Production and identification of heavy Ni isotopes: evidence for the doubly magic nucleus 28 78 Ni

Abstract

Abstract. We report the first observation of the doubly magic nucleus 78 Nis0 and the heavy isotopes 77Ni, 73,74,7s Co, so Cu. The isotopes were produced by nuclear fission in collisions of 750 A-MeV projectiles of 23su on Be target nuclei. The fully-stripped fission products were separated in-flight by the fragment separator FRS and identified event-by-event by measuring the magnetic rigidity, the trajectory, the energy deposit, and the time of flight. Production cross-sections and fission yields for the new Ni-isotopes are given.PACS: 21.60.Cs; 27.50+e; 25.85-w Doubly-closed shell-nuclei and the neighboring isotopes are intensively investigated as the nuclear shell-model residual interaction is calculated from excitation energies and quantum numbers of their well-characterlzed levels. 7s Ni is the heaviest of three doubly magic Ni isotopes: is Ni20, 56Ni2s and 78 Nis0 and, with 100 Sn, the second doubly closed-shell nucleus among the N=50 isotones. Neutron-capture and ~3-decay properties around Z=28 and N=50 are of great importance with respect to the stellar r-process needed to explain the relative abundances of nuclei in the solar system. It has been a long standing challenge to observe rSNi because of its very large A/Z-ratio of 2.79. As refractive elements are extracted with low efficiency by ISOL-methods until now, recoil separators are better suited to investigate these elements. The most neutron-rich Ni isotopes were produced either by low-energy fission or by projectile fragmentation. The isotopes of 7~ were discovered [1], and their haM-fives were measured [2] in very asymmetric thermal-neutron induced fission of 235 U. Production of 75-7~Ni was investigated by 86Kr projectile-fragmentation at 500 A.MeV, and their half-lives were determined [3,4]. By extrapolating the production cross-section for S6Kr fragmentation along the line N=50 or Z=28, a value of 0.5 pb is expected for the production of 7SNi. A count rate of 0.5 7SNi/day follows for an effective luminosity of 103a cm-2s -a . The discovery of very n-rich fission fragments in the reaction of 750 A-MeV 238U on a Pb target [5], the production cross sections of 190 t~b for Ni isotopes and the higher luminosity in case of a Be target suggested to use the fission process for searching for 78Ni [6].Projectile fission at relativistic energies leads to forward-focussed fully-ionized fragments. Within the 30 mrad angular aperture and the 2% momentum acceptance of the FRS [7] only fragments emitted forward or backward in the U-frame of reference can be transmitted. The consequences are a clean selection of forwardemitted fission fragments at rigidities Bp larger than the projectile value (Bp)o, but a transmission of fission fragments through the FRS of a few percent only [6].238 U ions deliveredby a Penning-source were acceleratedin the UNILAC and the heavy-ion synchrotron SIS (GSI) to an energy of 750 A.MeV. Spills with a duration of 1 s and an average number of 10 g U-ions were extracted every 5 s and directed onto a 1 g/cm 2 Be target. Reaction product...

Radioactive decays at limits of nuclear stability

“…The resulting extreme sensitivity is one of the most important advantages of the medium-and high-energy In-Flight technique. Indeed, this was the key factor allowing the first observation of doubly-magic nuclei 100 Sn ( Lewitowicz et al, 1994;Schneider et al, 1994), 78 Ni (Engelmann et al, 1995), and 48 Ni (Blank et al, 2000), as well as the discovery of the two-proton radioactivity (Giovinazzo et al, 2002;Pfützner et al, 2002).…”

$^{78}$Ni revealed as a doubly magic stronghold against nuclear deformation

“…Although similar amounts of (p, 2p) and (p, 3p) events were detected, the findings were largely different. A transition remained visible at 1,067 (17) keV in the (p, 3p) spectrum (Fig. 3d), but no further prominent peak was observed in the energy range up to 2,600 keV, with transitions reported for the (p, 2p) channel possessing only S.L.…”

Fast-timing study of Ga81 from the β decay of Zn81

“…Although 78 Ni, with 28 protons and 50 neutrons (Z = 28, N = 50), is located 14 neutrons off the stability line, it is expected to be a doubly magic nucleus due to the robust shell gaps arising from the spin-orbit splitting both for protons (π f 7/2 − π f 5/2 ) and neutrons (νg 9/2 − νg 7/2 ). The first evidence for the existence of 78 Ni came from [2]. Afterward its half-life was reported [3] to be T 1/2 = 100 +100 −60 ms and more recently T 1/2 = 122 ± 5 ms [4].…”