Mitsuhiro Irako

Construction of polarized slow-positron beams using a compact cyclotron

Abstract We have constructed a polarized positron beam using positrons provided from the β+ decay of 27Si with the half-life of 4.1 s and the maximum β+ energy of 3.85 MeV. This isotope is produced via the 27Al(p, n)27 Si reaction caused by proton irradiation using a compact proton cyclotron. The intensity of this beam is measured to be 5 × 105 e+/s for protons with the energy of 18 MeV and the current of 30 μA. A pair of Monte Carlo simulation programs has been developed to calculate depolarization of positrons in our beam channel. One program simulates spin motion of the positron under electric and magnetic fields, while the other simulates depolarization due to multiple Coulomb scattering in matter. Using these simulators, we designed a special polarimeter by means of magnetic quenching of ortho-positronium and measured a quenched lifetime of ortho-positronium under the magnetic field of 0.4, 2.5 and 4.0 kG leading to the average polarization of 33.4 ± 5.3%, which is consistent with the estimation based on the Monte Carlo simulations.

The Static Operation Mass Spectrometer

A mass spectrometer has been designed for the studies on the adsorption of CO, CO2 and O2 in a closed system. The spectrometer is a 60° sector type of 10 cm in radius, and made of Pyrex glass. The electrodes are made of gold and silver, and a tin-oxide coating is adopted as the conductive coating. The field emission cathode operated at room temperature is used as the electron emitter of the ion source in stead of a glowing tungsten filament. The above construction is based on the results of the experiments on the activity of O2 on semi-clean surfaces of the electrode materials. The spectrometer can be operated statically in ultrahigh vacuum of 6×10-10 Torr. The static operations for Ar, H2, N2 and CH4 can be made in the best condition, and those for n–C4H10, CO, CO2 and O2, which could not be analyzed statically using a spectrometer with a glowing filament, can be also made in good condition. The detectabilities for the above gases are about 5×10-8 cc N.T.P., which are satisfactory for adsorption experiments in a closed system.

Study on laser cooling of ortho-positronium

Abstract We have been theoretically and experimentally studying details of laser cooling of ortho-positronium. Experimental apparatus consists of a pulsed positron beam generator, a long pulse wide bandwidth ultraviolet laser and a time-of-flight system to measure kinetic energy of positronium atoms is constructed and tested. Using this apparatus, production of thermally activated positronium is confirmed. Its production rate is larger as the target temperature increases and ratio of thermally activated ortho-positronium to all γ-ray annihilation events is estimated to be 32.3±6.7% for the target temperature 1000 K.

Construction of a time-of-flight measurement system to study the low energy positronium production

Abstract To investigates the low energy positronium (Ps) production in terms of target temperatures and materials, surface condition and so on, we adopt the time-of-flight (TOF) method and perform extensive Monte Carlo simulation to design an apparatus. Under reasonable assumptions in the simulation with respect to an initial energy distribution of Ps, we examine how cleanly ortho-positronium (oPs) is observed without being subject to large backgrounds of 2γ decay from e+e− annihilation. The simulation finds that the contribution of 2γ events is suppressed by 10−6 smaller than the generated Ps by using the 2.5-cm thick W and 4.6-cm Pb collimators with slit gap of 1 mm. We inject slow positron beams on the W target installed in the TOF system and move the position of the target. Then we expect the position resolution of 1.5 mm resulting the accuracy of determining the oPs velocity 20%, corresponding to 1.1×106 cm/s for thermal Ps with 30 meV.

Angular Distribution of Scattered Electron and Medium Energy Electron Spectroscopy for Metals

The angular distribution (AD) of scattered electrons produced by medium energy incident electrons ( E P =50∼300 eV) from polycrystalline Ti, Fe, Ni, Cu and Au were obtained by the angle-resolved medium energy electron spectrometer. The AD of the energy loss peaks are similar figures to AD of the elastically reflected electron peaks. Therefore, the exchanged electrons produced by the knock-on collision between the incident electrons and those of metals without momentum transfer are observed as the energy loss spectra (ELS). This interpretation differs from the inconsequent interpretation by the dielectric theory or the interband transition. The information depth and penetration length are obtained from AD of the Auger electron peaks. The contribution of the surface to spectra is 3% at the maximum for E P =50 eV. The true secondary peaks representing the secondary electron emission spectroscopy (SES) are caused by the emissions of the energetic electrons (k T e ≧4 eV), and SES is the inversion of ELS. The e...

Bunched beam generation of polarized positrons in TOPPS-II

Abstract We have constructed a bunched beam generator of polarized slow positrons, namely TOPPS-II (Tokyo Metropolitan University Polarized Positron Beam System) using the magnetic transportation. The positrons emitted from 100 mCi 22 Na are implanted on the tungsten moderator of 6 μm thickness. Reemitted positrons, accelerated to 200 eV and guided with 100 G magnetic field of solenoid coils, are chopped to be 40 ns every 200 ns and bunched into 2 ns time width. In this paper, we present the results of the overall performance of TOPPS-II.

Laser cooling system of ortho-positronium

Abstract We have studied laser cooling (Doppler cooling) of ortho-positronium to achieve Bose–Einstein condensation of positronium atoms. We have been developing a long-pulse laser with the wavelength of 243 nm , a bunched slow positron beam to realize the proposed experiment and a Monte Carlo simulation program for theoretical understanding. The Monte Carlo analysis clarified that we were able to obtain 7% of ortho-positronium atoms which were cooled down to 1 K and confined within a small volume without using a trap system.

Development of a positron trap for the laser cooling experiment of positronium

A cloud of ortho-positronium produced at the room temperature can be cooled down to 0.6 K utilizing a long pulse (180 ns) laser with 243 nm wavelength. We developed a time bunching system of a slow positron beam to synchronize production of positronium atoms to the laser pulse. After a theoretical examination using a Monte Carlo simulation, an experiment was made for confirmation.

Time bunching of slow positrons for lifetime and time-of-flight measurements of ortho-positronium

Abstract On the basis of velocity modulation, we have developed a positron-beam buncher for the measurement of ortho-positronium lifetime τ o−Ps and TOF. Since high permeability cores were used, the buncher was constructed compactly and was used to efficiently produce a bunched beam with a bunch width of 2.2 ns (FWHM) and a repetition period of 960 ns . We have measured the lifetime of ortho-positronium using the buncher and obtained τ o − Ps =127 ns . In this paper, we report on the technical details and performance of the buncher.

Study on Spin Precession of Polarized Slow Positrons in Solids

We have investigated spin precession mechanisms of polarized slow positrons in materials caused by multiple Coulomb scatterings, Bhabha scatterings and bremsstrahlungs. It is found that the degree of depolarization is strongly dependent on the atomic number of materials, and the depolarizations caused by the Bhabha scatterings and the bremsstrahlungs are negligible compared with those caused by multiple Coulomb scatterings. By incorporating the theoretical formulations of spin precession due to the three processes mentioned above into a Monte-Carlo simulation program, we can successfully describe depolarizations of polarized slow-positron beams in materials, i.e. a target, an absorber, and a moderator which are utilized for slow-positron production facilities. The magnitudes of the polarizations thus estimated are in good agreement with experimental measurements.