Production and application of pulsed slow positron beam using an electron linac

Radiat. Phys. Chem. Vol. 49, No. 6, pp. 651-659. 1997 Pergamon

P l h S0969-806X(97)00015-7

© 1997ElsevierScienceLtd. All rights reserved Printed in Great Britain o969-8o6x/97 $17.oo+ o.oo

PRODUCTION AND APPLICATION OF PULSED SLOW POSITRON BEAM USING AN ELECTRON LINACt TETSUO Y A M A Z A K I , I RYOICHI SUZUKI, l TOSHIYUKI OHDAIRA, I TOMOHISA M I K A D O ~ and YOSHINORI K O B A Y A S H F ~Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305, Japan and 2National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba-shi, Ibaraki 305, Japan (Received for publication 17 January 1997)

Al~tract--Slow positron beam is quite useful for non-destructive material research. At the Electrotechnical Laboratory (ETL), an intense slow positron beam line has been constructed by exploiting an electron linac in order to carry out various experiments on material analysis. The beam line can generate pulsed positron beams of variable energy and variable pulse period. Various capabilities of the intense pulsed positron beam is presented, based on the experience at the ETL, and the prospect for the future is discussed. © 1997 Elsevier Science Ltd

INTRODUCTION

Positrons are useful for medical diagnostics, material evaluation of microstructural defects, electronic structure in solids, and so on, owing to its peculiar interaction with solids such as trapping to vacancies, annihilation with electrons and successive emission of 7-rays which are easily detected. Slow positron beam is especially advantageous as a non-destructive probe to investigate the property of surface, thin films, and the depth dependence of the properties of solids, because it is quite sensitive to surface properties and to the energy of incidence (Mills, 1983; Schultz and Lynn, 1988; Vehanen, 1989). When a slow positron enters into solid, it rapidly ( ~ 1 0 ps) loses its kinetic energy, diffuses within the sample, and after a certain time, annihilates with an electron. Before the annihilation, positronium (Ps), which is an electron-positron bound state, can be formed in some cases. The positron and Ps decay with characteristic lifetimes. Various works have demonstrated the possibility of slow positron beams as a probe for studies on thin films, surface, and near surface regions (Kajcsos and Szeles, 1992). Recently, generation of positron beams by accelerators has become possible with intensity of 102-103 times the intensity obtained from radioisotopes (RIs). This has made the time for experiments remarkably shorter. Furthermore, various experiments which were inaccessible with RIs have become possible. Experiments with pulsed slow

tThis paper was presented at the 7th International Symposium on Advanced Nuclear Energy Research,

18-20 March, JAERI, Takasaki, Japan.

positron beam described below are practically impossible with RIs. At the Electrotechnical Laboratory (ETL), an intense slow-positron beam line was constructed in order to carry out various experiments on material research (Mikado et al., 1987). The beam line can generate a pulsed positron beam of variable energy and variable pulse period with high intensity by the use of an electron linac. This paper describes the positron beam line including a pulsing system, application of the pulsed positron beam (positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy (PAES) with a time-of-flight technique) and future prospects of the slow-positron beam.

PRODUCTION OF SLOW-POSITRON BEAM

Positron beam line

Electron linear accelerator (linac) is usually used at present to produce intense slow positron beams, though cyclotron is also a powerful candidate. At the ETL, a positron beam line was constructed in 1987 in cooperation with the National Institute for Research in Inorganic Materials, the Institute of Physical and Chemical Research, and the Tsukuba University (Akahane et al., 1990). An electron beam of about 70 MeV extracted from the low-energy section of a 500 MeV electron linac is used to generate positrons, as shown in Fig. 1. The electron beam enters a tantalum converter to generate positrons, and a Venetian-blind-type tungsten moderator moderates the positrons down to below 10 eV. 651

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Tetsuo Yamazaki et al.

Figure 2 shows the cross-sectional view of the converter-moderator system. The slow positron beam is sent to a sample chamber through a vacuum transport line guided by Helmholtz coils. However, the time structure of the beam mimics that of the incident pulsed electron beam with 1-4 Us width and 100 pps, which is inconvenient for most positron experiments. Therefore, the slow-positron beam is stored temporarily in a linear storage section to generate the quasicontinuous beam. The linear storage section consists of a 4 m straight vacuum tube with Helmholtz coils and two grids. The positrons are trapped between the grids by means of electric field and magnetic field, then they are extracted gradually by changing the potential of exit electrode (Akahane et al., 1990). At the end of the beam line, slow positrons of about l0 s s- ~ are available. Pulsing system

For some measurements with slow positrons, such as positron lifetime spectroscopy, time-of-flight (TOF) measurements of positronium or secondary particles, it is necessary to know precisely the time at which a positron entered a sample to be used as a start timing signal. A few techniques to obtain the start signal have been developed (Schultz and Lynn, 1988) and several experimental results have shown the potential use of variable-energy pulsed positron beams for surface and near-surface studies (Sch6dlbauer et al., 1987; K6gel et al., 1988, 1989; Steindl et al., 1992) though the beam intensity was insufficient for high quality measurements. Recently, a pulsing apparatus for the ETL intense positron beam has been developed to perform variable-energy positron lifetime spectroscopy as well as TOF experiments with high resolution, high peak-to-background ratio, high count rate, and wide time range for measurement (Suzuki et al., 1991a). The positron beam is initially pulsed corresponding to the linac pulse structure. However, the repetition rate of the linac is too low and the pulse width is too wide for positron experiments with high count rate. Thus, the quasi-continuous positron beam described above is used for the pulsing system. The pulsing system consists of a chopper, a sub-harmonic pre-buncher (SHPB) and a buncher as shown in Fig. 3. An axial 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 field longitudinal to the beam direction. This device generates a pulsed beam (-,,5 ns, ~250 eV) from the pulse-stretched beam. The main feature of this type of chopper is that the performance is not affected by the strength of the axial magnetic field. Thus, it can be operated at any frequency by applying the appropriate electric pulses. The SHPB compresses the pulse width of the chopped beam to ~ 1 ns (FWHM). This beam can be

used for time-of-flight experiments. For the positron lifetime experiments, the positron pulse is further compressed to ~ 150 ps at the sample position by the buncher. The fundamental frequency of the buncher, fn is 150 MHz; the operating frequency of the SHPB is a quarter of the buncher frequency, and the chopper frequency isfB/4n, (n = 1, 2 .... ). There is a drift tube between the buncher and the sample. By adjusting the voltage applied to the drift tube, the incident positron energy at the sample can be varied from 0.2 to 30 keV. Performance of the system

The above positron pulsing system generates a pulsed beam of high intensity with variable energy and pulse period. For the positron lifetime measurements, we can generate a pulsed positron beam of ~100 ps pulse width, with incident energy of 0.2-30 keV, and pulse period of 25 ns-10 ms. The intense pulsed positron beam enables us to perform positron lifetime measurements with variable-energy pulsed positron beam with high count rate of ~> 103 cps. For the TOF-PAES experiments, the pulsed positron beam with pulse width of ~ 5 ns, incident energy of 35-70 eV, and pulse period of ~ 1 ms is used. The pulsed positron beam makes it possible to measure PAES spectra with high count rate and high energy resolution. APPLICATION OF PULSED SLOW POSITRON BEAM

Positron lifetime measurement

A positron lifetime spectrum is obtained by measuring the time interval between the timing signal of the pulsing system and the signal of an annihilation ~-ray detected with a BaF2 scintillation detector. At present, the count rate of annihilation y-ray is 2 x 103 cps for the positron lifetime spectroscopy. A peak-to-background ratio of 104 has been achieved by adjusting the storage time of the linear storage section and the gate timing for a time-to-amplitude converter. With the system, both the long- and short-lifetime components in near surface regions, surface, interfaces, and thin films can be measured. Several experimental results with the pulsed positron beam have proved the usefulness of variable-energy positron lifetime spectroscopy. Especially, the high peak-to-background ratio and wide time range for measurement enable us to observe long-lived o-Ps components in many specimens, for example, a-Si:H, diamond films, SiO2 films, porous Si, etc. (Suzuki et al., 1991b, 1992, 1995). Figure 4 shows an example of the results of positron lifetime experiments. The specimen is a dielectric multilayer cavity mirror (HfO2/SiO2) for ultraviolet (UV) free-electron-laser (FEL) experiments. For the short-wavelength FEL experiments, ultra-low-loss mirrors are indispensable because the FEL gain is rather low, and thus, the d e g r a d a t i o n of

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the mirrors due to the exposure to the UV and/or VUV radiation is a serious problem. We have devised a method of restoration of the degraded mirrors with oxygen-plasma treatment and annealing, and succeeded in the restoration, which has been confirmed by a precise cavity-loss measurement using a decay-time method.

The incident positron energy of Fig. 4 is 1 keV corresponding to the first SiO2 layer. The lifetime spectra before the FEL experiment, a high intensity of a long-lived positronium component (~25%) was observed. After the FEL experiment, the long-lifetime component has dramatically decreased. Furthermore, after the plasma treatment and

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Time (ns) Fig. 4. Positron lifetime spectra of a dielectric multilayer cavity mirror. annealing, the intensity of the long-lifetime component almost completely restored. The intensity of the long-lifetime component is highly correlated with the degradation and restoration of the efficiency of the mirror. Thus, the mechanism of degradation of the mirrors was confirmed with the series of experiments. Furthermore, this measurement can specify the damaged depth from the positron energy dependence of the intensity of the long-lifetime component; the damaged region is the first SiO2 layer. Age-momentum correlation spectroscopy

Age-momentum correlation spectroscopy (AMOCS) is known to provide information which is

unavailable from individual measurement of age (positron lifetime spectroscopy) and/or that of momentum (measurement of Doppler broadening or angular correlation of annihilation radiation). Several results have proved that AMOCS is especially useful to study the annihilation process of positronium (Hsu and Wu, 1967; MacKenzie and Sen, 1976; Kishimoto and Tanigawa, 1982; Chang et al., 1987). The conventional AMOCS system requires a triple coincidence technique for the start fl + or y-ray signal and two annihilation y-ray signals. Thus, the maximum count rate is limited to about 100 cps. On the contrary, an AMOCS system with a pulsed positron beam requires only a double coincidence technique, because the start signal can be obtained

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electronically from the pulsing system. Thus, it is possible to perform AMOCS with high count rate. Furthermore, the variable-energy pulsed positron beam gives information on the depth. Therefore, an age-momentum correlation measurement apparatus using the variable-energy pulsed positron beam is being constructed. The layout of the system is shown schematically in Fig. 5. The count rate of ,,~30 cps has been achieved so far with the pulsed beam. When a higher-intensity positron beam is available, the count rate will be easily increased.

ionization energy, which permits the elimination of the large secondary-electron background. 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 the feasibility of the PAES (Weiss et al., 1988; Mayer et al., 1990; Lei et al., 1989; Soininen et al., 1991; Weiss, 1992) although the energy resolution of these systems is lower than conventional AES.

Positronium time-of-flight measurement

Several experimental and theoretical results have shown that energy analysis of positronium emitted from solid surface gives valuable information on surface properties (Ishii et al., 1989; Mills and Pfeiffer, 1985; Howell et al., 1985; Sferlazzo et al., 1987). We have developed a Ps-TOF measurement system by using the intense pulsed positron beam. Figure 6 shows the experimental arrangement of the Ps-TOF system. Positronium atoms are detected by a micro-channel-plate (MCP) detector. TOF spectra are obtained by measuring the time interval between the signal of MCP detector and that of the pulsing system.

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Positron annihilation induced Auger electron spectroscopy

Positron annihilation-induced Auger electron spectroscopy (PAES) is known to have significant advantages over conventional methods of Auger electron spectroscopy (AES) (Weiss et al., 1988). In PAES, core electrons are removed by annihilation with positrons. Therefore, the incident beam energy in PAES need not be higher than the core level

I computer Fig. 6. Block diagram of the Ps--TOF system.

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trapped at the top layer with very weak binding energy or very low concentration. Though this measurement is preliminary, the result demonstrates the extremely high surface sensitivity of the PAES. Furthermore, the resolution of the carbon Auger peak ( ~ 50 eV) is significantly higher than that of the Auger peak of the PAES experiment (~200 eV) previously reported (Soininen et al., 1991). CONCLUSION

The intense positron beam line and positron pulsing apparatus at the ETL linac facility have been described. Application of the pulsed positron beam (positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique) were also discussed. The present pulsing efficiency for the positron experiments is not very high (only a few percent). One reason for this is that the energy spread of the positron beam is too large. In order to reduce the energy spread, a new positron beam line which has a new type of linear storage section, as shown in Fig. 9, is being constructed. We expect that the energy modulation of the new system will be reduced to about 9 meV, and the pulsing efficiency will increase by about one order of magnitude with the new beam line. The main feature of the measurements using a pulsed positron beam is that there is almost no limitation on the count rate since we can obtain the start timing signal electronically from the pulsing system. Therefore, when much higher intensity positron beams, such as that of the JAERI positron factory project which is in the stage of design work, become available, very high count rate measurements

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will be possible. These high count rate measurements will make it possible to do not only the experiments on dynamical processes on short time scales but also three dimensional scanning positron spectroscopy (or positron microscopy using very small diameter positron beam) which will give us more detailed information on the microstructure of new materials. The usefulness of the slow positron beam will be greatly expanded if the beam is of high intensity, and processed (shaped) in all of the time domain, energy domain, and spatial domain. Acknowledgements--The authors would like to thank T. N. Shiotani, T. Akahane, T. Chiba, S.

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Tanigawa, T. Hyodo and A. Uedono for discussions and collaboration in some of the experiments.

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