{include 3_1.DOC|3.0 NUCLEUS-NUCLEUS REACTIONS 3.1 Distributions of fusion barriers extracted from cross section measurements on the systems {EMBED EQUATION |} J.D. Bierman, P. Chan, J.F. Liang, M.P. Kelly, A.A. Sonzogni and R. Vandenbosch As described in last year's report,1 the goal of this project is to experimentally determine the fusion cross sections of the two systems to high precision spanning the entire barrier region. These data may then be twice differentiated to yield the distribution of barriers. These distributions should differ as a result of the difference in permanent quadrupole deformations of the targets. This will be a valuable test of the sensitivity of the barrier distribution or "fingerprint" method of determining the relative importance of different coupling channels. We have measured fission fragment angular distributions at energies both well above and within the barrier region. The differences in these distributions were very small. More importantly, the center of mass angle where the angular distribution intersects a properly normalized isotropic distribution is constant over the entire energy region. As a result, rather than measure the entire distribution at each energy, we simply measure the differential cross section at this angle for each energy and then convert this measurement to a total fusion cross section as if the distribution were isotropic. This allows us to acquire data of sufficiently high statistics spanning the entire barrier region in fairly small energy steps in a reasonable amount of beam time. To this point, fusion cross sections have been measured for the {EMBED EQUATION |} system in roughly 1.25 MeV energy steps covering the entire barrier region. The measured cross sections range from 400 mb down to 0.1 mb. Measurements were also made at energies well above the barrier for this system. Analysis of these data is nearly complete. The product of energy and cross section has been differentiated to extract the distribution of barriers for the osmium system which is shown in Fig. 3.1-1. This "fingerprint" is similar to that expected for a spherical projectile and a target with a prolate quadrupole and small negative hexadecapole deformation. Data have also recently been taken for the {EMBED EQUATION |} system in roughly 1.25 MeV energy steps spanning its barrier region. The analysis of these data is currently being performed. {import fig3x1.eps \* mergeformat|} Fig. 3.1-1. Fusion barrier distribution extracted from experimentally measured cross sections for {EMBED EQUATION |}. } {include 3_2.doc|3.2 Sub-barrier fusion for {EMBED EQUATION |} J.D. Bierman, P. Chan, J.F. Liang, A.A. Sonzogni and R. Vandenbosch Several mechanisms have been used to explain the observed enhancement in the sub-barrier fusion cross section, among them nuclear deformation, coupling to inelastic channels and transfer reactions. The {EMBED EQUATION |}isotopes exhibit a decrease in {EMBED EQUATION |} with increasing mass number and simultaneously the neutron transfer Q-values become more positive, which implies that collective motion should counteract transfer reactions as responsible for the enhancement. A measurement of the enhancement for the three isotopes should indicate which process is stronger. The projectile was the doubly magic {EMBED EQUATION |} to minimize projectile structure effects. Details of the experiment can be found in a previous report.1 Briefly, evaporation residue angular distributions were taken at a selected set of energies and integrated over angles. From these complete angular distributions we calibrated the relation between the differential cross section at 5 degrees and the integrated one. For a far larger set of energies, the differential cross section at 5 degrees was measured and the integrated one was then obtained. Results for {EMBED EQUATION |} are presented in Fig. 3.2-1. The full line is from a coupled channel calculation, which includes inelastic and transfer couplings. For the inelastic part we considered the 2+ and 3+ states for both projectile and target with strengths taken from the literature. The pickup of 1 and 2 neutrons was considered for the transfer part and their coupling strengths were adjusted to match the data. The dashed line is from an uncoupled calculation. A preliminary analysis suggests that for these systems, neutron transfer reactions are playing the most important role in the sub-barrier enhancement of the fusion cross section. A future experiment is being planned, where we will extend and improve the data already taken. {import fig3_2.eps \* mergeformat|} Fig. 3.2-1. Fusion excitation function for {EMBED EQUATION |}. } {include 3_3.doc|3.3 Properties of light charged particles produced by 25 A MeV 16O on {EMBED EQUATION |} and 35 A MeV 14N on {EMBED EQUATION |} D. Bowman,* G. Cren,* R. DeSouza,† J. Dinius,* A. Elmaanni,‡ D. Fox,† C. K. Gelbke,* W. Hsi,* C. Hyde-Wright,§ W. Jiang, W. G. Lynch,* T. Moore,† G. Peaslee,* D. Prindle, C. Schwarz,* A. A. Sonzogni, M. B. Tsang,* R. Vandenbosch and C. Williams* In last year's Annual Report we reported on our analysis1 of an experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University using the miniball array.2 In this experiment we measured light charged particles (LCPs) in coincidence with fission fragments (FF tag) or evaporation residues (ER tag). The ER tagged events were from more central collisions than the FF tagged events and by varying the target mass we change the average impact parameter over a large range while changing the total fusion cross section only slightly.3 In last years report we stated that we observed a substantial variation in the LCP multiplicity depending on the tag. For every target the observed LCP multiplicity is higher for the ER tag than it is for the FF tag. We show the characteristics of these "extra" LCPs by plotting in Fig. 3.3-1 the number of observed protons as a function of energy for 10,000 ER and 10,000 FF tagged events from the 25 A-MeV {EMBED EQUATION |} on {EMBED EQUATION |}. At high energies the spectra are essentially identical. For lower, evaporative energies, there is a clear excess of protons. This excess is clearly observed for protons and alphas from all targets and beam energies used in this experiment with the exception of the 35 A-MeV {EMBED EQUATION |} beam on the {EMBED EQUATION |} target which has a very low ER cross section. The excess is also observed but not as pronounced for deuterium and tritium emission. It seems clear that the fission process must occur before the compound nucleus reaches the end of the particle emission de-excitation chain. We should be able to use the number of LCPs emitted after the time at which fission occurs to estimate how quickly fission must occur. We plan to pursue this through the use of statistical model calculations. {import fig3x3.eps \* mergeformat|} Fig. 3.3-1. Number of protons as a function of energy for 25 A-MeV {EMBED EQUATION |} on {EMBED EQUATION |} data. The solid line is ER tagged data and the dashed line is FF tagged data. In the panel on the right we show the difference of the histograms. } {include 3_4.doc|3.4 Entrance channel dependence of light particle emissions in the {EMBED EQUATION |} compound nucleus decay J.D. Bierman, P. Chan, M.P. Kelly, J.F. Liang, A.A. Sonzogni and R. Vandenbosch The decay of a compound nucleus can be described by a statistical theory where the decay processes are assumed to be independent of the formation channels.1 Measurements on the decay of the {EMBED EQUATION |} compound nucleus populated by {EMBED EQUATION |} and {EMBED EQUATION |} entrance channels gave mixed results as compared to the statistical model.2 The {EMBED EQUATION |} data were in good agreement with statistical model calculations. However, the statistical model overestimated the neutron multiplicity for the {EMBED EQUATION |}reaction over a wide range of excitation energies.3 A program of studying the light-charged particles emitted from the {EMBED EQUATION |} compound nucleus was initiated recently. The {EMBED EQUATION |} compound nucleus was populated by {EMBED EQUATION |} and {EMBED EQUATION |} reactions at an excitation energy of 113 MeV where the fission barrier falls below the neutron binding energy and the maximum angular momentum, in the spin distribution, leading to evaporation residues is reached. The experiment was carried out in the Nuclear Physics Lab of the University of Washington. The angular distributions of light-charged particles were measured in coincidence with the evaporation residues by CsI scintillators coupled to PIN diodes. The evaporation residues were separated from the incident beam by a pair of electrostatic deflector plates and identified by energy vs. time-of-flight using the linac rf signal as stop. The evaporated proton and {EMBED EQUATION |} particle energy spectra are shown in Fig. 3.4-1 for the two reactions studied. The spectral shape of the C+Sm system is harder than that of the Ni+Zr system. It suggests that the protons and {EMBED EQUATION |} particles were emitted from a hotter source for the mass asymmetric system. The result of statistical model calculations using the code PACE4 are shown by solid curves. The level density parameter used in the calculations was A/10 for both systems. Good agreement between the data and the calculations can be seen for the {EMBED EQUATION |} induced reaction. For the Ni+Zr reaction, the calculation predicts a less steep slope than the measurement. More analyses comparing the entrance channel dependence of the {EMBED EQUATION |} compound nucleus decay is underway. {IMPORT C:\\ANNUAL\\ANN95\\3_chp\\FELIX34.EPS \* mergeformat|} Fig. 3.4-1. Energy spectra of protons and {EMBED EQUATION |} particles emitted from the decay of the {EMBED EQUATION |} compound nucleus. } {include 3_5.doc|3.5 Bremsstrahlung and the GDR in {EMBED EQUATION |} reactions J.D. Bierman, M.P. Kelly, J.F. Liang and K.A. Snover Recent measurements on C+Mo1 and C+Sn2 systems in the region 5-11 MeV/u bombarding energy have suggested a large target isotope effect on the production of high energy {EMBED EQUATION |} {EMBED EQUATION |} These {EMBED EQUATION |} were presumed to be bremsstrahlung primarily from first chance neutron-proton collisions. Another measurement,3 on the C+Sn system at 10 MeV/u bombarding energy finds, in conflict with the earlier measurements, no evidence for an enhanced bremsstrahlung yield when using the heavier target isotope. More recently, the Stony Brook group has presented results backing up earlier findings on the same system together with an explanation of the isotopic effect.4 In this report, we present progress in the search for a similar effect in O+Mo systems. An important part of our procedure is the use of angular distributions to better define the relative contributions of bremsstrahlung and statistical decay. We have measured the {EMBED EQUATION |} produced in the four possible reactions of {EMBED EQUATION |} using the Seattle 10" x 15" NaI spectrometer. The incident energy for both {EMBED EQUATION |} projectiles is 9.4 MeV/u. Gamma ray yields were measured at five angles between 40{SYMBOL 176 \f "Symbol"} and 140{SYMBOL 176 \f "Symbol"} in the lab frame relative to the beam axis. These {EMBED EQUATION |} cross sections were transformed to the compound nucleus center-of-mass and fit to a linear combination of the first three Legendre polynomials. The coefficient of the first polynomial, {EMBED EQUATION |}, is a measure of the {EMBED EQUATION |} cross section, {EMBED EQUATION |}. The second coefficient, {EMBED EQUATION |}, is a measure of the forward-backward anisotropy relative to the compound nucleus center-of-mass. Fig. 3.5-1. shows the extracted {EMBED EQUATION |} and {EMBED EQUATION |} for the inclusive {EMBED EQUATION |} data. In the left panel, a bremsstrahlung component (solid line) has been determined by manual iteration of the strength and slope parameters. The dashed line in the right panel shows the {EMBED EQUATION |} that results from this bremsstrahlung component alone. The bremsstrahlung component will always have a positive {EMBED EQUATION |} (for {EMBED EQUATION |} since the nucleon-nucleon center-of-mass moves at half the beam velocity while the compound nucleus center-of-mass moves more slowly. In the region where GDR emission competes significantly with bremsstrahlung {EMBED EQUATION |} the {EMBED EQUATION |} bremsstrahlung component of course falls below the data points. Since GDR emission has no forward-backward anisotropy in the compound nucleus center-of-mass, its effect is to dilute the {EMBED EQUATION |}. Hence the data points in the right panel fall below the dashed curve. The bremsstrahlung {EMBED EQUATION |} diluted by the contribution of statistical {EMBED EQUATION |} (solid line, right panel) follows the data reasonably well. Observed nonzero values of {EMBED EQUATION |} below approximately 16 MeV are due to some other reaction mechanism. The use of a multiplicity gate on the gamma spectrum has been shown to greatly reduce the nonzero {EMBED EQUATION |} at low {EMBED EQUATION |}, but has little effect on the region {EMBED EQUATION |} other than to worsen the statistical errors.5 Efforts are underway using the CASCADE statistical model code to do a simultaneous fit of statistical and bremsstrahlung components to both the {EMBED EQUATION |} and the {EMBED EQUATION |} spectra. {import fig3_5.eps \* mergeformat|} Fig. 3.5-1. The points are the inclusive data for {EMBED EQUATION |} at 9.4 MeV/u bombarding energy and the curves are the calculated bremsstrahlung component. The dashed line in the right panel is the {EMBED EQUATION |} that results from the bremsstrahlung component alone, as shown by the solid line in the left panel. The solid line in the right panel is the bremsstrahlung {EMBED EQUATION |} diluted by the statistical {EMBED EQUATION |} contribution. } 3.6 APEX update T.A. Trainor and the APEX collaboration During the past year the full APEX detector system has accumulated several million positrons resulting from several-pnA beams of {SYMBOL 187 \f "Symbol"}6 MeV/u uranium projectiles incident on several targets. Under no conditions have narrow peaks in positron-electron coincidence or positron singles energy spectra been observed (except for the 206Pb results described below). This overall result now constitutes a major disagreement with the results of several collaborations at the GSI in Darmstadt, Germany. APEX was designed to investigate in a kinematically complete way the production mechanism for reported anomalous electron pairs in several very-heavy-ion collision systems. The initial null results from APEX reported last year prompted an extensive study by Monte Carlo techniques of the detailed acceptance of APEX, especially as compared to the GSI experiments. This study has revealed no fundamental reason for a failure to observe anomalous pairs from an acceptance standpoint. More recently two collision systems have been used both to calibrate the APEX ability to detect such pairs and to examine the 'best case' GSI result. For the former the collision system 206Pb- 206Pb was used to produce pairs from internal conversion of a 3- {SYMBOL 174 \f "Symbol"} 2+ transition. These pairs have been seen as a narrow peak in the coincidence spectrum. The detailed peak shape is now being compared with Monte Carlo studies to further elucidate the APEX acceptance. In an attempt to examine the best-case GSI result (narrow peaks in positron singles spectra where the question of acceptance is much simpler) data were acquired for the uranium-thorium system. A total of 37,000 positrons was detected, 200 times that for the original GSI (EPOS) result and 20 times that for a more recent EPOS result. After a preliminary analysis no peak structures have been observed outside statistical fluctuations. In light of these results the reality of the electron-pair phenomenon as reported by GSI is called into question. Further analysis of the APEX data and data acquisition are planned for the coming year. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 10. 1Nuclear Physics Annual Report, University of Washington (1994) p. 11. *National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI. †Indiana University Cyclotron Facility, Indiana University, Bloomington IN. ‡Present Address: Battelle Memorial Institute, Columbus, OH. §Department of Physics, Old Dominion University, Norfolk VA. 1Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 14. 2R. DeSouza et al., Nucl. Instrum. Methods A, 295, 109 (1990). 3D. Prindle et al., Phys. Rev. C 48, 192 (1993). 1N. Bohr, Nature (London) 137, 344 (1936). 2B. Fornal et al., Phys. Rev. C 42, 1472 (1990). 3R.V.F. Janssens et al., Phys. Lett. B 181, 16 (1986). 4A. Gavron, Phys. Rev. C 20, 230 (1980). 1C.A. Gossett et al., Phys. Rev. C 42, R1800 (1990). 2Vojtech et al., Phys. Rev. C 40, R2441 (1989). 3R. Pfaff et al., Z. Phys. A 347, 67-70 (1993). 4N. Gan et al., Phys. Rev. C 49, 298-303 (1994). 5Nuclear Physics Laboratory Annual Report, University of Washington (1994) p. 8. 18 19 20 21 22 24