Isotopic scaling of heavy projectile residues from the collisions of 25 M e V / n u c l e o n 86 Kr with 124 Sn , 112 Sn and 64 Ni , 58 Ni

The scaling of the yields of heavy projectile residues from the reactions of $25\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{n}{ }^{86}\mathrm{Kr}$ projectiles with ${}^{124}\mathrm{Sn},$ ${}^{112}\mathrm{Sn}$ and ${}^{64}\mathrm{Ni},$ ${}^{58}\mathrm{Ni}$ targets is studied. Isotopically resolved yield distributions of projectile fragments in the range $Z=10--36$ from these reaction pairs were measured with the MARS recoil separator in the angular range $2.7\ifmmode^\circ\else\textdegree\fi{}--5.4\ifmmode^\circ\else\textdegree\fi{}.$ For these deep inelastic collisions, the velocities of the residues, monotonically decreasing with Z down to $Z\ensuremath{\simeq}26--28,$ are employed to characterize the excitation energy. The ratios ${R}_{21}(N,Z)$ of the yields of a given fragment $(N,Z)$ from each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are $\ensuremath{\alpha}=0.43$ and $\ensuremath{\beta}=\ensuremath{-}0.50$ for the $\mathrm{Kr}+\mathrm{Sn}$ system and $\ensuremath{\alpha}=0.27$ and $\ensuremath{\beta}=\ensuremath{-}0.34$ for the $\mathrm{Kr}+\mathrm{Ni}$ system. For the $\mathrm{Kr}+\mathrm{Sn}$ system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for $Zl~26$ and $Nl~34.$ For heavier fragments from $\mathrm{Kr}+\mathrm{Sn},$ the parameters vary monotonically, $\ensuremath{\alpha}$ decreasing with Z and $\ensuremath{\beta}$ increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the $N/Z$ equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Interestingly, such high-resolution mass spectrometric data can provide important information on the role of isospin and isospin equilibration in peripheral and midperipheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.