Chapter 29Chapter 29

Nuclear PhysicsNuclear Physics

General Physics

Nuclear PhysicsNuclear Physics

Sections 1–4Sections 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

Z

N

A

A=100

Z=N = A

/2

Ato

mic

ele

men

t

Nuclear isotope

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

General Physics

3D-CRT Treatment Planning3D-CRT Treatment Planning