Practical problems with a proton probe

694

Practical

Nuclear

problems

Instruments

with a proton

and Methods

in Physics

Research

B56/57

(1991) 694-698 North-Holland

probe

J.L. Campbell, W.J. Teesdale and J.A. Maxwell Guelph- Waterloo Program for Graduate Work in Physics, University of Guelph, Guelph, Ontario, Canada NIG 2 WI

The Guelph proton microprobe has provided several thousand spot analyses of mineralogical, geochemical and metallurgical specimens, mainly through contract arrangements. User-friendly target chamber design with color TV viewing of the incident beam spot at magnification x 300 is crucial in this work. A new PC-based data accumulation system for both spot analysis and elemental mapping is described. Various enhancements to the GUPIX software for spectrum fitting and standardization are reviewed.

1. Introduction As in the case with various proton probe laboratories the Guelph facility is run partly as a conventional grant-funded academic research laboratory and partly as an analytical enterprise which generates contract revenue. These two missions are not compartmentalized; indeed there is a useful synergy since the contract problems frequently generate new research projects in the context of further development of PIXE. However, various special demands are made by the contract work; to ensure good economics the maximum possible automation and computer control of analytical functions is necessary; so also is immediate data processing to convert raw spectra to analytic concentrations. If collaborators are involved in the analyses the maximum userfriendliness is demanded of both hardware and software, and a willingness is necessary to adapt to the collaborator’s viewpoint without compromising on the accuracy and the detection limits afforded by the PIXE technique. Our mandate here is to present our own solutions to the practical problems inherent in this dual enterprise.

A microbeam with minimum dimensions 3 X 3 urn is obtained using a miniature air-cooled magnetic quadrupole doublet [l]. To minimize the need for retuning it is run only at accelerator voltages of 3.0 (most frequent) and 1.5 MV and the lens is continuously powered. The first setting provides 3 MeV protons. The second provides 0.75 MeV protons via the intense Hz beam produced by the rf ion source. To date the microbeam has been used for point analyses of some thousands of mineralogical and metallurgical specimens. Most of the contract work is concerned with trace levels (l-100 ppm) of PGE metals in various sulfide and 0 1991 - Elsevier Science Publishers

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oxide matrices; beam-induced damage limits the tolerable current density and there is rarely any point in reducing the beam to its minimum diameter. Typically we use a 10 X 10 pm beam with some 10 nA of current. In the interests of speed the specimen holder accepts multiple specimens and a miniature gate valve at the chamber entry assures rapid pump down. An on-demand beam deflector specially designed for the microprobe context [2] obviates the need for dead-time corrections and plays an important role in minimizing beam damage to the specimen. We have shown that the 3 urn beam width (FWTM) does not deteriorate due to the deflector. A practical problem with most geological targets is their lack of conductivity. We use an evaporated carbon coating to enable conventional integration of beam current. But problems can arise if secondary electron emis-

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Fig. 1. Optical systems for viewing specimen while beam is on target: (a) conventional microscope viewing 45” mirror; (b) reflecting objective; (c) microscope viewing along normal to specimen.

B.V. (North-Holland)

J. L. Campbell et al. / Practical problems with a proton probe

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3. Data analysis software

other PIXE codes GUPIX employs no variable parameters in a functional background description. Instead it uses a top-hat filter to remove the background, and so no initial guesses or estimates about the background are necessary. Another major practical aspect is speed, since it is often necessary to understand the results of one analysis prior to proceeding with the next. Successive moves to faster processor chips have markedly increased the speed. Presently on a 80386 computer with a Cyrix co-processor, an aerosol spectrum containing 44 potential elements is fit in 2.5 min. Use of assembly-language graphics and a laser printer speeds up the production of output. A deliberate sacrifice in speed is made in that the program is not allowed to set any peak height to zero intensity, but only to reduce it by a factor 4 down to a minimum of 0.1 counts. This is done to avoid premature elimination of elements in early iterations when the overall fit is poor. Only once the chi squared is minimum is the code permitted to drop peaks whose heights are < 1 count, and to perform a final iteration without them. A considerable gain in speed has recently been effected by modifying our approach to pileup. In the standard treatment [5] the spectrum of a pileup element is synthesized from the N (value chosen by operator) most intense peaks; comparing and ordering these is time consuming. The code has been changed so that it ignores peaks below a defined intensity in this process. In analysis of heavily loaded air filters with typically 44 elements present this manoeuvre reduced the time per iteration by a factor 2.5, for N = 10. A final point concerning the GUPIX program is its ability to treat selected elements as surface films instead of being homogeneously dispersed in the matrix. In recent analyses we have encountered cases where an element thought to be a bulk contaminant was identified by SIMS as a surface contaminant. It is then useful in practical terms for GUPIX to be able to make this distinction.

3.1. GUPIX

3.2. Detector description

Both least-squares fitting of PIXE spectra and conversion of the resulting peak areas to element concentrations are handled by the software package GUPIX [4]. The most recent improvements to GUPIX, which is in use at 16 laboratories, include extensive new documentation and an expansion to cope with 700 X-ray lines. The program requires 520 K of RAM on a PC compatible computer running under DOS 4.01. It is written in Lahey Fortran V.4.00 and Microsoft V.5.1 assembler. The fitting/standardization routine is of interactive nature but can be directed to fit batches of spectra, writing the results to printer or disk. An important aspect in terms of user-friendliness is the treatment of continuum background. Unlike several

In early GUPIX versions a change of detector necessitated changes to the Fortran code and hence recompiling, which is certainly a practical problem. Detector properties are now listed in a file created by the user with an editor or word processor and converted to DOS for access by GUPIX. The list comprises properties governing efficiency, e.g., dimensions, window thicknesses etc., followed by line-shape information. The non-Gaussian features of line-shape are usually described by exponential tails and flat shelves (with the sharp edges smoothed), but the admixture of these varies among detectors. Once all the parameters of the line-shape function are determined over the relevant range of X-ray energy there is no universal function for

sion varies among targets and is not fully suppressed. For this reason RBS intensity from a graphite vane rotating at 8 Hz through the beam provides alternative beam integration; this results in rather complex deadtime problems, whose solution we demonstrate elsewhere

PI. In mineralogy a major problem is to view the grains of interest with very high magnification and resolution while the beam is on target. The most frequent approach to viewing is to interpose a 45’ mirror with a very narrow hole and view this with a microscope placed at 90 o to the beam direction, as in fig. la. At the working distances involved a magnification of 150 is achievable. In the design of Ryan et al. [3], shown in fig. lb, the beam reaches the target via a hole drilled axially through a reflecting objective assembly; an Ealing X 15 reflecting objective provides overall magnification of 150 at WD = 24 mm. Other Ealing devices can provide magnification from 360 up to 740 but the concomitant working distances of 8.0 to 2.5 mm, together with the large-diameter assembly, prevent the use of a Si(Li) detector at 45” take-off angle. Our solution is to view the specimen along the normal with a 300 magnification microscope of WD = 14 mm. The compromise made to achieve this large magnification is the geometry of fig. lc where the beam has to be incident at 45’ to the normal. In terms of hardware user-friendliness this has proven extremely successful, providing earth science users with viewing facilities very similar to those they enjoy on electron probes. We have coupled a color TV camera to the microscope’s monocular and view the specimen on a high-resolution color monitor; with most earth science specimens the beam impact point is easily discerned, which is of great importance in analyzing small grains without interference from inclusions or from neighbouring grains.

VIII. PIXE

J. L. Campbell et al. / Practical problems with a proton probe

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describing the energy dependence of each parameter. GUPIX permits the user to employ any or all of two exponentials, a flat shelf and a truncated flat shelf, implying up to four height and two slope parameters. In the line-editor file a vector of X-ray energies is followed by six vectors of the numerical values of the corresponding parameters; if a feature is not used, the corresponding parameter vectors are filled with zeros. GUPIX then uses log-lin interpolation to extract the parameter values it needs. The user may describe as many detectors as he wishes; prior to a fit GUPIX will prompt for the detector designation. 3.3. Standardization We have described elsewhere [4] the H-value method of standardization for thick targets, H being a constant (subsuming solid angle and charge calibration) for a given PIXE setup. H can be determined by whatever standards the user favors, e.g. pure elements, synthetic standards, reference materials etc. A code HVAL in the software package deduces H from the fitted peak intensities in the standard spectra. To increase versatility H may now be determined from the results of thin (e.g. Micromatter) or thick standards, and analyses may be carried out in ng/cm2 or ppm terms; in either case the standard may be a pure element or a compound. If the detector description is not quite perfect the resulting H may exhibit a slight dependence upon X-ray energy below 5 keV or above 15 keV. In such situations the user may employ the line editor to create a vector of H values along with the correponding energy vector and GUPIX will interpolate as required. We have found this approach versatile and consistent. With the same basic software we can analyze air filters (using Micromatter standards, see fig. 2), alloys (via pure metal standards) and mineral grains (using

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3.4. Unknown matrix Conduct of a trace or minor element analysis using GUPIX requires that the matrix (major elements) concentrations be known a priori. A major element analysis can be done by first fitting to obtain the major element peak intensities and then supplying them to an iterative code GUMAT which solves in iterative fashion the equations connecting X-ray yield to concentration; secondary fluorescence is included in this calculation. Although it is accurate and reliable, this approach causes us practical problems of shuttling between different programs when analyzing large suites of minerals of variable matrix. At present therefore GUPIX and GUMAT are being merged to provide user-friendly matrix determination within a single code. This code will create a file of matrix element concentrations which will then function as input for the trace element run using the standard version of GUPIX.

4. Elemental mapping Most reported systems for elemental mapping with proton microprobes employ the VME bus with a computer of the VAX type. Our preference was to develop a PC-based system in whose software the existing GUPIX code could be embedded. The first element of the system is a hardware beam scanner which supplies sawtooth waveforms to pairs of X and Y plates sited between the quadrupole lens and the target. The frequency is variable from 4 to 40 Hz, and the maximum voltage rating of 2000 V provides a mapping area of 400 x 400 urn. This device provides continuous digital readouts of the two scanning voltages. The ADCs measuring X-ray or RBS spectra are read directly by an 80386 computer (33 MHz) by means of a CONTEC PI/O 96W board which fits in one slot of the computer. When an ADC event is processed, one of its output logic lines goes from + 5 V to 0 V and this is inverted to form an interrupt for the I/O board. On the PC an interrupt routine is installed on IRQ level 4 in place of the second communications port, which is not used while thisprogram is running. These IRQ levels on the AT are used by such things as the clock timer which interrupts about every 55 ms, by the disk drives when requesting information and by the keyboard. When an interrupt request is received, all foreground processing is stopped and the computer is directed to proceed to memory address that holds the code that should be executed for that interrupt. These interrupt requests have an hierarchy and so even if one request is being

J. L. Campbell et al. / Practical problems with a proton probe

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serviced, one of a higher priority can be requested and then proceed while the other is put on hold. The interrupt routine is as follows. When the routine is first called by Fortran it installs itself in memory in place of the routine which normally handles the COM2 port. From that point on, this routine works completely in the background, filling vectors and, in the case of scanning, writing to disk. When the computer is diverted to this routine, the I/O board is queried as to whether an ADC or charge event caused the interrupt. In the case of charge, channel five of the data spectrum is incremented. For an ADC event, channel three of the spectrum is incremented to provided total counts and then the ADC is requested to send out the converted channel information. After this is read, the ADC is cleared and control is returned to the foreground job. This process takes about 10 ps and so high count rates are not a problem; the 10 ys process period is taken care of by the fact that the on-demand beam deflector can be set for a “ time-off” period to exceed any process period. The code is also written so as to be re-entrant, allowing charge interrupts to be received during the reading of the ADC. In the case of scanning, upon receipt of an event, the scanning unit is asked for the digitized X and Y positions which are one-byte values from 0 to 255. These are recorded in list mode along with the two-byte channel number from the ADC in a file, This file is continually

updated in background every 15000 events and continuously read in foreground and processed to give the user up-to-date visual information. The data is archived for further processing later if desired.

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J. L. Campbell et al. / Practical problems with a proton probe

The reconstruction software provides simultaneously eight color maps via an enhanced VGA board and a multifrequency monitor; alternatively element ratios may be mapped. A Shinko printer coupled by a Graftel Video Processor to the VGA board provides color or gray-scale output. These maps are simply based on the total counts in a user-defined window set on the relevant Ka or La X-ray peak. An additional feature, not yet in place, will permit the user to delineate an area of interest on the color monitor, and call for GUPIX to fit the spectrum assembled from the data in the enclosed area, thus providing element concentrations. The example in fig. 3 is an elemental map of a particular area of a quartz specimen suspected to contain fluid inclusions. The decreased silicon content towards the upper right of the map, together with localized potassium and calcium reveals an inclusion. Fig. 4 shows PIXE spectra from the inclusion and from the neighbouring matrix; the inclusion contains sodium, sulfur, chlorine, potassium, magnesium, aluminum, calcium, iron, nickel, copper, zinc and tin. Conversion of the peak intensities to concentrations awaits a modification of GUPIX to deal with subsurface inclusions

whether solid or fluid. The combination of elemental mapping plus point analysis provides a powerful tool for the study of inclusions.

Acknowledgements This work is supported by the Natural Sciences and Engineering Research Council of Canada and by various contracts. We thank A. Anderson (St. Francis Xavier University) for the fluid inclusion specimen.

References [l] Dyer Energy Systems, Tyngsboro, MA, USA. [2] W.J. Teesdale and J.L. Campbell, Nucl. Instr. and Meth. B52 (1990) 93. [3] C.G. Ryan, D.R. Cousens, SW. Sie, W.L. Griffin, G.F. Suter and E. Clayton, Nucl. Instr. and Meth. B47 (1990) 55. [4] J.A. Maxwell, J.L. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B43 (1989) 218. [5] G.I. Johansson, X-Ray Spectrom. 11 (1982) 194.