Positronium Decay in Silica Sol-Gels
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POSITRONIUM DECAY IN SILICA SOL-GELS B. HOPKINS, C. A. QUARLES and T. W. ZERDA Physics department, Texas Christian University, Fort Worth, TX 76129 ABSTRACT Positronium lifetimes inside porous silica are measured and discussed in terms of surface interactions, oxygen and nitrogen induced quenching and pore sizes. Surface interactions are studied for silica gels having surfaces covered solely with hydroxyl or methoxy groups. A rapid decay of positronium inside the pores filled with gaseous or liquid oxygen is reported and explained in terms of quenching mechanisms. An increase in the measured lifetimes for pores filled with nitrogen is attributed to the change in the pore distribution function. INTRODUCTION Silica gels are networks of interconnected irregular silica particles with large numbers of voids or pores permeating the structure. The concentration of pores is on the order of 1018 pores/gram, with pore diameters ranging from about 10 to 1000 angstroms. The high concentration of pores accounts for the gels large surface area, in some cases in excess of 900 m2/g. These properties make the gels excellent hosts for studying surface interactions. Positronium is a short lived bound system composed of a positron and an electron. The positronium "atom" occurs in two ground states. Parapositronium (p-Ps) is the singlet state with total spin of zero. It has a self annihilation lifetime in vacuum of 0.125 ns and decays via 2 gamma emission. Orthopositronium (o-Ps) is the triplet state with total spin of one. Its free space lifetime is much longer, 140 ns, and decays via 3 gamma emission. The lifetime of parapositronium is comparable to the direct annihilation lifetime of positrons, making practical measurement difficult. O-Ps, however, survives long enough to interact with surrounding atomic and molecular electrons. This interaction leads to a shortening of its lifetime through various processes called quenching. Measurement of this shortened lifetime can thus yield information about the physical and chemical properties of the medium sampled by o-Ps. Three types of quenching are possible. The positron of the positronium atom may directly annihilate via 2 gamma decay with a nearby atomic or molecular electron. This process is known as pickoff quenching. Alternately, the positron may encounter an unpaired electron which converts the orthopositronium atom to parapositronium (or vice versa). O-Ps converted to p-Ps then quickly decays via 2 gamma emission. This process is called conversion quenching. O-Ps may also reduce its lifetime by reacting chemically with its environment. This process is known as chemical quenching. Positrons entering silica particles may form o-Ps and migrate into the pores, where they remain trapped and subsequently decay. These decays contribute to the long lived intensity component, I, characterized by an exponential decay I = B exp(-tlt) (1) At low densities, 1/i increases with the concentration of the quenching substance, and to a first approximation, for thermalized positronium, the decay rates
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