5.2. The Ar/ Ar Method - University of Notre Damensl/Lectures/phys10262_2014/art... · 2014. 4....
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5.2. The 39 Ar/ 40 Ar Method Method has advantage of parallel counting of 39 Ar and 40 Ar atoms in mass analyzer identical efficiencies, reduced uncertainties! Problem of generating 39 Ar and relating it to 40 K Fixed ratio of stable 39 K and radioactive 40 K isotopes 39 K 93.2581 40 K 0.0117 41 K 6.7302 Z=19 Conversion of 39 K into 39 Ar by neutron activation: 39 K(n,p) 39 Ar
Transcript of 5.2. The Ar/ Ar Method - University of Notre Damensl/Lectures/phys10262_2014/art... · 2014. 4....
5.2. The 39Ar/40Ar Method
Method has advantage of parallel counting of 39Ar and 40Ar atoms
in mass analyzer identical efficiencies, reduced uncertainties!
Problem of generating 39Ar and relating it to 40K
Fixed ratio of stable 39K and radioactive 40K isotopes
39K 93.2581
40K 0.0117
41K 6.7302
Z=19
Conversion of 39K into 39Ar by neutron activation:
39K(n,p)39Ar
Standard Reactor Techniques
Conventional TRIGA reactor
Neutron flux of Fn ~ 1014 n s-1 cm2
http://geology.cr.usgs.gov/facilities/gstr/
Conversion rate 39K39Ar
;1055.1)(
10544.11010)(
10
101
10544.1)(1
)()(
1939
22211422739
2114
227
),(
223939
39
),(
39
39
39
sArY
cmsncmArY
cmsnF
cmmb
KNKofg
KNFArY
n
pnK
npnK
Full conversion would take quite a long time, a 1 hour
generation of measurable 39Ar content is sufficient to
produce the measurable amount of N(39Ar)=5.5·1012 !
39Ar/40Ar Method
Advantages: Parallel release of 39Ar and 40Ar by heating
Parallel separation of 39Ar and 40Ar in mass spectrometer
Parallel counting in identical detector systems
Neutron flux dependent parameters are typically determined by
using well known 39K standard sample relative measurement
)(
)(
1)(
)(ln
1
40
39
),(
39
40
39
KN
KNtFJ
JArN
ArNt
irrpnKnn
n
Jn determined with a
standard sample that
is exposed to parallel
treatment Hornblende (mineral) from the McClure
Mountains, Colorado (a.k.a. MMhb-1).
The cradle of mankind
Ex Africa Semper Aliquid Novi
(Always something new out of Africa) Pliny the Elder
The Olduvai Gorge Olduvai Gorge is an archaeological site
located in the eastern Serengeti Plains in
northern Tanzania. The gorge is a very
steep sided ravine roughly 30 miles long
and 295 ft. deep. Exposed deposits show
rich fossil fauna, many hominid remains
belong to the one of the oldest stone tool
technologies, called Olduwan. The time
span of the objects recovered date from
2,100,000 to 15,000 years ago.
Fossil samples from Olduvai site
post-cranial remains
of homo erectus:
0.5 million years
Australopithecus boisei
1.8 million years
mineralized fossil
The Hadar Valley
1974 Discovery of skeleton by Don
Johannsen in the Hadar valley near
the Awash river. The Hadar valley is
part of the volcano rich African Rift.
australopethicus afarensis
3.18 Million years
The oldest hominoid
Lucy
Dated by K-Ar
method on layers
of volcanic ashes
above and below
the skeleton
The Discovery of Lucy
Chronology of Evolution
Documentation
Footprints of our Ancestors?
The Age of the Moment! 3,500,000 years ago, our very
remote ancient ancestors walked
through a landscape very like that
which we see today. On one
particular day the volcano Sadiman
puffed out a lot of gray ash, which
covered the plains. A rain shower
dampened and settled the ashes,
so that the local animals left their
crisp, clear tracks when they
walked. Through this desolated
grey landscape, traveled three
hominids walking on two legs. A
large, medium sized and small
individual walked together. Some
days later, a fresh ash fall buried
the tracks, until they were
excavated in 1978.
