e-Journal of Surface Science and Nanotechnology

30 June 2018

Conference - ISSS-8 -e-J. Surf. Sci. Nanotech. Vol. 16 (2018) 313-319

Observation of Low-Energy Positron Diﬀraction Patterns
with a Linac-Based Slow-Positron Beam∗
Ken Wada†
Takasaki Advanced Radiation Research Institute, Quantum Beam Science Research Directorate,
National Institutes for Quantum and Radiological Science and Technology,
1233 Watanuki, Takasaki, Gunma 370-1292, Japan

Tetsuroh Shirasawa
National Metrology Institute of Japan,
National Institute of Advanced Industrial Science and Technology,
Tsukuba, Ibaraki 305-8565, Japan

Izumi Mochizuki
Institute of Materials Structure Science,High Energy Accelerator Research Organization (KEK),
1-1 Oho, Tsukuba 305-0801, Japan

Masanori Fujinami
Department of Applied Chemistry, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan

Masaki Maekawa and Atsuo Kawasuso
Takasaki Advanced Radiation Research Institute, Quantum Beam Science Research Directorate,
National Institutes for Quantum and Radiological Science and Technology,
1233 Watanuki, Takasaki, Gunma 370-1292, Japan

Toshio Takahashi
Department of Physics, Tokyo Gakugei University,
4-1-1 Nukuikita, Koganei, Tokyo 184-8501, Japan

Toshio Hyodo
Institute of Materials Structure Science,High Energy Accelerator Research Organization (KEK),
1-1 Oho, Tsukuba 305-0801, Japan(Received 9 February 2018; Accepted 5 May 2018; Published 30 June 2018)
An experimental system has been developed for the study of low-energy positron diﬀraction (LEPD) with aslow-positron beam generated by a linear-electron-accelerator (linac). Diﬀraction patterns of a Ge(001)-2×1 surface
structure have been observed with a normal positron incidence. This is the first LEPD observation with a linacbased high-intensity pulsed slow-positron beam. [DOI: 10.1380/ejssnt.2018.313]
Keywords: Low energy positron diﬀraction (LEPD); Low energy electron diﬀraction (LEED); Positron spectroscopy; Germanium

I.

INTRODUCTION

Low-energy electron diﬀraction (LEED) is a powerfulwell-established tool for the structural determination of
crystalline surfaces [1]. However, diﬃculties can arise on
occasion in discerning the structure, especially of highatomic number (high-Z) materials, because of complex
multiple scattering.
Low-energy positron diﬀraction (LEPD), which is the
positron counterpart of LEED, oﬀers a solution to thisproblem and could become a more powerful tool for the
study of surface structure.
The first LEPD observation was carried out with achannel electron multiplier by the Brandeis group [2].
∗ This paper was presented at the 8th International Symposiumon Surface Science, Tsukuba International Congress Center,
Tsukuba, Japan, October 22-26, 2017.
† Corresponding author: wada.ken@qst.go.jp

The Brookhaven group first reported multiple-spot LEPD
pattern with oﬀ-normal incidence [3]. Only the Brandeis group succeeded in observing multiple-spot LEPD
pattern with normal incidence [4]. They developed anelectrostatic slow-positron beam with a radioisotope (RI)
source emitting positrons through β + -decay. The Brandeis group also demonstrated that in LEPD, experimental
intensity profiles were more closely reproduced by the dynamical diﬀraction theory than in LEED [5–10].
Later theoretical works revealed that LEPD combines
the advantages of (1) simple and smooth scattering factor,(2) less multiple scattering, and (3) higher surface sensitivity, so that the measured spectra can be accurately simulated for surface structural analysis. These advantages
are summarized by Tong [11]. Firstly, the scattering factor of positrons more closely resembles that of X-rays thanof electrons. An electron has sharp and complex angularanisotropies in magnitude and phase caused by bound andresonance states of the atomic potential. A positron andc 2018 The Japan Society of Vacuum and Surface Science
ISSN 1348-0391 ⃝313

Wada et al.

Volume 16 (2018)

FIG. 1. Schematic view of the Slow Positron Facility at KEK (as of April 2018). Radiation shields are not shown. A slowpositron beam generated with a ∼50-MeV dedicated linac is guided by a magnetic field to the experimental hall and branched
to stations, namely: the positronium negative ion (Ps– ) at the end of the SPF-B1; the positronium time-of-flight (Ps-TOF) at
SPF-B2; and the total-reflection high-energy positron diﬀraction (TRHEPD) at SPF-A3. A new beam-line branch, SPF-A4,
has been constructed for the LEPD experiment station.

a photon, however, have a smooth and simple scattering factor because of the absence of bound or resonancestates. Secondly, while electrons are forward focused by achain of atoms, thus producing strong multiple-scatteringresonances in solids, positrons are not. The positron’s
wavefield behind an atom is actually decreased, resulting in forward shadowing. This diﬀerence arises from thesign of the phase shifts; a repulsive (positron-atom) potential produces negative phase shifts while an attractive(electron-atom) potential produces positive phase shifts.
Thirdly, the positron’s mean-free path is shorter than thatof an electron’s below 200 eV because there are no excluded final states for the positron in the solid. Therefore,
the positron is extremely sensitive to atomic positions in
the first three or four layers of a solid surface.
Despite these promising properties as an ideal technique
for studying surface structure, no further experimental
LEPD research has followed the pioneering work of theBrandeis group. The main reason for this is the diﬃcultyin obtaining suﬃcient beam intensities to aﬀord reasonable measurement times giving the required data quality
for LEPD. It has long been hoped that the groundbreaking work of the Brandeis group over 20 years ago would
be continued [12].
An attempt to develop a LEPD experiment systemwith a slow-positron beam generated by a normalconducting linear-electron-accelerator (linac) was once
made at Lawrence Livermore National Laboratory [13].
They intended to take advantage of a high-intensity slowpositron beam generated by a linac, where positrons wereobtained through positron-electron pair creation by bombarding high-energy electrons into a heavy metal. However, no report was published of an observation of a LEPD
314

pattern with this linac-based system.
In the present study, we have developed a new LEPD
system with a slow-positron beam generated by a linac
at the High Energy Accelerator Research Organization(KEK), and succeeded in obtaining LEPD patterns of aGe(001)-2×1 structure with normal incidence.

II.

LEPD SYSTEM AT THE SLOW POSITRONFACILITY, KEKA.

Linac-based slow-positron beam

The Slow Positron Facility (SPF) at the Institute ofMaterials Structure Science (IMSS) in KEK provides ahigh-intensity pulsed slow-positron beam generated using a dedicated normal-conducting linac operated at∼50 MeV, < 0.6 kW with a repetition frequency of 50Hz
[18, 19]. Accelerated electrons are impacted on a tantalum (Ta) plate and deflected in the electric field aroundthe Ta atom, emitting Bremsstrahlung X-rays. Radiation with energy greater than 2mc2 = 2 × 511 keV causes
positron-electron pair creation in the same metal. Theenergy range of the generated positrons is large, up toalmost 50 MeV. The Ta plate used for this process iscalled the electron-positron converter, or simply the converter. A moderator, which is composed of tungsten (W)films with thickness 25 µm, is then used to obtain monoenergetic positrons (slow positrons). When high-energy
positrons penetrate a metal, they are thermalized in thebulk and most annihilate with electrons. However, in metals with a negative positron work function such as W, Ni,
J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

e-Journal of Surface Science and Nanotechnology

Gate
valve

Volume 16 (2018)

Chamber for atomic
hydrogen cleaning
125 mmMagnetic coils

Magnetic lens

Gate
valve

+
SampleholderRemoderator
Electrostatic lenses

DLD
MCP

Gap
EL1

Deflectors

EL2

SampleholderMu-metal
Ni Remoderator
W PlateMu-metal

FIG. 2. (a) The present LEPD experimental station. (b) Detailed figure of the area surrounded by the dashed line in (a)showing the electrostatic potential with a color gradient, and simulated beam trajectories.

and Cu, a proportion of the positrons are emitted afterthermalization with an energy corresponding to the work
function. This slow-positron generating process occurs
within picosecond time order, so the time structure of theslow-positron beam reflects that of the pulsed electron
beam accelerated by the linac.
The slow-positron production unit, composed of theconverter and the moderator, is kept at 0.1–35 kV with respect to the grounded slow-positron beam line, so that themoderated positrons are accelerated immediately after exiting the unit. The accelerated positrons, with an energyof 0.1–35 keV are transported by a magnetic field generated using a series of coils aligned along the beam-line
duct to the experimental hall and branched to diﬀerent
stations, as shown in Fig. 1. There are three experimental stations, namely: the positronium negative ion (Ps– )at the end of the SPF-B1; the positronium time-of-flight(Ps-TOF) at SPF-B2; and the total-reflection high-energy
J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

positron diﬀraction (TRHEPD) at SPF-A3.
For the LEPD experiment, a new beam-line branch
was constructed, SPF-A4, which was extended from thebranching point of SPF-A3. The vacuum pressure of thebeam-line duct is maintained at ∼1 × 10−7 Pa with ion
pumps after baking.
The diameter of the magnetically transported beamis ∼10 mm.
The beam intensity is approximately+
7
10 slow-e /s with an energy of 5 keV.

B.

Pulse stretcher for a linac-based slow-positron
beam with an energy of 5 keV

There are a number of diﬀerences between an RI-based
LEPD system and a linac-based one. One diﬀerence is thetime structure of the beam. While an RI-based systemprovides a continuous beam owing to the Poisson random
process for positron emission through β + -decay, a slow315

Wada et al.

Volume 16 (2018)

TABLE I. A list of voltages applied to the remoderator (RM), the first hemispherical grid nearest to the sample (Grid #1),
the second grid (Grid #2), the third grid (Grid #3), the front side of the MCP (MCP front), and the back side of the MCP(MCP back) in volts. The remoderator bias is 50 V < VRM < 300 V.

RM
VRM

Grid #1
GND

Grid #2
VRM − 10positron beam generated with a normal-conducting linac
has a pulsed time-structure reflecting that of the linac
beam. For example at the SPF in KEK, the maximum
beam intensity is obtained with a pulse width of 1.2 µs at
a rate of 50 Hz. There are ∼106 slow-e+ in every pulse,which is a positron density per unit time within a pulseof ∼1012 slow-e+ /s. For the LEPD experiment, a twolayer delay-line detector (DLD) with micro-channel plates(MCP) was introduced [16, 17]. This is a counting-based
detection system, which obtains the position of the particles incident on the MCP, and has a limited allowance of∼100 ns for the time between two hits. In this time range,∼105 slow-e+ exist in the primary beam, and those could
cause a multi-hit problem in this set-up. To address thisissue, a pulse stretcher was incorporated.
The pulse stretcher, developed for a linac-based slowpositron beam with an energy of up to 5 keV is a linear storage system for the positrons, made up of cylindrical electrodes and magnetic solenoid coils. Details
will be described elsewhere [20]. The stretcher consistsof a grounded electrode, an entrance electrode, a 6-mlong trapping electrode, an exit electrode, and anothergrounded electrode, all contained within the beam-line
duct. The solenoid coils for the beam transportation
are used to confine the positrons longitudinally along thebeam-line. Positrons with an energy of 5.0 ± 0.05 keV
are trapped between the entrance electrode at 5.75 kV
and the exit electrode at 5.2 kV. To allow the 1.2-µs wide
positron pulse into the trap, the entrance electrode voltage is temporarily lowered to 4.5 kV. Positrons are slowed
down in the gap between the grounded electrode and theentrance electrode, pass through the entrance electrode,and are slowed down further in the gap between the entrance and the trapping electrode held at 4.74 kV. The
positrons travel along the magnetic field down to the exit
electrode where they are reflected back. Before they reachthe entry point, the entrance electrode potential is raisedback to 5.75 kV thus trapping the positrons. By gradually
increasing the trapping electrode voltage, positrons spillover the exit electrode. The pulse width of the spilled
positrons is controlled by adjusting the sweeping speed of
the trapping electrode voltage.
For the present LEPD system, the pulse width wasstretched to ∼200 µs, which is the minimum width limitedby the frequency response of the high-voltage amplifier
used for controlling the ramping-electrode voltage. Thispulse width is suﬃciently large to avoiding the multiplehit problem at the DLD and will give the minimum background signals caused by the dark current in the MCP byfiltering the data with the positron detection timing as is
described in Sec. II D.
316

Grid #3
VRM − 10C.

MCP front
VRM − 330

MCP back
VRM + 2070

Brightness enhancement system
and LEPD optics

Another diﬀerence between the RI-based system andthe linac-based system for LEPD experiments is the beamtransportation methods. The RI-based LEPD system
employed electrostatic lenses along the whole beam-line,
while linac-based systems transport the beam along amagnetic field aligned with the beam-line. Since X/γrays and neutrons generated in a linac operation must
be shielded from and suﬃciently reduced at the point ofexperiment, a slow-positron beam is usually transported
some distance from its production unit, typically morethan 10 m. Such distances are diﬃcult with electrostatictransportation, therefore linac-based slow-positron beams
have been magnetically transported. However, for diﬀraction experiments, positrons must interact with the sample
in a non-magnetic space and have suﬃcient beam quality
in order to acquire meaningful data.
So far, two kinds of positron diﬀraction experimentswith a linac-based system have been realized: transmission positron diﬀraction with a transmission positron
microscope (TPM) [14] and total-reflection high-energy
positron diﬀraction (TRHEPD) [15]. Compared to thehigh-energy beams used in the other systems (30 keV for
TPM and 10 keV for TRHEPD), typical LEPD beam energies are low, from ∼50 eV to ∼300 eV. Since the beam
trajectory of such low-energy positrons can be significantly aﬀected by the magnetic field from the beam-line,
special care must be taken.
To obtain positrons in the non-magnetic space with
suﬃcient beam quality for LEPD, we have constructeda set-up using a transmission-type remoderator [21–25]
of Ni(100) foil with thickness 150 nm, a magnetic lens,and electrostatic lenses shielded by Mu-metal, as shownin Fig. 2. Coils before the magnetic lens produce a typical magnetic field along the beam line of ∼10 mT. The
last coil prior to the magnetic lens generates a field in
the reverse direction, so that the field strength decreases
rapidly downstream. The 5.2-keV positron beam is introduced into the magnetic lens, which focuses it onto the Ni
remoderator, where the beam diameter is ∼3 mm. A Ni
ring of diameter 2 mm is placed on the incident side of theremoderator to eliminate the outer positrons, which may
result in a beam with a large diameter and divergence.
Figure 2(b) shows the non-magnetic region in detail.
Here, the positrons are focused onto the remoderator,
thermalized, and a proportion are re-emitted with an energy of 1 eV, which corresponds to the negative positron
work function of Ni. Approximately 90% of the beam
intensity is lost in this process, however the brightness
J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

e-Journal of Surface Science and Nanotechnology

Volume 16 (2018)is enhanced by ∼1000 times. The brightness, B, of theslow-positron beam is defined as [26]B=I
r2 θ2 E,

where I is the beam intensity, r is the beam radius, θ isthe angular spread of the beam, and E is the beam energy. Liouville’s theorem shows that the denominator of
the brightness expression keeps a constant value in a nondissipative force field. After remoderation, the value ofthe denominator becomes of the order of 1/10000 of that
before the remoderation, with almost the same r, smaller
θ, and significantly reduced E by a dissipative force during
the thermalization. In the present system, the positron
injection energy onto the remoderator is 5.2 keV, and there-emitted positron energy is 1 eV, producing a higher
quality beam albeit at the expense of intensity. The reemitted positrons are extracted from the back of the Ni remoderator and electrostatic lenses transport them to thegrounded sample. The final kinetic energy of the beam
impinging on the sample is determined by the potential
at the remoderator, VRM , and the negative work function
to be (VRM + 1) eV. The re-emitted positrons are accelerated by the electric field between the cathode, on which
the remoderator is mounted, and a grounded extraction
electrode. Then two sets of einzel lenses transport thebeam to the sample. The electrostatic potentials for the
first einzel lens (EL1) and the second (EL2) are set tobe ∼55% and ∼60% with respect to that at the remoderator, respectively. Figure 2(b) shows the electrostatic
potential with a color gradient, and also simulated beam
trajectories. Two sets of 8-pole deflectors [27] are placed
between the two einzel lenses to adjust the beam trajectory. This electrostatic lens system keeps the full width of
the beam diameter at half maximum intensity (FWHM)
at ∼1.5 mm and the angular divergence < 2◦ at an energyof 50–300 eV on the sample.
The entire LEPD optics for the remoderated beam issurrounded by a Mu-metal cylinder of thickness 1 mm
to give shielding from the magnetic field produced by
the magnetic lens and coils. Mu-metal was also used forthe remoderator mount to cap the Mu-metal cylinder. A
small gap between the Mu-metal cathode and Mu-metal
cylinder keeps the electrostatic potential of the cathodefrom the grounded cylinder.
Prior to introduction into the LEPD system, the Ni
remoderator was annealed in a hydrogen furnace. Then,in a small chamber above the beam line, the surface of
the Ni remoderator was cleaned by exposure to atomic
hydrogen just before use.
The whole LEPD chamber is surrounded by correction
coils to cancel the eﬀect of the Earth’s magnetic field on
the beam trajectory.

D.

Detection system

The detection system is composed of three hemispherical grids, a chevron MCP stack, and a DLD, all with a
J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

FIG. 3. A 2D histogram of pulse heights of the MCP signals and the detection time relative to the linac trigger. A
1D histogram of the pulse height distribution (left) and thatof detected time relative to the linac trigger (bottom) are also
shown. The data used for analysis is shown by the solid rectangular in the 2D histogram.

central hole. The development of a LEED system witha DLD from RoentDek Handels GmbH is reported elsewhere in detail [28]. In the present study, a DLD-LEED
system from OCI Vacuum Microengineering Inc. is used
after removing the electron gun and placing in the LEPD
optics as described above.
The voltages applied to the remoderator (RM), the firsthemispherical grid nearest to the sample (Grid #1), thesecond grid (Grid #2), the third grid (Grid #3), the frontof the MCP (MCP front), and the back of the MCP (MCPback) are listed in Table I.
The remoderator bias, VRM , is set to a value of 50–
300 V. The sample and Grid #1 are grounded, while Grid
#2 and Grid #3 are held at 10 V below the remoderator bias to prevent inelastically scattered positrons fromreaching the MCP. The incident energy of the diﬀracted
positrons to the front of the MCP is (330 + 1) eV for all
VRM , so that the eﬃciency of the avalanche process in the
MCP is almost saturated for all the LEPD energies. The
negative bias voltages at the front of the MCP prevent thedetection of most of the secondary electrons from being
detected.
To improve the signal-to-noise ratio, the backgroundsignal caused by γ-rays from positron-electron pair annihilation must be reduced. A 10-mm thick W base plate isplaced in front of the DLD, as shown in Fig. 2(b), to shield
the detector from the annihilation γ-rays emitted at theremoderator and the electrostatic lenses. The detection
eﬃciency of the MCP for 511-keV γ-ray is of the orderof 0.1%. In addition, the background caused by annihilation γ-rays and the dark current is reduced by filtering
the data with the pulse height of the MCP signal and thedetection time relative to the linac trigger.
A 2D histogram of pulse heights of the MCP signals andthe detection time relative to the linac trigger is plottedin Fig. 3. A 1D histogram of the pulse height distribution317

Wada et al.

Volume 16 (2018)

FIG. 4. Diﬀraction pattern of a Ge(001)-2×1 structure observed with a 144.5-eV positron beam. The data was accumulated over a four hour period.

(PHD) and that of detected time relative to the linac
trigger are also shown. The PHD for the γ-rays and thatfor the dark current has an exponentially decaying shape,
but the PHD for charged particles is Gaussian [29]. The
minimum threshold for the MCP pulse height is set atthe 515th channel, which is shown by a horizontal dashedline in Fig. 3. Since the pulse width of the slow-positron
beam is adjusted to ∼200 µs with a 20-ms repetition, therelevant data is also filtered by setting the time window
as shown by the vertical dashed lines in Fig. 3. The datafiltered with these limits is shown by the solid rectangularin the 2D histogram.

III.

EXPERIMENTThe samples observed were cut from a mirror-polishedflat Ge(001) wafer (non-doped, R ∼ 2 kΩ at room temperature). A clean Ge(001)-2×1 surface was prepared by
sputtering with 800-eV Ar+ ions at 670 K for 10 min andannealing at the same temperature for 15 min. For the
LEPD observations, the sample temperature was lowered
to 120 K using a refrigerator in order to obtain brighter
spots by reducing ineleastic positron–phonon scattering,i.e. the Debye-Waller eﬀect. The base pressure of the
LEPD chamber was 2 × 10−8 Pa at room temperature.
LEPD observation run-times were several hours inlength. To check the signal/background count rate, a further background observation run was conducted with the
Grid #2 and #3 voltages at (VRM + 2) V, so that all
positrons were repelled.

IV.

RESULTS AND DISCUSSION

Figure 4 displays an example of a diﬀraction patternaccumulated over a five hour period with a 144.5-eV318

positron beam. Integer and fractional order spots fromthe 2 × 1 structure are clearly recognized.
There is a “+” shaped dead zone in the diﬀraction pattern obtained. The design of an DLD anode with a central
hole requires a gap in the wiring of the delay-line layers[28]. A LEPD I-V curve analysis, for example, is possible
with this dead zone because the intensity of chosen spots
are observed for the analysis.
Due to the detector being flat (consisting of the flat
MCP and the DLD), the appearance of the spots shownin Fig. 4 is elongated along the radius direction comparedto those from a conventional LEED system with a hemispherical screen. Some spots located near the edges of
the “+” shaped dead zone are distorted because of a nonlinearity near the edge of the gap on the DLD [28].
With the cut-oﬀ conditions of the PHD and the timingfrom the linac trigger, the number of γ-rays detected isreduced by ∼60%, while that of positrons is reduced by∼15%. The ratio of the count rate for the γ-rays to thatfor the positrons in the whole area of the detector with andwithout the cut-oﬀ conditions is ∼10 and ∼5, respectively.
In addition to the ordinary I-V curve analysis, possible applications with the present LEPD system is surface
holography [30] and the analysis with the surface Patterson function by inversion of LEPD I-V Spectra [31]. For
surface holography, adatoms and dimers on the surface ofthe sample substrate are used as a beam splitter for object and reference waves for holographic reconstruction. A
calculation shows that LEPD is better suited than LEED
for the holographic artifact-free reconstruction [30].

V.

CONCLUSION

An experimental system has been developed to obtain low-energy positron diﬀraction patterns using a normal conducting linac-based high-intensity slow-positronbeam. Pulsed magnetically guided positrons, have been
remoderated in an electrostatic system to produce a highintensity brightness-enhanced beam. Diﬀraction patterns
were obtained, showing clearly Ge(001)-2×1 structurewith integer and fractional order spots. This is the firstobservation of diﬀraction patterns using this method,
demonstrating the feasibility of developing this technique
as a tool for surface analysis.

ACKNOWLEDGMENTS

This work was supported by MEXT/JSPS KAKENHI
Grant Numbers JP24221007 and JP16K13692, Toray Science and Technology Grant, the Cross Ministerial Strategic Innovation Promotion Program (SIP, unit D66). Experiments were conducted under the approval of the PF
PAC (Proposal Nos. 2014G636, 2015S2-002, 2016S2-001,
2016S2-006) and under the auspices of the QST- KEK
Joint Development Research.

J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

e-Journal of Surface Science and Nanotechnology[1] M. A. Van Hove, H. W. William, and C. M. Chan, LowEnergy Electron Diﬀraction: Experiment, Theory andSurface Structure Determination (Springer, Berlin, Heidelberg, 1986).
[2] I. J. Rosenberg, A. H. Weiss, and K. F. Canter, Phys.
Rev. Lett. 44, 1139 (1980).
[3] W. E. Frieze, D. W. Gidley, and K. G. Lynn, Phys. Rev.
B 31, 5628 (1985).
[4] T. N. Horsky, Semiconductor Surface Structure Determination via Low Energy Positron Diﬀraction: Cleavage
Faces of Cadmium-Selenide, Ph. D. thesis, Brandeis University, 1989.
[5] T. N. Horsky, G. R. Brandes, K. F. Canter, C. B. Duke,
S. F. Horng, A. Kahn, D. L. Lessor, A. P. Mills, Jr., A. Paton, K. Stevens, and K. Stiles, Phys. Rev. Lett. 62, 1876
(1989).
[6] C. B. Duke, D. E. Lessor, T. N. Horsky, G. Brandes,
K. F. Canter, P. H. Lippel, A. P. Mills Jr., A. Paton,and Y. R. Wang, J. Vac. Sci. Technol. A 7, 2030 (1989).
[7] T. N. Horsky, G. R. Brandes, K. F. Canter, C. B. Duke,
A. Paton, D. L. Lessor, A. Kahn, S. F. Horng, K. Stevens,
K. Stiles, and A. P. Mills Jr., Phys. Rev. B 46, 7011
(1992).
[8] D. L. Lessor, C. B. Duke, X. M. Chen, G. R. Brandes,
K. F. Canter, and W. K. Ford, J. Vac. Sci. Technol. A
10, 2585 (1992).
[9] X. M. Chen, K. F. Canter, C. B. Duke, A. Paton,
D. L. Lessor, and W. K. Ford, Phys. Rev. B 48, 2400
(1993).
[10] C. B. Duke, A. Paton, A. Lazarides, D. Vasumathi, andK. F. Canter, Phys. Rev. B 55, 7181 (1997).
[11] S. Y. Tong, Surf. Sci. 457, L432 (2000).
[12] P. G. Coleman, Appl. Surf. Sci. 194, 264 (2002).
[13] A. Hamza, P. Asoka-Kumar, W. Stoeﬄ, R. Howell,
D. Miller, and A. Denison, Radiat. Phys. Chem. 68, 635(2003).
[14] M. Matsuya, S. Jinno, T. Ootsuka, M. Inoue, M. Doyama,
M. Inoue, and M. Fujinami, Nucl. Instrum. Methods Phys.
Res. A 645, 102 (2011).

J-Stage: https://www.jstage.jst.go.jp/browse/ejssnt/

Volume 16 (2018)[15] M. Maekawa, K. Wada, Y. Fukaya, A. Kawasuso,
I. Mochizuki, T. Shidara, and T. Hyodo, Eur. Phys. J.
D 68, 165 (2014).
[16] S. E. Sobottka and M. B. Williams, IEEE Trans. Nucl.
Sci. 35, 348 (1988).
[17] O. Jagutzki, V. Mergel, K. Ullman-Pfleger, L. Spielberger,
U. Spillmann, R. Dorner, and H. Schmidt-Böcker, Nucl.
Instrum. Methods Phys. Res. A 477, 244 (2002).
[18] K. Wada, T. Hyodo, A. Yagishita, M. Ikeda, S. Ohsawa,
T. Shidara, K. Michishio, T. Tachibana, Y. Nagashima,
Y. Fukaya, M. Maekawa, and A. Kawasuso, Eur. Phys. J.
D 66, 37 (2012).
[19] K. Wada, T. Hyodo, T. Kosuge, Y. Saito, M. Ikeda, S. Ohsawa, T. Shidara, K. Michishio, T. Tachibana, H. Terabe,
Y. Nagashima, Y. Fukaya, M. Maekawa, I. Mochizuki, and
A. Kawasuso, J. Phys.: Conf. Ser. 443, 012082 (2013).
[20] K. Wada, M. Masaki, I. Mochizuki, A. Kawasuso,
M. Kimura, and T. Hyodo, in preparation.
[21] A. P. Mills Jr. and R. J. Wilson, Phys. Rev. A 26, 490(1982).
[22] D. M. Chen, K. G. Lynn, R. Pareja, and B. Nielsen, Phys.
Rev. B 31, 4123 (1985).
[23] N. Zafar, J. Chevallier, G. Laricchia, and M. Charlton,
J. Phys. D: Appl. Phys. 22, 868 (1989).
[24] N. Oshima, R. Suzuki, T. Ohdaira, A. Kinomura,
T. Narumi, A. Uedono, and M. Fujinami, J. Appl. Phys.
103, 094916 (2008).
[25] M. Fujinami, S. Jinno, M. Fukuzumi, T. Kawaguchi,
K. Oguma, and T. Akahane, Anal. Sci. 24, 73 (2008).
[26] A. P. Mills, Jr., Appl. Phys. 23, 189 (1980).
[27] R. G. E. Hutter, in: Advances in Image Pickup and Display, edited by B. Kazan (Academic Press, New York,1974) p. 163.
[28] D. Human, H. F. Hu, C. J. Hirschmugl, J. Ociepa, G. Hall,
O. Jagutzki, and K. Ullmann-Pfleger, Rev. Sci. Instrum.
77, 023302 (2006).
[29] J. L. Wiza, Nucl. Instrum. Methods 162, 587 (1979).
[30] S. Y. Tong, H. Huang, and X. Q. Guo, Phys. Rev. Lett.
69, 3654 (1992).
[31] H. Wu and S. Y. Tong, Phys. Rev. Lett. 87, 036101(2001).

319