Investigation of the shell evolution in neutron-rich titanium nuclei using reduced transition strengths

This work describes experimental studies that contribute to the understanding of the shell structure in the Ti-Cr-Fe region beyond N=28. For exotic nuclei, as examined in this thesis, significant changes in the shell structure may occur. Therefore, such nuclei offer the possibility to investigate the properties and validity of nuclear models under extreme conditions. From the experimental data available so far on the 2+ states in N=32 isotones, the occurrence of a phase transition from predominant collective structures in 58Fe to a neutron subshell closure is predicted, which evolves as the number of protons in the πf7/2 orbital reduces, i.e. of 56Cr-> 54Ti->52Ca, due to the weakening of the monopole interaction between the πf7/2 orbital of protons and the vf5/2 orbital of neutrons. However, modern shell-model calculations with different interactions, e.g. the GXPF1A, KB3G and FPD6 interactions, were not able to satisfactorily describe the observed staggering of the B(E2;2+ -> 0+) values in 50,52,54Ti, which probably results from neutron subshell closures. Therefore, in this work reduced transition probabilities of the lowest transitions of the respective ground-state bands in 46,50,52,53,54Ti were investigated experimentally by lifetime measurements, partly for the first time. The multinucleon-transfer reaction mechanism, which is a promising approach to the production of difficult to access and exotic atomic nuclei, was applied to study the evolution from nuclei in the valley of stability towards neutron-rich exotic Ti isotopes at the Grand Accélérateur National d'Ions Lourds (GANIL) in Caen, France: In the Cologne plunger for deep-inelastic reactions, a radioactive 238U beam with an energy of 6.76 MeV/u impinged on a stretched 50Ti target with nat Cu layer in front of the target, while a nat Mg degrader foil was used to decelerate the reaction products for measuring lifetimes using the Recoil Distance Doppler-shift (RDDS) technique. The reaction products of interest were identified with the magnetic spectrometer VAMOS++, while the gamma radiation was measured with the gamma tracking array AGATA. During this experiment beam-induced changes of the target occurred. These resulted in the fact that the distances between the target and degrader foil could not be extracted exactly, which are essential for an RDDS lifetime analysis. Therefore, another RDDS experiment was performed at the Cologne FN tandem accelerator using a fusion-evaporation reaction. The aim of this experiment was to determine the lifetimes of the 2+ and 4+ states of the ground-state band in the stable nucleus 46Ti with high precision, in order to subsequently determine the unknown distances between the target and degrader foil of the experiment performed at GANIL. This nucleus was chosen because it was populated with sufficient statistics in the multinucleon-transfer reaction at GANIL and the lifetimes of its low-lying states are in the sensitive range for the distances chosen in this experiment. The data from the experiment on 46Ti in Cologne were analyzed using the Differential Decay Curve method in gamma-gamma coincidences. The unknown distances from the GANIL experiment were then determined by comparing the experimental gamma-ray spectra with those generated by Monte-Carlo simulations using the chi-squared method. The lifetimes of the 2+, 4+, 6+ and 8+ in 52,54Ti and the (5/2-), 7/2-, 9/2-, 11/2- and 13/2- states in 53Ti were determined using both the DDCM and the minimum chi-squared method with the distances obtained in this way. Furthermore, a third RDDS measurement was carried out within this work, also at the Cologne FN tandem accelerator, with the aim of determining the lifetime values of the lowest-lying states of the ground-state band in 50Ti. Although this nucleus was also populated during the measurement at GANIL, the additional measurement provided a more precise determination of the lifetimes based on the knowledge of the exact relative distances between the target and degrader foil. The lifetimes were analyzed in a particle-gamma evaluation. The transition probabilities determined from the level lifetimes are interpreted based on shell-model predictions of different interactions in the f7/2, p3/2, f5/2, p1/2 valence space for protons and neutrons

Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data

Lifetime measurement of the

A $$\gamma$$-$$\gamma$$ coincidence Recoil Distance Doppler-Shift measurement has been carried out on $$^{60}$$Ni to re-measure the lifetime of the $$2_1^+$$ and $$4_1^+$$ states. The new lifetime of the $$2_1^+$$ state supports the adopted NNDC value but disagrees with the results of two more recent Doppler-Shift Attenuation Method measurements, which suggested a longer lifetime. The new result for the $$4_1^+$$ state’s lifetime is significantly shorter than the one recommended in the latest NNDC compilation while also reducing its uncertainty. It therefore resolves an unclear situation, where an unexpected drop in transition strength appeared from $$^{58}$$Ni to $$^{60}$$Ni. Both values match very well with recently applied shell model calculations using the GXPF1A interaction

Preliminary results of lifetime measurements in neutron-rich 53Ti

To study the nuclear structure of neutron-rich titanium isotopes, a lifetime measurement was performed at the Grand Accélérateur National d'Ions Lourds (GANIL) facility in Caen, France. The nucleiwere produced in a multinucleon-transfer reaction by using a 6.76 MeV/u 238U beam. The Advanced Gamma Tracking Array (AGATA) was employed for the γ-ray detection and target-like recoils were identified event-by-event by the large-acceptance variable mode spectrometer (VAMOS++). Preliminary level lifetimes of the (5/2−) to 13/2− states of the yrast band in the neutron-rich nucleus 53Ti were measured for the first time employing the recoil distance Doppler-shift (RDDS) method and the compact plunger for deep inelastic reactions. The differential decay curve method (DDCM) was used to obtain the lifetimes from the RDDS data

Coulomb excitation of pear-shaped nuclei

International audienceThere is a large body of evidence that atomic nuclei can undergo octupole distortion and assume the shape of a pear. This phenomenon is important for measurements of electric-dipole moments of atoms, which would indicate CP violation and hence probe physics beyond the Standard Model of particle physics. Isotopes of both radon and radium have been identified as candidates for such measurements. Here, we have observed the low-lying quantum states in 224Rn and 226Rn by accelerating beams of these radioactive nuclei. We show that radon isotopes undergo octupole vibrations but do not possess static pear-shapes in their ground states. We conclude that radon atoms provide less favourable conditions for the enhancement of a measurable atomic electric-dipole moment