T. Bäck

Evidence for a spin-aligned neutron-proton paired phase from the level structure of 92Pd

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin–orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron–proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus 92Pd. Gamma rays emitted following the 58Ni(36Ar,2n)92Pd fusion–evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron–proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron–proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

SPIN-ALIGNED NEUTRON-PROTON PAIR MODE IN ATOMIC NUCLEI

Shell model calculations reveal that the low-lying spectrum of the N = Z nucleus 92 Pd is generated from a correlated isoscalar spin-aligned neutron-proton pair mode, exhibiting a new form of collectivity different from vibrational and rotational excitations. Already the ground state structure of 92 Pd is mostly built from isoscalar pairs each carrying angular momentum J = 9. This structure is different from all other even-even nuclei studied so far. The energy spectrum generated by the correlated neutron-proton pairs has two distinctive features: i) it is almost equidistant for low-lying energies and ii) the transition probability I ! I − 2 is approximately constant and independent of I. This exotic coupling scheme is predicted to correspond to the yrast structures of the heaviest nuclei approaching the doubly-magic 100 Sn.

Application of Ultra-fast Timing Techniques to the Study of Exotic and Weakly Produced Nuclei

Ultra-fast time-delayed techniques have been recently applied in a number of studies where exotic nuclei were identified using advanced selection techniques. These include large Compton-suppressed ...

Identification of excited states in 167Os and 168Os: shape coexistence at extreme neutron deficiency

Excited states in the very neutron-deficient isotopes Os-167 and Os-168 have been observed using the reaction Sn-112(Ni-58, 2pxn). The JUROSPHERE gamma -ray spectrometer array was used in conjuncti ...

Collective Rotational - Vibrational Transition in the Very Neutron-Deficient Nuclei

Excited states have been identified for the first time in very neutron deficient Pt-171.172 nuclei using the recoil-or-decay tagging technique. The ground-state band in Pt-172 has been established up to I-pi = 8+. A similar level sequence, presumably built on the I-pi = 13/2(+) state, is observed for Pt-171. The data are compared with theoretical calculations based on the mean field approach and the random phase approximation and are put into the context of the systematics of platinum isotopes

^171, 172

Abstract Excited states have been observed for the first time in 168 Pt and 170 Pt using the α -decay recoil-tagging technique. The trend of decreasing deformation moving away from the N =104 mid-shell continues for 170 Pt but the structure of 168 Pt is significantly different. The low spin level energy systematics in 168 – 184 Pt are presented and discussed within the framework of the interacting boson model.

Pt

A TOF-PET system has been designed and constructed for educational purposes. The aim of this system is to demonstrate the possibilities of positron emission tomography in general and the time-of-fl ...

FIRST OBSERVATION OF EXCITED STATES IN THE NEUTRON DEFICIENT NUCLEI 168PT AND 170PT

A detector system for positron emission tomography with time-of-flight capability has been built to serve as an educational tool for undergraduate students. The set-up consists of 48 BaF2 scintillator crystals, each coupled to a fast photo-multiplier tube, mounted in a circular geometry. The analogue detector pulses are handled by fast constant fraction discriminators. A dedicated unit reduces the 48 channels to eight channels via delay-fine encoding, and the signals are then fed to an eight channel fast time-to-digital converter. A VME processor sorts the events and sends them to a workstation where the coincident events are extracted. The time resolution of the detectors together with fast VME based electronics allows for time-of-flight measurements to improve on the signal-to-noise ratio in the, reconstructed images. The system can be used for different types of exercises for the students, varying from the fundamentals of scintillator detectors to advanced image reconstruction. The set-up is described and some results are presented

A TOF-PET system for educational purposes

Abstract Excited states in 171Ir have been observed for the first time. Gamma-rays were assigned to the nucleus by the recoil-decay tagging method. The ground-state band has a structure consistent with an h 11 2 proton coupled to a core of large triaxial deformation. At high spins, a bandcrossing occurs which is interpreted as a change in shape to a prolate deformation. Band-mixing calculations are performed for 171–175Ir. These show that shape-coexistence between triaxial and prolate states in these nuclei follows the same systematics found in their Pt and Os neighbours. The systematics are also compared with deformations calculated for 171–179Ir using the code “Ultimate Cranker”. Dipole bands were also observed, but tilted axis cranking calculations suggest that they are associated with a collective rotation.

An educational tool for demonstrating the TOF-PET technique

A method to extract depth of interaction information for gamma-rays in a segmented planar Ge detector is presented. The method is demonstrated on signals from a segmented detector which were stored ...