A novel technique to produce X-rays for XRF, medical, and scientific purposes

Transcription

1 A novel technique to produce X-rays for XRF, medical, and scientific purposes Carlos G Camara a, *, Seth J Putterman b and Andy Kotowski a ABSTRACT A long-standing mystery in science is the process whereby charge spontaneously exchanges between different materials that are brought into contact. After thousands of years of study there is no ab initio theory of tribocharging. As such it is an area of R&D that is not yet tethered to the first principles of physics and is wide open for new inventions. In 2008, Camara et al at UCLA discovered that tribocharging in a moderate vacuum could be used to take X-ray images. Since then, we have improved the X-ray output by 6 orders of magnitude and controlled the emission for use in a commercial product. Here we present an overview of this technology for use in X-ray fluorescence and X-ray imaging. Keywords: X-ray sources, X-ray fluorescence, X-ray imaging, Triboluminescence, Tribo-electrification 1. INTRODUCTION Triboluminescence, the emission of photons from friction, has been known since the Greeks. This effect can be readily observed as a blue glow when peeling adhesive tape in a dark room. In 1930 Obreimoff noticed that under a vacuum, luminescence emitted from splitting mica was similar to the glow of an x-ray tube [1]; then in 1953 Karasev et al. reported the observation of high energy electron emission from the separation of adhesive bonds [2]. The possibility that continuous media could concentrate diffuse mechanical energy to the point where high-energy particles are emitted motivated the work at UCLA that lead to the invention of a new triboelectric X-ray source [3]. We interpret visible triboluminescence as the tail end of a much more energetic effect based on tribo-electrification. As will be described in this manuscript, triboelectrification is such a strong effect that it generates X-rays of sufficient flux and energy for X-ray imaging, Figure 1. Figure 1. Left, first demonstration of X-ray imaging powered by Tribocharging. Right, recent image taken at Tribogenics with a soda can sized prototype X-ray source generating ~3 W of ~90 kv X-rays powered by a 24 VDC source. * carlos@tribogenics.com, a Tribogenics Inc, b UCLA Physics Department

2 2. BACKGROUND Conventional X-ray sources consist of two main components, an X-ray tube and a high voltage (HV) power supply, illustrated in Figure 2. The X-ray tube is a vacuum chamber where a cathode and anode are separated by a vacuum gap. Typically a thermionic emission filament at the cathode that serves as a controllable source of electrons is also included. The HV supply is used to apply a potential of say 100 kv, depending on the application, across the vacuum gap inside the tube. When the filament is energized electrons are liberated at the cathode and accelerated towards the anode with 100 kev of energy. When the electrons strike the anode and abruptly stop, bremsstrahlung radiation is emitted in the X-ray energy. A sufficient vacuum level is necessary to minimize the probability of an electron colliding with a gas particle before attaining the maximum potential energy. The anode, which is the electron target, is typically constructed with a heavy material such as Tungsten to maximize the X-ray conversion efficiency. Figure 2. Conventional X-ray source and power supply. Top left, an X-ray tube. Right, high voltage power supply. Bottom left, diagram of a conventional X-ray tube operation showing the thermionic emitting filament (c), the target/anode, water cooling of the anode, the glass vacuum chamber with an X-ray window and the power supply. In contrast to conventional X-ray sources, our technology is based on the charge transfer that takes place when surfaces come into contact, known as tribo-electrification. This process results in a surface charge density (σ). Separation of the charged surfaces requires work, which is converted into electrostatic potential energy at about 10 kv per mm of separation. When this process is carried out in a moderate vacuum, electrons are stripped from the negatively charged surface and accelerated onto a target resulting in the emission of bremsstrahlung radiation of X-ray energy. We can generate freshly charged surfaces by tapping (Figure 3), sliding, and rolling. All of these mechanical processes create X-ray emission without the need for a high voltage power supply. The key parameters that control the use of Tribocharging to make an X-ray source are (Figure 3): (1.) the charge density (σ) that can be achieved between various surfaces by Tribocharging, (2.) the subsequent charge separation and conversion of mechanical energy into high voltage potential energy, (3.) the rate at which the area of freshly charged surface can be exposed to generate a high voltage current, and (4.) the control of the X-ray generation region.

3 Figure 3. Fundamental parameters that control the generation of X-rays from Tribocharging. The Tribocharging is governed by the fundamental charge exchange that takes place when dissimilar materials are in contact and results in a charge density (σ). The energy conversion requires a machine that works to separate the charges a distance (d) to generate a high voltage potential energy (V), where ε is the permittivity of vacuum. The emission rate is controlled by the amount of exposed area per second (A/s). The X-ray emission area is controlled by the electron target material and geometry. The origin of the charge imbalance from contact is poorly understood to this day. A key parameters is the actual area where the materials come into atomic length distances of each other. This is affected by the surface roughness, the mechanical properties of the materials and the contact force (F). The conversion of mechanical energy into high voltage can be calculated given the charge density and geometry. For the purposes of this paper, we take the infinite parallel plate approximation and a uniform charge density. The emission of electrons from the charged surface is due to the high field and the material properties; the details of our understanding of this process will be presented in a later publication. The electrical power from our devices is given by the potential and the rate of exposed charged area. The X-ray emission mechanism from high energy electrons striking a target in a vacuum is equivalent to a conventional X-ray tube. 3. STATE OF THE ART Using sliding contact tribo-electrification, illustrated in Figure 4, we have manufactured compact sources suitable for X- ray fluoresce spectrometry (XRF), Figure 5. In this technique an X-ray source is used to excite the highest energy bound electrons of a sample. The subsequent recapture of electrons emits X-ray photons with energies characteristic to the atoms present in the sample. Analyzing the X-ray fluorescence spectrum can uniquely identify the elements present and their relative concentrations with enough accuracy to determine most metal alloys. The sources used for XRF generate about 3 μa at 60 kev when the mechanical motion separates fresh surface at a rate of ~100 cm/s. These prototypes operate with an effective charge density σ ~ 1.1X10 11 e - /cm 2 and a charge separation (d) of ~ 3mm. The same sliding contact architecture has been optimized to build soda can sized X-ray sources that generate 30 μa at 90 kev. With these sources we have taken rudimentary chest X-rays (Figure 6). To realize this goal the effective charge density needs to be σ ~ 1.5X10 11 e - /cm 2. In this prototype, mechanical motion exposes fresh surface at a rate of ~1000 cm 2 /s.

4 SLIDING STATIONAR Y σ e TARGE MOTIO d ~ 3 Figure 4. Diagram of a sliding contact architecture. A continuous band of polymer material slides over a stationary contact rod in the direction of motion. The charge density on the band is exposed by the motion and results in electron emission from the band to the grounded target where X-rays are generated. Figure 5. XRF Instrument using a tribo-electric X-ray source. Left is a picture of the Watson M1 XRF hand-held analyzer with cover removed to show the Tribogenics replaceable X-ray source. Also shown are two replaceable X-ray sources. Right is a typical X-ray fluorescence spectra from two different Stainless Steel (SS) samples normalized to peak intensity. The inset shows the presence of Molybdenum in SS 316.

5 12 cm Figure 6. Left image is a rudimentary X-ray picture taken with a Tribogenics X-ray source. Top right is a picture of the Tribogenics X-ray source, which is about the size of a soda can, weighs less than 500g and is powered by 24 VDC. Bottom right is the spectrum of this source showing the Pb k-edge from lead shielding as well as the characteristic Ag lines from the target. 4. FUTURE We believe that by simply optimizing the mechanics and efficiency of a sliding surface source we will realize a prototype capable of generating 100 μa at 100 kv or 10 Watts of power. A further improvement in charge density by a factor of 10 to σ = 1X10 12 e - /cm 2 when combined with a factor of 3 improvement in the rate of contacting area will realize > 100 W of > 120 kv X-ray power from ~1000 c.c. Utilizing rotating contacts will minimize losses due to friction and mechanical motion. Also, as is commonly done in current X-ray tubes, tilting the target relative to the detector plane can reduce the effective spot size. Therefore, taking a 3 mm by 10 mm target and angling it by 70 degrees relative to the detector will yield 100 Watts from a 3x3 mm effective spot in a 40-degree cone beam. Moving forward the tapping architecture is ideal for the construction of an addressable pixelated X-ray source [4] such as illustrated in Figure 7. We have achieved tapping tribo-charge densities of σ = 1X10 11 e - /cm 2 at an actuation rate of ~10 Hz. Prior research on contact electrification has shown that the surface charge densities that can be experimentally achieved are at least 100 times higher [5]: which is σ > 1X10 13 e - /cm 2. Taking the experimentally demonstrated charge density σ = 1X10 13 e - /cm 2, an area of 1 square mm, an actuation rate of 1 khz and 100 μm motion, we will generate over 1 Watt at over 100 kv from a single pixel. This corresponds to over 100 W per square cm. Increasing the separation distance will increase the high voltage and increasing the rate will yield more current. Furthermore, each square mm can be independently controlled by a low voltage mechanical actuation. This will enable arrays of independent X-ray sources that can have unique capabilities for industrial applications and X-ray imaging.

6 Force F = σ 2 A / 2ε σ+ σ d 1 cm Figure 7. Left is a picture of the world s smallest turn-key X-ray source developed by Tribogenics. The center diagram shows a basic tapping architecture and the force needed to separate two parallel surfaces with a high charge density. On the right is an illustration of an array of independently addressable miniature X-ray sources based on the tapping architecture. The colors represent X-ray pixels with different characteristic X-ray spectra. REFERENCES [1] Obreimoff, J.W. The Splitting Strength of Mica, Proc. Roy. Soc (1930). [2] Karasev, V. V., Krotova, N. A. & Deryagin, B. W. Study of electronic emission during the stripping of a layer of high polymer from glass in a vacuum [in Russian] Dokl. Akad. Nauk. SSR 88, (1953). [3] Carlos G. Camara, Juan V. Escobar, Jonathan R. Hird & Seth J. Putterman, Correlation between nanosecond X-ray flashes and stick slip friction in peeling tape, Nature 455, (23 October 2008) doi: /nature07378 [4] J. R. Hird, C. G. Camara, and S. J. Putterman, A Triboelectric X-ray Source, Applied Physics Letters 98, (2011); doi: / [5] Roger G. Horn and Douglas T. Smith, Contact Electrification and Adhesion Between Dissimilar Material, Science, Vol. 256, No (Apr. 17, 1992), pp