Transcript Chapter4
Ch. 4 Nuclei and radioactivity • The atomic hypothesis • Mass concentrated in tiny nucleus with electrons around it • Atom 10–10 m, nucleus is 10–15 m • Nucleus contains protons (positive) and neutrons (no charge) • # of protons = atomic number • protons + neutrons gives atomic weight Periodic Table • Elements arranged in a systematic way by their atomic numbers • Elements in a group have similar chemical properties because of the way their outermost electrons are arranged • Isotopes have different number of neutrons (same # protons) Interactive Periodic Table Radiation When energy comes out of an atom, it’s usually called radiation. • Caused by electrons: light, x-rays, low energy electrons • Caused by nuclear processes: helium nucleus electron or positron high energy photon – p,n proton or neutron – Other energetic particles possible Radiation Units • Geiger counter video - watch dial • curie (Ci) and becquerel (Bq) are certain numbers of decays per second • rad and gray (Gy) measure how much energy is deposited in a kg of tissue. • rem and sievert (Sv) measure how much damage is done. 1 Sv = 100 rem • rem = rad * RBE • It’s rems that harm you Radiation Sickness Consider whole body doses • < 100 rem no short-term illness • 100 - 200 nausea, hair loss • 300 LD50 • > 1000 survival unlikely Alexander Litvinenko Cancer and the linear hypothesis • 2500 rem whole body gives 100% chance of excess cancer • So 25 rem whole body gives 1% chance of excess cancer • Give 100 people 25 rem each, expect one extra cancer (20 from other causes) • Most people assume linear hypothesis at all dose levels, so 25 mrem gives 0.001% chance of excess cancer. Radioactive half-life • N(@ time T) = N(@T=0) e–constant*T • e–constant*T is called the decay factor • T1/2 is called the half-life • At T = T1/2, the decay factor = 1/2 • At T = 2 T1/2, the decay factor = 1/4 • Half lives can range from tiny fractions of a second to billions of years • Provides “clocks” for dating rocks, etc. Fission and fusion energy • Fission occurs when a nucleus splits • Fragments have less mass than the original nucleus; E = mc2 • Fusion occurs when two lighter nuclei fuse into a single heavier nucleus. • Again, mass goes missing and comes out as energy (lots of it) How can this happen in both cases? • Fusion generates lots of relatively clean power, but requires high temperature and probably magnetic confinement • Fusion is the power source of stars • Fusion bombs are the most powerful ever made