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| Dissertation / PhD Thesis | GSI-2019-00705 |
2014
Please use a persistent id in citations: urn:nbn:de:bsz:14-qucosa-157725
Abstract: Many neutron rich nuclei are involved in the astrophysical r-process (rapid neutron capture process). The r-process forms an important path for heavy element nucleosynthesis and runs along the neutron drip line. Astrophysicists suggested core-collapse supernovae within a neutrino-driven wind scenario where the neutrino wind dissociates all previously formed elements into protons, neutrons and α particles, to be a possible astrophysical scenario for the r-process. Furthermore, reaction network calculations reported a high impact of light neutron rich nuclei to the r-process abundance. Reactions on these exotic nuclei can only be studied with radioactive ion beams as their half lifes, in the order of a few hundred milliseconds (T$_{1/2}$,$_{19N}$=330ms), are too low to fabricate target material out of them. Two examples of reactions along the path of the r-process are the $^{19}$N(n,$\gamma$)$^{20}$N and the $^{20}$N(n,$\gamma$)$^{21}$N reactions. Using $^{20}$N (resp. $^{21}$N) as a beam, these reactions were studied at the GSI Fragment Separator (FRS) in time-reversed conditions via Coulomb dissociation in the S393 experiment exploiting the virtual gamma field of a lead target. The experiment was performed at the LAND/R3B setup (Large Area Neutron Detector, Reactions with Relativistic Radioactive Beams) in a kinematically complete measurement, i.e., detecting all particles leaving the nuclear reaction. The neutrons flying at relativistic velocity were observed by the LAND detector, the calibration of which plays a crucial role for the present reaction. The Smiley effect, meaning that the measured energy of impinging particles in long scintillators is not independent of the hit position of the particle, has been investigated. It will be shown that reflections of the light traveling through the scintillator and the resulting longer path length of the light when not emitted directly towards the ends of the bar were identified to cause the Smiley effect. Gamma spectra in coincidence with outgoing $^{19}$N (resp. $^{20}$N) were generated. These fit well to recent publications and were utilized to separate transitions of the projectile nucleus into the ground state or first excited state of the ejectile nucleus. The Coulomb dissociation cross section was calculated for the total reaction, transitions into the ground state and the first excited state of the ejectile nucleus. Furthermore, excitation energy spectra were derived for both reactions separately for ground state transitions and for the dominating transitions into the first excited state. In order to facilitate future experiments on exotic nuclei, two detector solutions for the NeuLAND detector (the successor of LAND) were investigated. Utilizing minimum ionizing electrons of 30MeV at the ELBE facility, time resolutions and detection efficiencies were studied for an MRPC (Multi-gap Resistive Plate Chamber) based neutron detector with passive iron converters, on the one hand, and a pure scintillator based neutron ToF detector on the other hand. The ELBE data show good time resolutions ($\sigma_{t,electron}\leq$ 120 ps) and detection efficiencies ($\epsilon_{electron}\geq$ 90%) for both systems. Small MRPC prototypes were irradiated with 175MeV quasi-monochromatic neutrons at The Svedberg Laboratory (TSL) in Uppsala measuring efficiencies of $\epsilon_{ MRPC,neutron}\approx$ 1.0%. It will be shown that MRPCs with passive steel converters may be included as neutron detectors in experiments where a lower multi-neutron capability than the one needed for NeuLAND is sufficient.
Note: Dissertation, Technische Universität Dresden, 2014
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