title: Evidence for a spin-aligned neutron-proton paired phase from the level structure of (92)Pd.
abstract: 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 (92)Pd. Gamma rays emitted following the (58)Ni((36)Ar,2n)(92)Pd 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.
authors.affiliation: Department of Physics, Royal Institute of Technology, SE-10691 Stockholm, Sweden. cederwall@nuclear.kth.se
authors.forename: B
authors.initials: B
authors.lastname: Cederwall
authors.forename: F Ghazi
authors.initials: FG
authors.lastname: Moradi
authors.forename: T
authors.initials: T
authors.lastname: Bäck
authors.forename: A
authors.initials: A
authors.lastname: Johnson
authors.forename: J
authors.initials: J
authors.lastname: Blomqvist
authors.forename: E
authors.initials: E
authors.lastname: Clément
authors.forename: G
authors.initials: G
authors.lastname: de France
authors.forename: R
authors.initials: R
authors.lastname: Wadsworth
authors.forename: K
authors.initials: K
authors.lastname: Andgren
authors.forename: K
authors.initials: K
authors.lastname: Lagergren
authors.forename: A
authors.initials: A
authors.lastname: Dijon
authors.forename: G
authors.initials: G
authors.lastname: Jaworski
authors.forename: R
authors.initials: R
authors.lastname: Liotta
authors.forename: C
authors.initials: C
authors.lastname: Qi
authors.forename: B M
authors.initials: BM
authors.lastname: Nyakó
authors.forename: J
authors.initials: J
authors.lastname: Nyberg
authors.forename: M
authors.initials: M
authors.lastname: Palacz
authors.forename: H
authors.initials: H
authors.lastname: Al-Azri
authors.forename: A
authors.initials: A
authors.lastname: Algora
authors.forename: G
authors.initials: G
authors.lastname: de Angelis
authors.forename: A
authors.initials: A
authors.lastname: Ataç
authors.forename: S
authors.initials: S
authors.lastname: Bhattacharyya
authors.forename: T
authors.initials: T
authors.lastname: Brock
authors.forename: J R
authors.initials: JR
authors.lastname: Brown
authors.forename: P
authors.initials: P
authors.lastname: Davies
authors.forename: A
authors.initials: A
authors.lastname: Di Nitto
authors.forename: Zs
authors.initials: Z
authors.lastname: Dombrádi
authors.forename: A
authors.initials: A
authors.lastname: Gadea
authors.forename: J
authors.initials: J
authors.lastname: Gál
authors.forename: B
authors.initials: B
authors.lastname: Hadinia
authors.forename: F
authors.initials: F
authors.lastname: Johnston-Theasby
authors.forename: P
authors.initials: P
authors.lastname: Joshi
authors.forename: K
authors.initials: K
authors.lastname: Juhász
authors.forename: R
authors.initials: R
authors.lastname: Julin
authors.forename: A
authors.initials: A
authors.lastname: Jungclaus
authors.forename: G
authors.initials: G
authors.lastname: Kalinka
authors.forename: S O
authors.initials: SO
authors.lastname: Kara
authors.forename: A
authors.initials: A
authors.lastname: Khaplanov
authors.forename: J
authors.initials: J
authors.lastname: Kownacki
authors.forename: G
authors.initials: G
authors.lastname: La Rana
authors.forename: S M
authors.initials: SM
authors.lastname: Lenzi
authors.forename: J
authors.initials: J
authors.lastname: Molnár
authors.forename: R
authors.initials: R
authors.lastname: Moro
authors.forename: D R
authors.initials: DR
authors.lastname: Napoli
authors.forename: B S Nara
authors.initials: BS
authors.lastname: Singh
authors.forename: A
authors.initials: A
authors.lastname: Persson
authors.forename: F
authors.initials: F
authors.lastname: Recchia
authors.forename: M
authors.initials: M
authors.lastname: Sandzelius
authors.forename: J-N
authors.initials: JN
authors.lastname: Scheurer
authors.forename: G
authors.initials: G
authors.lastname: Sletten
authors.forename: D
authors.initials: D
authors.lastname: Sohler
authors.forename: P-A
authors.initials: PA
authors.lastname: Söderström
authors.forename: M J
authors.initials: MJ
authors.lastname: Taylor
authors.forename: J
authors.initials: J
authors.lastname: Timár
authors.forename: J J
authors.initials: JJ
authors.lastname: Valiente-Dobón
authors.forename: E
authors.initials: E
authors.lastname: Vardaci
authors.forename: S
authors.initials: S
authors.lastname: Williams