Jay Davis Lawrence Livermore National Laboratory

Transcription

1 Accelerator Mass Spectrometry: Isotopic Science Tools from Archaeology to Zoology Jay Davis Lawrence Livermore National Laboratory AAPT/APT Meeting Sacramento August 2, 2004

2 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 ,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

3 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

4 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 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

5 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

6 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.

7 64-sample wheel mounted in a Cs-sputter ion source

8 Spectrometer in Radiocarbon Mode Vacuum Carbon Foil ( µm ) CH H C C 1 1+ H 12 CH 14 2 C 13 CH High Energy Mass Spectrometer C 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 C 14 P-10 Counting Gas 400V

9

10 Research Operations Stable Ion Research Number of Isotope Current Background Accuracy Samples 14 C BioChem 125 µa 12 C 1x % C Natural 280 µa 12 C ka 0.4% H 35 µa 1 H 2x % Be 25 µa 9 BeO 1x % Al 1.5 µa 27 Al 2x % Cl 45 µa 35 Cl 3x % Ca 0.8 µa 40 CaF 3 3x % I 8 µa 127 I 2x % Pu 40 na 239 PuO (eq.)<1x10 6 atoms 5% 1070

11 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

12 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 Year (AD)

13 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)

14 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

15 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

16 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 ) y = x R= Biogenic carbon mass Fossil carbon mass Total carbon content in aerosol ( µg/m 3 )

17 The global inventory of carbon isotopes changed with the advent of the atomic age ²14C (per mil) Year

18 Ocean uptake and redistribution of Bomb- 14 C Simulated Surface Ocean Response Atmospheric Forcing C ( ) NH C 0 SH Surface Ocean Response 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

19 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

20 Tritium levels in tree cellulose accurately reflect environmental releases Decay Corrected Tritium Activity (T.U) Total Tritium Released Decay Corrected Euc A Year 1969: Tritium 1 st used in Building : NTLF formally established

21 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 Cu) of 63 Ni were formed by the passage of fast neutrons through copper materials

22 63 Ni-AMS measurements validated neutron dose estimates 1.0E+07 Measured Atoms Ni-63 per g Cu 1.0E E+05 DS86 Nature 424: 539 (2003) 1.0E 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

23 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

24 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.

25 A central advantage: AMS enables long-term studies in humans Calcium ( 41 Ca) elimination Folate ( 14 C) disposition

26 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

27 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 ( and search on CAMS