CHEMICAL EFFECTS OF THE ⁷⁹Br(?, 2n) ⁷⁷Br NUCLEAR REACTION IN K₂ReBr₆-K₂ReCl₆ MIXED CRYSTALS

Abstract
In continuation of investigations of the chemical fate of (n, ?) and (?, n) produced Re recoil atoms in K₂ReBr₆-K₂SnCl₆ [1, 2] and K₂ReBr₆-K₂OsCl₆ [3] mixed crystals and IT and (n, ?) produced Br recoil atoms in K₂ReBr₆-K₂SnCl₆ [4] and K₂ReBr₆-K₂ReCl₆ [5] mixed crystals we have investigated recently the chemical effects of ⁷⁷Br recoil atoms in the K₂ReBr₆- K₂ReCl₆ system activated by the nuclear reaction ⁷⁹Br (?, 2n)⁷⁷Br. By using ion exchange column chromatography as a new method of separation of the mixed Re⁷⁷BrBr_5-nCl^2-_n species larger amounts of the irradiated material could be separated following the dissolution. The yield of Re⁷⁷BrBr^2-₅ increases, the yield of Re⁷⁷BrCl^2-₅ decreases significantly with increasing K₂ReBr₆ concentration, all the other mixed hexabromochlororhenates and the free bromide-(⁷⁷Br) are only little dependent of the mixed crystal composition at least at concentrations below 50 mol-% K₂ReBr₆. For zero K₂ReBr₆ content we obtained by extrapolation : ⁷⁷Br^- 16 % Re⁷⁷BrBr^2-₅ 3 % Re⁷⁷BrBr₄Cl^2- 5 % Re⁷⁷BrBr₃Cl^2-₂ 5 % Re⁷⁷BrBr₂Cl^2-₃ 6 % Re⁷⁷BrBrCl^2-₄ 16 % Re⁷⁷BrCl^2-₅ 49 % The total bromine content (active ⁷⁷Br and inactive ^79,81Br) of the recoil labelled species amounts to 31 % at zero K₂ReBr₆ concentration clearly indicating that on the average two bromide ligands (one of them being ₇₇Br, the other inactive) of the original six survive the nuclear process and are still bonded to their original Re central atom. Especially the appearance of the more intimately mixed species cannot be explained by models discussed hitherto in the literature [6] (billiard ball impact, ligand vacancy exchange followed by combination [7], hot spot reaction). We propose here a new mechanism which we want to call the Inverse Cage Effect. Following this model Br recoil atoms that are directed towards the central atom transfer their kinetic energy via the central atom to all or part of the other ligands which are driven in the surrounding lattice. The newly formed vacancies are then filled by halide ions from the environment including the ligands just pushed away. The experimental results may be summarized as follows : Besides a very small yield of primary retention (Re⁷⁷BrBr^2-₅) one third of the recoil bromine atoms react following the inverse cage effect, two third are caught as interstitials (⁷⁷Br^-) or undergo simple displacements (Re⁷⁷BrCl^2-₅). The recoil energy of the ⁷⁷Br is estimated to be not larger than ca. 1 keV. This suggests that the nuclear reaction ⁷⁹Br(γ, 2n)⁷⁷Br is far from a compound nuclear mechanism.