JAERI-Conf

95-021

3a2-3
PRESENT STATUS OF THE POSITRON FACTORY PROJECTAND DEVELOPMENT OF POSITRON BEAM TECHNIQUESSohei OKADA, Hirohisa KANEKO, Atsuo KAWASUSO, Shin-ichi MASUNO,Hlromi SUNAGA, Haruki TAKIZAWA and Keiichi YOTSUMOTO

Advanced Radiation Technology Center, Takasaki Establishment,
Japan Atomic Energy Research Institute1233 Watanuki, Takasaki, Gunma 3 70-7 2, Japan

Abstract
A preliminary experimental result confirmed apossibility to study defects in silicon with an internal
positron source produced by a proton beam from acyclotron. A pulsed MeV positron beam, which can beapplied for detailed bulk defect analysis of wider range ofmaterials, is under construction. In the design study for thePositron Factory, a feasibility of simultaneous extractionof multi-channel monoenergetic positron beams wasdemonstrated by an experiment using an electron linac.
1. Introduction
We have been developing positron spectroscopy
techniques combined with accelerator technology. Detailed
bulk defect analysis using an internal positron source insilicon produced with a cyclotron is now in progress. The
preliminary result is reported. We have been constructing
a pulsed MeV positron beam line for the same purpose,
which will be applied not only for silicon but for variousmaterials. Outline of the design is described.
We have been promotipg design studies for the'Positron Factory' [1], in which linac-based intensemonoenergetic positron beams are planned to be applied
for advanced materials characterization and new fields ofbasic research. A tentative goal of the slow (i.e. monoenergetic) positron beam intensity is lOlOJsec, which islarger by two orders of magnitude than those of existingstrongest beams in the world. We have proposed a conceptof simultaneous extraction of multi-channel monoenergetic positron beams, on the basis of a Monte Carlosimulation, in the design study. In this report, an
experimental result to confirm the feasibility of thisconcept is demonstrated.
2. Internal Positron Source Production with a Cyclotron
It is possible to produce a positron emitter 22Na insidesilicon from a nuclear reaction 28Si(p,7Be)22Na with aproton bombardment. This internal positron source
enables thermal defect studies by using positron lifetime
measurement at high tempell'ature where usual positronsources would be melt down.
Positron lifetime spectra of a high-purity FZ-silicon
irradiated by 70 MeV proton from the TIARA A VF
cyclotron in JAERI Takasaki Establishment are shown in
Fig.l. The fluence was 2.5X 10l7cm-2. The lifetime
measurement was carried out at room temperature. The
spectrum (a) is for the sample as irradiated. It is resolved
into three components: 7: t=170ps(ft=59.4%), 7: 2=284ps(h=39.0%) and 7: 3=1.73ns(h=l.6%), where 7: and I are
the lifetime and the intensity, respectively. The value of
7: 3 is too large for that corresponding to defects insilicon. When the irradiated specimen was sandwiched
between two unirradiated ones, this component disappeared

as shown in the spectrum (b). Consequently, this extra
component of small amount is assumed to come frompositrons emitted from the vicinity of the surface of theirradiated specimen into air and/or surrounding materials
like a detector. This is an important fact to which wemust pay attention in use of the internal positron source
technique.
The value of 7:2 indicates that vacancy clusters wereinduced by the irradiation, which survive even at room
temperature. We will further investigate the detail of thecluster with elevating the temperature and also thebehavior of thermal vacancies at much higher
temperatures.
10° ~r-r,....,,...,...,.....-.,..T""''................-.-r-r"T""''I"'"''"""r""'.....
........ 10"1

( \ "";_,.,. s;irradiated Si

.....

·c:

~~:::l.ci.... 10"2iI ll10"3.....
c:f<I"\~~~~/~~~·~-- l.
~..~~v-. .

:::10u 10"4(a). .. ~ . ~~,...~~~~.<:t;:~~--?~f.:~i::'::\if.:.'~~-~'~.:~·

10-50100

200300400

Channel Number
Fig.l Positron lifetime spectra of a high-purity FZ-silicon
irradiated by 70 MeV proton, which were measured byusing the internal positron source technique.

3. Construction of a Pulsed MeV Positron Beam LineThe above internal positron source technique has anadvantage that ion-induced defects can be studied inaddition to thermal defects at higher temperatures.
However, available materials are limited. A pulsed
positron beam, on the other hand, makes it possible toanalyze defects in a variety of materials at arbitrary
temperatures. Suzuki et. al. [2] constructed a pulsed slow
positron beam line connected to an electron linac andsucceeded in various sorts of surface characterizations. The
pulse width is about lOOps, and the beam energy range isfrom several tens of eV to several tens ofkeV.
To study defects in a 'bulk' as well as those at asurface is important. Fig.2 shows stopping probability of
1 MeV positron beam onto tungsten, which wascalculated with a Monte Carlo simulation system EGS4SPG developed by us[3]. The fraction of implanted
positrons was 0.63 and ltbe others were back-scattered. It is
deduced that almost all implanted positrons annihilate in abulk of the material. If the beam is accelerated not by anelectrostatic field but by RF, only a little portion of the
back-scattered positrons may reenter the material.

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JAERI-Conf 95-021.~:.c 0.08
ell.0~0.06tn 0.04

c·a.

a.
0 0.02......
(/)

00.05

0.1

0.15

0.2

Linear Depth in Tungsten (mm)Fig.2

Stopping profile of 1 MeV positron inside tungsten
calculated with EGS4-SPG.

On the basis of this concept, we have been
constructing a pulsed MeV positron beam line with apulse width of lOOps, for detailed bulk defect analysis.
Schematic view of the beam line is shown in Fig.3. The
parts of the positron source, the slow positron beam
generation and the beam transport were already installed.
The subharmonic buncher (178.5 MHz) and theacceleration cavity (2856 MHz) will be installed by theend of 1996 fiscal year.

channel moderator assemblies' to supply multiple slow
positron beams simultaneously as shown in Fig.4. The
slow positron yield, that is a ratio of the number of slow
positrons emitted. from each tungsten moderator assemblyto that of incident electrons onto the tantalum converter,
was estimated using the EGS4-SPG. The result is shownin Fig.5. The contribution by energetic positrons from theconverter to generate slow positrons drastically decreased
at the assemblies distant from the converter. It wasdeduced from tracking of the particles that this is causedby spatial spread of the positron beam. On the contrary,
there still were sufficient slow positron yields originatingin energetic photons, even at the rear assemblies. This isbecause the photons go almost straightforward and cause
pair production reactions uniformly in every assembly.
Thus produced positrons h~tve comparatively lower
energies, which results in higher probabilities to bethermalized in each moderator foil.

1stassembly

3rd
assembly

2nd
assemblytungsten (50mmx50mmx25um 1 l x1 ofoils)<gap: 8mm)
( Ta converter-W moderator assembly: 27.7mm
assembly-assembly: 27.7mm

Fig.4 A concert of the simultaneous multi-channel extraction ofslow poS1tron beams by multiple moderator assemblies.
Slow Positron Yield (slow positrons/incident electrons)-9-10

1010-81010III

1st
assemblyFig.3 Schematics of a pulsed MeV positron beam line.

I

2ndassembly

4. Design Study for the Positron Factory
We have performed design studies for the Positron
Factory until 1994 as follows[4]:1) An optimum electron beam energy for slow positron
generation was estimated to be around 100 to 150
MeV.
2) It was calculated that a tentative goal of the slow
positron beam intensity (lQlOfsec) could be attainedwith a linac of 100 kW class with the above energy
range.
3) A technical survey study confirmed a feasibility ofmanufacturing such a state-of-the-art linac.
4) Further detailed analyses were carried out concerningthermal deformation of the accelerator structures, beam
instability, reliability of the components, down-sizingof the machine and a computer-aided control system.
5) A 'self-driven rotating converter' suitable for the highpower beam was proposed and successfully tested.
In the design study, we have also proposed 'multi-3rdassembly

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10I

I by

Iby positrons-4-5

1010I.bypositrons
IY photon

positrons- b y photons
by photons

Fig.5 Slow positron yield calculated with EGS4-SPG for themultipfe moderator assemblies shown in Fig.4. The incident
electron beam energy is 100 MeV. Contributions byenergetic positrons and photons from the converter areestimated separately.

To demonstrate a feasibility of the simultaneous
extraction of multi-channel slow positron beams, wefabricated a set of 2 channel tungsten moderator
assemblies as shown in Fig.6. The set was composed of
18 tungsten foil layers of 25 JJ. m in thickness. Slow
positrons from each 9 layers were separately extracted by 2tungsten mesh grids. Each moderator layer was divided
into 3 parts, electrically separated and biased to drift
emitted slow positrons by sloping the electric field toward
the extraction grids. We observed the slow positron beamprofile from the assemblies with a MCP (micro channel-423-JAERI-Conf 95-021

plate), using a 100 MeV electron beam from a S-band
electron linac at Osaka University.
The result is shown in Fig. 7. Three peaks wereobserved in the slow positron beam intensity. The largest
one was attributed to slow positrons from the first channel
which was nearer to the tantalum converter. The second
and third peaks were both attributed to slow positrons
from the second channel. It. is assumed that back-scatteredpositrons and pair production reactions by photons giverise to the third peak, because thick tungsten plates wereplaced at the end of the second moderator assembly. Thismeans that positrons and photons passing through the frrst
and second assemblies still have a potential to generate
slow positrons, and also that it will be efficient to place aheavy metal at the end in fabrication of moderator
assemblies.

The intensity of slow positrons from the second
channel was smaller only by a order of magnitude thanthat from the first channel, which agreed well with thesimulation result. It was concluded that such an extra
beam will be useful for preliminary or potential researches
which are promoted simultaneously with main
experiments using the strongest beam.
5. Conclusion
We confirmed a possibility to study defects in siliconwith an internal positron source produced by proton
bombardment. In the positron lifetime spectrum, a small
extra component which may originate in escaping
positrons from the irradiated specimen was found.
Construction of a pulsed MeV positron beam, which isapplicable for defect analysis of various materials, will becompleted at the end of 1996 fiscal year. In the design
study for the Positron Factory, we demonstrated afeasibility of simultaneous extraction of multi-channelmonoenergetic positron beams using an electron linac, by
an experiment.
The experiments of the internal positron source
production and the multi-channel positron beam extraction
were carried out in cooperative researches with Tohoku
University and Osaka University, respectively. Theauthors wish to thank Prof. M.Hasegawa, Prof.
S.Tagawa, Dr. Y.Honda and their colleagues for theircooperation.
·References· Fig.6 Experimental setup of 2-channel moderator assemblies forthe demonstrative experiment of simultaneous extractionof multi-channel monoenergetic positron beams.

[1] S.Okada and H.Sunaga: Nucl. Instr. and Meth. ~(1991) 604-609[2] R.Suzuki et. al.: Japanese Journal of Applied Physics
JQ (1991) L532-L534[3] S.Okada and H.Kaneko: Appl. Surface Science B.l(1995) 149-153[4] S.Okada et. al.: Proc. 1994 Int. Linac Conference(Tsukuba, 1994)570-572

from 1 st channel
from 2nd channelFig.7

The intensity of slow positrons extracted from themoderator assemblies shown in Fig.6, observed with aMCP.

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