MOXTEK S NEW ULTRA-LITE X-RAY SOURCES: PERFORMACE CHARACTERIZATIONS

Transcription

1 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN MOXTEK S NEW ULTRA-LITE X-RAY SOURCES: PERFORMACE CHARACTERIZATIONS S. Cornaby, S. Morris, J. Smith, D. Reynolds, K. Kozaczek Moxtek Inc. Orem UT USA ABSTRACT Moxtek is a manufacturer of miniature x-ray sources which are used in portable and handheld XRF instruments. Moxtek is undertaking a substantial effort to fully characterize their mini x- ray sources, with the intent to provide confidence for those using Moxtek s X-ray sources. The characterization test fall into three categories: environmental stress, X-ray flux and spot stability and repeatability, and a multi-settings evaluation. Examples are given of data collected from these characterization tests from evaluation of Moxtek s newest x-ray source, the ULTRA-LITE, which is a 50 kv, 200 µa source with a maximum power of 4 watts and a weight of only 250 grams. INTRODUCTION There have been tremendous advancements in miniature x-ray tube technology over the past ten years, and it has been increasingly critical to predict a miniature x-ray tube s performance in a number of applications. Moxtek is undertaking a substantial effort to fully characterize its mini x-ray sources during their development with the intent to provide confidence for using these X-ray sources in a variety of applications and environments. The basic methodology for testing involves using several detectors and instruments simultaneously measuring and evaluating the x-ray source. The characterization tests fall into three categories: environmental stress, X-ray 46 mm 25 mm 148 mm 94 mm Figure 1. Image of the ULTRA LITE X-ray source in two different orientations. HVPS output kv range 5-50 kv HVPS emission current range µa Anode Material Ag, W, and others HVPS power limit on x-ray tube 4 Watts HVPS power input-voltage range 6-18 V DC Total Weight 250 gm (8.8 oz) Table 1. The ULTRA LITE X-ray source s basic parameters. HVPS =High Voltage Power Supply

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 JCPDS-International Centre for Diffraction Data 2013 ISSN flux and spot stability and repeatability, and a multi-settings evaluation. Moxtek s most recent source is the ULTRA-LITE, which in development was subjected to Moxtek s new characterization testing (Figure 1 and Table 1). Each of the following sections explain these tests, what their benefits are, and show examples of data collected during the development of the ULTRA-LITE source. ENVIROMENTAL STRESS TEST One of the more critical constraints on portable x-ray sources is the environment. These small x-ray sources need to operate in the field, wherever they are taken. This translates into having a storage temperature range of -40 C to 80 C and an operational range of -10 C to 70 C. While developing a source, the design is considered to be robust when the x-ray source can survive a 120 hour environmental test during which the temperature profile in Figure 2 is repeated twenty times. In the environmental test we can test up to eight x-ray sources at the same time. Temp C ON Storage Temp. Range Enviromental stress test Temp. profile Operational Temp. Range Source on/off state OFF hours 4 5 Figure 2. The temperature profile from a single cycle of the environmental stress test & a profile showing the on/off state of the X-ray source. This cycle is repeated 20 times for the full environmental test. ULTRA-LITE sources were tested with the environmental stress test. Several iterations of this environmental test were performed. Each testing cycle found weaknesses in the design, and the failure points were renovated. This cycle continued until we had an ULTRA-LITE design able to survive this testing. X-RAY FLUX AND SPOT STABILITY AND REPEATABILITY TEST The stability and repeatability test evaluates how the x-ray source operates over time. For the stability test, the x-ray source is left on for the entire test. For the repeatability test, the x-ray source is periodically cycled on and off, typical off cycles are 2 to 10 seconds and typical on cycles are 30 to 120 seconds. The test durations can be from an hour to days. For the stability and repeatability tests, several detectors and instruments are simultaneously measuring and evaluating the x-ray source (Figure 3).

4 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN For both the stability and repeatability tests, we measure the flux directly from the source, which gives the flux stability (Figure 4), and the high voltage stability is measured using the bremsstrahlung edge (Figure 5). We also measure the flux off a 316 stainless steel secondary target, simulating XRF instruments stability. We measure the flux with a photo-diode in current mode, which provides x-ray flux stability information a third way. We measure a time series of pinhole images of the x-ray tube s anode for x-ray spot stability (Figure 6). In Figure 6, the ULTRA-LITE source generates a spot size of 390x340 µm FWHM, a 70% x-ray spot centered flux diameter of 600 um, and a 70% anode centered flux diameter of 950 um (D. Caruso et al., 2010). We have five thermocouples on different locations of the source tube and high voltage power supply, and we monitor every signal and power line going into and out-of the high voltage power supply. X-ray source XRF target Imaging pinhole 2D X-ray imaging camera Photo-diode XRF detector Direct detector & pinhole Figure 3. A simple schematic of the stability and repeatability test showing all the major instruments, three flux detectors and an imaging detector, simultaneously measuring and evaluating the x-ray source.

5 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN Figure 4. An X-ray flux measured directly from ULTRA-LITE source, set at 50 kv and 80 µa (the full 4 watts) during a repeatability test, cycled on for 120 seconds and off for 5 seconds for 2400 cycles. This test measured an x-ray flux repeatability of 0.10% RSD over 85 hours. During the source s warm-up time of minutes, the x-ray flux drifted by 0.45%. Figure 5. A high voltage repeatability test from ULTRA-LITE source from same experiment in Figure 4. The top left figure shows a single X-ray energy spectrum collected directly from the source. The top right figure shows the bremsstrahlung edge of the same spectrum. An algorithm finds the edge, and records it for each of the 2400 spectra taken over 85 hours. This source achieved a voltage of 49.5 kv (set at 50 kv), with a standard deviation of 50 V. 205

6 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN mm Figure 6. An X-ray spot repeatability test from ULTRA-LITE source, taken simultaneously with information in Figures 4 and 5. To the left is a single pinhole camera image of the X-ray spot, with a spot size of 390x340 µm FWHM. The small white dot in the center of the image represents the physical center of the anode. The graph to the right shows the drift of the spot over time; it drifts ~20 µm as the source warms up over the first 30 to 60 minutes and has a constant position thereafter over 85 hours. Figures 4-6 represent just a few highlights of data collected simultaneously from the x-ray source. Collecting all this information simultaneously allows for finding correlations in the data, which is critical for trouble shooting issues as problems are found. All the detectors and other hardware used are controlled with LabVIEW. In addition, LabVIEW writes a final report for each test, the graphs and images in Figures 4-6 are from this self-generated report. MULTI-SETTINGS TEST In the multi-settings test, we run the X-ray source over its entire range of voltage and emission current settings, to evaluate the source. We go to each setting in a random order, turn the source on and off, and run the source for about five seconds. For example, one series of tests done on the ULTRA-LITE, the source was tested at every setting in 1µA steps (ranging from µa) and 0.5 kev steps (ranging from 5-50 kev). This resulted in testing over 13,000 individual settings, with the entire multi-settings test taking about 24 hours. For this test, the flux is measured with a photo-diode in current mode, Figure 7. This shows the x-ray flux signal measured over time by the photo-diode as the source turns-on. For this setting of 50 kv and 80 µa, to the flux is fully on after 0.51 seconds.

7 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN Maximum Emission current 5 µa Emission kv ~ µa ~0.7 sec ~ µa ~0.6 sec ~ µa ~0.6 sec ~ µa ~0.5 sec ~ µa ~0.8 sec ~ µa ~1.1 sec ~ µa ~1.4 sec Table 2. This shows the ULTRA-LITE source turning on time over a few selected settings. A turn-on speed of less than 1 second is considered an adequate turn-on time for a handheld XRF instrument. which provides x-ray flux stability information over a short time frame and the turn on speed of the x-ray source. Again we measure as much as we can simultaneously, we have thermocouples on different locations of the source, and we monitor every signal and power line going into and out-of the high voltage power supply. This provides a great deal of good information about the functionality of the source over all the potential settings. Figure 7 and Table 2 gives information about the turn-on speed of the ULTRA-LITE source, which is defined to be the time between time between the high voltage TTL being enabled to the x-ray flux coming on to 99% of its maximum value. Figure 8. An example of data collected from the multi-settings test where the set voltage, and the set current are plotted against the standard deviation of the x-ray flux measured with a photodiode. An unstable flux output region is clearly seen inside the red circle. This issue was resolve in later designs. Figure 8 gives one example of the flux stability at a number of different settings after the source has turned on at each setting. A low standard deviation in the flux signal represents the desired stable flux output, and is represented in blue dots in the plot of Figure 8. A high standard deviation in the flux signal represents the undesired unstable flux output, and is represented in either red or yellow dots in the plot of Figure 8. In Figure 8, settings with unstable flux stability are clearly identified in the region circled in red. This graph is data from an earlier ULTRA- LITE source design; we improved the design of the analog control board to resolve this flux instability issue.

8 Copyright JCPDS-International Centre for Diffraction Data 2013 ISSN kv & 80 µa 4 40 kv & 100 ua 4 20 kv & 200 µa 2 40 kv & 50 µa 2 10 kv & 200 µa 1 10 kv & 100 µa 1 5 kv & 200 µa 18 V 10.4 W 9.9 W 10.3 W 7.3 W 7.5 W 5.5 W 6.0 W 12 V 9.2 W 8.8 W 9.1 W 6.0 W 6.3 W 4.1 W 4.6 W 9 V 8.6 W 8.1 W 8.5 W 5.5 W 5.7 W 3.7 W 4.3 W 6 V 8.6 W 8.2 W 8.6 W 5.4 W 5.6 W 3.5W 4.0 W Table 3. This table shows the total power drawn by the ULTRA-LITE x-ray source at different X-ray tube settings (in each column) and at different DC power input settings (in each row) ranging from 6 to 18 Volts DC. One last example of data collected during the multi-settings test is the total power drawn by the source. Table 3 gives information about the total power draw of the source depending on the both the x-ray tube power and the input DC voltage supplied to the ULTRA-LITE source. The multi-settings test has been good at identifying issues with the source, as well as providing how inputs and outputs of the source change over the X-ray source s range of settings. SUMMARY AND CONCLUSIONS Moxtek is undertaking a substantial effort to fully characterize their mini x-ray sources during development with the intent to provide confidence for those using these X-ray sources in a variety of applications and environments. Our methodology for testing involves using several detectors and instruments simultaneously measuring and evaluating the x-ray source. The ULTRA-LITE source has been rigorously tested in development over these three categories of tests: environmental stress tests, X-ray flux and spot stability and repeatability tests, and multi-settings tests. We have found the ULTRA-LITE source to be very stable, with a 0.10% RSD over several hours (Figure 4 gives one example). Also, the ULTRA-LITE has as very stable high voltage, not varying more than 50 V while set at 50 kv, measured using the bremsstrahlung edge. REFERENCES Caruso D., Dinsmore M., and Cornaby S. (2010), Miniature X-ray sources and the effects of spot size on system performance, 58 th Denver X-ray Conference, Advances in X-ray Analysis. 53, pp LabVIEW