Center For Accelerator Mass Spectrometry Research Articles

Use of a correlated compound-binomial model to assess absence of non-counting noise in Pu-isotope ratios measured by AMS at LLNL

Methods for error analysis pertaining to isotopic ratio measures made by accelerator mass spectrometry (AMS) typically focus on accuracy/precision of detected counts of a single rare isotope (e.g. 14C), relative to a current strength indicating the abundance of an isotope present in far greater amounts (e.g. 13C). Because AMS analysis of the relative abundance of actinide isotopes involves comparing detected counts that each correspond to a relatively rarely occurring isotope, error analysis is complicated by correlations between isotope-specific counts and by isotope-specific dead times, that affect each pair of isotope-specific counts. A new method of error analysis described here was developed specifically to estimate and characterize error in AMS measures of actinide isotope ratios. The method models a series of detected pairs of isotope counts as correlated compound-binomial (censored trinomial contingency) data, provides an approximately unbiased moment-method estimator of a common isotopic proportion (or ratio) corresponding to any data-pair series, and provides a corresponding homogeneity test for isotopic proportions observed within any data-pair series. Applications of this method to AMS measures of the relative abundance of plutonium isotopes, in samples analyzed at the Lawrence Livermore National Laboratory Center for AMS, revealed that observed measurement errors were nearly all attributable to modeled counting errors.

Ion source modeling and design at PRIME Lab

The design and ion optics of a new high intensity cesium ion source and sample changer being built at PRIME Lab will be discussed. The ion source is based on the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory design and the sample changer is based on a sample changer developed at Chalk River Nuclear Laboratories in Canada. The ion optics are being modeled using Simion 7.0, which iteratively solves Laplaces' equation. Simion 7.0 does not require cylindrical symmetry so the effects of non-symmetric components, in particular the sample insertion rod and immersion lens can be explored. Other aspects that are being modeled are small shifts in the position of the ionizer as it is heated; shape of the immersion lens; distance from the cathode to the immersion lens; distance from the ionizer to the cathode; the effect of voltage (relative to the cathode) on the immersion lens; the shape of the cathode; and space charge.

Ion source modeling and design at PRIME Lab

The design and ion optics of a new high intensity cesium ion source and sample changer being built at PRIME Lab will be discussed. The ion source is based on the Center for Accelerator Mass Spectrometry (CAMS) at Lawrence Livermore National Laboratory design and the sample changer is based on a sample changer developed at Chalk River Nuclear Laboratories in Canada. The ion optics are being modeled using Simion 7.0, which iteratively solves Laplaces' equation. Simion 7.0 does not require cylindrical symmetry so the effects of non-symmetric components, in particular the sample insertion rod and immersion lens can be explored. Other aspects that are being modeled are small shifts in the position of the ionizer as it is heated; shape of the immersion lens; distance from the cathode to the immersion lens; distance from the ionizer to the cathode; the effect of voltage (relative to the cathode) on the immersion lens; the shape of the cathode; and space charge.

Use of a correlated compound-binomial model to assess absence of non-counting noise in Pu-isotope ratios measured by AMS at LLNL

Methods for error analysis pertaining to isotopic ratio measures made by accelerator mass spectrometry (AMS) typically focus on accuracy/precision of detected counts of a single rare isotope (e.g. 14 C), relative to a current strength indicating the abundance of an isotope present in far greater amounts (e.g. 13 C). Because AMS analysis of the relative abundance of actinide isotopes involves comparing detected counts that each correspond to a relatively rarely occurring isotope, error analysis is complicated by correlations between isotope-specific counts and by isotope-specific dead times, that affect each pair of isotope-specific counts. A new method of error analysis described here was developed specifically to estimate and characterize error in AMS measures of actinide isotope ratios. The method models a series of detected pairs of isotope counts as correlated compound-binomial (censored trinomial contingency) data, provides an approximately unbiased moment-method estimator of a common isotopic proportion (or ratio) corresponding to any data-pair series, and provides a corresponding homogeneity test for isotopic proportions observed within any data-pair series. Applications of this method to AMS measures of the relative abundance of plutonium isotopes, in samples analyzed at the Lawrence Livermore National Laboratory Center for AMS, revealed that observed measurement errors were nearly all attributable to modeled counting errors.

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.

Preparation of 41Ca AMS standards

Absolute accelerator mass spectrometry (AMS) measurements require reliable standards for calibration. The Space Sciences Laboratory at the University of California, Berkeley, and the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory (LLNL) have prepared new 41Ca AMS standards. The highly enriched 41CaCO 3 was obtained from Oak Ridge National Laboratory. After purification of the CaCO 3, the precise 41Ca/Ca ratio in the original material was determined by surface ionization mass spectrometry. The 41Ca was sequentially diluted with natural Ca to obtain 41Ca/Ca ratios of 1.155×10 −10, 9.755×10 −12, 5.139×10 −12, 1.095×10 −12 and 5.884×10 −13. A solution with a 41Ca/Ca ratio of 10 −6 was also prepared for biomedical research. The solutions having 41Ca/Ca ratios between 10 −10 and 10 −12 were measured by AMS. Although the present blank level, 5×10 −14 41Ca/Ca, was higher than anticipated, excellent linearity was obtained from 6×10 −13 to 1.2×10 −10 41Ca/Ca.

A compact tritium AMS system

Tritium ( 3H) is a radioisotope that is extensively utilized in biological and environmental research. For biological research, 3H is generally quantified by liquid scintillation counting requiring gram-sized samples and counting times of several hours. For environmental research, 3H is usually quantified by 3He in-growth which requires gram-sized samples and in-growth times of several months. In contrast, provisional studies at LLNL’s Center for Accelerator Mass Spectrometry have demonstrated that accelerator mass spectrometry (AMS) can be used to quantify 3H in milligram-sized biological samples with a 100 to 1000-fold improvement in detection limits when compared to scintillation counting. This increased sensitivity is expected to have great impact on the biological and environmental research community. However, in order to make the 3H AMS technique more broadly accessible, smaller, simpler, and less expensive AMS instrumentation must be developed. To meet this need, a compact, relatively low cost prototype 3H AMS system has been designed and built based on an LLNL ion source/sample changer and an AccSys Technology radio frequency quadrupole (RFQ) linac. With the prototype system, 3H/ 1H ratios ranging from 1×10 −10 to 1×10 −13 have be measured from milligram-sized samples. With improvements in system operation and sample preparation methodology, the sensitivity limit of the system is expected to increase to approximately 1×10 −15.

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.

Technetium measurements by accelerator mass spectrometry at LLNL

Technetium-99 is a long-lived, high-abundance fission product which has been widely distributed in the environment through atmospheric testing, the nuclear fuel cycle, and nuclear medicine. It has a high potential for migration in the environment as the pertechnetate anion. At the Center for Accelerator Mass Spectrometry, methods are being developed for the detection of this radionuclide by accelerator mass spectrometry (AMS), including extraction from environmental samples, concentration and purification of the 99Tc, conversion to a form appropriate for AMS analysis, and quantification by AMS. Besides interference from the stable (though relatively rare) atomic isobar 99Ru, the detection of 99Tc by AMS presents some technical challenges which are not present for the other radionuclides typically measured by AMS. These challenges are related to the lack of a stable Tc isotope. Here we present the status of our 99Tc methods including discussion of interferences and sensitivity, and recent results for environmental samples and the IAEA reference material IAEA-381, Irish Sea Water. Sensitivity is presently ∼10 μBq (∼1×10 8 atoms) per sample, limited primarily by 99Ru introduced from process chemicals, and precision/reproducibility is ∼15–25%.

Optics elements for modeling electrostatic lenses and accelerator components: III. Electrostatic deflectors

Ion-beam optics models for simulating electrostatic prisms (deflectors) of different geometries have been developed for the envelope (matrix) computer code TRACE 3-D as a part of the development of a suite of electrostatic beamline element models which includes lenses, acceleration columns, quadrupoles and prisms. The models for electrostatic prisms are described in this paper. The electrostatic prism model options allow the first-order modeling of cylindrical, spherical and toroidal electrostatic deflectors. The application of these models in the development of ion-beam transport systems is illustrated through the modeling of a spherical electrostatic analyzer as a component of the new low-energy beamline at the Center for Accelerator Mass Spectrometry. Although initial tests following installation of the new beamline showed that the new spherical electrostatic analyzer was not behaving as predicted by these first-order models, operational conditions were found under which the analyzer now works properly as a double-focusing spherical electrostatic prism.

Comparison of cosmogenic radionuclide production and geomagnetic field intensity over the last 200 000 years

The production rate of cosmogenic radionuclides such as 10Be or 14C is known to vary as a function of the geomagnetic field intensity. It should, therefore, be possible to extract a record of palaeofield intensity from the deposition record of these radionuclides in marine or terrestrial sediments and ice cores. Field intensity variations, however, are not the only factor that has influenced the cosmogenic radionuclide records. In the case of 14C, variations of the global carbon cycle, caused by reorganization of the ocean circulation patterns from the last glacial to the present interglacial, are superimposed. 10Be is not affected by these variations because it is not part of the carbon cycle, but its deposition rates in marine sediments vary as a function of lateral sediment redistribution and boundary scavenging intensity. A global stacked record of 10Be deposition rates, corrected for sediment redistribution by normalizing to 230Thex, was shown to remove most of the disturbances, and provides a record of 10Be production rate variations over the last 200 000 years, which translates into geomagnetic field intensity variations. This dataset is compared with palaeofield intensities reconstructed from marine sediments by palaeomagnetic methods, from variations in atmospheric 14C/12C derived from independent calibrations of 14C ages, such as U/Th dating and tree ring chronology, and from 36Cl and 10Be fluxes in polar ice cores. Potential influences of the Earth’s orbital parameters and insufficient correction for orbitally triggered climate variations on the palaeointensity reconstructions are assessed. It is argued that the palaeointensity records derived from marine sediments are not significantly affected by these factors.

A compact 14C Isotope Ratio Mass Spectrometer for biomedical applications

During the last two decades the unparalleled sensitivity of accelerator mass spectrometry (AMS) has allowed major developments in many areas of geoscience and archeology. It is projected that in the near future a similar potential for AMS expansion is likely in the field of biomedical research leading, ultimately, to clinical applications. As an example of the growth of this new field, at the Lawrence Livermore National Laboratory Center for AMS the number of biomedical 14C measurements already represents a significant fraction of the total for all 14C. Widespread adoption of AMS in the biomedical field does require, however, the availability of instruments that are small in comparison with existing AMS systems and that will operate in a manner that is simple and user-friendly. To meet this demand, High Voltage Engineering Europa (HVEE) has developed a miniature, high-efficiency AMS instrument designed to provide for biomedical samples 14C12C ratios with an accuracy better than 2% and with backgrounds limitations below 0.1 Modern. The current status of this program will be presented in the present paper.

Biomedical accelerator mass spectrometry

Ultrasensitive SIMS with accelerator based spectrometers has recently begun to be applied to biomedical problems. Certain very long-lived radioisotopes of very low natural abundances can be used to trace metabolism at environmental dose levels (⩾ z mol in mg samples). 14C in particular can be employed to label a myriad of compounds. Competing technologies typically require super environmental doses that can perturb the system under investigation, followed by uncertain extrapolation to the low dose regime. 41Ca and 26Al are also used as elemental tracers. Given the sensitivity of the accelerator method, care must be taken to avoid contamination of the mass spectrometer and the apparatus employed in prior sample handling including chemical separation. This infant field comprises the efforts of a dozen accelerator laboratories. The Center for Accelerator Mass Spectrometry has been particularly active. In addition to collaborating with groups further afield, we are researching the kinematics and binding of genotoxins in-house, and we support innovative uses of our capability in the disciplines of chemistry, pharmacology, nutrition and physiology within the University of California. The field can be expected to grow further given the numerous potential applications and the efforts of several groups and companies to integrate more the accelerator technology into biomedical research programs; the development of miniaturized accelerator systems and ion sources capable of interfacing to conventional HPLC and GMC, etc. apparatus for complementary chemical analysis is anticipated for biomedical laboratories.

The new LLNL AMS sample changer

The Center for Accelerator Mass Spectrometry at LLNL has installed a new 64 position AMS sample changer on our spectrometer. This new sample changer has the capability of being controlled manually by an operator or automatically by the AMS data acquisition computer. Automatic control of the sample changer by the data acquisition system is a necessary step towards unattended AMS operation in our laboratory. The sample changer uses a fiber optic shaft encoder for rough rotational indexing of the sample wheel and a series of sequenced pneumatic cylinders for final mechanical indexing of the wheel and insertion and retraction of samples. Transit time from sample to sample varies from 4 to 19 s, depending on distance moved. Final sample location can be set to within 50 microns on the x and y axis and within 100 microns in the z axis. Changing sample wheels on the new sample changer is also easier and faster than was possible on our previous sample changer and does not require the use of any tools.

Materials impurity analysis by means of nuclear resonance reactions

An automated beam line for measurement of trace impurities in materials is installed on the FN tandem in the Center for Accelerator Mass Spectrometry, LLNL. The major purpose of the beam-line system is to determine the diffusion of water into glasses by use of the 19F(p, αγ) 16O reaction. This study is stimulated by the radiation-waste-isolation program; however, the system is applicable to studies of other impurities in many material matrices. The target wheels, each of which holds twenty-four samples, can be changed in less than an hour. The beam-line control system both indexes the target wheel and controls the tandem energy as it steps through the requested measurement protocol. The tandem control system, in response to a request, adjusts the terminal potential and other transport elements to maintain constant spot size and location.