Improved Limit on DirectαDecay of the Hoyle State

Abstract

The current evaluation of the triple-reaction rate assumes that the decay of the 7.65 MeV, 0 þ state in 12 C, commonly known as the Hoyle state, proceeds sequentially via the ground state of 8 Be. This assumption is challenged by the recent identification of two direct -decay branches with a combined branching ratio of 17ð5Þ%. If correct, this would imply a corresponding reduction in the triple-reaction rate with important astrophysical consequences. We have used the 11 Bð 3 He; dÞ reaction to populate the Hoyle state and measured the decay to three particles in complete kinematics. We find no evidence for direct -decay branches, and hence our data do not support a revision of the triple-reaction rate. We obtain an upper limit of 5 Â 10 À3 on the direct decay of the Hoyle state at 95% C.L., which is 1 order of magnitude better than a previous upper limit.

New Measurement of the Direct 3α Decay from the C12 Hoyle State

“…The 18 keV width of the triangle is due to the experimental resolution; the dominant contribution is the angular granularity of the DSSDs. The excellent resolution is obtained without the requirement for kinematic fitting [14]. Conservation of energy and linear momentum constrains the data to lie within a circle of radius 1=3, marked in Fig.…”

“…Recent experimental efforts have placed an upper limit on the direct decay contribution at 0.2% [14,15]. Here, we report an improvement on the upper limit for the direct 3α decay from the Hoyle state via a measurement of the 12 Cð 4 He; 3αÞα reaction.…”

“…However, the additional angular momentum barrier will suppress this channel. Furthermore, the 8 Be ground state ghost anomaly only contributes at the level of 6 × 10 −5 in this reaction channel [14,18]; below the sensitivity of the experiment.…”

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

“…The large solid angle combined with the high segmentation of the DSSSDs gives complete kinematics information on the triple-α breakup of populated resonances in 12 C. A detailed description of the setup and of the analysis techniques is given in Ref. [24] (see also the already mentioned papers [19,21,22]). The 12 C excitation energy is determined from the momentum of the outgoing light ion, i.e., the proton in the reaction with the 10 B target and the deuteron in the reaction with the 11 B target.…”

α -conjugate neck structures in the collisions of 35 MeV/nucleon Ca40 with Ca40

“…Given that near Fermi energy nuclear collisions can drastically modify the temperatures, densities and cluster properties of nucleonic matter, the possibility that short-lived Bose Condensates might be fleetingly produced in such collisions is an intriguing idea. Recently the emission of three α from the Hoyle state has been characterized for 12 C produced in several different reactions [16][17][18][19]. Results for the ratio of simultaneous to sequential de-excitation differ and the influence of medium or proximity effects on the de-excitation modes of that state in complex reactions remains an open question.…”