J.E. McAninch

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.

Biological sample preparation and 41Ca AMS measurement at LLNL

Abstract Calcium metabolism in biology may be better understood by the use of 41Ca tracer, although requiring detection by accelerator mass spectrometry (AMS). Methodologies for preparation of urine samples and subsequent AMS measurement were investigated. Novel attempts at preparing CaH2 were unsuccessful, but CaF2 of sufficient purity could be produced by precipitation of calcium from urine as oxalate, followed by separation of calcium by cation exchange chromatography and washing the CaF2 precipitate. The presence of some remaining impurities could be compensated for by selecting the appropriate accelerated ion charge state for AMS. The use of projectile X-rays for isobar discrimination was explored as an alternative to the conventional d E d x detector.

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).

Accelerator mass spectrometry of 63Ni using a gas-filled magnet at the Munich Tandem Laboratory

Abstract The detection of 63 Ni (T1/2=100.1 yr) by means of accelerator mass spectrometry (AMS) using a gas-filled magnet (GFM) is described. The experimental setup includes a dedicated ion source, a 14 MV MP tandem, a GFM and a multi-anode ionization chamber. First results indicate a background level of 63 Ni/Ni ratios as low as 2×10−14. This sensitivity will allow – for the first time ever – to detect 63 Ni induced by fast neutrons in copper samples from Hiroshima and Nagasaki, even for distances beyond 1500 m from the hypocenters. Thus, it will be possible to reconstruct experimentally the neutron doses of the A-bomb survivors from Hiroshima and Nagasaki.

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.

Elements in biological AMS

AMS (Accelerator Mass Spectrometry) provides high detection sensitivity for isotopes whose half-lives are between 10 years and 100 million years. {sup 14}C is the most developed of such isotopes and is used in tracing natural and anthropogenic organic compounds in the Earth`s biosphere. Thirty-three elements in the main periodic table and 17 lanthanides or actinides have long lived isotopes, providing potential tracers for research in elemental biochemistry. Overlap of biologically interesting heavy elements and possible AMS tracers is discussed.

Detection of99Tc by accelerator mass spectrometry: Preliminary investigations

Accelerator mass spectrometry (AMS) is an established technique for the detection of long-lived radionuclides at environmental levels. At LLNL, planned facility upgrades and advances in detection techniques are allowing us to explore the applicability of AMS to isotopes not previously pursued. One such isotope is99Tc. We have performed a number of preliminary tests to examine the technical feasibility of AMS for the detection of99Tc. The questions addressed were negative ion production in the cesium sputter source, transport efficiency for the ions through the spectrometer, and detection efficiency for99Tc ions after the spectrometer. Based on the positive results of these tests, we have begun to develo measurement protocol.

Accelerator mass spectrometry measurements of actinide concentrations and isotope ratios

Accelerator mass spectrometry (AMS) is an established technique for high throughput measurements of long-lived radioisotopes at very low abundance. At the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory (LLNL), we are extending our AMS capabilities to the measurement of Plutonium and other actinides for application in a number of fields such as environmental fate and transport and bioassays of potentially exposed populations.