Two-neutron alignment and shape changes in69As

The nucleus ${}^{69}\mathrm{As}$ was studied using the ${}^{40}\mathrm{Ca}{(}^{32}\mathrm{S}{,3p)}^{69}\mathrm{As}$ reaction at a beam energy of 105 MeV. An extension of the band built on the $g_{9/2}$ orbital was observed to exhibit a band crossing at a rotational frequency of 0.511 MeV with an associated alignment of $7ħ.$ This alignment is interpreted as being due to a pair of $g_{9/2}$ neutrons. Total Routhian surface calculations have been carried out which confirm that the shape of this nucleus changes from oblate at low spin to a triaxial prolate shape at intermediate spin.