No abstract

A fit to pionic atom data is used to determine four of the parameters of the low energy pion-nucleus optical potential, while the other parameters are taken from theory. The resulting potential is used to predict elastic scattering from 30-50 MeV pions. The effects of extrapolating the parameters to 50 MeV with a simple energy dependence are examined.NUCLEAR REACTIONS Optical potential, fit to pi-mesic atoms. Calculated elastic scattering with extrapolated potential at 30, 40, 50 MeV on individual targets.

The energy dependence of the optical model parameters for low energy, 0 -50 MeV, pions was determined by a compromise fit to pionic atoms, m+ elastic scattering, and m+ absorption measurements. NUCLEAR REACTIONS Pion-nucleus optical potential, elastic scattering and absorption 0 -50 MeV on various targets.

Measurements are reported of the differential cross section for the reaction T -p -7 -p , Ton, and ~n at three angles close to 180" and for incident momenta in the range 0.6 to 1.0 GeV/c. The three measurements were made simultaneously at 1% intervals of beam momentum. The data on elaqtic scattering resolve a discrepancy between two earlier experiments. They also show clearly the effect of the opening of the t) n channel. The charge-exchange data show that I-spin bounds are not violated in the kinematic region covered. The 7 ) n data can be adequately described with known s-channel resonances. No evidence for narrow N*'s is seen in any channel. I. EXPERIMENTWe present the results of an experiment performed at the Rutherford High Energy Laboratory to measure the differential cross sections of the reactions at three angles close to 180" in the center-of-mass system, for pion momenta in the range 0.6-1.0 G e~/ c . Figure 1 shows a plan of the apparatus, much of which was previously used for a study of meson thresholds.' A beam of negative pions was incident upon a liquid-hydrogen target 29.4 cm long, and the recoiling nucleons were detected in an array of ten "neutron" counters situated 7 m downstream of the target. A set of counters, A,-,,, covering the front faces of the neutron counters, determined the charge state of the nucleon. Reactions were further identified by a measurement of the recoil particle's time of flight, and by the detection of reaction products in an a r r a y of y detectors and charged-particle detectors surrounding the hydrogen target. The direct detection of the nucleon enabled a precise determination of the scattering angle to be made and also allowed all three reactions to be observed simultaneously. This feature was combined with a fine resolution on the incident momentum with measurements at closely spaced intervals.The beam of negative pions was produced by collisions of the circulating proton beam in Nimrod on an internal copper target, and was transported to the hydrogen target by a two-stage beam line. The first stage accepted a 3% bite of momenta and formed an image at an intermediate focus. The particles were momentum analyzed in the second stage and brought to a second focus in the hydrogen target. The spectrometer consisted of a bending magnet and quadrupole doublet with hodoscopes at conjugate foci of the doublet. With this system, beam particles were assigned to momentum bins of 1% separation; the momentum distribution in each bin being approximately triangular upon a base of width 2%. Three adjacent bins were accepted for each setting of the spectrometer central momentum.Apart from some changes to the hodoscope counters, the techniques used were similar to those described in Ref. 2. Electrons in the beam were vetoed using a CO, gas cerenkov counter, situated upstream of the spectrometer, and contamination by muons was measured in a separate exp_eriment using a high-pressure Freon-13 gas Cerenkov counter. Typical numbers of useful beam particles accepted during the Nimrod spill ...