Statistical behavior of nucleon transfer to highly excited states in heavy-ion collisions

Abstract

The $Q$-value dependence of the cross section of heavy-ion transfer reactions to high-lying states is studied. The continuum energy spectra are measured for each exit channel while systematically varying the bombarding energy, scattering angle, target and projectile: $^{16}\mathrm{O}$ + $^{232}\mathrm{Th}$, $^{15}\mathrm{N}$ + $^{232}\mathrm{Th}$ and $^{181}\mathrm{Ta}$, and $^{35}\mathrm{Cl}$ + $^{181}\mathrm{Ta}$ at energies ranging up to 1.4 times the Coulomb barrier, at and about the grazing angle. The transfer reactions exhibit common properties which depend mainly on the number of transferred nucleons $\ensuremath{\Delta}N$. The average amount of energy dissipated, which depends strongly on $\ensuremath{\Delta}N$, is interpreted with a semiclassical model that assumes momentum matching and the presence of a velocity-dependent frictional force. Also, the branching ratios of the exit channels are observed to depend systematically on $\ensuremath{\Delta}N$, which suggests a statistical process. The energy spectra are examined in terms of a phenomenological constrained phase-space analysis. The dominant constraint is found to be the optimal $Q$ value. The description of data with at most two constraints implies that essentially all of the information content of the energy distribution is contained in these constraints.NUCLEAR REACTIONS Heavy ions, $^{232}\mathrm{Th}$($^{16}\mathrm{O}$,$X$), $X=^{9,10}\mathrm{Be}, ^{11\ensuremath{-}13}\mathrm{B}, ^{12\ensuremath{-}16}\mathrm{C}, ^{15\ensuremath{-}17}\mathrm{N}, ^{17\ensuremath{-}19}\mathrm{O}$, $E=97,105,115,125$ MeV. $^{181}\mathrm{Ta}$, $^{232}\mathrm{Th}$($^{15}\mathrm{N}$,$X$), $X=^{9\ensuremath{-}11}\mathrm{Be}, ^{11\ensuremath{-}13}\mathrm{B}, ^{12\ensuremath{-}15}\mathrm{C}$, $E=86,95,103$ MeV. $^{181}\mathrm{Ta}$($^{35}\mathrm{Cl}$,$X$), $X=^{30\ensuremath{-}32}\mathrm{Si}, ^{31\ensuremath{-}34}\mathrm{P}, ^{33\ensuremath{-}36}\mathrm{S}, ^{36,37}\mathrm{Cl}$, $E=205$ MeV. Energy and angular distributions of the ejectiles. Statistical aspects. Surprisal analysis.