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