F. Navach

A large electromagnetic shower detector with high rejection power against pions

Abstract A large electromagnetic shower detector for identification and energy measurements of γ-rmrays (between 150 and 1600 MeV) and electrons (between 400 and 1100 MeV), in the presence of high pion background, is described. The detector is based on the principle of simultaneous measurement of the spatial development of the electromagnetic cascade and of its energy release. It consists of 1) two six-gap thin-plate spark chambers for the reconstruction of the incoming particle trajectories; and 2) nine elements, each made of a lead foil, a spark chamber, and a plastic scintillator, all sandwiched together; here the shower development is studied. When used for γ-detection, a 0.5 cm Pb foil is placed in front of the thin-plate spark chambers, in order to allow the detection of the γ-conversion process and the identification of the γ-direction. The dimensions of the detector are 60 x 120 cm 2 front face, and 50 cm depth along the electromagnetic shower development. A pion rejection power of the order 5 x 10 −4 between 400 and 1100 MeV, for electron efficiencies varying from 70% to 80%, is obtained. The pion rejection efficiency in the γ-case is highly improved by the anticoincidence efficiency factor, while the γ-detection efficiency depends on the precision required in the reconstruction of the γ-ray direction. The γ-ray and electron energy resolution is about ±15%.

Accurate position determination of particle trajectories using plastic scintillator counters

Abstract Using the CDF technique for locating charged particles in plastic scintillator counters, it has been found that the accuracy in determining the position of an incident particle in the counter is essentially a function of the distance between the two photomultipliers. Moreover it has been shown that a position resolution of Δx = ±0.15 cm and a time resolution of Δt = ±0.07 nsec can be achieved in a simple way. Finally it has been shown that it is possible to measure simultaneously the coordinates of an incident particle in two orthogonal directions. This would allow to measure particle trajectories in high rate beams, when spark chambers cannot be used.

A Review of the Results Obtained by The Bologna‐CERN Multipurpose NBC Set‐Up

The Bologna‐CERN multipurpose NBC set‐up has allowed a systematic study of some intrinsic properties of the η, S0, ρ, ω, X0, φ, and A20 mesons. These are presented in a synthetic way.

An accurate measurement of the branching ratio η→neutrals/η→total

SummaryThe branching ratioR=Γ(η→neutrals)/Γ(η→total) has been measured with counter techniques. The η-mesons were produced in the reaction π−+p→ n+MM at 0.8 GeV/c. A total of ∼24 000 η-events have been observed. The value found isR=(70.5±0.8)%. The 90% confidence level for the branching ratioR’=Γ(η→π0γγ)/Γη→→ total is found to beR′≦10%.RiassuntoStudiando la reazione π−+p→ n+MM a 0.8 GeV/c sono stati osservati circa 24 000 mesoni η, ed è stata ottenuta una nuova misura accurata del «branching ratio»,R=Γ(η→neutri)/Γ(η→totale)=(70.5±0.8)%.РезюмеС помощью счетчиков было измерено отношение ветвей распадовR=Г(η→нейтральные)/Г(η→полный). η мезоны рождались в реакции π−+p→ n+MM при 0.8 ГэB/c. Было наблюдено полное число η-событий ∼24 000. Измеренная величинаR составляетR=(70.5±0.8)%.