Search Results (28 total)

The results of extensive coupled-reaction-channel calculations are compared with the cross section and new analyzing power data for the ¹²C(⁶Li,d)¹⁶O reaction leading to the 0.0-MeV 0⁺, 6.13-MeV 3^-, 6.92-MeV 2⁺, 8.87-MeV 2^-, and 10.35-MeV 4⁺ states of ¹⁶O at ⁶Li bombarding energies of 34 and 50 MeV. All the analyzing power data at both energies and all the cross section data at 50 MeV, with the exception of that for the ¹²C(⁶Li,d)¹⁶O transition to the 0.0-MeV 0⁺ state of ¹⁶O are presented here for the first time. These results suggest that there are significant multistep contributions to transfers leading to the 0⁺ and 3^- states, while those leading to the 2⁺ and 4⁺ states may be reasonably well described by simple direct α transfer. The importance of multistep effects is found to increase with increasing bombarding energy.

We measured the strength of the 0_gs⁺→2₁⁺ excitations in the radioactive mirror nuclei ³²Ar and ³²Si using the techniques of intermediate-energy Coulomb excitation for ³²Ar and inelastic proton scattering in inverse kinematics for ³²Si. The ³²Ar measurement, taken together with previously existing Coulomb excitation data for ³²Si, yields the isoscalar and isovector multipole matrix elements for the 0₁⁺→2₁⁺ transition between T  =  2 states in the A  =  32 system. The proton scattering measurement for ³²Si, when combined with the Coulomb excitation data for this nucleus, yields a ratio of neutron and proton matrix elements, M_n/M_p, for ³²Si.

We have measured the electromagnetic matrix element B(E2;0_g.s.⁺→2₁⁺) in the radioactive nucleus ²⁶Si using the method of intermediate energy Coulomb excitation. Our result, B(E2;0_g.s.⁺→2₁⁺)=336(33) e² fm⁴, resolves a conflict in previous measurements of this matrix element. In addition, the present measurement allows us to determine M_n/M_p for ²⁶Mg using the mirror nucleus method. The mirror method result of M_n/M_p=1.05(6) is consistent with the most recent pion scattering results and is near the simple collective model expectation.

The 0_g.s.⁺→2₁⁺ excitation in the proton-rich nucleus ¹⁸Ne has been studied via intermediate energy heavy-ion scattering with a beam of this radioactive isotope. The observed γ-ray yields have been combined with coupled channels calculations of the inelastic scattering reactions to obtain the electromagnetic matrix element B(E2;0_g.s.⁺→2₁⁺). This result is combined with the corresponding results in the mirror nucleus ¹⁸O and the T=1 states in the N=Z nucleus ¹⁸F as a test of isospin purity.

The band structure of ²¹⁹Ra has been studied via the ²⁰⁸Pb(¹⁴C,3n) reaction using both γ-γ and conversion electron-γ coincidence measurements. This nucleus is at the lower mass limit of a region where static octupole deformation may occur. The conversion coefficients of seven transitions and two upper limits on conversion coefficients have been determined for the first time. The results establish the alternating parity nature of a sideband in this nucleus and also resolve an apparent conflict between radioactive decay data and heavy-ion reaction data. The resulting level scheme of ²¹⁹Ra can be explained either by intermediate coupling of the odd neutron to a reflection symmetric core and a low frequency octupole vibrational mode or by strong coupling to a statically octupole deformed core.

The neutron-rich argon isotope ⁴³Ar has been studied by quasielastic and inelastic proton scattering performed in inverse kinematics. The measured inelastic angular distribution for the second excited state is in good agreement with an L=2 transition. Assuming this transition to be E2, yields a β₂ value for this state of 0.25±0.03 when compared with distorted-wave Born approximation calculations. This value is comparable to the one reported for the stable isotope ⁴⁰Ar. Moreover it is similar to those measured by Coulomb excitation for the neighboring even-even isotopes ⁴²Ar and ⁴⁴Ar indicating that the structure of the argon isotopes is stable as a function of neutron number.

Elastic and inelastic proton scattering has been measured in inverse kinematics on the unstable nucleus ⁴⁰S. A phenomenological distorted wave Born approximation analysis yields a quadrupole deformation parameter β₂=0.35±0.05 for the 2₁⁺ state. Consistent phenomenological and microscopic proton scattering analyses have been applied to all even-even sulfur isotopes from A=32 to A=40. The second analysis used microscopic collective model densities and a modified Jeukenne-Lejeune-Mahaux nucleon-nucleon effective interaction. This microscopic analysis suggests the presence of a neutron skin in the heavy sulfur isotopes. The analysis is consistent with normalization values for λ_v and λ_w of 0.95 for both the real and imaginary parts of the Jeukenne-Lejeune-Mahaux potential.

The B(E2;0_g.s.⁺→2₁⁺) value for ³⁸Ca has been measured via the technique of intermediate energy Coulomb excitation using a beam of radioactive ³⁸Ca nuclei. The present result is used to test isospin purity in the mass 38 system by comparing the isoscalar multipole matrix element M₀ extracted from the 0_g.s.⁺→2₁⁺ transitions in ³⁸Ca and ³⁸Ar to the corresponding matrix element obtained from the T=1 states of the T_z=0 nucleus ³⁸K. A discrepancy between the two values of M₀ is found, suggesting that isospin symmetry is broken in A=38 nuclei. Similar discrepancies occur for A=34 and 42. Experiments for addressing these discrepancies are proposed.

The first complete set of analyzing powers for the ¹²C(⁶Li→,d)¹⁶O_g.s. reaction at E(⁶Li)=50 MeV is reported. It was possible to simultaneously describe ⁶Li+¹²C elastic scattering and its analyzing powers, d+¹⁶O elastic scattering, and the ¹²C(⁶Li→,d)¹⁶O_g.s. transfer data. This 0⁺→0⁺ transfer is used to probe the role of spin-dependent interactions in this reaction, including the ⁶Li D state, with finite-range-distorted-wave-Born-approximation calculations. It is found that the exit channel spin-orbit d+¹⁶O interaction produces the transfer analyzing powers, but that the shape and magnitude of the transfer angular distribution is determined by the central ⁶Li+¹²C and d+¹⁶O interactions. The limited knowledge of the size of the α+¹²C system is shown to be the major uncertainty in using the ¹²C(⁶Li,d)¹⁶O reaction to determine absolute α+¹²C spectroscopic factors.

High-spin states of ¹⁴³Nd have been studied using γ-γ and γ-conversion electron coincidences. The measurement of 16 new transitions and 8 new conversion coefficients allow revision of the high-spin portion of the level scheme and the determination of parities of six levels for the first time. While the yrast states up to J=43/2 can be described using the weak coupling of an f_7/2 neutron to the ¹⁴²Nd core nucleus, the new experimental results suggest that weak coupling breaks down above J=43/2. This behavior may signal that the closed N=82 neutron shell of ¹⁴²Nd is broken above J=16.

We have measured the parallel momentum distributions of outcoming fragments in the one-neutron breakup of the odd-mass carbon isotopes ^19,17,15C and ¹⁴B. Owing to their low neutron separation energies, the study of these nuclei is of particular interest regarding the appearance of the halo phenomenon. Discrepancies between experiment and theory observed for ¹⁵C and ¹⁴B indicate that their halos are not as prominent as in ¹¹Be or ¹¹Li, and that core excitations start playing a role in the breakup mechanism. In ¹⁷C the halo appears hindered by a d-wave neutron ground state configuration. Finally, the data on ¹⁹C suggests an s-wave neutron around the 2⁺ excited state in ¹⁸C.

The β-unstable nuclei ^32,34,36,38Si have been produced by projectile fragmentation and studied by in-beam Coulomb excitation. Excited states at 1399±25 keV and 1084±20 keV have been identified for the first time in ³⁶Si and ³⁸Si, respectively, and tentatively assigned J^π  =  2⁺. The B(E2;0₁⁺→2₁⁺) values leading to these states and the previously identified 2₁⁺ states in ^32,34Si have been measured, and are compared to shell model calculations. Our results indicate that the 2₁⁺ state in ³⁴Si has a large fp-shell intruder component, and that the 2₁⁺ states in the N>20 silicon isotopes can be reproduced assuming an N  =  20 shell closure.

A 39A MeV ³⁸S radioactive beam was used with inverse kinematics to measure angular distributions for elastic and inelastic proton scattering from a CH₂ target. Optical potential and folding model calculations are compared with the elastic distribution. Using coupled channel calculations, the β₂ value for the 2₁⁺ state is determined to be 0.35±0.04. This value, when compared with the corresponding result from a Coulomb excitation measurement, leads to M_n/M_p=(1.5±0.3)N/Z, indicating an isovector contribution to the 2₁⁺ state of ³⁸S.

An exotic beam of ¹¹Be at 57–60 MeV/nucleon has been used for a study of the excitation cross section of the 320 keV first excited state in collisions with targets of Pb, Au, C, and Be. The deexcitation γ rays were observed in an array of position-sensitive NaI(Tl) detectors in coincidence with the scattered fragment. The reduced transition strengths B(E1) extracted for the heavy targets suggest a predominantly electric excitation mechanism. Although the B(E1) value is marginally in agreement with that obtained from the lifetime of the state, the deviation is of the size expected from higher-order Coulomb and nuclear effects. Excitations on the lighter targets have sizable nuclear contributions.

The dependence of the spin polarization on the incident angle of the primary beam has been measured for ¹²B fragments produced in the fragmentation of an ¹⁸O beam on a Nb target at 80 MeV/nucleon. A change in sign of the spin polarization was observed with a change in sign of the incident angle of the projectile. This result is in agreement with the kinematical model presented recently to qualitatively describe the mechanism for the production of spin-polarized fragments following intermediate-energy heavy-ion reactions. The resonance curve for ¹²B has also been measured using a technique of multiple adiabatic fast passage with continuous implantation. This technique is briefly compared to the single-pass adiabatic fast passage method employed with beam-on/beam-off data acquisition.

The first excited state of ¹⁷Ne has been populated via relativistic Coulomb excitation with a radioactive beam of ¹⁷Ne on a ¹⁹⁷Au target and the subsequent γ-ray decay has been observed. This 3 / 2^- state is bound with respect to proton emission but unbound to two-proton decay. The measured γ-ray yield accounts for 43_-14⁺¹⁹% of the predicted yield from an excitation cross section of 28 mb. It is unlikely that the missing cross section can be attributed to two-proton emission because the lifetime of this branch would have to be a factor 1700 smaller than predicted by standard barrier penetration calculations.

Distributions of parallel and transverse momenta for ⁷Be fragments formed in the breakup of ⁸B have been measured at 41A MeV. The p_∥ distributions are narrow ( 81±4 and 62±3 MeV/c FWHM for Be and Au targets, respectively), comparable to those of neutron halo nuclei. Reaction mechanisms influence the ⁷Be momentum distributions, so they do not directly reflect the valence proton momentum wave function. We present reaction models that reproduce the distributions.

The ¹H(⁶He, ⁶Li)n reaction was studied at 0° with the NSCL A1200 fragment separator in the energy loss mode. A ⁶He secondary beam at E / A=93 MeV was used to measure the Gamow-Teller and Fermi strengths between the ground state of ⁶He and the ground and excited states of ⁶Li, in inverse kinematics. At 0° the ground-state cross section is measured to be dσ_GS / (dΩ)=43±16 mb/sr, which is dominated by systematic error in the secondary beam flux. The ratio of Gamow-Teller to Fermi strength is not sensitive to this error and is found to be (87±6)% of that expected from (p, n) systematics and β decay. Angular distributions have been measured between 0° and 10° in the center of mass.

To gain a better understanding of the production of exotic isotopes and provide information on the stability of nuclei along the path of the rapid-proton capture process, isotopic cross sections from the reaction ⁷⁸Kr + ⁵⁸Ni at 75 MeV/nucleon were measured at 0° using the A1200 fragment separator. Most notably the particle stability of ⁶⁹Br was thoroughly probed during this experiment and it appears to be particle unstable. The experimental production cross section data are compared to previous krypton isotope fragmentation data to explore the dependence of the N/Z ratio of the projectile on the observed isotopic distributions (‘‘memory effect’’) as well as with an intranuclear cascade code developed for higher energies (≳ 200 MeV/nucleon) and a semiempirical parametrization derived from high energy systematics. © 1996 The American Physical Society.

An attempt was made to find the very neutron-rich isotope ²⁶O among the fragmentation products of a 90 MeV/nucleon ⁴⁰Ar beam. This isotope has been predicted to be bound but was not observed in a previous experiment by Guillemaud-Mueller et al. As part of the search, the momentum distributions of all the oxygen isotopes in the range from ¹⁷O to ²⁴O were carefully determined so that the optimum separator setting for ²⁶O could be used. From an extrapolation of the counting rates of the lighter oxygen isotopes, we expected to observe several hundred events of ²⁶O during the measurement. However, no events could be attributed to ²⁶O, thus indicating the particle instability of this isotope. The results for the production cross sections of 72 neutron-rich isotopes, ranging from ³⁸P to ¹¹B, are presented and compared to predictions. © 1996 The American Physical Society.

Differential cross sections have been measured near 0° for the dominant channels in the mirror reaction ¹³C(¹³N,¹³C)¹³N at E/A  =  57 and 105 MeV. The cross sections of the peaks in the excitation spectrum are discussed in terms of Gamow-Teller and Fermi transition strength. The cross section per unit Gamow-Teller strength is found to be enhanced relative to that for unit Fermi strength when compared with previous results from (p,n) reactions. The present work represents the first use of a mirror-symmetric projectile and target system in a charge-exchange reaction as well as the first application of the developing radioactive nuclear beam field to this area.

We have measured a complete isotope distribution of projectile-like nuclear-charge pickup products, formed by bombarding a ¹⁷Al target with 790A MeV ¹²⁹Xe ions. The shape of the cross-section distribution indicates a dominant influence of evaporation processes during the formation of the final cesium fragments observed, thus masking to a large extent the primary processes involved in the charge exchange. We can show, however, that an intranuclear-cascade-plus-evaporation calculation can reproduce the observed yields, and that the effect of Δ-formation during the first stage of the reaction is visible even in the inclusive cross sections. The same model can explain the strong increase in total charge-pickup cross sections with increasing projectile mass noted previously by other authors. It is therefore not necessary to invoke coherent processes to explain this increase as has been suggested previously.

We have measured the longitudinal momentum distribution of ¹⁸C, ¹⁷C, and ¹⁶C after the one-neutron breakup of ¹⁹C, ¹⁸C, and ¹⁷C, respectively, using the A1200 fragment separator as an energy-loss spectrometer. The observed narrow width of 44.3±5.9 MeV/c for the ¹⁸C fragments indicates that ¹⁹C is a new example of a one-neutron halo, so far the heaviest ever observed. The consequences of the obtained results on the structure of the three isotopes ^19,18,17C are discussed together with shell-model calculations.

Projectilelike residue yields and momentum distributions from the reaction of 70 MeV/nucleon ⁸⁶Kr with ²⁷Al at 0° were studied. The data is compared with a model developed originally for higher energies (>200 MeV/nucleon), data from prior experiments performed at both higher and lower bombarding energies, and a semiempirical parametrization derived from high energy systematics.