Lecture 354/26/06

What does it mean to be radioactive? Radioactivity vs. nuclear power

Radioactive emissions Alpha particle (α) Helium nucleus

Radioactive emissions Beta particle (β) electron neutron  electron (β) + proton

Gamma (γ) doesn’t change atomic mass or number Radioactive emissions

positron (β + ) electron proton  positron (β) + neutron

Radioactive Electron capture Electron + proton  neutron

Rates of radioactive decay Decay is not affected by temperature, pressure, or state of chemical combination

If you start with 1.5 mg of tritium 3 H, how much is left after 49.2 years? t 1/2 =12.3 years

Belt of stability (empirically derived) Belt of stability ends at element 83 elements ≥ 84 protons are radioactive Type of decay nuclei above belt of stability (high N/Z) Beta emission 131 I  131 Xe + β decreases N/Z nuclei below the belt of stability (low N/Z) loss of positron of electron capture 11 C  11 B + β + or 81 Rb + e  81 Kr increases N/Z nuclei with atomic # ≥ 84 alpha emission 238 U  234Th + α

Belt of stability

Carbon dating in atmosphere t 1/2 =5730 years Ratio of 14 C/ 12 C constant until death, then 14 C/ 12 C decreases At death, 14 C has about 14 disintegrations per minute/gram (dpm/g)

Carbon-14 limitations 1. assume 14 C in atmosphere is constant 2. can’t date object < 100 years old 3. accuracy only ± 100 years 4. only good back to ~ 40, 000 years

Stability of Nuclei Out of > 300 stable isotopes: Even Odd Odd Even Z N P 19 9 F 2 1 H, 6 3 Li, 10 5 B, 14 7 N, Ta