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  • 1
    ISSN: 1572-9540
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Optical isotope shift measurements have been performed at the 244f Am fission isomers (T 1/2=1 ms) with low target production rates of 10/s employing resonance ionization spectroscopy in a buffer gas cell. The isotope shift ratio IS244f,241/IS243,241 = 42.0(9) has been measured at the 500.02 nm transition. A change in the nuclear mean charge radius $$\delta \langle r^2 \rangle _{{\text{opt}}}^{{\text{224}}f,241}$$ = 5.41(31) fm2 and an intrinsic quadrupole moment $$Q_{{\text{20}}}^{{\text{224}}f}$$ = 34.4 eb have been deduced with the deformed Fermi model. The small difference of the nuclear parameter Λ244f,242f = 0.069(21) fm2 between the 244f Am and 242f Am fission isomers results in a change of the quadrupole moment $$\delta Q_{{\text{20}}}^{{\text{224}}f,242f}$$ eb only and demonstrates the stability of the superdeformation in the second potential minimum if two neutrons are removed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The atomic force microscope (AFM) was used to explore the nature of features formed on the surfaces of cracks in soda–lime–silicate glass that were held at stress intensity factors below the crack growth threshold. All studies were conducted in water. Cracks were first propagated at a stress intensity factor above the crack growth threshold and then arrested for 16 h at a stress intensity factor below the threshold. The stress intensity factor was then raised to reinitiate crack growth. The cycle was repeated multiple times, varying the hold stress intensity factor, the hold time, and the propagation stress intensity factor. Examination of the fracture surface by optical microscopy showed surface features that marked the points of crack arrest during the hold time. These features were identical to those reported earlier by Michalske in a similar study of crack arrest. A study with the AFM showed these features to be a consequence of a bifurcation of the crack surface. During the hold period, waviness developed along the crack front so that parts of the front propagated out of the original fracture plane, while other parts propagated into the plane. Crack growth changed from the original flat plane to a bifurcated surface with directions of as much as 3° to 5° to the original plane. This modification of crack growth behavior cannot be explained by a variation in the far-field stresses applied to the crack. Nor can the crack growth features be explained by chemical fluctuations within the glass. We speculate that changes in crack growth direction are a consequence of an enhancement in the corrosion rate on the flank of the crack at stresses below the apparent crack growth threshold in a manner described recently by Chuang and Fuller.
    Type of Medium: Electronic Resource
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