Applied Surface Science 255 (2008) 217–219
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Spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces Yasuyuki Nagashima *, Toshihide Hakodate, Takahiko Sakai Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
A R T I C L E I N F O
A B S T R A C T
Recently, the spontaneous emission of positronium negative ions from polycrystalline tungsten surfaces was observed. In the present work, the emission of these ions in ultra-high vacuum has been studied and the long-term stability of the emission efﬁciency has been investigated. ß 2008 Elsevier B.V. All rights reserved.
Available online 21 May 2008 PACS: 36.10.Dr 71.60.+z 78.70.Bj Keywords: Positronium negative ion Tungsten Work function
1. Introduction When slow positrons are incident on metal surfaces, some penetrate into the bulk, rapidly losing their kinetic energies until thermalized, and then diffuse back to the surface where there are several channels for the fate of the positrons . The positrons may get trapped in image-potential-induced surface states at the metal–vacuum interface. When the positron work function is negative, the positrons may be ejected from the surface spontaneously. The positrons may also be emitted as work function positronium (Ps) atoms. When the target is heated, thermally activated Ps are also observed. Recently, we have observed the spontaneous emission from polycrystalline tungsten surfaces of the Ps negative ion (Ps), which is a bound state of a positron and two electrons . In the present paper, we report the measurement of the Ps emission efﬁciency from tungsten surfaces in higher vacuum conditions than the previous work, in order to investigate the effect of surface contamination on Ps emission. 2. PsS emission from tungsten surfaces The existence of Ps was predicted by Wheeler  and conﬁrmed by Mills  in a study using the beam-foil method.
* Corresponding author. Tel.: +81 3 5228 8724; fax: +81 3 5228 8229. E-mail address: [email protected]
(Y. Nagashima). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.05.204
After the discovery, this method was employed to measure the Ps decay rate [5,6]. It is well known that Ps atoms are emitted from most metal surfaces spontaneously because the Ps formation potential, fPs = f+ + f 6.8 eV, is negative, where f+ and f are the positron and electron work functions, respectively. Ps would also be produced from the thermalized positrons at metal surfaces and emitted if the Ps formation potential, fPs
fPs ¼ fþ þ 2f EB
is negative , where EB is the binding energy of Ps (the energy required to break-up into an isolated positron and two electrons). The values of fPs estimated from the reported values of EB [8–10], f and f+ [7,11–18] for metals are listed in Table 1. The negative values for tungsten indicate that Ps would be emitted from tungsten surfaces. Wilson and Mills  have attempted to observe the emission of Ps from tungsten (1 1 1) surfaces and set an upper limit of 0.1% for the Ps branching ratio. Recently, Ps formed through this mechanism has been observed for the ﬁrst time from polycrystalline tungsten targets annealed at 1500 8C . The emission efﬁciency measured in this study was found to be dependent on the time of the measurement after annealing. This was attributed to adsorbate coverage of the target surface by residual molecules in the target chamber. The positron and electron work functions can be written using the corresponding chemical potentials, m+ and m, as follows:
fþ ¼ D mþ ;
f ¼ D m
Y. Nagashima et al. / Applied Surface Science 255 (2008) 217–219
Table 1 Experimental values of f+ and f. Values of fPs estimated using Eq. (1) are also listed Element
Al (polycrystalline) Al (1 0 0) Al (1 1 1) Cr (1 0 0) Fe (polycrystalline) Co (polycrystalline) Ni (polycrystalline) Ni (1 0 0) Ni (1 1 0) Cu (1 0 0) Cu (1 1 0) Cu (1 1 1) Mo (polycrystalline) Mo (1 0 0) Ag (1 0 0) W (polycrystalline) W (1 0 0) W (1 1 0) W (1 1 1) Pt (polycrystalline) Au (polycrystalline) Pb (polycrystalline)
0.2  0.16  0.065  1.76  1.2  0.8  1.2  1.0  1.4  0.3  0.2  0.4  2.2  1.7  0.6  2.75  3.0  3.0  2.59  1.8  0.9  0.9 
4.25  4.20  4.26  4.46  4.4  5.0  5.15  5.22  5.04  5.10  4.48  4.94  4.6  4.53  4.64  4.55  4.63  5.22  4.45  5.64  5.2  4.25 
1.2 1.11 1.46 0.03 0.5 2.1 2.0 2.3 1.6 2.8 1.6 2.4 0.1 0.2 2.8 0.78 0.9 0.3 0.82 2.4 4.2 2.3
where D is the surface dipole barrier . Therefore, the Ps formation potential can be written as
fPs ¼ mþ 2m EB þ D
This equation indicates that fPs is dependent on D, i.e., the Ps emission from metal surfaces is affected by the coverage on the target surfaces. An oxygen overlayer on the tungsten target increases the value of D and therefore can suppress Ps emission. Carbon contamination precipitated onto the tungsten target by annealing also affects the emission. In the present work, we have measured the Ps emission efﬁciency from tungsten polycrystalline target in a UHV chamber operating at pressures lower than the previous work in order to check the effect of overlayers due to the residual molecules in the chamber.
were that the Ge detector was moved to a position upstream in order to decrease the angle between the direction of the detector and the Ps velocity and, also, the grounded grid, which was ﬁxed in the previous work, was mounted on a linear transfer at a distance d from the target. The vacuum system was also improved; the base pressure was 3 108 Pa, which was attained using a turbo-molecular pump and two getter pumps. The beam intensity was 6 104 e+/s and the transport energy was 0.1 keV. The target bias, W, was 3 kV so that the positrons were incident on the target with an energy of 3.1 keV. The Ps emitted from the target would be accelerated by the electric ﬁeld between the grid and the target. The annihilation g-rays were detected by a Ge detector which would see the Ps coming towards to it. In order to detect the low count rate of the Ps emission events, the background was reduced by using a coincidence circuit with a NaI(Tl) scintillation detector place behind the target. The target was polycrystalline tungsten of thickness 25 mm (99.95% purity), which was purchased from a company different to that in the previous work. It was annealed in situ at 1500 8C by passing an electric current. The temperature was raised to 1500 8C, maintained for 30 min, and then gradually lowered to room temperature over a period of 9 min. The measurements were performed at room temperature over a period of 1.4 106 s. We also measured the spectrum from the same target before annealing for 2 105 s. 4. Results and discussion Fig. 2 shows the annihilation photon energy spectra obtained for the tungsten target. No signiﬁcant peak of the Doppler-shifted g-rays was observed for the target before annealing. In the spectrum after annealing, a low-intensity peak at about 543 keV can be clearly observed, which is consistent with the Dopplershifted energy of the g-rays emitted from the accelerated Ps E¼
1 pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ mc2 2 2l þ l cos u
where l = eW/3mc2, e is the charge of the positron, m is the rest mass of the positron and c is the speed of light.
3. Experimental procedure The experimental system is shown in Fig. 1 and is similar to that used in the previous work . The changes made to the set-up
Fig. 1. Schematic diagram of the target chamber for observing Ps emitted from tungsten surfaces. The apparatus is an improved version of that used in the previous work .
Fig. 2. The annihilation photon energy spectra for the tungsten target before annealing and after annealing. The arrow indicates the g-ray energy predicted by Eq. (2).
Y. Nagashima et al. / Applied Surface Science 255 (2008) 217–219
onto the target surface. An in situ analysis system for the target surface was not available in our apparatus. Therefore, we have treated the target according the instructions by Stern  and Erskine ; the target was annealed in oxygen of 6 104 Pa at 1300 8C for 60 min in a separate chamber, transported in air to the target chamber of the beam apparatus and then annealed again in vacuum at 1500 8C for 30 min. The estimated efﬁciency of the Ps emission was of the same order as the target without oxygen treatment. Longer treatment or in situ treatment in the target chamber might be necessary to remove carbon atoms sufﬁciently to obtain a higher Ps emission efﬁciency. Acknowledgements
Fig. 3. Ps fraction, f, plotted against the time after annealing.
It is to be noted that the shape of the peak around 511 keV for the spectrum after annealing is asymmetrical although that before annealing is almost symmetrical. This asymmetry is due to the emission of para-Ps from the target with a maximum energy of jfPsj. The symmetrical shape indicates that para-Ps atoms in this energy range were not emitted from the target surface before annealing. In order to obtain the fraction, f, of incident slow positrons yielding Ps from the tungsten surface, the shifted peak in the spectrum was ﬁtted with a Gaussian function. The value of f must be corrected for annihilation in the acceleration region. A fraction
& ¼ exp ð3G DEd=eWcÞ
of the Ps will emerge from the acceleration region before annihilating, where G is the Ps annihilation rate and DE is difference between E deﬁned by Eq. (4) and mc2. The corrected value of f was 7 105. The spectrum acquired after annealing was divided into seven time regions and the value of f for each time region was obtained. Although the value of f in the previous work decreased with time, Fig. 3 shows that f was almost constant. This is because the adsorbate coverage of the target surface was reduced due to the improvement of the vacuum conditions in target chamber. We have also measured the spectra for the target annealed in oxygen in order to remove the carbon which may have precipitated
We thank Prof. A.P. Mills Jr., Professor A. Weiss, Prof. K.G. Lynn, Dr. I. Shimamura and Prof. A. Igarashi for valuable discussions. We also thank Prof. T. Miyazaki for his encouragement and support. This work was partly supported by Reimei-Kenkyu of Japan Atomic Energy Agency. References  A.P. Mills Jr., Positron Solid-State Physics, in: W. Brandt, A. Dupasquier (Eds.), Proc. Int. School Phys. ‘Enrico Fermi’ Course 83, North-Holland, Amsterdam, (1983), p. 432.  Y. Nagashima, T. Sakai, N. J. Phys. 8 (2006) 319.  J.A. Wheeler, Ann. N.Y. Acad. Sci. 48 (1946) 219.  A.P. Mills Jr., Phys. Rev. Lett. 44 (1981) 717.  A.P. Mills Jr., Phys. Rev. Lett. 50 (1983) 671.  F. Fleischer, K. Degreif, G. Gwinner, M. Lestinsky, V. Liechtenstein, F. Plenge, D. Schwalm, Phys. Rev. Lett. 96 (2006) 063401.  R.J. Wilson, A.P. Mills Jr., Phys. Rev. B 27 (1983) 3949.  A.K. Bhatia, R.J. Drachman, Phys. Rev. A 28 (1983) 2523.  Y.K. Ho, J. Phys. B: At. Mol. Opt. Phys. 23 (1983) 1345; Y.K. Ho, Phys. Rev. A 48 (1993) 4780.  A.M. Frolov, A. Yu Yeremin, J. Phys. B: At. Mol. Opt. Phys. 22 (1989) 1263.  G. Fletcher, J.L. Fry, P.C. Pattnaik, Phys. Rev. B 27 (1983) 3987.  A.H. Weiss, P.G. Coleman, in: P.G. Coleman (Ed.), Positron Beams and their Applications, World Scientiﬁc, Singapore, 2000, p. 129.  D.R. Lide (Ed.), CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton, 2004.  M. Jibary, A. Weiss, A.R. Koymen, D. Mehl, L. Stiborek, C. Lei, Phys. Rev. B 44 (1991) 12166.  J.G. Ociepa, P.J. Schultz, K. Grifﬁths, P.R. Norton, Surf. Sci. 225 (1990) 281.  N.G. Fazreev, J.L. Fry, A.H. Weiss, Phys. Rev. B 57 (1998) 12506.  M.R. Poulsen, M. Charlton, G. Laricchia, J. Phys.: Condens. Mat. 5 (1993) 5209.  A.P. Knights, P.G. Coleman, Surf. Sci. 367 (1996) 238.  R.M. Stern, Appl. Phys. Lett. 5 (1964) 218.  J.L. Erskine, Phys. Rev. B 24 (1981) 2236.