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 1 st Nobel Prize in Physics (1901) X-ray micrograph of Mrs. Roentgen’s hand (1895)

3. 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)

4. Types of Radiation Alpha 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!)

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 (t 1/2 ) of an isotope is the time it will take an isotope sample to decay to ½ of its original value Examples: t 1/2 for 3 H is 12.43 years t 1/2 for 235 U is 704 million years t 1/2 for 14 C 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 14 C to 14 N 14 C  14 N +  {this is a beta decay} Example 2: Decay of 241 Am to 237 Np 241 Am  237 Np +  {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 1 st successful transmutation experiments Discovered the “chain-reaction” A project leader & important contributor on the “Manhattan Project”

10. Carbon Dating 14 C is produced in the upper atmosphere as 14 N is bombarded by cosmic rays The 14 C drops to the earth where it is absorbed by plants and animals The 14 C levels in an organism are constant throughout the organism’s life (since it continuously adds and removes 14 C through nutrition & respiration) –There is one 14 C atom for every trillion 12 C atoms 1 carbon-14 : 1 x 10 12 carbon-12 When an organism dies it can no longer replenish its 14 C levels & the 14 C begins to decay (remember, t 1/2 for 14 C is 5730 years) Radioactivity levels of 14 C are measured & the level of decay from the original value is used to estimate the organism’s age