D.W. Heikkinen

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 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.

Density and composition analysis using focused MeV ion mubeam techniques

Abstract Nuclear muscopy uses focused MeV ion mubeams to non-destructively characterize materials and components with mun scale spatial resolution. Although a number of accelerator-based mubeam methods are available for materials analysis, this paper centers on the techniques of Ion mutomography (IMT) and Particle-Induced X-ray Emission (PIXE). IMT provides quantitative three-dimensional density information with mun-scale spatial resolution and 1% density variation sensitivity. Recently, IMT has become more versatile because greater emphasis has been placed on understanding the effects of reconstruction artifacts, beam spatial broadening, and limited projection data sets. PIXE provides quantitative elemental information with detection sensitivities to 1 μg/g or below in some instances. By scanning the beam, two-dimensional maps of elemental concentration can also be recorded. However, since X-rays are produced along the entire path of the ion beam as it penetrates the sample, these measurements only give depth-averaged information in general. PIXE tomography (PIXET) is the natural extension from conventional PIXE analysis to the full three-dimensional measurement and forms the bridge linking the complementary techniques of PIXE and IMT. This paper presents recent developments and applications of these ion beam techniques in a diverse range of fields including characterizing metal-matrix composites, biological specimens and inertial confinement fusion targets.

The new nuclear microprobe at Livermore

Abstract Lawrence Livermore National Laboratory (LLNL) and Sandia National Laboratories/California have jointly constructed a new nuclear microprobe beamline. This beamline is located on the LLNL 10 MV tandem accelerator and can be used for multidisciplinary research using PIXE, PIGE, energy loss tomography, or IBS techniques. Distinctive features of the beamline include incorporation of magnet power supplies into the accelerator control system, computer-controlled object and image slits, automated target positioning to sub-micron resolution, and video optics for beam positioning and observation. Mitigation of vibrations was accomplished with vibration isolators and a rigid beamline design while integral beamline shielding was used to shield from stray magnetic fields. Available detectors include a wavelength dispersive X-ray spectrometer, a High-Purity Germanium detector (HPGe), a Lithium-Drifted Silicon X-Ray detector (SiLi), and solid state surface barrier detectors. Along with beamline performance, results from recent measurements on determination of trace impurities in an International Thermonuclear Experimental Reactor (ITER) super conducting wire strand, determination of Ca/Sr ratios in seashells, and determination of minor and trace element concentrations in sperm cells are presented.

Proton energy straggling measurements in aluminum, titanium, silver and tungsten foils

Abstract Measurements of the transmitted energy straggling are made using a silicon surface barrier detector for 2 to 7 MeV protons incident on thin foil targets of Al, Ti, Ag and W. The straggling widths are studied as a function of target thickness, proton energy and target species. The combination of several incident energies with many target thicknesses provides data for the investigation of energy straggling in the region where Tschalars theory is applicable. The theoretical estimates are found to agree with the measured values for higher energy beams, but are about 10% lower than the data for lower incident energies. A simple empirical expression for the straggling is given which agrees with the experimental data for energy losses up to 60%.

The RTNS-II fusion materials irradiation facility☆

The Rotating Target Neutron Source (RTNS-II) facility provides an intense source of 14 MeV neutrons for the fusion energy programs of Japan and the United States. Each of the two identical accelerator-based neutron sources is capable of providing source strengths in excess of 3 × 1013n/s using deuteron beam currents up to 150 mA. At the present time, the facility operates a minimum total of twenty shifts (8 h) per week using both neutron sources. Because of funding cut-backs, future operations will be limited to one neutron source. The possibility exists to increase neutron output by combining the high voltage power supplies, thereby increasing the current capability of the operating neutron source. This would allow approximately a factor of three increase in neutron output. Using existing equipment, an ion source test stand has been constructed to improve ion source output. Conceptual design studies have been made concerning a factor of ten increase in neutron output. Detailed studies of needed target improvements have been made and will be discussed. The present status of the facility, as well as the various upgrade options, will be described in detail.

Ion source development at RTNS-II☆

Abstract Results are reported for an ongoing effort to optimize D+ beam production by the MATS-III ion source used at RTNS-II. The characteristics of the source have been determined. Particular attention was paid to the extraction geometry and plasma production. The plasma spatial and temporal uniformity has been examined. The seven aperture triode geometry has been varied to optimize neutron production. This includes beamlet steering and electrode gapping as well as aperture shaping.

Materials analysis at the SNL/LLNL nuclear microprobe

Abstract A wide variety of materials have been analyzed using the nuclear microprobe in Livermore, including metals, low density foams, compound semiconductors for room temperature detectors, glasses and particulates. A brief overview of our recent work is given, in addition to a broader discussion of three specific applications. In the first application, Ion Microtomography (IMT) is used to produce three-dimensional mass density maps of low density foams being developed as targets for Z-pinch physics experiments. In the second application, we produce two-dimensional elemental maps of CdZnTe crystals being developed as room temperature radiation detectors. The maps help determine the effect of local variations in stoichiometry on detector performance. Finally, we discuss the measurement of trace elements and radial diffusion profiles in superconducting wire using micro-scale Particle-Induced X-ray Emission (PIXE).

The rotating target neutron source II facility: Operational summary☆

Abstract The Rotating Target Neutron Source II facility (RTNS-II) operated for over nine years. Its purpose was to provide high intensities of 14 MeV neutrons for materials studies in the fusion energy program. For the period from 1982–1987, the facility was supported by both the USA (Department of Energy) and Japan (Ministry of Education, Culture and Science). RTNS-II contains two accelerator-based neutron sources which use the T(d, n) 4 He reaction. In this paper we will summarize the operational history of RTNS-II. Typical operating parameters are given. In addition, a brief description of the experimental program is presented. The current status and future options for the facility are discussed.