Extremely asymmetric shears band in Sm143

Abstract

A dipole sequence has been observed and investigated in the 143 Sm nucleus populated through the heavy-ion induced fusion-evaporation reaction and studied using the Indian National Gamma Array (INGA) as the detection system. The sequence has been established as a Magnetic Rotation (MR) band primarily from lifetime measurements of the band members using the Doppler Shift Attenuation Method (DSAM). A configuration based on nine quasiparticles, with highly asymmetric angular momentum blades, has been assigned to the shears band in the light of the theoretical calculations within the framework of Shears mechanism with the Principal Axis Cranking (SPAC) model. This is hitherto the maximum number of quasiparticles along with the highest asymmetricity associated with a MR band. Further, as it has followed from the SPAC calculations, the contribution of the core rotation to the angular momentum of this shears band is substantial and greater than in any other similar sequence, at least in the neighbouring nuclei. This band can thus be perceived as a unique phenomenon of shears mechanism in operation at the limits of quasiparticle excitations, as manifested in MR band-like phenomena, evolving into collectivity.Recent experimental and theoretical studies on the weakly deformed nuclei, with a very few particles and holes outside the core, unambiguously establish [1-4] the "shears mechanism" as a general phenomenon of generating angular momentum in them. The shears mechanism in these nuclei, manifested in form of the magnetic rotational (MR) bands, has been observed in their level structures. These bands are characterized by strong intraband M1 transitions and weak/unobserved cross-over E2 transitions, the latter being commensurate with the small deformations of these nuclei. The band-head corresponds to the perpendicular alignment of the angular momentum vectors generated by the particle and the holes constituting the band configuration. The repulsive interaction between the particle and holes favours the perpendicular coupling, for minimum energy at the bandhead. Excited states with higher angular momenta along the MR band are generated by the gradual alignment of the angular momentum vectors that may eventually completely align to produce the maximum spin accompanied by the termination of the MR band [5].The mid-shell nuclei, on the other hand, have considerable number of valence nucleons outside the core that is consequently deformed owing to the polarizing effect of the former. The deformation breaks the rotational sym- * Electronic address: subhphy@gmail.com † Electronic address: h.pai@saha.ac.in metry and leads to the observation of the so-called rotational bands in the level structure of the corresponding nuclei. This may be perceived as a transition in the characteristic excitation pattern of the nuclei with increasing number of valence nucleons outside the core. The weakly deformed systems with a few valence particles exhibiting MR-bands, or similar single particle features, evolves into well deformed nuclei with in...

Energy staggering parameters in nuclear magnetic rotational bands *

“…In the 110, 130 and 190 mass regions, the MR bands in even-even, odd-odd and odd-A nuclei are presented respectively. The corresponding data are taken from the previous review [12] and recent observations [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][56][57][58]. [12] and recent observations [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][56][57][58].…”

“…The corresponding data are taken from the previous review Ref [12]. and recent observations[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][56][57][58].…”

Systematic study of magnetic and antimagnetic rotational bands

“…For magnetic rotational bands, the B(M1)/B(E2) values are generally more than 20 [μ N /(eb)] 2 [2]. However, some values of MR bands are around 10 [μ N /(eb)] 2 in 106,107,109 Ag, 112 Sb, 128 Ba, 139 Sm, 142 Gd, 144 Dy, 192,196 Hg,and 192,193,194 Pb, smaller than the criterion for magnetic rotation mentioned above. In contrast, the B(M1)/B(E2) values are much larger in some MR bands, for example in 83 Rb whose 106 Sn.…”

“…Similarly, the candidates are also suggested in 108 Ag, 137 Ce, 135,140 Nd, 140 Sm, 190 Pb, and 196,201 Bi. Moreover, it is interesting that in nuclei with MR 104−106 Ag, 105 Rh, 136−138 Nd, and 194 Tl there are reported also chiral bands constructed on similar particle and hole geometries [325]. steadily as the rotational frequency increases.…”