The dilatations (negative) caused by neutrons, 40–600 keV electrons, 140 keV H+, D+, He+, and gamma rays are given as a function of dose and are compared. The presaturation dependence is linear for neutrons, He+, and D+, and depends on the 0.5–0.7 power of dose for H+, electrons, and gamma rays. The dilatations are not temperature dependent from about 0°–100°C, hence are not thermally activated. The dilatations are explained as compaction of the silica structure resulting from oxygens moving into some of the ``free volume,'' the structural change being similar to that occurring on pressure, shock, or thermal compaction of vitreous silica. The mechanism given for the neutron-induced compaction involves the state of high vibrational excitation developed in the slowing down of scattered atoms. The compaction by gamma rays, electrons, protons, or deuterons is explained as caused by transient ionization relaxing Si–O binding to permit oxygens to move into the ``free volume'' where they may be locked-in on recombination by similar incidents occurring nearby. The reduced power dependence is explained as a ``hardening'' effect; succeeding events are more difficult to perform or are performed with lower probability. The compaction caused by He+ (140 keV) seems to possess an equal contribution from ionization compaction and that caused by energy transfer to atomic vibrations.
An absolute determination of the superconducting penetration length in niobium has been carried out by a novel technique, in which the reflectivity of a polarized-neutron beam from a polished surface is measured. The result thus obtained, A(0) = 410 ±40 A, is in substantial agreement with the earlier work.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations –citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.