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36Cl
Accelerator Mass Spectrometry (AMS): $125-$500/sample
37Cl
Approximately $200/sample
(See
UA
Laboratory of Isotope Geochemistry)
(See
also Zymax
Isotope Laboratory)
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36Cl - Natural Production
36Cl
is produced naturally in the atmosphere and within
solid materials at the earth's surface.
36Cl
- Atmospheric Production
Chlorine-36 is produced in the upper atmosphere
through spallation reactions. High-energy cosmic
ray particles collide with atoms in the earth's
atmosphere producing protons and neutrons. After
the emission of other particles to lower the energy
state, the final result is either a stable element
or a long-lived radioactive isotope. Roughly two
thirds of atmospheric 36Cl
is produced by the following spallation reaction:
40Ar
+ p ® 36Cl
+ n + a
The other third of the atmospheric
36Cl
is produced by this spallation reaction:
36Ar
+ n ® 36Cl
+ p
where p is a proton, n is a neutron, and a
is an alpha particle (or helium nucleus). These
reactions result in an average atmospheric deposition
rate of 12 to 20 atoms 36Cl
per second per square meter.
36Cl
- Earth Surface Production
Chlorine-36 can be produced in solid materials
on the surface of the earth in three ways: spallation
reactions, muon reactions and thermal neutron
adsorption.
Spallation reactions also occur
when gamma rays interact with minerals in the
top several meters of the earth's surface. The
following reactions can result:
35Cl
+ n ® 36Cl
+ y
39K
+ n ® 36Cl
+ n + a
40Ca
+ n ® 36Cl
+ p + a
where y is a gamma ray.
Chlorine-36 can also be produced
through muon reactions. Muons are negatively charged,
short-lived particles that are produced by high-energy
cosmic ray reactions. When produced at the earth's
surface, a muon can react with the nucleus of
an atom. When a muon interacts with a calcium
or potassium atom (both are commonly found in
minerals at the earth's surface), 36Cl
can be produced through the following reactions:
40Ca
+ µ-
® 36Cl
+ a
39K
+ µ-
® 36Cl
+ p + 2n
where µ-
is a muon.
Finally, 36Cl
is produced through thermal neutron absorption.
The 35Cl
isotope has a large neutron absorption cross-section,
making a relatively large target for collisions
with thermal neutrons. The following reaction
results in the production of 36Cl
from 35Cl
in groundwater:
35Cl
+ n ® 36Cl
+ y
where the neutrons in the above
reaction are produced in deep subsurface aquifers
from the spontaneous radioactive decay of uranium
and thorium. To a smaller extent, potassium can
absorb neutrons from these same decay reactions
and produce 36Cl
in the following reaction:
39K
+ n ® 36Cl
+ n + a
36Cl - Anthropogenic Source
36Cl
was also produced during nuclear bomb testing
in the middle of the 20th century. This thermonuclear
testing produced many tons of neutrons which could
readily react with 35Cl
to form 36Cl:
35Cl
+ n ® 36Cl
+ y
Early tests were conducted on Pacific
Ocean atolls. The neutrons from these earlier
tests were mostly absorbed by rock. Later tests
were conducted on barges in the Pacific Ocean,
which were surrounded by a ready supply of 35Cl
in seawater. The barge test explosions totaled
over 60 megatons and were responsible for most
of the stratospheric injection of 36Cl.
Approximately 17% of the neutrons from these tests
were absorbed by 35Cl
in the ocean.
(See
images
of nuclear testing from the DOE Nevada Operations
Office)
Fallout of 36Cl
can be seen in ice cores in Greenland (see diagram
below). The major fallout occurred between 1954
and 1958. 36Cl
did not stay in the atmosphere for long; its residence
time is approximately one week. The 36Cl
comes down to the earth as either dry fallout
or is cleaned out of the atmosphere by precipitation.
Since 1980, levels of 36Cl
have returned to near their natural atmospheric
concentrations. However, 36Cl
has been stored and recycled in the biosphere
and therefore elevated levels can still be found
in the hydrosphere as well.
Reprinted from Clark and Fritz 1997, p. 190. In
addition to 36Cl
fallout (data from Bentley et al. 1986) this also
shows tritium measured in precipitation at Ottowa,
Canada.
Decay
The ratio of 36Cl
to stable 37Cl
in the environment is ~700 * 10-15.
Because of this the abundance of 36Cl
for natural samples is most often reported as
36Cl/1015
Cl.
36Cl
spontaneously decays in two ways: 98% of it decays
through the emission of a beta particle, and the
remaining 2% decays through electron capture.
Beta
Emission
36Cl
® 36Ar
+ b-
Electron
Capture
36Cl
+ e-
® 36S
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Accelerator Mass
Spectrometry (AMS): 36Cl
is measured using AMS.
(See
the University of Arizona's Accelerator
Mass Spectrometry Lab for more information
on AMS dating)
Chlorine-36 is currently measured at two facilities
in the United States:
PRIME
Lab - Purdue University
Center
for Accelerator Mass Spectrometry - Lawrence
Livermore Ntl. Lab/University of California
(See
the New
Mexico Bureau of Geology and Mineral Resources'
Chemistry Laboratory for more information
on the preparation for solid phase samples for
36Cl)
Gas Source Mass Spectrometry: measures
d37Cl
Zymax
Laboratory
36
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36Cl
is a long-lived radioactive isotope. Due to its
long half-life (301,000 ± 4,000 years),
it can be used to date groundwater that is up
to a million years old. The chloride ion exists
in most natural waters in varying concentrations
due to the dissociation of sodium chloride. Most
silicate surfaces onto which chloride could adsorb
are negative. Due to their own negative charge,
chloride ions do not adsorb onto these silicate
surfaces and therefore move at approximatley the
same rate as the groundwater.
(See
Bentley et al., 1986b, for more information).
36Cl
can also be used as an indicator of modern recharge.
The testing of nuclear bombs in the 1950's put
high levels of 36Cl
into the stratosphere. High levels of 36Cl
in a groundwater sample indicate that recharge
occurred recently. Similarly, 36Cl
can be used to identify and quantify salinity
sources of a river system. 36Cl
will be used to identify salinity sources to the
Rio Grande in New Mexico. It is hoped that 36Cl
will be able to separate agricultural solutes
(which should be relatively young and have higher
levels of 36Cl)
from saline groundwater from deep groundwater
flow in sedimentary basins (which would have lower
levels of 36Cl).
(See
the SAHRA
Rio Grande page for more information on this
research project)
36Cl
can also be used to determine long-term average
groundwater recharge rates in arid regions or
more recent recharge rates using bomb pulse 36Cl
- Bentley, H.W., F.M. Phillips, and S.N. Davis,
Chlorine-36 in the terrestrial environment,
in Handbook of Environmental Isotope Geochemistry,
vol. 2, ed. by P.Fritz and J.-Ch. Fontes,
pp. 427-480, Elsevier, Amsterdam, 1986a.
- Bentley, H.W., F.M. Phillips, S.N. Davis,
P.L. Airey, G.E. Calf, D Elmore, M.A. Habermehl,
and T. Torgenson, Chlorine-36 dating of very
old ground water: I. The Great Artesian Basin,
Australia, Water Resour. Res. (22), 1991-2002,
1986b.
- Clark, I., and P. Fritz, Environmental
Isotopes in Hydrogeology, CRC Press, Boca
Raton, 1997.
- Phillips, F.M., Chlorine-36, in Environmental
Tracers in Subsurface Hydrology, ed. by
P.G. Cook and A.L. Herczeg, p. 299-348, Kluwer,
Boston, 2000.
USGS
Periodic Table - Chlorine
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