Study of Surfaces of Ultra-fine Particles by Using Positronium

在许多氧化物细颗粒聚集体中，发射到颗粒间自由空间的电子偶素（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）时间选择能谱仪的构建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，通过自旋交换作用，邻位Ps转变为对位Ps。这意味着，使用正电子素作为探针，它必须能够检测表面的顺磁中心从那些在块。在低温下测量了二氧化硅超细粒子在高能正电子辐照下的湮没辐射角关联和正电子寿命谱。我们发现，诱导顺磁中心可以检测到使用正电子素，和表面的顺磁中心是不稳定的，在室温下。在此基础上，我们测量了二氧化硅和氧化铝超细粉体在低温紫外光照射下的正电子寿命谱。紫外光照射后，表面产生了顺磁中心，表面顺磁中心的形成可归因于表面O-H或C-H键的断裂。(2)当细晶粒表面上不存在顺磁中心时，正Ps通过与表面电子的拾取湮灭而湮灭为两个g射线。用选时能谱仪观测了二氧化硅气凝胶的ortho-Ps拾取湮没过程，测量了气凝胶表面电子的动量分布。我们发现热处理前的样品和加热到800 ℃后的样品的动量分布有很大的不同。这可能是由于加热使表面的OH或CH_3基被除去所致。

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.