Study of Surfaces of Ultra-fine Particles by Using Positronium

在各种氧化物细颗粒聚集体中，发射到颗粒间自由空间的正电子(Ps)原子由于其负功函数而选择性地与表面原子相互作用。利用这一现象，我们进行了(1)氧化物颗粒表面顺磁中心的研究和(2)时间选择能谱仪的建立和二氧化硅超细颗粒表面电子的研究。(1)当颗粒表面存在顺磁中心时，正电子通过自旋交换作用转变为对电子。这意味着，使用正电子作为探针，必须有可能从块体中探测到表面顺磁中心。在高能正电子辐照下，测量了二氧化硅超细粒子在低温下的湮没辐射角关联和正电子寿命谱。我们发现利用正电子可以探测到诱导顺磁中心，并且表面顺磁中心在室温下是不稳定的。随后，我们测量了二氧化硅和氧化铝超细粉末在紫外光照射下的低温正电子寿命谱。还发现了紫外光照射产生的表面顺磁中心。表面顺磁中心的形成可以归因于表面上O-H键或C-H键的断裂。(2)当细晶表面不存在顺磁中心时，正电子通过与表面电子的拾取湮没而湮没成两条g射线。我们用时间选择能谱仪观测了正电子激发态吸收湮没，测量了二氧化硅气凝胶表面电子的动量分布。我们发现，样品在热处理前和加热到800℃后的动量分布有很大的不同。这被解释为由于加热从表面除去了OH或CH_3碱。

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.