M.L. Roberts

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.

Ten years of sourcery at CAMS/LLNL – Evolution of a Cs ion source

The present performance and status of the LLNL AMS ion source and the rationale for the series of changes which led to the present design are discussed.

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.

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.

Ion-Source Modeling and Improved Performance of the CAMS High-Intensity Cs-Sputter Ion Source

The interior of the high-intensity Cs-sputter source used in routine operations at the Center for Accelerator Mass Spectrometry (CAMS) has been computer modeled using the program NEDLab, with the aim of improving negative ion output. Space charge effects on ion trajectories within the source were modeled through a successive iteration process involving the calculation of ion trajectories through Poisson-equation-determined electric fields, followed by calculation of modified electric fields incorporating the charge distribution from the previously calculated ion trajectories. The program has several additional features that are useful in ion source modeling: (1) averaging of space charge distributions over successive iterations to suppress instabilities, (2) Child’s Law modeling of space charge limited ion emission from surfaces, and (3) emission of particular ion groups with a thermal energy distribution and at randomized angles. The results of the modeling effort indicated that significant modification of the interior geometry of the source would double Cs+ ion production from our spherical ionizer and produce a significant increase in negative ion output from the source. The results of the implementation of the new geometry were found to be consistent with the model results.

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.

The stand-alone microprobe at Livermore

Lawrence Livermore National Laboratory (LLNL) and Sandia National Laboratories/California have jointly constructed a new stand-alone microprobe facility. Although the facility was built to develop a method to rapidly locate and determine elemental concentrations of micron scale particulates on various media using PIXE, the facility has found numerous applications in biology and materials science. The facility is located at LLNL and uses a General Ionex Corporation Model 358 duoplasmatron negative ion source, a National Electrostatics Corporation 5SDH-2 tandem accelerator, and an Oxford triplet lens. Features of the system include complete computer control of the beam transport using LabVIEW TM for Macintosh, computer controlled beam collimating and divergence limiting slits, automated sample positioning to micron resolution, and video optics for beam positioning and sample observation. Data collection is accomplished with the simultaneous use of as many as four EG&G Ortec IGLET-X TM X-Ray detectors, digital amplifiers made by X-Ray Instruments and Associates (XIA), and LabVIEW TM for Macintosh acquisition software.

The LLNL ion source — past, present and future

Abstract The Multi-user Tandem Laboratory (MTL) at LLNL is a general purpose laboratory for analysis using ion beam techniques. Our initial interest in AMS was for large throughput with modest precision. Toward this goal, we purchased a prototype GIC spherical ionizer source with a 60 sample cassette changer. The source has been extensively modified to increase reliability and to adapt it for AMS operation. The sample changing mechanism was completely rebuilt, pumping was increased in critical areas, electrical stress has been reduced in areas where failures were frequent and protection of insulators from cesium vapor has been increased. We are limiting the divergence to 20 mrad to match the present injection system and are only able to get about 50 μA of stable carbon beam in this configuration. Details of failures, changes to date and planned improvements will be discussed.

129I interlaboratory comparison: Phase II results ☆

Large discrepancies seen in a Phase I 129I Accelerator Mass Spectrometry (AMS) interlaboratory comparison have prompted a subsequent Phase II comparison. In Phase II of the 129I AMS interlaboratory comparison, three separate laboratories prepared AgI from two environmental samples (IAEA 375 soil and maple leaves). Each laboratory used its own chemical preparation method; each of these methods being considerably different. The resulting six samples (two sets of three) were then distributed to the participating 129I AMS facilities and 129I/127I ratios measured. Results and discussion of the Phase II interlaboratory comparison are presented.

Ion-optics calculations of the LLNL AMS system for biochemical 14C measurements

A dedicated AMS system for biochemical 14C measurements is being built at the Center for Accelerator Mass Spectrometry. The system is centered around a National Electrostatics Corporation Model 3SDH-1 1-MV Pelletron accelerator and is designed to accept two ion sources. The LLNL 64-sample Cs-sputter ion source with its zoom lens beam line is attached to one port of the electrostatic switching element. To insure efficient coupling of the source to the acceptance of the accelerator, ion-optics calculations of the low-energy injection beam line have been conducted; the results of which were used to determine the layout of the ion-optical components of the beam line. Beam tests of the low-energy injection line show that beam behavior was accurately predicted by the calculations.