Quadrupole deformed and octupole collective bands in Ra-228

Abstract

Spins and parities for collective states in Ra-228 have been determined from conversion electron measurements with a mini-orange β spectrometer. The fast-timing βγγ(t) method has been used to measure lifetimes of T(1/2) = 550(20) ps and 181(3) ps for the 2(1)(+) and 4(1)(+) members of the K = 0(+) band, and T(1/2) ≤ 7 ps and ≤ 6 ps for the 1(1)(-) and 3(1)(-) members of the K = 0(-) band, respectively. The quadrupole moments, Q(0), deduced from the B(E2;2(1)(+) --> 0(1)(+)) and B(E2;4(1)(+) -->2(1)(+)) rates are in good agreement with the previously measured value and the systematics of the region. However, the B(EI) rates of ≥ 4 x 10(-4) e(2)fm(2), which represent the first B(E1) measurements for this nucleus, are at least 25 times larger than the value previously suggested for Ra-228. The new results are consistent with the B(E1) rates recently measured for the neighbouring Ra-227 and reveal octupole correlations in Ra-228.