Abstract

Confirmation of Irregular Signature Splitting in ⁷⁴As

Student: Bojana Ivanic (University of Texas)
Faculty Mentor: Robert Haring-Kaye (OWU Department of Physics and Astronomy)

Atomic nuclei in the so-called “Wild West” of the nuclear landscape are capable of possessing both strong and unusual deformations (departures from a spherical shape). Most arsenic (As) nuclei are known to have prolate shapes that resemble an American football. In contrast, the ⁷⁴As isotope has been proposed to have a completely asymmetric (triaxial) shape based on an irregular pattern of its energy differences as a function of angular momentum (related to how fast it rotates). However, this conclusion was based on uncertain angular momentum (spin) values, which this research sought to measure accurately. Our results confirm the existing spin assignments, supporting the interpretations of previous work.

Most odd-odd nuclei in the mass A ~ 70 region exhibit a very consistent alternating pattern in the energy differences between adjacent states (signature splitting) in their strongest positiveparity bands. In odd-odd ⁷⁴As, the pattern shows an irregular phase reversal at high spin, which has been attributed to the onset of an unpaired band crossing with an underlying triaxial deformation. However, this interpretation depends critically on the behavior of the signature-splitting pattern, which is based on uncertain spin assignments. The primary motivation for this study was thus to make firm spin assignments in the strongest positive-parity band of ⁷⁴As. High-spin states in ⁷⁴As were produced using the ¹⁴C(⁶²Ni, pn) reaction with a beam energy of 50 MeV at Florida State University. γ-γ coincidences were measured using a Compton-suppressed Ge detector array comprised of three Clover detectors and seven single-crystal detectors. Measurements of directional correlation of oriented nuclei (DCO) ratios were used to confirm most of the spin assignments in the strongest positive-parity band, supporting the interpretations of previous work.