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A great deal of research work has been undertaken in the alpha-clustering study since the pioneering discovery, half a century ago, of 12C+12C molecular resonances. Our knowledge of the field of the 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. In this work, the occurence of exotic shapes in light N=Z alpha-like nuclei is investigated. Various approaches of superdeformed and hyperdeformed bands associated with quasimolecular resonant structures are presented. Results on clustering aspects are also discussed for light neutron-rich Oxygen isotopes.
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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.
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.
We propose to study the partonic structure of $^4$He by measuring the Beam Spin Asymmetry (BSA) in coherent Deeply Virtual Compton Scattering (DVCS) and the differential cross-section of the Deeply Virtual Meson Production (DVMP) of the $phi$. Despite its simple structure, a light nucleus such as $^4$He has a density and a binding energy comparable to that of heavier nuclei. Therefore, by studying $^4$He nucleus, one can learn typical features of the partonic structure of atomic nuclei. The combination of CLAS12 and the ALERT detector provides a unique opportunity to study both the quark and gluon structure of a dense light nucleus. Coherent exclusive DVCS off $^4$He will probe the transverse spatial distribution of quarks in the nucleus as a function of the quarks longitudinal momentum fraction, $x$. In parallel, the average spatial transverse gluon density of the $^4$He nucleus will be extracted within a GPD framework using the measured longitudinal cross-section for coherent $phi$ production in a similar range of $x$. Additionally, threshold effects of $phi$ production can be explored by exploiting the ALERT detectors large acceptance for low $|t|$ events.
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.
A role of the unstable nuclei ${}^{6}$Be, ${}^{8}$Be and ${}^{9}$B in the dissociation of relativistic nuclei ${}^{7,9}$Be, ${}^{10}$B and ${}^{10,11}$C is under study on the basis of nuclear track emulsion exposed to secondary beams of the JINR Nuclotron. Contribution of the configuration ${}^{6}$Be + $mit{n}$ to the ${}^{7}$Be nucleus structure is 8 $pm$ 1% which is near the value for the configuration ${}^{6}$Li + $mit{p}$. Distributions over the opening angle of $alpha$-particle pairs indicate to a simultaneous presence of virtual ${}^{8}$Be$_{g.s.}$ and ${}^{8}$Be$_{2^+}$ states in the ground states of the ${}^{9}$Be and ${}^{10}$C nuclei. The core ${}^{9}$B is manifested in the {${}^{10}$C} nucleus with a probability of 30 $pm$ 4%. Selection of the ${}^{10}$C white stars accompanied by ${}^{8}$Be$_{g.s.}$ (${}^{9}$B) leads to appearance in the excitation energy distribution of 2$alpha$2$mit{p}$ quartets of the distinct peak with a maximum at 4.1 $pm$ 0.3 MeV. ${}^{8}$Be$_{g.s.}$ decays are presented in 24 $pm$ 7% of 2He + 2H events of the ${}^{11}$C coherent dissociation and 27 $pm$ 11% of the 3He ones. The channel ${}^{9}$B + H amounts 14 $pm$ 3%. The ${}^{8}$Be$_{g.s.}$ nucleus is manifested in the coherent dissociation ${}^{10}$B $to$ 2He + H with a probability of 25 $pm$ 5% including 14 $pm$ 3% of ${}^{9}$B decays. A probability ratio of the mirror channels ${}^{9}$B + $mit{n}$ and ${}^{9}$Be + $mit{p}$ is estimated to be 6 $pm$ 1.
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