Augusto Macchiavelli

Overview of neutron–proton pairing

Abstract The role of neutron–proton pairing correlations on the structure of nuclei along the N = Z line is reviewed. Particular emphasis is placed on the competition between isovector ( T = 1 ) and isoscalar ( T = 0 ) pair fields. The expected properties of these systems, in terms of pairing collective motion, are assessed by different theoretical frameworks including schematic models, realistic Shell Model and mean field approaches. The results are contrasted with experimental data with the goal of establishing clear signals for the existence of neutron–proton ( n p ) condensates. We will show that there is clear evidence for an isovector n p condensate as expected from isospin invariance. However, and contrary to early expectations, a condensate of deuteron-like pairs appears quite elusive and pairing collectivity in the T = 0 channel may only show in the form of a phonon. Arguments are presented for the use of direct reactions, adding or removing an n p pair, as the most promising tool to provide a definite answer to this intriguing question.

Direct Evidence for Octupole Deformation in Ba146 and the Origin of Large E1 Moment Variations in Reflection-Asymmetric Nuclei

Despite the more than 1 order of magnitude difference between the measured dipole moments in ^{144}Ba and ^{146}Ba, the octupole correlations in ^{146}Ba are found to be as strong as those in ^{144}Ba with a similarly large value of B(E3;3^{-}→0^{+}) determined as 48(+21-29)u2009u2009W.u. The new results not only establish unambiguously the presence of a region of octupole deformation centered on these neutron-rich Ba isotopes, but also manifest the dependence of the electric dipole moments on the occupancy of different neutron orbitals in nuclei with enhanced octupole strength, as revealed by fully microscopic calculations.

Development of next-generation nuclear physics integrated readout electronics for GRETINA

Advances in nuclear structure studies using gamma-ray spectroscopy are being driven by large-volume, electrically-segmented germanium detectors such as the Gamma Ray Energy Tracking In-Beam Nuclear Array (GRETINA) and, in the future, GRETA. GRETINA employs 28 close-packed high-purity germanium detectors with 36 segments each assembled on a support structure covering 1-π of the target position. GRETA will use the same type of detectors as GRETINA and it will cover the full 4-π. To fully realize the potential of these detectors in terms of energy and position resolution, event rate and usability, a next-generation integrated readout electronics system is required with an emphasis on channel density, data throughput, low noise, low power and testability. Addressing these areas of research will have a positive impact in these large spectrometer arrays. The present research includes (a) the design and characterization of charge sensitive amplifier application specific integrated circuit (ASIC) for smaller dimension, lower noise and power dissipation, (b) design of a detector-mounted, low-power small form factor signal digitization to address issues in the analog-to-digital (ADC) linearity and to reduce the cable plant, including the design and characterization of a high speed ADC ASIC and (c) strategies to filter the microphonic noise using adaptive filtering. In this paper we will report the status of this research. Also, observe that although the target of this research is GRETINA, these approaches could be used in other Nuclear Science experiments.

Partial-wave contributions to pairing in nuclei

We present a detailed study of partial-wave contributions of nuclear forces to pairing in nuclei. For T = 1, J = 0 pairing, partial waves beyond the standard 1S0 channel play an interesting role for the pair formation in nuclei. We explore the impact of including partial waves beyond the 1S0 channel on the odd-even mass staggering in semi-magic isotopic chains. The additional contributions are dominated by the repulsive 3P1 partial wave.

Shears mechanism in Cd-109

Lifetimes of high-spin states in two {delta}I=1 bands and one {delta}I=2 band in {sup 109}Cd have been measured using the Doppler shift attenuation method in an experiment performed using the {sup 96}Zr({sup 18}O,5n) reaction with the GAMMASPHERE array. Experimental total angular momenta and reduced transition strengths for both {delta}I=1 bands were compared with tilted axis cranking (shears mechanism) predictions and the {delta}I=2 band with principal axis cranking predictions, based on configurations involving two proton g{sub 9/2} holes and one or three valence quasineutrons from the h{sub 11/2} and mixed g{sub 7/2}/d{sub 5/2} orbitals. Good overall agreement for angular momentum versus rotational frequency has been observed in each case. The {delta}I=2 band is shown to have a large J{sup (2)}/B(E2) ratio suggestive of antimagnetic rotation. Additionally, both dipole bands show a decreasing trend in B(M1) strength as a function of spin, a feature of the shears mechanism. The experimental results are also compared with a semiclassical model that employs effective interactions between the proton holes and neutrons as an alternate interpretation for the shears mechanism. (c) 2000 The American Physical Society.

High-spin states in

The 16O+58Ni reaction was used to study yrast and non-yrast excitations in 71As, 72Se, and 72Br. High-spin yrast and negative-parity non-yrast bands were observed in 72Se. The f7/2 proton extruder orbital was identified in 71As. The odd-even staggering in the πg9/2νg9/2 decoupled band in 72Br is compared with similar structures in heavier Br isotopes.

Highlights of experimental results from gammasphere

The large improvement in theresolving power of the new generation of γ-ray arrays is providing new insights in the understanding of the interplay between collective and single-particle motion at extreme conditions. In this paper I will review some of the technical aspects ofGammasphere and discuss a few examples among the broad range of nuclear physics topics that can be addressed with such a detector system. These will include: (i) Superdeformation in the Mass 60 and 80 regions and (ii) Lifetime measurements ofmagnetic rotational bands.