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We describe an experiment optimized to determine the transition probabilities for excitation of the first excited 2+ state in 132Sn. The large excitation energy (4.04 MeV) and consequent small excitation cross-section, together with the modest beam intensity available makes this a challenging experiment. The preliminary result is B(E2; 0+ → 2+) = 0.11 ± 0.03e 2 b 2. The high efficiency and generalized nature of the setup enabled us to also measure the first 2+ state in the two-neutron nucleus 134Sn. We have determined a value of B(E2; 0+ → 2+) = 0.029 ± 0.005e 2 b 2 which shows no sign of the asymmetry with respect to the N = 82 shell closure exhibited by the Te isotopes.

We have measured the B(E;0+ → 2+) for the first excited 2+ states in the double-closed shell nucleus 132Sn and the two-neutron nucleus 134Sn. The results, based on a preliminary analysis are shown in Fig. 1 along with measurements on the stable Sn isotopes, and earlier results on 126,128,130Sn [1] . The experimental setup developed for the 132,134Sn measurements was also employed in a successful measurement of B(E;0+ → 2+) for the closed-neutron-shell nucleus 82Ge.

Evaporation residue cross sections have been measured with neutron-rich radioactive 132 Sn beams on 64 Ni in the vicinity of the Coulomb barrier. The average beam intensity was 2 × 10 4 particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. The measurement is compared to coupled-channel calculations taking into account inelastic excitation and nucleon transfer.

Complete events with at least 83% of the total charge of the 24Mg+12C system has been investigated at 45 A MeV with a large multidetector array. Central single-source events are selected by use of the Statistical Discriminant Analysis Method. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate-mass fragments from single-source events. Many-body trajectory calculations with a surface sequential model indicate an emission time of ≈400 fm/c suggesting that a large part of the single-source data can be described in a sequential decay picture. At the highest reconstructed excitation energy limit (≈8–10 A MeV), a mean emission time of ≈200 fm/c is deduced showing that part of the data may be explained by an instantaneous multifragmentation scenario.

The breakup of the projectile 24 Mg, excited in peripheral collisions on a gold target, has been investigated at 25 and 35 A MeV with a large scintillation-detector array allowing exclusive measurements. Absolute breakup cross sections were deduced and the projectile-like nucleus velocity and excitation energy have been reconstructed. The excitation energy partition between the projectile and the target is found to lay between the limits of equal excitation energy sharing and equal temperature with some evolution from one limit to the other. The statistical nature of the decay mechanism is inferred from global variables. Small-relative-angle analysis is applied to the six-alpha exit channel and the corresponding data were found to be consistent with a sequential evaporation decay mechanism, with some contribution from sequential fission at higher excitation energies. The time scale involved in the breakup of 24 Mg projectiles into the 6α and the 5αHH channels has been investigated by examining distortions in the fragment velocity distributions due to the Coulomb field of the target. A decrease in the quasi-projectile lifetime is observed as the mean excitation energy increases from 3.4 to 4.5 A MeV.

Light charged particle emission has been investigated as a function of excitation energy in exclusive experiments on the decay of $^{16}\mathrm{O}$, $^{24}\mathrm{Mg}$, and $^{35}\mathrm{Cl}$ projectiles between 25A and 70A MeV. The systematics of excitation energy removed by Z=1 and Z=2 particles were deduced. The results are similar to a previous study of proton and \ensuremath{\alpha}-particle evaporation in compound nucleus reactions at beam energies below 20A MeV, supporting the idea of a common statistical process.

The fragmentation of the projectile 24 Mg, excited in peripheral collisions on 197 Au and 12 C targets, has been investigated at 25A MeV and 35A MeV with a large scintillator detector array. Projectile breakup events were selected in the off-line analysis, by requiring that the total detected charge in one event be equal to that of the projectile. The projectile-like nucleus (PLN) excitation energies have been reconstructed and compared to results at 50A MeV and 70A MeV. The branching ratios of 2-, 4-, 5-, and 6-fold decays show a linear dependence, characteristic of statistical decay. Global variable analysis in terms of relative angles was applied to the six-alpha exit channel and the corresponding data have been compared to theoretical calculations based on simultaneous and sequential binary decay. The latter have been divided into sequential fission and sequential evaporation decays. The data has been found to be consistent with a sequential evaporation decay mechanism for a wide range of the intermediate-energy domain.

Evaporation residue and fission cross sections of radioactive $$^{132}$$Sn on $$^{64}$$Ni were measured near the Coulomb barrier. A large sub-barrier fusion enhancement was observed. Coupled-channel calculations including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, no extra fusion enhancement is observed in $$^{132}$$Sn+$$^{64}$$Ni with respect to stable Sn+$$^{64}$$Ni. A systematic comparison of evaporation residue cross sections for the fusion of even $$^{112-124}$$Sn and $$^{132}$$Sn with $$^{64}$$Ni is presented.

The B(E2;0(+)(1)-->2(+)(1)) values for the radioactive neutron-rich germanium isotopes (78,80)Ge and the closed neutron shell nucleus 82Ge were measured at the HRIBF using Coulomb excitation in inverse kinematics. These data allow a study of the systematic trend between the subshell closures at N=40 and 50. The B(E2) behavior approaching N=50 is similar to the trend observed for heavier isotopic chains. A comparison of the experimental results with a shell model calculation demonstrates persistence of the N=50 shell gap and a strong sensitivity of the B(E2) values to the effective interaction.

Alpha particles emitted from an excited projectile-like fragment (PLF*) formed in a peripheral collision of two intermediate-energy heavy ions exhibit a strong preference for emission towards the target-like fragment (TLF). The interplay of the initial deformation of the PLF* caused by the reaction, Coulomb proximity, and the rotation of the PLF* results in the observed anisotropic angular distribution. Changes in the shape of the angular distribution with excitation energy are interpreted as being the result of forming more elongated initial geometries in the more peripheral collisions., 4 figures

Isotopic yields for light particles and intermediate mass fragments have been measured for central $^{112}\mathrm{Sn}+^{112}\mathrm{Sn}$, $^{112}\mathrm{Sn}+^{124}\mathrm{Sn}$, $^{124}\mathrm{Sn}+^{112}\mathrm{Sn}$, and $^{124}\mathrm{Sn}+^{124}\mathrm{Sn}$ collisions at $E∕A=50\phantom{\rule{0.3em}{0ex}}\text{MeV}$ and compared with predictions of stochastic mean field calculations. These calculations predict a sensitivity of the isotopic distributions to the density dependence of the asymmetry term of the nuclear equation of state. However, the secondary decay of the excited fragments modifies significantly the primary isotopic distributions and these modifications are rather sensitive to theoretical uncertainties in the excitation energies of the hot fragments. The predicted final isotope distributions are narrower than the experimental data and the sensitivity of the predicted yields to the density dependence of the asymmetry term is reduced.

Evaporation residue cross sections have been measured with neutron-rich radioactive 1 3 2 Sn beams on 6 4 Ni in the vicinity of the Coulomb harrier. The average beam intensity was 2 × 10 4 particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. Coupled-channel calculations taking into account inelastic excitation significantly underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of 1 3 2 Sn and 6 4 Ni.

Evaporation residue and fission cross sections of radioactive 132Sn on 64Ni were measured. Statistical model calculations using parameters simultaneously fitting stable Sn+64Ni data reproduce the 132Sn induced reaction very well. A large sub-barrier fusion enhancement was observed. The enhancement can be accounted for by coupled-channels calculations including inelastic excitation of the projectile and target, and neutron transfer.

It is now well known that collisions between heavy ions in the Fermi energy domain produce mainly binary type events[1]-[5]. It seems that this binary character dominates even for the most violent reactions[1, 3, 4]. However, what is still not well understood is the deexcitation stage of the two principal emitters and the effects produced by the entrance channel dynamics. An important factor in this energy range is that many processes are possibly in competition and it is experimentaly difficult to isolate each of them. Processes such as the progressively vanishing fusion, binary deep inelastic collisions and the appearance of nucleon-nucleon scattering are all present in the Fermi energy range. Furthermore, detected particles could have been emitted on a large time scale from very different stages of the decay, ranging from pre-equilibrium process to evaporation. Within the statistical break-up hypothesis, where the two principal emitters are considered as thermalized nuclei, we expect only the excitation energy of each emitter, and not the way it is reached, to be a determinant quantity for the disintegration exit channels. On the other hand, typical violence of these collisions can also lead to important deformations of the two main products of the reaction. Such deformations were observed recently by the rupture of neck-like structures linking the reaction partners[6]-[9]. In an asymmetric collision, we could expect the biggest nucleus to sustain the largest deformation. By its subsequent disintegration toward a more stable state, it could be possible to observe the effects of such a deformation on the deexcitation mode of the nucleus.

The same hot nuclear system ({Sigma}{ital Z}=18) has been studied for two different entrance channels with reaction products detected in a forward array of scintillators: central collisions of {sup 24}Mg on a {sup 12}C target at 25{ital A} and 35{ital A} MeV and peripheral pickup reactions of {sup 35}Cl on a {sup 197}Au target at 43{ital A} MeV. The detection-efficiency-corrected charge distributions, multiplicity of charged particles and cross sections as a function of excitation energy are compared. The reaction mechanism is investigated, through comparison to simulations with statistical observables. The central reaction {sup 24}Mg+{sup 12}C at 35{ital A} MeV is well characterized by a dissipative binary collision scenario. Data at 25{ital A} MeV show less evidence of such dynamical characteristics. The intermediate-mass fragments (3{le}{ital Z}{le}8) production for each reaction is compared to model calculations for different values of excitation energy. The systems formed in the central collision at 25{ital A} MeV and the pickup reaction at 43{ital A} MeV show similar source characteristics, both statistically and in momentum space. However, the yields of the various exit channels, from evaporation and/or fission to multifragmentation and vaporization, differ for the two reactions. {copyright} {ital 1996 The American Physical Society.}

Intermediate-mass fragment (IMF) production, typically 3≤;Z≤20, is a widely observed decay mode in heavy-ion reactions[1]. Possible scenarios to explain such a decay mode include bulk instabilities based on the expansion of hot nuclear matter with an initial compression stage in near-central collisions [2]–[4]. However, for such collisions, dynamical IMF production is also present as is evident from the observation of neck emission[5]–[7]. Also, the persistence of binary dissipative collisions[8]–[11] leaves a very small cross section for forming a hot and dense single source[12, 13]

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of $^{35}$Cl+$^{197}$Au at 43 MeV/nucleon and $^{70}$Ge+$^{nat}$Ti at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN collaboration. A mass scaling, by $A_{source} \sim$ 35 to 190, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling., Comment: 13 pages LaTeX file, including 3 postscript figures (in .tar.gz fornmat), accepted in Phys. Rev. C . Also available at http://thomson.phy.ulaval.ca/ions_lourds/gil-en.html

The time scale involved in the breakup of 24 Mg projectiles into the 6α and the 5αpH channels has been investigated by examining distortions and shifts in the fragment velocity distributions due to the Coulomb field of the target. Assuming a fixed angular momentum range of 4 to 8 ħ strok; for both channels, we deduce time scales of (5.1 – 6.8) × 10 −22 s for the six-alpha channel at 3.4 MeV of excitation per nucleon and (3.0–5.9) × 10 −22 s for the 5αpH channel at 4.5A MeV of excitation.

Projectile-like fragments (PLF:15, 5 pages, 6 figures