Search

High-spin states in 221Ra were studied by the 210Pb(14C, 3n) reaction at beam energies between 61 and 75 MeV. Excitation functions, multiplicity distributions and γγ-coincidences were measured. The level scheme of 221Ra is found to be composed of two bands of alternating parity, characterized by the simplex numbers s = +i and s = −i, which could be followed up to I π = 27 2 + and I π = 27 2 − , respectively. The presence of parity doublets as well as enhanced E1 transactions suggests a reflection-asymmetric shape for this nucleus.

The high-spin state structure of the nucleus 221 Ac was investigated by means of the 209 Bi( 14 C, 2n) reaction at 63 and 65 MeV. Measurements consisted of in-beam γγ coincidences using the Château de Cristal multidetector array and of e − γ coincidences. Two alternating parity bands could be established in 221 Ac up to spin 35 2 h . Almost all of the states may be regrouped into parity doublets and the origin of these doublets is discussed.

The high-spin level structure of 217Ra has been established up to a spin of ( 47 2 ) by means of in-beam γ-ray and conversion electron spectroscopy with (12,13C, xn) reactions on 208Pb. Positive parity levels up to case27 2 + and negative-parity states up to ( case31 2 − ) can be qualitatively understood in terms of neutron excitations arising out of the shell-model configurations v g 3 9 2 , v g 2 9 2 i 11 2 and v g 2 9 2 j 15 2 . The isomeric 33 2 + state at 2.4 Me V and the levels above it require both neutron and proton excitations. The El transitions between the negative-parity and the v g 3 9 2 states suggest the presence of octupole-deformed configurations. Our shell-model calculations provide only a qualitative description of the levels. The experimental level spacings and their moderate agreement with the calculated ones suggest also the onset of deformation and of collective vibrational characteristics.

Yrast states in the nucleus 220 Ra were studied by means of the 208 Pb( 14 C, 2n) reaction at 61 and 64 MeV. A staggering sequence of levels of positive and negative parity has been observed up to spin and parity I π = 16 + (18 + ) and from I π = 5 − to I π = 17 − , respectively. These states are connected by strong E1 transitions competing with the stretched E2 transitions, the B( E 1) B( E 2) ratio being ~ 10 −6 fm −2 . The ratio of the excitation energy of the 4 + state to that of the 2 + state is close to the vibrational limit. The moment of inertia associated with the negative-parity yrast states is slightly increasing with the rotational frequency ω. It is considerably higher than that of the positive-parity states at lower spins, the difference decreasing monotonically with increasing ω. The data are discussed with reference to the octupole vibrational picture as well as to the results of recent models predicting reflection-asymmetric shapes in the Ra-Th region.

The kinematically reversed reaction 13 C( 208 Pb, 3n) 218 Ra was used at 5.31 MeV/u to measure lifetimes of states with spins up to 15 − in 218 Ra, employing the Doppler shift recoil distance method. The higher spin states in 218 Ra exhibit enhanced E1 decays with B (E1)≈6×10 −3 W.u.. These enhanced B (E1)'s suggest a new form of collective dipole deexcitation arising from a reflection asymmetric intrinsic state. The measured B (E1)'s increase with spin between the I π =6 + , 7 − and 8 + states. It is suggested that, at higher spins, reflection asymmetry is enhanced as a consequence of octupole deformations stabilized by rotations or equivalently because of larger wavefunction admixture from cluster states.

High-spin states up to I${=31}^{\mathrm{\ensuremath{-}}}$ have been studied in the $^{218}\mathrm{Ra}$ nucleus and the staggering sequence of levels of positive and negative parity persists up to highest level. This observation supports the idea that in some nuclei the high-spin excitations may primarily be dominated by deformations which do not conserve the intrinsic parity (such as octupole, \ensuremath{\lambda}=3, and higher odd-\ensuremath{\lambda} multipolarities) rather than by the ``usual'' quadrupole (\ensuremath{\lambda}=2) and possibly higher even-\ensuremath{\lambda} multipolarities.

The level scheme of230Th is presented, as deduced from Coulomb excitation experiments using32S,84Kr and142Nd projectiles. The ground-state band is followed up to spin (24+) and the octupole band up to 19−. A comparison is made with the analogous bands known in232Th and232U.

The γ-radiation following Coulomb excitation of 230 Th is investigated. The maesurements establish the ground-state rotational band up to I φ = 22 + (24 + ) and the rotational band built on the K φ = 0 - octupole vibration up to the 21 - level. The alignment pattern shows that 230 Th is near a minimum of the coupling strength between the paired ground state and the rotation-aligned j 15 2 two-quasineutron configuration.