Lecture 354/26/06. What does it mean to be radioactive? Radioactivity vs. nuclear power.
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Transcript of Lecture 354/26/06. What does it mean to be radioactive? Radioactivity vs. nuclear power.
Lecture 35 4/26/06
What does it mean to be radioactive?
Radioactivity vs. nuclear power
Radioactive emissions
Alpha particle (α) Helium nucleus
RnHeRa 22286
42
22688
Radioactive emissions
Beta particle (β) electron neutron electron (β) + proton
NpeU 23993
01
23992
Gamma (γ) doesn’t change atomic mass or number
Radioactive emissions
Radioactive emissions
positron (β+) electron proton positron (β) + neutron
BiePo 20783
01
20784
Radioactive
Electron capture Electron + proton neutron
LieBe 73
01
74
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 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
Stability Stability of Nucleiof Nuclei
• Out of > 300 stable isotopes:
EvenEven OddOdd
OddOdd
EvenEven
ZZNN
157157 5252
5050 55
31311515PP
191999FF
2211H, H, 66
33Li, Li, 101055B, B, 1414
77N, N, 1801807373TaTa