The two-photon decay in nuclei has been formally theorized for many years and attempted to be measured on numerous occasions. The special case of a 0 + → 0 + transition in nuclei was examined for certain isotopes, and a branching ratio for the two-photon decay determined for each isotopes. Measurements of the branching ratio in nuclei other than this special case had so far proven unsuccessful. Motivated to find the two-photon branching ratio in a case where the transition competes with the single photon transition, we study the 11/2 − 137 Ba isomer. The experiment was performed at the Technische Universität Darmstadt using the available LaBr 3 scintillation detectors. We first study the absorption of various gamma energies by lead and compare the resulting values to a GEANT4 simulation. With an ideal value for lead shield thickness, the experimental setup is built in order to obtain a high two-photon count rate, while suppressing direct Compton scattering between detector pairs and suppressing other background interference. In order to suppress the background, plastic scintillators were placed atop the experimental setup. To treat the daunting level of random coincidences measured with this setup, fine energy and time gates were placed on the processed events in order to limit observation to the region of interest. Throughout the experiment, three different detector pair angles were successfully examined: 72 ◦ , 120 ◦ , and 144 ◦ . With these three angles a partial representation of the angular distribution of the two-photon decay is observed. The branching ratios were measured to be 1.56(23)·10 −6 , 0.55(22)·10 −6 , and 0.70(18)·10 −6 for the angles of 72 ◦ , 120 ◦ , and 144 ◦ respectively, with the values of 72 ◦ and 144 ◦ recorded in Ref.[1]. This experiment therefore shows it is possible to obtain a value for the two-photon branching ratio in the 11/2 − excited state of 137 Ba . A precise determination of this value, and for that of other nuclei, might contribute to solve current fundamental open problems such as restricting the parameters of the equation of state, or accurately determining neutron skin thickness.

The first aim of this thesis is to perform relativistic calculation of atomic and ionic polarizabilities and two-photon decay rates. Hydrogenic systems are treated by the Lagrange-mesh method. The extension to alkali-like systems is realized by means of a semiempirical-core-potential approach combined with the Lagrange-mesh method. The studied systems are partitioned into frozen-core electrons and an active valence electron. The core orbitals are defined by a Dirac-Hartree-Fock (DHF) calculation using the GRASP2K package. The valence electron is described by a Dirac-like Hamiltonian involving a core-polarization potential to simulate the core-valence electron correlation. Polarizabilities appear in a large number of fields and applications, namely in cold atoms physics, metrology and chemical physics. Two-photon transitions are part of a priori highly unlikely processes and are therefore called forbidden radiative processes. Experimental situations report decays from metastable excited states through these channels. Long lifetimes were measured for highly charged Be-like ions in recent storage-ring experiments, but their interpretation is problematic. The study of the competition between forbidden (one-photon beyond the dipole approximation, or multi-photon) and unexpected (hyperfine-induced or induced by external magnetic fields) radiative processes is all obviously relevant. The second aim of this thesis is to perform relativistic ab initio calculations of electronic isotope-shift (IS) factors by using the multiconfiguration DHF (MCDHF) method implemented in the RIS3/GRASP2K and RATIP program packages. Using the MCDHF method, two different approaches are adopted for the computation of electronic IS factors for a set of transitions between low-lying levels of neutral systems. The first one is based on the estimate of the expectation values of the one- and two-body nuclear recoil Hamiltonian for a given isotope, including relativistic corrections derived by Shabaev, combined with the calculation of the total electron densities at the origin. In the second approach, the relevant electronic factors are extracted from the calculated transition shifts for given triads of isotopes. These electronic quantities together with observed ISs between different pairs of isotopes provide the changes in mean-square charge radii of the atomic nuclei. Within this computational approach for the estimation of the mass- and field-shift factors, different models for electron correlation are explored in a systematic way to determine a reliable computational strategy, and to estimate theoretical error bars of the IS factors. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished