1995 Fiscal Year Final Research Report Summary
Study of Surfaces of Ultra-fine Particles by Using Positronium
| Project/Area Number |
06452052
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| Research Category |
Grant-in-Aid for General Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Research Field |
固体物性Ⅱ(磁性・金属・低温)
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| Research Institution | The University of Tokyo |
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Principal Investigator |
HYODO Toshio The University of Tokyo, College of Arts and Sciences, Professor, 教養学部, 教授 (90012484)
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| Co-Investigator(Kenkyū-buntansha) |
SAITO Haruo The University of Tokyo, College of Arts and Sciences, Research Associate, 教養学部, 助手 (60235059)
NAGASHIMA Yasuyuki The University of Tokyo, College of Arts and Sciences, Research Associate, 教養学部, 助手 (60198322)
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| Project Period (FY) |
1994 – 1995
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| Keywords | positron annihilation / silica / alumina / ultra-fine particle / surface / positronium / ortho-para conversion / paramagnetic center |
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| Research Abstract |
In many kinds of aggregates of oxide fine grains, positronium (Ps) atoms emitted into free space between grains interact with atoms on surfaces selectively because of its negative work function. Utilizing this phenomena, we performed (1) study of paramagnetic centers on suraces of oxide grains and (2) construction of a time-selected energy spectrometer and study of the electrons on the surfaces of silica ultra-fine particles.
(1) When paramagnetic centers exist on the surfaces of grains, ortho-Ps turns into para-Ps through spin-exchange interaction. This means that, using positronium as a probe, it must be possible to detect surface paramagnetic centers from those in the bulk. We measured angular correlation of annihilation radiation and positron lifetime spectra for silica ultra-fine particles at low temperatures under energetic positron irradiation. We found that induced paramagnetic centers can be detected using positronium, and that the surface paramagnetic centers are unstable at room temperature. Following this, we measured positron lifetime spectra in silica and alumina ultra-fine powders at low temperatures under UV irradiation. Production of surface paramagnetic centers by the UV irradiation was also found. Formation of the surface paramagnetic centers can be attributed to the abscission of O-H or C-H bond on the surfaces.
(2) When no paramagnetic centers exist on fine-grain surfaces, ortho-Ps annihilates into two g-rays through pick-off annihilation with the surface electrons. We measured the momentum distribution of the surface electrons of silica-aerogel by observing the ortho-Ps pick-off annihilation with the time-selected energy spectro-meter. We found that the momentum distrbution for the sample before heat treatment and that after heating to 800C was quite different from each other. This is interpreted as due to the removal of OH or CH_3 bases from the surfaces by heating.
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