The majority of the light elements up to iron (Fe) are formed by successive rounds of ther-
monuclear fusion burning in the stellar interiors. The nuclei heavier than iron (Z>26) are
being synthesized mainly by neutron-capture reactions - the astrophysical r-and s-processes.
There are 35 neutron de¯cient stable isotopes between Se and Hg which are shielded from the
rapid neutron capture by stable isobars. These so-called p-nuclei are produced in explosive
stellar environments via photodisintegration reactions like (°,n), (°,p) and (°,®) on r- or s-
seed nuclei. The reaction rates of the p-nuclei are mostly based on theoretical parameteriza-
tions using statistical model calculations. At the bremsstrahlung facility of the superconducting
electron accelerator ELBE, photon-induced reactions of the p-nuclei are being studied.
In the scope of this thesis work, photodisintegration measurements of the p-nuclei 92Mo and
144Sm have been performed via the photoactivation technique. The residual nuclei resulting
from photoactivation were studied via °-ray spectroscopy. For the decay measurements of
short-lived nuclei, a pneumatic delivery system has been used. In the case of 144Sm(°,p) and
144Sm(°,®) reactions, the activated samarium samples with very low counting statistics were
measured at the underground laboratory "Felsenkeller" in Dresden. The experimental activa-
tion yields for the 144Sm (°,n), (°,p) and (°; ®) and the 92Mo(°; ®) reactions were determined.
It is to be emphasized that the (°,p) and (°; ®) reactions were measured for the ¯rst time in a
laboratory at astrophysically relevant energies.
In all the mentioned experiments, special care was taken to determine the endpoint energy of
the bremsstrahlung spectra by using the photodisintegration of deuteron. The 197Au(°,n)196Au
reaction has been established as an activation standard. The photoactivation yields for the
197Au(°,n) and 144Sm(°; n) reactions have been compared to the yield calculated using cross
sections from previous photoneutron experiments. A comparison of the two data sets leads to
a conclusion on the inaccuracies in previous data. The statistical uncertainties involved in the
activation experiments are very small except for the case of decay spectra with weak counting
statistics. The systematic uncertainties are mostly from the experimental determination of
photon °ux. A detailed discussion of the overall uncertainty is provided.
Hauser-Feshbach statistical model calculations using TALYS and NON-SMOKER codes have
been performed for all the concerned reactions. The experimental activation yields, in general,
agree within a factor of 2 to the simulated yields using statistical model predictions. The
sensitivity of the model codes to the nuclear physics inputs like optical-model potentials, nuclear
level densities and °-ray strength functions has been tested.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-24875 |
| Date | 19 October 2009 |
| Creators | Kumaran Nair, Chithra |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. E. Grosse, Prof. Dr. E. Grosse, Prof. Dr. A. G. Zilges |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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