Experimental study of precisely selected evaporation chains in the decay of excited ${}^{25}\mathrm{Mg}$

The reaction ${}^{12}\mathrm{C}+{}^{13}\mathrm{C}$ at $95\mathrm{MeV}$ bombarding energy is studied using the Garfield + Ring Counter apparatus located at the INFN Laboratori Nazionali di Legnaro. In this paper we want to investigate the de-excitation of ${}^{25}\mathrm{Mg}$ aiming both at a new stringent test of the statistical description of nuclear decay and a direct comparison with the decay of the system ${}^{24}\mathrm{Mg}$ formed through ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ reactions previously studied. Thanks to the large acceptance of the detector and to its good fragment identification capabilities, we could apply stringent selections on fusion-evaporation events, requiring their completeness in charge. The main decay features of the evaporation residues and of the emitted light particles are overall well described by a pure statistical model; however, as for the case of the previously studied ${}^{24}\mathrm{Mg}$, we observed some deviations in the branching ratios, in particular for those chains involving only the evaporation of $\alpha$ particles. From this point of view the behavior of the ${}^{24}\mathrm{Mg}$ and ${}^{25}\mathrm{Mg}$ decay cases appear to be rather similar. An attempt to obtain a full mass balance even without neutron detection is also discussed.

Figure 1

Q-value distribution of complete in charge events with six detected $\alpha$ particles. The experimental data referring to the ${}^{12}\mathrm{C}$+${}^{13}\mathrm{C}$ reaction are shown as a continuous black line. The Q-value distribution obtained from the ${}^{12}\mathrm{C}$+${}^{12}\mathrm{C}$ reaction is also shown (dashed magenta line). The vertical lines mark the expected values for the two channels ${}^{24}\mathrm{Mg}$ $\to$ $6\alpha$ (${\mathrm{Q}}_{6\alpha }$) and ${}^{25}\mathrm{Mg}$ $\to$ $6\alpha$+1n (${\mathrm{Q}}_{6\alpha +n}$).

Figure 2

(a) Charge distribution. (b, c) Proton and $\alpha$ angular distributions in the laboratory frame. Black dots are the experimental results and the red bold and blue thin line represent the $\mathrm{HF}\ell$ and Gemini++ results, respectively. Both simulated charge distributions are normalized to the measured number of fusion events complete in charge, while the proton and $\alpha$ angular distributions are normalized to unitary area for a comparison with the results in Ref. [4].

Figure 3

Kinetic energy distributions in the laboratory frame for protons (a–e) and $\alpha$ particles (f–j) identified in Garfield. Points represent experimental data, thin blue and bold red histograms represent the Gemini++ the $\mathrm{HF}\ell$ predictions, respectively. The distributions refer to different coincident ER, from carbon to neon, and are normalized to unitary area.

Figure 4

Laboratory kinetic energy (a, b) and angular (c, d) distributions for $\alpha$ particles for the channel with an oxygen ER. Experimental and simulated results are drawn according to the convention of the previous figures. The two main contributing decay channels are considered, in particular ${}^A\mathrm{O}+2\alpha$ (a, c) and ${}^A\mathrm{O}+\alpha$+2H (b, d). The distributions are normalized to unitary area.

Figure 5

Q-value distributions for the decay channel $\mathrm{C}+3\alpha +xn$ (a) and $\mathrm{O}+2\alpha +xn$ (b), for the ${}^{25}\mathrm{Mg}$ and ${}^{24}\mathrm{Mg}$ drawn with black continue and magenta dashed line, respectively. The vertical lines are in correspondence of the one neutron emission threshold for each system. The two distributions are shifted by the amount of the neutron separation energy $S_n$ in ${}^{25}\mathrm{Mg}$.

Figure 6

Q-value experimental distribution for the ${}^{25}\mathrm{Mg}{\to }^{A}F+p+\alpha +xn$ chain (open dots). On the figure also the result of the fit procedure is shown (blue line), which is the sum of the various contributions related to the different neutron multiplicities represented as explained in the legend.

Figure 7

Mass distributions for various ER reached in the chains Table 1. The ${}^{25}\mathrm{Mg}$ (black continuous line) case is compared to the ${}^{24}\mathrm{Mg}$ case (magenta dotted line). All histograms are normalized to unitary area.

Figure 8

Probability distribution of $cos\left({\theta }_k\right)$ for experimental (black dots) and $\mathrm{HF}\ell$ events of the type $\alpha \text{−}\alpha \text{−}1n$-oxygen: both distributions are normalized to unitary area. The figure also shows the various cases corresponding to the three emission orders of the neutron (see legend). The sum of these three cases gives the total $\mathrm{HF}\ell$ curve (red continuous line). The weights are those predicted by $\mathrm{HF}\ell$. Each contribution is scaled by its weight to show the relative contribution to the total $\mathrm{HF}\ell$ distribution, before and after the fit procedure. In the sub pad on the shape comparison for $\alpha \text{−}\alpha$-oxygen events is shown.

Figure 9

Same as described in the caption of Fig. 8 but with the weights assigned through the fit procedure explained in the text.