Search Results (19 total)

An experiment was performed to study the ⁹Be(⁶Li,⁶Li)⁹Be^* →α+α+n reaction. This experiment was designed to study the breakup of ⁹Be in an attempt to quantify the breakup yield for each of the decay channels (n+ ⁸Be ^g.s,n+ ⁸Be ²⁺, and α+ ⁵He ^g.s) from the low-lying states. The results suggest that the population of states in ⁹Be from 1.68 to 11.28 MeV can be identified. Branching ratios for each of the breakup channels have been estimated for these states. These results are compared with earlier experiments and with theoretical predictions. They confirm the theoretical claim that the n+ ⁸Be ²⁺ and α+ ⁵He ^g.s channels increase in importance at higher excitation energies.

An analysis of the ¹²C(¹²C,3α)¹²C reaction was made at beam energies between 82 and 106 MeV. Decays to both the ground state and the excited states of ⁸Be were isolated, allowing states of different characters to be identified. In particular, evidence was found for a previously observed state at 11.16 MeV. An analysis of the angular distributions of the unnatural parity states at 11.83 and 13.35 MeV, previously assigned J^π=2^-, calls into question the validity of these assignments, suggesting that at least one of the states may correspond to J^π=4^-. Evidence is also found for 1^- and 3^- strengths associated with broad states between 11 and 14 MeV.

Rotary and displacement tuners are described for multistub superconducting rf resonators. The effectiveness of these tuners is made possible because the resonators have low currents between their outer conductors and tuner elements. Computer simulations and experimental data show that the devices provide a tuning range up to 100 kHz with a frequency resolution of about 1 Hz. As well, only a small driving force is required allowing use of a low-backlash drive mechanism. The use of the rotary tuner is limited to the resonators with two loading elements such as the 2-stub quarter wave resonator, the conventional split loop resonator, and the 2-stub, half wave resonator. The displacement tuner is more versatile and can be used for any TEM-like quarter wave resonator or half wave resonator resonators with two or more loading elements.

An experiment was performed at the Australian National University to study the ⁹Be(⁶Li,⁶Li)⁹Be^*→α+α+n reaction. This experiment was designed to study the breakup of ⁹Be, in an attempt to quantify the contribution played by the ⁵He+α and ⁸Be²⁺+n channels for the low lying excited states. This information is required in order to resolve uncertainties in the α+α+n→ ⁹Be reaction rate in high-energy and neutron-rich astrophysical environments such as supernovae. Angular correlation measurements have been used to deduce that the 2.429 MeV state breaks up almost exclusively via the ⁸Be²⁺ channel. This method of identifying the break-up channel resolves the problem of distinguishing between the ⁸Be²⁺ and ⁵He^g.s. channels which are kinetically identical at this excitation energy.

This paper describes a concept for superconducting resonators for the acceleration of ions in the velocity range β=v/c=0.015–0.04. Such a resonator operates in λ/4 mode with three loading elements and so can be thought of as a triple quarter wave resonator (3-QWR) providing 4 accelerating gaps. The use of a column to support the three stubs provides a benefit beyond those of the two-stub design (2-QWR). In the 3-QWR, the rf mirror currents in the walls surrounding the stubs need only travel through 45° instead of the 90° in the 2-QWR thus further reducing the current in the demountable joints. As in the 2-QWR, the shape of the column allows control of the frequency splitting between the accelerating and other modes. The copper structure is designed to be coated by a thin superconducting film of niobium or lead for operation at 4.3 K. The particular device reported here operates at 150 MHz with an optimum β of 0.04. Its outer cylinder is the same size and shape as for the 2-QWR structure reported previously, in order to minimize construction and cryostat costs. A simple transmission line model is presented and the results of microwave studio and other numerical analyses are discussed. The 3-QWR resonators are appropriate for the upgrade of the low-velocity sections of the ANU Heavy Ion Accelerator Facility and other heavy ion accelerator boosters.

This paper describes the electromagnetic and mechanical properties of a 150 MHz λ/4, 3-gap structure with two loading elements, for the velocity range β=0.04–0.12 in the context of TEM-like λ/4 and λ/2 structures with multiple loading elements. A simple transmission lines model is presented and the results of Micro Wave Studio and simulations are discussed. The column of the multistub structures opens the opportunity to minimize current in locations allowing the exploitation of demountable joints and control the frequency splitting between the accelerating and other modes. These resonators are appropriate for the upgrade of the medium- and high-velocity sections of the ANU Heavy-Ion Accelerator Facility. Because of the broad velocity range for which such structures can be tailored, they can also be used in spallation neutron sources and rare isotope accelerators.

A study of the α+⁶Li, α+⁶Li^*, d+⁸Be and p+⁹Be decay of ¹⁰B, the α+⁷Li, t+⁸Be, and d+⁹Be decay of ¹¹B and the α+⁸Li and t+⁹Be decay of ¹²B has been performed using the ¹²C(⁷Li,¹⁰B^*)⁹Be, ¹⁶O(⁷Li,¹⁰B^*)¹³C, ⁷Li(⁷Li,¹¹B^*)t, and ⁷Li(⁷Li,¹²B^*)d reactions at 58 MeV. The excitation energy of the ^10,11,12B^* was determined following the coincident detection of the α+Li and H+Be decay fragments. A study of the relative yields for the decay of a number of excited states in ^10,11,12B^* indicates that the α-decay channel dominates in all cases.

The ¹²C(¹²C,⁸Be+¹²C(0₂⁺))⁴He reaction has been studied with beam energies of 82–120 MeV to search for excited states in ²⁰Ne that decay into ⁸Be+¹²C(0₂⁺). The detection of five of the six final-state α particles in an array of eight, sixteen-element, strip detectors permitted the full reconstruction of the reaction kinematics. Two ²⁰Ne resonances at 35.2 and 36.5 MeV are observed, particularly at beam energies less than 93 MeV. An analysis of the angular correlations associated with the observed resonances indicates that an angular momentum of 10  ℏ is important in this region, and thus it is possible that at least one of the resonances possesses J^π=10⁺.

The ¹²C(¹²C,⁸Be+⁸Be)⁸Be reaction has been studied with 18 beam energies from 82  to  120  MeV. The detection of four of the six final-state α-particles in an array of eight 16-element strip detectors permitted the full reconstruction of the reaction kinematics. ¹⁶O excitation energy spectra spanning 19–40  MeV were measured, which provided evidence for ¹⁶O excited states up to 35.1  MeV. Angular correlation analysis indicates the dominant spins are 6 and 8  ℏ, possibly extending to 10  ℏ at the highest excitation energies. The resulting energy-spin systematics are compared with the predictions of the α cluster model and cranked Nilsson-Strutinsky calculations.

The α+⁶He decay of ¹⁰Be has been studied via the ⁷Li(⁷Li,α  ⁶He)α reaction at 58  MeV. The excitation energy of the ¹⁰Be nucleus was determined following the coincident detection of the α and ⁶He decay products. A study of the fragment angular correlations has provided a spin assignment consistent with the previously reported value of 3⁻ for the 10.15  MeV state in ¹⁰Be. A tentative assignment of (4⁺), 6⁺ is proposed for the 11.76  MeV state.

An excitation function measurement has been performed over the laboratory range 52–94  MeV investigating resonance phenomena in the single excitation ¹⁶O (¹⁶O,¹⁶O^*→¹²C_g.s.+α)¹⁶O_g.s. breakup reaction. A number of enhancements are observed in these excitation functions for excitation to the 4₁⁺, 10.35  MeV and 4₂⁺, 11.09  MeV excited states in ¹⁶O. The overlap between these and previously measured enhancements is discussed. Excitation functions for the 2₂⁺, 9.84  MeV and 2₃⁺, 11.52  MeV excited states are also presented.

A study of the ⁷Li(⁹Be,⁴He¹⁰Be)²H reaction at E_beam=70 MeV has been performed using resonant particle spectroscopy techniques and provides the first measurements of α-decaying states in ¹⁴C. Excited states are observed at 14.7, 15.5, 16.4, 18.5, 19.8, 20.6, 21.4, 22.4, and 24.0 MeV. The experimental technique was able to resolve decays to the various particle bound states in ¹⁰Be and provides evidence for the preferential decay of the high energy excited states into states in ¹⁰Be at ∼6 MeV. The decay processes are used to indicate the possible cluster structure of the ¹⁴C excited states.

Angular distributions for elastic scattering and for excitation of the 1.78 MeV state in ²⁸Si and for elastic scattering and excitation of the 3.73 and 3.90 states in ⁴⁰Ca by ⁷Li scattering at a bombarding energy of 45 MeV have been measured and analyzed. Double folding model calculations using a realistic effective nucleon-nucleon interaction similar to that performed for ⁹Be + ²⁸Si and ⁹Be + ⁴⁰Ca scattering have been carried out for the elastic angular distributions and the real potential must be renormalized to yield agreement with the measured cross sections. Coupled channels calculations using a Woods-Saxon potential were performed in an effort to describe the inelastic angular distributions. The extracted deformation parameters are in reasonable agreement with those obtained from light and heavier ion scattering from the same target nuclei. The effect of strong excitation of the 0.48 MeV state in ⁷Li and of mutual excitation of target and projectile is considered in a qualitative manner.

NUCLEAR REACTIONS ²⁸Si(⁷Li, ⁷Li)²⁸Si, ²⁸Si(⁷Li, ⁷Li*)²⁸Si* Q=-0.48, -1.78, and -2.26 MeV, E(⁷Li)=45 MeV; ⁴⁰Ca (⁷Li, ⁷Li)⁴⁰Ca, ⁴⁰Ca(⁷Li, ⁷Li*)⁴⁰Ca* Q=-0.48, -3.73, and -3.90 MeV, E(⁷Li)=45 MeV; measured σ(θ), θ_lab=10-70°;; performed optical model calculations using Woods-Saxon potential and double folding procedure, deduced optical model parameters; performed coupled-channels calculations, deduced deformation parameters.

The energy level scheme for the low-lying collective levels of ²⁴Mg are reasonably described by an asymmetric rotor with β₂=0.52 and γ=22°. To further investigate possible triaxial structures in ²⁴Mg, we have measured elastic and inelastic scattering angular distributions for excitation of the lowest three excited states in ²⁴Mg at an ¹⁶O bombarding energy of 67 MeV in the angular range θ_c.m.=10-65°. The measured cross sections have been analyzed using coupled channels calculations assuming that ²⁴Mg behaves as (1) a symmetric rotor and (2) a triaxial rotor. Because of the fact that the 2₂⁺ and 4₁⁺ states in ²⁴Mg are unresolved, difficulty occurs in distinguishing between the two models. Considering ²⁴Mg to be a triaxial rotor, however, does yield a simple description of the bound states and scattering to the low-lying collective levels.

NUCLEAR REACTIONS ²⁴Mg(¹⁶O,¹⁶O)²⁴Mg^* elastic and inelastic scattering Q=0.00, -1.37, -4.12, and -4.23 MeV at E(¹⁶O)=67 MeV. Measured σ_el(θ) and σ_inel(θ) for θ_c.m.=10-65°, calculated σ_el(θ) and σ_inel(θ) using coupled channels and symmetric and triaxial rotor models.

The measurement and analysis of the differential cross sections for the elastic and inelastic scattering to the lowest 2⁺ states of ²⁴Mg and ²⁸Si using ²⁴Mg and ²⁸Si beams in conjunction with ²⁰⁸Pb targets are reported. The incident beam energies were 145 MeV for ²⁴Mg and 162 MeV for ²⁸Si. The measured cross sections were analyzed using coupled channels calculations assuming a rotational model description for the low-lying collective states of ²⁴Mg and ²⁸Si. The coupled channels calculations are compared with polarization potential calculations in which the experimental B(E2) values are used in both cases. The extracted deformation lengths and deformation parameters are compared to those obtained using similar data with light projectiles, although because of strong Coulomb excitation the present calculations are fairly insensitive to the nuclear deformation parameters.

NUCLEAR REACTIONS ²⁰⁸Pb(²⁴Mg, ²⁴Mg), ²⁰⁸Pb(²⁴Mg, ²⁴Mg^*) Q=-1.37 MeV, E=145 MeV; ²⁰⁸Pb(²⁸Si, ²⁸Si), ²⁰⁸Pb(²⁸Si, ²⁸Si^*) Q=-1.78 MeV, E=162 MeV, measured σ(θ), θ_lab=20-120°; optical model calculations including polarization potential, coupled channels calculations, extracted deformation parameters.

Elastic and inelastic scattering excitation functions for ⁵²Cr and ⁵⁴Fe by ¹⁶O and ¹⁶O have been measured. The inelastic data associated with ¹⁶O and ¹⁸O on common targets are very different. Optical model and distorted-wave Born approximation calculations indicate that multistep processes involving the projectile states are responsible.

NUCLEAR REACTIONS ⁵²Cr, ⁵⁴Fe (¹⁶O, ¹⁶O), (¹⁸O, ¹⁸O), E=30-42 MeV; measured σ(E). DWBA analysis, deduced optical parameters, B(E2), β_n.

Counter-telescope and recoil-coincidence techniques were used to observe the reaction ¹⁸O(¹⁸O, ¹⁴O)²²O at 100 MeV. These data indicate a mass excess for ²²O of 9.48±0.10 MeV, which differs by ∼2.0 MeV from the previous value, but is in excellent agreement with several theoretical predictions.

The energy spectra and angular distributions of the yrast and statistical cascades in ^160,162Yb have been obtained from measurements of the reactions ^147,149Sm(¹⁶O, 3n) and ^148,150Sm(¹⁶O, 4n). An average of about six yrast and six statistical γ rays occur in the 4n reaction. The data suggest that the yrast γ rays are mostly stretched E2.

An optical-model analysis of the scattering of 64.3-MeV α particles from ⁵⁸Ni and ⁵⁸Fe, and of 50.2-MeV α particles from ⁵⁸Ni, has been performed using independent real- and imaginary-potential-geometry parameters. The scattering is found to be sensitive only to the potential form at radial distances beyond about 6 F. Real-potential-parameter ambiguities have been studied with particular reference to the quality of the analyzed data. The potentials found are compared with expectations based upon a simple folding of matter sizes and effective two-body forces.