The protonh11/2intruder orbital: Evidence for collectivity and a strong proton-neutron interaction

Abstract

A rotational band based on the proton hn/ 2 orbital has been observed to high spin (^h) and high rotational frequency (hcu ~ 1.0 MeV) in the nucleus ^f Sb. The measured transition quadrupole moment is QQ = 4.4 ± 0.6 eb, consistent with an axial prolate deformation of $2 -0.32. A large interaction strength (360 ±60 keV) has been measured for the rotational alignment of hn/ 2 neutrons which, together with a considerable delay in the crossing frequency (Ahtu c± 0.09 MeV), is construed as the first direct evidence of a large high-jf proton-neutron interaction.PACS numbers: 21.10. Re, 21.30.+y, 23.20.Lv, 27.60,+j A knowledge of the forces which drive the few-body nuclear system into collective motion, together with an understanding of both collective and noncollective effects within a single unified model, are fundamental goals in the study of nuclear structure. At modest excitation energy and spin in the odd-mass antimony nuclei (Sb, Z = 51), collective band structures are observed to coexist side by side with states corresponding to singleparticle excitations [1,2]. The collective states have been interpreted as proton particle-hole excitations across the major shell gap, made energetically possible by the combination of strong proton-pair correlations and a postulated proton-neutron interaction (e.g., Ref.[3]). The onset of prolate deformation, and thus the appearance of collective rotational bands, has also been linked to the occupation of particular orbitals which are preferentially lowered in energy at large quadrupole deformations (e.g., Ref.[4]), and hence exert a driving force towards increased deformation. Such high-j "intruder" orbitals are characterized by large values of single-particle angular momentum aligned along the axis of rotation, leading to a further reduction in energy due to the Coriolis force at high rotational frequency.Before this study very little experimental information existed concerning states at high spin (7 > 20ft) in nuclei near the Z = 50 closed spherical shell. In addition, no lifetime data (and therefore no measure of the collectivity) existed for the known band structures. In this Letter we address the role of the proton hu/2 orbital as a highspin intruder configuration, and report the first measurement of a transition quadrupole moment for a collective band in the Z = 50 region. Moreover, we show that the observed characteristics of the /in/2 intruder band may now be used to test the standard mean-field approach to nuclear structure calculations.High-spin states in 113 Sb have been populated via the 94 Mo( 23 Na,2p2n) reaction, with a 117-MeV sodium beam provided by the Tandem Accelerator Superconducting Cyclotron (TASCC) Facility at Chalk River Laboratories. Experiments were performed with thin selfsupporting and Au-backed enriched target foils, to provide high energy-resolution data and Doppler-shift information, respectively. Approximately 2 x 10 8 (selfsupporting target) and 3.2 x 10 8 (backed target) 7-7 events were collected with the 8TT spectrometer, which comprises 20...

Highly deformed high-spin band in125I

“…These collective structures are associated with a prolate minimum in the potential energy surface at ǫ 2 ≃ 0.25 [6][7][8][9][10][11][12]. Most of these bands represent examples of so-called smooth band termination [13,14], where the collectivity decreases gradually with increasing spin.…”

Experimental study of the πh 11/2 band in 113Sb

“…K is the projection of total angular momentum on the symmetry axis. Although the average transition quadrupole moment and the quadrupole deformation for this band have been reported previously [6] from DSA data using cen- The present lifetime results lead to a large average quadrupole deformation of β 2 = 0.32 and transition quadrupole moment Q t = 4.70±0.50 eb. This result is consistent with the average quadrupole moment reported in [6].…”

“…Although the average transition quadrupole moment and the quadrupole deformation for this band have been reported previously [6] from DSA data using cen- The present lifetime results lead to a large average quadrupole deformation of β 2 = 0.32 and transition quadrupole moment Q t = 4.70±0.50 eb. This result is consistent with the average quadrupole moment reported in [6]. The low-energy states in 113 Sb (Z = 51) have single-particle character arising from the coupling of the proton g 7/2 , d 5/2 and h 11/2 orbitals to the spherical Sn core states.…”

“…These rotational states arise as a result of the coupling of the valance proton occupying g 7/2 , d 5/2 and h 11/2 orbitals with 2p-2h deformed Sn states. In a recent study of 113 Sb, Janzen et al [6] observed a rotational band up to a spin of 75/2 − and excitation energy of about 19 MeV based on π h 11/2 configuartion. An extracted average value of the transition quadrupole moment Q t of 4.4±0.6 [6] was also determined using the centroid shift method for this rotational band.…”

“…In a recent study of 113 Sb, Janzen et al [6] observed a rotational band up to a spin of 75/2 − and excitation energy of about 19 MeV based on π h 11/2 configuartion. An extracted average value of the transition quadrupole moment Q t of 4.4±0.6 [6] was also determined using the centroid shift method for this rotational band. However, lifetime results for the individual states are not known in this rotational band.…”