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A detailed quantitative experimental investigation of the influence of nuclear deformation on the angular distribution of a particles emitted by oriented nuclei is reported. The favored a transitions in the decay of the deformed nuclei 221 Fr, 227 Pa, and 229 Pa were studied. In all three cases, very large anisotropies have been observed. The results are in good agreement with calculations based on a particle tunneling through a deformed Coulomb barrier. [S0031-9007(99) Alpha decay is a textbook example of quantum mechanical tunneling of a particle through a potential barrier. The exponential energy dependence of the a decay rate is indeed well explained by the tunneling of a preformed a particle through the Coulomb barrier of atomic nuclei [1]. Hill and Wheeler [2] argued that in a nucleus with a deformed Coulomb barrier the tunneling probability becomes direction dependent, resulting in anisotropic a emission from an ensemble of oriented nuclei (i.e., nuclei with a preferential spin direction in space). A firmer theoretical framework was built later [3][4][5][6], in which the shell model-including Bardeen-Cooper-Schriefer pairing [7]-was used to compute the formation amplitude of the a particle at the nuclear surface while employing the Wentzel-Kramers-Brillouin approximation [8] to calculate tunneling through the (deformed) Coulomb barrier.Based on the works mentioned above, the observation of anisotropic a emission from heavy nuclei has often been attributed to the tunneling of the a particles through a deformed barrier, thus relating a anisotropies to nuclear deformation [9]. This relationship, however, has not been firmly established experimentally. Indeed, the only a anisotropy experiments on nuclei known to be deformed were performed on prolate actinide nuclei more than two decades ago [10]. As predicted, a preferential emission of the a particles along the nuclear symmetry axis was observed. However, at that time, the source preparation technique and the quality of the detectors available did not allow resolution of the different a transitions in the decays investigated and no detailed conclusions could be drawn. These problems were solved for the first time when high-resolution particle detectors operating near 4.2 K were linked with ion implantation techniques for sample preparation [11]. Using this combination we have recently shown that for nuclei near the N 126 and Z 82 shell closures, anisotropic a emission in favored decays, i.e., in transitions which are (almost) unhindered compared to the ground-state-to-ground-state transitions in neighboring even-even nuclei, is not dominated by deformation but rather by nuclear structure effects [12]. One is thus lead to the conclusion that the assumed relation between nuclear deformation and the angular distribution of a particles is not evident. It may be noted here that only the higher order partial a waves with angular momentum L fi 0 determine the a anisotropy. The a decay of unoriented nuclei is isotropic in space and hence decay rate experiments are i...

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α-particle angular distribution data from an extensive on-line nuclear orientation study of the favoured decay of nearly spherical, neutron deficient At and Rn nuclei near N=126, as well as of deformed Fr and Pa isotopes are presented. From the comparison of these data with various theoretical models it is found that for nearly spherical nuclei, anisotropic α-emission in favoured decays can be well explained within the shell model but not by a tunnelling model. On the other hand, for the deformed nuclei studied here, a description on the basis of α-particle tunelling through the Coulomb barrier is quite appropriate.The process of α-decay was the first known form of radioactivity. Nevertheless, the mechanisms that determine the α-decay observables are still poorly understood. Indeed, for the description of the total α-decay rate as well as for the angular distribution of α-particles emitted by oriented nuclei [1, 2, 3], various divergent theories exist among which, with the experimental information available up to now, no clear choice could be made.Here, we report on a systematic study of the relationship between the angular distribution of α-particles emitted by oriented nuclei and their shape and structure. For this purpose, we have experimentally determined the angular dependence of the α-radiation pattern of favoured transitions from oriented, nearly spherical nuclei (205)(206)(207)(208)(209)(210)(211)217 At, 205,207,209 Rn) as well as of nuclei with intermediate ( 221 Fr) and strong deformations ( 227,229 Pa). The measurements were performed using the technique of low-temperature nuclear orientation (LTNO) at the NICOLE refrigerator at ISOLDE (CERN) and at the KOOL-facility on-line to LISOL at CYCLONE (Louvain-laNeuve). In this type of experiments, the observed quantity is the angular distribution function W (θ) which is derived from the normalised radiation intensity as a function of the emission angle θ relative to the nuclear orientation axis. The latter is determined by the direction of the magnetic field B. In the case of α-decay in an axially symmetric geometry, W (θ) may be written as [4]: Table 1. The experimental mixing ratios δ 02 for the favoured α-transitions in nearly spherical At and Rn and in deformed Fr and Pa isotopes together with theoretical values calculated in the shell model[5] or in a tunelling model[6, 8]. The nuclear quadrupole deformation is indicated by β 2 nucleus δ 02 (exp) δ 02 (th) β 2 [7] 205 At 0.041(2) 0.035 207 At 0.077(3) -0.035 209 At 0.115(4) -0.035 211 At 0.201(5) 0.207[5] 0.008 205 Rn 0.089(4) 0.081[5] -0.079 207 Rn 0.098(4) 0.089[5] -0.053 209 Rn 0.108(3) 0.096[5] -0.044 221 Fr -0.216(9) -0.204[8] 0.120 (K=1/2) 227 Pa 0.364(27) 0.341[6] 0.168 229 Pa 0.75(16) 0.45[6] 0.190 W (θ) = 1 + fA 2 B 2 Q 2 P 2 (cosθ) + fA 4 B 4 Q 4 P 4 (cosθ) + . . .where f is a two-site model parameter which describes the distribution of the implanted atoms in the host lattice. B k and Q k account for the orientation of the nuclear ensemble and the detection geometry, respectively and P k are ...