T. Bressani

The use of multiwire proportional counters to select and localize charged particles

Abstract Properties of chambers made of planes of independent wires placed between two plane electrodes have been investigated. A direct voltage is applied to the wires. It has been checked that each wire works as an independent proportional counter down to separations of 0.1 cm between wires. Counting rates of 105/wire are easily reached; time resolutions of the order of 100 nsec have been obtained in some gases; it is possible to measure the position of the tracks between the wires using the time delay of the pulses; energy resolution comparable to the one obtained with the best cylindrical chambers is observed; the chambers operate in strong magnetic fields.

SOME READ-OUT SYSTEMS FOR PROPORTIONAL MULTIWIRE CHAMBERS.

Abstract Several methods are discussed, which reduce substantially the number of amplifiers required for the readout of a proportional multiwire chamber. 1. 1. The plane of wires is built as a lumped delay line. The difference in the arrival time of the pulse at the two ends of the line determines the position of the active wire. 2. 2. The wires are connected to a resistive attenuator network The ratio of the pulse heights arriving at the two ends localizes the active wire. 3. 3. The wires can be connected in different subgroups by transformer coupling.

Meson spectroscopy with antineutrons

Abstract First results on meson spectroscopy using antineutrons as projectiles are reported. Three-body (π + π + π − ) and five-body (π + π + π + π − π − ) final states following the annihilation of 100 to 290 MeV/ c antineutrons on hydrogen were analyzed. The (π + π − ) invariant mass spectrum in the three-body reaction is accounted for by the ϱ 0 anf f 2 (1270) resonances, together with an enhancement around 1540 MeV, in the region of the AX (1565) and the f 2 (1515) resonances, whereas the best fit for the (π + π + π − π − ) invariant mass in five-body events shows the already reported structure ζ(1480) and suggests the presence of a second structure around 1650 MeV.

The (π+, 2p) reaction in light nuclei

Abstract The (π + , 2p) reaction with 76 MeV pions was studied on targets of 2 H, He, 6 Li, Li, Be, B, C, N, O, F, A1, S, C1, Ca, Fe and Pb. A complete determination of all the kinematic variables was made, except in the case of Pb, where only the integrated cross section was measured. Coincident protons were detected in two ( E , d E /d X ) counters placed symmetrically with respect to the pion beam at a mean angle of 76°. We present cross sections as functions of missing mass, recoil momentum, recoil angle, and Treiman-Yang angle. The missing-mass spectra show features at low energies (⪅ 30 MeV) which vary from nucleus to nucleus. The recoil momentum spectra show that for ground state or low-excitation regions of the product nuclei, low recoil momenta are favoured, as one would expect from a quasi-free absorption of the pion on a closely correlated pair of nucleons. This picture is confirmed by the Treiman-Yang angle distributions. Various theoretical models, all based on absorption of a pion by two correlated nucleons, are compared with the experimental results. It appears that the models are successful for reactions leading to the low excitation regions of the product nuclei. We present some evidence which indicates that the cross sections in our geometry leading to high-excitation regions of the product nuclei are also due, in large measure, to pion capture by two correlated nucleons, but with considerable distortion of the incident pion and the emerging protons.

A measurement of the ratio from annihilation in deuterium and hydrogen gas

Abstract We have measured the K + K - /π + π - ratio R from p annihilations in gaseous D 2 and H 2 at atmospheric pressure. The measurement was performed with the OBELIX spectrometer. From the measured value in gaseous D 2 (0.27±0.02) we infer a P wave contribution to p −p annihilation in D 2 of (18±7)%.

Measurement of the pp annihilation cross-sections at very low energies

Abstract The p p annihilation cross-sections at 66.4–68.9–71.4 MeV/c have been measured using the presently instrumented part of the OBELIX spectrometer. The results have been obtained at p energies ( ≃ 2.5 MeV) by one order of magnitude smaller than in any existing experiment concerning the in-flight antiproton-proton annihilation. The average value obtained over the total interval 65.1–72.5 MeV/c, βσanT = 33.2 ± 1.7 (stat.) ±1.8 (syst.) mb, in agreement with a specific calculation, would favour for the spin-averaged S-wave scattering length the value (−0.7 + i0.8) fm. The branching ratios for the annihilations in two, four and six charged mesons turn out to be quite similar to those for annihilation at rest in liquid hydrogen.

Energy dependence of the quasi-free (π+,2p) reactions and evidence for the reaction π+ + (2N) → π + p + p in nuclei

Abstract The cross sections of the (π+,2p) reactions in nuclei, leading to low-lying excited states, exhibit a resonance near 150 MeV incident energy, in close similarity to the free reaction π++d→p+p. This confirms that they proceed by a quasi-free mechanism. The missing-mass spectra show a rise at high mass values, which indicates a quasi-free process in nuclei, corresponding to the free non-peripheral reaction of the type π++d→πo+p+p.

Nuclear structure effects in (π+, 2p) reactions

Abstract Pions of 80 MeV from the CERN Synchro-cyclotron have been used to investigate (π + , 2p) reactions on 16 target nuclei. Excitations spectra of the residual nuclei are presented for targets of 12 C, 14 N and 16 O and compared with predictions of Kopaleishvili et al. It appears that at low residual excitation energies (⩽20 MeV) nuclear structure effects play an important role (π + , 2p) reactions.

Differential cross sections for radiative capture of pions on hydrogen in the Δ (1232) region: A test of the isospin structure and t-symmetry of hadronic electromagnetic interactions

Abstract Radiation capture of π − on hydrogen has been measured in the momentum range from p π − = 210 MeV/ c to p π − = 385 MeV/ c and for c.m. angles between 30° and 120°, covering the Δ (1232) resonance. The unambiguous separation of the events from the charge exchange background is based on precise neutron time-of-flight measurements. Detector efficiencies were carefully determined in separate experiments. The experimental results are in good agreement with those of the inverse reaction and with most recent multipole analyses. An upper limit of ±2% can be set on the contribution of the isotensor term to the transition amplitude. A time reversal violating phase, when added to the resonant M 1+ 3 amplitude in the Donnachie-Shaw model, is found to be consistent with zero.

Measurement of the differential cross section for π−p → π0n around the Δ(1232) resonance and test of the charge independence principle

Abstract Differential cross sections for the charge-exchange reaction of π − on protons in the region of the Δ(1232) resonance (128 MeV ≦ T π − ≦ 246 MeV) are reported. Measurements were performed by detecting the neutron with a time-of-flight technique. The neutron counter efficiency was measured over the entire energy range. Particular care was devoted to eliminating possible sources of systematic error. Statistical errors are of the order of 4%, total errors of the order of 7%. The experimental results are in fair agreement with the most recent phase-shift analysis. Our data satisfy the triangle inequality. An analysis based on recent phase-shift values does not favour violation of charge independence.