Injection of refractory metals into EBIT using a Knudsen cell

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Injection of refractory metals into EBIT using aKnudsen cellTo cite this article C Yamada et al 2007 J Phys Conf Ser 58 403

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Injection of refractory metals into EBIT using a

Knudsen cell

C Yamada1 K Nagata1 N Nakamura1 S Ohtani1 S Takahashi1H Tobiyama1 M Tona1 H Watanabe1 N Yoshiyasu1 M Sakurai2A P Kavanagh3 and F J Currell31 CRESTJST Institute for Laser Science Department ot Applied Physics and ChemistryThe University of Electro-Communications Chofu Tokyo 182-8585 Japan2 Department of Physics Kobe University Kobe 657-8501 Japan3 Queenrsquos University of Belfast Belfast BT7 1NN UK

E-mail yamadailsuecacjp

Abstract A new method for injection of metallic elements into an electron-beam ion trap(EBIT) is described Injection of metallic elements into an EBIT has so far been mainly achievedby MEVVA (metal vapor vacuum arc) ion sources However continuous injection as is the caseof rare gases is sometimes desirable especially for stable extraction of highly charged ionsIn the course of DR (dielectronic recombination) study we have developed a method of sucha stable injection of metallic elements This method is applicable to any metallic elements ormetallic compounds that have vapor pressures of sim 01 Pa at a temperature lower than 1900 CWe have employed this method for the extraction of highly charged ions of Bi Er Fe Ho and W

1 IntroductionFor the injection of atomic elements into an electron-beam ion trap (EBIT)[1] the followingmethods have been developed Metal-vapor vacuum arc (MEVVA) ion sources[2] wire probe[3]organometallic compounds such as C3H9Sb and C2H6Te[4] metalocenes metalcarbonyls andmetal methyl compounds [5 6] methyl iodide[7] other than noble gas injection In this workmetallic vapor was directly injected into the EBIT from a side window of the drift tube assemblyThough already metals with low melting points eg K and Cs have been successfully injectedin the gas phase [8 9] In our recent study with the Tokyo EBIT we investigated resonantdielectronic recombination (DR) of electrons with highly charged ions in which study the con-centration of the ions of various charges were directly and simultaneously monitored by countingthe numbers of extracted ions as a function of electron-beam energy During this observationstable and continuous injection of seed atoms is essential For this purpose we developed a nobleinjection method using a Knudsen cell [10]

2 Experiments and resultsA Knudsen cell (Fig 1) bearing the name of M Knudsen who thoroughly studied fundamentalphysics of effusion of rarefied gases[11 12] was placed horizontally at the level of the centerof the drift tube The Knudsen cell has been extensively used in the study of molecular-beam

IOP Publishing Journal of Physics Conference Series 58 (2007) 403ndash406doi1010881742-6596581092 13th International Conference on the Physics of Highly Charged Ions

403copy 2007 IOP Publishing Ltd

epitaxy [13] Essentially it is a high temperatuer oven as has been employed in the electroncyclotron-resonance ion source [14] but having a small aperture to facilitate highly directionalbeams and having a precise temperature control unit The drift tube of the Tokyo EBIT haseight slit windows 2 mm wide and 10 mm high to facilitate observation of X-ray and visibleemissions and for injection of noble gases One of the windows was used in this study of themetallic vapor injection The opening of the cylindrical crucible was facing a window in thedrift tube with two pinholes having a diameter of 1 mm aligned on a straight line between thecrucible and the drift tube to ensure the ballistic flight of the atoms A molybdenum boat wasplaced inside the crucible in which molten bismuth was kept For metals having higher meltingpoints ie Er Fe and Ho no boat was used because the vapor pressure was high enough belowthe melting point

Figure 1 Detail of the Knudsen cell Thecrucible is set horizontally while the electronbeam in the drift tube assembly is verticalA molybdenum boat is used for Bi to holdthe liquid The temperature was controlledwithin 1 degree using a PID controller

The temperature was kept at 550ndash600C for Bi when the vapor pressure was estimated tobe 004ndash02 Pa as a sum of Bi and Bi2 calculated by the Harlacher-Braun formula [15] usingappropriate constants [16]

log P = A + BT + C log T + DT (1)

By simple gas kinetic theorythe flux coming out of the Knudsen cell is expressed as follows

φ = 264 times 1020Pradic

MT cmminus2 sminus1 (2)

where P is the pressure in Pa M the atomic weight and T the temperature in K Usingthis equation the number of the bismuth atoms entering the drift tube region is calculated tobe 2sim10times1014 s For iron the temperature was set to 1300C which is the upper limit for theKnudsen cell using a PBN (pyrolytic boron nitride) crucible according to the manufacturer[17]However the vapor pressure at 1300C was calculated to be 006 Pa and the number of Fe atomsentering the drift tube is 4times1014s Measurement of the DR cross sections of highly chargedFe ions is now underway In comparison with the organometallic sources [5 6] contaminationwith lower charge state ions of C and O has been eliminated Other metals Er and Ho werealso introduced into the Knudsen cell to observe that sufficient highly charged ions are extractedbelow 1300C If we use graphite or alumina crucibles temperatures as high as 1900C may beachieved Ho and Bi are single-isotope elements and are suited for the extraction measurement ofthe DR process Er was chosen as a candidate material for fabricating nano-structured electro-optic devices using highly charged ions Figure 2 shows the charge spectrum of the Bi ions

404

dispersed by an electromagnet inserted in the extracted beam The horizontal axis correspondsto the m q ratio and the vertical axis is proportional to the ion flux At the electron beamenergy of 50 keV Heliumlike Bi ions charged by 81+ are clearly observed

In summary we have injected various metal atoms into the EBIT using a Knudsen cell andextracted their highly charged ions The Knudsen cell may be used for other metallic elementsand also for some refractory metals eg tantalum tungsten and platinum by using oxideshalides etc as a starting material

Figure 2 Mass spectrum of Er ionsTemperatures are those of the Knudsen cell

Figure 3 Mass spectrum of Ho ionsTemperature was 1300C

AcknowledgmentsThis work was performed under the auspices of the CREST program ldquoCreation of Ultra-fast Ultralow Power Superperformance Nanodevices and Systemsrdquo of the Japan Science andTechnology Agency This work was a part of the 21st Century Center of Excellence ProgramldquoInnovation in Coherent Optical Sciencerdquo at the University of Electro-Communications

References[1] Marrs R E Levine M A Knapp D A and Henderson J R 1986 Phys Rev Lett 60 1715[2] Brown I G Galvin J E Gavin B F and MacGill R A 1986 Rev Sci Instrum 57 1069[3] Elliot S R and Marrs R E 1995 Nucl Instrum Methods B 100 529[4] Nakamura N Kinugawa T Shimizu H Watanabe H Ito S Ohtani S Yamada C Okazaki K Sakurai M

Tarbutt M R and Silver J 2000 Rev Sci Instrum 71 684[5] Werner T Zschornack G Groβman F Ovsyannikov V P and Ullmann F 2000 Rev Sci Instrum 71 2038[6] Beiersdorfer P Trabert E and Pinnington E H 2003 Astrophys J 587 836[7] Tona M Nagata K Takahashi S Nakamura N Sakurai M Yamada C and Ohtani S 2006 Surf Sci 600 124[8] Trabert E Beiersdorfer P Brown G V Boyce K R Kelly R L Kilbourne C A Porter F S and Szymkowiak

A 2006 Phys Rev A 73 022508[9] Trabert E Beiersdorfer P Brown G V Chen H Pinnington E H and Thorn D B 2001 Phys Rev A 64

034501[10] Yamada C Nagata K Nakamura N Ohtani S Takahashi T Tona M Watanabe H Yoshiyasu N Sakurai

M Kavanagh A P and Currell F J 2006 Rev Sci Instrum 77 066110[11] Knudsen M 1908 Ann Phys (Leipzig) 28 75[12] Knudsen M 1909 Ann Phys (Leipzig) 29 179

405

[13] Shukla A K Banik S Dhaka R S Biswas C Barman S R and Haak H 2004 Rev Sci Instrum 75 4467[14] Pardo R C and Billquist P J 1990 Rev Sci Instrum 61 239[15] Alcock C B Itikin V P and Horrigan M K 1984 Can Metallurgical Quarterly 23 309[16] Alcock C B in CRC Handbook of Chemistry and Physics 1999-2000 80th ed (CRC Press)[17] Vacuum Handbook 1985 2nd ed (Ulvac Corp) in Japanese[18] Weber A H and Kirsch S C Jr 1940 Phys Rev 57 1042[19] For example Eiko Engineering Japan

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