The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Present Status of the Positron Factory Projectand Development of Positron Beam Techniques (II)Sohei OKADA, Hlromi SUNAGA, Hirohisa KANEKO, Atsuo KAWASUSO, Takeshi HIRANO,
Hajime SANO, Haruki TAKIZAW A and Keiichi YOTSUMOTO
Advanced Radiation Technology Center, Takasaki Establishment,Japan Atomic Energy Research Institute1233 Watanuki, Takasaki, Gunma 370-12, Japan

Abstract
Thermal defects in silicon was studied with an internal
positron source produced by a proton beam from a cyclotron.
A pulsed MeV positron beam, which enables more accurate
defect analyses of wider range of materials, was almostconstructed. In the design study for the Positron Factory, theconceptual design has been performed, and efficientmoderator structures are proposed on the basis of both anexperiment and a Monte Carlo simulation.

1. Introduction
We have been promoting four different types ofactivities related both to accelerators and positron utilization:
positron spectroscopy using an internal positron source
produced with a cyclotron, construction of positron beamlines including a pulsed MeV positron beam, design studies
for the Positron Factory[ 1] (linac-based intensemonoenergetic positron beam facility) and ion beam induced
fine structures studied by positron spectroscopy. The last
one will be presented elsewhere. In this report, the present
status of the upper three activities is described.

The experiment suffered from squeezing-out of active
sodium atoms from the specimen, which might cause
uncertainty in the result. Farther detailed investigations will
be necessary to get a clear answer.

3. Construction of Positron Beam Lines
The above internal positron source technique hasadvantages in good SIN ratio and availability in extreme
conditions such as high temperature. But there is alimitation of materials to produce the internal source by ion
beam irradiation.
Positron lifetime measurements with a pulsed positron
beam have the same advantages and no such limitations.
Pulsed positron beams having energies up to several tens
keV for surface study exist in the world[2], whereas higher
energy pulsed beams for bulk analysis do not.
We have been constructing a pulsed MeV positron beamline (PUMPS: PUlsed MeV Positron Source) with a pulse
width of 100 ps. Schematic view of the beam line is shownin Fig.2. Slow positron generation and transportation withgood performance in the part from the source to the chopper
tube were confirmed in the beam experiments using a 3.7
MBq 22Na positron source and an electron gun.

2. Thermal Defect Study with an Internal
Positron Source Produced by a Cyclotron
We produced a positron emitter 22Na inside a FZsilicon specimen by a nuclear reaction 28Si(p,7Be)22Na witha proton bombardment from the JAERI A VF cyclotron.
This internal positron source enables thermal defect studies
by using positron spectroscopy at high temperature whereusual positron sources would be melt down. It is an urgent
issue in semi-conductor industries whether the thermal
defects are formed or not.
The radiation-induced defects were eliminated byannealing up to 900 deg.C. After the annealing the Dopplerbroadened annihilation radiation energy spectra weremeasured with elevated temperatures. The result is shown in
Fig.l. The S-parameter drastically increases around 1100
deg.C, which suggests formation of thermal defects. Theformation energy was evaluated to be about 4. 7 eV from theresult.
1.05

...

.l!:l

1.04
1.03

§ 1.02~rJ:,=-1.01

1.00

mp

0.990
0200

400

600

800

1000 1200 1400

Measurement Temoerature ldei!.C)

Fig.l S-parameter of a FZ-silicon with an internal positronsource measured with elevated temperatures.

Fig.2 Schematics of PUMPS.
The chopper performance was tested with a low
emittance electron beam which emulates a slow positron
beam emitted from the moderator surface by the negative
surface work function. The transmitted current of theaccelerated beam was zero when the chopper potential was
higher than the· acceleration voltage, and sharply increased
with lowering the chopper potential, as shown in Fig.3. The
chopping efficiency for a 500 eV beam was estimated to be
75 %. Here the pulse level was assumed to be 5 V, which isa maximum of our available pulse generator (HP8131A;
rising time< 200 ps). A well coincident chopped beam withthe pulse level change in a time range of 2 ns was also
observed as shown in Fig.4.
The subharmonic buncher (SHB; 178.5 MHz) and theacceleration cavity (2856 MHz) was already installed. In the-107-The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

operation test of the klystron (2856 MHz), the performancewith 8 J.!S pulse width andJOO pps repetition was obtained,
which is better than its specification (>300 kW, >4 Jls, >50pps). The acceleration efficiency of an injected 500 eV beamwith a pulse width of 2 ns from the chopper into the SHB
was estimated to be 70% from the acceleration phase width
calculated by a simulation as shown in Fig.5.
H---: High Emittance Beam & Acceleration Voltage
L---: Low Emittance Beam & Acceleration Voltage<-~e..e

3.0 ..............,..,..,..............,...........-.....-r-.......-.-,..................-r-........-.,.....;..~..., 1.5

2.5-0.2~~-L~~-L~~~~-L~~-L~~...c!:=-..!.- 2.0..

u=

1.5u

....

-~ 0.50

·e="'

~

E-<

....

~

E-<

1.0

0.5

c".

o.o L.................._...._.ir-'-J.....<..O......_""'"III._....J.....<...........-~..""'-11-'-U...._...J o-200

0200400600

800 1,000 1,200Chopper Voltage (-V)
Fig.3 Chopper transmission performance of PUMPS.

-150-100-50050100

SHB Injection Phase (degree)

Fig.5 Acceleration energy in the SHB and the acceleration
cavity vs. injection phase to the SHB.

The waveguide circuit will be installed, and the beamline will be completed by March 1998. After thecompletion, the electron gun will be replaced by a 3.7 GBq
22Na positron source.
We have also developed an isotope-based andelectrostatically guided slow positron beam. The beam line
was completed, and the overall efficiency of slow positron
generation and transportation to the specimen was measured
to be 2.6x J0-5 to 5.5x I0-5 depending on the accelerationenergy up to 30 keY. These are fairly good for anelectrostatic slow positron beam line.

4. Design Study for the Positron Factory.:::I:.- -lOOV(DC).
Pulse Width: 2.8ns
Pulse Interval: 5.6ns

Fig.4 Chopper timing performance of PUMPS.

Electron I Posi1ron Converter, Positron Moderator,
Local Shield, Remote Manipulation Mechanism

Isotope-based slow positron beams cannot be applied foradvanced analyses such as time-dependent observation oftransient phenomena and fine structure investigation of supmicron size local region of materials, because of theinsufficient intensity. We have been promoting design
studies for the Positron Factory, which will produce linacbased intense slow (i.e. monoenergetic) positron beams of
more than JOIOfs in intensity. The conceptual design of themain parts of the facility has been completed as shown inFig.6.
SLOW POSITRON BEAM LINE
Beam Transport, Beam Switching, Beam Stretching,
Beam Banching, Bringhtness Enhancement

HIGH-POWER ELECTRON LINAC& IELJ<X:TRON BEAM LINE
(Energy: lOOMeV,Current 1 rnA, Power: IOOkW,Pulse Width< 3.5Jls)
Injection System, Regular Acceleration Tube, BBU,
RF System, Safety System, Control System.
Beam

Fig.6 Conceptual design of the planned Positron Factory.

-108-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

In the design study, we have proposed 'multicchannel
moderator assemblies' to supply multiple slow positron
beams simultaneously, on the basis of the simulation
system EGS4-SPG developed by us[3]. The feasibility ofthe proposal was confirmed by a demonstrative experimentusing an electron linac[4], as shown in Fig.7, which wasreported previously. We estimated slow positron yields fromeach layer of tungsten moderator foils for the same structure
as that used in the experiment, using EGS4-SPG. The resultis shown in Fig.8, which agreed well with the experimentalresult.

is obvious that the structure effect is remarkable. Especiallya structure of honeycomb-like ass.embled tungsten foils
surrounded by thick tungsten reflectors is expected to
produce more than three times larger amount of slow
positrons compared to the usual structure.

W MOIERATOR FOILS (25p mt)(interval= !Omm)c) Hocizo n tal
W-foils (H)j) Horiz onta!W -foils& Reflectors(H+R)

f) Cross W -foils &Reflectors (C+R)~ 3.0 10"4. . - - - - - - - - - - - - - - -....
~
4.il 2.510"~~;.. 5 2.0 w·'
c:E

f le 1.5 w·'~-~­
jg 1.0=-~~w·•r;; ·:;: 5.o w· 5~~~0.0 lcfa(V)c(V+R)(H)

e(H+R)(C)(C+R)

Structure of Moderator Assembly

Fig.9 Moderator assembly structure effect.
Fig.7 Experimental setup of 2-channel moderator assemblies forthe demonstrative experiment of the simultaneous
extraction of multi-channel monoenergetic positron
beams and the intensity of extracted slow positronsobserved with a MCP.

w·' ...,.....,...T""T",......,....,......,..,....,.....,.....,.......,..,...,,....,.,,..,...,....,~ 6.0 w·'... Ol~i 5.0 w·'~ 7 .oeg~ 4.0 10"6!=..=~: 3.0 w·'=~~ 2.010- 6iii~ 1.0 w·'

i0.0 HfILILIIL&.IILIIL....LLU>LL:II..I.l.......OLJ.:I'-Ioi..J..........

2.22

6.22
10.22
14.22
18.22
Moderator Foil Position(Distance from Converter; em)

Fig.8 Calculated slow positron yields from tungsten foil layersof the 2-channel moderator assemblies used in theexperiment as indicated in Fig.7.
The demonstrative experiment result suggests usefulnessof a heavy metal plate for a reflector to enhance the slow
positron yield, which is indicated as 'recoil effect' in Fig. 7,
and importance of the moderator assembly structure. To
evaluate the structure effect, we calculated slow positronyields for various moderator assembly structures as shown in
the upper side of Fig.9, using EGS4-SPG.
The lower side of Fig.9 shows the calculation result. It

5. Conclusion
We have been developing accelerator related positron
spectroscopy techniques. Positron spectroscopy withelevated temperatures using an internal positron source insilicon produced with a cyclotron suggested a possibility ofthe thermal defect formation. A pulsed MeV positron beamline, which will be applied not only for silicon but forvarious materials, was almost constructed remaining the RF
waveguide circuit for RF feed to the acceleration cavity. An
electrostatically guided slow positron beam line was also
constructed.
In the design study for the Positron Factory, theconceptual design of the main parts has been completed. An
efficient moderator structure of honeycomb-like assembled
tungsten foils surrounded by thick tungsten reflectors isproposed on the basis of an experiment and a Monte Carlosimulation, which will give more than three times larger
slow positron yield compared to the usual one.
References[I] S.Okada and H.Sunaga: Nucl. Instr. and Meth.
B56/57 (1991) 604-609[2] R.Suzuki et. al.: Japanese Journal of Applied Physics
30 (1991) L532-L534[3] S.Okada and H. Kaneko: Appl. Surface Science 85(1995) 149-153[4] S.Okada et. al.: Proc. 1996 Int. Linac Conference, Vol.l(Geneva, 1996) 249-251-109-