Asim (2019). Extracting the spectral signature of alpha-clustering in 44,48,52Ti using the continuous wavelet transform. Physical Review C -Nuclear Physics, 100 (051302).
Background: Near-threshold α-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of α-particles which condense into the 0s orbital. Experimental evidence for such a dramatic phase change in the structure of the nucleus has not yet been observed. Method:To examine signatures of this α-condensation, a compound nucleus reaction using 160, 280, and 400 MeV 16 O beams impinging on a carbon target was used to investigate the 12 C( 16 O, 7α) reaction. This permits a search for near-threshold states in the α-conjugate nuclei up to 24 Mg.Results: Events up to an α-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 7α final states could be reconstructed to form 8 Be and 12 C(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 16 O, 20 Ne, and 24 Mg due to the effect of the N-α penetrability suppressing the total α-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 α-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in N-α systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the N-α decay channel. No evidence of a highly clustered 15.1 MeV state in 16 O was observed from ( 28 Si , 12 C(0 + 2 )) 16 O(0 + 6 ) when reconstructing the Hoyle state from 3 α-particles. Therefore, no experimental signatures for α-condensation were observed. arXiv:1907.05471v2 [nucl-ex]
The 12C(13C, 9Be)16O reaction has been used to populate excited states in 16O. The 9Be nuclei were detected in a magnetic spectrometer and the 12C decay product of the recoiling excited 16O nucleus was detected in an array of silicon strip detectors. The large angular coverage of the strip detector array allowed the α-decay widths of the 14.66 MeV, 5−, and 16.275 MeV, 6+, states to be determined with good accuracy. The present results, together with earlier measurements, allow precise values for the branching ratios to be calculated: Ex(16O) = 14.66 MeV, Jπ = 5−, Γα0/Γ = 0.951 ± 0.049 and Γα1/Γ < 0.05; Ex(16O) = 16.275 MeV, Jπ = 6+, Γα0/Γ = 0.982 ± 0.048 and Γα1/Γ < 0.02.
Two recent experiments have indicated that the break-up of the 12C Hoyle state is dominated by the sequential 8Be(g.s.) + α decay channel. The rare direct 3α decay was found to contribute with a branching ratio of less than 0.047% (95% C.L.). However, the ability of experimentalists to successfully disentangle these two competing decay channels relies on accurate theoretical predictions of how they each manifest in phase space distribution of the three break-up α-particles. The following paper reviews the current theoretical approaches to calculating the break-up of the Hoyle state and introduces a semi-classical WKB approach, which adequately reproduces the results of more sophisticated calculations. It is proposed that a more accurate upper limit on this branching ratio may be obtained if these new theoretical results are taken into account when analysing experimental data.
The 9 Be( 6 Li, d) 13 C * reaction at a beam energy of 42 MeV has been investigated using a large-acceptance silicon-strip detector array and the high-resolution Q3D magnetic spectrograph. The Q3D facilitated the unambiguous determination of the reaction channel via identification of the deuteron ejectile, thereby providing the spectrum of excited states in 13 C in the range from 10.7 to 15.0 MeV. The silicon array was used to detect and identify the 13 C recoil-breakup products with efficiencies of up to 49%. The results obtained for the absolute partial branching ratios represent the first complete measurements for states in this energy region and allow the extraction of reduced widths. The quantities measured for n0 / tot and n1 / tot are 0.91 ± 0.11 and 0.13 (10.753 MeV), 0.51 ± 0.04 and 0.51 ± 0.04 (10.818 MeV), 0.68 ± 0.03 and 0.42 ± 0.02 (10.996 MeV), 0.49 ± 0.08 and 0.71 ± 0.11 (11.848 MeV), and 0.49 ± 0.08 and 0.53 ± 0.08 (12.130 MeV), respectively. For the two observed higher-lying energy levels, α0 / tot and n1 / tot have been measured as 0.54 ± 0.02 and 0.45 ± 0.02 (13.760 MeV) and 0.94 ± 0.03 and 0.13 ± 0.02 (14.582 MeV), respectively. The consequences for the proposed molecular structures in 13 C are explored following the extraction of reduced widths.
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