Yoshiko Itoh

Positron annihilation in porous silicon

Three lifetime components, one of which is extremely long (25±2 ns), have been observed in experimental studies of positron annihilation in porous silicon, made by anodization in hydrofluoric acid. The Doppler‐broadened spectrum of the porous silicon is sharp compared with that of crystal silicon and becomes even narrower in an applied magnetic field. The positronium yield in the porous silicon therefore is concluded from the long lifetime, narrow Doppler spectrum and its narrowing in a magnetic field. The porous structure is the cause of positronium formation.

Behavior of oxygen in the crystal formation and heat treatment of silicon heavily doped with antimony

Using charged particle activation analysis, oxygen concentration in silicon crystals heavily doped with antimony has been shown to be considerably lower than in undoped crystals grown under otherwise the same conditions. The difficulty of oxygen precipitation in these doped crystals is therefore due to the lowered oxygen concentration and also to the difference in the dopant effect on precipitate formation.

CPAA Study on Carbon, Boron and Oxygen in LEC-GaAs

Carbon, boron and oxygen in liquid-encapsulated Czochralski(LEC) semi-insulating GaAs crystals grown in three different ambiences (Ar, Ar+1% O2, and Ar+1% CO) were analyzed by charged particle activation analysis (CPAA) using the 12C(d, n)13N, natB(d, xn)11C, and 16O(3He, p)18F reactions. An addition of 1% O2 or CO gas to a conventional Ar ambience during crystal growth was found to increase C contamination by a factor of up to 35 (1.8× 1016 cm-3), while B and O concentrations remained unchanged for the most part.

Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×1018 cm−3) LEC-GaAs was measured before and after irradiation with protons (dose 1×1015/cm2, 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite GaAs. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as SiGa-VGa complexes cannot be annealed out above 973 K.

Slow positron production using the RIKEN AVF cyclotron

Using the RIKEN AVF cyclotron, various targets were irradiated with proton or deuteron beams to produce β+-decay radioisotopes. Slow positrons are extracted backward at an angle of 30° to the incident ion beam using a well annealed W film, with a thickness of 10 μm, as a moderator. This geometry was chosen to allow for both the use of various kinds of target materials and the in-beam slow positron extraction during ion beam irradiation. In preliminary experiments, a slow e+ intensity of ∼5.6 × 104 e+/s was obtained from BN by irradiating protons with an energy of 14 MeV and a current of 4 μA. From an Al target irradiated with a proton beam with an energy of 10 MeV and a current 3.6 μA, a slow e+ intensity of ∼5.5 × 102 e+/s was also obtained. By irradiating a deuteron beam with 8 MeV energy and 1.3 μA current, slow e+ intensities of ∼4.2 × 103 e+/s and ∼3.4 × 103 e+/s were obtained from carbon and Si3N4 respectively. Using a pulsed ion beam, background noise was eliminated almost completely in three-coincidence measurement.

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was ∼0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×1018 cm−3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.

Positron annihilation study on nanometer cavities in porous silicon

Measurements were carried out on the positron lifetime and the Doppler broadening of its annihilation radiation in porous silicon. A very long lifetime of a few tens of nanoseconds was found. TheS parameter increased upon annealing in vacuum at 350 °C. It is pointed out that positron/positronium spectroscopy is very useful for the study of physical and chemical properties of porous silicon.

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.

18F intense spot positron source for spin polarized positron beam

Abstract An intense positron source, 18 F , with a half life of 110 min has been developed for spin polarized positron beams. The radioactive 18 F (ca. 30 GBq) is produced in 1 ml 18 O -water target via the 18 O (p,n) 18 F reaction with a 14 MeV proton beam at a current of 20 μA in the AVF cyclotron vault. An irradiated 18 O -water is transferred to an experimental vault downstairs to be trapped and placed just in front of the moderator. The electrodeposition method was used to collect and fix 18 F on a small spot from 18 O -water. Recovery of 18 F on the graphite cylinder of 5 mm ∅ is 69% and 35% for 3 mm ∅ cylinder, respectively.

Slow positron beam production by irradiation of p+, d+, and He2+ on various targets

Slow positron beams were extracted by irradiating various targets with p + , d + and He 2+ ion beams from the AVF (azimuthally varying field) cyclotron of RIKEN. The targets were selected considering the large cross section and high maximum energy of the β + -decay. Two different geometries of the target and moderator (one W sheet) arrangement, reflection-type and transmission-type were used and a yield of ∼ 10 4 slow e + /s/μA was obtained. However, in both geometries loss of a fraction of the fast positrons by self-absorption in the target is unavoidable. For increasing the number of available fast positrons we have designed a gas target (N 2 ) system. The estimated slow positron yield is 1.5 X 10 5 e + /s/μA using a 16 MeV proton beam and a one W foil moderator.