High-Spin States In Odd-Odd ¹⁶⁰Tm Nucleus

In past decades, comparing to the known rich information of the even-even and odd-A nuclei, the high-spin data of doubly-odd nuclei have been very limited. It is the result of a more complicated spectrum resulting from various couplings of valence proton and neutron, and various couplings of valence nucleons and the core in an odd-odd nucleus, and the difficulty of resolving these spectra with older generations of detector arrays. However, an odd-odd nucleus is a unique system that offers the possibility of experimentally probing residual p-n interactions.This thesis mainly focuses on the in-beamγ-ray spectroscopic study of high-spin level structures, electronic-magnetic transition probability and signature inversion features in odd-odd ¹⁶⁰Tm nucleus.I. Introduction.Since E. Rutherford introduced that an atom is composed of a nucleus and electrons circumrotating around the nucleus base theα-particle- scattering experiment, the study of nuclear structure have attracted much attention. With the development of experimental conditions and techniques, in particular, the completion of the large heavy-ion- accelerator and the using of large detector arrays, nuclei can gain large angular momentum, which makes studing the high speed rotational motion of nuclei to be possible, thus set a new study field of nuclear structure-studies of high-spin states in nuclei. In 1971, Johnson et al. observed the backbending phenomenon of the moment of inertia, and it was explained based on band crossing by Stephens et al., it made a big development in investigation of nuclear collective motion. From then on, many new physical phenomena were observed; it obtained more advancement in the experimental and theoretic investigations. However, still some new problems can not be understood by current theoretic models, such as signature inversion in odd-odd nuclei. It indicates more efforts must be carry into execution in the future.II. In-beamγ-spectroscopy techniques.As a experimental subject, the high-spin states study has a integrated experimental methods and techniques (?) in-beamγspectroscopy.To study high-spin states, one must use a reaction in which a larger amount of angular momentum can be transferred to the nucleus. Heavy-ion fusion reaction, multiple coulomb excitation and prompt fission of heavy nuclei are three primary methods in population of the high-spin states in experimental studies.Sometimes, it will cost hundreds of hours in high-spin states experiments and the data recorded can not be analyzed indirectly, thus it should be sorted into a matrix. The information of theγ-rays coincidence, intensity and lifetime will be extracted from the matrix with some computer operations. According to the information, establish the level scheme of the object nucleus, with that the relational problems of the high-spin states in object nucleus can be investigated and analyzed deeply.III. High-spin states in odd-odd ¹⁶⁰Tm nucleus.High spin states of 160Tm have previously been studied by the ¹⁵²Sm(¹⁴N, 6n) reaction (1986), ¹²⁸Te(³⁷Cl, 5n) reaction at 167 MeV (1989) and ¹³⁰Te(³⁵Cl, 5n) reaction (2005), respectively. Two rotational bands were established by these works.The high spin states in ¹⁶⁰Tm were populated through the ¹⁴⁶Nd(¹⁹F, 5n) reaction at an incident energy of 102 MeV provided by the HI-13 tandem accelerator at CIAE in Beijing. Theγ-γcoincidence events were collected with an array of twelve Compton-suppressed HPGe detectors. A total of ¹60×106 double and higher fold events were recorded for the off-line analysis.The 255.6 keV transition between the (12?) level and the (10?) level placed uncertainly previously can not be found and another 259.6 keV transition with the sum energy of the 176.6 and 83.3 keVγ?rays is observed. The 259.6 keV transiton is treated as an intra-band crossover transition with the analysis of coincidences, DCO ratio and moment of inertia. The 742.9 keV transition between the (25?) level and the (23?) level is replaced by a 731.2 keV transition. According to the intensity-balance and DCO ratio, the 742.9 keV transition is assumed to be aΔI = 2 transition feeding into the (23^-) level of band A.Several low-lyingγ- rays in coincidence with the yrast band are observed. Most likely the (8^-) state that close to the bandhead of the yrast band de-excites the 74.5 s isomer of spin (5) via these transitions, and the decay path probably proceeds via anΔI =3 transition to an intermediate state of spin (2) followed by an (M1+E2) or E1 transition to the g. s. (IÏ€=1^-) of ¹⁶⁰Tm.Band C feeds to band A via the 513.5 keV, 606.1 keV, 704.5 keV and 801.9 keV transitions with the DCO ratios of 0.97, 1.15, 0.97 and 1.11, respectively, indicate quadrupole character of all the transitions, and thus the parity of band C is assumed to be negative. The configuration of this band is assigned to beÏ€g_7/2(?)νh_9/2(?)(νi_13/2)² by considering the large alignment and the high-excitation energy, delayed band crossing frequency, population condition of the quasiproton and quasineutron in the neighboring odd-A nuclei, calculation of the alignment addition and comparison of the experimental B(M1)/B(E2) ratios and those in theory.On account of the population intensity, the decoupled feature, highly alignment properties, delayed band crossing frequency, and systematic analyses, it is reasonably that band D and Band E assigned to the nucleus ¹⁶⁰Tm. Based on the band crossing frequency, addition of the alignments and energy level systematics, theÏ€d_3/2(?)νi_13/2 andÏ€g_7/2(?)νi_13/2 configuration are assigned to band D and band E, respectively.It is systematically observed that the B(M1)/B(E2) plots with the increasing of rotational frequency, behaves as a so-called parabola-like shape in theÏ€h_11/2(?)νi_13/2 bands of rare-earth doubly-odd nuclei (i. e., the B(M1)/B(E2) ratios increase rapidly after a certain rotational frequency). Such a phenomenon discussed based on the formula of magnetic dipole reduced transition probability deduced from the Cranking Shell Model and Particle Rotor Model, respectively. It is pointed out that, the occurrence of this behavior is closely related to the alignment nature of theνi13/2 quasineutron. The increasing of B(M1)/B(E2) occurring at large frequency approaching the second BC crossing can be understood as mainly resulted from the mixing of wave-function with the 4-quasiparticle band caused by the band crossing. Insight into the angular momentum coupling scheme between the quasiparticles and collective core in theÏ€h_11/2(?)νi_13/2 structures of rare-earth doubly-odd nuclei is gained by analyzing the increasing behavior of B(M1)/B(E2) ratios occurring at low rotational frequency.IV. Observation of signature inversion in ¹⁶⁰Tm.The plots of signature splitting, defined as S(I)=E(I)-E(I-1)+1/2[E(I+1) -E(I)+E(I-1)-E(I-2)], as a function of level spin I is calculated for theÏ€h_11/2(?)νi_13/2 andÏ€h_11/2(?)νh_9/2 bands in ¹⁶⁰Tm. The expected favored signature isαf =0 for theÏ€h_11/2(?)νi_13/2 configuration. It can be found that, it is the unfavored-signature branch (i.e.,αu =1) that is favored energetically at low spins other than theαf =0 transition sequence and the two signature branches cross with each other at I=18.5 h beyond which normal signature splitting is observed. Such a behavior has been referred to as the low-spin signature inversion. The systematic feature of signature inversion has been investigated by Liu et al.. Based on the 2-quasiparticle plus retor model, Zheng et al. determined that the inversion phenomenon is caused by the competition between the p-n interaction and the Coriolis force in low-K space.The evolution of the level staggering of theÏ€h_11/2(?)νh_9/2 band (band B) presents interesting features. we expectαf =0 for the normal favored sequence in the band. The two signature embranchments cross each other in two points (i. e. I～19 h and 23 h). The levels with the favored signature, which is defined byαf = [(-1)^{jp–1/2} + (-1)^{jn–1/2}]·1/2 orαf = [(j_p + j_n) mod 2] are found to be energetically unfavoured at 19 h 23 h and higher spin.The signature staggering of theÏ€h_11/2(?)νh_9/2 bands in odd-odd nuclei around A¹60 mass region are calculated. A higher level scheme of ¹⁶⁰Tm established by K. Lagergren et al. is taken for calculation in order to get more information. It can be found that the bands in odd-odd nuclei 160,162Tm, 160,162Lu and 160Ho occurs signature inversion at I²0 h and the plots of signature splitting show a saddle-like shape in ¹⁶⁰Tm and 162Tm, i.e., the signature splitting increase with the increasing spin and cross each other at I=19.5 h, after which the signature splitting stagger around 0 keV. From a certain spin (28.5 h for ¹⁶⁰Tm and 30.5 h for 162Tm), the signature splitting begin to increase with the increasing spin.Based on the qualitative analysis, a possible mechanism of this anomalous behavior is given. It is likely caused by the competition between the p-n residual reaction and the Coriolis force for a dominant position, and other than this causation, the saddle-like shape of the signature splitting is related to the triaxially deformation, QnQp and the quadrupole-pairing force.The mechanism of this anomalous behavior is not clear and it needs more theoretical investigation.In summary, the high-spin states in ¹⁶⁰Tm have been investigated. Based on the in-beamγ-spectroscopy technique, the previously level scheme of ¹⁶⁰Tm is confirmed, new transitions and new bands are observed, the configuration of the bands and the spin of theγ-rays are assigned. The electronic-magnetic transition probability is investigated and it is pointed out that the B(M1)/B(E2) ratios increasing rapidly is related to the mixing of wave-function with the 4-quasiparticle band and the angular momentum coupling scheme between the quasiparticles and collective core in theÏ€h_11/2(?)νi_13/2 structures of rare-earth doubly-odd nuclei. The signature inversion features of theÏ€h_11/2(?)νi_13/2 andÏ€h_11/2(?)νh_9/2 also be reported and the competition between the p-n residual reaction and the Coriolis force is considered as a possible mechanism of the abnormal features.