A. Ahmidouch

Measurement of the analysing power and the differential cross section of the pp charge-exchange reaction at LEAR

Abstract As part of a programme to study the spin structure of the p p→ n n channel, we have measured the analysing power A 0 n and the differential cross section at an incident antiproton beam momentum of 704 MeV/ c . The analysing power exhibits a remarkable angular dependence, which is poorly reproduced by the existing potential models.

Plastic detectors for medium energy neutrons

Abstract Three neutron detectors made up of a total of fifty-three vertical plastic scintillator bars BC412 (equivalent to NE110) with 8 × 20 cm2 section and a sensitive height varying from 40 to 130 cm are used in the PS199 experiment at the CERN Low Energy Antiproton Ring (LEAR). Monitoring of the stability of the counters during data taking is done by a laser system. The TOF is calibrated with γ rays from the p p → π 0 ′s → γ′s reaction. A new method to calibrate the amplitude of the counters at low energy using 4.4 MeV gamma rays from an 241Am/Be source is described, as well as a high energy calibration using cosmic muons. The absolute neutron detection efficiency of the counters is measured for neutron energies varying from 33 MeV to 204 MeV. Measurements are performed at five counter thresholds of 1.5, 2.5, 4.5, 6.5 and 8.5 MeVee. A comparison of the experimental measurements with the predictions of a Monte Carlo computer code is presented.

Measurement of the analyzing power of the charge-exchange reaction in the momentum range 546-875 MeV/c at LEAR

Abstract Results are given for the analysing power of the p p→ n n charge-exchange reaction at four incident beam momenta. The measurement is part of an experimental programme to study the spin structure of this reaction at LEAR. The analyzing power shows strong angular and energy dependence, which at present is not reproduced by the existing meson-exchange potential models.

Measurement of the analysing power of the charge-exchange pp → nn reaction in the momentum range 546–1287 MeV/c at LEAR

Final results of the measurement of the analysing power AOn of the pp → nn reaction are presented. Eight measurements in the range 546–1287 MeV/c incoming p momentum have been performed over the full angular range using a solid polarized proton target and the Low Energy Antiproton Ring (LEAR) at CERN antiproton extracted beams.

High-precision measurement of the pp → nn charge-exchange differential cross-section

Abstract The differential p p → n n charge-exchange cross section has been measured at the CERN Low Energy Antiproton Ring (LEAR), at two incident p momenta, 601 and 1202 MeV/c. features of the differential cross-section near the forward direction, i.e. a sharp peak at 0° scattering angle followed by an energy dependent dip-bump structure, are confirmed and measured with good precision and high statistical accuracy. The data show very clearly that the shape of the cross-section is a manifestation of the pion-exchange amplitude, and a simple extrapolation to the pion pole already indicates that the pion-nucleon coupling constant f c 2 can be determined with good precision.

First measurement of the depolarization parameter D0n0n of the pp → nn charge-exchange reaction

The depolarization parameter D0n0n in the charge-exchange reaction has been measured for the first time at the CERN Low Energy Antiproton Ring (LEAR) at 875 MeV/c antiproton beam momentum, in the forward hemisphere. The measured values of D0n0n are always smaller than ±0.3, indicating that the two-spin amplitudes dominate the scattering matrix as suggested by the meson exchange potential models

Angular dependence of analyzing power in np elastic scattering between 0.312 and 1.10 GeV

Abstract We present a total of 427 np analyzing power data points in a large angular interval at 12 energies between 0.312 and 1.10 GeV. The SATURNE II polarized beam of free monochromatic neutrons was scattered either on the Saclay frozen-spin polarized proton target or on CH2 and C targets. Present results are compared with existing elastic and quasieleastic data.

A high-statistics measurement of the p−p → n−n charge-exchange reaction at 875 MeV/c

Abstract A new measurement of the differential cross section and of the analysing power A0n of the charge-exchange reaction p − p → n − n at 875 MeV/c is presented. The A0n data cover the entire angular range and constitute a considerable improvement over previously published data, both in the forward and in the backward hemisphere. The cross-section data cover only the backward region, but are unique at this energy. A careful study of the long-term drifts of the apparatus has allowed to fully exploit the good statistics of the data.

An antineutron detectors calibration method for anti-pp→anti-nn charge-exchange differential cross-section measurement

Two modular antineutron detectors have been built and used to detect the low energy antineutrons produced in the charge-exchange reaction pp → nn in an experiment at LEAR. A novel calibration method has been used to measure the absolute efficiencies of the detectors, and allows for a precise measurement of the reaction differential cross-section.

Measurement of the n̄-Fe absorption cross section between 125 and 780 MeV/c

Abstract The n-Fe absorption cross section σ abs n Fe has been measured at LEAR (the Low Energy Antiproton Ring at CERN) using tagged ns produced in the pp → nn charge-exchange reaction. With incident p momenta of 875 and 545 MeV/ c σ abs n Fe has been measured in the momentum interval between 125 and 780 MeV/ c , in a region where only a few measurements presently exist. The overall statistical and systematic errors range from 20 to 5%; the normalisation error is less than 3%. The measured cross sections are well reproduced by a parametrisation a + b p n , with a = 0.680 ± 0.051 barn and b = 0.228 ± 0.024 barn·GeV/ c .