SEARCH OF NEW COUPLING SCHEMES IN NUCLEI IN 110 MASS REGION

Abstract

Several interesting phenomena giving rise to rotation like band structure in nearly spherical nuclei have recently been observed; these phenomena arise due to very specific angular momentum couplings of the high-j neutron and proton orbitals. One such phenomenon is the magnetic rotation (MR) (with AI = 1 rotation-like bands) found in a large number of very weakly deformed nuclei. This phenomenon is now very well established and reasonably well understood. Another example of such an interesting phenomenon is the twin-shears mechanism dubbed as the anti-magnetic rotation (AMR), which came into picture much after the establishment of the MR bands. In a geometrical representation of AMR, the angular momentum vector of high-j proton holes are in a stretched mode whereas the neutron angular momentum vector stands in the middle and is perpendicular to both of them. It is, therefore, like having two shears like systems operating together. As a result there is no net magnetic dipole moment. The symmetry of the system demands that the AMR band levels should differ in spin by two units which will be connected by stretched E2 transitions. As the two proton vectors close towards the neutron angular momentum, the reduced transition probability, B(E2), values are expected to decrease rapidly. AMR band is also expected to have nearly constant dynamic moment of Inertia (2) and large :.1(2)/B(E2) values increasing with the increase in spin. The AMR is new, rare and a very less explored phenomenon, having been confirmed only in three even-even 106,108,110Cd nuclei. However, no AMR band has so far been observed in an odd-A nucleus. In A = 110 nuclei (Z ti 50), the high-a g9/2 proton holes(s) and low-Il h1112 and 97/2 neutrons play an active role favoring 11 Abstract the possibility of AMR in this mass region. Hence, this mass region is most suitable to study and understand this exciting phenomenon. The main aim of the present work is to explore some new and interesting coupling schemes of the neutron and proton orbitals in nuclei in A = 110 mass region, with a focus on exploring the AMR in some odd-A Cd isotopes. The nuclei chosen for this purpose were 105"°7Cd. Two experiments were performed for lifetime measurements of the levels in the chosen nuclei by using the Doppler Shift Attenuation Method (DSAM). The first experiment was carried out by using the heavy-ion fusion evap-oration reaction "Zr (160, 501°5Cd with 93 MeV beam, at ILIAC, New Delhi. The decaying gamma rays were detected by the Indian National Gamma Array (INCA) (phase-III) comprising of 14 Compton suppressed germanium clover detectors ar-ranged in five rings viz. 2 detectors at 32°, 2 at 57°, 4 at 90°, 2 at 123° and 4 at 148° with respect to the beam direction. We performed our second experiment using the reaction 94Zr(180, 5n)107Cd at 85 MeV beam energy. The experiment was per-formed at TIFR, Mumbai. The emitted 7-rays were detected by the collaborative INGA setup (phase-III) comprising of 18 Compton suppressed germanium clover detectors arranged in six rings viz. 3 detectors at 157°, 3 at 140°, 3 at 115°, 4 at 90°, 2 at 65° and 3 at 40° with respect to the beam direction. The data collected in list mode form were analyzed offline, establishing the level scheme in the selected nuclei (by using coincidence analysis and intensity measurements) followed by the measurements of various properties of the levels. These include the spins, parities and lifetimes of the levels. The spins of the levels were established using the method of Directional Correlation from Oriented nuclear state (DCO). For establishing the parities of the levels, the linear polarization measurements were carried out. Finally the DSAM was used to determine the lifetimes of the high spin levels. The earlier published level scheme of the odd-A 1°5Cd nucleus has been confirmed from the present work, along with few modifications and addition of three new transitions. The lifetimes of the high spin states (above 23/2-) in the yrast band of '°5Cd have been measured. The obtained values of the reduced transition probability Abstract iii B(E2) are small and show a decrease with an increase in spin as we go up the band. This has been interpreted by using the semiclassical model of the twin-shears mechanism. At spin 23/2-, two proton angular momentum vectors corresponding to two proton holes in the high-S/ g9/2 orbitals are nearly perpendicular with the total neutron angular momentum vector corresponding to a neutron configuration v(g7/2 h211/2). Generation of high spin states beyond spin 23/2- occurs due to the gradual closing of the proton vectors towards the aligned neutron vectors, i.e., by AMR, based on the chosen configuration v(g7/2h211/2) 0 ir(g972) along with small contribution from the core rotation. An excellent agreement between the theoretical (semiclassical model) and experimental results along with a nearly constant dynamic moment of inertia a(2) and large :1(2) B(E2) ratio for the states confirm that the structure of the levels beyond spin 23/2 has the character of a twin-shears type AMR band resulting from the coupling of a pair of g9/2 proton holes with aligned h1112 and (g712)2 neutron particles, along with some contribution from the core rotation. This establishes, for the first time, the antimagnetic coupling scheme in an odd-A nucleus. We present the results of our calculations based on a semiclassical model and also compare them with the recent microscopic calculations which lends further support to the AMR nature of the band in 105Cd. For 1°7Cd, we have confirmed the earlier published level scheme of 107Cd with several modifications and addition of 7 new transitions. The lifetimes of the high spin states above 23/2- in the negative parity yrast band 2 and levels above 27/2+ in the positive parity signature partner bands 5 and 6 of 107Cd have been measured by using the Doppler shift attenuation method and the corresponding B(E2) values have been extracted for these levels. From the level structure for bands 2 and 3 obtained from the present data, we find that the band 2 has become the yrast band instead of the band 3 as was the situation prior to this work. On the basis of the lifetime data obtained by us and other features of these bands, it is suggested that the bands 2 and 3 do not appear to have an AMR nature. In fact, the two negative parity A/ = 2 bands 2 and 3 have iv Abstract same spin and parity and also nearly identical 'y-transitions from 23/2- to 39/2-, which suggests the possibility of a special symmetry-breaking being responsible for the generation of these two bands. We suggest that, such a situation, if further confirmed by experiments, could lend to an interpretation in terms of a new type of symmetry breaking not observed so far. The measured B(E2) values for the levels in the positive parity partner bands 5 and 6 have been found to decrease rapidly with increase in spin. This has been interpreted by the semiclassical model for AMR using the proposed configuration (vg7/2h11/2 7rgg-72)• Generation of high spin states beyond spin 23/2- occurs due to the gradual closing of the proton vectors towards the aligned neutron vectors along with small contribution from the core rotation. A good agreement of the the theoretically calculated I vs. w and B(E2) vs. I values (using the semiclassical model) with the experimental results, along with the constant a(2) 20 h2MeV-1] and large a(2)/B(E2) ratio (> 100 h2MeV'(eb)-2) increasing with spin, for both the bands 5 and 6 establish that the bands are an example of signature partner AMR bands. This is the first definitive result indicating the existence of a pair of AMR bands having nearly similar properties, operative in a single nucleus. We note that the AMR feature was expected in the band 3 of 107Cd before doing these measurements. However, we did not find any strong evidence for AMR in band 3. On the other hand, we have found the existence of AMR in bands 5 and 6, quite an unexpected and new result. In conclusion, the thesis presents the first evidence of an AMR band in an odd-A nucleus, i.e. 105Cd. It also presents the first evidence of multiple AMR bands in any nucleus studied so far, in this case 1°7Cd. These findings are supported by our calculations based on a simple semiclassical model and the recent microscopic calculations published by another group. Some new spectroscopic features observed in 1°7Cd could not be fully understood. These and several other questions still remain unanswered and require further experiments and calculations.