Accelerator Mass Spectrometry: Isotopic Science Tools from Archaeology to Zoology Jay Davis Lawrence Livermore National Laboratory AAPT/APT Meeting Sacramento August 2, 2004
Accelerator Mass Spectrometry was one of the most remarkable inventions of the 1970s Counting rare long-lived isotopes atom by atom, rather than observing their beta decays, produces gains in sensitivity of 1000-1,000,000 in detection For naturally occurring cosmogenic isotopes such as 10 Be, 14 C, 26 Al, 36 Cl, and 129 I, new uses of these isotopes as tracers and chronometers are possible The impacts in archaeology and the geosciences are immense For man-made (or introduced) isotopes such as 3 H, 14 C, 41 Ca and 239 Pu, new opportunities for tracer experiments emerg The impacts in the biosciences and forensic sciences are immense as well
The impacts of AMS are worth serious discussion In archaeology, the ability to carbon date with milligram or smaller samples vastly expanded the quantitative state of the field In many cases, one did not have to choose between the sample and the date In the geosciences, the ability to track 14 CO 2 flow from nuclear testing into the oceans with 1 liter samples rather than 1000 liter samples allowed inference of much more detail in global carbon cycle studies In the biosciences, the ability to use 3 H and 14 C as tracers at virtually no dose risk and at higher sensitivity makes possible animal and human experiments at the environmentally relevant doses
Radiocarbon dating is the best known application of Accelerator Mass Spectrometry Cosmic ray secondaries continuously produce 14 C in the atmosphere The 14 C then oxidizes to 14 CO 2 All living things thus contain approximately one 14 C atom per 10 12 12 C atoms as a result of respiration When anything living dies, the 14 C is no longer steadily replenished, so its decay half-life of 5730 years provides a clock. Depending on the sample, radiocarbon dates out to about 60,000 years before present are possible
Accelerator mass spectrometry was originally developed to advance applications of carbon dating Shroud of Turin (14 th Century AD) This is the classic case of the artifact that would have been unacceptably modified to get sufficient material for conventional radiocarbon dating
Sample preparation for AMS generally requires the conversion of samples to a solid matrix Organic sample Heat/CuO Combustion CO 2 + N 2 + H 2 O Heat/Zn Reduction Carbon (graphite) Cathode Ion source 14 C sample prep and a mpressor picture.
64-sample wheel mounted in a Cs-sputter ion source
Spectrometer in Radiocarbon Mode Vacuum Carbon Foil ( 0.02-0.05 µm ) 13 + + 12 141 4 + + 12 + 14 13 + 13 CH 2+ 1 1+ H C 12 3+ 14 4+ C 1 1+ H 12 CH 14 2 C 13 CH High Energy Mass Spectrometer C 1+...6 + +7 MV C 64 samples Negative Ion Source 13 C 4+ Faraday Cup Tandem Electrostatic Accelerator Low Energy Mass Spectrometer 14 C 4+ Ion Identification Detector Rigidity Filter Velocity (Wien) Filter +300V Vacuum 14 14 14 4+ C 14 P-10 Counting Gas 400V
Research Operations 2000-2001 Stable Ion Research Number of Isotope Current Background Accuracy Samples 14 C BioChem 125 µa 12 C 1x10 14 2% 7450 14 C Natural 280 µa 12 C 52-57 ka 0.4% 13940 3 H 35 µa 1 H 2x10 15 5% 700 10 Be 25 µa 9 BeO 1x10 14 3% 2540 26 Al 1.5 µa 27 Al 2x10 15 5% 1940 36 Cl 45 µa 35 Cl 3x10 15 3% 1010 41 Ca 0.8 µa 40 CaF 3 3x10 13 5% 340 129 I 8 µa 127 I 2x10 14 5% 100 239-244 Pu 40 na 239 PuO (eq.)<1x10 6 atoms 5% 1070
Famous things that have been radiocarbon dated Kennewick Man Found in July 1996 Almost immediately controversial Who owns? Indian tribes? Local officials? Scientists? Bone dated - 9,000 years old Clearly pre-columbian
Reconstruction of climate history West Walker River, California Drought-tolerant trees must have grown when the river bed was dry Carbon dating reveals that California has experienced 100-year droughts Drought Periods 900 1000 1100 1200 1300 1400 1500 1600 1700 Year (AD)
Water Resources: When does recharge occur? When: Western Nevada Carbon-14 ages indicate recharge of water existing in valleys was during the Pleistocene (>10,000 yrs)
Assessment of air quality in National Parks Fossil fuel contributions are 14 C-free Biogenic sources (e.g., trees) contain modern 14 C Accordingly, sources of organic aerosols can be determined based on 14 C content Collaborations with UC Davis & Colorado State University
Assessment of air quality in National Parks Fossil fuel contributions are 14 C-free Biogenic sources (e.g., trees) contain modern 14 C Accordingly, sources of organic aerosols can be determined based on 14 C content Collaborations with UC Davis & Colorado State University
Biogenic carbon was the major contributor to Yosemite aerosols during the Summer of 02 Samples collected July-August 02 Biogenic or fossil carbon content in aerosol ( µg/m 3 ) 10 8 6 4 2 0 y = -0.69929 + 0.99973x R= 0.99069 Biogenic carbon mass Fossil carbon mass 2 3 4 5 6 7 8 9 10 Total carbon content in aerosol ( µg/m 3 )
The global inventory of carbon isotopes changed with the advent of the atomic age 1000 900 800 700 600 500 ²14C (per mil) 400 300 200 100 0-100 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Ocean uptake and redistribution of Bomb- 14 C Simulated Surface Ocean Response 1000 800 Atmospheric Forcing 250 14C ( ) 600 400 NH 150 200 50 14 C 0 SH Surface Ocean Response -50-100 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000-150 The timing and amplitude of the post-bomb maximum of 14 C is a diagnostic of air-sea exchange and the mixing of bomb 14 CO 2 LLNL Coupled OBGCM
LBNL Tritium Release Reconstruction PhD thesis for Adam Love (UC Berkeley) Currently a postdoc in the Env. Sci. Div. investigating env. fate of CW Stack Hypothesis: trees will record tritium signal in cellulose Tritium samples measured by AMS at sub-annual intervals throughout the core
Tritium levels in tree cellulose accurately reflect environmental releases Decay Corrected Tritium Activity (T.U) 50000 45000 40000 35000 30000 25000 20000 15000 10000 5000 0 700 600 500 400 300 200 100 0 1960 1965 1970 1975 1980 1985 1990 1995 2000 Total Tritium Released Decay Corrected Euc A Year 1969: Tritium 1 st used in Building 75 1982: NTLF formally established
Measurement of 63 Ni as a surrogate for neutron flux from the Hiroshima atomic bomb Survivors of the atomic bomb are used to estimate cancer risks due to radiation exposure Doubts have been raised over the accuracy of calculated neutron doses Trace amounts (parts per 10 18 Cu) of 63 Ni were formed by the passage of fast neutrons through copper materials
63 Ni-AMS measurements validated neutron dose estimates 1.0E+07 Measured Atoms Ni-63 per g Cu 1.0E+06 1.0E+05 DS86 Nature 424: 539 (2003) 1.0E+04 0 500 1000 1500 2000 Distance from hypocenter (m) Measured levels of 63 Ni are in line with calculations at distances relevant to survivors, so concerns about large dose discrepancies can be discounted
AMS allows us to tackle a set of hard biomedical problems In toxicology, pharmacology and nutrition Are data from high dose experiments relevant? Can we connect high and low dose regimes? Can we connect animal models with human risk? Can we conduct studies with human subjects? We can do all of these -- but the penetration time across disciplinary membranes has been longer than you might think -- or than we expected
AMS enables the tracing of isotope-labeled compounds at environmental doses Exposure Metabolism Body distribution and fate studies Tri-Valley Herald, 8/21/00 Metabolism studies DNA adducts Molecular studies Response Risk assessment Dose Science, vol.. 271, 1996.
A central advantage: AMS enables long-term studies in humans Calcium ( 41 Ca) elimination Folate ( 14 C) disposition
Forensic techniques being currently employed Determining the age of recently grown biological materials using the radiocarbon bomb curve To assist in identifying possible sources To aid in reconstruction of operations or facilities subsequently decontaminated Determining Plutonium or Uranium isotopics of weapon debris To identify fuel characteristics of unexploded (or dud) devices To understand fuel burn in exploded devices Assessing Pu body burdens of individuals exposed to Pu in handling weapons or fuel materials To identify sources of proliferant activities
The uses of this tool continue to expand To the applied physicist, the discovery of accelerator mass spectrometry is almost as exciting as the discovery of radiation itself Use of the technique is limited by the cleverness of the investigator and his or her understanding of problems in allied fields For further information, go to the LLNL website (www.llnl.gov) and search on CAMS