Lecture physics a2 intro to nuclear physics huynh quang linh

Intro to Nuclear Physics

1


Nuclear Physics Topics
Composition of Nucleus
features of nuclei
Nuclear Models
nuclear energy
 Fission
 Fusion
 Summary
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
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

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About Units

 Energy - electron-volt
 1 electron-volt = kinetic energy of an electron when
moving through potential difference of 1 Volt;
o 1 eV = 1.6 ì 10-19 Joules
o 1 kWãhr = 3.6 × 106 Joules = 2.25 × 1025 eV
o 1 MeV = 106 eV, 1 GeV= 109 eV, 1 TeV = 1012 eV

 mass - eV/c2

o
o
o
o

1 eV/c2 = 1.78 × 10-36 kg
electron mass = 0.511 MeV/c2
proton mass = 938 MeV/c2 = 0.938 GeV/ c2
neutron mass = 939.6 MeV/c2

 momentum - eV/c:

o 1 eV/c = 5.3 × 10-28 kg m/s
o momentum of baseball at 80 mi/hr  5.29 kgm/s  9.9 × 1027
eV/c

 Distance

o 1 femtometer (“Fermi”) = 10-15 m

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Radioactivity
 Discovery of Radioactivity
 Antoine Becquerel (1896): serendipitous discovery of radioactivity:
penetrating radiation emitted by substances containing uranium
 A. Becquerel, Maria Curie, Pierre Curie(1896 – 1898):
o also other heavy elements (thorium, radium) show radioactivity
o three kinds of radiation, with different penetrating power

(i.e. amount of material necessary to attenuate beam):
 “Alpha (a) rays” (least penetrating – stopped by paper)
 “Beta (b) rays” (need 2mm lead to absorb)
 “Gamma (g) rays” (need several cm of lead to be attenuated)

o three kinds of rays have different electrical charge:
a: +, b: -, g: 0

 Identification of radiation:
 Ernest Rutherford (1899)
o Beta (b) rays have same q/m ratio as electrons
o Alpha (a) rays have same q/m ratio as He
o Alpha (a) rays captured in container show He-like emission spectrum
4


Proton

 “Canal rays”
 1898: Wilhelm Wien:
opposite of “cathode rays”
 Positive charge in
nucleus (1900 – 1920)
 Atoms are neutral

o positive charge needed to cancel electron’s negative charge
o Rutherford atom: positive charge in nucleus

 periodic table  realized that the positive charge of
any nucleus could be accounted for by an integer

number of hydrogen nuclei -- protons

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Neutron

 Walther Bothe 1930:
 bombard light elements (e.g. 49Be) with alpha particles 
neutral radiation emitted
 Irène and Frederic Joliot-Curie (1931)
 pass radiation released from Be target through paraffin wax 
protons with energies up to 5.7 MeV released
 if neutral radiation = photons, their energy would have to be 50
MeV -- puzzle
 puzzle solved by James Chadwick (1932):
 “assume that radiation is not quantum radiation, but a neutral
particle with mass approximately equal to that of the proton”
 identified with the “neutron” suggested by Rutherford in 1920
 observed reaction was:
a (24He++) + 49Be  613C*
13
 612C + n
6 C*
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Beta decay -- neutrino
 Beta decay puzzle :

o decay changes a neutron into a proton

o apparent “non-conservation” of energy

o apparent non-conservation of angular momentum

 Wolfgang Pauli predicted a light, neutral, feebly
interacting particle (called it neutron, later called
neutrino by Fermi)

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Positron


Positron (anti-electron)
 Predicted by Dirac (1928) -- needed for relativistic
quantum mechanics
 existence of antiparticles doubled the number of
known particles!!!
Positron track going
upward through lead
plate

 P.A.M. Dirac
 Nobel Prize (1933)
 member of FSU faculty
(1972-1984)
 one of the greatest physicists of the 20th century

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Structure of nucleus
 size (Rutherford 1910, Hofstadter 1950s):
 R = r0 A1/3, r0 = 1.2 x 10-15 m = 1.2 fm;
 i.e. ≈ 0.15 nucleons / fm3
 generally spherical shape, almost uniform density;
 made up of protons and neutrons
 protons and neutron -- “nucleons”;
are fermions (spin ½), have magnetic moment
 nucleons confined to small region (“potential well”)
  occupy discrete energy levels
 two distinct (but similar) sets of energy levels,
one for protons, one for neutrons
 proton energy levels slightly higher than those of
neutrons (electrostatic repulsion)
 spin ½  Pauli principle
 only two identical nucleons per eng. level

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Nuclear Sizes - examples
1
3

r  ro (A )

ro = 1.2 x 10-15 m

Find the ratio of the radii for the following nuclei:
1H, 12C, 56Fe, 208Pb, 238U

1
3

1
3

1
3

1
3

1
3

1 : 12 : 56 : 208 : 238

1 : 2.89 : 3.83 : 5.92 : 6.20
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