A Simple 2-D Microfluorescence

Transcription

1 A Simple 2-D Microfluorescence Unit By J.Carsello Department of Materials Science and Engineering, The Robert R. McCormick School of Engineering and Applied Sciences, Northwestern University, Evanston, IL 60208, USA ABSTRACT A micro-fluorescence X-ray attachment has been built for general use in an X-ray diffraction facility. The X-ray source is a Elliott microfocus rotating anode machine with a target size of 200 micrometer, fitted with a tapered glass capillary tube. The capillary tubes are fabricated in the facility and produce a beam size of about 10 microns. The sample which is optically viewed by a video microscope is carried on a X-Y stage and is computer controlled. This enables the operator to select particular features of interest for X-ray study. X-ray fluorescence detection is performed using an energy dispersive solid state type X-ray detector and a multichannel analyzer. The data are presented in a contour map relating composition to position. The matrix effects are corrected using a commercial software package. INTRODUCTION It was decided to build a dedicated micro-xrf instrument for general use in our user X-ray diffraction facility. Since the introduction of capillary optics, some problems of conventional slit methods for X-ray microbeam work have been overcome and micro-xrf instruments have been built taking advantage of this feature. This paper describes the unit built at Northwestern University. A micro-fluorescence unit can be built using equipment found in most diffraction labs. Being a user facility, keeping the instrument easy to use was always a consideration in its construction. The instrument is of modular construction with components both fabricated and purchased. An overview is depicted in figure 1. A-X-RAY SOURCE B- GIMBAL AND CAPILLARY C- SAMPLE STAGE D-X-RAY DETECTOR E-VIDEO CAMERA Figure 1 Copyright 0 JCPDS-International Centre for Diffraction Data 1997

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 THE X-RAY SOURCE An available copper point x-ray source on an Elliott micro-focus rotating anode was selected. This machine has a source size of 200 micrometers and is in a point focus configuration with a 5 degree take-off angle. A linear tapered profile for the capillary tube was chosen as it yields an increased flux over a straight capillary tube3, of the same diameter, at the expense of greater beam divergence. The profile was determined using a commercial X-ray tracing program. Parameters selected took into consideration the physical instrument size and X-ray flux. Straight capillary tubes were purchased with a 200 micrometer inside diameter and a 5500 micrometer outside diameter. Borosilicate glass was chosen because of the low cost and low melting point. The capillaries were drawn to a linear taper using a vertically mounted coaxial furnace. These had to be drawn several times in different regions to get the desired linear profile. The profile of the actual capillary was determined by measuring the outside diameter of the capillary with a dial caliper. In drawing the capillary tubes, the ratio of inside to outside diameter remains constant. This was confirmed by cutting one capillary into 2 centimeter sections and measuring the inside diameter directly with a calibrated microscope and the outside with a caliper. The profile is 200 micrometers to 10 micrometers in 30 centimeters. The capillary is then inserted into a gimbal mount that includes X-Y translations. First though, a small straight section of the as received capillary is inserted in the gimbal mount. This is to set the appropriate X-ray take-off angle, roughly align the gimbal mount, and it is used to measure the intensity gain of the of the tapered capillary tube over that of a straight tube. This is done by calculating : Gain = ( intensity from tapered capillary / intensity from straight capillary) x (diameter of straight capillary / diameter of tapered capillary) 2 Intensity gains of 2.5 times were observed for these conditions, and the divergence is 0.1 degrees for this size capillary. SAMPLE STAGE AND COMPUTER CONTROL The sample stage is placed 1 millimeter from the capillary and perpendicular to it. The user must take care when inserting the sample not damage the capillary tip. The computer controlled sample stage has 400 step / rev stepper motors and 50 tpi lead screws. This gives a limit of 0.6 micrometers per step for the X-Y motions. The user has the choice of moving the sample in steps to locate a feature of interest or scan a region about a particular feature. The scan range has a maximum of 6 X 6 centimeters.

4 OPTICAL SYSTEM The optical system consists of a long working distance objective and 5X zoom module. This is coupled to a high resolution, 750 line, low light 0.1 lux monochrome CCD camera and 13 inch video monitor. The system magnification is times. A coaxial illumination was not used as the optical coupling because it absorbs 50% of incident light. The optical system is used to both locate features on the sample and locate the direct beam. This is done using a dot of a visible light fluorescing media and marking the location on the monitor. THE X-RAY DETECTOR, MCA, AND XRF SOFTWARE This system uses a energy dispersive solid-state x-ray detector with 155ev energy resolution. A 2048 channel 100 MHz Wilkinson (ADC) multichannel analyzer collects the spectral data and moves the motors with other peripheral software. The system is calibrated with an iron-55 source for energy. Centroids and respective integrated intensities are used to determine the peak location in energy units and intensities. Spectra can be collected on an individual point basis or scanned with the output going to a data file. This is then plotted using a surface plotting program yielding contour maps of intensities vs. position for a given element. The software allows the use of many regions of interest so contour plots of more than one element can be performed simultaneously. Commercial software permits corrections for matrix effects. Figure :: X Fositinn (cm) Manganese in Stainless Steel Contour lines show intensity of manganese. n-.m,rinht h h-mnc _Intnm3tinnral fbntr0 fnr niffr5-tinn i-m 1997

5 SPECTRAL DATA Collection times of 100 data point scans took about 112 hour. Count times were on the order of 10 seconds. The sample is a commercial 304 stainless steel containing 19% Cr, 9% Ni, 2% Mn, balance Fe. A typical contour plot is shown in figure 2. The sample was scanned in a 5 X 5 centimeter region.this plot has a contour interval of 2% which indicates uniformity in the sample. To simulate a feature, a momentary shutter closure is shown, which appears on the lower right. CONCLUSION Purchasing a complete Micro-XRF system to perform these measurements may be cost prohibitive for small X-ray facilities. However, a relatively inexpensive micro-xrf system can be built from many of the existing components found in most X-ray diffraction labs. The cost of this system not including the MCA and solid state detector system was about $7000. Future improvements to the system would be further reduction of the X-ray source size and increased X-ray flux. The Elliot X-ray generator is capable of a 100 micron point focus source by installing the appropriate filament assembly. ACKNOWLEDGMENTS Portions of this research were supported by the National Science Foundation through Northwestern University s Materials Research Center and Northwestern University. The author would like to thank John P. Quintana, Kevin F. Peters and Jonathan D. Almer for all their time and help with the computer programming. And I m very grateful to Dean Jerome B. Cohen for all the help and guidance without which none of this would have been at all possible. REFERENCES A. Rindy, Nuclear Instruments and Methods Vol. A249,536 (1986) D.A Carpenter, X-ray Spectrometry Vol. 22,229 (1993) Thiel, Bilderback, Lewis, Stern, Rich, Applied Optics Vol. 31, NO. 7,987 (1992)