S.M. Tang

The National University of Singapore nuclear microscope facility

Abstract The National University of Singapore nuclear microscope facility is based around a HVEC AN2500 single ended Van de Graaff accelerator and an Oxford Microbeams coupled quadrupole triplet focusing system. Particle induced X-ray emission (PIXE), nuclear or Rutherford backscattering spectrometry (RBS) and scanning transmission ion microscopy (STIM) can be carried out simultaneously. Data acquisition is carried out using a simple but flexible PC based system (Oxford Microbeams DAQ) and the data is analysed using a combined RUMP and GUPIX PC based interactive package (NUSDAN) acting under WINDOWS. Resolution tests using a calibration grid and a multi layer integrated circuit have shown the facility to be capable of 600 nm spot sizes for 2 MeV protons at currents suitable for microanalysis.

Two years of aerosol pollution monitoring in Singapore: a review

Abstract An aerosol sampling campaign was initiated more than two years ago in Singapore. The aim was to determine the average elemental concentrations in fine and coarse aerosol fractions as well as to identify major pollution sources and their impact. For that purpose, two air samplers were employed at two different sampling locations; one sampler was a fine particulate aerosol sampler (PM2.5) located at the vicinity of a major industrial area. The other was a stacked filter unit (SFU) sampler designed for collection of fine and coarse fractions (PM2.5 and PM10) and installed in the residential area. Samples were taken typically twice a week and in several occasions daily. During the period of two years more than 700 aerosol samples were collected and analyzed using PIXE and RBS techniques. All samples were analyzed for 18 elements ranging between Na, Mg, Al, etc. up to As and Pb. Large daily and seasonal variations were found for most of the elements. These variations are attributed mainly to meteorological changes, in particular changes in wind speed and direction. On several occasions, short term sampling was performed to identify fingerprints of major pollution sources such as road traffic, refineries, as well as the rain-forest fires in neighboring countries. A summary of our findings is presented and discussed.

Parametrization of the total photon mass attenuation coefficients in the energy range 0.1–1000 keV

Abstract It is convenient to generate mass attenuation coefficients using semi-empirical schemes. The validity of most of the existing schemes is limited to a relatively narrow energy interval (1–40 keV) and their accuracies are poor in some energy regions. In this work, a semi-empirical scheme flexible enough to give a good fit to data in a very wide photon energy range (0.1–1000 keV) was employed. Fitting coefficients for the entire range were obtained by utilizing mass attenuation data from two sources: (1) semi-empirical data of Henke et al. in the low photon energy region, and (2) theoretical values generated with the XCOM code for fitting in the high energy region. The root mean square of the fit is generally less than 0.2% except for energies below 1 keV where the available data are scattered. A computer code for generating mass attenuation coefficients based on the proposed scheme has been developed.

New parameters for the calculation of L subshell ionization cross sections

Abstract New parameters for the calculation of L subshell ionization cross sections using a semiempirical expression are presented. The parameters were obtained by fitting an analytical function to the experimentally obtained values of L subshell ionization cross sections. The experimental data included compilation of the published data from 1975 to 1982 by Sokhi and Crumpton as well as our newly compiled data published in 1982–1991. The number of data employed for the fitting was 2295, 3 1 2 times more than that used in Miyagawas work. The data were divided into six groups according to the Z values: 14 ≤ Z ≤ 42, 43 ≤ Z ≤ 50, 51 ≤ Z ≤ 60, 61 ≤ Z ≤70, 71 ≤ Z ≤ 80 and 81 ≤ Z ≤ 92. Each was fitted by the analytical function to obtain the parameters for the calculation of L1, L2 and L3 subshell ionization cross sections. For data in the group with Z ≤ 42, only parameters for the calculation of L total ionization cross sections were determined. A comparison of the various theoretical predictions with the experimental data and the fits is also presented.

SEMIEMPIRICAL FORMULAS FOR CALCULATION OF L SUBSHELL IONIZATION CROSS SECTIONS

Presented are new parameters for the calculation of L subshell ionization cross sections for proton impact using a semiempirical expression. A similar paper was published by our group in early 1993 but the fitting parameters were obtained by using only 2295 experimental L shell cross section data. Since then a large number of experimental data have become available and therefore a new fitting has been performed employing more than 5000 data points. All available data were fitted separately for L1, L2 and L3 subshells. For targets with low atomic numbers (14≤Z≤42), only coefficients for Ltot were obtained. Because of the slight Z dependence of the universal function, data were also devised into five sub-groups according to their atomic numbers and fitted separately within each group for L1, L2 and L3 subshells. To extend the energy range of validity of the new fitting function theoretical values were used in the high energy region where experimental data were lacking. Results are compared with ECPSSR predictions and discussed.

NUCLEAR MICROPROBE ANALYSIS AND IMAGING : CURRENT STATE OF THE ART PERFORMANCES

Abstract The current state-of-the-art performances claimed for nuclear microbeam spatial resolutions are: (a) 400 nm spot sizes for high current (100 pA) proton beams, (b) 400 nm for high current alpha particle beams, (c) 100 nm for low proton current (

Analysis of Burmese and Thai Rubies by PIXE

Trace element analysis of 60 Burmese and Thai rubies by using the technique of proton-induced x-ray emission was carried out. It was found that the Burmese rubies contained higher concentrations of all impurities except iron. Our results also reveal that vanadium and iron are useful indicators for Burmese and Thai ruby attribution. The Thai rubies have high contents of iron and very low concentrations of vanadium. On the other hand, all the Burmese rubies examined contain significant amounts of vanadium, and their iron contents are, on the average, four times lower than those in the Thai rubies.

ELEMENTAL DEPTH PROFILES IN MARINE SEDIMENTS OF SINGAPORE COASTAL WATERS

Abstract Thirty-eight core sediment samples were recently collected from different locations of the Singapore coastal region. The aim of the project was to trace the history of marine pollution in various coastal regions and to determine the impact of industrial activities. Two nuclear analytical techniques were employed in this study: particle induced X-ray emission (PIXE), Rutherford backscattering (RBS) as well as X-ray fluorescence (XRF). Combined together these techniques provide an excellent tool to determine elemental concentrations of more than 30 elements with detection limits as low as few ppm. Our results show that elemental concentrations in most of the regions do not show a significant variation with depth. However, in regions where industrial and shipping activities are high, for example the Port of Singapore area and the northern part of Johore Straits, the concentrations of metals like Cr, Ni, Cu, Zn, Sn and Pb were found to have an obvious decreasing trend with the depth. In these cores, concentrations in the top 10–15 cm were sometimes ten times higher than the corresponding base line concentrations. Elemental depth profiles of Ni, Cu, Zn, As, Sn and Pb and their mean concentrations in various regions are reported and discussed.

Micro-PIXE analysis of trace element concentrations of natural rubies from different locations in Myanmar

Abstract The trace element concentrations found in geological samples can shed light on the formation process. In the case of gemstones, which might be of artificial or natural origin, there is also considerable interest in the development of methods that provide identification of the origin of a sample. For rubies, trace element concentrations present in natural samples were shown previously to be significant indicators of the region of origin [S.M. Tang et al., Appl. Spectr. 42 (1988) 44, and 43 (1989) 219]. Here we report the results of micro-PIXE analyses of trace element (Ti, V, Cr, Fe, Cu and Ga) concentrations of a large set ( n = 130) of natural rough rubies from nine locations in Myanmar (Burma). The resulting concentrations are subjected to statistical analysis. Six of the nine groups form clusters when the data base is evaluated using tree clustering and principal component analysis.

The effect of external fields and slit scattering on the beam spot profile of the coupled triplet system

Abstract The effect of external AC magnetic fields and slit scattering on the beam spot profile of the high excitation coupled triplet focusing system has been investigated. A 2 MeV proton beam has been exposed to an AC magnetic field generated from an air-cored solenoid positioned at various positions along the beam line. The resultant increase in beam spot size due to this external field was observed to be highly dependent on the field position along the beam line, with the region encompassing the collimation apertures found to be the most sensitive to stray fields. The effects of slit scattering was more pronounced for small aperture dimensions, but overall the beam quality at these small dimensions (1 μm) was satisfactory, with only a small fraction of the beam showing energy loss.