RELOCATION OF THE KEK SLOW-POSITRON FACILITY
T. Shidara, T. Kurihara, A. Shirakawa, A. Enomoto, H. Kobayashi and K. Nakahara,KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan
Abstract
The KEK slow-positron facility, aiming at the use ofslow-positron beams (ranging from eV to keV) in various
fields of solid-state physics, is now undergoing arelocation process to the 1.5-GeV point of the upgradedlinac relevant to the KEKB project. A dedicated linac forslow-positron use is also installed utilizing the remnantsof the 2.5-GeV linac reformation. We expect a slowpositron intensity of more than 108 positrons/sec with amaximum primary beam power of 1 kW.

the upgrade plan of the KEK 2.5-GeV linac. There are
two major goals of the upgrade [13,14]: 1) to increase theenergy of electrons and positrons to 8 and 3.5 GeV,respectively, and 2) to increase the bunch intensities ofpositrons by roughly one order. In accordance with thisupgrade plan, we must relocate our KEK-PF slowpositron facility to the 1.5-GeV point of the upgradedlinac (the KEKB J-linac ; see fig.1).
We describe here the relocated KEK-PF slow-positron
facility.
2

1 INTRODUCTION
A positron beam is a useful probe for investigating theelectronic states in solids, especially concerning thesurface states. However, such studies, as low-energy
positron diffraction, positron microscopy andpositronium spectroscopy, are very limited due to thepoor intensity obtained from a conventional radioactiveisotope-based positron source. We, therefore, investigatedaccelerator-based slow-positron sources [1-6], andconstructed the KEK-PF slow-positron facility [7,8],
aiming to produce more than 2x109 e+/s slow-positrons,
utilizing our 2.5-GeV electron linac [9,10] as its primarybeam source.
At the very initial stage of its performance tests, theobserved positron yield was 1/20 of the estimated value[8]. This discrepancy was thought to be due to thecondition of the moderator, since we put the moderator
into the target-moderator chamber without performing
any thermal treatment. An excellent improvement in thepositron yield was achieved by annealing of the moderator
assembly (tungsten foils) at 2270 K for 10 minutes underultra-high-vacuum conditions. A slow-positron flux of
1x108 e+/s was successfully achieved with a 2.0-GeV, 2kW primary electron beam power.
The energy of the positron beam was successfully
varied from 400 eV to 40 keV by applying a voltage tothe high-voltage station at the initial part of the slowpositron beam-transport line.
A slow-positron beam with a beam energy of 800 eV
was successfully switched from one direction to anotherutilizing the slow-positron beam-switch system. We cantherefore supply slow-positron beams to several
experimental stations without any waste of time.
Penning-trap electrodes were successfully operated inorder to make a dc positron beam from a pulsed primaryelectron beam.
Although we had opened this facility to slow-positronusers[11,12], we had to relocate our facility relevant toLAYOUT OF THE RELOCATED KEK-PF
SLOW-POSITRON FACILITY

There are two primary-electron-beam sources for our slow
positron facility; the 1.5-GeV beam of the KEKB linac
and the test linac. The nominal beam power of the 1.5GeV beam is 0.75 kW (an energy of 1.5 GeV, charge of
10 nC, a pulse length of 10 ps and a pulse repetition
rate of 50 pulse/s). Since the injection interval of the Jlinac for the KEKB rings might be relatively short, adedicated linac for slow positron use only (the test linac)
was installed utilizing the remnants of the J-linac upgrade
plan. An average beam power of 1 kW can be expectedfrom the test linac.
Figure 2 shows the relocated KEK-PF slow-positron
facility, which locates at the 1.5-GeV point of the KEKB
J-linac. It comprises beam lines for the primary electron
beams, a target-moderator assembly, a slow-positron
beam-transport line and relevant assemblies.
The primary electron beam is injected into the target.
The extracted slow-positron beam is directed by a 30-m
long beam-transport line with an axial magnetic field of
100 G to an experimental area. Twelve sets of steeringcoils were installed along the slow-positron beamtransport line in order to adjust the slow-positron beam
trajectory. A high-voltage station capable of applying 60
kV was installed in order to vary the energy of thepositron beam. A device controller, combining a personal
computer and a programmable sequence controller
through optical fiber, has been adopted to control themonitors and power supplies at a high-voltage potential.
Penning-trap electrodes are also installed at this station inorder to make a dc beam from a pulsed beam.
At an experimental area, a slow-positron beam switch
system, which comprises a pair of beam deflecting coils
and two pairs of Helmholtz coils with magnetic-field
directions crossing each other, was installed. This systemenables us to direct slow-positron beams to several
experimental stations one by one without breaking thevacuum.

As for the beam monitors, channel-electron
multipliers (CEM) for the beam intensity and microchannel plates (MCP) for the beam profile are intensivelyused.
3PRESENT STATUS

Almost all components of the KEK-PF slow-positron
facility were relocated. The accelerating structures andnecessary magnets of the test linac have already been
installed in the linac tunnel.
The comissioning of the slow positron beam isplanned to start from this April. We expect a slowpositron intensity of more than 10 8 positrons/sec
utilizing the 1.5-GeV J-linac beam as its primary beamsource.
4FUTURE PLAN
In order to produce more than 109 e+/s slow-positrons in
our slow-positron facility, it is necessary to reinforce its
primary beam source. If we add another two accelerating
units to the present test linac, we will be able to achieve
this positron intensity utilizing a beam power of 6 kW
from the upgraded test linac.
5SUMMARY

The KEK slow-positron facility was relocated to the 1.5GeV point of the KEKB J-linac relevant to the KEKB
project. A dedicated linac for slow-positron use is also
installed utilizing the remnants of the KEKB J-linac
upgrade plan. We expect a slow-positron intensity ofmore than 108 positrons/sec with a maximum primarybeam power of 1 kW.
6ACKNOWLEDGMENTS

The authors are greatly indebted to the Director General,
Prof. H. Sugawara as well as to the staff of the KEK
administration department for their encouragement andcontinuous support of this slow-positron project. They
should also express their gratitude to prof. Y. Kimura, thedirector of the Institute of Materials Structure Science andto Prof. M. Kihara, the director of the Accelerator
Laboratory, KEK, for their support. The staff of the KEK
electron/positron linac is also gratefully acknowledged formachine operation and support.
7

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Extended Area of the Linac Tunnel

1–5

1 –4

1–3

1–2

1 –1

C–8

C–7

C– 6

C –5

C–4

C–3

C–2

C–1

Slow-Positron
Experimental Area
A–1

Electron Gun
Room

A–2

A–3

A–4

B–1

Accelerating Structures

B–2

B–3

B–4

B–5

Test Linac

B–6

B–7

B–8

Bending
Mag.
Q-Mag.

Beam Dump

SP–1 (SP–2) (SP–3)
Slow-Positron Production Target

Figure 1. Extended area of the KEKB J-linac.
The extended area of the J-linac comprises three sectors (A, B and C). The positron production target chamber
locates at the beam dump of the J-linac. A dedicated linac for slow-positron use is installed at sector-B just beside theJ-linac.

Figure 2. Bird's-eye view of the KEK slow-positron facility.
It comprises beam lines for the primary electron beams, a target-moderator assembly, a slow-positron beamtransport line and relevant assemblies.