No abstract

No abstract

The influence of the atomic electron cloud on the gamma decay constant 2~ was evaluated in third-order approximation according to the theory of Krutov for some highly converted nuclear transitions (resonance case only). The results are 2~3)= 1.5 x l0 s 2~ 1) for the 73 eV transition of U-235m and 2~,,. 3J= 1.01 j[~l) for the 2.17 keV transition of Tc-99 m. Values calculated for Z= 80, E~ =2 keV are in good agreement with those of Krutov. Experimental evidence of the calculated effects does not seem to be obtainable at present because of the extremely low gamma intensity of the transitions in question.

No abstract

The mobilization of dust (i.e., detachment and removal of dust grains from a substrate) and the exfoliation of a film of erosion products in tokamaks have been studied theoretically. The following mecha nisms of dust mobilization have been taken into account: (i) sharp heating (thermal shock) as a result of, e.g., plasma disruption and edge instabilities; (ii) substrate vibrations; and (iii) gas and plasma flow (wind) action. The most effective mobilization takes place under the action of sharp heating. Power fluxes that are charac teristic of edge instabilities can mobilize dust grains with dimensions within or even greater than 0.1-1 µm. The velocities of detached grains reach m/s for heavy grains and up to v ~ 300 m/s for the light ones. Conditions favoring the exfoliation of a film of erosion products are determined. It is shown that exfoliation under the action of edge instabilities can take place at a film thickness of h > 1 µm. Under the action of ther mal shock induced stresses, the exfoliated film flakes with a size ranging from fractions of a millimeter to sev eral centimeters break into pieces.

This paper is intended to provide a concise introductory review of the fundamental principles underlying the higher-order effects of nuclear decay rate variations which may be produced by physical or chemical means. The first part of the paper embraces the theoretical foundations of the subject matter, and the second deals with methodological and experimental questions. Several tables summarize published experimental results and the pertinent literature.V a r ia tio n s in N u c le a r D e c a y R a te s IntroductionThis paper aims at providing an introductory review of the fundamental principles underlying the small but measurable variations in nuclear decay rates produced by physical or chemical means. Special emphasis is laid on consistency and con ciseness. For this reason the discussion has been restricted to the principal modes of disintegration of alpha, beta, and gamma decay including electron capture and internal conversion for which decay rate changes were predicted by Daudel and Segre as long ago as 1947.For ease of reading, the paper has been split into two main parts the first one embracing the theoreti cal fundamentals, the second dealing with method ological and experimental questions. Several tables have been included in the second part summarizing published experimental results and the pertinent literature.Earlier reviews of the current subject have been presented by several authors, among them DeBenedetti et al.1 and Emery 2. References on work pub lished before 1947 will be found in those articles and have not been replated in our list of references. Theoretical Fundamentals of Nuclear DecayRate Variations General RemarksThe time-dependent Schrödinger equation applied to a perturbed quantummechanical system and solved by Dirac's perturbation method yields in first-order approximation for the transition probability per unit time for a change in the system from the initial state i to one of the accessible final states f 3 Wn = (2 jz /h ) \H'n \*-Q(E{)Reprint requests to Dr. M. Nagel, Zentralinstitut für Iso topen-und Strahlenforschung der AdW der DDR, Permoserstr. 15, DDR-705 Leipzig.where Q (Ef) is the energy level density of the final states and U \x the matrix element of the perturba tion H' causing the transition. (1) is known as Fermi's "Golden Rule No. 2". It is this expression which will be the starting point of the present dis cussion of nuclear decay rate variations. Only alpha decay will be an exception since this case is most conveniently discussed within the framework of Gamov's theory. In deriving explicit expressions for the decay constant A, a rigorous treatment must always take into account the influence of the atomic electrons. However, a rough approximation of the type nor mally found in textbooks on nuclear physics is not sufficient for the present purpose. Variations in the decay rate dealt with in this paper are due to changes in the structure of the atomic electron cloud which may be caused by chemical binding effects etc. Therefore, sufficiently accurate formulae must be d...

The tetrahedral and octahedral complex ions TcOi-, TcC12 . TcBr 2-, TcI 2-, which exhibit experimentally determined decay constant variations of Tc-99m [1], meet the conditions for the application of the X~-SW-method. The results of the calculations are given, including the case of TcF62 . The decay constant variations are evaluated using the one-electron functions in a partial-wave expansion. The results are compared with the measured values and a detailed discussion gives a comprehensive picture of the decay constant variation induced by the chemical surroundings considered.

The chemical induced decay constant variations for some complex compounds of Tc-99 m were measured by means of a semidifferential approach, using the scintillation technique. The results areTheoretical estimations by means of atomic structure calculations show that Tc04~ should have the greatest decay constant among the compounds investigated, in accordance with the measured values, and that the decay constants of the octahedral complexes should follow the spectrochemical series, partly confirmed by the experiments.