A high-energy-resolution magnetic spectrometer has been used to measure the 12 C excitation energy spectrum to search for the 2 + excitation of the 7.65 MeV, 0 + Hoyle state. By measuring in the diffractive minimum of the angular distribution for the broad 0 + background, evidence is found for a possible 2 + state at 9.6(1) MeV with a width of 600(100) keV. The implications for the 8 Be + 4 He reaction rate in stellar environments are discussed.One of the mysteries of nuclear structure is the nature of the 7.65 MeV, 0 + state in 12 C. Its existence is innately tied to that of organic life as it is the portal through which the most abundant isotope of carbon ( 12 C) is synthesized. The existence of the state was originally proposed by Hoyle [1] to address the question as to the abundance of 12 C, which could only be accounted for if a resonance were to lie close to the Gamow window. The anthropic power of this argument was demonstrated when the state was discovered by Cook and co-workers [2] with precisely the predicted properties.The structure of this state has, however, remained something of a mystery. What is known is that it must have an unusual nature, which is probably a well-developed 3α-cluster structure. Evidence for this comes from several sources. First, it is known that the optimal conditions for the formation of clusters is that a state should lie close to the associated cluster decay threshold [3]; in the present instance, the Hoyle state lies just 375 keV above the 3α-decay threshold. Shell model calculations, for example, those of Ref.[4], reproduce rather well the energy of the first 2 + (4.44 MeV) excitation. However, in the region of the second 0 + state (0 + 2 ), the Hoyle state, there is a void in the calculations; the energy of this state cannot be reproduced. A similar conclusion is reached in the no-core shell model calculations [5]. Analysis of electron inelastic-scattering data [6,7] indicates that the Hoyle state has a volume some 3.4 times larger than the ground state. This larger volume reduces the overlap of the α particles and may allow them to obtain their quasifree characteristics in something approaching an α-particle gas or perhaps a bosonic condensate (BEC) [8]. This latter possibility is intriguing, as it would correspond to a new form of nuclear matter in which the bosonic nature of the α particles would allow the constituents to all occupy the lowest energy level of the mutual interaction potential-unlike fermions. Fermionic molecular dynamics (FMD) calculations also find that the 7.65 MeV state has a similar structure [9].From an experimental perspective, one key ingredient in pinning down the structural properties of the state is finding the location of its collective (2 + ) excitation. A state in which the three α particles are arranged in a linear fashion (3α chain) would have a 2 + excitation at 0.8 MeV above the 0 + state [10]. On the other hand, BEC calculations predict an energy difference of 1.3 MeV [11], the FMD predict 2.3 MeV [9], and the separation is 1.6-2.8 MeV...

The 12 C excitation energy spectra populated in both proton and α-particle inelastic scattering measurements are examined. The data indicate the existence of a 2 + state at Ex=9.75(0.15) MeV with a width of 750(150) keV. It is believed that this state corresponds to the 2 + excitation of the 7.65 MeV, 0 + , Hoyle-state, which acts as the main path by which carbon is synthesised in stars. A simultaneous R-matrix analysis of the two sets of data indicates that the 2 + state possesses a very large α-reduced width, approaching the Wigner limit. This would indicate that the state is associated with a highly clustered structure. The potential geometric arrangements of the clusters is discussed.

Proton inelastic scattering experiments at energy E p = 200 MeV and a spectrometer scattering angle of 0 • were performed on 144,146,148,150 Nd and 152 Sm exciting the IsoVector Giant Dipole Resonance (IVGDR). Comparison with results from photo-absorption experiments reveals a shift of resonance maxima towards higher energies for vibrational and transitional nuclei. The extracted photo-absorption cross sections in the most deformed nuclei, 150 Nd and 152 Sm, exhibit a pronounced asymmetry rather than a distinct doublehump structure expected as a signature of K-splitting. This behaviour may be related to the proximity of these nuclei to the critical point of the phase shape transition from vibrators to rotors with a soft quadrupole deformation potential. Self-consistent random-phase approximation (RPA) calculations using the SLy6 Skyrme force provide a relevant description of the IVGDR shapes deduced from the present data.

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The16 O(α,α ′ ) reaction was studied at θ lab = 0• at an incident energy of E lab = 200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton decay and α decay from the natural parity states were observed in a large-acceptance silicon strip detector array at backward angles. The coincident charged-particle measurements were used to characterize the decay channels of the 0 Table I). The 0 the four-α-particle breakup threshold and has a large radius of 5 fm, indicating a dilute density structure. Ohkubo and Hirabayashi showed in a study of α + 12 C elastic and inelastic scattering [9] that the moment of inertia of the 0 + 6 state is drastically reduced, which suggests that it is a good candidate for the 4-α cluster condensate state. Calculations performed with the Tohsaki-Horiuchi-Schuck-Röpke (THSR) α-cluster wave function [10] also support this notion with an estimated total width of 34 keV for the 0 + 6 state [11], much smaller than the experimentally determined value of 166(30) keV [12].Recent unsuccessful attempts to measure particle decay widths of the 0 + 6 state in 16 O[17,18] highlighted the need for an experiment that combines α-particle decay measurements with a high-energy-resolution experimental setup and a reaction capable of preferentially populating 0 + states. In contrast to transfer reaction measurements, inelastic α-particle scattering at zero degrees has the advantage that it predominantly excites low-spin natural parity states. A measurement of the 16 O(α,α ′ ) reaction at zero degrees, coupled with coincident observations of the 16 O decay products, was performed at the iThemba Laboratory for Accelerator-Based Sciences (iThemba LABS) (7) 162 (