Search Results (22 total)

The α decay of ¹⁵⁹Re has been observed for the first time in reactions of 300 MeV ⁵⁸Ni ions with an isotopically enriched ¹⁰⁶Cd target. The ¹⁵⁹Re ions were separated in-flight using the RITU separator and implanted into the GREAT spectrometer. The α decay emanates from the proton-emitting πh_11/2 state in ¹⁵⁹Re with an energy of E_α=6776±26 keV and a branching ratio of 7.5 ± 3.5%. This α decay populates a state in the closed neutron shell nucleus ¹⁵⁵Ta, which decays by emitting 1444 ± 15 keV protons with a half-life of 2.9_-1.1^+1.5 ms. These values are consistent with the emission of the proton from a πh_11/2 orbital. These results fit in with the systematics of proton and α-particle separation energies in the region, but disagree with the previously reported decay properties of ¹⁵⁵Ta.

The elastic scattering of the halo nucleus ⁶He from a proton target at 717 MeV/nucleon is investigated within three different multiple-scattering formulations of the total transition amplitude. The factorized impulse approximation (FIA) and the fixed scatterer approximation (FSA) of the multiple-scattering expansion are used to evaluate accurately the single-scattering terms and to test the validity of a few-body Glauber approach. The latter also includes terms beyond single scattering and the importance of these terms is investigated. The differential cross section is calculated for proton scattering from ⁶He at 717 MeV in inverse kinematics and compared with recent data.

We provide an analysis of vector meson photoproduction in the channel of the vector meson decaying into a pseudoscalar meson plus a photon (i.e., V→Pγ). It is shown that nontrivial kinematic correlations arise from the measurement of the Pγ angular distributions in the overall c.m. system in comparison with those in the vector meson rest frame. The implication of such kinematic correlations in the measurement of polarization observables is discussed in terms of the vector meson density matrix elements. For ω meson production, because of its relatively large branching ratios for ω→π⁰γ, additional events from this channel may enrich the information about the reaction mechanism and improve the statistics of the recent measurement of polarized beam asymmetries by the GRAAL Collaboration. For ϕ→ηγ,ρ→πγ, and K^*→Kγ, we expect that additional information about the spin structure of the vector meson production vertex can be derived.

The β decay of ¹¹Li has been investigated at TRIUMF-ISAC using a high-efficiency array of Compton suppressed HPGe detectors. From a line-shape analysis of the Doppler-broadened peaks observed in the ¹⁰Be γ spectrum, both the half-lives of states in ¹⁰Be and the energies of the β-delayed neutrons feeding those states were obtained. Furthermore, it was possible to determine the excitation energies of the parent states in ¹¹Be with uncertainties comparable to those obtained from neutron spectroscopy experiments. These data suggest that the β decay to the 8.81  MeV state in ¹¹Be occurs in the ⁹Li core and that one neutron comprising the halo of ¹¹Li survives in a halolike configuration after the β-delayed neutron emission from this level.

We derive the formalism for the leading order corrections to the adiabatic approximation to the scattering of composite projectiles. Assuming a two-body projectile of core plus loosely bound valence particle and a model (the core recoil model) in which the interaction of the valence particle and the target can be neglected, we derive the nonadiabatic correction terms both exactly, using a partial wave analysis, and using the eikonal approximation. Along with the expected energy dependence of the corrections, there is also a strong dependence on the valence-to-core mass ratio and on the strength of the imaginary potential for the core-target interaction, which relates to absorption of the core in its scattering by the target. The strength and diffuseness of the core-target potential also determine the size of the corrections. The first order nonadiabatic corrections were found to be smaller than qualitative estimates would expect. The large absorption associated with the core-target interaction in such halo nuclei as ¹¹Be kills off most of the nonadiabatic corrections. We give an improved estimate for the range of validity of the adiabatic approximation when the valence-target interaction is neglected, which includes the effect of core absorption. Some consideration was given to the validity of the eikonal approximation in our calculations.

We present a detailed quark-model study of pion photoproduction within the effective Lagrangian approach. Cross sections and single-polarization observables are investigated for the four charge channels, γp→π⁺n, γn→π^-p, γp→π⁰p, and γn→π⁰n. Leaving the πNΔ coupling strength to be a free parameter, we obtain a reasonably consistent description of these four channels from threshold to the first resonance region. Within this effective Lagrangian approach, strongly constrained by the quark model, we consider the issue of double counting which may occur if additional t-channel contributions are included.

An improved version of a recently developed quark model approach to vector meson photoproduction is applied to the investigation of contributions of vector meson photoproduction to the Gerasimov-Drell-Hearn (GDH) sum rule. We find that the sum rule converges at a few GeV’s. Contributions to the proton channel are found to be small while to the neutron are relatively large.

The photoproduction of K^* vector mesons is investigated in a quark model with an effective Lagrangian. Including both baryon resonance excitations and t-channel exchanges, observables for the reactions γp→K^*0Σ⁺ and γp→K^*+Σ⁰ are predicted, using the SU(3)-flavor-blind assumption of nonperturbative QCD.

The importance of an explicit treatment of continuum channels in the (d,²He) charge-exchange reaction is investigated. The continuum channel effects are clarified by a comparison of the full three-body results with calculations which use the distorted waves approximation to the three-body model. Continuum channel effects are shown to reduce the calculated ¹²C(d,²He)¹²B(g.s.) differential cross section at 270 MeV incident deuteron energy. This reduction is consistent with that from an earlier ad hoc modification of the absorptive content of an assumed ²He optical potential within distorted waves Born approximation calculations.

Calculations which improve upon the eikonal model description of the scattering of loosely bound n-cluster composite nuclei at low and medium energies are studied. Each cluster-target eikonal phase shift is replaced by the continuation of the corresponding exact partial wave phase shift to noninteger angular momenta. Comparisons with fully quantum mechanical calculations for two-body projectiles show that this yields an accurate practical alternative to few-body adiabatic model calculations. Calculations are shown to be accurate for projectile energies as low as 10 MeV/nucleon at which the eikonal approximation is no longer reliable.

Few-body Glauber theory calculations of reaction cross sections for the proposed single neutron halo nucleus ¹⁹C are presented for an incident energy of 960 MeV/nucleon on a ¹²C target. The calculated reaction cross sections are shown to be significantly smaller than those obtained using the optical limit approximation to the Glauber theory elastic profile function. The implications of these differences upon the deduced size and structure of the extended ¹⁹C ground state, from newly reported interaction cross-section measurements, are discussed, including the sensitivity of the cross sections to the assumed ¹⁹C single neutron separation energy.

We present a theoretical analysis of recently published experimental data for the elastic scattering of protons from the helium isotopes ⁶He and ⁸He at energies near 700 MeV per nucleon. The analysis treats the few-body degrees of freedom of these light neutron-rich nuclei explicitly and is developed in terms of three- and five-body wave functions for ⁶He and ⁸He, respectively. Comparisons of calculations with the data show that the sizes of the He nuclei consistent with such an analysis are larger by about 0.2 fm than those deduced from a more approximate procedure in which the structure of the weakly bound nuclei enters only through an assumed nuclear matter density.

We present new calculations and experimental measurements of the quasielastic cross section angular distribution for ⁸He scattering from ¹²C at 60 MeV/nucleon. ⁸He is treated as a five-body α+4n system and the six-body ⁸He+target scattering calculations make use of the eikonal few-body method and the cluster orbital shell model approximation for the ⁸He wave function. The qualitative features of the new data are successfully described without parameter variation. The sensitivity of the calculations to correlations in the ⁸He wave function is assessed.

We show that under certain conditions a simple relationship exists between the elastic scattering of a composite halo nucleus and of its core from a stable target. The coupling of the elastic and projectile excitation channels is crucial to the analysis, which is particularly useful when the ratio of the halo to the core mass is small. In the case of ¹¹Be elastic scattering the cross section relationship is quite well satisfied. For both ¹¹Be and ¹⁹C our analysis reveals a significant sensitivity of elastic scattering data to the halo size and structure.

A physical prescription to improve the accuracy of few-body Glauber model calculations of reactions involving loosely bound projectiles is presented in which the eikonal phase shift function of each projectile constituent is modified to account for curvature of its trajectory. Noneikonal effects due to both nuclear and Coulomb interactions are treated on an equal footing. The proposed method is assessed quantitatively by comparison with full quantum mechanical calculations in the case of ¹¹Be+¹²C elastic scattering, treated as a three-body ¹⁰Be+n+target problem, at energies of 25 and 49.3 MeV/nucleon. Calculated cross-section angular distributions which include the noneikonal modifications are shown to be accurate to larger scattering angles, and for lower incident projectile energies.

The root mean square matter radii of halo nuclei provide a basic measure in constructing, constraining, and assessing theoretical models of halo structures. We consider corrections to static density (optical limit) Glauber model calculations of reaction cross sections of such nuclei at high energy giving careful consideration to their intrinsic few-body structure and the adiabatic nature of the halo nucleus-target interaction. We take as important examples the loosely bound two- and three-body systems ¹¹Be, ⁶He, ¹¹Li, and ¹⁴Be. The contribution of the valence particles to the calculated reaction cross sections are shown to be significantly reduced, requiring increased halo radii to reproduce experimental data. The implications of these changes for structure models of extended two- and three-body systems are discussed. © 1996 The American Physical Society.

The quasielastic scattering of ⁹Li on a ¹²C target has been measured at an incident energy of 540 MeV. The new experimental data are used to extract an effective interaction for ⁹Li + ¹²C scattering. The uncertainty in this interaction was previously a major obstacle to extracting information on the structure of ¹¹Li from existing ¹¹Li + ¹²C quasielastic-scattering data. © 1996 The American Physical Society.

Measurements of reaction cross sections are routinely used to deduce effective nuclear root mean square (rms) radii by comparison with theoretical model predictions. Cross sections calculated using the optical limit Glauber model depend strongly on the rms radius of the density assumed for the projectile nucleus. We investigate such calculations by assuming a range of projectile density distributions. We show that calculated ¹¹Li-target cross sections at fixed rms radii retain a significant sensitivity to higher radial moments of the projectile density which is quite different for light and heavy targets.

We reexamine the matter radii of diffuse halo nuclei, as deduced from reaction cross section measurements at high energy. Careful consideration is given to the intrinsic few-body structure of these projectiles and the adiabatic nature of the projectile-target interaction. Using ¹¹Li, ¹¹Be, and ⁸B as examples we show that data require significantly larger matter radii than previously reported. The revised value for ¹¹Li of 3.55 fm is consistent with three-body models with significant 1s-intruder state components, which reproduce experimental ⁹Li momentum distributions following ¹¹Li breakup, but were hitherto thought to be at variance with cross section data.

We calculate the dynamic polarization potential for ¹¹Li scattering from ¹²C at 637 MeV using both two- and three-body descriptions of the ¹¹Li structure and the Glauber theoretical treatment of the scattering and breakup processes. We conclude that the recent observation of Mermaz, that the quasielastic scattering data for this system require a ¹¹Li optical potential with long range attractive surface terms in the real and imaginary parts of the interaction, cannot be attributed to inelastic excitation and breakup of the weakly bound neutron halo. Such a strong surface attraction is difficult to reconcile with the best few-body description of the reaction where the real part of the dynamic polarization potential is found to be consistently repulsive in the far nuclear surface over a wide range of theoretical inputs.

The quasielastic scattering of ¹¹Li from ¹²C at 60 MeV/nucleon is calculated in a four-body Glauber approximation. Different ¹¹Li ground-state wave functions are used, including those calculated using Faddeev three-body models. The calculated quasielastic cross sections, including the 2⁺ and 3^- states of ¹²C in a distorted wave Born approximation, reproduce the experimental data over most of the angular range, the differences between theoretical models being less than the quoted statistical errors on the available data.

The effects of breakup to singlet spin states on elastic scattering observables for the ⁵⁸Ni(d→,d)⁵⁸Ni reaction at 400 MeV is studied. The singlet breakup contribution to the elastic amplitude is estimated through an approximate two-step calculation, in which we exploit an adiabatic approximation and closure over the intermediate continuum states. The inclusion of the singlet channel coupling has a large effect on the reaction tensor analyzing power A_yy originating from a dynamically induced T_L tensor interaction.