J.W. Nelson

Cascade impactor aerosol samples for PIXE and PESA analyses

Abstract Single orifice impactors have been modified to fractionate a full range of aerosol aerodynamic diameters into eight closed and two open ended intervals (>16, 16-8, 8-4, 4-2, 2-1, 1.0-0.5, 0.50-0.25, 0.12-0.06 and

Advances In The Use Of Pixe And Pesa For Air-Pollution Sampling

Abstract Proton induced X-ray emission (PIXE) and proton elastic scattering analysis (PESA) are complementary ion beam techniques well suited to giving total elemental analysis of small samples. Air particulate samplers designed to yield ideal small samples have been developed at Florida State University to give size and time distributions of the particulates. The two methods are discussed using samples taken mostly at Kawecki Berylco Industries.

PIXE ANALYSIS OF ATMOSPHERIC AEROSOLS FROM A SIMULTANEOUS THREE SITE SAMPLING DURING THE AUTUMN OF 1993 IN MEXICO CITY

Abstract A simultaneous three site aerosol sampling was performed during the autumn of 1993 in Mexico City. Stacked filter units of the Davis design were used to collect atmospheric aerosol samples, providing two samples per unit and separating particles into “coarse” and “fine” according to size. The samples were taken simultaneously at three different sites from 6:00 to 12:00 h, on seven alternate days. Elemental concentrations of aerosol contents were determined by PIXE analysis performed both at the Florida State University and at the ININ facilities. 18 elements were found systematically in most of the samples, including S, Cl and heavy metals. Fine mass concentrations of sulfur were more abundant than coarse mass concentrations at all three sites. Enrichment factors and correlation matrices were computed to identify possible pollutant sources.

Light element analysis by proton scattering

Abstract A system for quantitative elemental analysis by proton scattering at 16 MeV has been developed. Samples of thicknesses up to 1 mg/cm2 may be analyzed for the light elements up to Cl. Examples of spectra for atmospheric particulate matter and biological specimens are shown.

PIXE analysis of intermediate and thick targets via line intensity ratios

Accounting for target self-absorption of X rays and the variation of the production cross section with proton energy, calculated yield ratios LK for 3.2 MeV protons for elements Fe through Br and K(3.2 MeV)K(1.6 MeV) for elements S through Br show sufficient variation with target thickness to permit a determination of effective thickness ranging from thin to infinitely thick. For uniform composition targets of a known fraction of elements Z < 10 PIXE quantitative analyses are possible on the basis of a yield ratio for an intense line in the spectrum of a target under analysis. Expressed in terms of thin target yield, those of thicker targets are smaller by a factor whose value is dominated by self-absorption in the case of low Z elements and by the decrease of production cross section for high Z elements. This line intensity ratio method is advantageous in that it involves only the use of PIXE spectra and their analyses and in that it results in an effective thickness for geometrically non-uniform targets when a proton beam of uniform areal density is used. Application of the method to intermediate thickness samples of air paniculate matter from an inertial impactor is discussed.

A “leaky” integrator circuit with automatic switching

Abstract An electromechanical system for cross section and excitation curve measurements by counting residual radioactivity is described. The principle of operation of the “leaky” integrator is discussed. Circuits are described in detail and examples of experiments performed with the aid of the equipment are given.

An intercomparison of pixe and xrf for statistically resolving aerosol components in leningrad and environs

Abstract PIXE and X-ray fluorescence methods for elemental analysis of aerosol samples have been intercompared from the standpoint of suitability for resolving principal components using multivariate statistical procedures. Agreement between the two analytical methods is excellent. However, important differences can result from differences in methods of aerosol sampling chosen for the two analytical methods, especially by sampling in separated coarse and fine particle size fractions by 2-stage streaker for PIXE, compared with single filter sampling for XRF without particle size selectivity. The PIXE streaker approach, requiring greater analysis effort in the laboratory but simpler sample collection procedures in the field owing to its automatic operation, provided more detailed statistical resolution of aerosol principal components than was attained by the XRF filter approach. The physical significance of these components is supported by independent particle number concentrations counted in 11 size ranges and resolution of principal components and calculation of their particle size distributions. Rather than emphasizing absolute accuracy of concentrations in air, our intercomparison depends on interelemental correlations of samples collected in time series and the precision of relative elemental determinations in relation to the sample-to-sample variability, the degree to which the elements are characteristic of the components and are determined with precision sufficient for their resolution, and the precision of component resolution in relation to the numbers of these elements and of the numbers of samples analyzed. A linear relation between characteristic X-rays detected and abundance of an element is required, but absolute calibration of X-rays with respect to amounts of an element is not. Atmospheric concentrations were measured in 56 consecutive 3-h samples collected for one week at two sites, in urban Leningrad and at nonurban Gorkovskaya 65 km north. Two-stage circular streaker samples and manually changed filters were analyzed by PIXE and XRF, respectively. Then a combination of factor and multiple linear regression analysis was carried out using a microcomputer and Statgraphics software for an IBM PC, assuming a linear receptor model in which data variance is accounted for by mixing components of fixed and distinctive elemental compositions.

PIXE calibration and absorption corrections with the aid of proton scattering

Abstract The computer programme HEX (a modification of programme REX) is a nonlinear, least squares, matrix minimization code for evaluation of Si(Li) detector X-ray spectra induced by charged particle bombardment. Correction to the Gaussian areas to allow for low energy tailing of peaks and secondary fluorescence has been included in the calculation. For our detector, this correction is largest (9%) for phosphorous and decreases with X-ray energy to less than 1% for copper. Since our present range of energies observed extends from 0.6–40 keV, absorption errors are usually large for the softer X-rays. Inclusion of L X-ray line intensities (down to 0.897 keV, Z = 28) into the fitting process allows for evaluation of absorption. In addition, the amount of absorption correction for sodium in air particulate field samples (where particle size effects dominate) has been assessed by comparison with results from the proton scattering (PESA) method.

A bombardment chamber for PIXE analyses

Abstract A target chamber for PIXE irradiations is described. This compact aluminum chamber has a small volume for rapid evacuation and a short (30 mm) target to detector distance. Beam collimators and detector filter/collimators are changed simultaneously by rotating either a six or nine position disc. A circular array of eight 25 mm diameter targets or an 82 mm diameter sample holding frame is rotated by a 192 step solenoid-operated ratchet which also functions as an angular position encoder. Other options include provisions for mounting a particle backscatter detector, backfill with helium gas, and vertical variation of target position by motor driven cam.

The use of PIXE in generating evidence for primordial superheavy elements

Abstract The physical and chemical nature of the monazite inclusions studied through PIXE in the search for superheavy elements sharply constrained the nature of the analysis and set strict limits on the nature fo the ion beam and X-ray detection techniques.