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.

The CNO cycle consists of a set of nuclear reactions that convert hydrogen into helium and releases energy in stars. The cycle contributes less than 1% to our Sun's luminosity, but it is responsible for detectable neutrino fluxes that can bring direct information of the physical conditions in the solar core, provided that the nuclear reaction rate is known with sufficient precision. The 14N(p,γ)15O is the slowest reaction in the CNO cycle and estabilishes its rate. The experimental study has been performed both at the LUNA 400 kV accelerator deep underground in the Gran Sasso mountain in Italy and at a 3 MV Tandetron in the Helmholtz-Zentrum Dresden-Rossendorf. A proton beam was sent on solid TiN targets and the prompt photons were collected by a composite HPGe detector (at LUNA) or by up to four HPGe detectors (Dresden). The obtained results improve the fit of the excitation function in the R-matrix framework, that is used to extrapolate the S-factor at the very low astrophysical energies. In addition, the strength of two resonances at Ep = 430 and 897 keV of the 15N(p,αγ)12C reaction were measured, improving the precision for hydrogen depth profiling.