The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

Generation of an Intense Pulsed Positron Beam and Its Applications
Ryoichi Suzuki, Yoshinori Kobayashi*, Tomohisa Mikado, Hideaki Ohgaki,Mitsukuni Chiwaki and Tetsuo Yamazaki' Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan*National Chemical Laboratory for Industry, 1-1 Higashi,Tsukuba, Ibaraki 305, Japan

Abstract

chopper

A positron pulsing system for an intense positron
beam generated by an electron Jinac has been developed atthe Electrotechnical Laboratory. The pulsing system generates an intense pulsed positron beam of variable energy
and variable pulse period. The pulsed positron beam isused as a non destructive probe for various materials researches. In this paper, we report the present status of thepulsed positron beam and its applications.

c--\
5 ns

150 psI. INTRODUCTION

1 nsFigure. 1. The positron pulsing system.

Positrons have been established to be a very sensitive
probe to study microstructural defects, electronic structure in solids, and surface or near surface phenomena.(!]In 1987, a facility to produce an intense slow positron
beam was constructed at the Electrotechnical Laboratory(ETL) [2}. We have developed a pulsing system for the intense positron beam, which enables us to perform variableenergy positron lifetime spectroscopy (VEPLS) and timeof-flight(TOF) experiments with high peak-to-backgr01md
ratio, high count rate, and wide measurable time range (3].
In this paper, we report the present status of thepositron pulsing system and some application examples ofthe pulsed positron beam.

II. POSITRON PULSING SYSTEM
An intense positron beam of ..... 107 e+ /sis producedwith a high energy ( ..... 70 MeV) electron beam of the ETL
Jinac. The positron beam is initially a pulsed beam corresponding to the linac pulse. However, the repetition rateof the Jinac is too low (50 pps - 100 pps) and the pulse
width is too wide (1 ItS - 4 ps) for high count-rate positron
experiments. Thus, the positron beam is stored temporarily in a linear storage section (4], and the pulse-stretchedbeam is used for the pulsing system. The pulsing system
consists of a chopper, a sub-harmonic pre-buncher (SHPB)
and a double harmonic buncher as shown in Fig.l (5]. Anaxial magnetic field of .....0.01 T is applied to the pulsing
system by Helmholtz coils for beam guiding. The chopper

consists of three grids, and utilizes a pulsed electric fieldlongitudinal to the beam direction. This device generates
a pulsed beam ("'"'5 ns, ..... 250 eV) of variable pulse period
from the pulse-stretched beam. The SHPB compresses thepulse width of the chopped beam to ..... 1 ns (FWHM). Thisbeam can be used for time-of-flight experiments. For thepositron lifetime spectroscopy, the positron pulse is further compressed to .....150 ps at the sample position by thebuncher. The fundamental frequency of the buncher, fa, is150 MHz; the operating frequency of the SHPB is a quarter of the buncher frequency, and the chopper frequency is/o/(4·n), (n=l, 2, ... ). There are accelerating rings and adrift tube between the buncher and the sample. By adjusting the applied voltages to the drift tube and the accelerating rings, the incident positron energy at the sample canbe varied from 0.5 keV to 30 keV. As a result, the positronpulsing system generates a pulsed beam of high intensity,variable energy, and variable pulse period.

III. APPLICATIONS OF THE PULSEDPOSITRON BEAM

A. Positron Lifetime Measurement
A positron lifetime spectrum is obtained by measuring the time interval between the timing signal of the pulsing system and timing signal of an annihilation 7-ray de--237-

tected with a BaF 2 scintillation detector. At present, theconnt rate of annihilation 1-rays is 100-500 cps for thepositron lifetime experiments. By using this system, wecan measure both long lifetime and short lifetime components in near surface regions, surfaces, interfaces, andthin films. Several experimental results using the pulsed
positron beam have proved the usefulness of variableenergy positron lifetime spectroscopy [6-9]. We observed
long-lived o- Ps components of high intensity in many specimens, for example, a-Si:H [6], diamond films [8], Si0 2 films[7], porous Si (Fig.2), etc. These long-lived components arestrongly influenced by the deposition condition, annealing
temperature, and other conditions .

...

Figure. 3. Block diagram of the AMOCS system.

c81o· e---1\\----t---i1C. Positron Annihilation Induced Auger Electron
Spectroscopy

040
20time (ns)60

Figure. 2. Positron lifetime spectra of porous Si
at the sample temperatures of 300 ° C, 400
oc, and 600 °C.

B. Age-Momemtum Correlation Spectroscopy
Age-momentum correlation spectroscopy (AMOCS) is
known to provide information which is unavailable fromindividual measurement of age (positron lifetime spectroscopy) and/or that of momentum (measurement ofDoppler broadening or angular correlation of annihilationradiation)[10]. The conventional AMOCS system requiresa triple coincidence technique for the start f3+ or 1 ray
signal and two annihilation 1-ray signals. Thus, the maximum connt rate is limited to about 100 cps. On the contrary, an AMOCS system with a pulsed positron beamrequires only a double coincidence technique, because thestart signal can be obtained electronically from the pulsing system. Thus, it is possible to perform AMOCS withhigh connt rate. We have developed a AMOCS system forthe variable-energy pulsed positron beam [9]. The connt
rate of -10 cps was achieved with the pulsed beam. If ahigher-intensity positron beam is available, the connt rate
can be easily increased.

Positron annihilation-induced Auger electron spectroscopy (PAES) is known to have significant advantages
over conventional methods of Auger electron spectroscopy[11]. In PAES, core electrons are removed by annihilation with positrons. Therefore, the incident beam energy
in PAES need not be larger than the core level ionizationenergy, permitting the elimination of the large secondary
electron backgronnd. Furthermore, the PAES signal originates almost exclusively from the topmost atomic layer
since the positrons are trapped in an image correlation well
just outside the surface. Several works using a position sensitive or single slit energy analyzer have demonstrated thefeasibility ofPAES [11-13]. However, the energy resolutionof these systems is lower than conventional Auger electron
spectroscopy since the resolution is limited by the energy
analyzer.
Recently, we have developed a PAES system whichmeasures the energy of electrons by means of a TOF technique with the pulsed positron beam [9]. Figure 4 shows
an example of PAES spectra from highly oriented pyrolyticgraphite (HOPG). A significant peak at aronnd 250 eV,corresponding to the Auger peak of carbon was observedin the spectrum of Figure 4. This measurement is preliminary, nevertheless, the resolution of the carbon Auger
peak is significantly higher than that of previously reported
Auger peak of the PAES experiment [13].

D. Positronium Time-of-Flight Measurement
Several experimental and theoretical results haveshown that energy analysis of positronium emitted fromsolid surfaces gives valuable information on surface properties [14-15]. We have developed a Ps-TOF measurement
system by using the pulsed positron beam [9).
Figure 5 shows the experimental set-up of the Ps--238-

caxry out variable-energy positron lifetime spectroscopy,
age-momentmn correlation spectroscopy, TOF-PAES, andPs-TOF measurements.

energy (eV)
1~0 fto_]JO

10~0 5~0 4Jl0 ~0 ~030-(--V. ACKNOWLEDGEMENTS~--The authors would like to thank T. Noguchi, S.
Sugiyama, K. Yamada, N.Sei for helpful assistance.

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VI. REFERENCES............. ............... !, ·•• ···\·~t.......
I0 .....

- - ---

50'100[1] P. J. Schultz and K. G. Lynn, Rev. Mod. Phys. 60 (1988)
701.

150time (ns)[2] T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki, N.
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Joint Symposium on Accelerators for Nuclear Science andTheir Applications (Riken, Wako, 1987) p. 163.

Figure. 4. TOF-PAES spectrmn of HOPG.

[3] R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamasaki and T. Tomimasu, Jpn. J. Appl. Phys.
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[7] R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, and T. Tomimasu, Materials ScienceForum, 105-110 (1992) 1459.

Figure. 5. Schematics of the Ps-TOF system.
TOF system. Positronimn atoms axe detected by a microchannel-plate (MCP) detector of single anode type. TOF
spectra axe obtained by measuring the time interval between the signal of MCP detector and that of the pulsing
system.
The present connt rate of both the Ps-TOF system
and TOF-PAES system is insufficient for high statisticmeasurements. One reason for this is that the pulsing efficiency of the present system, which is optimized for lifetime measurements but not for TOF measurement, is low(a few %). Therefore, if we construct an optimized pulsing
system for TOF experiments, the pulsing efficiency will increase by about one order of magnitude.

[8] R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, A. Uedono, S. Tanigawa and H. Funamoto, Jpn. J. Appl. Phys., 31 (1992) 2237.
[9] R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M._ Chiwaki and T. Yamazaki, to be published in Proc. 5-th Int.
Workshop on Slow-Positron Beam Techniques for Solids &Surfaces.
[10] F.H.H. Hsu and C.S. Wu, Phys. Rev. Lett., 18 (1967)
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[13] E. Soininen, A. Schwab, and K.G. Lynn, Phys. Rev. B 43
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IV. CONCLUSIONS,[14] A. Ishii, J.B. Pendry and D.K. Saldin, Positron Annihilation, eds L. Dorikens-Vanpraet, M. Dorikens and D.
Segers, (World Scientific, Singapore, 1989) p. 326.

A slow positron pulsing system and application examples of the pulsed positron beam were _described. By theuse of the pulsed positron beam, it has become possible to[15] R.H. Howell, P. Meyer, I.J. Rosenberg and M.J. Fluss,Phys. Rev. Lett. 54 (1985) 1698.

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