A 50-MEV ELECTRON LINAC FOR THE KEK SLOW-POSITRONFACILITY
T. Shidara, I. Abe, M. Akemoto, S. Anami, A. Enomoto, S. Fukuda, K. Furukawa, H. Honma, M. Ikeda, K. Kakihara,
N. Kamikubota, T. Kamitani, H. Katagiri, H. Kobayashi, T. Kuriharai, Y. Ogawa, S. Ohsawa, T. Oogoe, T. Matsumoto,
S. Michizono, H. Nakajima, K. Nakao, A. Shirakawa, T. Suwada, Y. Takeuchi, S. Yamaguchi, and Y. Yan, KEK, HighEnergy Accelerator Research Organization, Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract
A 50-MeV electron linac is constructed as a primarybeam source for the production of slow-positron beams at
the KEK slow-positron facility. This linac comprises agun, a pre-buncher, a buncher and a 4-m accelerating
structure with a 40-MW klystron unit. Utilizing a 1-kW
electron beam, more than one hundred million slow
positrons per second will be available in various fields ofsolid state physics.

1 INTRODUCTION
A positron beam is a useful probe for investigating theelectronic states in solids, especially concerning thesurface states and defect distribution. However, suchstudies, as low-energy positron diffraction, positron
microscopy, positron reemission 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 slow-positron facility [7, 8], utilizing our
2.5-GeV electron linac [9, 10] as its primary beam source.
Although we had opened this facility to slow-positronusers [11,12], we had to relocate our facility to the southof our linac, relevant to the upgrade plan of the KEK 2.5GeV linac. The upgraded linac (the KEKB linac) supplies
8-GeV electron and 3.5-GeV positron beams to theasymmetric collider KEKB [13-15]. There is a heavy
competition between the KEKB and the PEPB, theKEKB linac is and will be almost fully occupied by thecontinuous (top-up) injection to the KEKB rings in orderto achieve a possible maximum luminosity. We therefore
decided to install a 50-MeV electron linac as a dedicatedprimary electron beam source for the KEK slow-positron
facility utilizing the remnants of the KEK 2.5-GeV linac.
We describe here this 50-MeV electron linac and ourslow-positron facility.

2 KEK SLOW-POSITRON FACILITY
The KEK slow-positron facility consists of a 50-MeV
electron linac as a primary beam source, a targetmoderator assembly, a slow-positron beam-transport line
and relevant assemblies. The 50-MeV linac comprises agun, a pre-buncher, a buncher and a 4-m accelerating
structure with a 40-MW klystron unit. This linac can
supply a 1-kW long and a 100-W semi-long beams to thepositron production target. Characteristics of the 50-MeV
linac are listed in Table 1. Utilizing a 1-kW electron

beam, more than one hundred million slow positrons per
second will be achievable.
Table 1: Characteristics of the 50-MeV linacLong
Semi-long
25 MeV
50 MeV
Beam energy
Pulse length
20 ns
2 µs
Beam current
400 mA2A
Pulse repetition
50 pulses/s
50 pulses/s
Beam power
1 kW
100 W
Positron
10*8 positrons/s 10*7 positrons/s
intensityThe primary electron beam is injected into the target.
The extracted slow-positron beam is directed by a 10-mlong beam-transport line with an axial magnetic field of
100 G to an experimental area. Several sets of steeringcoils were installed along the slow-positron beamtransport line in order to adjust the slow-positron beam
trajectory. Penning-trap electrodes are also installed inorder to make a dc beam from a pulsed beam. As for thebeam monitors, micro-channel plates (MCP) for the beamprofile are intensively used.

3 PRESENT STATUS
Almost all components of the 50-MeV electron linac, atarget-moderator assembly and a slow-positron beamtransport line are installed. The commissioning of the 50MeV electron linac started from this March. At the initialstage, a slightly low-power primary beam of 15 W (anenergy of 40 MeV, a pulse length of 15 ns, a bunch
current of 30 nC and a repetition rate of 12.5 pulses/s)
was successfully accelerated and an appreciable numberof slow positrons were observed by MCP which locates atthe end of the 10-m slow-positron beam-transport line.
Since the insufficiency of the radiation shielding wasrecognized during this commissioning, we are planning toreinforce the shielding in the near future.

4 FUTURE PLAN
Linac based high-intensity slow-positron beam enables
us to investigate the defects mapping (two dimensional or
three dimensional information of defects distribution).
More intense slow-positron beam will be achievableutilizing a primary electron beam power of 5 kW byupgrading the rf source of this test linac.

5 SUMMARY
A 50-MeV electron linac is constructed as a dedicatedprimary beam source for the KEK Slow-Positron Facility.
During its preliminary performance test, a low-power
primary electron beam of 15 W was injected into thepositron production target and slow-positron beams weresuccessfully generated. After reinforcing the radiation
shield, we will open this facility to slow-positron users.

6 ACKNOWLEDGMENTS
The authors are greatly indebted to the director of theInstitute of Materials Structure Science, Prof. Y. Kimura
as well as to the staff of the KEK administration
department for their encouragement and continuous
support to this slow-positron project. They also express
their gratitude to Drs. M. Yoshioka, H. Hayano, S. Ban,K. Takahashi, and T. Sanami for their support and help toreinforcing the radiation shield of our 50-MeV electronlinac.

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