I.D. Proctor

LLNL/UC AMS facility and research program

Abstract The Lawrence Livermore National Laboratory (LLNL) and the University of California (UC) now have in operation a large AMS spectrometer built as part of a new multiuser laboratory centered on an FN tandem. AMS measurements are expected to use half of the beam time of the accelerator. LLNL use of AMS is in research on consequences of energy usage. Examples include global warming, geophysical site characterization, radiation biology and dosimetry, and study of mutagenic and carcinogenic processes. UC research activities are in clinical applications, archaeology and anthropology, oceanography, and geophysical and geochemical research. Access is also possible for researchers outside the UC system. The technological focus of the laboratory is on achieving high rates of sample throughput, unattended operation, and advances in sample preparation methods. Because of the expected growth in the research programs and the other obligations of the present accelerator, we are designing a follow-on dedicated facility for only AMS and microprobe analysis that will contain at least two accelerators with multiple spectrometers.

Ion microbeam tomography

Abstract Proton beams with energies of 5 and 7 MeV are focused to 5 μm and used to produce tomograms of capillary tubes and low-density foams. In this energy range, proton energy loss is primarily due to interactions with electrons. Therefore, by measuring the residual energy of protons transmitted through samples in a manner similar to that used for Scanning Transmission Ion Microscopy (STIM), and reconstructing a cross-sectional image from multiple projections, we can map out spatial variations in electron density due to sample geometry and composition. In our experimental arrangement, the sample is translated and rotated in a stationary proton beam. Transmitted proton energies are measured using a silicon surface barrier detector. Tomographie reconstructions are produced from the calculated line-average densities using a procedure based on a filtered backprojection algorithm developed for X-ray computed tomography (CT) systems. The technique is especially useful in characterizing samples where large variations in Z or low total density limit the applicability of X-ray CT analysis.

The LLNL AMS facility

The AMS facility at Lawrence Livermore National Laboratory (LLNL) routinely measures the isotopes 3H, 7Be, 10Be, 14C, 26Al, 36Cl, 41Ca, and 129I. During the past two years, over 30000 research samples have been measured. Of these samples, approximately 30% were for 14C bioscience tracer studies, 45% were 14C samples for archaeology and the geosciences, and the other isotopes constitute the remaining 25%. During the past two years at LLNL, a significant amount of work has gone into the development of the Projectile X-ray AMS (PXAMS) technique. PXAMS uses induced characteristic X-rays to discriminate against competing atomic isobars. PXAMS has been most fully developed for 63Ni but shows promise for the measurement of several other long lived isotopes. During the past year LLNL has also conducted an 129I interlaboratory comparison exercise. Recent hardware changes at the LLNL AMS facility include the installation and testing of a new thermal emission ion source, a new multi-anode gas ionization detector for general AMS use, re-alignment of the vacuum tank of the first of the two magnets that make up the high energy spectrometer, and a new cryo-vacuum system for the AMS ion source. In addition, we have begun design studies and carried out tests for a new high-resolution injector and a new beamline for heavy element AMS.

The new LLNL AMS spectrometer

Abstract The multi-user tandem laboratory at Lawrence Livermore is a new facility dedicated to AMS and a variety of other ion-beam analysis techniques. The AMS spectrometer design aims and implementation are presented here, and present performance and planned improvements are discussed.

Application of AMS to the biomedical sciences

Abstract Radio-isotopic tracers are a useful tool in numerous areas of biomedical research, including metabolism, pharmacokinetics, and the detailed study of biomolecular interactions. Accelerator mass spectrometry was suggested as a tool for the biomedical sciences shortly after its invention, but few attempts to use its sensitivity in such research have been reported. We have examined some of the strengths and limitations of the technique and find that AMS has a sensitivity advantage over decay-counting for the long-lived radio-isotopes and for shorter-lived, common radiotracers. The advantage can be translated into the use of much smaller sample sizes and much lower radio-isotope concentrations, both of which present new opportunities for biochemical tracing and human research. New approaches to separation and preparation of the material to be assayed for radio-tracers will be developed to take advantage of the sensitivity and specificity. Most biochemical laboratories have used radioactive isotopes as tracers and their facilities have been contaminated with unacceptably high levels of these tracers. Careful protocols and/or new facilities are required to prevent contamination of the AMS samples.

129I interlaboratory comparison

An interlaboratory comparison exercise for 129I has been organized and conducted. A total of seven laboratories participated in the exercise to either a full or limited extent. In the comparison, a suite of 11 samples was used. This suite of standards contained both synthetic “standard type” materials (i.e., AgI) and environmental materials. The isotopic 129I127I ratio of the samples varied from 10−8 to 10−14. Preliminary results of the comparison are presented.

Accelerator mass spectrometry in the biomedical sciences: applications in low-exposure biomedical and environmental dosimetry☆

Abstract We are utilizing accelerator mass spectrometry as a sensitive detector for tracking the disposition of radioisotopically labeled molecules in the biomedical sciences. These applications have shown the effectiveness of AMS as a tool to quantify biologically important molecules at extremely low levels. For example, AMS is being used to determine the amount of carcinogen covalently bound to animal DNA (DNA adduct) at levels relevent to human exposure. Detection sensitivities are 1 carcinogen molecule bound in 1011 to 1012 DNA bases, depending on the specific activity of the radiolabeled carcinogen. Studies have been undertaken in our laboratory utilizing heterocyclic amine food-borne carcinogens and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent environmental carcinogen, to study the metabolism of carcinogens at low doses. In addition, AMS is being used to detect the presence of rare proteins (mutant forms of protamine) in human sperm. Approximately l per 106 sperm analyzed contain the rare form of the protamine. Protamine isolated from this small number of cells is being analyzed by AMS, following 14C labeling. Thus, AMS can be used to verify the identity of an extremely small amount of biological material. Furthermore, an additional improvement of 2 orders of magnitude in the sensitivity of biomedical tracer studies is suggested by preliminary work with bacterial hosts depleted in radiocarbon. Other problems in the life sciences where detection sensitivity or sample sizes are limitations should also benefit from AMS. Studies are underway to measure the molecular targeting of cancer chemotherapeutics in human tissue and to pursue applications for receptor biology. We are also applying other candidate isotopes, such as 3 H (double labeling with 14 C) and 41 Ca (bone absorption) to problems in biology. The detection of 36 Cl and 26 Al have applications for determination of human neutron exposure and understanding neurological toxicity, respectively. The results described here with 14 C-labeled molecules coupled with new isotope applications clearly show AMS technology to be an important new tool for the biomedical sciences community.

Measurement of 63Ni and 59Ni by accelerator mass spectrometry using characteristic projectile X-rays

The long-lived isotopes of nickel ({sup 59}Ni, {sup 63}Ni) have current and potential use in a number of applications including cosmic radiation studies, biomedical tracing, characterization of low-level radioactive wastes, and neutron dosimetry. Methods are being developed at LLNL for the routine detection of these isotopes by AMS. One intended application is in Hiroshima dosimetry. The reaction {sup 63}Cu(n,p){sup 63}Ni has been identified as one of a small number of reactions which might be used for the direct determination of the fast neutron fluence emitted by the Hiroshima bomb. AMS measurement of {sup 63}Ni(t{sub 1/2} = 100 y) requires the chemical removal of {sup 63}Cu, which is a stable isobar of {sup 63}Ni. Following the electrochemical separation of Ni from gram-sized copper samples, the Cu concentration is further lowered to < 2 x 10{sup -8} (Cu/Ni) using the reaction of Ni with carbon monoxide to form the gas Ni(CO){sub 4}. The Ni(CO){sub 4} is thermally decomposed directly in sample holders for measurement by AMS. After analysis in the AMS spectrometer, the ions are identified using characteristic projectile x-rays, allowing further rejection of remaining {sup 63}Cu. In a demonstration experiment, {sup 63}Ni was measured in Cu wires (2-20 g) which had been exposed to neutrons from a {sup 252}Cf source. We successfully measured {sup 63}Ni at levels necessary for the measurement of Cu samples exposed near the Hiroshima hypocenter. For the demonstration samples, the Cu content was chemically reduced by a factor of 10{sup 12} with quantitative retention of {sup 63}Ni. Detection sensitivity (3{sigma}) was {approximately}20 fg {sup 63}Ni in 1 mg Ni carrier ({sup 63}Ni/Ni {approx} 2 x 10{sup -11}). Significant improvements in sensitivity are expected with planned incremental changes in the methods. Preliminary results indicate that a similar sensitivity is achievable for {sup 59}Ni (t{sub 1/2} = 10{sup 5} y).

PXAMS — projectile X-ray AMS: X-ray yields and applications☆

Characteristic X-rays have recently been explored as a method for the detection and identification of ions in accelerator mass spectrometry (AMS) [H. Artigalas et al., Nucl. Instr. and Meth. B 92 (1994) 227; M. Wagner et al., Nucl. Instr. and Meth. B 89 (1994) 266]. After analysis in the AMS spectrometer, the ions stop in an appropriately chosen target and the induced X-rays identify the ions by atomic number. For the application of AMS to higher mass isotopes, characteristic X-rays allow significantly better discrimination of competing atomic isobars than is possible using energy loss detectors. Characteristic X-rays also show promise as a convenient component in hybrid detection systems. Measurements of X-ray yields are presented for Si, Fe, Ni, Se, Mo, and Pd ions of 0.5 – 2 MeV/amu. The yields rise by more than a factor of 10 over this energy range, and approach 1 X-ray per incident ion at 2 MeV/amu for the lighter ions. Preliminary work on the application of PXAMS to the detection of 79Se is described.

ULTRA-SEPARATION OF NICKEL FROM COPPER METAL FOR THE MEASUREMENT OF 63NI BY AMS

Measurements of 63Ni (t12 = 100 yr) produced by the reaction 63Cu(n,p)63Ni could be used in the assessment of fast-neutron fluence from the Hiroshima atomic bomb. Such measurements would add new information to help resolve the current discrepancy between measured thermal neutron activation values and those calculated with the DS86 dosimetry system. It has been estimated that the 63Ni production at 5 m from the hypocenter was (1.4 ± 0.1) × 107 atoms/g Cu. Because of its sensitivity, accelerator mass spectrometry (AMS) is ideal for measurements at this low level. However, 63Ni has to be separated from large amounts of stable atomic isobar 63Cu (69% of pure Cu). In this study, a procedure is presented for the electrochemical separation of ultra-low amounts of Ni from Cu. The method was developed using samples of electrical Cu wire that were irradiated with fission neutrons from a 252Cf source. The wire samples were electrochemically dissolved in a solution containing 1 mg of Ni carrier. The Cu was selectively deposited on a cathode at controlled potential. Measurements of total Ni after electroseparation indicate ∼ 100% carrier recovery. To prevent Cu contamination, AMS targets were prepared by nickel carbonyl generation. The AMS results show a successful quantitative separation of ∼ 107 atoms of 63Ni from 2–20 g samples of Cu.