Technological Applications of Ion Tracks in Insulators
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Hole Engineering Track etching depends on accelerated dissolution occurring along each damage track at a rate i>T that is greater than the general attack rate vG of dissolution of undamaged material. The result is that a conical hole with a half cone angle 6 [= sin"1 (vG/vr)] is formed.4 In some materials such as calcite (CaCO3), lithium fluoride (LiF), and some glasses, vT is only somewhat greater than vG and 8 therefore is large (~45° for lime glass). In others, such as polycarbonate plastic, phosphate glasses, and muscovite mica, Vj » Vc- Therefore, 0is nearly zero, giving straight holes of nearly constant bore.3 In some plastics, exposure to ultraviolet light or aging in an oxygen-rich atmosphere further reduces 6. Filters of controlled cylindrical porosity and highly uniform hole diameters are made by irradiating thin sheets of plastics to a specified dose and then etching until the desired hole diameters are produced. 5 Such filters were first made experimentally at the General Electric Research Laboratory as PIE (plastics—irradiated and etched) Filters, then at General Electric's Vallecitos Nuclear Laboratory as Nuclepore Filters,
MRS BULLETIN/DECEMBER 1995
which is the name they are now sold under by the Nuclepore Corporation in Pleasanton, California. In a continuous process, a roll of plastic sheet is threaded past plates containing 235U within a nuclear reactor, and the fissions of the uranium supply heavy ions that cross the plastic. Filters with hole sizes ranging from 10 nm to more than 10 jtm are available commercially. Figure 1 shows 5-/um holes through a polycarbonate film. These filters have a diversity of uses that include straining out particles from solutions that are to be used on delicate semiconductor surfaces for electronics applications, isolating oversized biological cells or aerosol particles on simple
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Figure 1. A filter with uniform 5-^j.m holes made by irradiating a polycarbonate membrane with fission fragments at normal incidence and then etching the resulting radiation damage tracks.5
surfaces that are suitable for viewing with electron microscopes, and a multitude of scientific tests of physical principles on samples of small dimensions.
Single Holes The most economical uses of holes made by track etching employs a single hole to measure small objects with high precision by monitoring the electrical resistance of an electrolyte in a hole as the object passes through.6 This procedure, the resistive pulse technique or "DeBlois-Bean counter," named after its inventors, can recognize odd shapes. Figure 2 shows sea-urchin sperm and the resistive trace as two sperms pass through the hole—one head first, the other tail first. The large part of the pulse is from the head passing through the hole, and the longer "tail" on the pulse is from the time when only the tail of the sperm is in the hole. From these measurements, DeBlois7 could infer that the volume of the head is 5.5 (±0.5) ftm' and that the tail has a uniform diameter of 0.26 (±0.01) fim. The same technique, by using
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