Approaching neutron-rich nuclei toward the $r$-process path in peripheral heavy-ion collisions at 15 MeV/nucleon

The production cross sections of projectilelike fragments from the interaction of 15 MeV/nucleon ${}^{86}$Kr with ${}^{64}$Ni and ${}^{58}$Ni and with ${}^{124}$Sn and ${}^{112}$Sn were measured using a magnetic separator with emphasis on the neutron-rich isotopes. Neutron pickup isotopes (with up to six neutrons picked up from the target) were observed with large cross sections. Calculations involving the the deep-inelastic transfer model as well as the microscopic constrained molecular dynamics model (CoMD) for the primary interaction stage coupled to the statistical deexcitation code gemini were used to describe the gross properties of the product distributions. The present results were also compared with our previous data of the same reactions at 25 MeV/nucleon. The data at 15 MeV/nucleon show enhanced production of neutron-rich isotopes very close to the projectile, relative to the corresponding data at 25 MeV/nucleon. This enhancement in the cross sections is possibly associated with very peripheral collisions and longer interaction times of the neutron-rich ${}^{86}$Kr projectile with the neutron-rich targets. The large cross sections of such reactions well above the Coulomb barrier, but below the Fermi energy, involving peripheral nucleon exchange, suggest that both the $N\text{/}Z$ of the projectile and the $N\text{/}Z$ of the target may properly be exploited in the production of neutron-rich rare isotopes. These reactions offer a novel and attractive route to access extremely neutron-rich rare isotopes toward the the astrophysical $r$-process path and, possibly, the neutron drip-line.

Figure 1

Schematic diagram of the MARS experimental setup for rare isotope production. The main elements of the MARS (“Q” for quadrupoles, “D” for dipoles) and the location of the detectors are indicated (see text).

Figure 2

Mass distributions of projectilelike fragments from the reaction of ${}^{86}$Kr (15 MeV/nucleon) with ${}^{64}$Ni. The vertical dashed line indicates the mass of the projectile. Diamonds: Measured differential cross sections (mb/msr) at 4${}^{\circ }$. Triangles: Measured differential cross sections at 7${}^{\circ }$. Solid circles: Cross sections (mb) from the combined data at 4${}^{\circ }$ and 7${}^{\circ }$. Open circles: Total cross sections (mb), (i.e., measured cross sections corrected for the acceptance of the measurements with the MARS). Solid lines: DIT/gemini calculations. Dashed lines: CoMD/gemini calculations. For each model calculation, thick lines represent total cross sections and thin lines are cross sections filtered by the acceptance of the experimental setup (see text).

Figure 3

Mass distributions of projectilelike fragments from the reaction of ${}^{86}$Kr (15 MeV/nucleon) with ${}^{124}$Sn. The meanings of the points and lines are as in Fig. 2.

Figure 4

Mass distributions of elements $Z=35$–30 from the reactions of ${}^{86}$Kr (15 MeV/nucleon) with ${}^{64}$Ni and ${}^{58}$Ni. The data are shown by solid circles for ${}^{86}\mathrm{Kr}+{}^{64}\mathrm{Ni}$ and open circles for ${}^{86}\mathrm{Kr}+{}^{58}\mathrm{Ni}$.

Figure 5

Mass distributions of elements $Z=35$–30 from the reactions of ${}^{86}$Kr (15 MeV/nucleon) with ${}^{124}$Sn and ${}^{112}$Sn. The data are shown by solid circles for ${}^{86}\mathrm{Kr}+{}^{124}\mathrm{Sn}$ and open circles for ${}^{86}\mathrm{Kr}+{}^{112}\mathrm{Sn}$.

Figure 6

Mass distributions of elements $Z=36$–39 from the reactions of ${}^{86}$Kr (15 MeV/nucleon) with ${}^{64}$Ni and ${}^{58}$Ni. The data are shown by solid circles for ${}^{86}\mathrm{Kr}+{}^{64}\mathrm{Ni}$ and open circles for ${}^{86}$Kr $+$ ${}^{58}$Ni.

Figure 7

Comparisons of mass distributions of elements $Z=35$–30 from the reaction of ${}^{86}$Kr with ${}^{64}$Ni at 15 MeV/nucleon of this work (circles) with those of our previous work [29] at 25 MeV/nucleon (diamonds). The dotted lines are the EPAX expectations [55].

Figure 8

Comparisons of mass distributions of elements $Z=35$–30 from the reaction of ${}^{86}$Kr with ${}^{124}$Sn at 15 MeV/nucleon of this work (circles) with those of our previous work [29] at 25 MeV/nucleon (diamonds). The dotted lines are the EPAX expectations [55].

Figure 9

Representation of the production cross sections of projectile fragments from the reaction ${}^{86}$Kr (15 MeV/nucleon) $+$ ${}^{64}$Ni on the $Z$-$N$ plane. The cross-section ranges are shown by open circles according to the key. The solid squares show the stable isotopes. The solid line shows the astrophysical $r$-process path and the dashed line shows the location of the neutron drip-line [56]. The horizontal and vertical dashed lines indicate, respectively, the proton and neutron number of the ${}^{86}$Kr projectile.

Figure 10

Representation of the production cross sections of projectile fragments from the reaction ${}^{86}$Kr (15 MeV/nucleon) $+$ ${}^{124}$Sn on the $Z$-$N$ plane. The cross-section ranges are shown by open circles according to the key. The solid squares show the stable isotopes. The solid line shows the astrophysical $r$-process path and the dashed line shows the location of the neutron drip-line [56]. The horizontal and vertical dashed lines indicate, respectively, the proton and neutron number of the ${}^{86}$Kr projectile.