V.M. Prozesky

The NAC nuclear microprobe facility

The NAC NMP facility is discussed, showing that it is a well characterised analytical tool, and some novel features are incorporated, such as on-demand beam deflection and a new lid for the sample chamber.

Biological applications of the NAC nuclear microprobe

The nuclear microprobe (NMP) is an established analytical instrument for the determination of minor and trace elements. It allows measurements with a spatial resolution of the order of 1 μm and minimum detection limits down to few ppm by weight, with excellent scanning capabilities. The nuclear microprobe of the National Accelerator Centre, South Africa, is being used in a wide number of applications in the biosciences. The complementarity of proton-induced x-ray emission and backscattering spectrometry in a wide range of biological applications is shown. The advantages and restrictions of true elemental imaging are also discussed.

Quantitative micro-PIXE comparison of elemental distribution in Ni-hyperaccumulating and non-accumulating genotypes of Senecio coronatus

The Ni hyperaccumulator, plant species Senecio coronatus (Thunb.) Harv., Asteraceae is an example of plant adaptation mechanisms to different ecological conditions. This widespread species can inter alia be found on serpentine outcrops and the genotypes growing in serpentine soils show different ways of adaptation. The populations from two distant localities take up and translocate Ni in concentrations which are normally phytotoxic, while plants growing on a different site, in the vicinity of another hyperaccumulating species, absorb amounts which are typical for most of the plants found on serpentine soils. The NAC nuclear microprobe was used to compare the distribution of Ni and other elements in selected organs and cells with simultaneous use of PIXE and proton BackScattering (BS). Quantitative maps of stems showed large differences in concentrations and distributions of major and trace elements. In hyperaccumulating genotypes Ni is present everywhere within stem tissues, but the highest concentrations were found in the epidermis, cortex and phloem. In non-accumulating plants Ni was concentrated in the phloem. In the leaf epidermis Ni was concentrated in the cell walls for both accumulating and non-accumulating plants. These results suggest that biochemical diversity is more than morphological, because investigated genotypes belong to the same taxon.

Botanical applications in nuclear microscopy

Abstract An overview of botanical applications in the field of nuclear microscopy is presented. A wide range of applications has been studied up to now and included an array of very thin to thick samples. The relevant methods of sample preparation which are critical to many botanical applications are discussed. Quantitative mapping techniques allowing simultaneous quantitative studies of major and trace elements offer exciting possibilities in various fields of application, from science to industry and agriculture. In these studies PIXE and RBS were typically used to obtain complementary information. Recent examples are presented.

Bayesian PIXE background subtraction

Abstract The subtraction of the X-ray background in a PIXE spectrum has been the subject of many investigations and different techniques have been developed. These techniques vary from filtering to fitting polynomial functions. The promising Bayesian Statistics technique has been used in this study to eliminate the background from the spectrum in a rigorous and self-consistent manner. We compare the results of the Bayesian background subtraction method to that obtained by stripping and the “rolling ball”; method.

ERDA measurement of hydrogen isotopes with a ΔE-E telescope

Abstract A ΔE-E telescope for mass separation of the isotopes of hydrogen is described for a 4He bombarding energy of 4.0 MeV. The use of a 13.6 μm Si transmission detector as ΔE detector enabled the separation of ERDA signals from the three isotopes of hydrogen in thick samples containing hydrogen (H), deuterium (D) and tritium (T). Whereas a ΔE detector is normally used as an atomic charge (Z) separator in heavy ion spectroscopy, in this study it was used to separate the different masses of the hydrogen isotopes. H events were fully separated from the other isotopes and the separation between D and T events was such that more than 95% of the signals corresponding to the two isotopes could be extracted for analytical purposes, with the maximum depth information obtainable for a single isotope being about 4.1 × 1018 Ti at./cm2 for H, 5.3 × 1018 Ti at./cm2 for D and 4.8 × 1018 Ti at./cm2 for T. The technique, which is especially useful in targets containing all the isotopes of hydrogen, is illustrated by means of a study of the depth distribution of hydrogen isotopes in solid thick TiDx and TiTx targets.

Elemental mapping analysis of recurrent calcium oxalate human kidney stones

Abstract Elemental maps from a series of recurrent calcium oxalate kidney stones were obtained by PIXE with 3 MeV protons, using the nuclear microprobe at Faure. Trace elemental patterns in the stones were identified. Good positive correlations between Mn, Fe, Cu, Zn and Sr relative to Ca were identified. These correlations showed two types of distributions which could be attributed to the COM or COD phase-matrix framework. Some trace elements such as As, Se, Br and Pb were distributed homogeneously in the scanned micro-regions and possibly deposited as single ion-entities in the mixed stone matrix. It is concluded that micro-PIXE can provide useful data for deducing the mechanisms by which kidney stones are induced and how they grow.

THE USE OF MAXIMUM ENTROPY AND BAYESIAN TECHNIQUES IN NUCLEAR MICROPROBE APPLICATIONS

Abstract This paper presents an overview of the Bayesian Formalism (BF) and Maximum Entropy (ME), as well as applications of the theory to various ion-beam and nuclear microprobe (NMP) related work. The Bayesian formalism is an efficient and theoretically sound technique of information recovery, with special applications in ill-posed inverse problems, such as detector function deconvolution. Results are presented showing the promise of Bayesian Statistics and Maximum Entropy in PIXE spectrum deconvolution, deconvolution of the beam profile from one-dimensional scans and the recovery of depth profiles in RBS.

Study of the nutrient distribution in root tips of the tropical forage Brachiaria

A Brachiaria breeding project is being conducted at CIAT, Colombia to combine acid soil adaptation of B. decumbens with other favourable traits of Brachiaria species that are less adapted to acid soils. Micro-PIXE was applied to investigate nutrient uptake and distribution in root tips of different species grown in hydroponic culture under control and simulated acid soil stress conditions. Different sample measurement approaches were evaluated, including: (1) linear scans with single point measurements along the root axis; (2) mapping of whole root tips; and (3) mapping of root cross sections. Different tissue types could be distinguished on the base of differences in nutrient concentrations and/or Al stress. Al, if supplied under nutrient stress conditions, increased P accumulation in the central vascular tissue of the meristematic and elongation zone of B. decumbens and B. brizantha. Furthermore, a negative correlation was found between Al and Cl accumulation in the root cap of B. decumbens.

NUCLEAR MICROPROBE (PIXE) ANALYSES OF ZIRCONS AS INDICATORS OF GRANITE TYPE

Abstract Petrochemical investigation of the Cape Granite Suite illustrated that three major granitoid types exist namely S-, I-and A-types. Studies of zircon typologies of these granites confirmed that the three major types and their subtypes could be distinguished with relative ease. Using the typological classification as a base, zircons from the three granite groups were analysed by proton microprobe. The proton microprobe was utilized to enable the detection of elements normally present in low quantities in zircon. The results clearly illustrated that PIXE analyses may readily be used to discriminate zircons from S-, I- and A-type granites using the elements Th, Y and Yb.