$\alpha$-conjugate neck structures in the collisions of 35 MeV/nucleon ${}^{40}\mathrm{Ca}$ with ${}^{40}\mathrm{Ca}$

The deexcitation of alpha-conjugate nuclei produced in reactions of 35 MeV/nucleon ${}^{40}\mathrm{Ca}$ with ${}^{40}\mathrm{Ca}$ has been investigated. Particular emphasis is placed on examining the dynamics of collisions leading to projectile-like fragment exit channels. A general exploration of the reaction systematics reveals the binary dissipative character of the collisions and a hierarchy effect similar to that seen for heavier systems. Investigation of the subset of events characterized by a total $\alpha$-conjugate mass ($\alpha$ particles plus $\alpha$-conjugate fragments) equal to 40 and atomic number equal to 20 reveal a dominance of $\alpha$-conjugate exit channels. The hierarchy effect for these channels leads to the production of $\alpha$-clustered neck structures with potentially exotic geometries and properties.

Figure 1

Yields of the three heaviest fragments in the event as a function of the fragment parallel velocity in different windows of initial apparent excitation energy $E^{*}/A$. The projectile velocity is 8.0 [cm/ns]. The c.m. velocity is 4.0 [cm/ns].These two velocities are indicated by vertical lines in each panel.

Figure 2

Filtered amd-gemini results, similar as Fig. 1.

Figure 3

Average excitation energy vs exit channel separation energy for the deexcitation channels of $A=40$, $Z=20$ nuclei selected as described in the text. Data are represented by small filled dots. The linear least squares fit to the data is represented by the solid line.

Figure 4

Detected number of events yielding $\alpha$ particles or $\alpha$-conjugate nuclei in the collision of ${}^{40}\mathrm{Ca}$ with ${}^{40}\mathrm{Ca}$ at $35A$ MeV, plotted against total detected mass of $\alpha$-conjugate nuclei. Small filled circles represent total yields. Open circles represent yields for events in which only $\alpha$ particles contribute to the $A_L$.

Figure 5

Ikeda-like diagram for the possible $\alpha$-conjugate components of ${}^{40}\mathrm{Ca}$. Separation energy $-Q$ in MeV for each decay channel is shown.

Figure 6

Excitation energy distributions for $A=40$, $Z=20$ derived as indicated in the text. The blue area represents the data for all such events. The hatched area represents the data for $A_L=40$ events.

Figure 7

Excitation functions for the $A_L=40$ events discussed in the text.

Figure 8

Percentages of $A_L=40$ events appearing in the possible exit channels. The experimental results are represented by solid dark grey bars. The filtered amd-gemini results are represented by solid silver bars.

Figure 9

The fraction of exit channel events as a function of the ratio of the average excitation energy to the separation energy. Top panel shows experimental data, bottom panel shows amd calculation. Each solid circle represents an exit channel. The $A_L=40$ channels are identified by using large open diamonds. Events identified by open circles may be $A_L=40$ channels which have undergone secondary decays with $d$ and ${}^6\mathrm{Li}$ emissions (see text).

Figure 10

Sphericity coplanarity plots for the $A_L=40$ exit channels. Two-body exit channels are excluded. See text.

Figure 11

Source-frame-invariant velocity plots for the $\alpha$-like exit channels containing $\alpha$ particles. Vertical lines at 0 are to aid the eye in comparisons of these distributions.

Figure 12

Source-frame-invariant velocity plots for the two-fragment $\alpha$-like exit channels. Vertical lines at 0 are to aid the eye in comparisons of these distributions.

Figure 13

Parallel velocity distributions for the $\alpha$-conjugate exit channels. In each panel, distributions are color coded for different products. Solid red diamonds are for the heaviest fragment in the event, black lines are for the second-heaviest fragment, open blue circles are for $\alpha$ particles.