Investigation Of Level Structure In¹¹⁴In And Shape Evolution In¹⁷⁴Os

The main content of this thesis could be divided to three sections as follows: Thefirst part introduces the level scheme and magnetic rotation in doubly odd nucleus¹¹⁴In; the second part investigates the anomalously high transition strength for the first2+state and critical-point symmetry in174Os through delayed coincidence technique;The last part concerns in nuclear shape changes that occur from Coriolis andcentrifugal forces through lifetime measurements of high spin states in the yrast bandof174Os using Doppler-shift attenuation method.1. The level structure of doubly odd nucleus¹¹⁴InIn recent years, a variety of nuclear structure phenomena in the A～110massregion have attracted great interest. Particularly, a great deal of effort has been madeto study newly observed quantized modes of rotation characterized as “magneticrotation”. In this case, sequences of M1transitions are observed with weak or absentcrossover E2transitions, indicating small deformation of the structures involved.Lifetime measurements show that the M1transitions are strongly enhanced whereasthe E2transition probabilities are very small as expected for almost spherical nuclei.These so-called magnetic rotation bands were described in the tilted axis cranking（TAC） model and represent a novel rotational mechanism called “shears mechanism”.In the case of these bands, the angular momentum is generated by gradual alignmentof spins of the deformation aligned and the rotation-aligned quasiparticles and theangular momentum is not along any of the principal axes of the nucleus. The M1bands in the indium isotopes have been observed for configurations where the protonholes are in high Ω orbitals of g9/2while the neutron particles occupy the low Ωorbitals of h11/2. In even-In isotopes the shears mechanism has been observed in thelight indium isotopes^106–112In, whereas in odd-In isotopes it has been observed up tostable¹¹³In. It is of particular interest to investigate the shears mechanism along an isotopic chain moving away from the shell closure at the same time coming close tothe N=66neutron mid-shell. For the¹¹⁴In nucleus the previous information onhigh-spin states was very scarce. The motivation of the present experiment was toextend the level scheme of¹¹⁴In for the systematic search of ΔI=1dipole bands andanti-magnetic （ΔI=2） rotational bands.High-spin states in¹¹⁴In were populated using the reaction¹¹⁰Pd （7Li,3n）¹¹⁴In.The target consisted of a2.45mg/cm²¹¹⁰Pd （enrichment97.2%） rolled onto a10.55mg/cm²Au backing. The experiment was performed at the HI-13tandemaccelerator of the China Institute of Atomic Energy. The γ-rays were detected with anarray consisting of12Compton-suppressed HPGe detectors and2planar-type HPGedetectors. Five of these detectors were placed at90, three at150, two at42and140,and one each at34and127with respect to the beam direction. Estimation ofcross-sections and fusion products were done using the code PACE. Based on thesecalculations, relative excitation functions for the emitted γ-rays were measured atbeam energies of22,24,26,28and32MeV. The maximum of¹¹⁴In productionoccurred at26MeV, and thus this energy was selected for all in beam experiments.The detectors were calibrated for γ-ray energies and efficiencies using the¹¹³Ba and¹⁵²Eu radioactive sources. A total of35million γ-γ coincidence events were recorded.The data were sorted into symmetrized and angular correlation γ-γ matrices foroff-line analysis. An asymmetric angular correlation matrix between the detectors at42and at90was constructed and used for the directional correlation of orientedstates （DCO） ratio analysis to distinguish between quadrupole and dipole transitions.menaejrogTryhit y～e o l5ef vtMelhee s Vchob saeenmrdev es odpf isn1e1q～4Iu ne1n6cc oe ns s wisinitti1hn1g4tIh onef four bands is established up to excitation haadvdei tbioenen o sfu agbgoeustt e5d0c noenwfi gturarnatsiiotinosn. sT. hTrheeesequences of dipole bands and two bands of E2γ-ray transitions have been placed inthe level scheme for the first time. The I （ω） values of yrast band are reasonablyreproduced by TAC-RMF calculations on the basis of the proposed configurationsbefore and after backbend. The experimental results of positive-parity dipole bandhave been discussed and show the characteristic features of magnetic rotation. The comparison of the characteristics of experimental results in the band2with thepredictions of the TAC-RMF calculations shows that the band can be described bythis tilted configuration that implies a strong magnetic component. It is an example ofrAme ag～gnio etic rotation that microscopic TAC-RMF model can describe the shears band inn1,10e mspaescsi arlelyg ioanf.t eInr tihnec lfuudtuinreg, itth ies npeacierisnsgar yc toor reexlaatmioinn e iont hethr iiss omtoopdese li. n Tthhieseffective-interaction model was also successfully applied to interpret shears bandfrom a phenomenological perspective. Both models predict that the majority of theangular momentum in band2is generated by alignment of proton–hole and neutronblades. However, lifetime measurements will be useful to understand the structure ofthese closely spaced dipole bands. It is to be noted that the positive dipole band seemsto indicate the onset of further alignment at the highest observable frequency. Thus,this band needs to be extended further in order to investigate the nature of alignment.2. Investigation of anomalously high transition strength for the first2+state and critical-point symmetry in174Os through lifetimemeasurementMeasurements of excitation transition probabilities are useful for studyingnuclear structure. Reduced transition probabilities of first2+states B（E2）, foreveneven nuclei are important in evaluating nuclear collectivity. Among thesemeasurements, nuclei in the transitional region of isotopes having a mass number ofA～160-190still attract attention because they provide an opportunity to study thetransition from spherical structures to deformed axially symmetric rotors.Very recently, a new experimental method with LaBr3（Ce） detectors incombination with the power of theGe array, employing triple-γ coincidencemeasurements, has been developed to deduce state lifetimes longer than tens ofpicoseconds. The method is especially suitable for in-beam γ-ray spectroscopymeasurements. In this newmethod, triple-γ coincidences are measured with an arraycontaining both HPGe and LaBr3（Ce） detectors. The high-energy resolution of theGedetectors is used to select the desired γ-ray cascade, and the array of fast LaBr3（Ce） to build the delayed coincidence time spectra for selected levels. We used this newtriple-γ coincidences method to remeasure the lifetime of the2+1state in¹⁷⁴Os. Thismethod applied in a delayed coincidence technique provides a more reliable valuemvcoaelmausepu arroeefd with earlier measurements with the plunger method because the measuredm e～n1t onf st hiiss realta ttihvee lyli lmonitg oliff etthime e.technique, causing a large error in theThe excited states in¹⁷⁴Os were populated using the¹⁵⁰Sm (²⁸Si,4n)¹⁷⁴Os)reaction at a beam energy of140MeV. The target consisted of a0.9mg/cm2¹⁵⁰Smrolled onto a12.5mg/cm2Pb backing. The experiment was performed at the HI-13tandem accelerator of the China Institute of Atomic Energy. The lifetimes of the statesof interest were determined by using a fast timing setup consisting of four LaBr3（Ce）scintillator detectors working in coincidence with11Compton-suppressed HPGedetectors. The seven LaBr3（Ce） detectors were mounted below the target chamber ona ring of approximately90with respect to the beam axis. Four of LaBr3（Ce） detectorshave crystals with a size of30mm in height and a diameter of40mm and they werecoupled to an XP2020Q photomultiplier tube. At first, we performed off-line testswith standard radioactive sources (²²Na,60Co,133Ba, and152Eu) to optimize theperformance of our fast timing setup.The lifetime of the2+state in¹⁷⁴Os has been remeasured in a fast timingexperiment and determined with better precision to be τ=513（20） ps. The small errormakes this value sufficiently precise to serve as a normalization parameter formeaningful tests of relevant models. The systematics of B（E2） indicate that thedeformation reaches a maximum at N=98for W and Os isotopes, which can beattributed to both the effect of the deformed subshell at N=98and the proton“intruder” h9/2orbital.An initial comparison of¹⁷⁴Os to the predictions of the X（5） critical-point modelpoints to¹⁷⁴Os lying slightly to the spherical-vibrator side of the phase transition. TheX（5）-β2model, which describes the structure between a spherical vibrator and X（5）,well describes the energies in¹⁷⁴Os but shows large discrepancies with the relativeB（E2） transition strengths. These disagreements perhaps can be attributed to¹⁷⁴Os having a potential in the γ degree of freedom which is softer than the axiallysymmetric potential assumed in the X（5） and X（5）-βnmodels. This idea was tested bytrying to describe¹⁷⁴Os using two-parameter IBA-1calculations. The agreement,overall, is reasonable and the resulting χ value points to a structure that is intermediatebetween a γ-soft and axially symmetric potential in the γ degree of freedom. While thepresent work provides some evidence that the γ degree of freedom may need to beconsidered in this mass region, more extensive data are needed on the off-yraststructures of other potential X（5） candidates in this region.3. Lifetime measurements probing shape changes in¹⁷⁴OsTransition quadrupole moments from lifetime mearurements of states with spinup to I=20in collective bands near the yrast line have proved to br a value tool fortracing nuclear shape changes that occur from Coriolis and centrifugal forces. Areduction of collectivity at high spin has been observed in several N=104、106and108muclei. The effect in^182-186Pt is qualitatively caused by the deformation drivingforce of the aliged neutrons when the Feimi level lies high in the i13/2shell. Thenucleus changes from prolate shape at low spin to a triaxial with reduced collectivity,i.e. γ>0at high spin.The calculated systemmatics of triaxial-shape driving orbitals suggests that thecollectivity should not decrease at higher neutron number around N=96and98wherethe aliged i13/2orbitals are situated near the middle of the shell and negative γ valueare energetically favored. However, Cranked HFB calculations indicate that there alsooccurs a reduction of β2in collectivity. The latter effect approximately cancels out theincrease in collectivity caused by the negative γ values. In line with these predictions,our mearsurement show no significant reduction of the transition quadrupole momentsfor spin I≥10.