Yrast structures in the neutron-rich isotopes Fe59,60 and the role of the g9/2 orbital

The structure of the neutron-rich isotopes Fe59,60 has been studied with the Gammasphere detector array using fusion-evaporation reactions. Level schemes for these nuclei are presented which have been extended to spins of ∼20. Both isotopes exhibit regular, near-yrast γ-decay sequences which are generated by the intrusion of the g9/2 orbital into the fp shell-model space. Lower-spin, natural-parity levels are discussed within the context of shell-model calculations using the GXPF1A interaction in the full fp model space. Experimental features of the high-spin bands are compared with total Routhian surface calculations

β-decay of odd-A Ti57 and V59

The β-decay of odd-A, neutron-rich Ti57 and V59 are studied. More precise β-decay half-lives of 98±5 and 97±2 ms are deduced for Ti57 and V59, respectively. In addition, β-delayed γ-ray spectroscopy is used to deduce β-decay branching ratios and establish the low-energy-level structures of the daughter nuclides. The new data for levels in V57 and Cr59 are compared with the results of shell-model calculations completed in the full pf model space. Both V57 and Cr59 show evidence of modest oblate deformation near the ground state

Magnetic rotation and quasicollective structures in 58Fe: Influence of the νg9/2 orbital

The structure of 58Fe was investigated at Gammasphere using 48Ca(13 ,14C,xn) fusion-evaporation reactions at a beam energy of 130 MeV. The level scheme has been revised and extended to J∼17 and an excitation energy of 16.6 MeV. Regular band structures consisting of low-energy ΔJ=1 transitions have been observed at moderate spin (J∼8-15) and are candidates for magnetic rotational bands. Self-consistent tilted-axis-cranking calculations within a relativistic mean-field theory were applied to investigate these bands and were found to reproduce the experimental results well. In other parts of the level scheme, quasirotational bands composed of stretched-E2 transitions have been extended to high spin, and other new bands have been identified. Positive-parity experimental states were compared to predictions of the spherical shell model using the GXPF1A, KB3G, and FPD6 effective interactions in the fp model space. The projected shell model, with a deformed quasiparticle basis including the neutron νg9/2 orbital, was applied to interpret regular ΔJ=2 band structures that extend beyond the maximum spin available for π[(f7/2)-2]- ν[(p3/2f 5/2p1/2)4] configurations and exhibit features characteristic of rotational alignment. It is clear that the νg9/2 intruder orbital plays a crucial role in describing the quasirotational structures in this nucleus, even starting as low as J∼5

Intruder configurations of excited states in the neutron-rich isotopes P 33 and P 34

Excited states in the neutron-rich isotopes P33 and P34 were populated by the O18+O18 fusion-evaporation reaction at Elab=24 MeV. The Gammasphere array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus S36. The use of Microball enabled the selection of the proton emission channel. It also helped in determining the exact position and energy of the emitted proton; this was later employed in kinematic Doppler corrections. 16 new transitions and 13 new states were observed in P33 and 21 γ rays and 20 energy levels were observed in P34 for the first time. The nearly 4π geometry of Gammasphere allowed the measurement of γ-ray angular distributions leading to spin assignments for many states. The experimental observations for both isotopes were interpreted with the help of shell-model calculations using the (0+1)ω PSDPF interaction. The calculations accounted for both the 0p-0h and 1p-1h states reasonably well and indicated that 2p-2h excitations might dominate the higher-spin configurations in both P33 and P34

Cross-shell excitations near the "island of inversion": Structure of Mg30

Excited states in Mg30 have been populated to ~6 and 5 MeV excitation energy with the C14(O18,2p) reaction. Firm spin assignments for states with J2 have been made in this nucleus. The level scheme is compared to shell-model calculations using the Universal sd effective interaction and the Monte Carlo shell model method. Calculations employing a full sd model space fail to reproduce the observed levels. The results indicate that excitations across the N=20 gap are required at relatively low excitation energy to achieve a description of the data. The incorporation of the f7/2 and p3/2 orbitals into the model space gives improved results but indicate the need for further refinement of the models to reproduce the observed spectra

Multi-intruder structures in 34P

The available experimental information on 34P has been greatly increased through the analysis of γ decays in coincidence with protons from the interaction of an 18O beam at 24 MeV with an 18O target. Light charged particles from the reaction were detected with Microball, and multiple γ-ray coincidences with Gammasphere. Many observed γ transitions have been identified and placed in the level scheme. Additionally, for most states, spins have been assigned based on measured γ-ray angular distributions while parities were inferred from lifetimes determined through Doppler-broadened line-shape analysis. Most of the states observed have been interpreted in terms of shell-model calculations using the WBP-a and SDPF-NR interactions having one particle in the 0f7/2 or 1p3/2 orbital. The two calculations agree almost equally well with the data resulting in root-mean-square differences of about 200 keV. However, a few high-lying states observed with long lifetimes challenge current calculations. Two of these may be associated with stretched πf7/2⊗νf7/2 states, but the calculations overpredict their energies by 2-3 MeV. Furthermore, a long-lived 7919-keV state is established for which no explanation is available at present

The polarization sensitivity of GRETINA

Compton polarimeters have played an important role in the study of nuclear structure physics, but have often been limited in their applications because of relatively low γ-ray detection efficiency. With the advent of γ-ray tracking detector arrays, which feature nearly 4π solid angle coverage and the ability to identify the location of Compton-scattering events to within a few millimeters, this limitation can be overcome. Here we present a characterization of the performance of the Gamma Ray Energy Tracking In-beam Nuclear Array (GRETINA) as a Compton polarimeter using the 24Mg(p,p′) reaction at 2.45 MeV proton energy. We also discuss a new capability added to the simulation package UCGretina to simulate the emission of polarized photons, and compare it to the measured data. Finally, we use these simulations to predict the performance of the Gamma Ray Energy Tracking Array (GRETA)