Kazuo Nakahara

Intense positron beam at KEK

Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e + /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

Trigger control system of the PF linac

Abstract The trigger control system of the Photon Factory 2.5 GeV electron linac at KEK is described. The trigger system is composed of a pulse repetition rate generator, a storage ring rf synchronizing system, main delay modules and a sector trigger system. Some trigger modules have microprocessors and are fully controllable by the linac control system through a serial communication line. Some other trigger modules are planned to be remotely controlled using CAMAC modules or RS232C communication lines.

Control system for the photon factory 2.5 GeV electron linac

Abstract The 2.5 GeV electron linac of the KEK Photon Factory is controlled by a distributed processor network composed of eight minicomputers and many microprocessors. Each individual linac equipment has autonomous operation capability with a micro-processor-based device controller. The device controllers are interconnected to six subcontrol stations and a main operators console through a fiber optic communication network; it is effective to prevent control system malfunction by strong electromagnetic noises arising from pulse operation of high-power klystrons. The system configuration and performance are described.

An overview of the slow-positron beam facility at the photon factory, KEK

The KEK slow-positron source is in the final stage of construction. The beam line comprises a 31 m long vacuum duct within an axial magnetic field and a following electrostatic guided section. In order to vary the energy of a positron beam dedicated to depth-profile measurements, a high voltage station capable of applying 60 kV has been installed in the beam transport system. The target assembly (a water-cooled tantalum rod of 5 radiation lengths and a moderator with multiple tungsten vanes) and the following straight section (8 m; used for positron storage) are under high voltage. The beam duct located downstream is at ground potential. Positron beams passing through this region have a high kinetic energy. A focusing triplet quadrupole lens and a moderator on the retarding electrode are located at the end of the magnetic transport. This beam line has 9 right-angle-curved ducts, comprising a radius of curvature of 40 cm. Positrons with a maximum energy of 60 keV are guided by bending magnets attached to the beam-transport ducts. A transport system to switch from magnetically guided to electrostatically guided has been installed. The design of the brightness-enhancement stage of the positron beam for positron re-emission microscopy is in progress. In a preliminary experiments at 2.0 GeV with a 2 kW primary beam, 4×106e+/s of slow positrons were observed by detecting annihilation γ-rays at the end of the magnetic beam-transport line. Further improvements are expected by careful surface and thermal treatments of the moderator.

Klystron modulator controller for the KEK 2.5 GeV electron linac

Abstract The microprocessor based klystron modulator controller of the KEK Photon Factory injector linac has been designed, constructed and operated. Design philosophy, equipment configuration, functions and the present status of the system are described. With careful consideration to noise problems, the klystron modulator controller has been running quite successfully and reliably in the severe electromagnetic noise environment of the klystron modulator.

Multiple single-board-computer system for the KEK positron generator control

Abstract The KEK positron generator is controlled by means of a distributed microprocessor network. The control system is composed of three kinds of equipment: device controllers for the linac equipment, operation management stations and a communication network. Individual linac equipment has its own microprocessor-based controller. A multiple single board computer (SBC) system is used for communication control and for equipment surveillance; it has a database containing communication and linac equipment status information. The linac operation management that should be the most soft part in the control system, is separated from the multiple SBC system and is carried out by work-stations. The principle that every processor executes only one task is maintained throughout the control system. This made the software architecture very simple.

An operator-console system of the photon factory injector linac

Abstract It is sometimes difficult to unify accelerator control systems constructed in different ways. This problem arose in unifying the control systems of the injector linac and the storage ring making up the Photon Factory of the National Laboratory for High Energy Physics. One easy approach is to unify only the operator consoles; the unified console is connected to both separate control systems using gateways. The operator-console system of the Photon Factory injector linac has been designed and constructed using this approach. It consists of several workstations interconnected via a local-area network, a gateway to the old linac control network and a CATV system for the real-time display of the accelerator status. In this way the linac will be controlled from the control center of the Photon Factory storage ring.

Recent progress in the control system of the KEK 2.5 GeV e−/e+ linac

Abstract The KEK 2.5 GeV linac is controlled by a distributed processor network having minicomputers and microcomputers interconnected with fiber-optic networks. This system has operated for seven years; however, the system capability has become inadequate for the increasing demands. Thus a new network system, using the TCP/IP protocol suite over Ethernet, has been introduced. New control software to enable more reliable operation of the linac is now under development.

Positron microscopes at KEK

Abstract This paper reports the plans of positron microscopes being built at KEK, Tsukuba, Japan, employing used electron microscopes. The kinetic energies of positrons produced by accelerators or by nuclear decays do not have a unique value but are spread in a wide range. The positron beam will be guided near electron microscopes, such as a transmission electron microscope (JEM100S) and a scanning electron microscope (JSM25S). Positrons are slowed down by a tungsten foil, accelerated and focused on a nickel sheet. The monochromatic focused beam will be injected into an electron microscope. The focusing of positrons and electrons are the same in the magnetic system. The characteristic feature of this plan is that the electron microscopes can be used for positrons without changing the focusing system. The electron microscope is shielded from earth’s magnetic field and thus it is convenient to use electron microscopes. Imaging plates are used to detect positron images for the transmission positron microscope.

Development of a position-sensitive gamma detector based on a scintillating fiber for a 2DACAR study

Abstract A position-sensitive gamma detector based on a scintillating fiber is described. At the KEK PF slow-positron facility, an intensity of more than 108 e+/s has already been achieved at the target point of the sample chamber utilizing the PF 2.5 GeV electron linac as its primary beam source, and studies for producing a positron microbeam have been carried out. In parallel with this, we have tried to realize a position-sensitive gamma detector for a positron microbeam. We attempted to use a fiber-optic scintillator for a position-sensitive detector. The glass compositions were alkaline earth, gadolinium, and terbium loaded silicate glasses. The glass was used as the core glass to fabricate bundles of fibers, with each fiber surrounded by cladding glass. Fiber sizes of 6 μm were used, which allowed an image resolution of greater than 20 line pairs per mm. The glass-fiber optic scintillating plate was coupled to an image intensifier. The image-intensifier output image was observed by a CCD camera.