Lecture 365/2/05 Tuesday 2:30 TSB 006

What does it mean to be radioactive?

Radioactive emissions Alpha particle (α) Helium nucleus Mass: Atomic #: 88286

Radioactive emissions Beta particle (β) electron neutron  electron (β) + proton Mass: Atomic #:

Gamma (γ) Radioactive emissions

positron (β + ) electron proton  positron (β) + neutron Mass: Atomic #:

Radioactive Electron capture Electron + proton  neutron Mass: 707 Atomic #: 4-13

Rates of radioactive decay Decay is not affected by temperature, pressure, or state of chemical combination N 0 = # of radioactive nuclei at t = 0 N t = # of radioactive nuclei at t = t 1 K = decay constant t = time

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