Microscopic approach to pion-nucleus dynamics

Elastic scattering of pions from finite nuclei is investigated utilizing a contemporary, momentum--space first--order optical potential combined with microscopic estimates of second--order corrections. The calculation of the first--order potential includes:\ \ (1)~full Fermi--averaging integration including both the delta propagation and the intrinsic nonlocalities in the $\pi$-$N$ amplitude, (2)~fully covariant kinematics, (3)~use of invariant amplitudes which do not contain kinematic singularities, and (4)~a finite--range off--shell pion--nucleon model which contains the nucleon pole term. The effect of the delta--nucleus interaction is included via the mean spectral--energy approximation. It is demonstrated that this produces a convergent perturbation theory in which the Pauli corrections (here treated as a second--order term) cancel remarkably against the pion true absorption terms. Parameter--free results, including the delta--nucleus shell--model potential, Pauli corrections, pion true absorption, and short--range correlations are presented. (2 figures available from authors)Comment: 13 page

Transverse emittance of a 2. 0-MeV RFQ beam with high brightness

One major purpose of the accelerator test stand (ATS) is to see if particle simulation codes can correctly predict the output beam parameters of our radio-frequency quadrupole (RFQ) linac. By injecting a very bright 100-keV beam (100 mA, 0.02 ..pi.. x cm x mrad) and comparing the measured output beam parameters with those predicted (using PARMTEQ), we can investigate PARMTEQ's performance in a high space-charge regime. PARMTEQ has successfully simulated the RFQ beam transmission and energy distribution; it is more difficult to simulate output transverse emittance. Transverse emittance is also difficult to measure because of the high-power-density beam. Special modification of our interceptive slit and collector equipment was required. This paper will compare measured transverse emittances with those predicted. 3 refs., 7 figs

Noninterceptive techniques for the measurement of longitudinal parameters for intense H/sup -/ beams

With increasing brightness, beam diagnostic techniques requiring interception of the beam become impractical. For H/sup -/ particle beams, solutions for this problem based on the phenomenon of photodissociation are currently under investigation at the Los Alamos National Laboratory accelerator test stand (ATS). A laser can be used to selectively neutralize portions of the beam that can be characterized after the charged particles have been swept away. We have used this technique for the measurement of current density versus longitudinal phase and the longitudinal phase-space distribution at the output of the ATS radio frequency quadrupole (RFQ). The results of our measurement are compared with the predictions of the particle-dynamics code, PARMTEQ. 2 refs., 6 figs

Nuclear pion photoproduction in the Delta resonance region

A measurement of the /sup 12/C( gamma , pi /sup +/n)/sup 11/B reaction in quasifree pi -production kinematic regimes has been performed using tagged photons in conjunction with large solid angle pi and n detectors. The aim of the experiment was to investigate predicted modifications to the Delta excitation of nucleons and their subsequent propagation and decay, brought about by the nuclear medium. Differential cross sections are presented for photon energies spanning the Delta (1232) excitation region. The measurements are consistent with distorted wave impulse approximation calculations in which the amplitude for proton Delta excitation, followed by Delta propagation and decay to pi /sup +/+n, is reduced compared to that for a free p. However, because of uncertainties in the magnitudes of the final state interactions, it is concluded that improved calculations are required to obtain a quantitative estimate of Delta - medium effects