The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

DEVELOPMENT OF A SYNCHRONIZATION SYSTEM BETWEENPOSITRON BEAM AND LASER
K. Kannot, Y. Honda, M. Tashiro, T. Yamaguchi, G Isoyama, S. Tagawa
The Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Abstract
The new experiment is planed to understand thebehavior of positroniums in materials by direct excitationof positroniums using laser. In this experiment, the highdensity bunched positron beam is required. The newfacility for this experiment was designed. And it wasfound that the numerically estimated number of available
positrons were about 1.I X I 0 5 and the pulse width was
200 ps.

Trigger~~M:::r:i~~;Y- ray1 INTRODUCTION~/

It is known that energetic positrons injected into amaterial lose their kinetic energy rapidly by inelastic
scattering with atomic electrons dominantly until theirenergy fall less than the ionization energy. And these
decelerated positrons reach to thermal energy, so-called
thermalization, by generation of the exciton and thephonon excitation. After thermalization they diffuse andannihilate with electrons. The annihilation rate depends
on the electron density in the material. If there are freevolumes in the material, the positrons can be trapped
there efficiently and the positron lifetime becomes longer
because of lower electron density. In polymer, somepositrons can form a bound state with electrons known as
positroniums (Ps) which are trapped in the free volume.
As the lifetime of the orthopositronium (o-Ps), whose spin
direction is in parallel, reflects the size of the free volume,the positron annihilation lifetime spectroscopy plays animportant role in detecting the free volume of atomic
sizes. This is the powerful tool ·to characterize the freevolume. Various interactions concern the factor of theo-Ps lifetime definition. Our purpose is to investigate thebehavior of the o-Ps in materials and its relationship withlifetime. As an excited o-Ps is easier to be influenced by
the surrounding potential barrier than the o-Ps on theground state, it is possible to obtain the information
concerning this potential by the use of direct excitation ofthe o-Ps. By changing the irradiation timing andwavelength of laser on the bunched positron beamsystematically, several kinds of interactions take place
between the Ps and the materials. In this experiments, thebunched positron beam which consists of I~ I 0 thousandsof positrons are required. However, it is impossible toobtain the high-density positron beam by using present
system based on the S-hand electron linac at ISIR, Osaka
University [1].

Figure 1: Measuring system based on the L-band linac.
Therefore we decided to use the L-band electron linac
at !SIR, which can produce high-density electron single
bunch beam.
The number of the Coulomb per pulse of this linac is 91
nC, and pulse width is 20 ps.
The block diagram of measuring system is shown inFig. I.

2 THE TRANSPORT SYSTEM FOR THE
ELECTRON BEAM PASSING FROM THE
BENDING MAGNET TO THE POSITRONGENERATOR
The distance between the bending magnet and wall ofthe linac room is about 4 m as shown in Fig. 2. A
converter that is used to generate the positrons and somequadrupole magnets are placed on this length. The beam
size of the electron, which is injected into the converter, isrequired to be as small as possible.
The L-band linac is operated at 28 MeV and its
emittance is about 5n mm · mrad.

tkanno25@sanken.osaka-u.ac.jp-435-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

New Positron Gene1·ator~iFF1~1'1 1 1 1 1\l\1:,:J: Ij

L-band Linac

Ti-Sapphirc Laser

1;>0 550~50 5!;>0"'X[~ ~J,50 "'5 0 "'

so b5 0 "'The motion of the electron beam in the xx' phase space~=JtEFFN\FH\jl]-50 5-50sso 5

50 5.Y[:-r, ;.;..;]!50 550 5!50 !550 5The motion of the electron beam in the yy' phase space

Figure 2: Facility section

The trajectories of electrons were calculated by the useof transfer matrices, and the transport system wasoptimized in order to minimize the beam size on theconverter by using simplex method [2]. When adapting
only quadrupole triplet to this system, one of the exciting
current values of the coil of quadrupole magent was overits capacity. When adapting quadrupole triplet with singlet
to this system, it was found that the combination of theparameters, which were every exciting current of coils of
Quadrupole magnet and the position of quadrupole
magnets and the converter, existed as the values whichwere able to realize. Design of the transport system andthe motion of the electron beam in the phase space areshown in Fig. 3 and Fig. 4, respectively.

Positron Generator

BMFigure 4: The motion of the electron beam in the phasespace passing from the BM to the converter. Thesecomponents are perpendicular to the beam axis. From left
to right, each graph shows diagram at exit of the BM, theentrance and the exit of 4 QMs, and on the converter,respectively.

3 DESIGN OF THE POSITRONGENERATOR
We decided to use only the converter as the positron
generator, since the moderator causes widening of thepulse width to the bunched positron beam.
EGS4 program [3] was used to estimate the number ofpositrons produced in the converter. The tungsten discwas selected as the converter. In this program, the cut-offenergies for an electron and a positron are limited toIO keY. Actually, the positrons still go further loosing
their energy less than I 0 ke V by inelastic collisions.
However, this distance was no need to be taken into
account, since it could be estimated less than 100 nm [4],and this length was small compared with diffusion length.
In the region with thickness of 1j!m from the end
surface of the converter, the number of positrons wasestimated by varying thickness of the converter. The
dependence of the production efficiencies on thethickness of the converter are shown in Table I, wherethese efficiencies are defined as the number of produced
positrons in this region divided by the number of theincident electrons
Table 1: Dependence of the production efficiencies on thethickness of the converter
Converter thickness
Efficiency(mm)

123
456

Figure 3: Design of the beam line-436-(Xl0-6 )0.8± 0.3
2.4± 0.5
2.8±0.8
2.8± 0.6
2.8± 0.8
1.8± 0.6

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Table 2: Production efficiencies of positrons fromelectrons when equipped the reflector::0(Converter thickness: 4 mm)

,--.

0e2

14.7±0.91380:..

0cG)·;:; 34603'403<;::G)

22.8± 1.0 6.4±1.6

48±1.4~36.4±116.0±1.1

4

5.2±1.040±1.4> 2'+'

2.4±1.0OS>·;:

7.7±1.6 6.2±1.1G)

20c0100

200

300

4000500

Time [ps]Water inlet

D>

.,
.,

0....,a;·::l(')'<,.....,~

Figure 6: Derivative efficiency corresponding to time

It was found that optimum thickness for the converter
was 4 mm and conversion rate from an electron to auniform and was 1.5 em 2 Is, lifetime was 150 ps. The
results showed that about 4.5 % of positrons included in
1 11m thick region could reach the end surface atmaximum and also about 94 % of these positrons could
reach surface within 100 ps and about 99 % within 200 ps
as shown in Fig. 6. The total conversion rate from anc::ll(')

0positron was (2.8 ± 0.6) X 10 - 6 . Furthermore, theconversion rate when the reflector was equipped was also
estimated for the sake of increasing efficiency, and theseresults are shown in Table 2.
It was found that optimum reflector thickness and gap,between the converter and the reflector, were 3 mm and3mm, respectively, and then conversion rate was(7.7± 1.6)X 10-6 • The design of the positron generator isshown in Fig. 5.
The arriving rate at end surface of positrons staying inthis region was also estimated by solving one dimensional
diffusion equation, in which several assumptions wereadopted such that the diffusion constant for positron was

3D>
X!Ew

3.6±1.2

gtJ

D>

§: 4

Reflector thickness (rnm)
0.1...
100 ~....,::r5incident electron to positrons arriving at end surface could
be deduced from above results to be about 3.5 X 10-7 .
When the number of incident electrons is 3 X 10 11 , about
1.1 X 10 5 positrons are expected to obtain within about
200 ps.
The pulse width will be stretched to some in thetransport tube, since each positron travels along thedifferent field line. To suppress this stretching effect, asimple beam line composed of two bending sections wasconsidered and it was found that the smaller diameter andhigher transport energy would suppress the stretching inthis beam line. When the positron beam is 1Omm diameter,
the transport energies: 100 e V, 1 ke V, 10 ke V
correspond to time spread: 68 ps, 26 ps, 4ps, respectively.
The emerging positron beam from the end surface of theconverter has wide energy spread, since the most ofpositrons cannot be thermalized in the converter. To
separate these high-energy components, several apertures
and a set of collimators will be equipped, since the highenergy components shift normal to the axis more atbending section.

4 CONCLUSION
To obtain high-density bunched positron beam, the newtransport system based on the L-band linac at ISIR wasdesigned in order to understand the behavior of Ps inmaterials. The new facility for this experiment wasdesigned, and the number of positrons are expected to beTungsten
converter(4nn)

about 1.1 X 10 5 and the pulse width is 200 ps.

5 REFERENCES
Ceramics
Figure 5: Design of the positron generator[I] M. Tashiro, et al., Proceedings of the 261h Linear
Accelerator Meeting in Japan, Tukuba, Augustl-3,
2001, pp.309-310.
[2] William H. Press, et al., "Numerical Recipes inFortran" The Art of Scientific Computing.
[3] W. R. Nelson, H. Hirayama and D. W. 0. Rogers.,"SLAC-265, December 1985".
[4] A. P. Mills, et al., "Phys. Rev. A26 (1982), p490".

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