R-matrix analysis of theβdecays ofN12

Abstract

The β decays of 12 N and 12 B have been studied at KVI and JYFL to resolve the composition of the broad and interfering 0 + and 2 + strengths in the triple-α continuum. For the first time a complete treatment of 3α decay is presented including all major breakup channels. A multilevel, many-channel R-matrix formalism has been developed for the complete description of the breakup in combination with the recently published separate analysis of angular correlations. We find that, in addition to the Hoyle state at 7.65 MeV, more than one 0 + and 2 + state is needed to reproduce the spectra. Broad 0 + 3 and 2 + 2 states are found between 10.5 and 12 MeV in this work. The presence of β strength up to the 12 N Q-value window suggests the presence of additional 0 + and 2 + components in the 12 C structure at energies above 12.7 MeV.

Breakup ofC12resonances into threeαparticles

“…6 parity were not well established prior to this work. In the most recent compilation [25], it is tentatively assigned spin-parity 2 − , but theoretical considerations 1 and recent experimental findings [26] seem to favor a 4 − assignment. As we will later show, the present work firmly establishes 4…”

“…6 is essentially free of background [24]. The intensity present in addition to the five visible peaks does not constitute background in the usual sense of the term 1 Cf. footnote to 6 Li were easily removed by gating on the corresponding peak in the d + α relative energy spectrum).…”

“…The coexistence of two such different excitation modes makes the low-level resonance structure of 12 C a particularly interesting and challenging case, since theoretical models are usually tailored for the description of either shell-model type or cluster-type excitations but seldom both. Experimentally, the study of the low-level resonance structure of 12 C is complicated by the large natural width of cluster states [1]. Motivated by these considerations, we shall attempt to address the following question: What does the energy distribution of the 3α breakup tell us about the properties of the initial 12 C resonance?…”

Properties of12C resonances determined from the10B(3He,Alcorta1, Borge2, Cubero3 et al. 2012Phys. Rev. CSelf Cite

“…The partial branches given in Table II used the typical reduced radius of r 0 = 1.41 fm, giving an exit-channel radius of a c = r 0 (4 1/3 + 4 1/3 ) = 4.5 fm which corresponds to the best-fit found in Ref. [36]. Table II shows the deduced branching ratios α 0 / to the ground state of 8 Be with and without corrections due to the ghost effect.…”

“…of broad 0 + and 2 + resonances in 12 C identified in β-decay studies [36]. Nevertheless, an attempt has been made to analyze the region between 12 and 13 MeV through the use of Dalitz plots in a separate publication [44], and there is evidence for a broad resonance centered around 12.4 MeV of unnatural parity with J 4, i.e., the possibilities are 4 − , 5 + , 6 − , 7 + , etc.…”