STUDY OF SHAPE EFFECTS AND MAGNETIC ROTATION IN N = 79 AND 47 NUCLEI

Abstract

11 ABSTRACT Fermi surface originating from the g712, d5/2, d3/2 and si/ 2 spherical shell model states which determine the nuclear structures, whereas for the negative parity states there is only the hii /2 orbital. One of the objectives of our experiments was to obtain detailed structure information of low and high spin states in 139Nd in order to systematically understand the evolution of nuclear structure from low to high spin in N = 79 isotones. The experimental details, results of our measurements, discussions based on systematics and comparison with the Tilted Axis Cranking (TAC) calculations are reported in this work. The high spin states of 139Nd nucleus were populated in our study through the 128Te(I60,5n)139Nd reaction by using 85 MeV 160 beam from the 14-UD BARC-TIFR Pelletron at Mumbai. The earlier known level scheme has been substantially extended to 6 MeV excitation energy and 41/2 h spin with the addition of about 50 new transitions. The yrast levels of 139Nd represent the coupling of odd neutron hole in h11/2 shell to the yrast states of I40Nd core. At low spins, this nucleus shows a triaxial shape, similar to the other neighbouring nuclei with N = 79. Two dipole bands have been observed in the present work and compared with the tilted axis cranking calculations. The negative parity band has been assigned the 3 quasiparticle(qp) configuration 7t(hT1/ 2) vh111/2. This configuration appears to cross the 5qp configuration 7c(h211/2g27/2) vh1-11/2 at an angular momentum of 33/2- h. The TAC calculations suggest that both the bands are magnetic rotational bands. The positive parity band has, similarly, been assigned the 3qp configuration 7t(hi112g712) -1 vh11/2 and may also be magnetic rotational in nature. We further present the 7-ray spectroscopy of another N = 79 nucleus i.e. 135Ba to understand the N = 79 systematics and shape changes from low spin states to high spin states. In the previous works, some lower spin levels were reported by the (9 Be, xn), p-decay and (n, y) reactions. The high spin states of 135Ba were populated in the 130Te(9Be, 4n)I35Ba reaction at a beam energy of 42.5 MeV obtained from the 15UD Pelletron at Inter University Accelerator Centre (IUAC), New Delhi and studied by using the GDA set-up. The earlier known level scheme has been substantially extended to 6 MeV excitation 111 energy and 37/2- h spin with the addition of about 25 new transitions. The low spin states upto 19/2- h indicate a triaxial shape from a systematic comparison with the neighbouring N = 79 even-Z isotones. A positive parity dipole band which is triaxial, a near prolate band and an oblate band have been identified in the present work. Configurations of the various low spin states have been discussed from a systematic comparison with the neighbouring N = 79 even-Z isotones. Results of the Tilted Axis Cranking (TAC) calculation are used to support the various band configurations and the shape co-existence. The calculations support a band structure, which results from the existence of multiple minima in the y-deformation while £2 ", 0.09 remains nearly constant. The structure of the nuclei in Sr isotopic chain is seen to vary from strongly collective through the moderately-deformed to near spherical, with an increase in the neutron number towards N = 50. Although the neighbouring odd-A isotones like 83Kr, 87Zr and 89Mo have been studied in detail, information about high-spin states in 85Sr is scanty. The 83Kr nucleus is probably a good example of MR band based on a shape mixing configuration and experimentally known B(M1) and B(E2) values. We have undertaken a study of 85Sr in order to test whether a similar behaviour exists in 85Sr. The high spin states of 85Sr were populated in the 76Ge(13C, 4n) reaction at a beam energy of 52 MeV obtained from the 15UD Pelletron at IUAC, New Delhi and studied by using the GDA set-up. We have placed the new y-rays on the basis of y-y coincidence with the known y-rays. The yy-coincidence measurements were used to establish the level scheme upto (33/2+) and (35/2- ). Eighteen new levels were identified and most of the previously know levels confirmed. A systematic comparison of the positive parity and the negative parity levels suggests that the proton number Z = 38 appears to exhibit a `magicity' with collectivity increasing on either side for N = 47 isotones. One positive parity band is assigned a 3qp configuration which may have magnetic rotation. Another negative parity band, also as-signed a 3qp configuration, and expected to show a magnetic rotation, however, does not exhibit the magnetic rotation feature. It appears that a minimum critical deformation is required for activating the shears mechanism.