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