Chapter 29 Nuclear Physics. General Physics Nuclear Physics Sections 1–4.
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Transcript of Chapter 29 Nuclear Physics. General Physics Nuclear Physics Sections 1–4.
General Physics
Milestones in Nuclear Physics
1896 – the birth of nuclear physics Becquerel discovered radioactivity in uranium compounds
1911 Rutherford, Geiger and Marsden performed scattering experiments Established the point mass nature of the nucleus Nuclear force was a new type of force
1919 Rutherford and coworkers first observed nuclear reactions Naturally occurring alpha particles bombarded nitrogen
nuclei to produce oxygen
General Physics
HistoryHistory Becquerel – discovered radioactivity (1896) (from radiare = emit rays)Becquerel – discovered radioactivity (1896) (from radiare = emit rays) Curie – discovery of polonium and radium (1898) Curie – discovery of polonium and radium (1898) Rutherford – nuclear modelRutherford – nuclear model
classified classified ,,,, radiation, radiation, particle = particle = 44He nucleus (nuclear transmutation)He nucleus (nuclear transmutation) used used scattering to discover the nuclear model scattering to discover the nuclear model postulated ‘neutrons’ A=Z+N (1920); bound ppostulated ‘neutrons’ A=Z+N (1920); bound p+ + ee-- state? state?
Mosley – studied nucleus via X-ray spectraMosley – studied nucleus via X-ray spectra correlated (Z = charge of nucleus) with periodic tablecorrelated (Z = charge of nucleus) with periodic table extra particles in nucleus: A = Z + ?extra particles in nucleus: A = Z + ?
Chadwick – discovered neutron (1932)Chadwick – discovered neutron (1932) Pauli – postulated neutral particle from Pauli – postulated neutral particle from -decay (1930)-decay (1930) Fermi – theory or weak decay (1933) ‘neutrino’Fermi – theory or weak decay (1933) ‘neutrino’ Fission – Hahn, Strassmann, (&Meitner!) (1938)Fission – Hahn, Strassmann, (&Meitner!) (1938)
first reactor (chain reaction), Fermi (1942)first reactor (chain reaction), Fermi (1942) Bohr, Wheeler – liquid drop modelBohr, Wheeler – liquid drop model Mayer, Jensen – shell model (1949)Mayer, Jensen – shell model (1949) Hofstadter – electron scattering (1953-)Hofstadter – electron scattering (1953-)
measured the charge density of various nucleimeasured the charge density of various nuclei discovered structure in the proton (not point-like particle)discovered structure in the proton (not point-like particle)
General Physics
Some Properties of Nuclei All nuclei are composed of protons and
neutrons Exception is ordinary hydrogen
with just a proton Atomic number, Z
Number of protons in the nucleus Neutron number, N
Number of neutrons in the nucleus Mass number, A
Number of nucleons in the nucleus: A = Z + N Nucleon is a generic term used to refer to
either a proton or a neutron in the nucleus The mass number is not the same as the mass
General Physics
Symbolism
Symbol:
X is the chemical symbol of the element
Example: Mass number: A = 27 nucleons Atomic number: Z = 13 protons Neutron number: N = 27 – 13 = 14 neutrons
The Z may be omitted since the element can be used to determine Z
XAZ
Al2713
General Physics
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:
See Appendix B – An Abbreviated Table of Isotopes
CCCC 146
136
126
116
Radioactive Stable Stable Radioactive
General Physics
Charge and Mass The proton has a single positive charge, +e
e = 1.60217733 x 10-19 C The electron has a single negative charge, –e The neutron has no charge – difficult to detect unified mass unit: u
1 u = 1.660559 x 10-27 kg Based on definition that mass of one atom of 12C is
exactly 12 u E = m c2 1 u = 931.494 MeV/c2
General Physics
Summary of Charges & Masses
Charges Masses
Particle
C kg uMeV/c2
Proton+1.6022 x
10-19
1.6726 x 10-
27
1.007276
938.28
Neutron
01.6750 x 10-
27
1.008665
939.57
Electron
–1.6022 x 10-
19
9.109 x 10-
31
5.486x10-4 0.511
General Physics
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
General Physics
Binding Energy per Nucleon Except for light nuclei,
the binding energy is ~ 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
General Physics
Size and Density of Nuclei Since the time of Rutherford, many
other experiments have concluded: Most nuclei are approximately spherical
Average radius is
ro = 1.2 x 10-15 m or 1.2 fm
13
or r A
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
General Physics
N
Z
Nuclear decay modes:Nuclear decay modes: ++++ decay decay - - decay (isobar)decay (isobar) ++ decay (isobar) decay (isobar) electron capture (isobar)electron capture (isobar) p decay (isotone)p decay (isotone) n decay (isotope)n decay (isotope) decay (isomers)decay (isomers) electron conversion (EC)electron conversion (EC) spontaneous fission (SF)spontaneous fission (SF) double beta decay (2double beta decay (2)) neutrino-less double beta decay neutrino-less double beta decay
(0(0)) beta-delayed n,p,beta-delayed n,p, decay decay
ISOBARS
ISOTOPES
ISO
TO
NES
ISOMERS
General Physics
Chart of Nuclides – decay Chart of Nuclides – decay modemode
http://www.nndc.bnl.gov/chart
stable nuclide- decay, electron capturedecayp decayn decayspontaneous fission
magic numbers
General Physics
Chart of Nuclides – island of Chart of Nuclides – island of stabilitystability
http://en.wikipedia.org/wiki/Island_of_stability
magic numbers
General Physics
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
X is called the parent nucleus Y is called the daughter nucleus
HeYX 42
4A2Z
AZ
General Physics
Alpha Decay – Example
Decay of 226 Ra
Half life for this decay is 1600 years
Excess mass is converted into kinetic energy
Momentum of the two particles is equal and opposite
HeRnRa 42
22286
22688
Active Figure: Alpha Decay of Radium-226
General Physics
Beta Decay Beta– decay: a neutron is transformed into a proton,
and an electron and antineutrino are emitted A stays the same, Z Z+1
Beta+ decay: a proton is transformed into a neutron, and a positron and neutrino are emitted A stays the same, Z Z–1
Symbolically
Energy must be conserved is the symbol for the neutrino (carries away excess KE) is the symbol for the antineutrino (carries away excess KE)
eYX
eYXA1Z
AZ
A1Z
AZ
General Physics
Beta Decay – Example Radioactive Carbon-14 decay Used to date organic samples
eNC 147
146
General Physics
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
General Physics
Gamma Decay – Example Example of a decay sequence
The first decay is a beta decay emission
The second step is a gamma decay emission
C* indicates the Carbon nucleus is in an excited state
Gamma emission doesn’t change either A or Z
eCB *126
125
CC 126
126 *
General Physics
Medical Applications of Radiation
Tracing Radioactive particles can be used to trace
chemicals participating in various reactions Example, 131I to test thyroid action
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
General Physics
Medical Applications of Radiation
CAT scans Computed Axial Tomography Produces pictures with greater clarity and
detail than traditional x-rays
General Physics
Medical Applications of Radiation
MRI scans Magnetic Resonance Imaging When a nucleus having a
magnetic moment is placed in an external magnetic field, its moment precesses about the magnetic field with a frequency that is proportional to the field
Transitions between energy states can be detected electronically to produce cross-sectional images
General Physics
3D-CRT Treatment Three-dimensional conformal radiation therapy
uses sophisticated computers, CT scans and/or MRI scans to create detailed 3-D representations of a tumor and surrounding organs
Radiation beams are then shaped exactly to treat the size and shape of the tumor – nearby normal tissue receives less radiation exposure
Medical Applications of Radiation