Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre-including this research content-immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active.

Abstract. Nuclear levels in 23~ have been investigatedin the fl decay of 23~ the a3~ pnT) reaction and the 232Th(p, t) reaction. The K'=0~ -, 2 + , and 2 3 bands with band heads at 635, 781, and 1010 keV were observed up to the 8 +, 9 +, and 7 + levels, respectively. A second excited 0 + level was identified at 1297 keV which might be interpreted as the usual fl shape-oscillation. The branching ratios of the E2 transitions from the 0~-, 2~, and 2+ bands are explained in the framework of the rotational model by taking into account the coupling of these bands with the ground-state band, and the coupling between the 0~ + and 2~ + band. A strong enhancement of E2 transitions from the 2 + to the 0 + band reported earlier is not confirmed. For the octupole vibrations with K ~ = 0-, 1 -, and 2-the E 1 branching ratios are analyzed in terms of the Coriolis coupling of these bands. An almost complete experimental set of E1 transition moments from these negative-parity bands to the 0 § 0~, and 2~-bands was g obtained. It is suggested that octupole correlations might be important in explaining these E 1 moments.

No abstract

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...