Search Results (9 total)

We present results from an experimental study of the beam halo in a high-current 6.7-MeV proton beam propagating through a 52-quadrupole periodic-focusing channel. The gradients of the first four quadrupoles were independently adjusted to match or mismatch the injected beam. Emittances and beamwidths were obtained from measured profiles for comparisons with maximum emittance-growth predictions of a free-energy model and maximum halo-amplitude predictions of a particle-core model. The experimental results support both models and the present theoretical picture of halo formation.

Macroparticle simulation plays an important role in modern accelerator design and operation. Most linear rf accelerators have been designed based on macroparticle simulations using longitudinal position as the independent variable. In this paper, we have done a systematic comparison between using longitudinal position as the independent variable and using time as the independent variable in macroparticle simulations. We have found that, for an rms-matched beam, the maximum relative moment difference for second, fourth moments and beam maximum amplitudes between these two types of simulations is 0.25% in a 10 m reference transport system with physical parameters similar to the Spallation Neutron Source linac design. The maximum z-to- t transform error in the space-charge force calculation of the position dependent simulation is about 0.1% in such a system. This might cause a several percent error in a complete simulation of a linac with a length of hundreds of meters. Furthermore, the error may be several times larger in simulations of mismatched beams. However, if such errors are acceptable to the linac designer, then one is justified in using position dependent macroparticle simulations in this type of linac design application.

Results are presented for the spin-spin correlation parameters C_SS and C_LS for free np elastic scattering at neutron beam kinetic energies of 484, 634, 720, and 788 MeV and c.m. angles between 25° and 80°. The measurements were performed with a polarized neutron beam and a polarized proton target. These are the first measurements of this type to be reported in the forward angular region with a free polarized neutron beam. The observables C_SS and C_LS are both small at all energies, except for C_LS at 788 MeV, which is larger than phase-shift analysis predictions by more than one standard deviation for most of the measured points.

Final results are presented for the spin-spin correlation parameters C_SL and C_LL for np elastic scattering with a polarized neutron beam incident on a polarized proton target. The beam kinetic energies are 484, 634, and 788 MeV, and the c.m. angular range is 80°-180°. These data will contribute significantly to the determination of the isospin-0 amplitudes in the energy range from 500 to 800 MeV.

Elastic and inelastic π^± scattering by ²⁰⁶Pb has been studied to measure the isospin character of transitions to bound states. The data have been interpreted using both distorted wave impulse approximation and optical model potentials. The data for the collective states at 2.647 MeV (3^-) and 4.111 MeV (2⁺) are well reproduced with δ_l⁺=δ_l^-=δ_l^p, i.e., assuming that these transitions are isoscalar. For the 0.803-MeV, 2⁺ level we deduce M_n/M_p=2.6±0.3 which is in excellent agreement with a value obtained from inelastic heavy-ion scattering.

In a series of experiments, differential cross sections for elastic scattering of π⁺ and π^- from ⁴He were measured at the Clinton P. Anderson Meson Physics Facility (LAMPF). Data were taken at incident energies between T_π=90 and 240 MeV using different experimental setups which covered the angular range from far forward (≊10°) to far backward angles (≊170°). A phase-shift analysis of the data was carried out. The experimental phase shifts are compared with values predicted by a first-order optical model program.

The spin-spin correlation parameters C_LL=(L,L;0,0)=A_LL and C_SL=(S,L;0,0)=A_SL for np elastic scattering were measured for incident polarized-neutron–beam kinetic energies of 484 and 634 MeV over the center-of-mass angles from ≃80° to 180°. The data are important for determining the I=0 nucleon-nucleon amplitudes. These results are compared with phase-shift calculations.

Measurements of cross sections for the (π⁺,π^-) reaction to the double isobaric analog state on targets of ³⁰Si, ³⁴S, ⁴⁴Ca, ⁵⁰Ti, ⁵¹V, ⁵²Cr, and ⁵⁸Ni at θ_lab=5° and an incident pion kinetic energy of ∼292 MeV are presented. We also present limits for the cross section to the residual ground state for the four T>1 targets. The data are compared with a phenomenological two-amplitude model and with a two-amplitude model that uses seniority-zero shell-model wave functions. The latter model provides expressions for both ground state and double isobaric analog state cross sections.

The excitation function for π⁺ inelastic scattering to the 0⁺, T=1, 3.563-MeV level of ⁶Li has been measured at a constant momentum transfer q≃109 MeV/c for incident pion energies from 100 to 260 MeV. Although the differential cross sections extracted for the natural-parity transitions to the 3⁺, T=0, 2.185-MeV and 2⁺, T=0, 4.25-MeV levels are well reproduced within the framework of the distorted-wave impulse approximation, distorted-wave impulse approximation calculations fail to reproduce the anomalous excitation function observed for the transition to the 3.563-MeV level. The shape of the 3.563-MeV excitation function is similar to that previously observed for π^± inelastic scattering to the 1⁺, T=1, 15.11-MeV state of ¹²C [C. L. Morris et al., Phys. Lett. 108B, 172 (1982)]. The same mechanism may be responsible for the observed excitation functions of both ΔS=ΔT=1 transitions. A possible mechanism is the previously proposed direct excitation of Δ-particle-nucleon-hole (Δ-h) components in the wave functions.