USING A CHARGE-COUPLED DEVICE (CCD) TO GATHER X-RAY FLUORESCENCE (XRF)AND X-RAY DIFFRACTION (XRD) INFORMATION SIMULTANEOUSLY

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1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol USING A CHARGE-COUPLED DEVICE (CCD) TO GATHER X-RAY FLUORESCENCE (XRF)AND X-RAY DIFFRACTION (XRD) INFORMATION SIMULTANEOUSLY A. Reyes-Mena 1, S. Cornaby 2, H. K. Pew 1, P. W. Moody 1, T. Hughes 2, A. Stradling 2, and L. V. Knight 2 1 MOXTEK Inc., 452 West 1260 North, Orem, UT Department of Physics and Astronomy, Brigham Young University, Provo, UT ABSTRACT A breadboard setup constructed at MOXTEK, Inc. is capable of capturing both XRF and XRD information using a charge-coupled device (CCD) as the x-ray detector. This preliminary setup will lead to a prototype simultaneous XRD/XRF instrument. NASA is funding the instrument s construction because of its capabilities and small size; it could be used for future Mars missions for rock analysis. The instrument uses a CCD to capture both the energy information of an incoming x-ray and the position where the x-ray hit. A powdered sample of material is placed in front of the CCD and then a collimated x-ray beam bombards the sample. The energy information is used to accumulate the x-ray events caused by diffraction, thus enabling us to construct x-ray diffraction patterns. We are also able to discard split events and use all the single x-ray events to create an x-ray fluorescence spectrum from the data. The focus of this paper is how a front-sideilluminated (FSI) CCD and back-side-illuminated (BSI) CCD gather information and how this information is sorted. A split event comparison is made between the two types of CCDs. INTRODUCTION A first prototype XRF/XRD instrument was developed at Ames National Laboratory and is currently in use at Los Alamos National Laboratory [1,2]. The intended applications for this instrument are planetary exploration and a portable instrument for terrestrial use. NASA funding for the instrument s construction is based on its possible use in future Mars missions. Mars missions have previously only used methods that give the elemental composition, which are subject to sizable uncertainties for mineral identification. XRF gives the elemental composition and suggests certain minerals, but the traditional well-tested method of definitive mineral identification is by XRD [3]. With a small, compact XRF/XRD instrument, both the diffraction and the fluorescence information can be practically gathered on Mars as well as on Earth. The purpose of the breadboard setup constructed at MOXTEK is to incorporate and test some of the various key components, including some of MOXTEK s products, involved in an XRF/XRD instrument design. The information gained from this breadboard will lead to a prototype instrument which will be small and portable when a final design is completed.

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 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol EXPERIMENTAL SETUP The breadboard setup for simultaneously capturing both XRF and XRD consists of a commercial CCD camera, a high brilliance Rotaflex Rigaku rotating copper anode x-ray generator (model RU-200), a beam collimating system, a shutter and a sample support (Fig. 1). Both a Princeton Instruments FSI CCD (which consists of an EEV 576 x 384 CCD with 22 x 22-µm pixels) and a BSI CCD (which consists of an EEV 512 x 512 CCD with 25 x 25-µm pixels) were used in the breadboard instrument to compare split events. The x-ray source is run at 30 kv and 30 ma to generate x rays. To capture the XRD information the x rays are collimated with a 100-µm tungsten and a 300-µm lead pinholes. The collimated x-ray beam irradiates the sample, and the CCD camera is used to collect the scattered x rays. The sample is mounted outside the CCD camera. A vacuum environment is maintained inside the camera with the use of an x-ray transparent window mounted on the camera flange. MOXTEK s DuraBeryllium TM and UltraThin TM polymer AP1.3 windows are used in the test setup (Fig. 2) [4]. The information is collected and sorted using algorithms. X-RAY DETECTION USING A CCD Figure 1. Components of the XRF/XRD breadboard instrument. Figure 2. Transparency of different x-ray windows. There is an ideal situation for obtaining both the XRF and the XRD information simultaneously. This situation is to know both the energy of each incoming x ray and the location where each x ray hits. When a sample is exposed to a collimated x-ray beam, both XRF and XRD can be observed. By knowing the energy of the incoming x-rays, the XRD and XRF events can be distinguished. By creating a histogram of all the energies of x-rays events, a fluorescent spectra is observed. By filtering out only the Cu K events and displaying the location of these events spatially, a diffraction image can be observed. A CCD is a detector that has these capabilities. It can be used as an energy-dispersive detector that will give the energy of an x ray and the location where an x ray hit. Each pixel on the CCD can be used as an individual energy-dispersive detector. Each pixel also has a different location in space allowing spatial information to be captured.

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Each pixel on a CCD can be used as an energy dispersive detector because in the soft x-ray range the generation of electron-hole pairs is dominated by the direct band gap. The direct band gap in silicon has an energy gap of 3.65 ev [5]. In the x-ray range the number of electron-hole pairs created in silicon when an x ray is absorbed is proportional to the energy of the x ray. In order to use each pixel as an energy dispersive detector, the pixels on the CCD must work in single-event count mode. In order to acquire the energy information from an x ray, only one x ray must be captured in a pixel. If more than one x ray is deposited in a pixel, the pixel will contain the energy information from both of the events and there is no way to separate the energy. As long as one x-ray event is captured in one pixel, then the CCD can be used as an energy dispersive detector. split events single events Figure 3. Single and split events. (a) total events (b) single events (c) split events, two or more Not all of the x-ray events that hit the CCD are collected into a single pixel. There are a number (d) split events, three or more of events that are collected into two or more adjacent pixels; these events are called split events (Fig. 3). The split events do not give the Figure 4. Number of counts per exposure. energy information needed to know the energy of the x ray and are therefore eliminated from the data set. In order to get the XRF and the XRD information, we keep only the single events. A single event is a pixel with a count of electrons above the noise level and has all of the eight neighboring pixels with an electron count at the noise level. Also, in order to get a large number of both the XRF and XRD events, multiple exposures need to be taken by the CCD. The number of x-ray events that we typically capture per exposure for the FSI CCD is shown in Figure 4. Figure 4(a) shows the total number of events that hit the CCD per exposure; 4(b) represents the total number of single events; 4(c) shows the total number of events split between two or more pixels; 4(d) shows the number of events split between three or more pixels. With other samples taken there have been between 400 to 700 single events captured on a single CCD exposure. The CCD exposure time presently runs between five to eight seconds. To get approximately 300,000 events (enough events to start to view Cu K diffraction rings), 400 to 600 exposures need to be taken. Minimizing the number of split events is crucial in making the CCD an effective energy-dispersive detector.

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Figure 5. XRF spectrum collected with the FSI CCD. Figure 6. XRF spectrum collected with the BSI CCD. CCD COMPARISON In order to determine the optimum CCD detector for gathering the XRF/XRD information for the instrument, we compared the performance of both the FSI CCD and the BSI CCD. We found that the FSI CCD had a fewer number of split events then the BSI CCD. Figure 5 shows a typical energy spectrum collected with the FSI CCD with 277-five second exposures while doing simultaneous diffraction. The upper function in this graph represents the energy spectrum with both the single and the split events present. The lower function shows the spectrum with only the single events remaining. Eliminating the split events reduced the background noise in the energy spectrum. Figure 6 shows a typical energy spectrum collected by a BSI CCD in a single fivesecond exposure. The upper function shows the spectrum with all the events included and the lower function includes only single events. It is clear that the FSI CCD has fewer split events than the BSI CCD. Since the FSI captures more single events per exposure than the BSI CCD, fewer exposures are needed to capture the same number of single events with the FSI CCD. For this reason we have chosen to use the FSI CCD for the XRF/XRD instrument. RESULTS AND DISCUSSION The energy information for each x ray is captured, along with the location where the event happened. Using this energy information as a filter for x-ray events, we can construct the diffraction pattern from a sample. Since all the x-ray events are captured, this allows the acquisition of multiple diffraction patterns. Figures 7 and 8 show diffraction patterns collected simultaneously from a KCl sample. Figure 7 is constructed from the K events, and Figure 8 is constructed from the K events. The nondiffracted beam that irradiates the sample is blocked with a beam stop to keep the CCD from being damaged. The shadow of the beam stop is the vertical line where there are few x-ray events (Figs. 7 and 8). Once these images are created they can be radially integrated to give the diffraction intensities as a function of the 2 angle. Figure 9 shows the intensity versus 2 plot for the K events, and Figure 10 shows the plot for the K events. Figure 11 shows the XRF for the KCl sample. The camera was fitted with an AP1.3 window when data for this sample were taken. This causes a Si K peak because of the silicon

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol rib structure of the x-ray window on the CCD. This rib structure also affects the XRD images and can be seen in the spotty change of intensity in the diffraction rings (Figs. 7 and 8). If the rib structure was not present, then each diffraction ring would have a uniform intensity. Data collected with a beryllium window do not have a Si K peak present and the diffraction rings have uniform intensities. With the breadboard XRF/XRD instrument, all the information in Figures 7-11 was gathered simultaneously. Figure 7. Cu K events for KCl sample. Figure 8. Cu K events for KCl sample. Figure 9. Cu K events in a 2 plot for a KCl sample. Figure 10. Cu K events in a 2 plot for a KCl sample. CONCLUSIONS Both the XRF and XRD information can be captured simultaneously using a CCD as the detector. A CCD can be used to gather both the diffraction information and the fluorescence information when the CCD is able to capture single events within a single pixel. Also, in comparing FSI and BSI CCDs it was found that the FSI CCD had fewer split events than the BSI CCD. This allows the FSI CCD to capture more single Si K Cl K K K Cu K Cu K Figure 11. XRF for the KCl sample.

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol events for a given exposure time. The breadboard instrument has shown adequately that the construction of a small, portable XRF/XRD instrument is feasible for terrestrial applications and possibly for planetary missions to Mars. ACKNOWLEDGMENTS This work is being supported by NASA under SBIR contract number NAS We thank Brigham Young University for student partial-financial support and for use of equipment. REFERENCES [1] Blake, D.F. et al., Proceedings of the LPSC XXIII, 1992, [2] Blake, D.F. et al. X-ray Diffraction Apparatus, 1993, U.S. Patent No. 5,491,738. [3] Vaniman, D.; Bish, D.; Blake, D.F.; Elliott, S.T.; Sarrazin, P.; Collins, S.A.; Chipera, S., J. Geophys. Res., 1998, 103(E13), 31,477-31,489. [4] MOXTEK, Inc., UltraThin Soft X-ray Window Data Sheet, [5] Sweedler, J.V.; Ratzlaff, K.L; Denton, M.B.; eds. Charge-Transfer Devices in Spectroscopy, VCH Publishers Inc.: New York, 1994, 12.