Phy 102: Fundamentals of Physics II

1. Phy 102: Fundamentals of Physics II Chapter 33: The Atomic Nucleus & Radioactivity Lecture Notes

2. 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)

3. Marie Curie (1867-1934) • 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)

4. Types of Radiation Alpha particles (a): • Helium nuclei • positive charge • Occurs when the nucleus of an atom ejects 2 protons & 2 neutrons (the identity of the atom changes!) Beta particles (b): • Electrons • Negative charge • Usually occurs when a neutron transmutes to a proton (the identity 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!)

5. 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

6. 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

7. The Nuclear Decay for Tritium 1 half-life 2 half-lives 3 half-lives

8. Transmutation of Elements • When radioactive isotopes decay they become different elements. This process is called transmutation. Example 1: Decay of 14C to 14N 14C ®14N + b {this is a beta decay} Example 2: Decay of 241Am to 237Np 241Am ®237Np + a {this is an alpha decay}

9. 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”

10. 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