Toshikazu Kurihara

High-precision pepper-pot technique for a low-emittance electron beam

Abstract An emittance measurement system employing a high-precision pepper-pot technique has been developed for an electron beam with low emittance, less than π mm mrad. Since luminous spots on a scintillator are very small, each one was magnified and observed individually with high spatial resolution by scanning an optical system mounted on a precise movable platform. The beam from a high-brightness electron gun was evaluated using this system.

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.

Pore interconnectivity of nanoclustering silica porous films as studied by positronium time-of-flight spectroscopy

Positronium time-of-flight spectroscopy with improved stability and signal-to-noise ratio, achieved by a developed off-line digital data analysis, was applied to the characterization of three types of nanoclustering silica porous films with different relative dielectric constants and refractive indices. The emission of triplet ortho-positronium (o‐Ps) from the film surface was examined as a function of incident positron energy (Ein). It was found that the o‐Ps emission peak energies from two highly porous films with similar total porosities decrease similarly to each other with increasing Ein up to 1.50keV. On the other hand, o‐Ps emission intensities from the two films differed considerably in the range between 0.5keV<Ein<4keV, which reflects a difference in pore interconnectivity between the two films with different mean secondary particle sizes. Some interconnected pores are expected to be closed by the necking at the particle contacts as calcination proceeds, possibly leading to more necks in the pore...

Positronium time of flight measurements of an open-pored spin-on low-k mesoporous film

Depth-profiled positronium time of flight (Ps-TOF) was measured by implanting slow positron pulses at variable energies into an open-pored spin-on low-k mesoporous film with bimodal pore size distribution arising from zeolite micropores and interparticle mesopores. We estimate the energy of the positronium (Ps) that diffuses out of the target into the vacuum to investigate the slowing down of Ps by collisions with the walls of the micropores (0.5 nm in diameter) and mesopores (4 nm in diameter). The obtained Ps-TOF spectra showed that the temperature of the emitted Ps depends on the positron implantation depth, i.e., the number of collisions with the walls. The data indicate that the pore tortuosity is low in the present sample. However, the slowing down rate seems to increase when the Ps temperature is high. This is probably because when the Ps temperature is high, the Ps energy is higher than the Ps confinement energy for micropores (~3 eV), and the apparent tortuosity increases because there are millions of micropores. The Ps formed in this film has to travel a long distance to escape from the same depth into the vacuum until it slows down below the confinement energy of the zeolite micropores.

Development of beryllium-based neutron target system with three-layer structure for accelerator-based neutron source for boron neutron capture therapy

The iBNCT project team with University of Tsukuba is developing an accelerator-based neutron source. Regarding neutron target material, our project has applied beryllium. To deal with large heat load and blistering of the target system, we developed a three-layer structure for the target system that includes a blistering mitigation material between the beryllium used as the neutron generator and the copper heat sink. The three materials were bonded through diffusion bonding using a hot isostatic pressing method. Based on several verifications, our project chose palladium as the intermediate layer. A prototype of the neutron target system was produced. We will verify that sufficient neutrons for BNCT treatment are generated by the device in the near future.

Present Status of the Slow Positron Facility at KEK

There has been increasing interest in the use of slow positron beams in various fields of science. However, because of its poor intensity a radioactive-isotope-based positron beam is not so versatile as a laboratory-based electron beam. To obtain intense beams accelerator-based positron sources[1,2,3] and reactor-based ones[4] have been developed and used successfully in studies of various materials. In this brief report a new slow positron facility at KEK is described. The facility consists of a dedicated 50 MeV linac, an assembly of slow positron generator, a slow positron transport line and an experimental station for positronium time-of-flight (Ps-TOF) spectroscopy. Figure 1 shows an overview of the facility.

Recovery of 18F from [18O] water by electrochemical method

An electrochemical method for producing 18F sources for the slow positron beam was applied to the recovery of 18F from H2(18)O water. The 18F of activities 150-227 mCi (5.55-8.40 GBq) was electro-deposited on a graphite rod and then emitted into pure water. The best result of the efficiency for the electro-deposition for 5 min was 97% and that for the electro-emission for 5 min was 89%. The H2(18)O water is expected to be reused much more easily by this method than by the ion exchange resin method. The metal impurities contained in the 18F solution were considerably reduced by using this method.

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.

The Defect Characterization of Heavily Si-doped Molecular Beam Epitaxy-Grown GaAs by the Monoenergetic Positron Method

Monoenergetic (100 eV-30 keV) positrons were used as a defect probe for heavily Si-doped GaAs with a thickness of 300nm grown by molecular beam epitaxy (MBE). The Doppler broadening parameters showed quite different profiles for two kinds of samples prepared at 450°C and 650°C, respectively. It is indicated that the surface states are quite different for preserved GaAs and the as-etched one. The high growth temperature introduced point defects with a higher concentration. It is assumed that the defect type of X in the Si-X complex should be a gallium vacancy or a vacancy complex which suppresses the free carrier concentration in heavily Si-doped GaAs.

KEK-IMSS Slow Positron Facility

The Slow Positron Facility at the Institute of Material Structure Science (IMSS) of High Energy Accelerator Research Organization (KEK) is a user dedicated facility with an energy tunable (0.1 – 35 keV) slow positron beam produced by a dedicated 55MeV linac. The present beam line branches have been used for the positronium time-of-flight (Ps-TOF) measurements, the transmission positron microscope (TPM) and the photo-detachment of Ps negative ions (Ps−). During the year 2010, a reflection high-energy positron diffraction (RHEPD) measurement station is going to be installed. The slow positron generator (converter/ moderator) system will be modified to get a higher slow positron intensity, and a new user-friendly beam line power-supply control and vacuum monitoring system is being developed. Another plan for this year is the transfer of a 22Na-based slow positron beam from RIKEN. This machine will be used for the continuous slow positron beam applications and for the orientation training of those who are interested in beginning researches with a slow positron beam.