Phys 328 Nuclear Physics and Particles

Introduction

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COURSE OUTLINE

PHYS328

NUCLEAR PHYSICS AND PARTICLES

Instructor: Prof. Dr. A. Murat Güler

Room: 333, e-mail: mguler@newton.physics.metu.edu.tr

Class Schedule:

Monday 9:00-11:30 in P350

Wednesday 9:40-10:30 in P350

Text Book

B.R. Martin, Nuclear and Particle Physics

Syllabus

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Topics:

– Introduction

– Basic Concepts

– Nuclear Phenomenology

– Particle Phenomenology

– Experimental Methods

– Applications of Nuclear Physics

Syllabus

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Grading • Midterm : 25 %

• Final : 40%

• Project work: 30 %

• Attandance : 5%

Dates:

• Midterm Examination First week of May.

History

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-Provides historical introduction to the field of nuclear and

particle physics.

-A number of concepts and tools are introduced concerning

the nuclear and particle physics.

-Give the basic principles and interpretations in nuclear and

particle physics

Basic Concepts

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19th Century : atoms are indivisible

History

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Radioactivity In 1896: Henry Becquerel dicovers

radioactivity (β-radiation from uranium salts)

When the salts were placed near to a

photographic plate covered with opaque

paper, the plate was discovered to be

fogged. The phenomenon was found to be

common to all the uranium salts studied and

was concluded to be a property of the

uranium atom.

History

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The Nobel Prize in Physics 1903

Henri Becquerel

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Rutherford and Pierre & Marie

Curie establish existence of α and β

rays and nature of radiactivity

History

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Through a long series of experiments he

realized that there were two kinds of

radiation emitted from Uranium. Rutherford

called them alpha and beta.

In a few years it was concluded that beta

rays were cathode rays, that is, electrons.

The precise nature of alpha particles

remained a mystery although both

Rutherford and the Curies suspected they

were particles, atoms electrically charged

and projected at high speed. They also

knew they could be stopped by extremely

thin shields (e.g. paper) and were deflected

to only a small extent in a magnetic field.

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In 1896 Roentgen discovered X-rays

History

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• X-rays were first discovered accidentally by Wilhelm Conrad Röntgen.

• X-rays are waves of electromagnetic energy that have a shorter wavelength than normal light

• He discovered that these new invisible rays could pass through most objects that casted shadows including human tissue but not human bones and metals.

• Within a year of the discovery many scientists replicated the experiment Röntgen performed and began using it in clinical settings

• In 1901 Röntgen won the

first Nobel Prize in Physics

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X-ray production

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History

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Television Computer Monitor

Cathode ray tubes pass electricity through a gas that is contained at a

very low pressure.

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In 1898 Pierre and Marie Curie α-radiation

History

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m1 m2 M

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1900 Villard … γ-radiation Villard investigated the radiation from radium salts that

escaped from a narrow aperture in a shielded container

onto a photographic plate, through a thin layer of lead

that was known to stop alpha rays. He was able to

show that the remaining radiation consisted of a second

and third type of rays. One of those was deflected by a

magnetic field (as were the familiar "canal rays") and

could be identified with Rutherford's beta rays. The last

type was a very penetrating kind of radiation which had

not been identified before...

History

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1897: Thomson discovers the electron – cathode

rays, and measured mass and charge

‘Plum pudding model’ of atom

History

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Poor agreement with experiment

JJ Thomson

Nobel Prize in Physics ,1906

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• 1911: Rutherford experiments ‐‐> positive

nucleus orbited by electrons (planetary model)

History

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(Nobel Prize in Chemistry, 1908)

Ernest Rutherford

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History

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Most of the particles passed right through

A few particles were deflected

VERY FEW were greatly deflected

a) The nucleus is small

b) The nucleus is dense

c) The nucleus is positively charged

Conclusions:

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History

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• Based on his experimental evidence:

– The atom is mostly empty space

– All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus”

– The nucleus is composed of protons and neutrons (they make the nucleus!)

– The electrons distributed around the nucleus, and occupy most of the volume

– His model was called a “nuclear model”

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1913: Bohr model of atom: first window into

quantum physics

History

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Atom was like a miniature planetry system

with electrons circulating about the

nucleus (like planets circulating about

Sun).

But accelerated charge radiates

electromagnetic energy. As it radiates this

energy its total energy would decrease

and electron spirals in toward the nucleus

and atom would collapse.

Nobel prize in Physics in 1922 Prize motivation: "for his services in the

investigation of the structure of atoms and

of the radiation emanating from them"

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History

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Only orbits of certain radii and thus only certain only quantized energies

are allowed.

Allowed radii is n= 1,2,3...

a0 is Bohr radius

Quantized radiation is emitted /absorbed if an electron changes its orbit.

Bohr proposed that there are certain special states called stationary states.

İn which angular momentum of electron may have magnitude h, 2h,

...(quantization of angular momentum).

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Where does this go wrong?

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• The Bohr model’s successes are limited:

• Doesn’t work for multi-electron atoms.

• The “electron racetrack” picture is incorrect.

• That said, the Bohr model was a pioneering,

“quantized” picture of atomic energy levels.

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Discovery of Neutron

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Outside the nucleus, free

neutrons are unstable and have

a half life of 611.0 ±1.0s.

James Chadwick

Nobel Prize in 1935

• Discovery of the neutron, by J. Chadwick in

1932

1) Neutral

Gamma? No! protons too energetic

2) mn ~ mp

Interpretation:

radiation ionizing-nonBe

CnBeHe 12

6

1

0

9

4

4

2

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Discovery of Isotopes

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Frederick Soddy (1877-1956) proposed the idea of

isotopes in 1912

Isotopes are atoms of the same element having

different masses, due to varying numbers of

neutrons.

Soddy won the Nobel Prize

in Chemistry in 1921

• We can also put the mass number after the

name of the element:

carbon-12

carbon-14

uranium-235

"for his contributions to our knowledge of the chemistry of radioactive

substances, and his investigations into the origin and nature of isotopes".

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Isotopes

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Isotope Symbol Composition of

the nucleus

% in nature

Carbon-

12

12C 6 protons

6 neutrons

98.89%

Carbon-

13

13C 6 protons

7 neutrons

1.11%

Carbon-

14

14C 6 protons

8 neutrons

<0.01%

Atomic mass is the average of all the naturally occurring

isotopes of that element

Carbon = 12.011

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• 1930s – The known 'Elementary Particles' were :

– electron

– proton

– neutron (inside the nucleus)

– 'neutrino' (now anti-neutrino) in beta decay

– photon (γ)– the quantum of the electromagnetic field

Elementary particles

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Heisenberg et al applied QM to nucleons – bound

together byshort‐range strong nuclear force

1930’s: Nucleus is composite consisting of nucleons:

protons and neutrons

1960’s: Nucleons are bound states of quarks which

have fractional

electric charge – Gell‐Mann (NP 1969)

1930: Neutrino postulated by Pauli to save energy

conservation in

β‐decay

1956: Reines and Cowan discover neutrino (Reines

NP 1995)

Elementary particles

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Elementary particles

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Questions that are asked since 2000 years

What are the building blocks of matter ?

What forces act on matter ?

Demokritos

atom

Newton

forces

Maxwell

electromagnetism

Einstein

a lot …

400 v.Chr. 1687 1864 1905

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Standard Model

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Elementary particle:

•mass, charge, spin

Fundamental forces:

•Electromagnetic

•Weak

•Strong

•Gravity

Fundamental particles

Fermions and bosons. There are also

antiparticles.

Proton (uud) and neutron (udd) are not

elementary particles.

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We know that E=mc2 and λ=h/p –High energies are required to make new particles

Proton radius is ~10‐15m, >103 me energy required –So relativistic effects are important

Quantum Theory + Special Relativity: each particle must have an

antiparticle with opposite quantum numbers: electric charge, spin, lepton

charge, etc. Described by Dirac equation:

Particle+antiparticle → annihilation (γ’s or other)

Symmetry between particles and antiparticles

Dirac made theoretical prediction of anti‐particles

1933 Anderson discovered positron (NP 1936)

1959 Segre & Chamberlain NP for discovery of anti‐proton

Relativity and Antiparticles

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• Emmy Noether’s theorem

• Space, time translation & orientation symmetries are

all continuous symmetries

– Translational invariance → p

– Rotational invariance → L

– Time invariance → E

• Additional symmetries of interest for nuclear

and particle physics are:

– parity

– charge conjugation

– time‐reversal

Space‐Time Symmetries and Conservation Laws

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Discrete Symmetries

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• Parity, P

– Parity reflects a system through the origin. Converts right-handed coordinate systems to left-handed ones.

– Vectors change sign but axial vectors remain unchanged

• x -x , p -p, but L = x p L

• Charge Conjugation, C

– Charge conjugation turns a particle into its anti-particle

• e + e- , K - K +

• Time Reversal, T

– Changes, for example, the direction of motion of particles

• t -t

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Parity Refers to spatial reflection r →‐ r

P eigenvalue is called intrinsic parity, or simply

parity.

Strong and EM interactions conserve parity.

Weak interaction violates parity.

Leptons and quarks have parity +1

Antileptons and antiquarks have parity ‐1

Additional contribution to parity from orbital

angular momentum

Parity

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Charge Conjugation Changes particles into antiparticles

Charge Conjugation

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a – electrically neutral particles, b – electrically

charged particles

Ca – phase factor

Weak interaction violates parity

EM and strong preserve parity

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• t→t’ = ‐t • If conserved

Time Reversal

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T is not Hermitian, so it is not observable when

conserved.

However, rates of time reversed processes must be the

same(if no weak interaction is present – violates parity).

There is a general CPT theorem – any relativistic field

theory is invariant under the combined CPT.

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Particle Reactions

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• Scattering Reactions

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Decays

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Feynman Diagrams

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Feynman Rules

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Feynman Rules

M= g g

1/(q2-m2)

p1

p2 p3

• How to calculate amplitude M ?

• The ‘drawing’ is a mathematical object!

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Weak Interactions

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Strong Interactions

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Four Vectors

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Four Vectors

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Particle Exchange

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Klein-Gordon Equation

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Yukowa Potential

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Units

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Units

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