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Beta-delayed proton emission may occur at very low rates in the decays of the light nuclei $^{11}$Be and $^8$B. This paper explores the potential physical significance of such decays, estimates their rates and reports on first attempts to detect them: an experiment at ISOLDE/CERN gives a branching ratio for $^{11}$Be of $(2.5 pm 2.5) cdot 10^{-6}$ and an experiment at JYFL a 95% confidence upper limit of $2.6 cdot 10^{-5}$ for $^8$B.
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The dynamics present in the fusion of neutron-rich nuclei is explored through the comparison of experimental cross-sections at above-barrier energies with measurements of the interaction cross-section at relativistic energies. The increase of fusion dynamics with increasing neutron excess is clearly demonstrated. Experimental cross-sections are compared with the predictions of a Sao Paulo model using relativistic mean field density distributions and the impact of different interactions is explored.
The current status of the experimental searches for rare alpha and beta decays is reviewed. Several interesting observations of alpha and beta decays, previously unseen due to their large half-lives ($10^{15} - 10^{20}$ yr), have been achieved during the last years thanks to the improvements in the experimental techniques and to the underground locations of experiments that allows to suppress backgrounds. In particular, the list includes first observations of alpha decays of $^{151}$Eu, $^{180}$W (both to the ground state of the daughter nuclei), $^{190}$Pt (to excited state of the daughter nucleus), $^{209}$Bi (to the ground and excited states of the daughter nucleus). The isotope $^{209}$Bi has the longest known half-life of $T_{1/2} approx 10^{19}$ yr relatively to alpha decay. The beta decay of $^{115}$In to the first excited state of $^{115}$Sn (E$_{exc} = 497.334$ keV), recently observed for the first time, has the $Q_beta$ value of only $(147 pm 10)$ eV, which is the lowest $Q_beta$ value known to-date. Searches and investigations of other rare alpha and beta decays ($^{48}$Ca, $^{50}$V, $^{96}$Zr, $^{113}$Cd, $^{123}$Te, $^{178m2}$Hf, $^{180m}$Ta and others) are also discussed.
Since the discovery of molecular resonances in $^{12}$C+$^{12}$C in the early sixties a great deal of research work has been undertaken to study alpha-clustering. Our knowledge on physics of nuclear molecules has increased considerably and nuclear clustering remains one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. Occurrence of exotic shapes and Bose-Einstein Condensates in light alpha-cluster nuclei are investigated. Various approaches of superdeformed/hyperdeformed shapes associated with quasimolecular resonant structures are discussed. The astrophysical reaction rate of 12C+12C is extracted from recent fusion measurements at deep subbarrier energies near the Gamov window. Evolution of clustering from stability to the drip-lines is examined.
Method: To examine signatures of this alpha-condensation, a compound nucleus reaction using 160, 280, and 400 MeV 16O beams impinging on a carbon target was used to investigate the 12C(16O,7a) reaction. This permits a search for near-threshold states in the alpha-conjugate nuclei up to 24Mg. Results: Events up to an alpha-particle multiplicity of 7 were measured and the results were compared to both an Extended Hauser-Feshbach calculation and the Fermi break-up model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multi-particle Fermi break-up model. Examination of how these 7 alpha final states could be reconstructed to form 8Be and 12C(0_2+) showed a quantitative difference in which decay modes were dominant compared to the Fermi break-up model. No new states were observed in 16O, 20Ne, and 24Mg due to the effect of the N-alpha penetrability suppressing the total alpha-particle dissociation decay mode. Conclusion: The reaction mechanism for a high energy compound nucleus reaction can only be described by a hybrid of sequential decay and multi-particle breakup. Highly alpha-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in N-alpha systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the N-alpha decay channel. No evidence of a highly clustered 15.1 MeV state in 16O was observed from (28Si*,12C(0_2+))16O(0_6+) when reconstructing the Hoyle state from 3 alpha-particles. Therefore, no experimental signatures for alpha-condensation were observed.
The neutron-rich 6He and 8He isotopes exhibit an exotic nuclear structure that consists of a tightly bound 4He-like core with additional neutrons orbiting at a relatively large distance, forming a halo. Recent experimental efforts have succeeded in laser trapping and cooling these short-lived, rare helium atoms, and have measured the atomic isotope shifts along the 4He-6He-8He chain by performing laser spectroscopy on individual trapped atoms. Meanwhile, the few-electron atomic structure theory, including relativistic and QED corrections, has reached a comparable degree of accuracy in the calculation of the isotope shifts. In parallel efforts, also by measuring atomic isotope shifts, the nuclear charge radii of lithium and beryllium isotopes have been studied. The techniques employed were resonance ionization spectroscopy on neutral, thermal lithium atoms and collinear laser spectroscopy on beryllium ions. Combining advances in both atomic theory and laser spectroscopy, the charge radii of these light halo nuclei have now been determined for the first time independent of nuclear structure models. The results are compared with the values predicted by a number of nuclear structure calculations, and are used to guide our understanding of the nuclear forces in the extremely neutron-rich environment.
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