Vasileios Rakopoulos Measurements of Isomeric Yield Ratios of Proton-Induced Fission of natU and natTh at the IGISOL-JYFLTRAP facility Abstract This thesis presents the measurements of isomeric yield ratios of fission products in 25 MeV proton-induced fission of nat U and nat Th, performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyväskylä. Knowledge of the relative intensities of metastable states produced in fission is of importance for various fields of nuclear physics, both fundamental and applied. The angular momentum of fission fragments is regarded as an important quantity in order to understand the fission mechanism because it can provide information on the scission configuration. One of the means to deduce the angular momentum of highly excited nuclei is by determining the yield ratio of low lying isomeric states. Isomeric yield ratios are also important themselves for simulations of processes such as the r-process, which is believed to be terminated by the fission of very neutron-rich heavy nuclei, and the neutronics and decay heat of nuclear reactors. In addition, proper simulation of the effect of delayed neutrons in a reactor requires accurate knowledge of the population of isomeric states, since the b-delayed neutron emission probability from the isomeric state can be an order of magnitude different from that of the ground state. The measurements were performed from 2010 to 2014, both at IGISOL-3 and at the re- cently upgraded and relocated IGISOL-4 facility. With the IGISOL method short-lived fission product yields can be measured and, by employing the high resolving power of the Penning trap JYFLTRAP, isomeric states separated by a few hundred keV from the ground state can be observed. Thus, a direct determination of the isomeric yield ratios by means of ion counting, registering the products in less than a second after their production has been accomplished for the first time. In addition, g-spectroscopy was employed in order to verify the consistency of the experimental method. Isomeric yield ratios of fission products were measured in a wide mass range (A = 81 to 130) for 25 MeV protons on nat U and nat Th. Specifically, six isomeric pairs (81Ge, 96Y, 97Y, 97Nb, 128Sn and 130Sn) with suitable half-lives were measured and indications of a dependence of the production rate on the fissioning system were observed. A 25 MeV proton beam was selected as there are experimental data available in the literature, determined by means of g-ray spectroscopy, so that a comparison of the results could be performed. "survive another winter" to Katerina and Elias-Sebastian List of papers List of papers is not included in this thesis. Contents Preface ................................................................................................................. 1 1 Introduction .................................................................................................. 3 1.1 A (brief) introduction to fission ....................................................... 3 1.2 Isomers .............................................................................................. 9 1.3 The importance of isomeric yields ................................................ 10 1.4 Fission yields measurements techniques ...................................... 13 2 Experimental Facility ................................................................................ 17 2.1 The IGISOL technique combined with JYFLTRAP .................... 17 2.2 Description of experimental elements .......................................... 18 2.2.1 The fission ion guide ....................................................... 18 2.2.2 Mass separator ................................................................. 21 2.2.3 Radio-Frequency cooler and buncher ............................. 21 2.2.4 Isobaric purification with JYFLTRAP ........................... 22 2.2.5 Timing Structure of the measurement ............................ 25 2.3 Chemical effects of IGISOL and JYFLTRAP .............................. 26 3 Data Analysis ............................................................................................. 29 3.1 Penning Trap Data .......................................................................... 29 3.1.1 Time of flight selection ................................................... 30 3.1.2 Peak intensity determination ........................................... 31 3.1.3 Corrections due to radioactivity ...................................... 34 3.2 g-spectroscopy Data ....................................................................... 36 3.2.1 Efficiency calibration ....................................................... 37 3.2.2 Decay corrections ............................................................ 39 3.2.3 Transport efficiency ......................................................... 42 3.2.4 Uncertainties .................................................................... 42 4 Results and Discussion .............................................................................. 43 4.1 Presentation of the results .............................................................. 43 4.2 Discussion and comparison ........................................................... 44 4.2.1 Mass 81 ............................................................................ 46 4.2.2 Mass 96 ............................................................................ 48 4.2.3 Mass 97 ............................................................................ 48 4.2.4 Mass 128 .......................................................................... 49 4.2.5 Mass 130 .......................................................................... 50 4.3 General remarks ............................................................................. 50 5 Summary and Conclusions ....................................................................... 53 5.1 Summary ......................................................................................... 53 5.2 Conclusions .................................................................................... 53 References ........................................................................................................ 56 List of Figures Fig. 1.1: Nucleus deformation in terms of a liquid drop model ................... 4 Fig. 1.2: Potential energy surface of deforming nucleus .............................. 6 Fig. 1.3: Double-humped fission barrier ........................................................ 7 Fig. 1.4: Time scale of fission fragments de-excitation ................................ 9 Fig. 1.5: Decay paths of nuclides ................................................................. 10 Fig. 1.6: De-excitation of the fission fragments .......................................... 11 Fig. 1.7: Decay scheme of mass chain A=115 ............................................ 12 Fig. 2.1: IGISOL and JYFLTRAP facility .................................................. 19 Fig. 2.2: The fission ion guide at IGISOL ................................................... 20 Fig. 2.3: Ion’s trajectory in the Penning Trap ............................................. 23 Fig. 2.4: Conversion of the ion’s motion in the trap ................................... 24 Fig. 2.5: Timing structure of the measurement ........................................... 26 Fig. 3.1: Time of flight distribution of mass A=96 ..................................... 30 Fig. 3.2: Mass spectrum without and with TOF gating .............................. 31 Fig. 3.3: Frequency distribution of mass A=96 ........................................... 33 Fig. 3.4: g-ray spectrum of mass A=128 ..................................................... 38 Fig. 3.5: HPGe intrinsic efficiency curve .................................................... 39 Fig. 4.1: Isomeric yield ratios ...................................................................... 45 Fig. 4.2: Frequency spectrum for mass A=81 ............................................. 47 Fig. 4.3: g-ray spectrum for mass A=81 ...................................................... 47 Fig. 4.4: The case of multiple results ........................................................... 49 Fig. 4.5: Investigation of the IYR dependence on the fissioning system ... 51 Fig. 4.6: IYR as a function of the spin difference of the states .................. 52 Preface This thesis reports on the experimentally deduced isomeric yield ratios from proton-induced fission on natU and natTh. All the measurements were per- formed at the IGISOL-JYFLTRAP facility at the University of Jyväskylä over a span of four years, from April 2010 to May 2014. This work was accomplished as part of the collaboration between Uppsala University and the University of Jyväskylä that aims at high precision measure- ments of fission yields. Since the fission yields are an important characteristic of the fission process, a brief introduction of the latter is attempted in Chap- ter 1, where a description of the time evolution of the fission and a definition of the fission yields are given. In addition, the importance of the knowledge of the population of the isomeric states for both fundamental and applied physics is emphasised, as motivation for the present work. At the end of the chapter different techniques of measuring fission yields are described. During this period of four years, a lot have changed at the IGISOL facility since a major upgrade was realised, both in the IGISOL and JYFLTRAP fa- cilities. Chapter 2 gives an overview