In many applications to finite Fermi-systems, the pairing problem has to be treated exactly. We suggest a numerical method of exact solution based on SU(2) quasispin algebras and demonstrate its simplicity and practicality. We show that the treatment of binding energies with the use of the exact pairing and uncorrelated monopole contribution of other residual interactions can serve as an effective alternative to the full shell-model diagonalization in spherical nuclei. A self-consistent combination of the exactly treated pairing and Hartree-Fock method is discussed. Results for Sn isotopes indicate a good agreement with experimental data.Comment: 10 pages, 2 figure

ERRATACorrections to the Fermi Matrix Element for Superallowed p Decay. W. E. ORMAND and B. A. BROWN [Phys. Rev. Lett. 62, 866 (1989)].

In-beam ␥-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from A = 20 to 24. For the produced fragments, ␥-ray energies, intensities, and ␥-␥ coincidences have been measured. Based on this information new level schemes are proposed for

Measurements of the ground-state nuclear spins and magnetic and quadrupole moments of the copper isotopes from 61 Cu up to 75 Cu are reported. The experiments were performed at the CERN online isotope mass separator (ISOLDE) facility, using the technique of collinear laser spectroscopy. The trend in the magnetic moments between the N = 28 and N = 50 shell closures is reasonably reproduced by large-scale shell-model calculations starting from a 56 Ni core. The quadrupole moments reveal a strong polarization of the underlying Ni core when the neutron shell is opened, which is, however, strongly reduced at N = 40 due to the parity change between the pf and g orbits. No enhanced core polarization is seen beyond N = 40. Deviations between measured and calculated moments are attributed to the softness of the 56 Ni core and weakening of the Z = 28 and N = 28 shell gaps.

The quenching of the N =20 shell gap in neutron-rich nuclei is investigated by studying the singleparticle structure of 27 Ne via neutron transfer using a 26 Ne beam. Two low-lying negative parity intruder states have been observed, the lowest of which is identified as J π = 3/2 − , confirming earlier speculations. A level identified as 7/2 − is observed higher in energy than the 3/2 − , contrary to the ordering at β-stability and at an energy significantly different to the predictions of previous shellmodel calculations. The measured energies and deduced spectroscopic factors are well reproduced in full (0,1)-ω 0s-0p-0d-1s-0f -1p calculations in which there is a significant ad-hoc reduction (∼ 0.7 MeV) in the N =20 shell gap. Neutron-rich nuclei often exhibit structural behaviour significantly different to stable nuclei, with a striking example being the "island of inversion" in the A ≃ 32 region of neutron rich nuclei [1,2]. The nuclei in this "island" are deformed rather than spherical owing to residual interactions and quenching of the N =20 magic number through the migration in energy of the shell-model orbitals [3]. This migration is known to be due, in part, to nucleon-nucleon tensor forces and to three-body N N N forces [4,5]. In adjacent more weakly bound nuclei, the migration may also be affected by the proximity of the continuum [6].The N =20 shell gap seen in nuclei near stability arises from the separation of the 0d 3/2 orbital and the negative parity orbitals (0f 7/2 , 1p 3/2 ,.. [10]. Guided by shell model expectations, the 885 keV level was tentatively identified as the lowest 1/2 + state and the 765 keV level was inferred to have a negative parity of 1/2 − , 3/2 − or 5/2 − . The 1/2 + is weak since it is populated mainly via pair-excited components in the 26 Ne ground state, in single-step transfer. In single-neutron knockout at higher energies, both levels were seen and the angular momentum of the removed nucleon was assigned as ℓ=0 or 1 [11], consistent with the results from the (d,p) reaction study [10]. These two excited states were also observed in the p( 28 Ne, 27 Neγ) reaction at intermediate energy [12] but no further information concerning spins was obtained [37]. No evidence for a 7/2 − state has been reported, despite a clear prediction that it should exist at an energy close to the 3/2 − state [11,15]. The most direct means to probe the evolution of orbital energies is to measure the strength of single-particle states where a neutron is transferred into orbitals that are otherwise empty. The present experiment was designed along these lines to populate the 7/2 − and 3/2 − 0f -1p shell states and any other strong single-particle levels via (d,p) transfer and to determine their properties.A beam of 26 Ne ions (∼100% pure) at 9.8A MeV was provided by the SPIRAL facility at GANIL and

No abstract

Relations between the total β+ Gamow-Teller (GT+) strength and the E2 strength are further examined. It is found that in shell-model calculations for N =Z nuclei, in which changes in deformation are induced by varying the single-particle energies, the total GT+ or GT− strength decreases monotonically with increasing values of the B(E2) from the ground state to the first excited J π =2 + state. Similar trends are also seen for the double GT transition amplitude (with some exceptions) and for the spin part of the total M1 strength as a function of B(E2).

An approach is presented to experimentally constrain previously unreachable (p, γ) reaction rates on nuclei far from stability in the astrophysical rp process. Energies of all critical resonances in the (57)Cu(p,γ)(58)Zn reaction are deduced by populating states in (58)Zn with a (d, n) reaction in inverse kinematics at 75 MeV/u, and detecting γ-ray-recoil coincidences with the state-of-the-art γ-ray tracking array GRETINA and the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The results reduce the uncertainty in the (57)Cu(p,γ) reaction rate by several orders of magnitude. The effective lifetime of (56)Ni, an important waiting point in the rp process in x-ray bursts, can now be determined entirely from experimentally constrained reaction rates.