Chapter 29 Nuclear Physics
Ernest Rutherford 1871 1937 Discovery that atoms could be broken apart Studied radioactivity Nobel prize in 1908
Some Properties of Nuclei All nuclei are composed of protons and neutrons Exception is ordinary hydrogen with just a proton The atomic number, Z, equals the number of protons in the nucleus The neutron number, N, is the number of neutrons in the nucleus The mass number, A, is the number of nucleons in the nucleus A = Z + N Nucleon is a generic term used to refer to either a proton or a neutron The mass number is not the same as the mass
Symbolism Symbol: X is the chemical symbol of the element Example: Mass number is 27 27 13 Al A Z X Atomic number is 13 Contains 13 protons Contains 14 (27 13) neutrons The Z may be omitted since the element can be used to determine Z
More Properties The nuclei of all atoms of a particular element must contain the same number of protons They may contain varying numbers of neutrons Isotopes of an element have the same Z but differing N and A values Example: 11 C 12 C 13 C 14 C 6 6 6 6
Charge The proton has a single positive charge, +e The electron has a single negative charge, -e The neutron has no charge Makes it difficult to detect e = 1.602 177 33 x 10-19 C
Mass It is convenient to use unified mass units, u, to express masses 1 u = 1.660 559 x 10-27 kg Based on definition that the mass of one atom of C-12 is exactly 12 u Mass can also be expressed in MeV/c 2 From E R = m c 2 1 u = 931.494 MeV/c 2
Summary of Masses
The Size of the Nucleus First investigated by Rutherford in scattering experiments He found an expression for how close an alpha particle moving toward the nucleus can come before being turned around by the Coulomb force The KE of the particle must be completely converted to PE
Size of the Nucleus, cont d gives an upper limit for the size of the nucleus Rutherford determined that d 4 e k Ze mv 2 2 For gold, he found d = 3.2 x 10-14 m For silver, he found d = 2 x 10-14 m Such small lengths are often expressed in femtometers where 1 fm = 10-15 m Also called a fermi
Size of Nucleus, Current Since the time of Rutherford, many other experiments have concluded: Most nuclei are approximately spherical Average radius is r r A o 1 3 r o = 1.2 x 10-15 m A is the number of nucleons
Density of Nuclei The volume of the nucleus (assumed to be spherical) is directly proportional to the total number of nucleons This suggests that all nuclei have nearly the same density Nucleons combine to form a nucleus as though they were tightly packed spheres
Maria Goeppert-Mayer 1906 1972 Best known for her development of shell model of the nucleus Shared Nobel Prize in 1963
Nuclear Stability There are very large repulsive electrostatic forces between protons These forces should cause the nucleus to fly apart The nuclei are stable because of the presence of another, short-range force, called the nuclear force This is an attractive force that acts between all nuclear particles The nuclear attractive force is stronger than the Coulomb repulsive force at the short ranges within the nucleus
Nuclear Stability, cont Light nuclei are most stable if N = Z Heavy nuclei are most stable when N > Z As the number of protons increase, the Coulomb force increases and so more nucleons are needed to keep the nucleus stable No nuclei are stable when Z > 83
Binding Energy The total energy of the bound system (the nucleus) is less than the combined energy of the separated nucleons This difference in energy is called the binding energy of the nucleus It can be thought of as the amount of energy you need to add to the nucleus to break it apart into separated protons and neutrons
Binding Energy per Nucleon
Binding Energy Notes Except for light nuclei, the binding energy is about 8 MeV per nucleon The curve peaks in the vicinity of A = 60 Nuclei with mass numbers greater than or less than 60 are not as strongly bound as those near the middle of the periodic table The curve is slowly varying at A > 40 This suggests that the nuclear force saturates A particular nucleon can interact with only a limited number of other nucleons
Marie Curie 1867 1934 Discovered new radioactive elements Shared Nobel Prize in physics in 1903 Nobel Prize in Chemistry in 1911
Radioactivity Radioactivity is the spontaneous emission of radiation Experiments suggested that radioactivity was the result of the decay, or disintegration, of unstable nuclei
Radioactivity Types Three types of radiation can be emitted Alpha particles The particles are 4 He nuclei Beta particles The particles are either electrons or positrons A positron is the antiparticle of the electron It is similar to the electron except its charge is +e Gamma rays The rays are high energy photons
Distinguishing Types of Radiation A radioactive beam is directed into a region with a magnetic field The gamma particles carry no charge and they are not deflected The alpha particles are deflected upward The beta particles are deflected downward A positron would be deflected upward
Penetrating Ability of Particles Alpha particles Barely penetrate a piece of paper Beta particles Can penetrate a few mm of aluminum Gamma rays Can penetrate several cm of lead
The Decay Constant The number of particles that decay in a given time is proportional to the total number of particles in a radioactive sample ΔN = -λ N Δt λ is called the decay constant and determines the rate at which the material will decay The decay rate or activity, R, of a sample is defined as the number of decays per second N R N t
Decay Curve The decay curve follows the equation N = N o e - λt The half-life is also a useful parameter The half-life is defined as the time it takes for half of any given number of radioactive nuclei to decay T 12 ln 2 0.693
Units The unit of activity, R, is the Curie, Ci 1 Ci = 3.7 x 10 10 decays/second The SI unit of activity is the Becquerel, Bq 1 Bq = 1 decay / second Therefore, 1 Ci = 3.7 x 10 10 Bq The most commonly used units of activity are the mci and the µci
Alpha Decay When a nucleus emits an alpha particle it loses two protons and two neutrons N decreases by 2 Z decreases by 2 A decreases by 4 Symbolically A Z X A Z Y He X is called the parent nucleus Y is called the daughter nucleus 4 2 4 2
Alpha Decay Example Decay of 226 Ra 226 88 Ra 222 86 Rn He Half life for this decay is 1600 years Excess mass is converted into kinetic energy Momentum of the two particles is equal and opposite 4 2
Decay General Rules When one element changes into another element, the process is called spontaneous decay or transmutation The sum of the mass numbers, A, must be the same on both sides of the equation The sum of the atomic numbers, Z, must be the same on both sides of the equation Conservation of mass-energy and conservation of momentum must hold
Beta Decay During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is changed by one Symbolically A Z A Z X X A Z 1 A Z 1 Y Y e e
Beta Decay, cont The emission of the electron is from the nucleus The nucleus contains protons and neutrons The process occurs when a neutron is transformed into a proton and an electron Energy must be conserved
Beta Decay Electron Energy The energy released in the decay process should almost all go to kinetic energy of the electron (KE max ) Experiments showed that few electrons had this amount of kinetic energy
Neutrino To account for this missing energy, in 1930 Pauli proposed the existence of another particle Enrico Fermi later named this particle the neutrino Properties of the neutrino Zero electrical charge Mass much smaller than the electron, recent experiments indicate definitely some mass Spin of ½ Very weak interaction with matter
Beta Decay Completed Symbolically A Z A Z X X A Z 1 A Z 1 Y Y e e is the symbol for the neutrino is the symbol for the antineutrino To summarize, in beta decay, the following pairs of particles are emitted An electron and an antineutrino A positron and a neutrino
Gamma Decay Gamma rays are given off when an excited nucleus falls to a lower energy state Similar to the process of electron jumps to lower energy states and giving off photons The photons are called gamma rays, very high energy relative to light The excited nuclear states result from jumps made by a proton or neutron The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission
Gamma Decay Example Example of a decay sequence The first decay is a beta emission The second step is a gamma emission 12 B C * e 5 12 6 C* 12 6 12 6 C The C* indicates the Carbon nucleus is in an excited state Gamma emission doesn t change either A or Z
Enrico Fermi 1901 1954 Produced transuranic elements Other contributions Theory of beta decay Free-electron theory of metals World s first fission reactor (1942) Nobel Prize in 1938
Uses of Radioactivity Carbon Dating Beta decay of 14 C is used to date organic samples The ratio of 14 C to 12 C is used Smoke detectors Ionization-type smoke detectors use a radioactive source to ionize the air in a chamber A voltage and current are maintained When smoke enters the chamber, the current is decreased and the alarm sounds
More Uses of Radioactivity Radon detection Radon is an inert, gaseous element associated with the decay of radium It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building May also come from the ground itself
Natural Radioactivity Classification of nuclei Unstable nuclei found in nature Give rise to natural radioactivity Nuclei produced in the laboratory through nuclear reactions Exhibit artificial radioactivity Three series of natural radioactivity exist Uranium Actinium Thorium See table 29.2
Decay Series of 232 Th Series starts with 232 Th Processes through a series of alpha and beta decays Ends with a stable isotope of lead, 208 Pb
Nuclear Reactions Structure of nuclei can be changed by bombarding them with energetic particles The changes are called nuclear reactions As with nuclear decays, the atomic numbers and mass numbers must balance on both sides of the equation
Nuclear Reactions Example Alpha particle colliding with nitrogen: He N X H 4 14 1 2 7 1 Balancing the equation allows for the identification of X X 17 X 17 O 8 8 So the reaction is He N O H 4 14 17 1 2 7 8 1
Q Values Energy must also be conserved in nuclear reactions The energy required to balance a nuclear reaction is called the Q value of the reaction An exothermic reaction There is a mass loss in the reaction There is a release of energy Q is positive An endothermic reaction There is a gain of mass in the reaction Energy is needed, in the form of kinetic energy of the incoming particles Q is negative
Threshold Energy To conserve both momentum and energy, incoming particles must have a minimum amount of kinetic energy, called the threshold energy KEmin 1 m Q M m is the mass of the incoming particle M is the mass of the target particle If the energy is less than this amount, the reaction cannot occur
Radiation Damage in Matter Radiation absorbed by matter can cause damage The degree and type of damage depend on many factors Type and energy of the radiation Properties of the absorbing matter Radiation damage in biological organisms is primarily due to ionization effects in cells Ionization disrupts the normal functioning of the cell
Types of Damage Somatic damage is radiation damage to any cells except reproductive ones Can lead to cancer at high radiation levels Can seriously alter the characteristics of specific organisms Genetic damage affects only reproductive cells Can lead to defective offspring
Units of Radiation Exposure Roentgen [R] That amount of ionizing radiation that will produce 2.08 x 10 9 ion pairs in 1 cm 3 of air under standard conditions That amount of radiation that deposits 8.76 x 10-3 J of energy into 1 kg of air Rad (Radiation Absorbed Dose) That amount of radiation that deposits 10-2 J of energy into 1 kg of absorbing material
More Units RBE (Relative Biological Effectiveness) The number of rad of x-radiation or gamma radiation that produces the same biological damage as 1 rad of the radiation being used Accounts for type of particle which the rad itself does not Rem (Roentgen Equivalent in Man) Defined as the product of the dose in rad and the RBE factor Dose in rem = dose in rad X RBE
Radiation Levels Natural sources rocks and soil, cosmic rays Background radiation About 0.13 rem/yr Upper limit suggested by US government 0.50 rem/yr Excludes background and medical exposures Occupational 5 rem/yr for whole-body radiation Certain body parts can withstand higher levels Ingestion or inhalation is most dangerous
Applications of Radiation Sterilization Radiation has been used to sterilize medical equipment Used to destroy bacteria, worms and insects in food Bone, cartilage, and skin used in graphs is often irradiated before grafting to reduce the chances of infection
Applications of Radiation, cont Tracing Radioactive particles can be used to trace chemicals participating in various reactions Example, 131 I to test thyroid action
Applications of Radiation, final MRI Magnetic Resonance Imaging When a nucleus having a magnetic moment is placed in an external magnetic field, its moment processes about the magnetic field with a frequency that is proportional to the field Transitions between energy states can be detected electronically