Publications (15)37.33 Total impact
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ABSTRACT: The capturefission cross section for the 36 S+ 208 Pb reaction was measured for seven centerofmass energies ranging from 147.5 to 210.2 MeV. A comparison of the deduced interaction barriers from "distribution of barriers" measurements and simple 1/E c.m. plots for 13 wellcharacterized systems shows the validity of the latter approach for deducing interaction barriers, especially for reaction systems involving radioactive beams where the former measurements are not currently feasible. Application of the 1/E c.m. plot technique for the 36 S+ 208 Pb reaction gives an interaction barrier height of 140.4 ± 1.4 MeV. This value as well as the deduced interaction barriers for all known studies of capture cross sections with radioactive beams are in good agreement with recent predictions of an improved isospindependent quantum molecular dynamics model and a modified version of capture crosssection systematics by Swiatecki et al. The deduced barriers for these nrich systems are lower than one would expect from semiempirical systematics based upon the Bass potential. In addition to the barrier lowering, there is an enhanced subbarrier cross section in these nrich systems not predicted by the Bass potential systematics. These enhanced subbarrier cross sections may be important in the synthesis of the heaviest nuclei. 
Article: Fusion of Li 9 with Pb 208
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ABSTRACT: We have measured the fusion excitation function for the Li9+Pb208 reaction for nearbarrier projectile centerofmass energies of 23.9 to 43.0 MeV using the ISAC2 facility at TRIUMF. The αemitting evaporation residues (At211–214) were stopped in the Pb208 target, and their decay was measured. The isotopic yields at each energy were in good agreement with the predictions of a statistical model code (HIVAP). The measured fusion excitation function shows evidence for substantial subbarrier fusion enhancement not predicted by current theoretical models. There is a suppression of the above barrier cross sections relative to these model predictions. The implications of this measurement for studying the fusion of Li11 with Pb208 are discussed. 
Article: Fusion of Li9 with Pb208
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ABSTRACT: We have measured the fusion excitation function for the Li9+Pb208 reaction for nearbarrier projectile centerofmass energies of 23.9 to 43.0 MeV using the ISAC2 facility at TRIUMF. The αemitting evaporation residues (At211214) were stopped in the Pb208 target, and their decay was measured. The isotopic yields at each energy were in good agreement with the predictions of a statistical model code (HIVAP). The measured fusion excitation function shows evidence for substantial subbarrier fusion enhancement not predicted by current theoretical models. There is a suppression of the above barrier cross sections relative to these model predictions. The implications of this measurement for studying the fusion of Li11 with Pb208 are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: As hindrance sets in for the fusion of heavier systems, the effect of large neutron excess in the colliding nuclei on their probability to fuse is still an open question. The detection of evaporation residues (ERs), however, provides indisputable evidence for the fusion (complete and incomplete) in the reaction. We therefore devised a system with which we could measure ERs using low intensity neutron‐rich radioactive ion beams with an efficiency close to 100%. We report on measurements of the production of ERs in collisions of 132,134Sn, 134Te and 134Sb ion beams with medium mass, neutron‐rich targets. The data taken with 132,134Sn bombarding a 64Ni target are compared to available data (ERs and fusion) taken with stable Sn isotopes. Preliminary data on the fusion of 132Sn with 96Zr target are also presented. 
Article: Fusion of 9Li with 208Pb
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ABSTRACT: We have measured the fusion excitation function for the Li9+Pb208 reaction for nearbarrier projectile centerofmass energies of 23.9 to 43.0 MeV using the ISAC2 facility at TRIUMF. The alphaemitting evaporation residues (At211214) were stopped in the Pb208 target, and their decay was measured. The isotopic yields at each energy were in good agreement with the predictions of a statistical model code (HIVAP). The measured fusion excitation function shows evidence for substantial subbarrier fusion enhancement not predicted by current theoretical models. There is a suppression of the above barrier cross sections relative to these model predictions. The implications of this measurement for studying the fusion of Li11 with Pb208 are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Capturefission cross sections were measured for the collision of the massive nucleus 132 Sn with 96 Zr at centerofmass energies ranging from 192.8 to 249.6 MeV in an attempt to study fusion enhancement and hindrance in this reaction involving very neutronrich nuclei. Coincident fission fragments were detected using silicon detectors. Using angle and energy conditions, deep inelastic scattering events were separated from fission events. Coupledchannels calculations can describe the data if the surface diffuseness parameter, a, is allowed to be 1.10 fm instead of the customary 0.6 fm. The measured capturefission cross sections agree moderately well with model calculations using the dinuclear system model. If we use this model to predict fusion barrier heights for these reactions, we find the predicted fusion hindrance, as represented by the extra push energy, is greater for the more neutronrich system, lessening the advantage of the lower interaction barriers with neutronrich projectiles. 
Article: Fusion of ^9Li with ^208Pb
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ABSTRACT: The fusion of weakly bound nuclei is one of the active areas of research with radioactive beams. The main issue is whether the fusion cross section will be enhanced due to large nuclear size of the halo nuclei or breakup of the weakly bound valence nucleons will lead to decreased fusion cross section. In the case of ^11Li with ^208Pb, differences between theoretical predictions are very large. We observed large sub barrier fusion enhancement in the case of ^9Li with ^70Zn. As an extension to this study, measurements were carried out at TRIUMF using ^9Li beams in the energy range 2545 MeV on ^208Pb. The alpha decay of the evaporation residues were detected using 16 silicon detectors placed close to the target. The study of ^9Li with ^208Li will be very important to understand the halo effect on fusion of ^11Li with ^208Pb. The experimental results will be presented along with theoretical model predictions.  [Show abstract] [Hide abstract]
ABSTRACT: The capture cross sections and fission fragment angular distributions were measured for the reaction of 50 Ti with 208 Pb at center of mass projectile energies (E c.m.. From fitting the backward angle fragment angular distributions, the cross sections for quasifission and fusionfission and P CN , the probability that the colliding nuclei go from the contact configuration to inside the fission saddle point, were deduced. These quantities, along with the known values of the evaporation residue production cross sections for this reaction, were used to deduce values of the survival probabilities, W sur , for this reaction as a function of excitation energy. The deduced values of P CN and W sur and their dependence on excitation energy differ from some current theoretical predictions of these quantities.  [Show abstract] [Hide abstract]
ABSTRACT: The cross section for the fusion of 9Li with 70Zn was measured for seven projectile energies spanning the subbarrier and nearbarrier region (Ec.m. ranging from 9.7 to 13.4 MeV) using the ISAC facility at TRIUMF. γray spectroscopy of the irradiated target foils along with β counting of the chemically separated Ge and As evaporation residues were used to measure the fusion cross sections. Statistical model calculations were used to correct for the yields of any unobserved nuclei. The observed fusion excitation function shows significant subbarrier fusion enhancement with a large deduced value of the fusion radius, RB=12.1±1.0 fm. Coupledchannels calculations do not account for the observed subbarrier enhancement. The implications of this finding for understanding the fusion of 11Li are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Capturefission cross sections were measured for the near symmetric reaction between the massive nuclei Â¹Â²â´Sn and â¹â¶Zr for center of mass energies from 195 to 265 MeV. Coincident fission fragments were detected and separated from elastic and deep inelastic scattering products by angle/energy/mass conditions. The measured capture cross sections agree quite well with calculations using the dinuclear system (DNS) model. The DNS model also predicts the fusion cross section for this reaction with a fusion barrier height of 208.0 MeV. The deduced extra push energy, corresponding to this barrier height, differs from that deduced from evaporation residue measurements. 
Article: Subbarrier fusion of Li9 with Zn70
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ABSTRACT: The cross section for the fusion of Li9 with Zn70 was measured for seven projectile energies spanning the subbarrier and nearbarrier region (Ec.m. ranging from 9.7 to 13.4 MeV) using the ISAC facility at TRIUMF. gammaray spectroscopy of the irradiated target foils along with beta counting of the chemically separated Ge and As evaporation residues were used to measure the fusion cross sections. Statistical model calculations were used to correct for the yields of any unobserved nuclei. The observed fusion excitation function shows significant subbarrier fusion enhancement with a large deduced value of the fusion radius, RB=12.1±1.0 fm. Coupledchannels calculations do not account for the observed subbarrier enhancement. The implications of this finding for understanding the fusion of Li11 are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: The capturefission cross section was measured for the reaction of 38S with 208Pb for centerofmass projectile energies Ecm of 160–265 MeV. The 38S beam was prepared by projectile fragmentation at the NSCL at higher energies and degraded to 4–7 MeV/nucleon. The time of flight (energy) of each interacting beam particle was measured along with the fission fragments. The data were compared to previous measurements of the capturefission excitation function for the 32S+208Pb reaction. The interaction barrier for the 38Sinduced reaction is 16.1±10.1 MeV lower than the 32Sinduced reaction whereas the reduced excitation functions for the two reactions are similar. A discussion of the systematics of barrier shifts in the fusion of nrich nuclei is given and the implications of this shift for the synthesis of heavy nuclei with radioactive beams are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Accurate measurements of prompt, scission, and preequilibrium neutron multiplicities from the nuclear fission process are of great importance to nuclear technology. The variation of these quantities with mass division, the excitation energy of the fissioning system E*, and the kinetic energy release to the fragments sheds light on the partition of energy during the large scale collective motion of the scission process. While some systems have been characterized very well, many others remain unknown. Instead of using the (n,f) reaction, we are using the surrogate (d,pf) reaction. Measurements performed recently at the CENPA yielded information on the ^237239U, ^236239Np, and ^240Pu fissioning systems. Analysis is in progress.  [Show abstract] [Hide abstract]
ABSTRACT: An attempt to confirm production of superheavy elements in the reaction of 48Ca beams with actinide targets has been performed using the 238U(48Ca,3n)283112 reaction. Two 48Ca projectile energies were used that spanned the energy range where the largest cross sections have been reported for this reaction. No spontaneous fission events were observed. No α decay chains consistent with either reported or theoretically predicted element 112 decay properties were observed. The crosssection limits reached are significantly smaller than the recently reported cross sections. 
Article: The fusion of ^9,11Li with ^70Zn
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ABSTRACT: The fusion of ^9,11Li with ^70Zn was studied at the ISAC1 facility at TRIUMF. Beams of ^9Li (1114.5 MeV) struck 1 mg/cm^2 ^70Zn targets in an evacuated scattering chamber. Beam intensities were monitored by measuring elastic scattering and by the use of a Faraday cup for ^9Li while the ^11Li beam was counted in a semiconductor detector placed behind the target. Typical ^9Li beam intensities were 5 x 10^6 particle/s while the ^11Li intensities were 800 particles/s. As and Ge evaporation residues were assayed using gamma and beta spectroscopy following postirradiation chemical separation from the irradiated targets. A seven point excitation function for the ^9Li + ^70Zn reaction was measured and compared to coupled channels calculations. Due to the low ^11Li beam intensity, only upper limits for fusion of 2b could be established for ^11Li.
Publication Stats
201  Citations  
37.33  Total Impact Points  
Top Journals
 Physical Review C (8)
 Physical Review C (2)
Institutions

20052010

Oregon State University
 Department of Chemistry
Corvallis, Oregon, United States
