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The use of an electromagnetic isotope separator in preparing special research samples
Abstract
Electromagnetic isotope separator facilities at Oak Ridge National Laboratory have been used for the “direct” preparation of special research samples. Description of the separator facilities and some of the techniques in preparing samples for a variety of research activities in an electromagnetic isotope separator are included. Isotope implants in thin and thick substrates have resulted in accelerator targets and doped solid state samples that cannot be produced effectively by other techniques.
The Department of Energy national laboratories and their predecessor
organizations have played a prominent role in the development of ion
implantation technology for the past four decades. Beginning with the
birth of ion beam technology in the large-scale electromagnetic
separation of isotopes in the 1940s, the national laboratories have made
major advances in ion source technology and in the use of ion beams for
materials processing. Many of these advances have found their way into
the private sector in the form of new equipment and processes. The
history of this technology transfer is reviewed along with examples of
recent successes in ion implantation technology at the national
laboratories. The positive impact of new legislation promoting
technology transfer and the increased emphasis on making unique
facilities at the national laboratories available to outside users is
also discussed.
In the present investigation backing materials for electromagnetically separated noble gas targets are discussed. Ion beams of 22Ne and 40Ar in the energy range 2–75 keV have been shot into the common backing materials tantalum and molybdenum, and the distributions of the collected noble gas atoms in saturated backings have been studied by means of (p,γ)-reactions. Tables of experimental half-widths of the distributions are given in μg/cm2. Some measurements have been made in order to elucidate the collection process.
Sputtering ratios for three target metals, Cu, Ag, and Ta, are
determined for 45 kev ions of about 70 elements. A formula for estimation of the
collected amounts of ions in a target material is given. From this the amounts
collected in Ta are calculated and compared with experimental determinations. In
addition, selfsputtering ratios are determined for about 25 elements at 45 kev
ion energy, and for some of them also at lower energies. (auth)
Foils of Al, Mg, Mo, Pt, Au, and MYLAR were bombarded with known fluxes of ions (3H, 4He, 22Ne, 37Ar, 85Kr), and accelerated to 1–40 keV in an electromagnetic isotope separator to determine the usefulness of a hypothetical experiment to bring home a sample of solar wind by exposing foils to the solar plasma current outside the earth's geomagnetic cavity. The amounts retained in such foils after bombardment and after subjecting them to particular heating cycles in vacuum and in air (1 atm) were determined. The problems of the loss of ions in space during the proposed space experiment and of extraction of ions in the laboratory subsequent to recovery were thus studied. Aluminum seems to be an acceptable collector material for solar‐wind ions. The trapping efficiency for other materials studied is variable and appreciably smaller than unity in some cases. However, since commercial aluminum can contain significant amounts of trapped rare gases, care must be taken to prepare the collector surfaces in their absence.
The feasibility of doping silicon by directly implanting impurity atoms has been demonstrated. Both phosphorus and boron ions have been successfully implanted in silicon to produce electrical junctions. Junctions as deep as 1.2 μm have been produced by phosphorus ions having energies the order of 80 keV.An electromagnetic separator has been used to controllably produce uniform. large area implantations for evaluation experimentation. Correlations between junction depths and ion energies have been obtained. Orientation effects have been observed and it has been found that the 〈 110 〉 direction is the easy direction for implanting.Monitoring of thermal treatments of implanted substrates by sheet resistance measurement indicates a very mild thermal anneal and removes most of the damage done during the implantation. These indications are further substantiated by reverse current measurements of junctions produced by implantation.Electrical characteristics of implanted junctions will be compared with those of diffused junctions.
The present paper describes experimental work on the collection of noble gas ions of energies 5–65 keV. We have used an electromagnetic isotope separator to measure saturation values in different materials. The dependence of saturation value on different experimental conditions is investigated, e.g. variations with current density, angle of ion beam incidence and target temperature. In order to provide a simple description of the collection process, we have also measured the sputtering ratios for noble gas ions bombarding different materials. Sputtering is discussed in the light of existing theories for which a few improvements are proposed.
An analysis of the optical properties of sector magnetic fields and of the field-shaping techniques used to produce them is presented in connection with their applications to high-purity, medium-current, isotopes separators. Particular emphasis is placed on the use of axially symmetric, inhomogeneous, magnetic sector fields. The Oak Ridge Sector Isotope Separator (ORSIS), which resulted from this study, has been constructed and has successfully operated at strontium beam currents of 6.4 mA for 65 h with a beam resolution of ≈ 3000 fwhm. This separation used natural strontium metal charge having 0.56% 84Sr and produced a purified sample of 99.40% 84Sr weighing 3.7 mg.The ORSIS operates with a beam-deflecting radius of 24-in., 180° of beam deflection, field inhomogeneity of n=0.8, object and image distances of 100-in., acceleration voltage of 40 kV, and uses an electron-bombardment ion source which employs a side-extraction slit geometry. The optical techniques, which are being used to improve the resolution and throughput of this separator, are reported along with the projected improvement in machine performance expected from these changes. The first test model of this separator presently averages 1–7 mA of total collected beam current with isotopic enrichments ranging from 1000 to 60 000. These figures vary with the element being separated and the program emphasis.