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...
