Quenching of positronium by surface paramagnetic centers in ultraviolet- and positron-irradiated fine oxide grains
Quenching of positronium by surface paramagnetic centers in ultraviolet- and positron-irradiated fine oxide grains
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DOI：
10.1103/physrevb.60.11070
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发表时间：
1999-10
影响因子：
3.7
通讯作者：
H. Saito;T. Hyodo
中科院分区：
文献类型：
--
作者：
H. Saito;T. Hyodo
用正电子湮没和电子自旋共振（ESR）方法研究了紫外光（254 nm）和22 Na正电子辐照引起的氧化物颗粒表面正电子偶素原子与顺磁中心的相互作用。所研究的氧化物是在200 ℃下热处理，然后在800 ℃下热处理的二氧化硅气凝胶、二氧化硅粉末（Cab-O-Sil EH-5）和氧化铝粉末（Degussa Alumina C）。在低温下，~（22）Na的正电子辐照引起了所有样品正电子湮没辐射谱角相关的显著变化，表明正电子偶素原子与辐照诱导的晶粒表面顺磁中心发生了自旋交换反应。在200 ℃下热处理的二氧化硅气凝胶和Degussa Alumina C上的正电子寿命谱也由紫外线照射引起显著变化，而在800 ℃下热处理的二氧化硅气凝胶和Cab-O-Sil EH-5中没有观察到紫外线照射的变化。ESR测量表明，在200 ℃下热处理的二氧化硅气凝胶表面上形成了− OCH 2自由基。Ps原子与− OCH 2自由基碰撞的自旋交换截面估计为10− 21 m 2量级。
Interactions between positronium atoms and paramagnetic centers on oxide grain surfaces induced by ultraviolet irradiation (254 nm) or positron irradiation from 22 Na have been studied with positron annihilation and electron spin resonance (ESR) methods. Oxides studied are silica aerogel heat treated at 200 C, then heat treated at 800 C, silica powder (Cab-O-Sil EH-5) and alumina powder (Degussa Alumina C). The positron irradiation from a 22 Na of∼ 10 mCi at low temperatures induced remarkable changes in the angular correlation of the positron annihilation radiation spectra for all the samples studied, showing the spin-exchange reaction of positronium atoms with irradiation-induced paramagnetic centers on the grain surfaces. Remarkable changes in the positron lifetime spectra were also induced by the ultraviolet irradiation on silica aerogel heat treated at 200 C and Degussa Alumina C, while no change by the ultraviolet irradiation was observed in silica aerogel heat treated at 800 C and the Cab-O-Sil EH-5. ESR measurements show formation of− OCH 2 radicals on the surfaces of silica aerogel heat treated at 200 C. The spin-exchange cross section for the collisions between the Ps atoms and the− OCH 2 radicals is estimated to be of the order of 10− 21 m 2.