T.M. Benjamin

Geochemical applications of nuclear microprobes

Abstract The Los Alamos nuclear microprobe has been used for a series of geochemical analyses by proton induced X-ray emission (PIXE). Spectra of geologic samples obtained with a Si(Li) detector are often complex, containing many overlapping peaks. To deconvolute these spectra, a fitting routine using only one variable parameter per element has been developed. This requires that the relative peak intensities, corrected for the sample matrix, be calculated independently for each element. Since thick samples are used, the calculations include the effects of decreasing beam energy with depth and of X-ray absorption in the sample. Deconvolution of the spectrum for Durango apatite, which contains over twenty overlapping peaks in the region frm 4.5 to 6.7 keV, provides a severe test of the spectrum fitting routine. The use of calibration standards with compositions similar to those of the unknown considerably simplifies quantitative measurements. However, such standards are not available for many trace elements. To overcome this problem, preliminary tests of an alternate, internal calibration scheme also are discussed.

Accuracy of standardless nuclear microprobe trace element analyses

Abstract The primary advantage of the PIXE technique is the ability to provide rapid, simultaneous, spatially resolved analysis for a large number of elements at the few μg/g level. Assessment of the accuracy of quantitative analysis of PIXE spectra obtained at the Los Alamos nuclear microprobe facility has been made by comparing accepted reference values to the analyses of NBS standard reference material #610, which contains 61 elements at nominal concentrations of 500 μg/g, and the C1 carbonaceous chondrite Orgueil. No systematic error was found in the analyses, however the uncertainty in an individual analysis was found to be about 10%. Long count time analyses of Orgueil show that minimum practical detection limits, in complex thick targets, for the PIXE technique are about 0.2 to 0.6 μg/g.

A test of the smoothness of the elemental abundances of carbonaceous chondrites

The identification of CI chondrite concentrations with average solar system abundances for heavy elements is based primarily on the smoothness of the CI abundance curves for odd mass nuclei. A good test of smoothness is measurement of all elements in a given mass range in the same sample with the same technique. High precision proton-induced X-ray spectra of CI chondrites yielded analyses of 17 elements (Ni through Ru, plus Fe and Pb) with precisions better than 10% for all except As, Pb, Nb, and Ru. Excellent theoretical descriptions of the spectra were obtained. Two independent estimates of precision agree well, giving confidence in the quoted errors. Intersample differences are the largest source of variability. Within these limits good agreement with literature results are obtained, except for As and Y. Although our Y values are 10 to 30% lower than previously adopted, amonoelemental s-process peak in the abundance curve at Y is still necessary. Except for Br (higher by 59% in Ivuna), there are no significant concentration differences between Orgueil and Ivuna. In general, our results confirm previous abundance curves. The abundances are exceptionally smooth and strongly decreasing in the mass 60–75 region. From mass 75–101 a smooth curve can be drawn, within limits of intersample variability, except for the Y peak. Over the whole periodic table a large number of peaks of probable nucleosynthetic origin can be identified, some understood, some not. These smoothness deviations are 10 to 30% and set an overall limit to the smoothness argument alone in justifying using CI abundances as average solar system values.

Geochemical utilization of nuclear microprobes

Abstract Analysis of geological materials may be the most challenging application for nuclear microprobes. Quantitative multi-element PIXE results are required on thick, compositionally complex, fine-grained materials. Major elements such as calcium and iron dominate the spectra that include numerous X-ray peak overlaps from a wide range of minor and trace elements. A concerted effort is required to optimize sample selection, data acquisition, and data reduction. Geochemical complexities can also make RBS and nuclear reaction analysis difficult. At Los Alamos, we are rebuilding our superconducting-solenoid nuclear microprobe beam line. We have moved to a new 3 MV tandem accelerator, simplified our ion optics, added two-dimensional beam scanning, and are working on a new precision stage and vacuum chamber. Continuing development of our software is aimed at obtaining maximum compositional information from PIXE spectra. In addition to the geochemical work of Cahill (UCD), Cabri (Ottawa), Clayton (Lucas Heights), and Minkin (USGS/Heidelberg), we (and our collaborators) have addressed several meteoritic and terrestrial problems. These range from early solar system condensation, planetary differentiation, and the possibility of “martian” meteorites, to in-situ retorting of oil shale, mid-ocean ridge hydrothermal processes, and the deduction of American indian trade routes through the characterization of the source minerals used in their copper metallurgy. Natural and synthetic standards have demonstrated that PIXE spectra can yield quantitative concentrations for more than 60 elements in a single spectrum and that, in favorable cases and with effort, sub-ppm detection limits are attainable.

The Los Alamos PIXE data reduction software

Abstract A fortran computer code has been written to perform standard-free analysis of PIXE spectra. The analysis is broken into three components: relative X-ray intensity calculation, detector calibration, and spectrum deconvolution. Nonlinear portions of the problem are limited to detector calibration. Actual spectrum deconvolution is performed as a linear least squares problem with linear inequality constraints on element concentrations and background. Up to 9 K and 15 L lines of 85 elements are available for spectrum fitting. The current model consists of Gaussian peaks and a polynomial background. Development work is underway to add low energy tails to the peak model and absorption edges to the background.

Calibration of micro-PIXE analysis of sulfide minerals

Abstract Two different standardization methods have been used in micro-PIXE analysis of several hundred sulfide mineral specimens. Their advantages and disadvantages are discussed and the compatibility of the two sets of results is investigated.

Microprobe analyses of rare earth element fractionation in meteoritic minerals

Two meteorites were analyzed by PIXE with the Los Alamos nuclear microprobe. The enstatite achondrite Pena Blanca Spring and the ordinary chondrite St Severin were chosen as likely candidates for use in ^(244)Pu(t_(l/2)=82 my) cosmochronology and geochronology. These applications require the meteoritic minerals to have unfractionated actinides and lanthanides relative to “cosmic” elemental abundance ratios. The PIXE analyses produced evidence of actinide-lanthanide fractionation in Pena Blanca Spring oldhamite (CaS) whereas the St Severin phosphates, whitlockite and chlorapatite, do not exhibit this fractionation.

High precision thick target PIXE analyses of carbonaceous meteorites

High precision proton-induced X-ray (PIXE) thick target spectra of carbonaceous meteorites have been deconvoluted using previously described programs. Even in cases of totally overlapping peaks excellent fits to spectra were obtained. Concentrations for 15 elements in the mass range 56–100, as well as Pb, agree well in most cases with previous literature values. The total concentration range studied was over 105, with the lowest concentrations precisely analyzed being around 1 ppm.

Tracing copper bell trade patterns in the greater Southwest using the Los Alamos nuclear microprobe

Abstract The Los Alamos nuclear microprobe has been used for trace-element analysis of cast copper crotals (bells) by proton induced X-ray emmission (PIXE). Small spot sizes were used because the copper bells have mineral inclusions and intercrystalline grain boundaries on the order of 40 μm. Information on the trace element metallurgy of copper bell castings is important for the establishment of early trade patterns. Results show that the PIXE technique has potential for tracking copper bell trade patterns using trace-elements at the 5 μg g level.

Trace element contents of primitive meteorites; A test of solar system abundance smoothness

Elemental abundances from Cl carbonaceous chondrite meteorites are thought to represent the average solar system (“cosmic”) composition, based on the agreement between Cl and solar photospheric abundances and on the smoothness of heavy element abundances of odd mass nuclei when plotted as a function of mass number. To test Cl elemental smoothness, we have analyzed Cl meteorites, using conventional PIXE (proton probe) techniques, where we have good sensitivity for the range of elements: Ni to Mo. Preliminary analyses indicate that elemental smoothness is only approximate, with possible deviations of 30% to perhaps even 50%. This may be due to chemical fractionation. Alternatively, explanations for nonsmooth behavior may be understandable with the aid of general ideas of n-capture nucleosynthesis. As Cl abundances are refined, it could be that the lack of elemental smoothness may provide the strongest argument for the identification of Cl with primordial solar system abundances.