http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Probing fission time scales and dynamics via GDR [] rays and neutron angular distributions
Probing fission time scales and dynamics via GDR Gamma rays and neutron angular distributions
This dissertation presents a study of assign dynamics and time scales for the following reactions; 133 MeV ¹⁶O + ²⁰⁸Pb, 104 MeV ⁴He + ²⁰⁹Bi, 133 MeV ¹⁶O + ¹⁷⁶Yb, and 104 MeV ⁴He + ¹⁸⁸Os. Two disparate means were utilized: fission coincidence measurements of pre- and post-scission neutrons and of [] rays. These measurements were accomplished simultaneously and analyzed similarly, so as to minimize experimental differences and systematic model-dependent biases. Fission fragments were detected via large x-y position-sensitive parallel plate avalanche counters (PPAC'S) mounted in compact geometry to maximize their geometric efficiency. The fission fragment emission angles and relative velocities were used to reconstruct masses via standard kinematics. The y rays were detected by 144 BaF[] crystals from the U. S. Barium Fluoride Array (BFA). These detectors were arranged into two pods of 72 crystals placed at backward angles to minimize the neutron flux. The high granularity made it possible to reconstruct the event showers to obtain the total energy deposited by each detected y ray. The reconstructed energy spectra were then analyzed in the giant dipole resonance (GDR) region with the aid of statistical model calculations, giving the time scales for fission, []. From the []-ray data, the time scales obtained for the reactions ¹⁶O +²⁰⁸Pb, ⁴He + ²⁰⁹Bi, and ¹⁶O + ¹⁷⁶Yb were []=67 [] 10 zs, 45 [] 9 zs, respectively. Neutrons were detected by 8 liquid scintillator detectors from the DEMON Array, which were positioned around the target to ensure separation of neutrons and y rays and to obtain the multiplicities of pre- and post-scission neutrons. With the help of statistical model calculations, these multiplicities were also analyzed to extract the fission time scales, []. From the neutron data, the time scales obtained for the reactions ¹⁶O + ²⁰⁸Pb, ⁴He + ²⁰⁹Bi, ¹⁶O + ¹⁷⁶Yb, and ⁴He + ¹⁸⁸Os were []= 105 [] 10 zs, 72 []7 zs, 112 [] 12 zs, and 31 [] 4 zs, respectively.
In title, Greek letter Gamma is a symbol.
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Includes bibliographical references (leaves 159-168).