Svenska fysikersamfundet

Kärnfysiksektionens hemsida

Kärnfysikermöte XXVI

Abstracts

Neutron-proton pairing in N~Z nuclei within the framework of HFB theory

Shufang Ban

KTH, Stockholm

In the original theory of nuclear pair correlation, each Cooper pair contained two neutrons or two protons. However, it was soon recognized that a Cooper pair might also be composed of one neutron and one proton. The ideal condition for finding neutron-proton (np) pairing occurs in N=Z nuclei, where the neutrons and protons occupy the same spatial orbitals and have the maximum spatial overlap. The np pairing was studied by Goodman in 1970's. Due to lack of experimental data, the theoretical progress has been very slow. Recently, more and more experimental data for N=Z nuclei have been collected and the quest for np pairing gains great importance.

It is known that the np pairing breaks the axial and signature symmetries, therefore, parity is the only preserved spatial symmetry in this case. Studies have shown that the np pairing is especially important at high angular momentum, hence the cranking approximation must be considered. Then, we need to study the np pairing in cranking triaxial-deformed theory. Based on the computer code for the cranking Hartree-Fock Bogoliubov (HFB) theory in coordinate space, we can make one computer with np pairing and to study its influence in N~Z nuclei, e.g., 24Mg, 48Cr.

Fission-Fragment Characteristics in the Reaction 238U(n,f) in the Vicinity of the Vibrational Resonances at En = 1 MeV

Evert Birgersson

Örebro Universitet

The description of the fission fragment mass and energy distribution in terms of fission modes in the nuclear energy landscape of the deforming nucleus between the ground state and the scission point was successfully describing fission of 235U and 237Np. Recent calculations predict observable changes in the mass and energy distributions for the reaction 238U(n, f) in the neighbourhood of the vibrational resonances around En = 1 MeV. Experiments were performed at the IRMM in Geel, Belgium, a Joint Research Centre of EC, using a 7 MV Van de Graaff accelerator for the production of quasi mono-energetic neutrons to prove the predictions. The experimental results will be presented as a function of neutron energy from 0.9 to 2.0 MeV and compared with the predictions of the multi-modal random neck-rupture model (MM-RNR).

AGATA

Bo Cederwall

KTH, Stockholm

In the history of nuclear science, major technical advances in gamma-ray detection techniques have always resulted in significant new insights into the structure of atomic nuclei. These advances have culminated in the construction of 4π arrays of escape-suppressed spectrometers that comprising high-purity Ge detectors and their associated scintillation detector escape suppression shields.

The next major step in gamma-ray spectroscopy involves achieving the ultimate goal of a solid 4π shell of Ge detectors by using the technique of gamma-ray energy tracking in electrically segmented Ge crystals. The resulting spectrometer will have an unparalleled level of detection power for nuclear electromagnetic radiation. Its sensitivity for selecting the weakest signals from exotic nuclear events will be enhanced by a factor of up to 1000 relative to its predecessors. It will also have an unprecedented angular resolution making it ideally suited for high-resolution spectroscopy even at source velocities up to 50% of the velocity of light. Therefore, it is ideally suited to be used both in conjunction with the new generation of European radioactive beam accelerators or existing stable beam facilities. A European collaboration has been established to construct a 4π tracking spectrometer called AGATA (Advanced Gamma Tracking Array). This collaboration is currently performing the research and development necessary to finalize the technology for gamma-ray tracking and hence fully specify the full 4π spectrometer. The status of this first phase of the AGATA project will be reported.

Towards an ab initio Description of Nuclear Reactions

Christian Forssén

Chalmers University of Technology

I will discuss present attempts to employ high-precision nuclear structure information in nuclear reaction calculations. The foundation of our approach is the ab initio no-core shell model (NCSM), which is a well-established theoretical framework aimed at an A-body description of nuclear structure starting from high-precision interactions between the nucleons. I will also outline our strategy for developing a fundamental theory for light-ion reactions based on realistic inter-nucleon forces.

Development of position sensitivity and imaging capabilities with HPGe detectors

Anton Khaplanov

KTH, Stockholm

High purity germanium detectors with 2-dimentional segmentation of the electrical contacts are becoming common in gamma-ray spectroscopy systems. Such detectors have also been suggested for gamma-ray imaging with applications in medicine among other fields. The segmentation, combined with the pulse shape analysis (PSA) techniques, has enabled the localization of the individual interactions between gamma photons and a germanium detector. This allows the tracks of individual photons to be reconstructed, enabling both the separation of multiple events as well as the determination of direction of the incoming photon.

From RISING to HISPEC/DESPEC

Zsolt Podolyak

University of Surrey

Results from recent experiments devoted to elucidate the structure of nuclei with extreme N/Z ratio will be presented. These measurements were performed at GSI using the RISING gamma-ray detection array. In the future similar types of experiments will be preformed at the FAIR facility within the HISPEC/DESPEC (High-resolutin in-flight spectroscopy/Decay spectroscopy) project. The physics case as well as the state-of-the-art experimental setup, now under design and construction, will be presented.

Transfer Reactions at REX-ISOLDE

Elisabeth Tengborn

Chalmers Tekniska Högskola

Transfer reactions in inverse kinematics are powerful tools to investigate the structure of light nuclei far from the line of stability and the island of inversion. The first case exhibits a large degree of clustering and in the second case it has been observed that the shell model needs to be modified. Examples are given from transfer experiments where radioactive beams of 8Li, 9Li and 30Mg were used. Spectroscopic factors are deduced and for the light nuclei comparisons with ab initio calculations are made.