Neutron-rich Cr Isotopes Research Articles

In-beam γ -ray spectroscopy of Cr62,64

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number $N=40$. We report here on the first $\ensuremath{\gamma}$-ray spectroscopy beyond the $({4}_{1}^{+})$ state in $^{62,64}\mathrm{Cr}$, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the ${6}^{+}$ state in $^{64}\mathrm{Cr}$ is presented as well as one for, possibly, the second excited ${0}^{+}$ state in $^{62}\mathrm{Cr}$. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the $N=40$ harmonic oscillator gap into the ${g}_{9/2}$ and ${d}_{5/2}$ orbitals. The calculated level schemes for $^{62,64}\mathrm{Cr}$ reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, $^{62}\mathrm{Cr}$ emerges as a transitional system on the path to the center of the $N=40$ island of inversion.

Reduced transition strengths of low-lying yrast states in chromium isotopes in the vicinity ofN=40

Background: In neutron-rich nuclei around $N=40$ rapid changes in nuclear structure can be observed. While $^{68}\mathrm{Ni}$ 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 $^{58,60,62}\mathrm{Cr}$ 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 $^{58,60,62}\mathrm{Cr}$. 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 $^{58,60,62}\mathrm{Cr}$ 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 ${\mathrm{B}}_{42}$ ratios suggest an evolution toward a rotational nature of the collective structures in $^{60,62}\mathrm{Cr}$. Compared to $^{58}\mathrm{Cr}$, experimental ${\mathrm{B}}_{42}$ and ${\mathrm{B}}_{62}$ values for $^{60}\mathrm{Cr}$ 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.

βdecay ofV61,63and low-energy level schemes ofCr61,63

Background: Near N = 40, rapid development of collectivity has been inferred based on the low-energy level schemes of the even-even Fe and Cr isotopes and attributed to deformation arising primarily from the influence of the $\ensuremath{\nu}{g}_{9/2}$ intruder orbital. The level schemes of the odd-A Co and Fe isotopes, as well as the odd-odd Co and Mn isotopes, are also influenced by the ${g}_{9/2}$ orbital and suggest prolate deformation. However, scarce information is available regarding the neutron-rich odd-A Cr isotopes.Purpose: Determine low-energy level schemes of the neutron-rich Cr isotopes approaching N = 40 and investigate the influence of the $\ensuremath{\nu}{g}_{9/2}$ orbital.Method: Neutron-rich V isotopes were produced at the NSCL through projectile fragmentation. The $\ensuremath{\beta}$ decay of the V isotopes into Cr isotopes was studied, and the observed $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays were used to determine the low-energy level schemes of the neutron-rich Cr isotopes.Results: A greatly expanded level scheme is constructed for $^{61}\mathrm{Cr}$, which has an increased low-energy level density relative to isotopic ${}^{55,57,59}$Cr. Excited states are discovered in $^{63}\mathrm{Cr}$ for the first time.Conclusion: The distinct difference between the low-energy level scheme of $^{61}\mathrm{Cr}$ and the lighter neutron-rich odd-A Cr isotopes is inferred to be due to the influence of the $\ensuremath{\nu}{g}_{9/2}$ intruder orbital and suggests the possibility of low-energy positive-parity states in $^{61}\mathrm{Cr}$, leading to the conclusion that a significant change in deformation and orbital occupancies has taken place when $N$ exceeds 36.

Microscopic approach to large-amplitude deformation dynamics with local QRPA inertial masses

We have developed a new method for determining microscopically the five-dimensional quadrupole collective Hamiltonian, on the basis of the adiabatic self-consistent collective coordinate method. This method consists of the constrained Hartree-Fock-Bogoliubov (HFB) equation and the local QRPA (LQRPA) equations, which are an extension of the usual QRPA (quasiparticle random phase approximation) to non-HFB-equilibrium points, on top of the CHFB states. One of the advantages of our method is that the inertial functions calculated with this method contain the contributions of the time-odd components of the mean field, which are ignored in the widely-used cranking formula. We illustrate usefulness of our method by applying to oblate-prolate shape coexistence in 72Kr and shape phase transition in neutron-rich Cr isotopes around N = 40.

Shape transition and fluctuations in neutron-rich Cr isotopes aroundN=40

The spherical-to-prolate shape transition in neutron-rich Cr isotopes from N = 34 to 42 is studied by solving the collective Schr\"odinger equation for the five-dimensional quadrupole collective Hamiltonian. The collective potential and inertial functions are microscopically derived with use of the constrained Hartree-Fock-Bogoliubov plus local quasiparticle random-phase approximation method. Nature of the quadrupole collectivity of low-lying states is discussed by evaluating excitation spectra and electric quadrupole moments and transition strengths. The result of calculation indicates that Cr isotopes around 64Cr are prolately deformed but still possess transitional character; large-amplitude shape fluctuations dominate in their low-lying states.

Shape transition and fluctuations in neutron-rich Cr isotopes around N = 40

Shape transition and fluctuations in neutron-rich Cr isotopes around N = 40

Shape changes and large-amplitude collective dynamics in neutron-rich Cr isotopes

Shape-phase transition in neutron-rich Cr isotopes around $N=40$ is investigated by employing the collective Hamiltonian approach. The inertial functions for large-amplitude vibration and rotation are evaluated by the local normal modes along the axial quadrupole collective coordinate using the Skyrme and pairing energy-density functionals. The time-odd components of the mean fields are fully included in the derived masses. The low-lying spectra obtained by requantizing the collective Hamiltonian show an excellent agreement with the recent experimental data. Gradual change from spherical to axially deformed shapes in between $N=34$ and 38 is well described.

Shell model study of single-particle and collective structure in neutron-rich Cr isotopes

The structure of neutron-rich Cr isotopes is systematically investigated by using the spherical shell model. The calculations reproduce well the known energy levels for the even-even $^{52\ensuremath{-}62}\mathrm{Cr}$ and odd-mass $^{53\ensuremath{-}59}\mathrm{Cr}$ nuclei, and predict a lowering of excitation energies around neutron number $N=40$. The calculated $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ systematics shows a pronounced collectivity around $N=40$; a similar characteristic behavior has been suggested for Zn and Ge isotopes. Causes for the sudden drop of the $9/{2}_{1}^{+}$ energy in $^{59}\mathrm{Cr}$ and the appearance of very low ${0}_{2}^{+}$ states around $N=40$ are discussed. We also predict a new band with strong collectivity built on the ${0}_{2}^{+}$ state in the $N=40$ isotope $^{64}\mathrm{Cr}$.

Deformation around Neutron-Rich Cr Isotopes in Axially Symmetric Skyrme-Hatree-Fock-Bogoliubov Method

We analyse the deformation mechanism in neutron-rich Cr, Fe and Ti isotopes with N = 32–44 using a Skyrme-Hartree-Fock-Bogoliubov mean-field code employing a twodimensional mesh representation in the cylindrical coordinate system. Evaluating the quadrupole deformation energy systematically, we show that the Skyrme parameter set SkM* gives a quadrupole instability around the neutron numbers N ∼ 38–42 in Cr isotopes, where the deformation energy curve suggests a transitional behavior with a shallow minimum extending to a large prolate deformation. The roles of a deformed N = 38 gap and the position of the neutron g 9 orbit are analysed in detail.

Structure of the neutron-rich Cr isotopes

The low-lying levels in 59,60Cr have been populated with 13,14C(48Ca, 2p) reactions using a beam energy of 130 MeV. Prompt electromagnetic radiation was detected using the Gammasphere array, in coincidence with recoiling ions measured with the Fragment Mass Analyzer. The residues were selected and identified on the basis of charge-to-mass ratio, energy-loss and time-of-flight measurements. Preliminary results for 60Cr, when compared to lighter even-even isotopes, indicate a softness in shape which increases with neutron number. The low-lying structure of 59Cr is clearly inconsistent with the results of a spherical shell-model calculation and requires the inclusion of the νg9/2 orbital. The sequence of states can be understood within the Nilsson model assuming a moderate oblate ground-state deformation. This is in contrast to lighter odd-Cr nuclei where there is evidence for prolate deformation after excitation of g9/2 neutrons.

Changes in νg9/2 shape polarisation across the odd neutron-rich Cr isotopes

Changes in νg9/2 shape polarisation across the odd neutron-rich Cr isotopes

New region of deformation in the neutron-rich 60 24Cr36 and 62 24Cr38

The neutron-rich nuclei 23 60-63V have been produced at GANIL via interactions of a 61.8A·MeV 76Ge beam with a 58Ni target. Beta-decay to 24 60-63Cr has been investigated using combined β- and γ-ray spectroscopy. Half-lives of the 60-63V nuclei have been determined, and the existence of a beta-decay isomer in the 60V nucleus is strongly supported. The observation of low-energy 2+ states in 60Cr (646keV) and 62Cr (446keV) suggests that these isotopes are strongly deformed with β2 ∼ 0.3. This is confirmed by shell model calculations which show the dominant influence of the intruder g and d orbitals to obtain low 2+ energies in the neutron-rich Cr isotopes.

Low-energy structure of neutron-rich Cr isotopes

Low-energy structure of neutron-rich Cr isotopes