| 1. |
- Aktas, Özge
(författare)
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Gamma-ray Spectroscopy ofNeutron-rich 111 Mo, 85,87 Ge andSelf-Conjugate 88 Ru Far From Stability
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The neutron-deficient self-conjugate (N=Z) nucleus 8844 Ru 44 was populatedvia the heavy ion fusion evaporation reaction 54 Fe( 36 Ar, 2n) 88 Ru in an ex-periment performed at the GANIL accelerator laboratory in France. Usingthe AGATA γ-ray spectrometer together with ancillary detectors, promptγ − γ − 2n coincidence and charge particle anticoincidence analysis was per-formed for the low-lying energy spectrum of 88 Ru. The results confirm thepreviously assigned γ-ray cascade and extend it to the 14 + level. The levelscheme is consistent with a deformed rotational system. However, the rota-tional frequency of the alignment of the valence nucleons has a significantlyhigher value than what is predicted by theoretical calculations performedwithout isoscalar neutron-proton pairing. By including isoscalar pairing, anagrement is obtained with the experimentally observed delayed rotationalalignment.Excited states in the neutron-rich nuclei 109 Mo and 111 Mo were studiedfollowing nucleon knock-out reactions. Seven γ-ray transitions, some of themin prompt mutual coincidence, were identified for the first time in 111 Mo usingthe DALI2 and MINOS detector systems at the BigRIPS and ZeroDegree elec-tromagnetic fragment separator at the RIBF, RIKEN, Japan. Total Routhiansurface (TRS) and Particle-Plus-Rotor calculations have been performed toinvestigate the predicted shape coexistence and its effect on the structure ofnuclei in this region of the nuclear chart. Following the results of the calcula-tions, theoretical level schemes are proposed for positive and negative paritystates and compared with the experimental findings.Gamma-ray transitions have been identified for the first time in the ex-tremely neutron-rich (N = Z + 25) nucleus 87 Ge following nucleon knockoutreactions studied at the RIBF, RIKEN, Japan. Previously unknown γ-raytransitions between excited states in 85 Ge were also observed and placed ina tentative level scheme. The results are compared with large-scale shell-model calculations and potential energy surface calculations based on thetotal Routhian surface formalism. The neutron-rich titanium isotopes havebeen studied, and preliminary results are presented in this work. For the odd-even 57,59,61 Ti isotopes several gamma-ray transitions has been identified forthe first time. For the even-even isotopes 56,58,60 Ti the previously knowndecays from 2 + and 4 + spin-parity states, are confirmed with the currentpreliminary analysis.
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| 2. |
- Ertoprak, Aysegul, 1987-
(författare)
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Experimental Studies of the Neutron Deficient Atomic Nuclei 94Ru, 95Rh and 172Pt via their Electromagnetic Properties : Du som saknar dator/datorvana kan kontakta Pär Olsson, polsson@kth.se för information
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis reports new results obtained from studies of the neutron deficient atomic nuclei 94Ru, 95Rh and 172Pt using two different experimental set-ups. In the first part, lifetimes of highly excited states in nuclei near the N=50 closed-shell (94Ru and 95Rh) were deduced from an analysis of the Doppler broadened transition lineshapes measured following the 58Ni(40Ca,4p) and 58Ni(40Ca,3p) fusion-evaporation reactions at the Grand Accelerateur National d'Ions Lourds (GANIL) accelerator complex situated in Caen, France. Doppler Shift Attenuation Method (DSAM) lifetime analysis was performed on the Doppler broadened peaks in energy spectra from γ-rays emitted from excited states in the nuclei of interest while they were slowing down in a thick 6 mg/cm2 metallic 58Ni target. For 94Ru, eight excited-state lifetimes in the angular momentum range I= (13-20)h have been measured in total, five of which were determined for the first time. For the lifetime analysis of 95Rh, three lifetime values have been obtained. One of them, the lifetime of the 39/21- excited state, has been measured for the first time. In the other cases, the lifetimes of the previously measured 29/22- and 37/21- excited states have been obtained. The corresponding B(M1) and B(E2) reduced transition strengths have been deduced and are discussed within the framework of large-scale shell model (LSSM) calculations. In the second part, the extremely neutron deficient 172Pt nucleus has been studied. Excited states in 172Pt were populated using the 96Ru(78Kr,2p) and 92Mo(83Kr,3n) reactions at the Accelerator Laboratory of the University of Jyväskylä (JYFL), Finland. Prompt γ-rays were detected using the JUROGAM high-purity germanium detector array at the target position while the identification and decay spectroscopy of 172Pt was performed using the RITU gas-filled separator in conjunction with the GREAT spectrometer. The Recoil Decay Tagging (RDT) technique was used for the selection of prompt γ-rays. The known positive-parity band has been extended and the negative-parity structure has been established on top of the lowest member of the negative parity band which has now been firmly assigned as spin-parity 3-. Moreover, the newly observed E3 transition provides a link between the negative parity band with the ground state. The observations of this E3 transition together with several E1 transitions connecting the negative-parity structure with the ground-state band is consistent with the presence of octupole collectivity in 172Pt. Furthermore, this is the first observation of an E3 transition connecting the negative parity band with the ground-state band in the Pt-Os-W region. The experimental results were interpreted in terms of LSSM and total routhian surface calculations. With the support of these theoretical calculations, evidence for octupole collectivity in 172Pt is proposed.
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| 3. |
- Liu, Xiaoyu
(författare)
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Experimental Studies of the Neutron Deficient Atomic Nuclei 88Ru and 87Tc, and the Diagonalization of the General Pairing Hamiltonian
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral thesis consists of two parts: the experimental study of the neutron-deficient atomic nuclei 88Ru and 87Tc, and the computational study on the diagonalization of general pairing Hamiltonian. In the first part, which constitutes the main content of the thesis, the low-lying excited states in the N = Z nucleus 88Ru and N = Z + 1 nucleus 87Tc were studied via fusion-evaporation reactions induced by 115 MeV 36 Ar ions bombarding6 mg/cm 2 thick metallic 54 Fe target foils at the Grand Accélérateur Nationald’Ions Lourds (GANIL), Caen, France. The prompt γγ-neutron and charged-particle coincidences from the de-excitation of the reactions were measured by the AGATA γ-ray spectrometer coupled to the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays. The results for 88 Ru confirmed and extended the previous level scheme to a tentative (14+) state. The constructed level structure exhibits a moderately deformed rotational behavior but shows a band crossing at a significantly higher rotational frequency compared with neighboring nuclei with N > Z. Such band crossings are associated with quasiparticle alignment within the standard isovector pairing scheme. The observation of a “delayed” rotation alignment in the deformed N = Znucleus is consistent with theoretical predictions related to the existence of strong isoscalar neutron-proton pair condensate. The yrast band in 87 Tc from the (9/2+) state to the (33/2+) state was established based on six mutually coincident γ-ray transitions. The constructed yrast band exhibits a sharp backbending at ~ω ≈ 0.50 MeV. In the odd-A isotonic chains around N = 44, approaching the N = Z line, the observed decrease in alignment frequency and increase in alignment sharpness were proposed as an effect of the enhanced isoscalar neutron-proton interactions. In addition to the yrast band in 87 Tc, six new mutually coincident γ-ray transitions were identified by comparing the γ-ray intensities in the spectra gated under different reaction channel selection conditions. The constructed level scheme was compared with shell model and TRS calculations. The results indicate that these low-lying states exhibit spherical behavior different from the previously identified oblate yrast band, and the band might be built on a (7/2 +1 ) ground state.In the second part, an OpenMP parallel Fortran program, PairDiag, for the diagonalization of the general pairing Hamiltonian in deformed systems was developed. In the program, the ‘01’ inversion algorithm is used to generate the seniority-zero basis vectors; all the non-zero Hamiltonian matrix elements are evaluated “on the fly” by the scattering operators and a search algorithm; the matrix diagonalization is achieved by the Lanczos + QR algorithm. The PairDiag program is packaged in a Fortran module and can be easily used to replace the BCS approximation in other nuclear structure programs.
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| 4. |
- Zhang, Wei
(författare)
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Experimental Studies of Neutron Deficient Nuclei in the A ≈ 110 and A ≈ 170 Mass Regions
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral thesis describes several experimental studies on the structures of neutron-deficient nuclei with mass numbers A≈110 and A≈170. The experiments were carried out at the Accelerator Laboratory of the University of Jyväskylä, Finland, using heavy-ion fusion-evaporation reactions induced by beams of energies around 5 megaelectronvolts/nucleon. The experimental setups used for these studies are based on the high-resolution germanium-detector array Jurogam in combination with the recoil ion separators RITU or MARA and their respective focal plane detector systems, as well as with other ancillary devices. Using the recoil distance Doppler shift technique, lifetimes of excited states in the neutron-deficient osmium isotopes 169,171,173Os were measured for the first time by coupling the Jurogam germanium detector array with the ancillary device DPUNS (differential plunger for unbound nuclear states), the gas-filled recoil separator RITU, and GREAT the decay spectrometer situated at the RITU focal plane. The differential decay curve analysis method was used to extract lifetimes of the excited states of interest. An anomalously low value was observed for the ratio of the reduced electric quadrupole transition probabilities between consecutive gamma-ray transitions in the yrast band of 169Os. This observation adds to the handful of nuclides known to exhibit such behavior, which cannot be explained by state-of-the-art theory. Based on data from the same experiment, new alpha decay fine structures were identified for the osmium isotopes 171,172Os and iridium isotopes 171,172,174Ir, relying on the correlations of the alpha decays with both prompt and delayed gamma rays. It is suggested that the relatively enhanced hindrance factors observed for some of the newly observed decays might be related to changes in the superposition degree of different geometrical shapes (shape coexistence) in the final-state wave functions of the respective daughter nuclei.Another experimental setup used in the studies was based on the vacuum-mode mass separator MARA and its focal plane detector system in conjunction with the germanium array Jurogam and the charged-particle detector JYTube which was placed around the target position. In this work, the extremely neutron deficient proton emitting lanthanum isotope 116La was discovered. Enhanced proton emission probability and smaller proton-decay Q value were observed for 116La compared with its nearest less neutron-deficient isotope 117La. This unusual circumstance was proposed to be a possible effect of rarely observed neutron-proton pair correlations, as a result of their effect on the spatial distribution of the valence proton wave function. Using the highly selective recoil-decay tagging technique, excited states in the extremely neutron-deficient tellurium isotope 107Te were identified from two independent experiments based on the two setups described above. The deduced level scheme was compared with shell model calculations and TRS (Total Routhian Surface) calculations, indicating that the valence neutron occupying the h11/2 orbit has an important effect on the collectivity. The thesis includes two published articles and two manuscripts in advanced preparation. The experimental techniques and data analysis used in this work are summarized and the theoretical background to the interpretations of the experimental results is briefly outlined.
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