0000000001025094
AUTHOR
T. Stora
Investigation of the Δn = 0 selection rule in Gamow-Teller transitions : The β-decay of 207Hg
Gamow-Teller β decay is forbidden if the number of nodes in the radial wave functions of the initial and final states is different. This Δn = 0 requirement plays a major role in the β decay of heavy neutron-rich nuclei, affecting the nucleosynthesis through the increased half-lives of nuclei on the astrophysical r-process pathway below both Z = 50 (for N > 82) and Z = 82 (for N >126). The level of forbiddenness of the Δn = 1ν1g9/2 → π0g7/2 transition has been investigated from the β− decay of the ground state of 207Hg into the single-proton-hole nucleus 207Tl in an experiment at the ISOLDE Decay Station. From statistical observational limits on possible γ-ray transitions depopulating the π0…
Evolution of quadrupole collectivity in N=80 isotones toward the Z=64 subshell gap: The B(E2;2+1→0+1) value of 142Sm
It was shown that the evolution of the B(E2; 2+ 1 → 0+ 1 ) values in N = 80 isotones from Te to Nd is affected by the underlying subshell structure. This manifests itself in the observation of the local suppression of the B(E2) value at Z = 58 with respect to the neighboring nuclei 136Ba and 140Nd. To investigate this shell sensitivity toward the Z = 64 subshell gap, the B(E2; 2+ 1 → 0+ 1 ) value of the unstable nucleus 142Sm was measured utilizing the projectile Coulomb excitation technique. The radioactive ion beam (RIB) experiment was performed at the REX-ISOLDE facility at CERN. The B(E2) value of 32 (4) W.u. reflects the impact of the π(1g7/2 2d5/2) subshell closure at Z = 64 with resp…
High intensity neutrino oscillation facilities in Europe
The EUROnu project has studied three possible options for future, high intensity neutrino oscillation facilities in Europe. The first is a Super Beam, in which the neutrinos come from the decay of pions created by bombarding targets with a 4 MW proton beam from the CERN High Power Superconducting Proton Linac. The far detector for this facility is the 500 kt MEMPHYS water Cherenkov, located in the Frejus tunnel. The second facility is the Neutrino Factory, in which the neutrinos come from the decay of mu(+) and mu(-) beams in a storage ring. The far detector in this case is a 100 kt magnetized iron neutrino detector at a baseline of 2000 km. The third option is a Beta Beam, in which the neu…
Structure of low-lying states in 140Sm studied by Coulomb excitation
The electromagnetic structure of 140Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The 2+ and 4+ states of the ground-state band and a second 2+ state were populated by multistep excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the 2+ 1 state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and alge…
Study of the Ti44(α,p)V47 reaction and implications for core collapse supernovae
The underlying physics triggering core collapse supernovae is not fully understood but observations of material ejected during such events helps to solve this puzzle. In particular, several satellite based γ -ray observations of the isotope 44 Ti have been reported recently. Conveniently, the amount of this isotope in stellar ejecta is thought to depend critically on the explosion mechanism. The most influential reaction to the amount of 44 Ti in supernovae is Ti44(α,p)V47 . Here we report on a direct study of this reaction conducted at the REX-ISOLDE facility, CERN. The experiment was performed with a 44 Ti beam at Elab=2.16 MeV/u , corresponding to an energy distribution, for reacting …
Competition between allowed and first-forbidden β decays of At208 and expansion of the Po208 level scheme
The structure of 208Po populated through the EC/β+ decay of 208At is investigated using γ-ray spectroscopy at the ISOLDE Decay Station. The presented level scheme contains 27 new excited states and 43 new transitions, as well as a further 50 previously observed γ rays which have been (re)assigned a position. The level scheme is compared to shell model calculations. Through this analysis approximately half of the β-decay strength of 208At is found to proceed via allowed decay and half via first-forbidden decay. The first-forbidden transitions predominantly populate core excited states at high excitation energies, which is qualitatively understood using shell model considerations. This mass r…
A concept for the extraction of the most refractory elements at CERN-ISOLDE as carbonyl complex ions
The European physical journal / A 58(5), 94 (2022). doi:10.1140/epja/s10050-022-00739-1
Local suppression of collectivity in the N=80 isotones at the Z=58 subshell closure
Background: Recent data on N=80 isotones have suggested that the proton π(1g7/2) subshell closure at Z=58 has an impact on the properties of low-lying collective states. Purpose: Knowledge of the B(E2;2+1→0+1) value of 140Nd is needed in order to test this conjecture. Method: The unstable, neutron-rich nucleus 140Nd was investigated via projectile Coulomb excitation at the REX-ISOLDE facility with the MINIBALL spectrometer. Results: The B(E2) value of 33(2) W.u. expands the N=80 systematics beyond the Z=58 subshell closure. Conclusions: The measurement demonstrates that the reduced collectivity of 138Ce is a local effect possibly due to the Z=58 subshell closure and requests refined theoret…
The structure of low-lying states in ${}^{140}$Sm studied by Coulomb excitation
The electromagnetic structure of $^{140}$Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The $2^+$ and $4^+$ states of the ground-state band and a second $2^+$ state were populated by multi-step excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the $2_1^+$ state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler …
Competition between Allowed and First-Forbidden β Decay : The Case of Hg208→Tl208
The β decay of 208Hg into the one-proton hole, one neutron-particle 20881Tl127 nucleus was investigated at CERN-ISOLDE. Shell-model calculations describe well the level scheme deduced, validating the proton-neutron interactions used, with implications for the whole of the N>126, Z<82 quadrant of neutron-rich nuclei. While both negative and positive parity states with spin 0 and 1 are expected within the Qβ window, only three negative parity states are populated directly in the β decay. The data provide a unique test of the competition between allowed Gamow-Teller and Fermi, and first-forbidden β decays, essential for the understanding of the nucleosynthesis of heavy nuclei in the rapid neut…
Shapes and Collectivity in Neutron Deficient Even-Mass 188–198Pb Isotopes
The neutron deficient 188−198Pb isotopes have been studied in a Coulomb excitation measurement employing the Miniball spectrometer and radioactive beams from REX-ISOLDE, CERN. These isotopes are of particular importance as they lie in a transitional region, where the intruding structures, associated with different deformed shapes, come down in energy close to the spherical ground state. For detailed analysis of the Coulomb excitation data, the understanding of the beam composition is essential. peerReviewed
Normal and intruder configurations in $^{34}$Si populated in the $\beta^-$ decay of $^{34}$Mg and $^{34}$Al
The structure of Si34 was studied through γ spectroscopy separately in the β− decays of Mg34 and Al34 at the ISOLDE facility of CERN. Different configurations in Si34 were populated independently from the two recently identified β-decaying states in Al34 having spin-parity assignments Jπ=4− dominated by the normal configuration π(d5/2)−1⊗ν(f7/2) and Jπ=1+ by the intruder configuration π(d5/2)−1⊗ν(d3/2)−1(f7/2)2. The paper reports on spectroscopic properties of Si34 such as an extended level scheme, spin and parity assignments based on log(ft) values and γ-ray branching ratios, absolute β feeding intensities, and neutron emission probabilities. A total of 11 newly identified levels and 26 tr…
The laser ion source trap for highest isobaric selectivity in online exotic isotope production
The improvement in the performance of a conventional laser ion source in the laser ion source and trap (LIST) project is presented, which envisages installation of a repeller electrode and a linear Paul trap/ion guide structure. This approach promises highest isobaric purity and optimum temporal and spatial control of the radioactive ion beam produced at an online isotope separator facility. The functionality of the LIST was explored at the offline test separators of University of Mainz (UMz) and ISOLDE/CERN, using the UMz solid state laser system. Ionization efficiency and selectivity as well as time structure and transversal emittance of the produced ion beam was determined. Next step aft…
Study of the Ti-44(alpha, p)V-47 reaction and implications for core collapse supernovae
The underlying physics triggering core collapse supernovae is not fully understood but observations of material ejected during such events helps to solve this puzzle. In particular, several satellite based γ -ray observations of the isotope 44Ti have been reported recently. Conveniently, the amount of this isotope in stellar ejecta is thought to depend critically on the explosion mechanism. The most influential reaction to the amount of 44Ti in supernovae is 44Ti(α, p)47V. Here we report on a direct study of this reaction conducted at the REX-ISOLDE facility, CERN. The experiment was performed with a 44Ti beam at Elab = 2.16 MeV/u, corresponding to an energy distribution, for reacting α-par…