TIME BUNCHING SYSTEM TO MODULATE LOW ENERGY POSITRON BEAMS

R. Suzuki, T. Mikado, H. Ohgaki, :IlL Chiwaki, T. Yamazaki, T. Tomimasu andY. Kobayashi*Electrotechnical Laboratory, 1-1-·1 Umezono, Tsnkuba, Ibaraki 305, Japan*National Chemical Laboratory for Industry, 1-1-·1 Higashi, Tsukuha, Ibaraki 305, JapanABSTRACT
A time bunching system to modulate low energy positronbeams has been designed and is being constructed for highThe bunching systemcount rate lifetime measurements.
consists of a dual mode chopper (continuous-wave and pulsemode) and a single cavity double harmonic buncher. This
paper describes the design procedure of the system.

intense positron beam. The bunching system consists· of two
main pulsing devices, i.e. the chopper and the buncher, andthe electronic system. In this paper, we discuss the design
procedure of the bunching system.

TIME BUNCHING SYSTEM
Chopper

INTRODUCTION
Lifetime measurement with a pulsed low energy positron
beam is known to be a powerful method to study defects
or electronic properties in solids at. the surface or close tothe surface. 1•2 Using this method, a depth dependence ofthe lifetime spectra can be obtained by means of controllingthe mean penetration depth of the positrons with variable
injection energy. Furthermore, very high count rate(> 10 6cps) measurement is possible when an intense pulsed positron
beam exists, because a start signal is derived electronicallyand the coincidence count rate is equal to the single countrate in one detector.
However, in the world, there exists only one system
which can measure lifetime spectra using a pulsed low energy
positron beam with a sufficient time resolution. This system
has been developed by Schodlbauer et a/. 2 •3 The performanceof the system is that the total time resolution is 22.5 psec andthat the count rate is 30- 70 cps with a 15 mCi 22 Na source.
Since one time spectrum requires about 106 counts, this countrate is insufficient to measure precisely the depth dependenceof the lifetime spectra.
The present authors plan to carry out high countrate(>103 cps) lifetime measurements with an intense slow
positron beam in the Electrotechnical Laboratory(ETL). Anintense slow positron beam(>10 8 positrons/sec) is generated
from the electron beam{70 MeV, .50 pps, 4!tsec) of the linac
Thein ETL by a Ta converter and a W moderator.
slow positrons are stored temporarily in a pulse stretcher ora linear-storage section. 4 The positrons can be extracted
from the stretcher as a continuous or pulsed (,,,J 0 nsec, 100 k-5 Mpps) beam. The pulsed beam is used to measurelong lifetime components( T > 1 nsec) such as positronium
components.
We are constructing a bunching system to modulate theA chopper is used to suppress a part of the beamoutside of the desired pulse by deflecting the beam acrossan aperture. Figure 1 shows a section of a chopper. Thischopper has two deflectors. The second deflector is used tocompensate the transversal energy of the beam, which is
given by the firstdeflector. This chopper can be operated inboth continuous wave(CW) mode and pulse mode. In thepulse mode, the pulsed beam extracted from the stretcher is
synchronized with the chopper. Electronic power is supplied
to the chopper from a linear amplifier through an impedance
transformer.

BEAM-====
DEFLECTORS

t-----i

20mm

Figure 1. Cross sectional view of the chopper.

Buncher
Since low energy positrons are treated as non relativistic
particles, the theory of bunching of low energy positrons
Theis similar to that of the heavy ion bunching.
ideal modulation energy bE at the gap of the buncher is given as 5

where Es is the energy of the positron entering the buncher-225-

at the time t, and rn is the time interval during which thepositron travels from the gap to the sample. However, becauseof the practical difficulty in applying the ideal waveform,bunchers generally use single sine-wave 3 or combined wave
with two or more harmonics. 5
We have chosen a single cavity buncher with twoharmonics for the reason of the easier fabrication andoperation. Several considerations have been involved in thejudgment of the injection energy of the buncher and theoperating frequency of the buncher.

4000u

Gl

(a)

Ulc.

lot)

...c110 psec

UlFWHM

2000::s0u

1. In order to achieve total time resolution of the lifetime
spectrum less than 200 psec, the pulse width of the beamat the sample should be less than 100 psec.

2. The energy acceptance of the buncher should belarger than 10 eV(FWHM) because there is an energy
spread due to the longitudinal electric field of the chopper.
3. The chopper and the ')'-ray detector should be separated
each other as long as possible in order to reduce
background ')'-ray signals caused by positrons being
annihilated in the chopper.

0.,-200200

04000(b)uGlUlAfter the series of parameter surveys, taking into account theabove conditions, it has been judged that the injection energyof the buncher is 250 eV and the fundamental frequency ofthe buncher cavity is 150 MHz.
Figure 2 shows the caluculated time spectra of theIn this calculation,bunched positrons at the sample.
we assumed that the pulse width at the chopper is 2.5
nsec(FWHM) and the energy spread of the positrons entering
the buncher is 10 eV(FWHM), the distance between the second
deflector of the chopper and the gap of the buncher is 500mm, and the mean drift time from the gap of the buncher tothe sample is 3.2 nsec. This result indicates that the bunchingefficiency of the first and third order harmonic buncher ishigher than that of the single wave buncher by 30%.
Figure 3 shows a section of the buncher cavity being
This cavity is a 1/4>. and 3/4J.. coaxial
fabricated.
resonator and the length is about 500 mm. This cavity
has a tuner to match the third order harmonic to theresonant frequency. The fundamental resonant frequency istuned by means of varying the frequency of the signal generator.

/COUPLER

96 psec
FWHMc.

lot)...c 2000
Ul::s0u0200~IDRIFT TUBE20mmt (psec)

Figure 2. Caluculated time spectra of the bunched positrons
at the sample (a) with only the fundamental wave;(b) with the first and third order harmonics. For
each spectrum, 105 trajectories are calculated bymeans of Monte Carlo method.

500mm

1---1t (psec)

TUNER

Figure 3. Cross sectional view of the buncher.

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REFERENCES

Electronic System
Figure 4 shows a block diagram of the electronic
The signal generator generates a fundamentalsystem.
frequency signal which matches the resonant frequency ofthe buncher cavity. The signal is split to two signals:Oneis fed to the buncher control unit; The other is transformed
to a 1/N (N=1,2,4,8,16,· · ·) frequency signal by a frequency
divider, and then, connected to a timing pulse generator
The buncher control unit consists ofand the chopper.
a harmonic generator, phase shifters, variable attenuators,
The output signal of the control
and a combiner.
Theunit is amplified and coupled into the buncher.
outputs of the timing pulse generator are connected toa time to amplitude converter(TAC), a gate generator,and the beam stretcher through a delay unit, a function
generator, and an amplifier. A positron lifetime spectrum isobtained by measuring the time interval between the. timing
pulse and the output signal of a fast -y-ray detector(BaF 2 +PMT).

1. G.Kogel, D.Schoolbauer, W.Trifthiiuser, and J. Winter:
Phys. Rev. Let. 60(1988) 1550.
2. D.Schoolbauer, G.Kogel, P.Sperr, and W.Trifthiiuser:
Phys. Stat. Sol. a102(1987)549.
3. D.SchOdlbauer, G.Kogel, P.Sperr, and W.Trifthiiuser:
Nucl. lnstr. and Meth. B34(1988)258.
4. T.Akahane and T.Chiba: "Positron Annihilation", Ed.
L.Dorikens-Vanpraet, M.Dorikens and D.Segers, (WorldScientific, Singapore, 1989), p.31.
5. F.J.Lynch, R.N.Lewis, L.M.Bollinger, W.Henning andO.D.Depse: Nucl. Instr. and Meth. 159(1979)245.
6. R.Suzuki, T.Mikado, H.Ohgaki, M.Chiwaki, T.Yamazaki,and T.Tomimasu: Advance Abstracts of the Autumn
Meeting of the Phys. Soc. of Jpn., 1989, Vol.3,
p.153(3a-N-2).

SUMMARY
In this paper, we present the design procedure of thebunching system of intense low energy positron beams. We
have carried out a pleliminary test of the bunching system usinga pliot model. 6 The performance of the lifetime measurementswith this system is expected that the injection energy at thesample is variable from "'500 eV to 30 keV, total time resolution is less than 200 psec, and count rate is more than 1000 cps.

BEAM
Function
Gen.

tt

STRETCHERICHOPPER I~
00.................... --- . ----------- ..... --- .. -- ..... -. --- .... -----Buncher Control UnitGateFiqure 4. Block diagram of the electronic system.

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