Search

The interplay of the effects of geometry and collective motion on d – α correlation functions is investigated for central Xe + Au collisions at E / A = 50 MeV . The data cannot be explained without collective motion, which could be partly along the beam axis. A semi-quantitative description of the data can be obtained using a Monte Carlo model, where thermal emission is superimposed on collective motion. Both the emission volume and the competition between the thermal and collective motion influence significantly the shape of the correlation function, motivating new strategies for extending intensity interferometry studies to massive particles.

A technique for measuring mass differences has been developed at the Holifield Radioactive Ion Beam Facility (HRIBF) that requires no specialized equipment. Mass differences are measured as position differences between known and unknown-mass isobars, dispersed at the image of the energy-analyzing magnet following the 25 MV tandem post-accelerator, and identified by an energy-loss measurement. The technique has been demonstrated on neutron-rich 77–79Cu and 83–86Ge isotopes produced using the isotope separator online (ISOL) method with the 238U(p,fission) reaction, where a mass accuracy of 500 keV was achieved. These nuclides are well suited to the measurement technique, as they readily migrate out of the production target and to the ion source and comprise the most neutron-rich elements of the isobarically mixed beam. Because modest precision mass values can be obtained with only a few tens of counts of the nuclide of interest among orders of magnitude more of the isobaric neighbors closer to stability, the sensitivity of this technique makes it appropriate for initial mass measurements far from stability.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation and single-nucleon transfer. The B(E2;0+ → 2+) values for first 2+ excited states of even-even neutron-rich 132-136Te and 126-130Sn have been measured by Coulomb excitation in inverse kinematics. Neutron transfer onto a 134Te beam from 9Be and 13C targets, to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the νi13/2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

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.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of measurements around the 132Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2+ excited states of even-even neutron-rich 132–136Te and 126–134Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single-particle states in 135Te, has also been studied. Gamma rays from the 13C(134Te, 12C) reaction were used to identify the ν i 13 / 2 state in 135Te, at an energy of 2109 keV. These and other results, and plans for future experiments with these neutron-rich beams, are presented.

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.

Neutron-rich radioactive ion beams available from the HRIBF allow a variety of exciting measurements around the 132 Sn region, including Coulomb excitation, fusion-evaporation, and neutron transfer. The B ( E 2 ; 0 + → 2 + ) value for first 2 + excited states of neutron-rich 132,134,136 Te and 126,128,130,132,134 Sn have been measured by Coulomb excitation in inverse kinematics. The results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134 Te beam, from 9 Be and 13 C targets to populate single-particle states in 135 Te, has also been studied. Results, and plans for future experiments with these neutron-rich beams, are presented.

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.

Projectile binary breakup has been investigated in 58 Ni+ 12 C, 24 Mg, 197 Au at 34.5 MeV/A and 58 Ni+ 70 Zn at 40 MeV/A. The fragment angular distributions exhibit an anisotropic pattern showing that breakup is aligned with the direction of scattered quasi-projectile (QP). The correlation functions of the two heaviest fragments have been studied as a function of charge asymmetry. They suggest that the QP decays while still in close proximity of the target. The correlation between the charge and velocity of the two heavy fragments shows that the binary breakup of the QP might originate from an important deformation of the projectile by the target, and that the lighter of the colliding partners also contributes to the aligned emission pattern.

Evaporation residue cross sections have been measured using neutron-rich radioactive 132Sn beams incident on a 64Ni target in the vicinity of the Coulomb barrier. This is the first experiment using accelerated 132Sn beams to study nuclear reaction mechanisms. The average beam intensity was 2x 104 particles per second and the smallest cross section measured was less than 5 mb. A large sub-barrier fusion enhancement was observed compared to evaporation residue cross sections for 64Ni on stable even Sn isotopes. The enhancement cannot be accounted for by a simple barrier shift due to the change in nuclear sizes. Coupled-channel calculations including inelastic excitation 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 132Sn and 64Ni.

An array for decay studies of neutron-rich nuclei has been commissioned for use at the UNISOR separator at Holifield Radioactive Ion Beam Facility. This array consists of three segmented clover Ge detectors, plastic scintillators, and a high-resolution (∼1 keV) Si conversion electron spectrometer. These detectors are mounted on a support that surrounds a moving tape collector. This system has been named clover array for radioactive decay studies. The detectors have been outfitted with digital flash ADCs (XIA DGFs) that fit the preamp signals, with built-in pileup rejection.

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.

A new high resolution, charged particle detector array, LASSA, has been developed. LASSA consists of nine individual telescopes that each utilizes a Si–Si–CsI(Tl) stack to provide isotopic identification of fragments (1⩽Z⩽8) with good angular resolution over a wide dynamic range in energy. The energy range covered is E/A=2.4– 140 MeV for protons and E/A=4.8– 335 MeV for 16O ions. The front portion of each telescope is comprised of a 65 μm Si(IP) strip detector backed by a 500 μm Si(IP) strip detector. This second detector provides position information in two dimensions. Behind the second silicon detector is a cluster of four 6 cm thick CsI(Tl) crystals, each read-out by a photodiode. The design, construction, and performance characteristics of this detector telescope are described.

This article describes the crystal selection and quality control utilized to develop and calibrate a high resolution array of CsI(TI) scintillator crystals for the detection of energetic charged particles. Alpha sources are used to test the light output variation due to thallium doping gradients. Selection of crystals with better than 1% non-uniformity in light output is accomplished using this method. Tests with 240 MeV alpha beam reveal that local light output variations within each of the tested CsI(TI) crystals limit the resolution to about 0.5%. Charge and mass dependences in the energy - light output relationship are determined by calibrating with energetic projectile fragmentation beams., Comment: 24 pages, 7 figures

The peripheral and semiperipheral reactions in ${}^{35}\mathrm{Cl}{+}^{197}\mathrm{Au}$ have been studied at 30 and 43 MeV/nucleon. The nonequilibrium $\ensuremath{\alpha}$ and IMF components have been observed in the experiment. The fraction of nonequilibrium emission decreases with an increase in the atomic number of the projectilelike fragments but, for a given projectilelike fragment, it increases with the charge of the emitted particles. The characteristics of quasiprojectiles reconstructed from their decay products reveal several features reminiscent of damped reactions at lower bombarding energies. The atomic number and deflection angle of projectilelike fragments depend strongly on their kinetic energy or dissipated energy. At 30 MeV/nucleon, the experimental data can be explained by a deep inelastic transfer model. One-body dissipation is still the main mechanism for the energy and angular momentum dissipation. However, at 43 MeV/nucleon, deep inelastic transfer models can predict only the experimental tendency. Two-body dissipation plays a more important role at higher incident energies. The similarity observed in the decay product distributions, as a function of excitation energy, suggests that the excited quasiprojectiles formed in binary collisions might approach thermal equilibrium for both incident energies. The decay products have been analyzed with sequential-binary and simultaneous-disassembly statistical decay models. Both statistical models are able to provide good agreement with the experimental observables except for the mean kinetic energy of the products.

The experimental signature of the formation of a necklike structure, with a velocity between that of the projectilelike emitter and that of the targetlike emitter, is investigated with the same beam and experimental setup for targets lighter and heavier than the projectile. The reactions are {sup 35}Cl on {sup 12}C and on {sup 197}Au at 43 MeV/nucleon. Particle velocity distributions are compared with two-source statistical simulations and the presence of a necklike structure is inferred from the data. In the second part of the paper, dynamical model simulations with the formation of a necklike structure are presented for the {sup 35}Cl+{sup 12}C system at 43 MeV/nucleon. {copyright} {ital 1997} {ital The American Physical Society}

The evolution of the πh 11/2 vh 11/2 and πh 11/2 vs 1/2 isomeric configurations was studied for the N = 77 isotones near the proton drip line. The decays of metastable levels in 140Eu, 142Tb, and 144Ho were measured by means of X-, gamma- and conversion electron spectroscopy at the Recoil Mass Spectrometer at Oak Ridge. The sequence of isomeric levels in 140Eu was experimentally determined. The half-life of the πh 11/2 vh 11/2 state in 142Tb was remeasured to be 25(1) µs. The spins and parities of 5− and 8+ for the πh 11/2 vs 1/2 and πh 11/2 vh 11/2 142Tb isomers, respectively, were established from measured multipolarities. No evidence for the expected 1+ ground state was found in the 144Ho decay data.

The g factors of the first-excited states of stable 124Sn and radioactive 126,128Sn were measured by the recoil in vacuum method with relatively high precision (particularly for 124,126Sn). The experiments were performed at the Holifield Radioactive Ion Beam Facility by Coulomb exciting 3 MeV/u beams in inverse kinematics on carbon and titanium targets. The results for 124Sn and 126Sn are in excellent agreement with recent shell model calculations. For 128Sn the experiment suggests an increase in the magnitude of g(2+1 ), as predicted by some models. The present results provide a sensitive probe of the valence orbitals that contribute to the 2+1 wavefunctions as one approaches the radioactive 132Sn double-shell closure.

The reverse kinematics light heavy-ion system 35 Cl+ 12 C at 43 MeV/nucleon is investigated in the framework of a dynamical-statistical hybrid model. The modified quantum molecular dynamics model which includes a Pauli potential is used to describe the initial non-equilibrium stage of the reaction and the formation of highly excited pre-fragments. The decay of pre-fragments is further described with a statistical sequential decay code. The distributions of global observables from filtered simulations, such as the center-of-mass velocity of products, fragment charge, anisotropy ratio and flow angle, reproduce the experimental results remarkably well. The evolutions between the number of sources and flow angle with impact parameter are investigated and compared with experimental results. For the light heavy-ion system, binary dissipative collision is a dominant reaction process. The process is still present even in central collisions. The formation of a neck-like structure can be observed at mid-central collision in the simulations. From a comparison to Boltzmann-Uehling-Uhlenbeck model calculations, a possible origin of the neck-like structure for the present system could be the mean-field instabilities which originate from dynamical fluctuations through the time evolution of the collision.

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.

Characteristics of {sup 35}Cl+{sup 12}C collisions at 43 MeV/nucleon have been studied for events with total charge detected ({ital Z}=23). It is shown that while single-source events are present in the data, the binary nature of the collision is dominant. For binary events, the emitting sources (projectile-like and target-like) were reconstructed independently allowing a direct measurement of the total dissipated energy. It is found that up to 75{percent} of the available energy is dissipated and the significant momentum transfer of the selected events leads to the {open_quote}{open_quote}equal temperature limit.{close_quote}{close_quote} {copyright} {ital 1996 The American Physical Society.}

The response of an array of plastic phoswich detectors to ions of $1\le Z\le 18$ has been measured from $E/A$=12 to 72 MeV. The detector response has been parameterized by a three parameter fit which includes both quenching and high energy delta-ray effects. The fits have a mean variation of $\le 4\%$ with respect to the data., Comment: 17 pages, 5 figures

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.

A study of the most violent collisions in the 24Mg+12C reactions at 25A and 35A MeV has been carried out. Experimental data, for those events in which the total charge of the system has been detected, are compared to simulations based on statistical fragmentation codes. For violent events, a binary mechanism appears to be competing successfully with compound nucleus formation.

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.

In this paper an original energy relation for light scintillation in a CsI(T1) detector is established. It is dependent on the charge and mass of the particle (fragment) and is a direct light-to-energy function suitable for the calibration of CsI(T1) detectors used in heavy-ion studies at intermediate energies. Resulting calibration with this function and energy spectra for light ions, obtained from a heavy ion experiment, are presented.

High-precision measurements of $\ensuremath{\langle}{0}_{1}||M(E2)||{2}_{1}\ensuremath{\rangle}$ matrix elements from the Coulomb excitation of ${}^{124,126,128}$Sn on a ${}^{12}$C target are presented. The extracted $B(E2)$ values decrease monotonically from the neutron midshell toward the ${}^{132}$Sn double-shell closure, despite a near constancy in the first ${2}^{+}$ level energy. Furthermore, Coulomb excitation of ${}^{124,126,128}$Sn on an enriched ${}^{50}$Ti target, combined with the results from the ${}^{12}$C target, provide a measure of the static quadrupole moments, $Q({2}_{1}{}^{+})$ (expected to be zero for a spherical shape). These new results confirm that the unstable neutron-rich ${}^{126,128}$Sn isotopes have deformations consistent with zero. The present study marks the first report on measured ${2}_{1}{}^{+}$ static quadrupole moments for unstable closed-shell nuclei.

We have re-investigated the beta decay of the three isomers of $$^{116}$$Ag at the Holifield Radioactive Ion Beam Facility (HRIBF). Through the use of half-life information, we have been able to construct individual decay schemes for each isomer, and correct what was a puzzling inconsistency with the published data, namely the beta feeding of 2$^+$ states by a 5$^+$ isomer. Our results indicate that the feeding of these levels arises from a 3$^+$ isomer in $$^{116}$$Ag. A total of 271 gamma-ray transitions (159 new) were assigned to 148 levels (94 new) from the beta-decay of $$^{116m1,m2,gs}$$Ag. Significant deviations are observed from expected U(5) symmetry in the 0$^+$ and 2$^+$ members of the previously assigned three-phonon quintuplet. Candidate states for the quadrupole-octupole quintuplet states and $$\pi$$g$$_{9/2}-\pi$$p$$_{1/2}$$, $$\pi$$g$$_{9/2}-\pi$$p$$_{3/2}$$, $$\nu$$h$$_{11/2}-\nu$$s$$_{1/2}$$, $$\nu$$h$$_{11/2}-\nu$$d$$_{3/2}$$, and $$\nu$$h$$_{11/2}-\nu$$d$$_{5/2}$$ broken pair states are assigned.

Collisions of 112Sn and 124Sn nuclei, which differ in their isospin asymmetry, provide information about the rate of isospin diffusion and equilibration. While several different probes can provide accurate diffusion measurements, the ratios of the mirror nuclei may be the simplest and most promising one. Ratios of the mass seven mirror nuclei yields are analyzed to show the rapidity, transverse momentum and impact parameter dependence of isospin diffusion., 22 pages, 7 figures

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.

A new isomer in {sup 116}Ag with a half-life of 20(1) s has been discovered through the use of conversion electron, {beta}, and {gamma} spectroscopy of on-line mass-separated radioactivities at the Holifield Radioactive Ion Beam Facility at ORNL. The observed electron peaks at 22.5, 44.42, and 47.33 keV were interpreted as the K,L, and M conversion electron lines resulting from a 47.9-keV E3 transition associated with the decay of a second isomeric level in {sup 116}Ag. A new level structure of {sup 116}Ag is proposed, with the levels identified as the 0{sup -} ground-state and isomers at 47.9 and 128.8 keV having spin/parities of 3{sup +} and 6{sup -} respectively.

The characteristics of intermediate mass fragments (IMFs: 3, 13 pages, 15 figures

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.

Simultaneous measurement of both neutrons and charged particles emitted in the reaction {sup 64}Zn + {sup 64}Zn at 45 MeV/nucleon allows comparison of the neutron to proton ratio at midrapidity with that at projectile rapidity. The evolution of N/Z in both rapidity regimes with increasing centrality is examined. For the completely reconstructed midrapidity material one finds that the neutron to proton ratio is above that of the overall {sup 64}Zn + {sup 64}Zn system. In contrast, the reconstructed ratio for the quasiprojectile is below that of the overall system. This difference provides the most complete evidence to date of neutron enrichment of midrapidity nuclear matter at the expense of the quasiprojectile.

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.

Particle correlation functions from central $^{129}\mathrm{Xe}+^{197}\mathrm{Au}$ collisions at $50A\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ have been measured with a large area $\text{silicon}\text{\penalty1000-\hskip0pt}\text{strip}∕\mathrm{Cs}\mathrm{I}$ detector array. A new technique of spin determination from particle correlation functions is proposed. Two examples of correlation functions are studied. The spin of the first excited level of $^{8}\mathrm{B}$ at $0.774\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ is determined as $J=1$.

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

Using symmetric 112Sn+112Sn, 124Sn+124Sn collisions as references, we probe isospin diffusion in peripheral asymmetric 112Sn+124Sn, 124Sn+112Sn systems at an incident energy of E/A=50 MeV. 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.

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.