Search

Inverse kinematics scattering of 18O on 6Li at Elab(18O) = 114 MeV was measured to obtain elastic and inelastic scattering cross sections. In this way cross sections for excited states in 6Li and 18O were determined. The data were analyzed within the optical model and coupled reaction channel method. The 6Li + 18O optical potential as well as the 6Li and 18O deformation parameters were deduced. Contributions of different nuclear processes to the 6Li + 18O elastic and inelastic scattering were explored. The isotopic differences between the 6,7Li + 18O and 6Li + 16,18O potential parameters were determined.

A. Gillibert, N. Alamanos, M. Alvarez, F. Auger, D. Beaumel, E. Becheva, Y. Blumenfeld, R. Dayras, F. Delaunay, A. Drouart, G. de France, L. Giot, B. Jurado, N. Keeleya∗ , K.W. Kemper, V. Lapoux, W. Mittig, X. Mougeot, L. Nalpas, A. Obertelli, N. Patronis, A. Pakou, E.C. Pollacco, R. Raabe, P. Roussel-Chomaz, F. Rejmund, M. Rejmund, H. Savajols, J.A. Scarpaci , J.L. Sida, F. Skaza, S. Stepantsov , Ch. Theisen and R. Wolski DAPNIA/SPhN, CE Saclay, 91191 Gif-sur-Yvette, France Institut de Physique Nucleaire, 91406 Orsay ,France GANIL, BP 5027, 14021 Caen Cedex, France Department of Physics, Florida State University, Florida 32306-4350 USA University of Ioannina, 45110 Ioannina, Greece JINR,FLNR 141980 Dubna, Russia The Henryk Niewodniczanski Institute of Nuclear Physics, Krakow, Poland

The reaction 13 C( 6 Li, t) 16 O has been studied in the incident energy range 24–26 MeV. Complete angular distributions have been measured at E 6 Li , = 25 MeV in the angular range θ lab = 8°–172°, with the reaction 6 Li( 13 C, t) 6 O being used for the backward angle measurements. Cross sections for evaporation residues from the fusion of the 6 Li + 13 C system have been measured in the incident 6 Li energy range 9.2–35.1 MeV. Compound nuclear contributions to the transfer cross sections have been calculated using the Hauser-Feshbach statistical theory with the assumption that the compound-nucleus formation cross section is equal to the measured fusion cross section. By comparison of the compound nuclear calculations with backward angle data it is found that the sharp cutoff approximation commonly used to represent the initial angular momentum distribution of the compound nucleus is not adequate for the 13 C( 6 Li, t) 16 O reaction. Good fits to the backward angle data can be obtained by using a smooth cutoff approximation. The forward angle cross sections have been compared with exact finite-range distorted-wave Born approximation calculations to extract transferred angular momenta and spectroscopic strengths. The present results differ from those of an earlier study. These differences are due to the inclusion of forward angle data in the present study.

Detailed angular distributions for the elastic and inelastic scattering of 7 Li at E = 36, 42, and 48 MeV and of 6 Li at E = 38, 44, and 50 MeV by 54 Fe have been measured. It is not possible to describe both sets of data with the same set of optical-model parameters. The ratio of U W is 0.8 for 7 Li and 1.4 for 6 Li at the strong absorption radius, implying stronger absorption for 7 Li than for 6 Li. No energy dependence in the optical-model parameters was necessary for either 6 Li or 7 Li. The inelastic scattering from the 54 Fe 2 + , 1.41 MeV state was well described by the DWBA and the extracted deformation length ( βR = 0.62) was the same for both 6 Li and 7 Li scattering. It was not possible to describe the 7 Li projectile excitation data with collective-model DWBA calculations showing that more detailed calculations for the projectile excitation are necessary.

The reaction /sup 12/C(/sup 19/F,..cap alpha..)/sup 27/Al was studied over the same energy range, E/sub c.m./ = 16--22 MeV, where previous elastic and inelastic measurements were found to exhibit large back angle oscillatory motion. Excitation functions and angular distributions were measured for a number of alpha particle groups. These excitation functions do not show a strong correlation among themselves, nor do they correlate strongly with the elastic and inelastic scattering data. The angular distributions for two resolved states and two unresolved groups have the same average slope without any discernible pattern. Hauser-Feshbach calculations are structureless and the calculated angular distributions have a much smaller slope than the data. Overall, we conclude that within the present study it has not been possible to identify unequivocally any resonances in the /sup 12/C+ /sup 19/F system.

Complete elastic scattering angular distributions have been measured for 6 Li + 39 K at 28 and 34 MeV to determine whether the large-angle enhancements reported earlier for 6 Li + 40 Ca are observed also for 39 K. At 34 MeV the angular distribution data for 39 K are almost identical to those previously reported for 40 Ca over the entire angular range. Rather poor descriptions of the data are obtained with the standard Woods-Saxon optical potential, especially in the interference region between 90° and 120°. A better reproduction of the data was obtained with a double folded microscopic real potential and an imaginary Woods-Saxon potential. A real force normalization of about 0.6 was found for 39 K just as for 6 Li scattering by 40 Ca.

The forward-angle j -dependence of the 48 Ca( 7 Li, 6 He) reaction has been used to determine the spins of the states populated. Spectroscopic factors are in good agreement with previous ( 3 He, d) values when the new spin values are used. The centroid energies obtained show that the 1 f 5 2 and 2 p 1 2 states are nearly degenerate in 49 Sc. The value obtained for the 1 f 7 2 strength shows that the calculated magnitude of the ( 7 Li, 6 He) reaction is correct to within 10 % when Cohen and Kurath Spectroscopic factors are used in the exact finite-range DWBA calculations.

Angular distributions have been measured for transitions to states in 13N and 13C populated by the 12C(7Li, 6He)13N and the 12C(7Li, 6Li)13C reactions at E = 48 MeV. Elastic scattering of 7Li at 48 MeV on 12C and 12Li on 13C at 40 MeV were measured. Optical-model parameters extracted from fits to these data were used in the DWBA analysis of inelastic scattering to levels in 12C and 7Li and for particle transfers to levels in 13N and 13C. The calculations generally reproduced the data, except for the 3.09 MeV 1 2 + level in 13C, which was seriously out of phase. Spectroscopic factors were obtained from the transfer data and deformation lengths from the inelastic scattering to the 4.44 MeV 2+, and the 9.64 MeV 3− levels in 12C and to the 0.48 MeV 1 2 − level in 7Li.

Excitation functions have been measured for reaction products from the $^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}$ induced reactions $^{6}\mathrm{Li}$+$^{12}\mathrm{C}$, $^{6}\mathrm{Li}$+$^{13}\mathrm{C}$, $^{7}\mathrm{Li}$+$^{12}\mathrm{C}$, and $^{7}\mathrm{Li}$+$^{13}\mathrm{C}$. These data cover the energy range from ${E}_{\mathrm{lab}}=9 \mathrm{to} 36$ MeV. It was found that the maximum fusion cross sections for these reactions are 780 and 770 mb for the $^{6}\mathrm{Li}$+$^{12}\mathrm{C}$ and $^{6}\mathrm{Li}$+$^{13}\mathrm{C}$, respectively, and 960 and 930 mb for the $^{7}\mathrm{Li}$+$^{12}\mathrm{C}$ and $^{7}\mathrm{Li}$+$^{13}\mathrm{C}$, respectively. The relative uncertainties for these maximum fusion cross sections are about 5%. Comparisons between optical model calculations of the total reaction cross sections and the measured fusion cross sections indicate that total fusion cross sections are substantially smaller than the total reaction cross sections for all four reactions at all energies. For each of the four entrance channels, energy and angular distributions have been measured at four energies for individual mass groups between $A=9 \mathrm{and} 19$ u. Most of these products appear to be evaporation residues. The critical angular momenta deduced from the experimental fusion cross sections are discussed in terms of entrance channel and compound nucleus limitation models for compound nucleus formation. The individual mass groups are discussed in terms of systematics for evaporation residues.NUCLEAR REACTIONS $^{6}\mathrm{Li}$+$^{12}\mathrm{C}$, ${E}_{^{6}\mathrm{Li}}=10 \mathrm{to} 36$ MeV; $^{6}\mathrm{Li}$+$^{13}\mathrm{C}$, ${E}_{^{6}\mathrm{Li}}=9.23 \mathrm{to} 35.08$ MeV; $^{7}\mathrm{Li}$+$^{12}\mathrm{C}$, ${E}_{^{7}\mathrm{Li}}=10 \mathrm{to} 38$ MeV; $^{7}\mathrm{Li}$+$^{13}\mathrm{C}$, ${E}_{^{7}\mathrm{Li}}=10 \mathrm{to} 34$ MeV, measured $\frac{{d}^{2}\ensuremath{\sigma}}{d\ensuremath{\Omega}\mathrm{dE}}$ for reaction products from $A=9 \mathrm{to} 19$. Extracted ${\ensuremath{\sigma}}_{\mathrm{fus}}$.

Selective population of states in 15N has been observed up to 24.5 MeV in excitation in 15N with the 12C(7Li, α) reaction at E( 7 Li ) = 48 MeV . Strong states are observed at 17.95, 18.70, 19.68, 20.93 and 24.75 MeV. The width of the 18.70 MeV state is 100 keV or less even though it is 4 MeV above the 12C + t breakup threshold. Comparison between the 12C(7Li, α) and previously reported 12C(α, p) data shows only the 17.95 MeV state to be strongly excited in (α, p).

The /sup 11/B+/sup 12/C and /sup 10/B+/sup 13/C total fusion cross sections have been measured over a laboratory energy range from 10 to 54 MeV. It has been found that the limitation in the fusion cross sections for these systems cannot presently be explained by the properties of the compound nucleus.

Angular distributions for the transfer reactions 40 Ca( 7 Li, 6 Li) 41 Ca and 40 Ca( 7 Li, 6 He) 41 Sc have been measured at 34 MeV, in small angular increments out to angles where the cross section has fallen by four orders of magnitude. Finite range DWBA calculations show a high sensitivity to different distorting potentials at the largest angle cross sections. It is found that a combination of Woods-Saxon and double-folded potentials does the best job of reproducing the data over the whole angular range.

Enhancements are observed in the /sup 12/C(/sup 12/C,d)/sup 22/Na reaction for the population of the J/sup ..pi../+ and 7/sup +/ states at E/sub x/ = 1.53 and 4.52 MeV in the energy region from 13.0 to 16.8 MeV (c.m.). A comparison of /sup 12/C(/sup 12/C,/sup 8/Be) and /sup 12/C(/sup 12/C,d) suggests that the selective population of the states in /sup 22/Na arises from the deuteron exit channel penetrabilities for grazing partial waves rather than nuclear structure similarities between the /sup 12/C + /sup 12/C and d + /sup 22/Na systems.

A measurement of the residues from the 12 C + 7 Li reaction has been obtained for 7 Li energies from 10 to 38 MeV. From these measurements the fusion cross sections and critical angular momenta for the 12 C + 7 Li system have been deduced. Cross sections for the 7 Li( 12 C, t) 16 O reaction have been obtained for 12 C energies from 54 to 62 MeV at θ lab = 2.7°. The critical angular momenta obtained from the fusion cross sections have been used to perform Hauser-Feshbach calculations for the 12 C( 7 Li, t) 16 O reaction. These calculations have been compared to measured angular distributions over a wide energy range. By comparing the fusion cross sections required by the Hauser-Feshbach calculations to fit the 12 C( 7 Li, t) 16 O(8.87 MeV) reaction and the measured residue cross section it is estimated that at least 80 % of the measured residues are fusion products. The calculations also indicate that direct processes dominate the population of many 16 O levels at forward angles and the 10.35 MeV state at backward angles. The necessity for using a critical angular momentum in Hauser-Feshbach calculations is discussed.

The dependence of the vector polarization produced in the weak field transition region on the direction of the static field gradient is investigated by means of experimental measurements and numerical calculations for a lithium source. The measurements employ a weak field transition apparatus whose tapered magnet is outside the vacuum and hence easily reversed and calculations are performed for several existing polarized ion sources. The results demonstrate that while the vector polarization depends on the direction of the static field gradient under nonadiabatic conditions, this dependence vanishes in the adiabatic limit. It is shown that the positive field gradient consistently produces higher vector polarization and that the independence of the vector polarization on the direction of the field gradient under adiabatic conditions provides a sensitive experimental test of the adiabatic operation of a vector polarized ion source.

Forward-angle 48Ca(7Li,6He) data show the 4.49 MeV state in 49Sc to be 2p1/2 and not 2p3/2 as reported in an earlier 48Ca(15N,14C) work. The (3He,d) spectroscopic factor of 0.55 indicates that this state has the major component of the 2p1/2 strength in 49Sc.

Angular distributions for the reactions 9Be(α, t)10B and 13C(α, t)14N have been measured at Eα = 27 MeV in the angular range 15°–110° c.m. for transitions to the first four states in 10B and the first five states in 14N. Excitation functions for 9Be(α, t)10B measured at the lab angles 25° and 45° in the energy range 26–27.5 MeV showed little structure indicating a small compound nucleus contribution. For both reactions the transition to the 0+T = 1 state is suppressed by at least a factor of five compared to the transition to the T = 0 states. DWBA and CCBA calculations gave a poor description of the data and did not predict the suppression of the T = 1 transitions in the (α, t) reactions. A reanalysis of more recent (d, n) and (τ, d) data in terms of the Cotanch-Robson isospin dependent DWBA model, which explains the spectroscopic factor ratio for T = 1 T = 0 transitions in the two reactions, is also presented.

A ground state atomic beam source for producing polarized 6 Li nuclei is described. Representative data which cross calibrates three existing sources for 6 Li → + 12 C at 20 MeV are presented. Possible future systems for producing polarized heavy-ion beams are discussed.

Angular distributions have been measured for transitions to low-lying states in 119Sb populated in the 118Sn(7Li, 6He)119Sb reaction at E7Li = 48 MeV. Elastic scattering of 7Li at 48 MeV and 6Li at 42 MeV was measured and optical model parameters determined by fits to the angular distributions. The optical model parameters deduced were used in a finite-range DWBA analysis of the transitions to states at 0.0, 0.27, 0.66 and 1.35 MeV excitation energy in 119Sb. Generally the data exhibited less structure than was predicted by the DWBA calculations, but the extracted spectroscopic factors are generally in good agreement with other results. The same types of discrepancies between calculations and data observed with heavier projectiles, are also observed for 7Li induced reactions.

The inclusion of the quadrupole moment of /sup 7/Li and /sup 9/Be in coupled-channels calculations removes the need to renormalize the real double-folded potential, obtained from an effective nucleon-nucleon interaction, for /sup 7/Li+/sup 54/Fe, /sup 40/Ca and /sup 9/Be+/sup 40/Ca elastic scattering.

A line shape calculation combining geometric and straggling effects for the energy spectrum of nuclear scattering from a gas target is described. The calculated full widths at half maximum are in agreement with values extracted from scattering data to within about 1%. The nearly Gaussian calculated line shape is shown to accurately describe a measured spectral line shape over an energy range of ±2 σ of a fitted Gaussian. The effect of large angle double scattering originating outside the geometric target volume, which is the principle contribution near three standard deviations from the centroid, has not been included.

The beta-gamma directional correlation involving the first-forbidden non-unique outer beta group of 140 La has been re-measured with a precision magnetic spectrometer and found to be 10 % lower than previous measurements. Values of the matrix elements determined from this measurement and existing data for other observables are presented. In terms of the extended shell model, the deduced matrix elements indicate the ground state configuration of 140 La to be an f 7 2 neutron coupled to a g 7 2 proton hole.

Statistical model calculations for the interaction of /sup 6/Li with /sup 12/C at 30 MeV show that a considerable fraction of the carbon production is produced via complete fusion. This result suggests that the continuum cross section of 700 +- 125 mb measured earlier for /sup 6/Li+ /sup 12/C has carbon fusion residuals included in the sum and hence does not arise solely from direct breakup. Statistical model calculations are able to predict the background continuum in the carbon energy spectra and when used with measured angular distributions determine a direct breakup cross section of 390 mb. Measured alpha spectra for the /sup 6/Li+ /sup 12/C reaction are also presented. They provide a further test of the statistical model predictions for /sup 6/Li+ /sup 12/C.

The Harwell proton microprobe has been used to study a giant halo inclusion which had previously provided inconclusive evidence for superheavy elements. X-ray spectra obtained from the inclusion during bombardment with 3 MeV protons showed no indications of the anomalous x-rays seen during the previous experiments. Also absent from the spectra was any evidence for some elemental constituents strongly seen in the earlier work. Implications of these results to the earlier analyses include confirmation of strong spatial inhomogeneities among trace constituents in this inclusion and absence of cerium-enriched surface layers that could have increased the relative yield of the gamma ray near 27.3 keV from the 140Ce(p, nγ) reaction.

The inner bremsstrahlung contribution to the unique β - γ correlation in 144 Pr has been determined as function of beta ray energy. The corrected correlation agrees with the theoretical prediction for a first forbidden unique correlation to within 5%.

It has been observed experimentally that the pulse-height obtained from a Si surface barrier detector for 9.500 MeV incident alpha particles and protons is different by 56±9 keV, with the alpha particle appearing to have the higher energy. This result indicates that Q -value determinations obtained by comparing the energy of different particles can be seriously in error.