- Email: [email protected]
Production of an intense slow positron beam by using an electron LINAC and its applications
surface ELSEVIER
science
Applied Surface Science 85 (1995) 124-131
Production of an intense slow positron beam by using an electron LINAC and its applications I. Kanazawa ,,a y. Ito b M. Hirose b H. Abe c, O. Sueoka 0, S. Takamura e, A. Ichimiya f, Y. Murata g, F. Komori g, K. Fukutani g, S. Okada h, T. Hattori i a Department of Physics, Tokyo Gakugei University, Koganei, Tokyo 184, Japan b Research Center for Nuclear Science and Technology, The University of Tokyo, Tokai, Ibaraki 319-11, Japan c Faculty of Engineering, Tokai University, Kitakaname 1117, Hiratsuka, Kanagawa 259-12, Japan d Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan e Physics Department, Tokai Establishment, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan f Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan g Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo 106, Japan h Takasaki Radiation Chemistry Establishment, Japan Atomic Energy Research Institute, Watanuki, Gunma 370-12, Japan i Faculty of Engineering, Musashi Institute of Technology, Tamazutsumi, 1-28-1 Setagaya, Tokyo 158, Japan
Received 18 May 1994
Abstract
An image of reemitted positrons has been produced in a proto-type positron transmission-type reemission microscope by using the electrostatic intense slow positron beam from an electron LINAC. RHEPD experiments have then been performed on H-terminated Si(ll 1).
1. Introduction
Advancement in the production of intense slow positron beams is required, so that they can provide new analytical methods in physics, chemistry, and materials science. Basically there are two methods with which to obtain intense slow positron beams. One is to prepare highly intense radioactive sources. Radioactive copper is produced in the Brookhaven National Laboratory (BNL) high-flux reactor (1015 n / c m 2. s), which can supply a few hundred Ci of 64Cuper gram of natural copper. Since
* Corresponding author. Fax: +81 423 24 9832.
the half-life of 64Cu is not short (12.8 h), an intense slow positron beam can be obtained for ~ 10 h [1]. Also, highly intense short-lived isotopes (e.g. 11C) can be prepared using ion accelerators. For these short-lived isotopes it should be possible to obtain intense slow positron beams in an on-line configuration [2]. The other method is to use positrons from positron-electron pairs produced using an electron LINAC [3-7]. High-energy electrons are stopped in high-Z materials like Ta and produce a bremsstrahlung photon shower. Positron-electron pairs are created from this photon shower. In this study, we used an electron LINAC for production of the intense slow positron beam [8].
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD1 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 0 3 2 0 - 3
1. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
The beam was transported through a solenoid magnetic field and then transferred to a field-free region by using a 'magnetic field splitter'. In the field-free region, the beam was brightness-enhanced for diffraction and microscope experiments by using electrostatic lenses. The positron reemission yield gives unique information concerning subsurface defects, because thermalized positrons are trapped at those defects. In addition, positrons have negative work functions in many substances so that they are reflected more efficiently at the outermost layers and can give more selective information on surface structures than can electron diffraction. This is the reason why reflection high-energy positron diffraction (RHEPD) is useful for surface science.
In the present study, we have performed a transmission-type reemission positron yield measurement where the image of reemitted positrons is produced in a proto-type positron reemission microscope, using an intense slow positron beam produced by an electron LINAC. RHEPD experiments have then been performed using this intense slow positron beam.
2. Beam line configuration and brightness enhancement Positrons were produced by bombarding a watercooled convertor with the electron beam from a LINAC, Typical electron beam parameters were 100 MeV energy, 1 ixs pulse width, 50 pps, and 10 txA
L
(i3
125
r
ri
Fig. 1. The reemiUedpositron beam profilesfor positron incident energies of (a) 3.5 keY, (b) 4.0 keV, and (c) 5.0 keV. (d) The reemitted positron beam profile for a positron incident energy of 5.0 keV in the deflection by the weak magneticfield.
126
L Kanazawa et aL/AppliedSurface Science 85 (1995) 124-131
p,•'.••'•`.••`'•••,•`•.•••`•'•`•.•••.`•`
phosphor
\ MCP
••••`••`•
lens / \ mesh
~f0il
Fig. 2. Assembly of the lens configuration and detector for the proto-type transmission-type reemission positron microscope.
average current. The converter, which is cooled by water, was made of several Ta plates (12 mm overall thickness) with the last Ta plate constituting the wall of the vacuum chamber. Tungsten sheets with a thickness of ~ 25 Ixm were used a s the positron moderator. After annealing at ~ 2000°C under ultra-highvacuum conditions, the moderator was placed behind the last Ta plate either in the Venetian-blind or in the transmission-type geometry. The slow positrons were transported into the experimental room through solenoid tubes, 5 m long with two 90 ° bends. The magnetic field (60-80 G) was supplied from solenoid coils directly wound around the vacuum transport tube, and by Helmholtz coils set at several positions
along the tube. Slow positrons were extracted from the moderator by the tungsten mesh held at a potential of 30-60 eV, and were guided along the magnetic field. The intensity of the slow positron beam was determined before the stage of brightness enhancement either by using a Faraday cup or by stopping all the positrons and measuring the annihilation gamma-rays with an ionization chamber gauge. After the electrons had been carefully eliminated, the output of the Faraday cup was 5 pA or ~ 3 X 107 e + / s (at 1 / 1 0 of full power operation of the LINAC, i.e., 1 kW of the electron beam power). From the ionization chamber reading, the intensity of the slow positrons was calculated to be ~ 2 X 107 e+/s, in rough agreement with the measurement from the Faraday cup. At the end point of the solenoid transport tube, an assembly of an accelerator in the magnetic field and electrostatic lenses in a field-free region was placed, which enabled the beam to be extracted from the magnetic field and focused onto the reemission moderator. Positrons extracted by the 'magnetic field splitter' were focused onto the first remoderator (W 1) with an energy in the range 3-5 keV. The remoderator was a single crystal of W ( l l 0 ) onto which the focused positrons entered at an angle of 30 ° to the normal. The reemitted positrons were again accelerated and focused onto the second remoderator (W2) and the final beam profile, measured by MCPA, was 0.5 mm in diameter (at FWHM).
3. Reemitted positron yields and the positron teemission microscope Fig. 3. A photograph of the proto-type transmission-type reemlssion positron microscope.
A single crystal of W(100) with a thickness of 2000 A was set at the position where the first
L Kanazawaet al./AppliedSurfaceScience85 (1995)124-131
127
be observed. The positron implantation profile P (Z, E) is calculated as follows [9,10]:
P(Z, E) = - ( d / d z )
where Z = ZoF[1 + ( l / m ) ] = ( a / p ) (E)", Z is the mean range of positrons at an implantation energy E (3.5-5 keV), p is the density of the foil and a, m, and n are the fitting parameters. It is estimated that the implanted positrons do not directly reach the opposite surface of the foil which has a thickness of 2000 A. The positrons implanted into one side of the foil thermalize and diffuse to the other side of the foil, and then reemit with an energy determined by the negative work function of the positrons. The fraction of transmitted positrons is given by
Fig. 4. The image of the W mesh formed by the reemitted positrons. The magnification is 10 X. It took 3 s to obtain this image.
r2o00
J=Jo remoderator was located. Positrons extracted by the 'magnetic field splitter' were focused onto the W foil. The diameter of the beam on the W foil was about 3 mm (at FWHM). A microchannel plate assembly (MCP) was set behind the W(100) foil so that the transmission-type reemitted positrons could
.
P ( Z , E ) exp[-(2OOO-Z)/L] dz,
where L is the diffusion length in W. The values of J for an accelerating voltage of 4 kV are 0.05 and o 0.16 for L = 600 and 1000 A, respectively. Figs. l a - c show the reemitted positron beam profiles versus the positron incident energy (3.5-5 keV) for the
field-free
electro-static lens
MCPA
exp[-(Z/Z0)m],
magnetic field
accelerator
tO Li noc
O~QOOOOQO
III
7
0766Q
sampleI 1]
o 6 QQ
magnetic lens slow
deflector
modified section
l SxlO z e+/s lOmmtb I Fig. 5. Horizontalview of the RHEPD system.
128
L Kanazawa et aL/Applied Surface Science 85 (1995) 124-131
W(100) foil with a thickness of ~ 2000 ,~. Fig. ld shows the reemitted positron beam profile in the deflection by the weak magnetic field for a positron incident energy of 5 keV. It is seen that the intensities of the beams become stronger as the incident energy increases. Using a retarding grid it was confirmed that the particles giving rise to these profiles have a positive charge and a low kinetic energy (below ~ 10 eV). Furthermore, by comparing the beam profile in Fig. lc with that in Fig. ld, a shift of the beam position by the weak magnetic field is observed. This means that these particles are not heavy positive particles such as protons. Now, we have performed a preliminary positron reemission microscopy experiment. The assembly of the lens electrodes and a photograph of the positron transmission-type reemission microscope are shown in Figs. 2 and 3, respectively. The lenses are of the symmetrical three-electrode type and have symmetry with respect to the midplane of the central electrode. The electrode aperture diameter is 2 mm, the interelectrode spacing is 2 mm and the thickness of the electrode is 1 mm. The end electrodes were held at ground potential. The distance between the W foil and the objective lens was 2 mm and the distance between the lens electrodes and the MCP was 60 mm. The MCP had a phosphor screen anode which generated a spot of light when hit by incident particles. The spot was detected by a TV camera and recorded by an image analysis system. The foil overlying the W mesh was mounted on the stainless steel holder having a 7 mm diameter opening and the holder was installed on the sample manipulator. The mesh period was 250 ixm. A voltage was applied to the W foil so that the accelerating voltage between the foil and the objective lens was 2 kV. The image of the mesh placed behind the W foil was formed by adjusting the voltage applied to the central electrode. The image displays the shadows of the mesh as shown in Fig. 4. The magnification was determined from the distance between the wires in the W mesh and those of the formed image on the detector. Although the magnification at present is 10, it takes only 3 s to obtain this image. In order to get the magnifications which are obtained by using a radioactive source [11,12], we must
RHEPD
on
H-terminated
0°
to
(112)
30 °
to
(112)
58 °
to
(112)
Si
direction
direction
Fig. 6. RHEPD patterns of H-terminated S i ( l l l ) for a variety ol incident beam directions.
I. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
RHEPD
129
on H-terminated Si
~pw glancing angle
II
!
high glancing angle Fig. 7. RHEPD patterns of H-terminated Si(111) for a variety of incident beam glancing angles. The beam was incident at 30° to [112].
130
I. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
eliminate the relatively high background and instability in the beam profile, which are characteristic of an electron LINAC, and we must improve the lense system of the microscope.
4. Reflection high-energy positron diffraction (RHEPD) It is planned to use the intense high-brightness positron beam for RHEPD (reflection high-energy positron diffraction). In addition to the requirements for high intensity and small beam size, the beam must be highly parallel (long focal length). The assembly for RHEPD has been constructed using a series of electrostatic lenses and a magnetic lens, as shown in Fig. 5. In this study, we have measured RHEPD patterns of a H-terminated S i ( l l l ) crystal. The energy of the beam was 6 keV. Fig. 6 shows changes in the RHEPD patterns of H-terminated S i ( l l l ) with the incident beam direction. A shadow edge can be clearly seen. Also, a specular beam spot was observed in the incident beam direction, 0 ° and 58 ° to [11,2]. This beam spot is derived from the intersection of the Ewald Sphere and the reciprocal lattice section containing the zeroth-order Laue zone, where the direction of the reciprocal rod is along the surface normal. In fact, it is known that the intensity of this peak is relatively strong in the incident beam direction [112] in RHEED. This means that the positron beam is diffracted dominantly by the surface atoms. Although the beam spot is a little bit obscure, it looks like that there exists a beam spot, which is derived from the intersection of the Ewald sphere with the 01 (01) rods, in the incident beam direction, 0° and 58 ° to [112]. Also, there appears to be a circle which surrounds the specular beam spot. It is thought that this circle corresponds to a zeroth-order Laue zone. Fig. 7 shows the changes in the RHEPD patterns of H-terminated S i ( l l l ) with the incident beam glancing angle. The beam was incident at 30 ° to [11,2]. It is very interesting to note that there exists a dark region in the RHEPD pattern. The distance between the direct beam spot and the dark spot does not depend on the incident beam
glancing angle. Furthermore, this dark region could not be observed in the RHEPD pattern of a clean S i ( l l l ) surface. Thus, we infer that diffracted positrons of a given momentum have been used in the desorption of hydrogen from the S i ( l l l ) surface. A plausible explanation for this interesting observation is that the diffracted positrons of a particular momentum are lost, giving rise to the dark region, probably by giving the momentum to the desorption of hydrogen atoms from the S i ( l l l ) surface. It has often been observed by us and others [13] that slow positrons somehow induce emission of positive charge from surfaces. We propose that the dark region in the RHEPD pattern is strongly related to such efficient desorption, and this new aspect of RHEPD offers an important distinction between this method and LEPD (low-energy positron diffraction) for which transfer of such a large momentum is impossible.
Acknowledgements This work has been carried out as one of the subjects of the Universities-JAERI Collaborative Project Research Program. It is also supported financially in part by a Grant-in-Aid of the Ministry of Education, Science and Culture.
References [1] J.W. Humberston and M.R.C. McDowell, Eds., Positron Scattering in Gases (Plenum, New York, 1984). [2] Y. Itoh, K.H. Lee, T. Nakajou, A. Goto, N. Nakanishi, M. Kase, I. Kanazawa, Y. Yamamoto, N. Ohshima and Y. Ito, to be published. [3] R.H. Howell, R.A. Alvarez and M. Stanek, Appl. Phys. Lett. 40 (1982) 751. [4] G. Graft, R. Ley, A. Osipowicz and G. Werth, Appl. Phys. A 33 (1984) 59. [5] O. Sueoka, Y. Ito, T. Azuma, S. Mori, K. Katsumura, H. Kobayashi and Y. Tabata, Jpn. J. Appl. Phys. 24 (1985) 222. [6] L.D. Hullett, Jr., T.A. Lewis and D.L. Donohue, in: Proc. 8th Int. Conf. Positron Annihilation, Ghent, Belgium 1988, Eds. L. Dorikens-Vanpraet, M. Dorikens and D. Seger (1989) p. 586. [7] T. Akahane, T. Chiba, N. Shiotani, S. Tanigawa, T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki and T. Tomimasu, Appl. Phys. A 51 (1990) 146.
L Kanazawa et al./Applied Surface Science 85 (1995) 124-131 [8] Y. Ito, M. Hirose, S. Takamura, O. Sueoka, I. Kanazawa, K. Mashiko, A. Ichimiya, Y. Murata, S. Okada, M. Hasegawa and T. Hyodo, Nucl. Instrum. Methods A 305 (1991) 269. [9] S. Valkealahti and R.M. Nieminen, Appl. Phys. A 35 (1984) 51. [10] A. Vehanen, K. Saarinen, P. Hautojarvi and H. Huomo, Phys. Rev. B 35 (1987) 4606.
131
[11] J.V. House and A. Rich, Phys. Rev. Lett. 61 (1988) 488. [12] G.R. Brandes, K.F. Canter and A.P. Mills, Jr., Phys. Rev. Lett. 61 (1988) 492. [13] L.D. Hulett and F. Jacobsen, private communications.
science
Applied Surface Science 85 (1995) 124-131
Production of an intense slow positron beam by using an electron LINAC and its applications I. Kanazawa ,,a y. Ito b M. Hirose b H. Abe c, O. Sueoka 0, S. Takamura e, A. Ichimiya f, Y. Murata g, F. Komori g, K. Fukutani g, S. Okada h, T. Hattori i a Department of Physics, Tokyo Gakugei University, Koganei, Tokyo 184, Japan b Research Center for Nuclear Science and Technology, The University of Tokyo, Tokai, Ibaraki 319-11, Japan c Faculty of Engineering, Tokai University, Kitakaname 1117, Hiratsuka, Kanagawa 259-12, Japan d Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan e Physics Department, Tokai Establishment, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan f Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan g Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo 106, Japan h Takasaki Radiation Chemistry Establishment, Japan Atomic Energy Research Institute, Watanuki, Gunma 370-12, Japan i Faculty of Engineering, Musashi Institute of Technology, Tamazutsumi, 1-28-1 Setagaya, Tokyo 158, Japan
Received 18 May 1994
Abstract
An image of reemitted positrons has been produced in a proto-type positron transmission-type reemission microscope by using the electrostatic intense slow positron beam from an electron LINAC. RHEPD experiments have then been performed on H-terminated Si(ll 1).
1. Introduction
Advancement in the production of intense slow positron beams is required, so that they can provide new analytical methods in physics, chemistry, and materials science. Basically there are two methods with which to obtain intense slow positron beams. One is to prepare highly intense radioactive sources. Radioactive copper is produced in the Brookhaven National Laboratory (BNL) high-flux reactor (1015 n / c m 2. s), which can supply a few hundred Ci of 64Cuper gram of natural copper. Since
* Corresponding author. Fax: +81 423 24 9832.
the half-life of 64Cu is not short (12.8 h), an intense slow positron beam can be obtained for ~ 10 h [1]. Also, highly intense short-lived isotopes (e.g. 11C) can be prepared using ion accelerators. For these short-lived isotopes it should be possible to obtain intense slow positron beams in an on-line configuration [2]. The other method is to use positrons from positron-electron pairs produced using an electron LINAC [3-7]. High-energy electrons are stopped in high-Z materials like Ta and produce a bremsstrahlung photon shower. Positron-electron pairs are created from this photon shower. In this study, we used an electron LINAC for production of the intense slow positron beam [8].
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD1 0 1 6 9 - 4 3 3 2 ( 9 4 ) 0 0 3 2 0 - 3
1. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
The beam was transported through a solenoid magnetic field and then transferred to a field-free region by using a 'magnetic field splitter'. In the field-free region, the beam was brightness-enhanced for diffraction and microscope experiments by using electrostatic lenses. The positron reemission yield gives unique information concerning subsurface defects, because thermalized positrons are trapped at those defects. In addition, positrons have negative work functions in many substances so that they are reflected more efficiently at the outermost layers and can give more selective information on surface structures than can electron diffraction. This is the reason why reflection high-energy positron diffraction (RHEPD) is useful for surface science.
In the present study, we have performed a transmission-type reemission positron yield measurement where the image of reemitted positrons is produced in a proto-type positron reemission microscope, using an intense slow positron beam produced by an electron LINAC. RHEPD experiments have then been performed using this intense slow positron beam.
2. Beam line configuration and brightness enhancement Positrons were produced by bombarding a watercooled convertor with the electron beam from a LINAC, Typical electron beam parameters were 100 MeV energy, 1 ixs pulse width, 50 pps, and 10 txA
L
(i3
125
r
ri
Fig. 1. The reemiUedpositron beam profilesfor positron incident energies of (a) 3.5 keY, (b) 4.0 keV, and (c) 5.0 keV. (d) The reemitted positron beam profile for a positron incident energy of 5.0 keV in the deflection by the weak magneticfield.
126
L Kanazawa et aL/AppliedSurface Science 85 (1995) 124-131
p,•'.••'•`.••`'•••,•`•.•••`•'•`•.•••.`•`
phosphor
\ MCP
••••`••`•
lens / \ mesh
~f0il
Fig. 2. Assembly of the lens configuration and detector for the proto-type transmission-type reemission positron microscope.
average current. The converter, which is cooled by water, was made of several Ta plates (12 mm overall thickness) with the last Ta plate constituting the wall of the vacuum chamber. Tungsten sheets with a thickness of ~ 25 Ixm were used a s the positron moderator. After annealing at ~ 2000°C under ultra-highvacuum conditions, the moderator was placed behind the last Ta plate either in the Venetian-blind or in the transmission-type geometry. The slow positrons were transported into the experimental room through solenoid tubes, 5 m long with two 90 ° bends. The magnetic field (60-80 G) was supplied from solenoid coils directly wound around the vacuum transport tube, and by Helmholtz coils set at several positions
along the tube. Slow positrons were extracted from the moderator by the tungsten mesh held at a potential of 30-60 eV, and were guided along the magnetic field. The intensity of the slow positron beam was determined before the stage of brightness enhancement either by using a Faraday cup or by stopping all the positrons and measuring the annihilation gamma-rays with an ionization chamber gauge. After the electrons had been carefully eliminated, the output of the Faraday cup was 5 pA or ~ 3 X 107 e + / s (at 1 / 1 0 of full power operation of the LINAC, i.e., 1 kW of the electron beam power). From the ionization chamber reading, the intensity of the slow positrons was calculated to be ~ 2 X 107 e+/s, in rough agreement with the measurement from the Faraday cup. At the end point of the solenoid transport tube, an assembly of an accelerator in the magnetic field and electrostatic lenses in a field-free region was placed, which enabled the beam to be extracted from the magnetic field and focused onto the reemission moderator. Positrons extracted by the 'magnetic field splitter' were focused onto the first remoderator (W 1) with an energy in the range 3-5 keV. The remoderator was a single crystal of W ( l l 0 ) onto which the focused positrons entered at an angle of 30 ° to the normal. The reemitted positrons were again accelerated and focused onto the second remoderator (W2) and the final beam profile, measured by MCPA, was 0.5 mm in diameter (at FWHM).
3. Reemitted positron yields and the positron teemission microscope Fig. 3. A photograph of the proto-type transmission-type reemlssion positron microscope.
A single crystal of W(100) with a thickness of 2000 A was set at the position where the first
L Kanazawaet al./AppliedSurfaceScience85 (1995)124-131
127
be observed. The positron implantation profile P (Z, E) is calculated as follows [9,10]:
P(Z, E) = - ( d / d z )
where Z = ZoF[1 + ( l / m ) ] = ( a / p ) (E)", Z is the mean range of positrons at an implantation energy E (3.5-5 keV), p is the density of the foil and a, m, and n are the fitting parameters. It is estimated that the implanted positrons do not directly reach the opposite surface of the foil which has a thickness of 2000 A. The positrons implanted into one side of the foil thermalize and diffuse to the other side of the foil, and then reemit with an energy determined by the negative work function of the positrons. The fraction of transmitted positrons is given by
Fig. 4. The image of the W mesh formed by the reemitted positrons. The magnification is 10 X. It took 3 s to obtain this image.
r2o00
J=Jo remoderator was located. Positrons extracted by the 'magnetic field splitter' were focused onto the W foil. The diameter of the beam on the W foil was about 3 mm (at FWHM). A microchannel plate assembly (MCP) was set behind the W(100) foil so that the transmission-type reemitted positrons could
.
P ( Z , E ) exp[-(2OOO-Z)/L] dz,
where L is the diffusion length in W. The values of J for an accelerating voltage of 4 kV are 0.05 and o 0.16 for L = 600 and 1000 A, respectively. Figs. l a - c show the reemitted positron beam profiles versus the positron incident energy (3.5-5 keV) for the
field-free
electro-static lens
MCPA
exp[-(Z/Z0)m],
magnetic field
accelerator
tO Li noc
O~QOOOOQO
III
7
0766Q
sampleI 1]
o 6 QQ
magnetic lens slow
deflector
modified section
l SxlO z e+/s lOmmtb I Fig. 5. Horizontalview of the RHEPD system.
128
L Kanazawa et aL/Applied Surface Science 85 (1995) 124-131
W(100) foil with a thickness of ~ 2000 ,~. Fig. ld shows the reemitted positron beam profile in the deflection by the weak magnetic field for a positron incident energy of 5 keV. It is seen that the intensities of the beams become stronger as the incident energy increases. Using a retarding grid it was confirmed that the particles giving rise to these profiles have a positive charge and a low kinetic energy (below ~ 10 eV). Furthermore, by comparing the beam profile in Fig. lc with that in Fig. ld, a shift of the beam position by the weak magnetic field is observed. This means that these particles are not heavy positive particles such as protons. Now, we have performed a preliminary positron reemission microscopy experiment. The assembly of the lens electrodes and a photograph of the positron transmission-type reemission microscope are shown in Figs. 2 and 3, respectively. The lenses are of the symmetrical three-electrode type and have symmetry with respect to the midplane of the central electrode. The electrode aperture diameter is 2 mm, the interelectrode spacing is 2 mm and the thickness of the electrode is 1 mm. The end electrodes were held at ground potential. The distance between the W foil and the objective lens was 2 mm and the distance between the lens electrodes and the MCP was 60 mm. The MCP had a phosphor screen anode which generated a spot of light when hit by incident particles. The spot was detected by a TV camera and recorded by an image analysis system. The foil overlying the W mesh was mounted on the stainless steel holder having a 7 mm diameter opening and the holder was installed on the sample manipulator. The mesh period was 250 ixm. A voltage was applied to the W foil so that the accelerating voltage between the foil and the objective lens was 2 kV. The image of the mesh placed behind the W foil was formed by adjusting the voltage applied to the central electrode. The image displays the shadows of the mesh as shown in Fig. 4. The magnification was determined from the distance between the wires in the W mesh and those of the formed image on the detector. Although the magnification at present is 10, it takes only 3 s to obtain this image. In order to get the magnifications which are obtained by using a radioactive source [11,12], we must
RHEPD
on
H-terminated
0°
to
(112)
30 °
to
(112)
58 °
to
(112)
Si
direction
direction
Fig. 6. RHEPD patterns of H-terminated S i ( l l l ) for a variety ol incident beam directions.
I. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
RHEPD
129
on H-terminated Si
~pw glancing angle
II
!
high glancing angle Fig. 7. RHEPD patterns of H-terminated Si(111) for a variety of incident beam glancing angles. The beam was incident at 30° to [112].
130
I. Kanazawa et al. /Applied Surface Science 85 (1995) 124-131
eliminate the relatively high background and instability in the beam profile, which are characteristic of an electron LINAC, and we must improve the lense system of the microscope.
4. Reflection high-energy positron diffraction (RHEPD) It is planned to use the intense high-brightness positron beam for RHEPD (reflection high-energy positron diffraction). In addition to the requirements for high intensity and small beam size, the beam must be highly parallel (long focal length). The assembly for RHEPD has been constructed using a series of electrostatic lenses and a magnetic lens, as shown in Fig. 5. In this study, we have measured RHEPD patterns of a H-terminated S i ( l l l ) crystal. The energy of the beam was 6 keV. Fig. 6 shows changes in the RHEPD patterns of H-terminated S i ( l l l ) with the incident beam direction. A shadow edge can be clearly seen. Also, a specular beam spot was observed in the incident beam direction, 0 ° and 58 ° to [11,2]. This beam spot is derived from the intersection of the Ewald Sphere and the reciprocal lattice section containing the zeroth-order Laue zone, where the direction of the reciprocal rod is along the surface normal. In fact, it is known that the intensity of this peak is relatively strong in the incident beam direction [112] in RHEED. This means that the positron beam is diffracted dominantly by the surface atoms. Although the beam spot is a little bit obscure, it looks like that there exists a beam spot, which is derived from the intersection of the Ewald sphere with the 01 (01) rods, in the incident beam direction, 0° and 58 ° to [112]. Also, there appears to be a circle which surrounds the specular beam spot. It is thought that this circle corresponds to a zeroth-order Laue zone. Fig. 7 shows the changes in the RHEPD patterns of H-terminated S i ( l l l ) with the incident beam glancing angle. The beam was incident at 30 ° to [11,2]. It is very interesting to note that there exists a dark region in the RHEPD pattern. The distance between the direct beam spot and the dark spot does not depend on the incident beam
glancing angle. Furthermore, this dark region could not be observed in the RHEPD pattern of a clean S i ( l l l ) surface. Thus, we infer that diffracted positrons of a given momentum have been used in the desorption of hydrogen from the S i ( l l l ) surface. A plausible explanation for this interesting observation is that the diffracted positrons of a particular momentum are lost, giving rise to the dark region, probably by giving the momentum to the desorption of hydrogen atoms from the S i ( l l l ) surface. It has often been observed by us and others [13] that slow positrons somehow induce emission of positive charge from surfaces. We propose that the dark region in the RHEPD pattern is strongly related to such efficient desorption, and this new aspect of RHEPD offers an important distinction between this method and LEPD (low-energy positron diffraction) for which transfer of such a large momentum is impossible.
Acknowledgements This work has been carried out as one of the subjects of the Universities-JAERI Collaborative Project Research Program. It is also supported financially in part by a Grant-in-Aid of the Ministry of Education, Science and Culture.
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