The rotating ring-disk electrode (RRDE), first described almost forty years ago by Frumkin and Nekrasov, 1 has proved an invaluable tool for the study of electrochemical reactions. The RRDE consists of two electrodes in a cylindrical holder: a disk of radius r 1 (see Fig. 1) surrounded by a concentric ring with inner and outer radii r 2 and r 3 , respectively. The area between the disk and the ring is electrochemically inactive. The electrodes face downward in solution. The rotation axis passes through the center of the disk (r ϭ 0) and is perpendicular to its surface. Products generated at the disk are swept outward by the convection caused by rotation; at the ring they can be detected electrochemically. Using a bipotentiostat one measures a disk current which is generally due to two or more electrochemical reactions. By fixing the potential of the ring it may be possible to distinguish between the products from the disk, i.e., the ring current at a given potential is a measure for the rate of production of a specific product at the disk. In this way complex reactions can be studied both under steady-state and transient conditions. 2,3 For quantitative analysis the collection efficiency for the RRDE geometry used must be known; this is the fraction of product produced at the disk that is detected at the ring. Early theory of the RRDE was developed by Leveque and co-workers. 4,5 Albery 6 and Albery and Bruckenstein, 7 using boundary layer theory and applying the Levèque approximation to describe mass transport, succeeded in setting up a complete and accurate theory for determining the collection efficiency. The theory has been confirmed by experiments in which a single reaction occurs at the disk and its product is measured at the ring. 7 A survey of work on the RRDE is to be found in books by Albery and Hitchman 2 and Pleskov and Filinovski. 8 As part of a project devoted to etching under well-defined hydrodynamic conditions we were interested in etching a ring in a masked rotating disk. In order to follow the dissolution reaction in the ring shaped opening we use an outer concentric ring to detect the etch products. This is, in fact, a rotating ring-ring electrode (RRRE) geometry. To characterize this system we first considered the case in which a simple redox reaction occurs at the inner ring, i.e., no etching occurs. In this paper, we show how the theory of Albery et al. can be extended to cover the RRRE and we derive an expression for the collection efficiency as a function of the ring geometry. Details of the mathematical treatment are presented in a number of Appendices. The experimental setup used to check the validity of the theory is described in the experimental section together with the experimental results.Unstable products or intermediates formed at the disk of an RRDE can be detected at the ring. 2 Clearly, the transit time of the species, i.e., the time required for passing from disk to ring, must be shorter than the lifetime of the species. In a separate Appendix, we show that the RRRE can offer...

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The 197Au(n,?) reaction was studied with the aid of polarized thermal neutron beams. Two kinds of experimental data were obtained: i) circular polarization of gamma rays of high energy due to capture of polarized neutrons by unoriented 197Au nuclei, and ii) angular distribution of such gamma rays observed after capture of polarized neutrons by polarized 197Au nuclei. Spins of several levels of 198Au could be uniquely assigned or could be restricted. The sign of the hyperfine field of Au in AuFe has been determined to be negative.

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