Chapter 30 nuclear energy and elementary particles
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Chapter 30: Nuclear Energy and Elementary Particles. Homework : 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.

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Chapter 30: Nuclear Energy and Elementary Particles

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Chapter 30 nuclear energy and elementary particles

Chapter 30: Nuclear Energy and Elementary Particles

Homework : 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 :

NuclearFission


Chapter 30 nuclear energy and elementary particles

Nuclear Fission

  • Nuclear fission (in some detail)

  • Sequence of events in the nuclear fission

  • 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.


Chapter 30 nuclear energy and elementary particles

Nuclear Fission

  • Nuclear fission (cont’d)

  • Energy released in the nuclear fission

  • 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


Chapter 30 nuclear energy and elementary particles

Nuclear Fission

  • Example 30.1 : Fission of uranium

  • How many neutrons are produced in the fission process

  • ?

  • 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?

Number of nuclei in

1.0 kg of uranium

Total energy released

  • How many kilograms would provide for world’s annual energy needs

  • (4x1020 J)?

Total energy released from

Nkg kg of uranium


Chapter 30 nuclear energy and elementary particles

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).


Chapter 30 nuclear energy and elementary particles

Nuclear Reactors

  • Nuclear reactors

  • A nuclear reactor is designed to control nuclear reactions and maintain

  • a self-sustained chain reaction.

  • 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.

cadmium

D2O


Chapter 30 nuclear energy and elementary particles

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)

  • 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 by

gamma rays, positrons and neutrinos.


Chapter 30 nuclear energy and elementary particles

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:

  • 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.


Chapter 30 nuclear energy and elementary particles

Particle Physics

What is the world made of?

nucleus

Model of Atoms

Old view

proton

electrons e-

nucleus

quarks

Modern view

Semi-modern view


Chapter 30 nuclear energy and elementary particles

Particle Physics

What is matter made of?

Building Blocks of Matter

Discoveries of too many “elementary” particles and anti-particles lead to more fundamental model the Standard Model. Anti-particle

has the same property as particle except that charge is opposite to that of particle.

Hadrons

electric charge

Proton p : uud

Neutron n : udd

-

Pion p+ : ud

Particles made of quarks are called

hadrons and among them they

interact through strong force.

+(2/3)e

-(1/3)e

Leptons

0

neutrinos n : feel only weak force

charged lepton : feel electromagnetic

e-,m-,t- and weak force

+e


Chapter 30 nuclear energy and elementary particles

Particle Physics

How many kinds of forces are there?

Fundamental Forces

There are four know fundamental forces:

An example:

Free neutron decay


Chapter 30 nuclear energy and elementary particles

Particle Physics

Fundamental Forces

Examples of weak interaction

-

Free neutron decay: n -> p + e- + ne

-

Muon decay: m- -> e- + ne + nm


Chapter 30 nuclear energy and elementary particles

Particle Physics

What is our dream?

Unification of Forces

Grand Unified Theories (GUTs)

Strong

Electric

Electromagnetic

19th c.

Magnetic

Electroweak

GUTs

21st c.?

20th c.

Weak

GUTs predict:

Neutrino mass/oscillation (found)

hard

Nucleon decays (not yet found)

Gravitational


Chapter 30 nuclear energy and elementary particles

Particle Physics

What is neutrino oscillation?

Neutrino Oscillation

There are three kinds of neutrinos:

ne

nm

nt

(flavours)

If neutrinos have mass, they can change their identities (flavours)

ne

nm

nt

A simple example:

nm

nt

nm

n1

- sin q

n2

cos q

=

nt

sin q

n1

+ cos q

n2

=

n1,2

neutrinos with definite mass

Probability

nm

nm

Probability

It depends on

neutrino energy,

masses, q and

distance it travels

nm

nt

1-Probability

~Earth’s diameter 12,000 km

Neutrino pathlength (km)


Chapter 30 nuclear energy and elementary particles

Atmospheric Neutrinos

Source of atmospheric neutrinos

Earth’s atmosphere is constantly

bombarded by cosmic rays.

Energetic cosmic rays (mostly

protons) interact with atoms in

the air.

These interactions produce many

particles-air showers.

Neutrinos are produced in decays

of pions and muons.


Chapter 30 nuclear energy and elementary particles

50,000 tons of pure water equipped with 12,000 50 cm photomultipliers

and 2,800 20 cm photomultipliers (PMTs).

Physicists are having fun on a boat in Super-Kamiokande


Chapter 30 nuclear energy and elementary particles

Atmospheric Neutrinos

How does a water Cherenkov detector work?

Water Cherenkov Detector: Kamiokande,IMB,Super-Kamiokande,SNO

Water is cheap and easy to handle!

When the speed of a charged

particle exceeds that of light

IN WATER, electric shock

waves in form of light are

generated similar to sonic boom

sound by super-sonic jet plane .

These light waves form a cone

and are detected as a ring by

a plane equipped by photo-

sensors.


Chapter 30 nuclear energy and elementary particles

Atmospheric Neutrinos

How do we detect atmospheric muon and electron neutrinos ?

muon-like ring

Major interactions:

ne

+ n -> p +

e-

nm

+ n -> p +

m-

Most of time invisible

electron-like ring


Chapter 30 nuclear energy and elementary particles

Atmospheric Neutrinos

How do we see neutrino oscillation in atmospheric neutrinos?

a

cos q

= a/b

q

q

b

Neutrino pathlength

downward-going

upward-going

cos (zenith angle)

Probability (nm->nm)

Actual probability for measured zenith angle

due to measurement errors


Chapter 30 nuclear energy and elementary particles

Atmospheric Neutrinos

Evidence of neutrino oscillation/mass

with oscillation

without oscillation

low energy nm

low energy ne

high energy ne

high energy nm

First crack in the Standard Model!!!


Chapter 30 nuclear energy and elementary particles

Solar Neutrinos

How does the Sun shine?

Nuclear fusions generate:

- energy/heat/light

- neutrinos

Kamiokande

1 MeV = 1x106 eV


Chapter 30 nuclear energy and elementary particles

Solar Neutrinos

How does the neutral current confirm neutrino oscillation?

n + e- -> n + e-

Elastic scattering

-This reaction is available only for ne .

-Available for both water and heavy

water.


Chapter 30 nuclear energy and elementary particles

Solar Neutrinos

How do we see the Sun underground?

Image of Sun by Super-Kamiokande

Solar neutrinos

background

e

e

Seeing the Sun underground


Chapter 30 nuclear energy and elementary particles

Solar Neutrinos

How do we see neutrino oscillation with solar neutrinos?

Flux: measured/expected

Neutrino deficit!!!

Super-Kamiokande : 0.465+-0.005+0.016-0.015

nm is not visible to all

experiments above


Chapter 30 nuclear energy and elementary particles

Supernova

How do we know detected neutrinos are from a supernova?

Birth of a supernova witnessed with neutrinos

A few hours before optical observation

Kamiokande

Number of photomultipliers fired

Taken by Hubble Telescope

( 1990)

Background level


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