper left of the symbol is the mass number, A. • This notation takes the form Az X • For example, two isotopes of neon with atomic 22 number 10 are written 10Ne and 20 10Ne Radioactive Decay • In 1896, Henri Becquerel discovered that uranium samples would “fog” photographic plates. • This suggested that some type of ray was passing through the plates. This is called radiation. • Several other materials had this same effect; these materials are called radioactive and therefore undergo radioactive decay. Types of Radioactive Decay • In 1899, Rutherford discovered that Uranium compounds have three types of radiation. • These are separated by their penetrating ability, α (alpha) being the weakest, followed by β (beta) and finally γ (gamma), the strongest. α Alpha Decay • An Alpha particle is the nucleus of a Helium atom, 24He • This particle contains two protons and two neutrons, and can be stopped by a simple piece of paper. • This particle comes from within the nucleus of the decaying material • The mass number of the original nucleus is therefore decreased by four, while the charge decreases by two β Beta Decay • A beta particle is simply a high speed ejected electron, -10e • Beta decay can be stopped with about six millimeters of aluminum • The charge of the original atom will increase by one, since it is losing a negative charge. γ Gamma Decay • A Gamma particle is a high energy photon. • This particle has negligible mass and no charge: 00 γ • It requires several centimeters of lead to stop gamma radiation. • The original atom does not change mass or charge. Nuclear Reactions and Equations • A nuclear reaction occurs whenever the number of neutrons protons in a nucleus changes. • Just as in chemical reactions, some nuclear reactions occur with a release of energy in the form of the kinetic energy of the released particles. • Others occur only when energy is added to the nucleus. • Nuclear reactions can be expressed in words. • The word equation for the change of uranium to thorium due to α decay: • Uranium 238 yields Thorium 234 plus one α particle. • Nuclear reactions are more easily expressed as equations. • The same reaction expressed is equation form: 234Th+ 4He • 238 U 90 2 92 • No nuclear particles are destroyed during the reaction, thus the sums of the superscripts are equal on both sides, as well as the sums of the subscripts. Half-Life The time required for half of the atoms in any given quantity of a radioactive isotope to decay is the Half-Life of that element. Each particular isotope has its own half-life Half-Life of Selected Isotopes Activity The decay rate, or number of decays per second, of a radioactive substance is called it’s activity. Activity is proportional to the number of radioactive atoms present. Therefore, the activity of a particular sample is also reduced by one half in one half-life. 131 Consider 53 I with a half-life of 8.07 days. If the activity of a certain sample is 8x10^5 decays per second when the 131 53 I is produced, 8.07 days later its activity will be 4x10^5 decays per second. After another 8.07 days, its activity will be 2x10^5 decays per second. The SI unit for decays per second is a Bequerel, Bq. Nuclear Bombardment • Rutherford bombarded many elements with α particles, using them to cause a nuclear reaction. • For example, when nitrogen gas was bombarded, he noted that high energy protons were emitted from the gas. • A proton has a charge of 1, while an α particle has a charge of 2. • Rutherford hypothesized that the nitrogen had been artificially transmuted by the α particles. • The unknown results of the transmutation A can be written ZX, and the nuclear reaction can be written…. 14 4 A 1 2He + 7N 1H + ZX • Simple arithmetic shows that the unknown isotopes number is Z = 2 + 7 - 1= 8 and the mass number is A = 4 + 14 - 1= 17 • By looking at the appendix D-5 in the 17 book, we find that the isotope must be 8O • Bombarding Be with α particles produced a radiation more penetrating that any previously discovered. • In 1932, Irene Curie and her husband, Frederic Joliot, discovered that high speed protons expelled from paraffin wax that was exposed to this new radiation from beryllium. 9 4 • That same year, James Chadwick showed that the particles emitted from the beryllium were uncharged, but had approximately the same mass as protons. • In other words, the beryllium emitted the particle Rutherford had theorized must be in the nucleus, the neutron. • The reaction can be written using 1 the symbol for the neutron, 0n 4 1 12 9 2He + 4Be 6C + 0n • Since neutrons are uncharged and are not repelled by the nucleus, neutrons are often used to bombard nuclei • Alpha particles from radioactive materials have fixed energies • Also, sources that emit large numbers of particles per second are hard to produce • Because of this, methods of artificially accelerating particles to higher energies are needed… Linear Accelerators • A linear accelerator consists of a series of hollow tubes within a long evacuated chamber. • The tubes are connected to a source of high frequency alternating voltage. • There is no electric field within the tube, so a proton or electron can move at a constant velocity, inside the tube • When the first tube has a negative potential, protons are accelerated into it. • When the protons is at the end of one tube, the potential of the second tube is negative with respect to the first tube. • This accelerates the proton into the second tube. • This continues, and the proton keeps accelerating, gaining 105 eV every time. • Linear accelerators can be used with both protons and electrons • The largest linear accelerator is at Stanford University in California. It’s 3.3 km long and accelerates electrons to energies of 20 GeV (1x1010 eV) The Synchrotron • A smaller accelerator by bending the path for the particles into a circle • The bending magnets are separated by accelerating regions • In the straight regions, high frequency alternating voltage accelerates the particles. • The strength of the magnetic field and the length of path are chosen so that the particles reach the location of the alternating electric field exactly when the field’s polarity will accelerate them. • One of the largest synchrotrons is at the Fermi National Accelerator lab near chicago, where protons there can reach energies of 1 TeV (1x1012eV) Particle Detectors Photographic films become “fogged,” or exposed when α particles, β particles, or y rays strike them. Thus, photographic film can be used to detect these particles and rays. Many other devices are used to detect charged particles and rays. Most of these devices make use of the fact that a collision with a high speed particle will remove electrons from atoms. That is, the high speed particles ionize the matter that they bombard. In addition, some substances fluoresce when exposed to certain types of radiation. Thus, fluorescent substances can be used to detect radiation. Types of detectors In the Geiger-Mueller tube, particles ionize gas atoms. The tube contains a gas at low pressure (10 kPa). At one end of the tube is a very thin “window” through which charged particles or gamma rays pass. Modern experiments use spark chambers that are like giant Geiger-mueller tubes. Plates several meters in size are separated by a few centimeters. The gap is filled with a low-pressure gas. A discharge is produced in the path of a particle passing through the chamber. A computer records the discharge which is later used for analysis. Fundamental Particles • The atom was thought to be the smallest particle into which matter could be divided. • Rutherford then discovered that the atom was a nucleus surrounded by electrons. • Once protons were discovered, they too were thought to be indivisible. • Experiments involving the bombardment of protons by other protons or neutrons showed that these particles were also made up of even smaller particles. Quarks and Leptons • Protons and neutrons are composed of Quarks. • Leptons are particles like electrons and neutrinos. • Other particles carry or transmit forces between particles. Other Subatomic Particles • Photons carry electromagnetic forces. • There are eight types of gluons, which carry the strong forces that bind quarks into protons and neutrons and that bind the nucleus together. • Three types of weak bosons, which carry a weak force involved in beta decay. • The gravitron is the particle responsible for causing gravity. Antiparticles (Antimatter) • For every particle, there is an identical antiparticle. • These antiparticles differ from their matching particles only in charge. • When a particle and its matching antiparticle collide, they annihilate each other and are transformed into photons (particle-antiparticle pairs) and a massive amount of energy • Antimatter rockets and bombs are theoretical designs that would yield incredibly powerful results (1Kg of matter and 1Kg of antimatter=47 Megatons of TNT) Particles and Antiparticles • α particles and γ rays emitted by radioactive nuclei have single energies that depend on the decaying nucleus. • β particles, however, are emitted with a wide range of energies. • Niels Bohr suggested that this would mean that nuclear reactions did not follow the law of conservation of energy. • Wolfgang Pauli in 1931 and Enrico Fermi in 1934 suggested instead that an unseen particle was emitted along with the β particle. • Fermi called it the neutrino. However, it was actually the antineutrino that was emitted. This was first observed in 1956. • Neutrons in an unstable nucleus decay by emitting a β particle and an antineutrino. • Antineutrinos have no charge and zero mass, but like a photon carry momentum and energy. • A proton in an unstable nucleus decays into a neutron by emitting a positron and a neutrino. • Positrons are positive electrons. • The energy equivalent of the positron and electron can be calculated with E=mc2. E = 2(9.11x10-31 kg)(3.00x108 m/s)2 = (1.64x10-13 J)(1 eV/1.60x10-19 J) = 1.02x106 eV or 1.02 MeV • Just as there are positive electrons, there are also negative protons called antiprotons. The pair was first created in the lab by Berkeley in 1955. • Antiparticles are antimatter. When they collide with their matter counterparts, they annihilate each other and release energy in γ rays. • Just as matter can be converted to energy, energy can be converted to matter. For example, if a γ ray with at least 1.02 MeV energy passes close to a nucleus, a positron and electron pair can be produced. The pair must always be created together and cannot be created individually. The Quark Model of the Nucleons • The Quark model describes the structure of the proton and neutron. • Each nucleon is divided into three quarks. • There are both up quarks and down quarks. • An up quark has a positive 2/3 e value. • A down quark has a negative 1/3 e value. The Quark Model of the Nucleons (Cont.) • The proton has two up quarks (2/3 e*2=4/3) and one down quark (-1/3 e) for a balanced charge of +1 e. • The notation to describe this is p=(uud). • The neutron has one up quark and two down quarks. • The charge therefore is 2/3 – 1/3 – 1/3, or zero. • The notation for this is n=(udd). The Quark Model of the Nucleons (Cont.) • In the Quark Model the force that holds the quarks together is a result of gluons which are emitted and absorbed. • The farther away the quarks become the greater the force, unlike electric force. • Weak interaction is caused by three forces: W+, W-, Z0 bosons. • Weak interaction is witnessed through beta decay.