X-Ray Spectra from a Brass-Target Plasma Triode Abstract in This
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X-ray Spectra from a Brass-target Plasma Triode Eiichi SATO, Haruo OBARA, Toshiyuki ENOMOTO, Etsuro TANAKA Hidezo MORI, Toshiaki KAWAI, Toshio ICHIMARU, Akira OGAWA Shigehiro SATO, Kazuyoshi TAKAYAMA Jun ONAGAWA I) Department of Physics , Iwate Medical University 2) Department of Radiological Technology , College of Medical Science, Tohoku University 3) The 3rd Department of Surge ry, Toho University School of Medicine 4) Department of Nutrition al Science, Faculty of Applied Bioscience, Tokyo University of Agriculture 5) Department of Cardiac Physiology , National Cardiovascular Center Research Institute 6) Electron Tube Division #2, Hamamatsu Photonics K. K. 7) Department of Radiologi cal Technology, School of Health Sciences, Hirosaki University 8) Department of Neurosurgery , School of Medicine, Iwate Medical University 9) Department of Microbiolog y, School of Medicine, Iwate Medical University 10) Tohoku University Biomedical Engineering Research Organization II) Department of Electronics , Faculty of Engineering, Tohoku Gakuin University Research Code No.: 200, 204.1 Keywords:Flash x-ray,Brass target, Weaklyionized linear plasma, Characteristicx-rays, Absorptionof zinc Kj3rays Abstract In this paper, we describe a recentlydeveloped table-top plasma x-ray generator utilizing a brass-targettriode, which we used in preliminary experiments for the superpositionof K-series characteristicx-rays in weakly ionized plasma and for producing their higher harmonics. In the plasma flash x-raygenerator, a 200 nF condenser was charged,and flash x-rays were producedby discharging.The x-ray tube was a demountabletriode with a brass target containing65% copper and 35% zinc by weight, and a turbomolecularpump evacuatedair from the tube with a pressure of 1 mPa. Targetevaporation led to the formationof weakly ionized linear plasma, consistingof metal ions and electrons,around the rod target, and intense characteristicx-rays were produced. At a chargingvoltage of 50 kV,the maximum tube voltage was almost equal to the charging volt- age of the main condenser,and the peak current was 15 kA. When the charging voltage was in- creased, the linear plasma formed, and the K-series characteristicx-ray intensities of zinc Ka , copper K,8,and copper K/3 lines increased substantially. On the other hand, hardly any zinc KP lines were detected. In particular, we confirmedthe irradiationof the second unsharp harmonics of the fundamental Ka lines of copper and zinc. The x-ray pulse widths were 700 ns, and the time-integratedx-ray intensitywas 1.2mGy at 1.0m per pulse with a charging voltageof 50 kV. Received May 17, 2007; revision accepted October 25, 2007 *岩 手医 科 大学 共 通 教 育 セ ン ター物 理 学 科 〔〒028-3694岩 手 県 紫 波 郡 矢 巾 町 西 徳 田2-H〕: Department of Physics, lwate Medical University E-mail: dresato@iwate-med .acjp 163 1. Introduction ray intensities from evaporating plasma target. Synchrotrons produce monochromatic par- In particular, the absorption of K-series char- allel beams using single crystals, and the pho- acteristic x-rays in the plasma consisting of ton energy is selected by Bragg's angle. In par- electrons and two-element metal ions requires ticular, the beams with photon energies of ap- investigations, since all K-rays of two ele- proximately 35 keV have been employed for ments are produced from the two-element enhanced K-edge angiography") using iodine solid alloy target. Furthermore, because we contrast media, because iodine media with a have confirmed the irradiation of higher har- K-edge of 33.2 keV absorb the beams effec- monic hard x-rays using nickel and copper tar- tively. Thus, we have developed a steady state gets, the x-ray spectra with photon energies cerium x-ray generator4-6)and have succeeded beyond the K-edges should be measured. in carrying out cone-beam enhanced angiogra- In this paper, we describe a recently devel- phy using cerium K-series characteristic x- oped table-top plasma x-ray generator utilizing rays. a brass-target triode. We used it to carry out Conventional flash x-ray generators') utilize preliminary experiments for the superposition high-voltage condensers and cold-cathode x- of K-series characteristic x-rays in weakly ion- ray tubes, and produce extremely short x-ray ized plasma and for producing their higher pulses with durations of less than 1 us. For use harmonics. in biomedical radiography, we have developed several different flash x-ray generators8-1°)cor- 2. Generator responding to specific radiographic objectives, Figure 1 shows a block diagram of a high- and have succeeded in producing clean K-se- intensity plasma flash x-ray generator. This ries characteristic x-rays of nickel and copper generator consists of the following essential from a weakly ionized linear plasma using a components: a high-voltage power supply, a plasma triode 11-14).We have confirmed the ir- high-voltage condenser with a capacity of radiations of clean K-series characteristic x- 200 nF, a turbomolecular pump, a krytron rays of molybdenum using a compact flash x- pulse generator as a trigger device, and a flash ray generator with a disk-cathode diode15'16), x-ray tube. A low-impedance transmission line and we have developed an intense plasma is employed in order to increase the maximum diode to produce high-photon-energy charac- tube current in the generator. The high-voltage teristic x-rays of molybdenum, cerium"), main condenser is charged to 50 kV by the gadolinium, tantalum18)and tungsten19).In par- power supply, and electric charges in the con- ticular, the tantalum and tungsten Ka rays denser are discharged to the tube after trigger- have been applied to perform high-speed K- ing the cathode electrode with the trigger de- edge angiography using gadolinium-based vice. The plasma flash x-rays are then pro- contrast media. duced. In order to carry out wide-photon-energy or The x-ray tube is a demountable cold-cath- energy subtraction radiography, we are very ode triode that is connected to the turbomolec- interested in the variations of characteristic x- ular pump with a pressure of approximately I 164 mPa. This tube consists of the following major a high-voltage divider with an input imped- parts: a pipe-shaped graphite cathode with a ance of 1 GO and a current transformer, re- bore diameter of 10.0 mm, a trigger electrode spectively. Figure 2 shows the time relation be- made from copper wire, a brass focusing elec- tween the tube voltage and current. The tube trode, a stainless steel vacuum chamber, a voltage and current roughly displayed damped nylon insulator, a polyethylene terephthalate oscillations at the indicated charging voltages. (Mylar) x-ray window 0.25 mm in thickness, When the charging voltage was increased, both and a 4.0-mm-diameter rod brass target con- the maximum tube voltage and current in- taining 65% copper and 35% zinc by weight. creased. At a charging voltage of 50 kV, the The distance between the target and cathode is maximum tube voltage was almost equal to the 20 mm, and the trigger electrode is set in the charging voltage of the main condenser, and cathode electrode. As electron beams from the the maximum tube current was 15 kA. cathode are roughly converged to the target by the focusing electrode, evaporation leads to the 3.2. X-ray output formation of a weakly ionized linear plasma, X-ray output pulse was detected using a consisting of metal ions and electrons, around combination of a plastic scintillator and a pho- the target. tomultiplier (Fig. 3). The x-ray pulse height substantially increased with corresponding in- 3. Characteristics creases in the charging voltage. The x-ray 3.1. Tube voltage and current pulse widths were 700 ns, and the time-inte- Tube voltage and current were measured by grated x-ray intensity measured by a thermolu- 165 3.3. X-ray source In order to roughly observe images of the plasma x-ray source in the detector plane, we employed a l00-um-diameter pinhole camera and an x-ray film (Polaroid XR-7) (Fig. 4). When the charging voltage was increased, the plasma x-ray source grew, and both spot di- mension and intensity increased. Because the x-ray intensity was the highest at the center of the target, both the dimension and intensity de- creased according to both increases in the thickness of the filter for absorbing x-rays and decreases in the pinhole diameter. The mini- mum dimension was equal to the target diame- ter of 3.0 mm. 3.4. X-ray spectra X-ray spectra from the plasma source were measured by a transmission-type spectrometer Fig. 2 Tube voltages and currents with a charging voltage of (a) 35.0 kV, (b) 42.5 kV and (c) with a lithium fluoride curved crystal 0.5 mm 50.0 kV. in thickness. The spectra were taken by a com- puted radiography (CR) system (Konica minescence dosimeter (Kyokko TLD Reader Regius 150)20)with a wide dynamic range, and 1500 with MSO-S elements without energy relative x-ray intensity was calculated from compensation) had a value of 1.2 mGy at 1.0 Dicom digital data. m per pulse with a charging voltage of 50 kV. Figure 5 shows measured spectra from 166 weakly ionized metal plasma. We observed wires coiled around a pipe made of polymethyl clean copper Ka, copper K/3 and zinc Ka methacrylate. Although the image contrast in- lines. However, zinc K/3 and bremsstrahlung creased with increases in the wire diameter, a rays were barely detected.The characteristicx- 50-pm-diameter wire could be observed. Next, ray intensity substantially increased with in- the image of aluminum grains falling into a creases in the charging voltage.