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Ultra-Fast Timing Study of Exotic Neutron-Rich Fe Isotopes Estudio De

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Ultra-Fast Timing Study of Exotic Neutron-Rich Fe Isotopes Estudio De Universidad Complutense de Madrid Facultad de Ciencias Físicas Dpto. de Física Atómica, Molecular y Nuclear Ultra-fast timing study of exotic neutron-rich Fe isotopes Estudio de coincidencias ultra- rápidas de isótopos exóticos de Fe ricos en neutrones CERN-THESIS-2013-264 20/09/2013 Bruno Olaizola Mampaso PhD Thesis supervisors: Dr. Luis Mario Fraile Prieto Dr. Henryk Mach Madrid, 2013 Ultra-fast timing study of exotic neutron-rich Fe isotopes Bruno Olaizola Mampaso Supervisor Co-supervisor Luis Mario Fraile Henryk Mach Madrid, September, 2013 Dpto. F´ısicaAt´omica,Molecular y Nuclear Universidad Complutense de Madrid Contents Summary i Resumen en castellano xv Outline of this Thesis xxix 1 Introduction 1 1.1 The shell model . .2 1.1.1 The nuclear shell model . .2 1.1.2 The spin-orbit interaction . .4 1.1.3 Deformed nuclei . .6 1.2 The neutron rich N = 40 region . .9 1.2.1 The region below 68Ni ...................... 10 1.2.2 The fp shell and the 0g9=2 and 1d5=2 intruders . 10 1.3 Experimental motivation . 13 2 The Advanced Time Delayed βγγ(t) method 15 2.1 Experimental setup . 16 2.1.1 Detector selection . 17 2.1.2 Detector arrangement . 18 2.2 Techniques of data evaluation . 19 2.2.1 Convolution technique . 20 2.2.2 Centroid shift technique . 21 CONTENTS 2.3 Concluding remarks . 26 3 Technical details 29 3.1 ISOLDE . 29 3.1.1 Linac 2 . 30 3.1.2 PS-Booster and its proton supercycle . 31 3.1.3 The ISOLDE target . 33 3.1.4 RILIS . 36 3.1.5 Mass separators and beam line . 39 3.2 Experimental setup . 41 3.2.1 The detector system . 41 3.2.2 Electronics . 43 3.2.3 Digital Gamma Finder Pixie-4 . 46 3.3 Sorting software . 48 3.3.1 Pixie data writing structure . 49 3.3.2 Presorting . 52 3.3.3 Sorting . 54 3.4 Concluding remarks . 55 4 Calibrations 57 4.1 Energy calibration . 57 4.1.1 LaBr3(Ce) energy calibration and stability . 58 4.1.2 HPGe energy calibration and stability . 60 4.1.3 HPGe efficiency . 62 4.2 Timing calibration . 66 4.2.1 TAC calibrations . 66 4.2.2 β-walk . 67 4.2.3 Compton-walk . 68 4.2.4 Prompt curve . 71 4.3 Concluding remarks . 75 CONTENTS 5 Structure of 65Fe 77 5.1 Information on 65Fe from previous studies . 77 5.2 Results . 78 5.2.1 65Mn half-life . 79 5.2.2 γγ coincidences . 79 5.2.3 The exact energy of the β-decaying isomer . 90 5.2.4 The 420-ns isomer . 91 5.2.5 Absolute β and γ intensities . 93 5.2.6 β-n branch directly feeding the g.s. of 64Fe........... 95 5.3 Ultra-fast timing measurements . 96 5.3.1 Half-life of the 455.6-keV level . 97 5.3.2 Half-life of the 363.7-keV level . 98 5.3.3 Half-life of the 561.0-keV level . 99 5.3.4 Half-life of the 569.1-keV level . 100 5.3.5 Half-life of the 683.3-keV level . 100 5.3.6 Half-life of the 894.8-keV level . 101 5.4 Discussion . 101 5.5 Calculations . 107 5.6 Conclusion . 109 6 β decay of 63Mn and structure of 63Fe 111 6.1 Previous knowledge of 63Fe........................ 111 6.2 Experimental results . 112 6.2.1 63Mn half-life . 113 6.2.2 63Fe levels and transitions . 114 6.2.3 The 475.0-keV isomer . 123 6.2.4 Ground state feeding . 126 6.2.5 β-delayed neutron emission branch . 127 6.3 Fast-timing measurements . 128 6.3.1 451.1-keV level half-life . 129 CONTENTS 6.3.2 357.3-keV level half-life . 130 6.3.3 681.3-keV level half-life . 131 6.4 Discussion . 132 6.5 Conclusions . 134 7 Structure of 66Fe 137 7.1 Existing information on 66Fe ...................... 137 7.2 Experimental results . 138 7.2.1 Measurement of the 66Mn and 66Fe half-lives . 139 7.2.2 66Fe level scheme . 142 7.2.3 Direct beta-feeding to the ground state . 146 7.2.4 β-delayed neutron emission branch . 148 7.3 Fast-timing analysis . 150 7.3.1 Half-lives measured via βγ .................... 150 7.3.2 Half-lives measured via βγγ ................... 151 7.3.3 Check via centroid shift . 151 7.3.4 Half-lives results . 153 7.4 Shell model calculations . 153 7.5 Discussion . 158 7.6 Conclusions . 161 8 Conclusions and outlook 163 Appendix 169 65Co preliminary results . 169 Preliminary half life in 66Ni .......................... 171 Summary The cornerstone of nuclear structure, as we know it from stable nuclei, is the exist- ence of magic numbers. The most stable nuclei arise for completely occupied shells, closed shells, and give rise to the magic numbers. At the Valley of Stability their values are 8, 20, 28, 50, 82 and 126. The steady development of the production, separation and identification of exotic nuclei, together with the improvement of the detection techniques, makes it possible to experimentally explore nuclei further away from the Valley of Stability. These exotic nuclei with nucleon numbers supposed to be magic do not always have the properties one would expect. As extra nucleons are added (or removed) from stable nuclei, the single particle energies are modified and strong quadrupole correlations appear, which may neutralize the spherical mean-field shell gaps. The investigation of the evolution of shell structure far from stability has be- come a major subject in Nuclear Physics. Research in this field has strong implic- ations also in nuclear astrophysics, because exotic nuclei have a crucial role in the processes of stellar nucleosynthesis leading to the formation of the nuclei present in the Universe. The IS474 experiment was performed at the ISOLDE facility, CERN, where 59−66Mn isotopes were created and their β-decay chains studied. In this PhD Thesis we focused on the study of the neutron-rich iron isotopes (odd-A 63;65Fe and even-A 66Fe) by means of gamma and Advanced Time-Delayed spectroscopy. Introduction In the harmonic oscillator potential N = 40 is a magic number. However, in the nuclear framework it is weakened by the spin-orbit interaction, which lowers the orbitals with the highest j in the next major oscillator shell, which in the present case is the g9=2 orbital. The g9=2 orbital is a primary reason for a strong collectivity in the neutron-rich nuclei of Fe and Cr around N = 40. As protons are added to the f7=2 i ii Summary orbital, the spin-orbit splitting of the negative parity pf neutron orbitals increase due the strongly attractive neutron-proton tensor interaction [1]. Therefore, as the neutron pf orbits are filled, the excitation to the intruder positive parity orbital g9=2 becomes more and more accessible. Shell model calculations using only the pfg (1p3=2, 1p1=2 0f5=2 and 0g9=2) neut- ron valence space fail to correctly describe the collectivity at N = 40 [2, 3], and + 66 specifically the low energy of the 573-keV 21 state in Fe [4]. As pointed out in [5], a proper description of the strong quadrupole collectivity in this region requires also the inclusion of the neutron 1d5=2 orbital. Recently Lenzi et al. [6] have developed shell-model calculations in a large valence space that encompasses the pf shell for protons and the 1p3=2, 1p1=2 0f5=2, 0g9=2 and 1d5=2 orbitals for neutrons, by using an effective interaction based on G matrix and with the monopole part empirically tuned to reproduce the experimental single-particle energies. With this approach a very good agreement with the available experimental data was obtained, not only for excitation energies but also for transition rates. A strong quadrupole deformation, coming close to the rotational regime, is predicted for Cr isotopes. The onset of collectivity in Fe isotopes is shown to develop at N = 40 (66Fe), with multiparticle- multihole configurations playing a key role in the ground state wave functions, and as many as four neutrons present in the intruder g9=2 and d5=2 orbitals coming from the upper oscillator shell. The region below 68Ni The N = 40 nucleus 68Ni (Z = 28) shows some of the characteristics of a doubly + magic nucleus. It has a large E(21 ) energy of more than 2 MeV [7] and a small + + 2 4 value of B(E2; 01 ! 21 ) = 265 e fm [8] corresponding to 3.2 W.u. However, mass measurements have showed that the shell gap at N = 40 is weak for 68Ni [9] and imply that the small B(E2) value does not indicate a sub-shell gap. One should note that neutrons from the valence νp1=2 orbital, which is completely filled in a spherical N = 40 system, cannot couple together to form a J = 2 state, which explains the + 68 high energy of the 21 state in Ni. + As protons are removed from the f7=2 orbital, the energies of the 21 states drop sharply and the B(E2) values increase. By removing only two protons from 68 + 66 Ni the energy of the 21 state drops from 2033 keV to only 573 keV in Fe [4]. By removing two more protons the energy decreases even further to 420 keV in 64Cr, + which is in the middle of the proton shell (Z=24). This is the lowest-lying 21 level experimentally reported so far in the region [10].
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