Transcript Age of our galaxy - Physics

Purdue Rare Isotope Measurement
Laboratory (PRIME Lab)
•
•
Purdue University is home of the
only university-based
accelerator-mass-spectrometry
(AMS) multi-isotope facility in the
United States
PRIME Lab has facilities support
from the NSF geosciences
program and facilities upgrade
funds from NASA
Measurements performed at PRIME Lab enable Purdue research
endeavors and research activities from numerous research
groups outside Purdue University
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What is Accelerator Mass Spectrometry
• One of the main uses is Geologic dating
• Use elements that decay radioactively as clocks to measure
elapsed time and hence the age of an object
• Measure concentrations of specific radioactive element to a
precision of a few times 10-15. This is like finding a few grains of
sand in a Ross Ade stadium full of sand
• This means the sample size can be very tiny and not
destructive of the original object e.g. shroud of Turin
• We need to know when the clock started and that it has
measured time without variation
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Radioactive decay
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Age of the earth
The best age for the Earth comes not from dating individual rocks but by
considering the Earth and meteorites as part of the same evolving system in which
the isotopic composition of lead, specifically the ratio of lead-207 to lead-206
changes over time owing to the decay of radioactive uranium-235 and uranium-238,
respectively. These calculations result in an age for the Earth and meteorites, and
hence the Solar System, of 4.54 billion years with an uncertainty of less than 1
percent. To be precise, this age represents the last time that lead isotopes were
homogeneous throughout the inner Solar System and the time that lead and
uranium was incorporated into the solid bodies of the Solar System. The age of 4.54
billion years found for the Solar System and Earth is consistent with current
calculations of 11 to 13 billion years for the age of the Milky Way Galaxy (based on
the stage of evolution of globular cluster stars) and the age of 14 billion years for
the age of the Universe (based on the recession of distant galaxies and analysis of
the Cosmic Microwave background).
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Age of our galaxy
Rhenium-187 (187Re), an element similar in mass to gold, was produced
in the first stellar explosions after the birth of our galaxy. 187Re betadecays into osmium-187 (187Os) with a half-life of 41 billion years—
slowly enough so that the relative abundance of 187Re to 187Os provides a
good measure of the time that has elapsed since our galaxy first formed.
However, additional nuclear processes, other than the decay of 187Re,
change the abundance of 187Os, which can cause error in the rheniumosmium clock. Accelerators were used to precisely determine the
neutron-capture cross sections of 187Os and its adjacent osmium
isotopes, 186Os and 188Os, which then allows for an accurate subtraction
of this direct contribution.
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Carbon 14 dating
Carbon 14 is a radioactive form of carbon which is continually
being produced by the interaction of cosmic rays in the
atmosphere. The chemistry of C14 is the same as that of stable
C12. We ingest C14 by breathing and eating so at the time we die
we have a fixed ratio of C14/C12 . The C14 then decays with a half
life of 5730 years. So measuring this ratio at a later time
detemines the age at which the living organism died. One C14
atom exists in nature for every 1,000,000,000,000 C12 atoms in
living material.
Accuracy of ±0.4% or ±35 years using a 0.5-milligramsample
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Analyzing the surface of rocks
When cosmic rays hit rock surfaces they produce very
small amounts of trace elements called cosmogenic
nuclei. The amount of the elements depend on the length
of exposure. This technique can be used to measure the
age of meteorite impacts, volcanic eruptions and
movement of glaciers since the rocks now on the surface
were first exposed when the event happened. Have to
worry about the climate, weathering etc.
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Galactic Cosmic Rays
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Radionuclides measured routinely
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Detection Natural
limit (10- level (1015)
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Radionuclide
Half-life
(years)
10Be
1,600,000
3
14C
5,730
2
26Al
730,000
5
36Cl
300,000
1
500
41Ca
100,000
1000
10
129I
16,000,00
0
20
1000
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Sample
Age in Years
Cloth wrappings from a mummified bull. [Samples taken from a pyramid in Dashur, Egypt. This date
agrees with the age of the pyramid as estimated from historical records.]
2,050
Charcoal. [Sample, recovered from bed of ash near Crater Lake, Oregon, is from a tree burned in the
violent eruption of Mount Mazama which created Crater Lake. This eruption blanketed several
States with ash, providing geologists with an excellent time zone.]
6,640
Charcoal. [Sample collected from the “Marmes Man” site in southeastern Washington. This rock
shelter is believed toe among the oldest known inhabited sites in North America.]
10,130
Spruce Wood. [Sample from the Two Creeks forest bed near Milwaukee, Wisconsin, dates one of the
last advances of the continental ice sheet into the United States.]
11,640
Bishop Tuff. [Samples collected from volcanic ash and pumice that overlie glacial debris in Owens
Valley, California. This volcanic episode provides an important reference datum in the glacial
history of North America.]
700,000
Volcanic ash. [Samples collected from strata in Olduvai Gorge, East Africa, which sandwich the fossil
remains of Zinjanthropus and Homo habilis<possible precursors of modem man.]
1,750,000
Monzonite. [Samples of copper-bearing rock from vast open-pit mine at Bingham Canyon, Utah.]
37,500,000
Quartz monzonite. [Samples collected from Half Dome, Yosemite National Park, California.]
80,000,000
Conway Granite. [Samples collected from Redstone Quarry in the White Mountains of New
Hampshire.]
180,000,000
Rhyolite. [Samples collected from Mount Rogers, the highest point in Virginia.]
820,000,000
Pikes Peak Granite. [Samples collected on top of Pikes Peak, Colorado.]
1,030,000,000
Gneiss. [Samples from outcrops in the Karelian area of eastern Finland are believed to represent the
oldest rocks in the Baltic Region.]
2,700,000,000
The Old Granite. [Samples from outcrops in the Transvaal, South Africa. These rocks intrude even
older rocks that have not been dated.]
3,200,000,000
Morton Gneiss. [Samples from outcrops in southwestern Minnesota are believed to represent some of
the oldest rocks in North America.]
3,600,000,000
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Requirements for Dating
There are several critical elements to determine the
date at which a particular event occurred
 Reliable clock --- Use radioactive decay
 Knowing when the clock started
Death of a living organism
First exposure to cosmic rays
calibration
compare to a known standard sample
precise and reproducible measurements
measure ratios e.g. 12C/14C
control of machine parameters
statistically significant number of ions
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Timeline on earth
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How is it done
We cannot manipulate neutral atoms. The only tools
we can use are electric and magnetic fields. Electric
fields accelerate charged particles and magnetic
fields bend particles and act like prisms and lenses
for light
qEd = 1/2mv2
qvB = mv2/r
E
v2 = 2Edq/m
v = Brq/m
So if we use electric and magnetic fields
We can select atoms with the same q/m
Use energy loss in a gas which depends on q2 to
separate the ionized atoms with the same q/m
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Accelerator mass spectrometry
Add negative charge to the atoms
AMS allows the counting of individual atoms of a radionuclide
rather than the decay
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Accelerator Components
The ion source
Hot cesium ions are used to produce negative ions
with q = -1 from a few milligrams of solid material
such as a rock or meteorite. which are accelerated
down an evacuated beam line.
The injector magnet
This magnet bends the ions by 90 degrees and is
tuned to select the element of interest.
The accelerator
The accelerator is a large tank containing the beam
vacuum tubes and CO2 and N2 insulating gas at 10
atmospheres.The center of the tank is at a positive
voltage of up to 10 million volts.
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Accelerator components
The stripper
At the center of the tank the ions pass through a foil which
strips electrons from the negative ions and the ions become
positively charged and are accelerated again by the 10 million
volt potential. The ions have a distribution of charges and
They choose the most populous charge state
Carbon +3 Beryllium +3 Aluminum +7 Chlorine +7
The analyzing magnets
These select the radionuclide of interest and
further reduce the intensity of the wrong elements.
The electrostatic analyser
This uses electric fields to select ions with the correct energy
and remove background.
The gas ionization chamber
Counts ions one at a time by measuring the energy deposited
in propane gas which determines the nuclear charge.
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Possible Backgrounds
ISOTOPES
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ISOBARS
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MOLECULES 
same q different m
different q same m
could have same q/m
The measurement of a particular element requires a
detailed knowledge of both the element and possible
backgrounds and the incorporation of many different
techniques to reduce or eliminate the background.
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Research Topics
14C dating death of living organisms.
10Be, 26Al, 36Cl in rocks chronologies of glaciations, volcanic
eruptions and impact craters. Also erosion rates and burial
histories of sediments.
36Cl, 129I Release of radioactivity into the environment
 36Cl
Tracing groundwater movements
10Be, 26Al, 36Cl meteorites, exposure history, terrestrial
residence times.
Use for measuring biological tracers e.g Calcium for
osteoporosis
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Surface exposure dating
Meteor Crater in Arizona
Typical glaciated surface
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Glacial chronology has been redefined
around the world.
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Dating terraces above the
goosenecks of the San Juan River
River gravels
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GENESIS
Our Sun belongs to the generation of stars created 4.6 billion years ago, when our galaxy
was roughly half its present age. A cloud of interstellar gas, dust, and ices containing
several generations of material collapsed to form the nebula from which the Sun and the
rest of the solar system grew. This collapse may have been triggered by a nearby
supernova. The conservation of angular momentum confined some of the material to a
flat, spinning disk around the young Sun. As time went on, the grains and ices in that
disk bumped into and stuck to one another. As they grew larger, their gravitational
forces increased, attracting more matter from the disk and gradually building up
kilometer-sized bodies called planetesimals, some of which in turn formed the nuclei of
the planets as we know them today. Other planetesimals either became comets or
asteroids.
About 4.6 billion years ago, the solar nebula transformed
into the present solar system. Fortunately for us the average isotopic and elemental
composition of the solar system of 4.6 billion years ago, is largely preserved in the
surface layers of the Sun. The Sun contains most of the mass of the original nebula, and
while nuclear reaction has modified the composition at the core, the surface layers
which do not mix with the core have preserved the original nebula composition. NASA’s
Genesis sample-return mission 1 is designed to give us just such a baseline composition
2. It has collected solar wind, material which is ejected fromthe outer portion of the sun,
and returned it to Earth. This material can be thought of as a fossil of our nebula
because the preponderance of scientific evidence suggests that the outer layer of our sun
has not changed measurably for billions of years. Moreover, for most rock-forming
elements, there appears to be little fractionation of either elements or isotopes between
the sun and the solar system
http://genesismission.jpl.nasa.gov/
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