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23. Once the fullerenes \:!ere depos~ted In the Sudbury structure (Onapng Formaton) they were protected from oxdaton and rradaton by a surroundng sufde-slcate matrx, whch allowed them to su~iive to the present (7) 24. Th~s calculat~on uses an average cosm~c-ray-~nduced crustal production [T. E Cering and H. Cra~g Annu. Rev. Earth Planet Sci. 22, 273 ( I 994)J of 3He determined for slcates (rate of 100 atoms per gram per year) at the latitude and alttude of Sudbury The efficency of the recoil of cosmc-ray 'He from the slcate matrx Into the C , , structure s not possbe to evaluate. 25, D Lal. Eanh Planet Sci. Lett. 96. 1 (1989). The rate of 3He producton n t h s nuclear reaction s a com-pl~cated functon of LI, U, and Th concentratons. but for average crustal rocks (such as the Black Member Onapng Formaton) w t h a U content of 1.25 ppm Th/U = 4, and a Li content of 20 ppm. the rate of producton can be approxmated by the U concentration, the 'He producton rate [4He (in cubc centmeters per gram) = 2.35 X 1 0 -l 3 X U (ppm) x time (In years) and the average 3Hei'He ratlo for product~on in crustal rocks (1 x

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The size and depth distributions of pores in silica-based intermetal-dielectric materials were studied using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons were measured for methyl-silsesquioxane (MSSQ) spin-on-glass films. The size distribution of pores in the MSSQ films fabricated with 8% porogen load was found to be bimodal, with the major peaks located at 3 and 8 nm3. Increasing the porogen load from 8% to 40% caused the smaller pores (3 nm3) to disappear and 30-nm3 ones to appear; these pores were considered to be interconnected, and this structure makes it possible for positronium (Ps) atoms to find paths towards the surface and to escape into vacuum. The 8%-porogen MSSQ films had low porosity near the Si substrate. From measurements of the temperature dependence of the self-annihilation rate of ortho-Ps, we discuss the relationship between o-Ps emission into vacuum and the pore structure.