Four α -particles as a ﬁnal state of 16 O ∗ Quasi Projectile decay

. Four α -particles as a ﬁnal state of 16 O ∗ quasi-projectile decay produced in peripheral 16 O + 12 C reactions at 130 MeV is thoroughly studied. The di ﬀ erent decay channels leading to the four α -particles ﬁnal state are reconstructed by carrying out an event-by-event analysis of α correlations in the population of intermediate 8 Be and 12 C. Although small, a non negligible contribution due to 8 Be g s evaporation is found. A comparison between predictions of an accurate Hauser-Feshbach decay code and branching ratios of the di ﬀ erent decay channels is performed. Signiﬁcant deviations are observed, among these the Hoyle state population which is considerably lower than the one predicted according to the statistical model, thus suggesting possible structure e ﬀ ects in the Coulomb barrier and / or in the transmission coe ﬃ cients.


Introduction
The aim of this experimental campaign carried out by the NUCL-EX collaboration is investigating the properties of light nuclei [1]. In this contribution we will present results of peripheral 16 O+ 12 C collisions with the decay of the excited 16 O * Quasi Projectile (QP) in a final state of four αparticles. A comparison with the predictions of a statistical decay code (HF ) based on Hauser-Feshbach formalism and including all the discrete levels of light nuclei, taken from the NUDAT2 database [2], was made. Agreement or disagreement with statistical decay code could be a hint of the importance of non statistical effects. The analysis explores the two channels of the decay [3]: the first foresees the formation of two 8 Be gs which both decay in two α particles 16 O * → 8 Be gs + 8 Be gs → α + α + α + α, the second consists in the emission of an α-particle leaving a 12 C * as a residue, which in turn decays in three αparticles, through a 8 Be intermediate state.

The experiment and the event selection
Measurements were carried out at the INFN laboratories in Legnaro using a pulsed beam of 16 O at 130 MeV [5]. The apparatus consists of GARFIELD, a multi-detector located inside a large scattering chamber, coupled with the Ring Counter (RCo), a three-stage annular detector with a truncated cone geometry aimed for the detection at forward angles [6]. ∆E-E correlation and fast-slow PSA techniques were used for particle identification. The selection of events of interest has been performed by considering only events which feature four α-particles, in the final state, detected at forward angles (by the RCo in 5 • ÷ 17 • angular range) and no product detected by GARFIELD (30 • ÷ 150 • ).

Analysis
From energy and momentum conservation one obtains the target and projectile excitation energy: where E α i is kinetic energy of the i th α-particle, E exc t the QT excitation energy, Q is the reaction Q-value and E rec is the QT recoil energy.
Taking into account also the energy loss in the target, E rec turns out to be always lower than the threshold for 12 C detection. In the energy spectrum of the QT, shown in Fig. 1, it is possible to observe the corresponding peaks at the ground state and first γ excited state at 4.4 MeV of the QT ( 12 C) nucleus. A tail to higher excitation energies can be seen as well. The analysis presented here includes only the events in which the QT remains in its ground state. However, other analyses that consider the other QT excitation energies have been carried out [7]. In order to determine the decay channel in two 8 Be gs , the relative energy of the pair of α-particles providing the lowest value out of the four is considered together with the relative energy of the two complementary α-particles. The vertical red lines in Fig. 2 Energia relativa minore represent the selection performed in order to consider the two relative energies corresponding to the ground state of the 8 Be. In this way, only events with the production of two contemporary 8 Be gs are taken into account. This accounts for 7.5 % of the total number of events with the QT in the ground state.
To consider the events corresponding to the second decay channel, from the excitation energy of three αparticles out of the four, the excitation energy of carbon is calculated as the minimum energy of the four combinations. This amount is added to the energy of the fourth α particle to obtain the excitation energy of the 16 O.
In Fig. 3 we show the excitation energy of the 12 C*. The two excited carbon states (Hoyle 0 + 2 and 3 − state, labelled as C * 1 and C * 2 ) can be distinguished. The selection of the data that fall within the 7.65 MeV or 9.64 MeV peaks enables a separate analysis of the two decay channels in which an excited carbon nucleus is formed. The decay process with the formation of 12 C in the Hoyle state accounts for 32 % of the occurrences, whereas the one with carbon in 3 − state represents 49 %. This results in the preferential decay of 16 O * through these decay channels.
Finally, we have performed the comparison with the HF predictions where we considered the decay of a mixing of excited levels with characteristics extracted from experimental data. Table 1 shows the results of this comparison: the observed branching ratios differ significantly from the theoretical ones, especially as far as Hoyle state is concerned. A possible explanation might be the influence of the well-known α-cluster structure of the Hoyle state on the Coulomb barrier and the associated transmission coefficients of the corresponding evaporation channel.