ch33 atomic nucleus & radioactivity (physics)

7/28/2019 Ch33 Atomic Nucleus & Radioactivity (Physics)

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Phy 102: Fundamentals of

Physics II

Chapter 33: The Atomic Nucleus & Radioactivity

Lecture Notes


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Wilhem Rontgen(1845-1923)

German physicist Discovered x-rays &

studied ability of x-rays topenetrate solid materials

Refused to file patents forhis work on x-ray devices

Winner of the 1st Nobel

Prize in Physics (1901)

X-ray micrograph of Mrs.

Roentgens hand (1895)


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First woman to win the Nobel Prize

Won 2 Nobel Prizes Physics (1903): the discovery of polonium and

radium

Chemistry (1911): the isolation of radium and

the study of its chemical properties Some of her contributions:

Discovered radium and polonium

Studied the properties of radioactivity

During WWI proposed the use of x-rays tolocate bullets and facilitate surgery

Invented x-ray vans

Died from leukemia (thought to be due to

radiation exposure)

Marie Curie (1867-1934)


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Types of RadiationAlpha particles (a):

Helium nuclei

positive charge

Occurs when the nucleus of an atom ejects 2 protons & 2neutrons (the identity of the atom changes!)

Beta particles (b): Electrons

Negative charge

Usually occurs when a neutron transmutes to a proton (theidentity of the atom changes!)

Gamma rays (g): High frequency electromagnetic (X ray) radiation (e.g. light)

No charge

Usually occurs when an excited nucleus relaxes to a

lower state (the identity of the atom does not change!)


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Isotopes & Radioactivity

Isotopes are atoms of the same element that havedifferent numbers of neutrons (& different masses)

The skinny:

Two fundamental forces involved: Electric and Strong force

Electric force acts over longer distances than Strong force

Protons repel each other (Electric force)

Protons and neutrons attract each other (Strong force)

The more nucleons that are in the nucleus the greater the

distance of separation between protons

At some point, the electric force repels the nucleons and theStrong force cannot balance it out

The nucleus is susceptible to nuclear decay


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Half-Life

The nuclei of radioactive isotopes are unstableand inevitably decay to produce smaller nuclei(and some nuclear radiation)

The half life (t1/2) of an isotope is the time it willtake an isotope sample to decay to of its originalvalue

Examples:

t1/2 for3H is 12.43 yearst1/2 for

235U is 704 million years

t1/2 for14C is 5730 years


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Decay of 1000 Tritium Atoms

0

200

400

600

800

1000

1200

0 10 20 30 40 50 60

time (years)

#o

fTritiumat

oms

The Nuclear Decay for Tritium

1 half-life

2 half-lives3 half-lives


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Transmutation of Elements

When radioactive isotopes decay they

become different elements. This process is

called transmutation.

Example 1: Decay of14C to 14N

14C 14N + b {this is a beta decay}

Example 2: Decay of241Am to 237Np

241Am237Np + a {this is an alpha decay}


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Enrico Fermi (1901-1954)

Italian-American physicist

Early work was theoretical

Explained the statistics of nuclearparticles that obey the Pauli ExclusionPrinciple (now called Fermions)

Discovered artificial radioactivity

produced by bombarding elements withneutrons

Performed 1st successful transmutation

experiments

Discovered the chain-reaction

A project leader & importantcontributor on the Manhattan

Project


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ch33 atomic nucleus & radioactivity (physics)

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