chapter 30: nuclear energy and elementary particles nuclear fission homework : read and understand...
TRANSCRIPT
Chapter 30: Nuclear Energy and Elementary Particles
Nuclear FissionHomework : Read and understand the lecture note.
What is nuclear fission?• Nuclear fission occurs when a heavy nucleus, such as , splits, or fissions, into two smaller nuclei.
• The fission of by slow (low-energy) neutron can be represented symbolically by : : an intermediate excited and short-lived state X, Y : fission fragments that satisfy conservation of energy and charge
• A typical reaction of this type is :
U23592
neutrons*23692
23592
10 YXUUn
*23592 U
U23592
nKrBaUUn 10
9236
14156
*23692
23592
10 3
Nuclear fission (in some detail)
Nuclear Fission
• Sequence of events in the nuclear fission neutrons*23692
23592
10 YXUUn
- The nucleus captures a thermal (slow-moving) neutron.- An excited state is formed and the excess energy cause oscillation of the nucleus.- The nucleus highly elongated, and the repulsive force among protons enhances the deformation.- The nucleus splits into to fragments, emitting several neutrons.
*23592 U
*23592 U
U23592
Nuclear fission (cont’d)
Nuclear Fission
• Energy released in the nuclear fission neutrons*23692
23592
10 YXUUn
- The binding energy per nucleon for heavy nuclei (mass~240) :~7.2 MeV- The binding energy per nucleon of intermediate mass :~8.2 MeV- Nuclei of intermediate mass are more tightly bound than heavy nuclei.- For a total of 240 nucleons, the energy released (Q-value) in a fission: Q=240 nucleons/(8.2 MeV/nucleon – 7.2 MeV/nucleon) = 240 MeV
Nuclear Fission Example 30.1 : Fission of uranium
• How many neutrons are produced in the fission process ?)x(1
09438
14054
23592
10 nSrXeUn
2941402351 xx
Example 30.2 : A fission-powered world• Calculate the total energy released if 1.00 kg of undergoes fission, taking the disintegration energy per event to be Q = 208 MeV?
U235
nuclei 102.56g) 1000.1(g/mol 235
nuclei/mol 1002.6 24323
NNumber of nuclei in
1.0 kg of uranium
MeV 105.32s)MeV/nucleu 208nuclei)( 1056.2( 2624 NQETotal energy released
• How many kilograms would provide for world’s annual energy needs (4x1020 J)?
Total energy released from Nkg kg of uranium
J 104 20 totkgkg ENE
kg 105J/eV) 100eV/kg)(1.6 1032.5(
J 104 619-32
20
kg
totkg E
EN
Nuclear Reactors Nuclear chain reaction
• Neutrons emitted when undergoes fission can in turn trigger other nuclei to undergoes with the possibility of a chain reaction.• Calculations show that without control the chain reaction goes out of control and results in the sudden release of enormous amount of energy (1 kg of would produce energy equivalent to 20 ktons of TNT).
U235
U235
Nuclear Reactors Nuclear reactors
• A nuclear reactor is designed to control nuclear reactions and maintain a self-sustained chain reaction.
D2O
cadmium
- Moderator slows down neutrons so that they can be absorbed by uranium more easily.- Control rods absorb very efficiently neutrons to control the reaction rate.- The reproduction constant K, defined as the average number of neutrons from each fission event that will cause another event. A self-sustained chain reaction is achieved when K=1.
Nuclear Fusion Nuclear fusion
• The binding energy of light nuclei (mass number <20) is much smaller than that of heavier nuclei.• When two light nuclei combine to form a heavier nucleus, the process is called nuclear fusion.• Because the mass of the final nucleus is less than the masses of the original nuclei, there is extra energy released.
Nuclear fusion in Sun (thermal nuclear fusion reactions)
eeDHH 21
11
11
HeDH 32
21
11
)(2or 11
42
32
32
42
32
11 HHeHeHeeHeHeH e
• proton-proton chain: to sustain the nuclear fusion - the temperature needs to be high enough to overcome the repulsive Coulomb force between protons - the density of nuclei must be high enough to ensure a high rate of collisions
The liberated energy is carried bygamma rays, positrons and neutrinos.
Nuclear Fusion Fusion reactors
• Scientists and engineers have been trying to create similar conditions to those in the interior of Sun to achieve self-sustained nuclear fusion reactions on Earth.
• Most promising reactions as fusion reactors are:
MeV) 27.3( 10
32
21
21 QnHeDD
MeV) 03.4( 11
31
21
21 QHTDD
MeV) 59.17( 10
42
31
21 QnHeTD
• Deuterium is abundant on Earth but tritium is radio active with T1/2= 12.3 yr and undergoes beta decay to 3He. So tritium is rare on Earth.
• One of the major problems to achieve fusion reactors is to give to the nuclei enough kinetic energy to overcome the repulsive Coulomb force.
Particle Physics
Model of Atoms
electrons e-
nucleus
Old view
Semi-modern view
Modern view
nucleusquarks
prot
on
What is the world made of?
Pion + : ud
Particle PhysicsBuilding Blocks of Matter
Discoveries of too many “elementary” particles and anti-particles lead to more fundamental model the Standard Model. Anti-particlehas the same property as particle except that charge is opposite to that of particle.
Proton p : uudNeutron n : udd
-
Particles made of quarks are calledhadrons and among them theyinteract through strong force.
What is matter made of?
Hadrons
Leptonsneutrinos : feel only weak forcecharged lepton : feel electromagnetic e-,-, and weak force
2MeV/c 3.938pm2MeV/c 6.939nm
2MeV/c 6.139m
2MeV/c 511.0em2MeV/c 7.105m
2MeV/c 1777m
+(2/3)e
-(1/3)e
+e
0
ele
ctri
c ch
arg
e
but tiny 0m
Particle Physics
Fundamental Forces
There are four know fundamental forces:
An example:Free neutron decay
How many kinds of forces are there?
Particle Physics
Fundamental Forces
Examples of weak interaction
Free neutron decay: n -> p + e-e
-
Muon decay: -> e-e + -
Particle Physics
Unification of Forces
Grand Unified Theories (GUTs)
Strong
Electric
Magnetic
Electromagnetic
Weak
Electroweak
Gravitational
GU
Ts
hard
19th c.
20th c.
21st c.?
GUTs predict:
Nucleon decays (not yet found)
Neutrino mass/oscillation (found)
What is our dream?
.,,0 etcKvep
Particle Physics
Neutrino Oscillation
There are three kinds of neutrinos: e
If neutrinos have mass, they can change their identities (flavours)
e
A simple example:
=
2
cos
+ cos
- sin
sin =
1
1
2
neutrinos with definite mass
Probability
1-Probability
Pro
babi
lity
Neutrino pathlength (km)
It depends onneutrino energy,masses, anddistance it travels
What is neutrino oscillation?
(flavours)
~Earth’s diameter 12,000 km
Atmospheric Neutrinos
Source of atmospheric neutrinos
Earth’s atmosphere is constantlybombarded by cosmic rays.
Energetic cosmic rays (mostlyprotons) interact with atoms inthe air.
These interactions produce manyparticles-air showers.
Neutrinos are produced in decaysof pions and muons.
Physicists are having fun on a boat in Super-Kamiokande
50,000 tons of pure water equipped with 12,000 50 cm photomultipliersand 2,800 20 cm photomultipliers (PMTs).
Atmospheric Neutrinos
Water Cherenkov Detector: Kamiokande,IMB,Super-Kamiokande,SNO
Water is cheap and easy to handle!
When the speed of a chargedparticle exceeds that of lightIN WATER, electric shockwaves in form of light are generated similar to sonic boomsound by super-sonic jet plane .
These light waves form a coneand are detected as a ring bya plane equipped by photo-sensors.
How does a water Cherenkov detector work?
Atmospheric Neutrinos
How do we detect atmospheric muon and electron neutrinos ?
electron-like ring
muon-like ring
+ n -> p +
e
+ n -> p +
e-
Major interactions:
Most of time invisible
Atmospheric Neutrinos
How do we see neutrino oscillation in atmospheric neutrinos?
Pro
babi
lity
(
N
eutr
ino
path
leng
th
cos (zenith angle)
downward-goingupward-going
Actual probability for measured zenith angledue to measurement errors
a
b
cos = a/b
Atmospheric NeutrinosEvidence of neutrino oscillation/mass
low energye
high energye
low energy
high energy
with oscillation
without oscillation
First crack in the Standard Model!!!
Solar Neutrinos How does the Sun shine?
Nuclear fusions generate: - energy/heat/light - neutrinos
1 MeV = 1x106 eV
Kamiokande
Solar Neutrinos How does the neutral current confirm neutrino oscillation?
Elastic scattering
-This reaction is available only for e .
-Available for both water and heavy water.
+ e- -> + e-
Solar Neutrinos
Solar neutrinos
background
Seeing the Sun underground
Image of Sun by Super-Kamiokande
How do we see the Sun underground?
e
e
Solar Neutrinos How do we see neutrino oscillation with solar neutrinos?
Super-Kamiokande : 0.465+-0.005+0.016-0.015
Flux: measured/expected
Neutrino deficit!!!
is not visible to allexperiments above
Supernova
Background level
Birth of a supernova witnessed with neutrinos
How do we know detected neutrinos are from a supernova?
Kamiokande
Num
ber
of p
hoto
mul
tipl
iers
fir
edA few hours before optical observation
Taken by Hubble Telescope ( 1990)