Phy 102: Fundamentals of Physics II

Chapter 33: The Atomic Nucleus & Radioactivity

Lecture Notes

Wilhem Rontgen(1845-1923)

• German physicist• Discovered x-rays &

studied ability of x-rays to penetrate solid materials

• Refused to file patents for his work on x-ray devices

• Winner of the 1st Nobel Prize in Physics (1901)

X-ray micrograph of Mrs. Roentgen’s hand (1895)

• 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 to locate bullets and

facilitate surgery– Invented x-ray vans

• Died from leukemia (thought to be due to radiation exposure)

Marie Curie (1867-1934)

                               

Types of RadiationAlpha particles ():

– Helium nuclei– positive charge– Occurs when the nucleus of an atom ejects 2 protons & 2 neutrons (the identity of the

atom changes!)

Beta particles (): – Electrons– Negative charge– Usually occurs when a neutron transmutes to a proton (the identity of the atom changes!)

Gamma rays (): – 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!)

Isotopes & Radioactivity• Isotopes are atoms of the same element that have different

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 the Strong

force cannot balance it out• The nucleus is susceptible to nuclear decay

Half-Life

• The nuclei of “radioactive” isotopes are unstable and inevitably decay to produce smaller nuclei (and some nuclear radiation)

• The half life (t1/2) of an isotope is the time it will take an isotope sample to decay to ½ of its original value

Examples:t1/2 for 3H is 12.43 years

t1/2 for 235U is 704 million years

t1/2 for 14C is 5730 years

Decay of 1000 Tritium Atoms

0

200

400

600

800

1000

1200

0 10 20 30 40 50 60time (years)

#o

f T

riti

um

ato

ms

The Nuclear Decay for Tritium

1 half-life

2 half-lives3 half-lives

Transmutation of Elements

• When radioactive isotopes decay they become different elements. This process is called transmutation.

Example 1: Decay of 14C to 14N14C 14N + {this is a beta decay}

Example 2: Decay of 241Am to 237Np241Am 237Np + {this is an alpha decay}

Enrico Fermi (1901-1954)• Italian-American physicist• Early work was theoretical

– Explained the statistics of nuclear particles that obey the Pauli Exclusion Principle (now called Fermions)

• Discovered artificial radioactivity– produced by bombarding elements with neutrons– Performed 1st successful transmutation

experiments

• Discovered the “chain-reaction”• A project leader & important contributor on

the “Manhattan Project”

Carbon Dating• 14C is produced in the upper atmosphere as 14N is bombarded by

cosmic rays• The 14C drops to the earth where it is absorbed by plants and animals• The 14C levels in an organism are constant throughout the

organism’s life (since it continuously adds and removes 14C through nutrition & respiration)

– There is one 14C atom for every trillion 12C atoms

1 carbon-14 : 1 x 1012 carbon-12• When an organism dies it can no longer replenish its 14C levels &

the 14C begins to decay (remember, t1/2 for 14C is 5730 years)

• Radioactivity levels of 14C are measured & the level of decay from the original value is used to estimate the organism’s age