View
215
Download
0
Embed Size (px)
Citation preview
Lecture 36 5/2/05
Tuesday 2:30 TSB 006
What does it mean to be radioactive?
Radioactive emissions
Alpha particle (α) Helium nucleus
RnHeRa 22286
42
22688
Mass: 226 4222
Atomic #: 88 2 86
Radioactive emissions
Beta particle (β) electron neutron electron (β) + proton
NpeU 23993
01
23992
Mass: 239 0239
Atomic #: 92 -1 93
Gamma (γ)
Radioactive emissions
Radioactive emissions
positron (β+) electron proton positron (β) + neutron
BiePo 20783
01
20784
Mass: 207 0207
Atomic #: 84 +1 83
Radioactive
Electron capture Electron + proton neutron
LieBe 73
01
74
Mass: 7 0 7
Atomic #: 4 -1 3
Rates of radioactive decay
Decay is not affected by temperature, pressure, or state of chemical combination
ktN
Nln
0
t
N0 = # of radioactive nuclei at t = 0
Nt = # of radioactive nuclei at t = t1
K = decay constant
t = time
2
1
21
21
21
0
0
0
t
t
693.0k
kt693.0
kt2
1ln
ktN
N2
1
ln
ktN
Nln
tt
693.0
0t
kt0t
0
t
21
eNN
eNN
ktN
Nln
If you start with 1.5 mg of tritium 3H, how much is left after 49.2 years? t1/2=12.3 years
tt
693.0
0t2
1
eNN
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 131I 131Xe + β decreases N/Z
nuclei below the belt of stability (low N/Z) loss of positron of electron capture 11C 11B + β+ or 81Rb + e 81Kr increases N/Z
nuclei with atomic # ≥ 84 alpha emission 238U 234Th + α
Belt of stability
Carbon dating
eNC
pCnN
01
147
146
11
146
10
147
in atmosphere
t1/2=5730 years
Ratio of 14C/12C constant until death, then 14C/12C decreases
At death, 14C has about 14 disintegrations per minute/gram (dpm/g)
Carbon-14 limitations
1. assume 14C in atmosphere is constant2. can’t date object < 100 years old3. accuracy only ± 100 years4. only good back to ~ 40, 000 years