The partial widths of the 16.1 MeV 2+ resonance in 12C

Abstract

The 16.1 MeV 2 + resonance in 12 C situated slightly above the proton threshold can decay by proton-, α-, and γ emission. The partial width for proton emission cannot be directly measured due to the low proton energy and the small branching ratio. Instead it must be indirectly derived from other observables. However, due to several inconsistent data the derived partial width varies by almost a factor 2 dependent on the data used. Here we trace the majority of this inconsistency to different measurements of the (p, α) cross sections. We have remeasured this cross section using modern large area silicon strip detectors allowing to measure all final state particles, which circumvents a normalization issue affecting some of the previous measurements. Based on this we determine Γp = 21.0(13) eV. We discuss the implications for other observables related to the 16.1 MeV 2 + resonance and for isospin symmetry in the A = 12 system. In addition, we conclude that the dataset currently used for the NACRE and NACRE II evaluation of the 11 B(p, 3α) reaction should be scaled by a factor of 2/3. This impacts the reaction rate accordingly.PACS. 23.20.+e α decay -27.20.+n Properties of specific nuclei listed by mass ranges; 6 ≤ A ≤ 19

Cross-Section Measurements of the 11B(p,α)2α Reaction near the First Resonant Energy

“…Based on the integral formula for thick target yield and the α particle energy spectrum, we obtain the peak value of the cross-section σ � 45.6 ± 12.5 mb near 156 keV. Te peak value is consistent with previous results [8][9][10]18], and the position of the resonance is slightly shifted to the left. Te shift of the resonance peak is mainly due to the very limited data near the resonance peak and the use of a thick target.…”

“…Te shift of the resonance peak is mainly due to the very limited data near the resonance peak and the use of a thick target. According to Munch [18], the resonance width in the lab system of 163 keV is 5.76 keV, while the energy interval of proton beams in our experiments is 4 keV, which means that there are too few energy points measured near the resonance peak. Terefore, this causes a large error in the ftting of the peak position, thus leading to the shift of the peak.…”

“…Te red line is the curve ftting. (b) p 11 B cross-section (the data are from References[8][9][10]18]). Here, we have corrected Becker's value by multiplying a factor of 2/3.…”

Exclusive decay study of the 16.62\,MeV (2$^-$, T=1) resonance in $^{12}$C

“…The experiment was conducted at the 5 MeV Van de Graaff accelerator at Aarhus University. Other results from the same experimental campaign have been published elsewhere [22,23].…”

Clusters in light nuclei: history and recent developments

“…At still higher excitation energies, a revision of the partial decay widths of the 16.1 MeV 2 + state in 12 C was discussed in Ref. [483], as a result of a re-measurement of the 11 B(p,α) reaction. A complete kinematics experiment on the 11 B(p,3α)γ reaction allowed a revision of the γ partial widths for excited states in 12 C above the α threshold and pointed out the possible existence of a natural parity state at ≈ 11.8 MeV [484].…”