School of Physics - Theses

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Type: PhD thesis × Start date: 1980 × End date: 1999 × Author: Braunshausen, Gerhard ×

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    Direct observation of charged particle tracks in crystals
    Braunshausen, Gerhard ( 1995)
    From a general point of view in material science, it is interesting to know, how swiftly moving, heavy ions (such as those ejected from spontaneously occuring nuclear fission events inside natural minerals) interact with solid bulk matter. For example, in geological mineral deposits (such as zircon, apatite, and sphene) one often finds that naturally radioactive isotopes (uranium, thorium etc.) are included in varying degrees of abundance. In particular, single 238 U atoms have a small but finite probability to disintegrate through a spontaneous nuclear fission event, thereby ejecting two highly charged and comparably heavy nuclear fragments with about 250Me V kinetic energy. This fact became known only about 30 years ago when straight-line, needle-like contrast features were observed in electron micrographs of mica (S&B 59, P&W 62). These so called "fission tracks" originate through the damage incurred on the host material containing naturally radioactive isotopes by the fast moving, charged and heavy fission-fragments penetrating the matrix in two oppositely directed straight lines. The length of the tracks is related to the penetration range of the ions, which in turn is determined through the initially available kinetic energy, the atomic number, and charge-state of the projectile as well as the mean density of the target material. (Because the energy level difference between the initial and final state of the nuclear decay event is constant on average, a fixed amount of kinetic energy is available for the two fragments. This fact requires the fission tracks to have a more or less constant longitudinal dimension of around 50µm.) Their thickness, however, could not be determined accurately up to very recently because the image quality remained poor due to the technical limitations in electron-optical engineering in the first few generations of electronrnicroscopes which were commercially marketed after their pioneering development by Ruska some sixty years ago (Rus 49). With the improved performance of the recent generation of high-voltage microscopes a renewed attempt was made to determine a) the electron optical conditions of making fission tracks (or at least short sections thereof) visible in their latent form through transmission electron microscopy, employing a variety of contrast mechanisms, b) the geometrical parameters of the track morphology in the particular instance of 208Pb20+ ions penetrating slabs of zircon crystals at an initial kinetic energy of 14MeV/u, c) the structural conditions as well as the nature and mechanism of the damage formation. Lastly, it was also hoped to be able to image the damaged regions showing atomic-size detail in High Resolution Electron Microscopy (HRTEM), thus making the native structure of the damaging effects of nuclear radiation penetrating bulk matter directly visible for the first time.