DEPARTMENT OF PHYSICS UNIVERSITY OF JYVASKYL¨ A¨ RESEARCH REPORT No. 3/2007 PRECISION MASS MEASUREMENTS OF NEUTRON-RICH NUCLIDES AROUND A = 100 BY ULRIKE HAGER Academic Dissertation for the Degree of Doctor of Philosophy To be presented, by permission of the Faculty of Mathematics and Natural Sciences of the University of Jyv¨askyl¨a, for public examination in Auditorium FYS-1 of the University of Jyv¨askyl¨aon July 9, 2007 at 12 o’clock noon Jyv¨askyl¨a, Finland July 2007 Preface The work reported in this thesis was carried out during the years 2004-2007 at the Department of Physics of the University of Jyv¨askyl¨a, and at the ISOLDE facility at CERN, Geneva. I would like to thank the whole staff of the Department of Physics for the friendly working atmosphere. I wish to thank my supervisor professor Juha Ayst¨ofor¨ giving me the opportunity to carry out this work, and for his guidance and support. I would also like to thank the members of the IGISOL group, past and present, especially Dr. Stefan Kopecky, Dr. Ari Jokinen, Tommi Eronen, Jani Hakala, Viki Elomaa, Dr. Iain Moore and Thomas Kessler. Part of this work was carried out at CERN, and I would like to thank the whole ISOLTRAP group for their warm welcome. Martin Breitenfeldt, Romain Savreux, M´elanie Marie-Jeanne, Michael Dworschak, Dr. Chabouh Yazidjian and Dr. Alexander Herlert have made my time at CERN not only educational but also fun. In addition, I would like to express my warmest thanks to Dr. habil. Klaus Blaum for his support and the interest he showed in my work. The financial support from the Graduate School of Particle and Nuclear Physics is gratefully ac- knowledged. Finally, I would like to thank my family, especially my brothers, for their support and encouragement. i Abstract The masses of 73 neutron-rich fission products were measured with high precision using the JYFLTRAP Penning trap setup at the IGISOL facility in Jyv¨askyl¨a. These new data enable mapping of the mass surface in this region of the nuclear chart. The data are compared to nuclear model calculations, and various systematic trends are analysed. Since these measurements are part of one of the first precision mass measurement campaigns conducted at JYFLTRAP, the systematic uncertainties of the setup were explored and quantified as far as possible. In addition, analysis programs had to be written to handle the raw data efficiently. At the mass spectrometer ISOLTRAP, CERN, a new project was initiated to install a tape station at the end of the setup. Exploiting the high resolving power of this setup, this will provide the pos- sibility for decay spectroscopy with pure samples. A first design for the new apparatus is presented in this work. ii Contents 1 Introduction 1 2 Nuclear masses 3 2.1 Globalmassmodels ................................... 3 2.2 Pairinganddeformation ............................... ... 5 2.3 The astrophysical r-process . 7 3 Mass measurement techniques 11 3.1 Indirect techniques . 11 3.2 Direct techniques . 11 3.2.1 Time-of-flight measurements . 11 3.2.2 Storagerings .................................... 12 3.2.3 Radio frequency transmission spectrometers . 12 3.2.4 Penningtraps.................................... 12 3.2.5 Comparison..................................... 13 4 Experimental methods 15 4.1 Radioactive ion beam production . 15 4.1.1 Overview ...................................... 15 4.1.2 The ISOL technique . 15 4.1.3 IGISOL ....................................... 15 4.1.4 ISOLDE....................................... 16 4.2 Radio frequency-coolers/bunchers . 18 4.3 Penning traps . 19 4.3.1 Principles...................................... 19 4.3.2 Precision mass measurement . 22 4.3.3 Cooling and isobaric purification . 22 4.3.4 Decay spectroscopy with pure samples . 25 4.3.5 JYFLTRAP..................................... 26 4.3.6 ISOLTRAP..................................... 28 5 Precision mass measurements at JYFLTRAP 31 5.1 Evaluation of the statistical uncertainties . ....... 31 5.2 Systematic uncertainties . 32 5.2.1 Countrate ..................................... 32 5.2.2 Fluctuations .................................... 33 5.2.3 Mass-dependent uncertainties . 33 5.3 Results........................................... 35 5.4 Discussion........................................ 39 5.4.1 Comparison to previous data . 39 5.4.2 Comparison to global models . 44 iii iv CONTENTS 5.4.3 Study of systematic trends . 49 6 Spectroscopy at ISOLTRAP 63 6.1 Currentsituation................................... 63 6.2 Designing the new extraction beamline in SIMION . 63 6.3 Construction of the new detection setup . 65 6.3.1 Differential pumping section . 66 6.4 Status ............................................ 66 7 Summary and outlook 73 A Analysis programs 75 A.1 Frequency calibration and contaminations . 75 A.2 Datafitting ........................................ 75 List of Figures 2.1 Nuclear binding energy per nucleon . 4 2.2 Schematic view of a deformed nucleus . 7 2.3 Nilsson diagram for neutrons and protons with Z or N ≤ 50 ............. 8 2.4 Nilsson diagram for neutrons 50 ≤ N ≤ 80........................ 9 2.5 r-process path . 10 3.1 Uncertainties of different mass measurement techniques . 13 4.1 ISOLmethods ....................................... 16 4.2 IGISOL working principle . 17 4.3 Fissionionguide ................................... 17 4.4 Schematic layout of the ISOLDE hall . 18 4.5 Two possible Penning trap electrode structures . 19 4.6 MotionofanioninaPenningtrap . 21 4.7 Time-of-flight resonance spectrum . 23 4.8 Motion of ions in the trap without and with buffer gas . 24 4.9 Ion motion in the trap with buffer gas and ωc-excitation . 24 4.10 Cooling resonances for A =101.............................. 25 4.11TheJYFLTRAPsetup.................................. 26 4.12 Sketch of the IGISOL RFQ cooler/buncher . 27 4.13 Electrode configuration of the JYFLTRAP purification trap . 27 4.14 Sketch of the ISOLTRAP triple trap setup . 29 5.1 Countrateclassanalysis............................. ..... 34 5.2 Field fluctuations . 35 5.3 Previous measurements of strontium . 40 5.4 Previous measurements of zirconium . 41 5.5 Previous measurements of molybdenum . 43 5.6 Comparison to the HFB2 model predictions. 44 5.7 Comparison to the HFB9 model predictions. 45 5.8 Comparison to the HFBCS-1 model predictions. 45 5.9 Comparison to the FRDM model predictions. 46 5.10 Comparison to the ETFSI-1 model predictions. 46 5.11 Comparison to the ETFSI-2 model predictions. 47 5.12 Comparison to the 10 parameter Duflo-Zuker model predictions. 47 5.13 Comparison to the 28 parameter Duflo-Zuker model predictions. 48 5.14 Comparison to the KTUY model predictions. 48 5.15 S2n as a function of N ................................... 49 5.16 S2n as a function of Z ................................... 51 5.17 S2n and the E2 transition energy . 52 2 N−50,N 5.18 S2n and < δ > ................................... 53 v vi LIST OF FIGURES 5.19 Sn .............................................. 54 ee 5.20 δVpn ............................................. 55 eo 5.21 δVpn ............................................. 56 oe 5.22 δVpn ............................................. 56 ee 5.23 δVpn against N and Z ................................... 57 + + 5.24 E(41 )/E(21 )........................................ 57 ee 5.25 δVpn , HFB-9 for even-even nuclei. 59 ee 5.26 δVpn , FRDM for even-even nuclei. 59 ee 5.27 δVpn , Duflo-Zuker (28 parameters) for even-even nuclei. 60 ee 5.28 δVpn , ETFSI-2 for even-even nuclei. 60 ee 5.29 δVpn , KTUY for even-even nuclei. 61 5.30 Extrapolated binding energies using theoretical δVpn values .............. 61 5.31 Neutron odd-even staggering three-point indicator ∆ν ................. 62 6.1 Yields at ISOLDE for mercury, thallium and francium . 64 6.2 PictureofISOLTRAP ................................... 64 6.3 Current SIMION IOB of the precision trap and extraction . 65 6.4 SIMION, new ISOLTRAP extraction geometry . 65 6.5 SIMION IOB of the new extraction geometry with Channeltron . 66 6.6 Design of both new crosses and the two valves after the ISOLTRAP precision trap . 67 6.7 The trap and the detector/tape station table with three detectors . ..... 68 6.8 Differential pumping/slit system to guide the tape into vacuum . 69 6.9 Differential pumping/slit system: inner part . 70 6.10 Differential pumping/slit system: outer part . ..... 71 6.11 Lower cross and extension piece . 71 6.12 New beamline parts with flanges and electronic feedthroughs . 72 7.1 ν+ Ramsey scan in the precision trap . 74 7.2 Time-of-flight resonance spectra of 115Sn and 115In using Ramsey cleaning . 74 A.1 Frequency calculator GUI . 76 A.2 FittingprogramGUI................................. 77 List of Tables 5.1 Nuclides studied in June 2005 . 36 5.2 Results........................................... 37 5.3 Previous measurements of strontium. 39 5.4 Previous measurements of zirconium. 42 5.5 Previous measurements of molybdenum. 42 5.6 Comparison of δVpn and R4/2 ............................... 58 vii 1 Introduction An important parameter in studies of nuclear structure and in nucleosynthesis is the mass of the nucleus, which is connected to the mass of its constituents and the binding energy: M(N,Z) = N · mn + Z · mp − B(N,Z) (1.1) with mn the mass of the neutron, mp the mass of the proton and B(N,Z) the binding energy of a nucleus with N neutrons and Z protons. Therefore, from the exact knowledge of the nuclear masses follow the binding energies, which in turn are needed for models of nuclear interactions. For several decades the neutron-rich region between A ≈ 80 and A ≈ 130 has been subject to intense study via decay spectroscopy. The reason for this is the suggested shape transition in the rhodium, palladium region from an SU(5) vibrational state to a O(6) γ-unstable state, as noted by Stachel et al. [1], though no sharp onset of deformation was expected. Indication of this transition was also found by Aryaeinejad et al. [2] for neutron-rich palladium, suggesting that the occupation of the νh11/2 orbital may cause the very slow change in excitation energies from N = 64 to N = 70 and lead to oblate deformation.