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Project
Two-neutron transfer reactions with radioactive beams.
The region around the nucleus 68Ni, with a shell closure for its protons at Z=28 and a harmonic oscillator shell gap for its neutrons at N=40, has drawn considerable interest over the past decades. 68Ni has properties that are typical for a doubly-magic nucleus, such as a high excitation energy and low B(E2:2+->0+) transition probability for the first excited 2+ level and a 0+ level as the first excited state. However, it has been suggested that the magic properties of 68Ni arise due to the fact that the N=40 separates the negative parity pf-shell from the positive parity 1g9/2 orbital, and indeed, recent mass measurements have not revealed a clear N=40 energy gap. Despite all additional information that was acquired over the last decade the specific role of the N=40 is not yet understood and a new experimental approach to study 68Ni was proposed. Namely, a two-neutron transfer reaction on 66Ni to characterize and disentangle the structure of the low-lying 0+ and 2+ states in 68Ni.
The experiment was performed at the ISOLDE facility at CERN, Geneva, Switzerland. A radioactive 66Ni beam was produced in several steps. It started with an impingement of high energetic protons on a thick uranium-carbide target, after which the desired isotopes were ionized and accelerated to 30keV and finally, to eliminate contamination of the beam, the beam was passed through a mass separator. However, in order to perform transfer experiments a higher beam energy is required and thus the 66Ni beam was post-accelerated to 2.6MeV/u with the REX linear accelerator. The 66Ni beam was then guided towards a radioactive tritium-loaded titanium foil, where the reaction took place. The reaction products were detected with T-REX and Miniball. T-REX is a position-sensitive particle detection array, consisting out of several silicon detectors, while Miniball is an array of position-sensitive gamma ray detectors consisting of high-purity germanium detectors.
A first step in the analysis was thecalibration of the data and performing a particle identification. From proton-gamma and proton-gamma-gamma coincidences a level scheme of 68Ni could be constructed. No new levels were identified in this research, however, the excitation energy of the level that is populated in 68Ni can be deduced from the detected proton energy. Out of the probability of populating different states, structure information can be derived. By looking at the excitation energy spectra it was clear that most of the feeding in the two-neutron (t,p) transfer reaction to 68Ni goes to highly excited levels between 5-9MeV. Also, a strong feeding to the ground and a direct population of the first excited 0+2 at 1604keV and 2+1 state at 2033keV was observed, namely respectively 4.2(16)% and 29.3(29)% of the ground state feeding. Direct population of other known 0+ and 2+ states in68Ni was not detected, only upper limits could be determined.
In a second step of the analysis the angular distributions constructed for the ground state and first excited 0+ and 2+ state were compared with theoretical DWBA calculations performed with Fresco, where input from the shell-model code Nushell was used. The predicted magnitude of theangular distributions for the ground and 0+2 state is in good agreementwith the data, while that for the 2+1 state is an order of magnitude too small. This discrepancy is currently not understood. The agreement of the feeding of the 0+ states with the calculations indicates that the structure of the 0+2 state consists dominantly out of two neutrons in the g9/2 orbital.
Further, the obtained results for 68Ni were compared to the systematics of the (t,p) reactions on the lighter, stable nickel isotopes and to its valence counterpart 90Zr, which has a shell closure for its neutrons at N=50 and a harmonic oscillator shell closure for its protons at Z=40. An outlook for new experiments to study 68Ni closes the thesis. .
The experiment was performed at the ISOLDE facility at CERN, Geneva, Switzerland. A radioactive 66Ni beam was produced in several steps. It started with an impingement of high energetic protons on a thick uranium-carbide target, after which the desired isotopes were ionized and accelerated to 30keV and finally, to eliminate contamination of the beam, the beam was passed through a mass separator. However, in order to perform transfer experiments a higher beam energy is required and thus the 66Ni beam was post-accelerated to 2.6MeV/u with the REX linear accelerator. The 66Ni beam was then guided towards a radioactive tritium-loaded titanium foil, where the reaction took place. The reaction products were detected with T-REX and Miniball. T-REX is a position-sensitive particle detection array, consisting out of several silicon detectors, while Miniball is an array of position-sensitive gamma ray detectors consisting of high-purity germanium detectors.
A first step in the analysis was thecalibration of the data and performing a particle identification. From proton-gamma and proton-gamma-gamma coincidences a level scheme of 68Ni could be constructed. No new levels were identified in this research, however, the excitation energy of the level that is populated in 68Ni can be deduced from the detected proton energy. Out of the probability of populating different states, structure information can be derived. By looking at the excitation energy spectra it was clear that most of the feeding in the two-neutron (t,p) transfer reaction to 68Ni goes to highly excited levels between 5-9MeV. Also, a strong feeding to the ground and a direct population of the first excited 0+2 at 1604keV and 2+1 state at 2033keV was observed, namely respectively 4.2(16)% and 29.3(29)% of the ground state feeding. Direct population of other known 0+ and 2+ states in68Ni was not detected, only upper limits could be determined.
In a second step of the analysis the angular distributions constructed for the ground state and first excited 0+ and 2+ state were compared with theoretical DWBA calculations performed with Fresco, where input from the shell-model code Nushell was used. The predicted magnitude of theangular distributions for the ground and 0+2 state is in good agreementwith the data, while that for the 2+1 state is an order of magnitude too small. This discrepancy is currently not understood. The agreement of the feeding of the 0+ states with the calculations indicates that the structure of the 0+2 state consists dominantly out of two neutrons in the g9/2 orbital.
Further, the obtained results for 68Ni were compared to the systematics of the (t,p) reactions on the lighter, stable nickel isotopes and to its valence counterpart 90Zr, which has a shell closure for its neutrons at N=50 and a harmonic oscillator shell closure for its protons at Z=40. An outlook for new experiments to study 68Ni closes the thesis. .
Date:1 Oct 2009 → 28 Jan 2014
Keywords:Radioactive beams, (t, p) reaction, MINIBALL, ISOLDE
Disciplines:Nuclear physics
Project type:PhD project