Using symmetric ${}^{112}\mathrm{S}\mathrm{n}+{}^{112}\mathrm{S}\mathrm{n}$, ${}^{124}\mathrm{S}\mathrm{n}+{}^{124}\mathrm{S}\mathrm{n}$ collisions as references, we probe isospin diffusion in peripheral asymmetric ${}^{112}\mathrm{S}\mathrm{n}+{}^{124}\mathrm{S}\mathrm{n}$, ${}^{124}\mathrm{S}\mathrm{n}+{}^{112}\mathrm{S}\mathrm{n}$ systems at an incident energy of $E/A=50\mathrm{M}\mathrm{e}\mathrm{V}$. Isoscaling analyses imply that the quasiprojectile and quasitarget in these collisions do not achieve isospin equilibrium, permitting an assessment of isospin transport rates. We find that comparisons between isospin sensitive experimental and theoretical observables, using suitably chosen scaled ratios, permit investigation of the density dependence of the asymmetry term of the nuclear equation of state. DOI: http://dx.doi.org/10.1103/PhysRevLett.92.062701 © 2004 The American Physical Society

Isotopic distributions for light particles and intermediate mass fragments have been measured for ${}^{112}\mathrm{Sn}{+}^{112}\mathrm{Sn}$, ${}^{112}\mathrm{Sn}{+}^{124}\mathrm{Sn}$, ${}^{124}\mathrm{Sn}{+}^{112}\mathrm{Sn}$, and ${}^{124}\mathrm{Sn}{+}^{124}\mathrm{Sn}$ collisions at $E/A=50\mathrm{}\mathrm{MeV}$. Isotope, isotone, and isobar yield ratios are utilized to estimate the isotopic composition of the gas phase at freeze-out. Analyses within the equilibrium limit imply that the gas phase is enriched in neutrons relative to the liquid phase represented by bound nuclei. These observations suggest that neutron diffusion is commensurate with or more rapid than fragment production.

We have measured the scintillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field by exposing a dual-phase liquid argon time projection chamber (LAr-TPC) to a low energy pulsed narrow band neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC and to select the energy of the recoiling nuclei. We report measurements of the scintillation yields for nuclear recoils with energies from 10.3 to 57.3 keV and for median applied electric fields from 0 to $970\mathrm{V}/\mathrm{cm}$. For the ionization yields, we report measurements from 16.9 to 57.3 keV and for electric fields from 96.4 to $486\mathrm{V}/\mathrm{cm}$. We also report the observation of an anticorrelation between scintillation and ionization from nuclear recoils, which is similar to the anticorrelation between scintillation and ionization from electron recoils. Assuming that the energy loss partitions into excitons and ion pairs from ${}^{83m}\mathrm{Kr}$ internal conversion electrons is comparable to that from ${}^{207}\mathrm{Bi}$ conversion electrons, we obtained the numbers of excitons ($N_{\text{ex}}$) and ion pairs ($N_{\mathrm{i}}$) and their ratio ($N_{\text{ex}}/N_{\mathrm{i}}$) produced by nuclear recoils from 16.9 to 57.3 keV. Motivated by arguments suggesting direction sensitivity in LAr-TPC signals due to columnar recombination, a comparison of the light and charge yield of recoils parallel and perpendicular to the applied electric field is presented for the first time.

Different statistical multifragmentation models have been used to study isoscaling, i.e., the factorization of the isotope ratios from two reactions, into fugacity terms of proton and neutron number, $R_{21}{\mathrm{}(N,Z)=Y}_2{\mathrm{}(N,Z)/Y}_1\mathrm{}(N,Z)=C\exp (\alpha N+\beta Z).\mathrm{}$ Even though the primary isotope distributions are quite different from the final distributions due to evaporation from the excited fragments, the values of $\alpha$ and $\beta$ are not much affected by sequential decays. $\alpha$ is shown to be mainly sensitive to the proton-to-neutron composition of the emitting source and may be used to study isospin-dependent properties in nuclear collisions, such as, the symmetry energy in the equation of state of asymmetric nuclear matter.

The two reactions ${}^{12}\mathrm{C}$(${}^4\mathrm{He}$,${}^4\mathrm{He}$ + ${}^4\mathrm{He}$ + ${}^4\mathrm{He}$)${}^4\mathrm{He}$ and ${}^9\mathrm{Be}$(${}^4\mathrm{He}$, ${}^4\mathrm{He}$ + ${}^4\mathrm{He}$ + ${}^4\mathrm{He}$)$n$ were measured using an array of four double-sided strip detectors. Excited states in ${}^{12}\mathrm{C}$ were reconstructed filtered by the condition that the $\alpha$-decay proceeded via the ${}^8\mathrm{Be}$ ground state. In both measurements, evidence was found for a new state at 13.3(0.2) MeV with a width 1.7(0.2) MeV. Angular correlation measurements from the ${}^{12}\mathrm{C}$(${}^4\mathrm{He}$, ${}^4\mathrm{He}$ + ${}^4\mathrm{He}$ + ${}^4\mathrm{He}$)${}^4\mathrm{He}$ reaction indicates that the state may have $J^{\pi }=4^{+}$.