Transcript Physics 362 – week 13

Physics 362 – week 13
Modern Physics Seminars
How to measure the Lifetime of an
Isotope


Several measurements over a period of
time
Single measurement of radioactivity and
mass
Dating with radioactive nuclei


Dating of fossils
Dating of rocks
Carbon Nuclear Data
Three principal isotopes of carbon occur naturally
12C
- 98.89%
13C
- 1.11%
14C
- (radioactive) 0.00000000010%.
Atomic
Mass (u)
Nuclear
Mass(GeV/c2 )
Binding
Energy(MeV)
Spin
Natural
Abund.
Half-life
Decay
Q
MeV
6 11
11.011433
10.2541
73.44
3/2
...
20.3m
b+
1.98
6 12
12.000000
11.1750
92.16
0
0.989
stable
...
...
6 13
13.003355
12.1096
97.11
1/2
0.011
stable
...
...
6 14
14.003241
13.0410
105.29
0
...
5715y
b-
0.016
Z
A
Carbon-14 Decay
14C
 14N + e- + ne
t1/2=5715 yr
MC14=14.003242 u
MN14=14.003074 u
Q=0.016 MeV
Carbon Dating
Libby, Anderson and Arnold (1949)
N  N0  e

t
t
 t has been measured in the laboratory
 We can measure N in the sample
 If we know N0 we can derive t
Carbon Dating
Measuring Carbon-14 activity
R

t

t

t
dN
N
 R0  e t    N 0  e t   0  e t
t
dt
The sample is disintegrated and the activity
is measured using a radiation detector
t1/2=5715 yr  Only good up to about 50,000 yr
Dating of rocks and meteorites
Potassium-Argon: 40K  40Ar + e+ + ne;
t1/2=1.28 109 yr
Rubidium-Strontium: 87Rb  87Sr + e- + ne; t1/2=4.8 1010 yr
Samarium-Neodymium: 147Sm  143Nd + a; t1/2=10.5 1010 yr
Lutetium-Hafnium: 176Lu  176Hf + e- + ne; t1/2=3.8 1010 yr
Rhenium-Olmium: 187Re  187Ol + e- + ne; t1/2=4.2 1010 yr
Determining N and N0
N  N0  e

t
t
We measure the ratio between parent and daughter
abundances:
t
N0  N N0

 1  et  1
N
N
How do we evaluate N0-N?
Potassium-Argon
Argon is a gas. Whenever rock is melted to become
magma or lava, the argon tends to escape. Once the
molten material hardens, it begins to trap the new
argon produced since the hardening took place. In this
way the potassium-argon clock is clearly reset when an
igneous rock is formed.
In nearly all of the dating methods, except potassiumargon and the associated argon-argon method, there is
always some amount of the daughter product already in
the rock when it cools.
Rubidium-strontium
Rubidium-Strontium