Radionuclides in the Environment and the Arizona TAMS...
Transcript of Radionuclides in the Environment and the Arizona TAMS...
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Radionuclides in the Environment and the Arizona TAMS Facility
Dr. Dana L. Biddulph
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Accelerator Mass Spectometry LabThe NSF-Arizona Accelerator Mass Spectrometry (AMS) Laboratory has operated as an NSF research and service Facility since 1981. It is jointly operated by the Physics and Geosciences Departments at the University of Arizona, serving as an interdisciplinary hub for a broad range of research and educational activities. During the last five years, 76 students have utilized this facility for their research, leading to 32 Doctoral and 13 Master's degrees completed from various universities. The facility has two tandem accelerators with terminal potentials of up to 3 million volts that are used for measuring cosmogenic isotopes with ultra-low abundances, such as 14C, 10Be, 129I, and 26Al. These are used to investigate many research topics, including tracer studies, radiometric dating, carbon cycle dynamics, terrestrial magnetic field, solar wind, ocean sciences, cosmic ray physics, meteoritics, geology, paleoclimate, faunal extinctions, hydrologic balance, forest fire-frequency, archaeology, art history, forensic science, and instrument development.
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Our Team
Research Scientists Staff
D. J. Donahue, Prof Emeritus L. ChengA. J. T. Jull, PI R. CruzJ. W. Beck L. HewittG. Hodgins S. LaMottaG. S. Burr T. LangeN. Lifton A. LeonardL. McHargue M. de MartinoD. Biddulph R. Watson
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Why Study Radioisotopes?The study of radioisotopes can reveal information about:
Abundances of target nucleiPathways in natureChanges in climate, ocean and atmospheric circulation patternsChanges in Earth’s magnetic fieldChanges in cosmic ray flux
Certain isotopes can also be used in radiometric dating techniques
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Production of Radioisotopes
FissiogenicCosmogenicAnthropogenic
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Radioisotopes Measured at Arizona
10Be, t½ = 1.5 x 106 years14C, t½ = 5730 years26Al, t½ = 7.3 x 105 years129I, t½ = 1.6 x 107 years
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Radiocarbon Dating14C dating can be used to date material that was living in the last few ten thousands of years and that received its carbon from the air14C is produced in the atmosphere
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Radiocarbon DatingThe production rate of 14C in the atmosphere has been relatively constant for tens of thousands of years
14C:12C = 1:1012
14C decays via beta decay to 14N with t1/2 = 5730 yearsWhen living, there is an equilibrium of 14C:12C When dead, the 14C decays away while the 12C doesn’t
Measurement of the activity of 14C or the amount of 14C determines the object’s age
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Figure 3. Increase of carbon-14 in the atmosphere due toatmospheric nuclear weapons testing in the period 1950-1963 AD.
Subsequently, the level has declined to about 110% of thepre-bomb level, due to re-equilibration with the ocean.
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14C Dating Applications
ArchaeologyOceanographyForensicsMeteorologyMeteoritesPaleoclimateArt History
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129I Applications
Cosmogenically produced when thermal neutrons strike a Xenon target in the atmosphere
May be used to track changes in Earth’s magnetic field or cosmic ray flux
Tracer studies from anthropogenic releases
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Measurement of Radioisotopes
Make an ion beam and separate the ions with magnetic and electric fieldsFirst the sample under study must be cleaned and purified. This is done with a variety of chemical extraction techniques.
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Sample Preparation for 14C Analysis
ABA PretreatmentCombust sample in vacuum chamber with CuOCollect liberated CO2 gasReduce CO2 with Zn in hot chamberAttach C with Fe in vessel
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Sample PreparationSample Preparation
CarrierCarrier--free iodine extraction and AMS target preparation.free iodine extraction and AMS target preparation.Annual sample sizes roughly 30 g; iodine content 3 to 5 ppm.Annual sample sizes roughly 30 g; iodine content 3 to 5 ppm.Clean samples with 10% acid dissolution and sonication.Clean samples with 10% acid dissolution and sonication.Rinse in distilled water, dissolve in 85% HRinse in distilled water, dissolve in 85% H33POPO44 + H+ H22O.O.Oxidize IOxidize I-- to Ito I22 with 2 drops 1 M NaNOwith 2 drops 1 M NaNO22, 5 to 10 ml CHCl, 5 to 10 ml CHCl33 for for iodine extraction.iodine extraction.Drain CHClDrain CHCl33 + I+ I22 into glass vial containing 10 to 15 mg 120 mesh into glass vial containing 10 to 15 mg 120 mesh Ag powder. Repeat extraction until fresh CHClAg powder. Repeat extraction until fresh CHCl33 remains clear.remains clear.Allow to settle overnight, then evaporate CHClAllow to settle overnight, then evaporate CHCl33..Rinse Ag + IRinse Ag + I22 mixture three times with distilled water and dry.mixture three times with distilled water and dry.Press into Al cathode for AMS measurement.Press into Al cathode for AMS measurement.
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Ion Source
Cesium sputter ion source – AMS
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Ion Beam Analysis
Rare isotope analysis not possible with just magnetsFor example, setting a magnet to pass 14C-1 allows all atomic and molecular species with mass 14 and charge –1 through
This includes 14N-1, 13CH-1 and 12CH2-1
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Solution: Break Molecules Apart
Accelerate ions to high energy (a few MeV)Collide ions with low pressure gas (Ar)Electrons removed from ions to create positively charged ionsMolecules with a net high positive charge are unstable and break up
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Electrostatic AcceleratorCockroft – Walton type accelerator shown here
High voltage is created by movingcharge up a ladder of diodes.
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Tandem Van de Graaf Accelerator
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AMS Facility
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AMS Facility
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F = q(E + v x B) = ma
For Magnetic Case Only, E = 0, and vand B are perpendicular
F = qvB = ma = mv2/r for a constant radius of curvature r
Squaring and arranging terms,
B2r2 = m2v2/q2 = 2mE/q2, for kinetic energy E
Therefore, magnetic analysis selects all ions with the same
mE/q2
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F = q(E + v x B) = ma
For Electric Case Only, B = 0
F = qE = ma = mv2/r for a constant radius of curvature r
Er = mv2/q = 2E/q, for kinetic energy E
Therefore, electrostatic analysis selects all ions with the same
E/q
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Pre-acceleration energyE = qVNominally 60 keV
Post-acceleration energyE = qV = TV(1 + q)Example: 14C+3
E = 2.5MeV(1 + 3) = 10MeV
Energy Calculations
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Scaling
Passing 13C+3 through magnet, B = 4190 gTo pass 14C+3, must change magnetic field
B2)14C+3 / B2)13C+3 = ME/q2)14C+3/ME/q2)13C+3
= 14E/q2 / 13E/q2
or B)14C+3 = sqrt(14/13)B)13C+3
= 1.038*4190 g = 4348 g
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Now that the Molecules are Gone…
Still not done!Interference from isobars and molecular fragments can still overwhelm signal if m/q = M/Q
Examples: 10B+2 and 10Be+2, 7Li+2 and 14C+4, 97Mo+3 and 129I+4
For these cases nuclear physics techniques can be employed
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Ion Charge Transmission (%) Interferences
+1 Not Measured Molecular
+2 30 Molecular
+3 17 43Ca+1, 86Kr +2, 86Sr+2
+4 7.8 97Mo+3
+5 4.1 103Rh+4
+6 2.1 43Ca+2, 86Kr+4, 86Sr+4
+7 0.7 37Cl+2, 92Zr+5, 92Mo+5, 111Cd+6
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surface barrier “dE/dx” detector
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Gas ionization detector: 10Be detection
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Questions?