2 Nuclear Physics Back to Rutherford and his discovery of the nucleus Also coined the term proton in 1920 and described a neutron in 1921 Neutron discovered by Chadwick in 1932 Ernest Rutherford ID: 643079
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Nuclear and Particle PhysicsSlide2
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Nuclear PhysicsBack to Rutherford and his discovery of the nucleusAlso coined the term “proton” in 1920, and described a “neutron” in 1921Neutron discovered by Chadwick in 1932
Ernest Rutherford
1871-1937
m
e
=
9.1 x 10
-31 kg mN = 1.6749 x 10-27 kgmP = 1.6726 x 10-27 kg
James Chadwick 1891-1974
nucleonsSlide3
3
Nuclides and IsotopesTo specify a nuclide:
Z is the atomic number = number of electrons
or
protons
A is the mass number = number of neutrons + protons
So number of neutrons = A-Z
Number of protons = Z
Isotopes – same atomic number, different mass numbere.g. carbon:
Many isotopes do not occur naturally, also elements > USlide4
4
SizesWe saw with the Bohr model that radius of the atom depended on atomic numberNucleus = protons + neutrons = mass numberThe volume of a nucleus is proportional to the mass numberSlide5
5
MassesMass spectrometer
1 atomic mass unit (u.) = 1.6606 x 10
-27
kg = 931.5 MeV
Fixed so that carbon = 12.00000 u
m
N = 1.6749 x 10-27 kg = 1.0087 u
mP = 1.6726 x 10-27 kg = 1.0078 uSlide6
6
Binding EnergyTotal mass of a nucleus < sum of massesExample:Mass of helium nucleus = 6.6447 x 10-27 kg
Contains 2 protons and 2 neutrons
Mass = 2 x (1.6749 x 10
-27
+ 1.6726 x 10
-27
) kg
= 6.6950 x 10-27 kg
Difference = (6.6950 – 6.6447) x 10-27 = 0.0503 x 10-27 kgEnergy = mc2 = 0.0503 x 10
-27 x c2 = 4.53 x 10-12 J = (4.53 x 10
-12) / (1.6 x 10-19) = 2.83 x 107 eV = 28.3 MeVSlide7
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Atomic Mass Units1 u = 931.5 MeVmN = 1.6749 x 10-27 kg = 1.0087 um
P
= 1.6726 x 10
-27
kg = 1.0078 uMass of helium nucleus = 4.0026 uSlide8
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Atomic Mass UnitsSame calculationMass of 2p + 2n = 2 x (1.0078 + 1.0087) = 4.0330 u
Difference = 0.0304 u
Binding energy = 0.0305 x 931.5 = 28.3 MeV
4.0330 uSlide9
9
Average Binding EnergyGraph
He – 4 nucleons, 28.3 MeV total: average = 7.075MeVSlide10
10
Attractive?How does nucleus stay together? Like charges repel!Force stronger than electric force Strong nuclear forceShort range (~10-15
m)
Stable nuclides N = Z
A > 30-40 – more neutrons
Z > 82 – no stable nuclides
Strong force can’t overcome repulsionSlide11
11
RadioactivityBecquerel, 1896Emission of radiation without external stimulusCuries – polonium (Po) and radium (Ra)
Henri Becquerel
1852-1908
Marie Curie
1867 - 1934
Pierre Curie
1859 - 1906
1903 (Physics)
1911 (Chem)
Radioactivity unaffected by heating, cooling, etc.Slide12
12
ClassificationRutherford classified 3 types of radioactivity according to penetration powerAlso different charge
Important factor: Conservation of
nucleon number
(neutrons + protons) = (neutrons + protons)
Video:
“
People Pretending to be Alpha Particles
”Slide13
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Alpha DecayLeast penetrating – nucleus of
Radium 226 is an alpha emitter:
Parent
Daughter
transmutation
Mass of parent > mass of daughter + mass of alpha
Difference = kinetic energySlide14
14
Example232.03714 u 228.02873 u + 4.002603 u
total = 232.03133 u
Lost mass = 232.03714 – 232.03133 = 0.00581 u
0.00581u
x
931.5 MeV/u
= 5.4 MeV (some recoil)Slide15
15
Beta decay One electron
What is lost is NOT an orbital electron
Instead a neutron changes to a proton + electron
So (6p + 8n) => (7p + 7n) + e
-
- decaySlide16
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ExampleKeep track of electrons!Carbon 14 has m = 14.003242 u 6 electronsNitrogen 14 has m = 14.003074 u normally 7 electronsBut in the decay, the nitrogen would have 6 electronsHowever the total on the r.h.s. of the equation has 7
So difference = 0.000168 u = 0.156 MeV = 156 keVSlide17
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Conservation of energyEnergy of decay = 156 keV = problem!
?Slide18
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A new particleProposed by Pauli (1930) - neutrinoTheory by FermiDiscovered 1956Zero charge, ~0 rest mass
Wolfgang Pauli
1900-1958
Enrico Fermi
1901-1954
antineutrino
“Zero rest mass” – speed of light
1998 – Super Kamiokande – some mass
Cosmic neutrino detectionSlide19
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More on positronsMany isotopes have more neutrons than protonsDecay by emission of electronOther isotopes have more protons than neutronsDecay by emission of positron
Proton changes to a neutron + positron
+
decaySlide20
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AnnihilationProton changes to a neutron + positron
+
decay
Positron annihilation
Application – positron emission tomographySlide21
21
Positron Emission TomographyPET – basis – use radio-labelled compounds, i.e. those containing a radionuclide.Positron emitters:
As an example, oxygen-15 can be used to look at oxygen metabolism and blood flow. Fluorine-18 is commonly used to examine cancerous tumours.Slide22
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PET - methodAnnihilation produces two back-to-back 511 keV photons
Simultaneous detectionSlide23
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Electron captureNucleus absorbs orbiting electron
Proton changes to neutron
Usually K electron
X-ray emission as outer electron jumps down to KSlide24
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Gamma decay
Most penetrating
= photon. High energy
*Excited nucleus lower energy state
Energy levels far apart = keV or MeV
-
(13.4 MeV)
-
(9.0 MeV)
(4.4 MeV)Slide25
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Homework . . . p.902,#6; p.908, Practice 25B; p.912,Section Review
p.928, 30-37;
p. 930, 56,60;
Read through lab for next time; answer
pre-lab questions