L. O. Love’s research while affiliated with Oak Ridge National Laboratory and other places

Isochronal anneal studies have been made on n- and p-type germanium bombarded with germanium and several substitutional impurities, using the four-point and thermoelectric probes as means of indicating changes in sheet resistivity and majority carrier type. Electrical properties of diodes made from type converted samples were also determined. All bombardments were performed at 40 keV in a 24-in.-radius electromagnetic isotope separator at room temperature. These studies revealed the following. (1) Heavy-particle bombardment of germanium introduces acceptor states regardless of the identity of the bombarding species. (2) Annealing curves for n-type germanium indicate a two-stage process; details of the annealing process are dependent on the ion species used and the crystal orientation bombarded. (3) There is a crystallographic and mass dependence on the amount of change in sheet resistivity produced in n-type germanium. (4) Type conversion temperatures in p-type germanium were observed to occur at 450–475 °C for phosphorus, ~500 °C for arsenic, and slightly greater than 500 °C for antimony, suggesting that the ease of moving a particular species into an active lattice position is related to its atomic size. (5) Voltage–current and capacitance measurements indicate that large-area diodes of relatively good properties and reproducibility can be made by ion implantation in germanium.

A description is given of the current isotope separations program at Oak Ridge National Laboratory emphasizing new techniques and developments which have demonstrated improvement in isotopic purity, ion output, and process efficiency. Among the topics discussed are the use of tungsten-wire electrodes as a replacement for conventional graphite electrodes; the in-situ use of ClF3 as a halogenating agent in the isotopic separation of Ru, Pd, and Os; and the production and enhancement of the radioactive isotopes, 41Ca and 205Pb, with higher process efficiencies than are normally attained. Developmental activities include the source-slit system modifications for the 255° separators. Brief descriptions are also given of special efforts to achieve higher purity materials such as 124Te, 122Sn, and 184W. The continued use of a 180° sector isotope separator for preparing high isotopically enriched targets for physics research will also be described.

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.

Numerous instruments have been constructed to achieve mass separation by electromagnetic means, the most productive being the calutron type. The sector instrument, used at many laboratories to supply smaller quantities of material, has been designed for economical construction, low beam current, and the achievement of a more nearly perfect mass separation. An engineering approach to improving enrichment capabilities in sector-type isotope separation involves the construction of multistage instruments. Extension of this concept has resulted in a multistage calutron-type separator, constructed for use in an existing calutron tank. The ion source accelerates the beam across magnetic field lines at a slight angle to produce a helical path having a reference radius of 24 in. Beam components focus at the 180° position where all isotopes are collectible except one which is allowed to continue its helical trajectory. This selected isotopic beam is allowed to pass through an additional 360° of travel before being collected at the 540° position. Ion output to the 540° collector has reached a maximum of 750 μA, and the isotopic purity of 125Te from the third separation run is 98.95% compared with 95% resulting from best performance in a 48-in.-radius calutron.

An ORNL electromagnetic isotope separator (calutron) has been operated with milligram amounts of uranium charge material. A series of nine experimental runs using 1 to 50 mg of charge (material) achieved process efficiencies ranging from 1 to 13%. In one of the experimental runs, 235U was enriched from 0.72 (normal abundance) to 94.3% in a single pass.

The electromagnetic separations group at Oak Ridge National Laboratory (ORNL) has collected and purified isotopes for special purposes, including 46Ca, 48Ca, 182W, 186W, 33S, 29Si and 30Si. Certain contaminants have been reduced to the ppb-range in some preparations. Experiences in preparing ∼ 10-g crystals of 48CaF2 and > 100-g cylinders of tungsten isotopes are given.The results of experimental ion collections on single crystals of copper and on various other substrates are presented. These collection techniques are applicable for preparation of specimens for diffusion studies, the formation of semiconductors, etc. Singly or doubly charged ions can be deposited with a considerable choice of ion velocity. In other preparations, actinide isotopes have been electrodeposited uniformly on a variety of metal forms.

Special equipment has been designed at the Oak Ridge National Laboratory (ORNL) for the calutron processing of moderate quantities of α,γ-emitting materials. A 73-h run having an average ion output of 2.9 mA provided 71 mg of 20.4% 242mAm from an americium sample containing ∼ 1% of the nuclide. Other elements which have been separated in the special electromagnetic containment facility at ORNL include plutonium, thorium and uranium. Some separations were made to reduce the unwanted isotope to the ppm- or ppb-level.

Electromagnetic isotope separations at Oak Ridge National Laboratory (ORNL) have been improved for a number of elements by more selective choice of charge materials, more precise vapor feed controls, more efficient pumping systems and more effective chemical trapping techniques in the collector.The use of special feed materials has resulted in increased outputs and higher isotopic purities for such elements as silicon, sulfur, ruthenium, tin and tungsten. An internally valved feed system has permitted the use of volatile fluorides of ruthenium and iridium in conventional sources and resulted in more successful separations at higher outputs. For some elements, particularly with isotopes of low abundance, losses from the receiver pockets due to the formation of volatile compounds by reaction with residual gases have been reduced by chemical pumping and use of different charge compounds. Chemical pumping has significantly reduced contamination of collected material by the normal element. New receiver concepts are being employed for collecting isotopes of high-vapor-pressure elements and are providing appreciable increases in isotopic purity.

Methods of identifying problems occurring in the various steps of the electromagnetic separation of isotopes at the Oak Ridge National Laboratory (ORNL) have been developed. Most variations in isotopic purity can be traced to separator (calutron) performance, with only minor variations in purity introduced by chemical or mass analysis procedures. The contamination approach used to evaluate and interpret routine and experimental results has been programmed for use in computers. The knowledge of calutron behavior gained in this manner has been applied in efforts to improve the separation process and minimize the number of mass assays required.Experimental work toward stabilizing ion beams is described; however, the improved stability appears to be of value only in separating isotopes of the heavy elements. Simple techniques for detecting charge exchange, for studying changes in contaminating ion currents produced by experimental modifications and for evaluating charge recovery processes are described.