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Self-conjugate nuclei in the A ��� 70���80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Zline. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2; 2��������� 0������)transition strength in ������Kr. The method is then applied to determine the B(E2; 2��������� 0������) transition strength in ������Rb, the heaviest odd���odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in ������Rb.

A new method of extracting the g-ray intensities necessary to perform lifetime measurements using the differential decay curve method (DDCM) is presented in this work, the unresolved Doppler-shifted components method (UDCM). The UDCM allows for a DDCM analysis to be performed using a g-ray transition for which the fully Doppler-shifted and degraded components are unresolvable in energy and so are detected as a single peak. This technique was used to measure the known lifetime of the yrast 2+1 state in 50Mn with a depopulating transition that does not have resolvable fully Doppler-shifted and degraded components. The lifetime measured through applying the UDCM was consistent with the standard DDCM measurement of the 2+1 state. Use of the UDCM allows for DDCM lifetime measurements to be made using transitions of smaller g-ray energies, smaller recoil velocities and, in some cases, with a smaller uncertainty. In contrast to a standard DDCM analysis, a UDCM analysis is also independent of the widths of the fully Doppler-shifted and degraded components and as a result they do not need to be determined.

Lifetimes of the T = 1 isobaric analog Ip = 2+ states in 50Mn and 50Cr were measured simultaneously by employing the recoil distance Doppler-shift (RDDS) technique. The states were populated in a fusion-evaporation reaction with a ¹²C beam on a 40Ca target. An analysis of the data and the calculations from the present work, together with the available B(E2 : 2+ RT 0+) data of the isobaric analog states in 50Fe and 50Cr, were used to study isospin symmetry in the three A = 50 isobaric nuclei. Shell-model calculations reproduce the magnitudes as well as the increasing trend of the B(E2) data with increasing Z. To draw a firm conclusion on the level of isospin mixing in the triplet, a new precision measurement of the 2+ RT 0+ transition rate in 50Fe will be required.

The lifetimes of the first excited 2+, 4+, and 6+ states in Zr98 were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in Zr98 were populated using the fission reaction between a 6.2 MeV/u U238 beam and a Be9 target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;21+→01+) value in Zr98, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state. © 2018 American Physical Society.

Lifetimes of the first excited 2þ and 4þ states in the extremely neutron-deficient nuclide 172Pt have been measured for the first time using the recoil-distance Doppler shift and recoil-decay tagging techniques. An unusually low value of the ratio BðE2∶4þ 1 → 2þ 1 Þ=BðE2∶2þ 1 → 0þ gsÞ ¼ 0.55ð19Þ was found, similar to a handful of other such anomalous cases observed in the entire Segr´e chart. The observation adds to a cluster of a few extremely neutron-deficient nuclides of the heavy transition metals with neutron numbers N ≈ 90–94 featuring the effect. No theoretical model calculations reported to date have been able to explain the anomalously low BðE2∶4þ 1 → 2þ 1 Þ=BðE2∶2þ 1 → 0þ gsÞ ratios observed in these cases. Such low values cannot, e.g., be explained within the framework of the geometrical collective model or by algebraic approaches within the interacting boson model framework. It is proposed that the group of BðE2∶4þ 1 → 2þ 1 Þ=BðE2∶2þ 1 → 0þ gs) ratios in the extremely neutron-deficient even-even W, Os, and Pt nuclei around neutron numbers N ≈ 90–94 reveal a quantum phase transition from a seniority-conserving structure to a collective regime as a function of neutron number. Although a system governed by seniority symmetry is the only theoretical framework for which such an effect may naturally occur, the phenomenon is highly unexpected for these nuclei that are not situated near closed shells.

The main topic of the present work is the experimental determination of level lifetimes in 58,60,62Cr by means of the recoil distance Doppler-shift technique. The corresponding experiment was performed at the National Superconducting Cyclotron Laboratory at Michigan State University (USA). Excited states in 58,60,62Cr were populated by one-proton knockout reactions from fast radioactive 59,61,63Mn beams. Level lifetimes were determined for the 2+ and 4+ states as well as - in case of 58,60Cr - for the 6+ states. The determined B(E2; 2+ -> 0+) values in 58,60,62Cr are in reasonable agreement with previous results from intermediate Coulomb-excitation experiments, although the present data suggests slightly enhanced collectivity. In 58Cr, the B(E2) values of the 4+ -> 2+ and 6+ -> 4+ transitions are comparable to the B(E2; 2+ -> 0+) value. From 58Cr to 60,62Cr, the B(E2; 4+ -> 2+) values evolve more rapidly compared to the B(E2; 2+ -> 0+) values which suggests rotor-like behaviour with pronounced quadrupole deformations ~0.3. Compared to the neighbouring iron isotones, the evolution of B(E2) values along the chromium isotopes toward N=40 indicates a comparable trend, although the collectivity in chromium appears to be mildly enhanced. The experimental results are discussed in the shell-model framework using the effective interaction LNPS. The comparison shows a pleasant agreement, although the experimental data suggests a slightly stronger enhancement of collectivity toward N=40. Calculations with the interacting boson model were performed for 56,58,60,62Cr. They are able to reproduce the experimental excitation energies and transition strengths with reasonable accuracy but do not indicate a significant degree of quadrupole deformation. Other isotopes that were produced in the experiment were investigated with respect to the identification of g-ray transitions and level lifetimes. Of particular interest are the neutron-rich 59,61Mn isotopes (Z=25), for which level lifetimes of the 7/2-, 9/2- and 11/2- states were determined (in 61Mn the assignment is only tentative). In addition, level lifetimes in 63Mn of the tentatively assigned (7/2-) and (9/2-) states were evaluated. The results are compared to shell-model calculations using the fp interaction KB3G as well as the state-of-the-art interaction LNPS-m. The present data indicates a structural change close to N=36. The experimental data for 61Mn suggests that the B(M1) value for the 7/2^- -> 5/2- is underestimated by the LNPS-m interaction when free g factors are used. In the second part of this thesis, a tool is presented which allows to generate g-ray spectra of lifetime studies based on the electromagnetic Doppler-shift using empirical parameters. The tool is highly flexible which enables the incorporation of various experimental bounding conditions. It is applied to investigate the influence of velocity distributions on the lifetime analysis and results indicate that systematic deviations are minimized if distance-dependent mean recoil velocities are taken into account.

The lifetimes of the 21+ , the 22+ and the 31- states of ²¹0Po have been measured in the ²08Pb(¹²C,¹0Be)²¹0Po transfer reaction by the Doppler-shift attenuation method. The result for the lifetime of the 21+ state is about three times shorter than the adopted value. However, the new value still does not allow for a consistent description of the properties of the yrast 21+ , 41+ , 61+ , and 81+ states of ²¹0Po in the framework of nuclear shell models. Quasi-particle Phonon Model (QPM) calculations also cannot overcome this problem thus indicating the existence of a peculiarity which is neglected in both theoretical approaches.

The lifetime of the 21+ state of ²¹²Po was measured in the ²08Pb(¹²C,8Be)²¹²Po transfer reaction by g-ray spectroscopy employing the recoil distance Doppler shift (RDDS) method. The derived absolute B(E2) value of 2.6(3) W.u. indicates a low collectivity and contradicts previous claims of a-cluster components in the structure of the 21+ state. It is demonstrated that a consistent description of the properties of the 21+− 41+− 61+− 81+ sequence in ²¹²Po cannot be achieved in the framework of a single- j shell-model calculation, either. This puzzle is traced to the properties of the seniority-2 configurations in ²¹0Pb and ²¹0Po.

Background: In neutron-rich nuclei around N=40 rapid changes in nuclear structure can be observed. While Ni68 exhibits signatures of a doubly magic nucleus, experimental data along the isotopic chains in even more exotic Fe and Cr isotopes - such as excitation energies and transition strengths - suggest a sudden rise in collectivity toward N=40. Purpose: Reduced quadrupole transition strengths for low-lying transitions in neutron-rich Cr58,60,62 are investigated. This gives quantitative new insights into the evolution of quadrupole collectivity in the neutron-rich region close to N=40. Method: The recoil distance Doppler-shift (RDDS) technique was applied to measure lifetimes of low-lying states in Cr58,60,62. The experiment was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the SeGA array in a plunger configuration coupled to the S800 magnetic spectrograph. The states of interest were populated by means of one-proton knockout reactions. Results: Data reveal a rapid increase in quadrupole collectivity for Cr58,60,62 toward N=40 and point to stronger quadrupole deformations compared to neighboring Fe isotopes. The experimental B(E2) values are reproduced well with state-of-the-art shell-model calculations using the LNPS effective interaction. A consideration of intrinsic quadrupole moments and B42 ratios suggest an evolution toward a rotational nature of the collective structures in Cr60,62. Compared to Cr58, experimental B42 and B62 values for Cr60 are in better agreement with the E(5) limit. Conclusion: Our results indicate that collective excitations in neutron-rich Cr isotopes saturate at N=38, which is in agreement with theoretical predictions. More detailed experimental data of excited structures and interband transitions are needed for a comprehensive understanding of quadrupole collectivity close to N=40. This calls for additional measurements in neutron-rich Cr and neighboring Ti and Fe nuclei.