P. F. Dalpiaz

A new large-acceptance and high-efficiency neutron detector for missing-mass studies

SummaryA large-acceptance and high-efficiency neutron detector is described. The sensitive surface and volume of the detector are 2.16 m2 and 0.78 m3, respectively. The detector consists of twenty-four elements of plastic scintillator, each having dimensions (100×18×18) cm3. The large volume of scintillator, in the particular geometrical arrangement chosen, allows a mean detection efficiency of about 25% in the range (70÷390) MeV neutron kinetic energy for a laboratory solid angle of 0.14 sr at 4 m radial distance. With the techniques adopted, calibrations with charged particles can be easily performed in a few hours using a low beam intensity. An interesting features of this instrument is the accuracy achieved in locating incident particles, which is ±1.4 cm for charged particles, and ±2.5 cm for neutrons. The accuracies achieved for the time-of-flight measurement are ±0.35 ns for charged particles and ±0.7 ns for neutrons. With these resolutions in the neutron time of flight and angle, the uncertainty in the missing mass is ±4 MeV for η, ±10 MeV for ω, and ±15 MeV for ϕ mesons.RiassuntoSi descrive un rivelatore di neutroni a grande accettanza ed alta efficienza. La superficie ed il volume sensibili del rivelatore sono rispettivamente 2.16 m2 e 0.78 m3. Il rivelatore consiste di ventiquattro elementi di scintillatore plastico, ciascuno avente le dimensioni (100×18×18) cm3. Il grande volume di scintillatore, nella particolare configurazione geometrica scelta, permette di ottenere una efficienza media di rivelazione di circa il 25% nellintervallo di energia cinetica del neutrone (70÷390) MeV, coprendo un angolo solido di 0.14 sr nel laboratorio a una distanza radiale di 4 m. Con le tecniche adottate, calibrazioni con particelle cariche possono essere fatte in poche ore usando un fascio di bassa intensità. Una caratteristica interessante di questo strumento è la precisione ottenuta nella localizzazione di particelle incidenti; essa à di ±1.4 cm per particelle cariche e di ±2.5 cm per neutroni. Le precisioni ottenute nella misura del tempo di volo sono ±0.35 ns per particelle cariche e ±0.7 ns per neutroni. Con queste risoluzioni nel tempo di volo e nellangolo del neutrone, lincertezza sul difetto di massa risulta essere ±4 MeV per il mesone η, ±10 MeV per il mesone ω, ±15 MeV per il mesone ϕ.

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%.

Measurement of the X0 cross-section in π−p interactions at 1.6 GeV/c and determination of the branching ratio (X0→neutrals)/(X0→total)

SummaryThe reactions π−p→ n+(X0→total) and π−p→ n+(X0→neutrals) have been studied at 1.6 GeV/c with the Bologna-CERN neutron missing-mass spectrometer. Both reactions have been detected without the use of visual techniques. The results are: σ(X0→total)=(108±14) μb and σ(X0→neutrals)=(20.0±3.5) μb, giving a branching ratio Γ(X0→neutrals)/Γ(X0→total)=(18.5±2.2)%. The branching ratio for other possible, so far undetected, neutral decay modes of the X0 turns out to be (2.4±1.9)%.RiassuntoStudiando le reazioni π−p→ n+(X0→total) e π−p→ n+(X0→neutrals) , con lo spettrometro a neutroni Bologna-CERN, sono state misurate le sezioni durto σ(X0→totale) e σ(X0→neutri) e il Branching Ratio Γ(X0→neutri)/Γ(X0→totale)=(18.5±2.2). (noltre il Branching Ratio per possibili decadimenti neutri del mesone X0, finora non rivelati, risulta essere (2.4±1.9)%.РезюмеБыли исследованы реакции π−p→ n+(X0→total) и π−p→ n+(X0→neutrals) при импулбсе 1.6ГэВ/с, с помощью нейтронного спектрометра недостающей массы «Болонья-ЦЕРН». Обе реакции детектировались без использования визуальной техники. Получены следующие результаты: σ(X0→полный)=(108±14) мб и σ(X0→нейтральные)=(20.0±3.5) мб, что приводит к отношению ветвей распадов, равному Γ(X0→нейтральные)/Γ(X0→ полный)=(18.5±2.2)%.

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 measurement of the branching ratio ω→Neutrals/ω→total

SummaryThe results of a counter experiment on the branching ratioR=Γ(ω→neutrals)/Γ(ω→total) are presented. The masses of particles produced in the reaction π− + p → n + (missing mass) at 2.12 GeV/c were determined with an uncertainty of ±15 MeV by using counter techniques to measure the time of flight and emission angle of the neutrons. Veto counters around the hydrogen target selected neutral decay modes. The peak values of ω/background were 0.4 for the neutral mode and 0.55 for the total decay. Corrections to the experimental value are discussed. The value of the branching ratio was found to be (8.4±1.5)%. This result is compared with various theoretical predictions.RiassuntoSi presentano i risultati di un esperimento eseguito con contatori sul rapportoR=Γ(ω→neutri)/Γ(ω→totale). Le masse delle particelle prodotte nella reazione π− + p → n + (masa mancante) a 2.12 Ge V/c sono state determinate con una incertezza di±15 MeV usando la tecnica dei contatori per misurare il tempo di volo e langolo di emissione dei neutroni. Contatori in anticoincidenza attorno alla targhetta di idrogeno selezionavano i modi di decadimento neutri. I valori al picco del rapporto ω/fondo erano 0.4 per il decadimento neutro e 0.55 per il decadimento totale. Si discutono correzioni al valore sperimentale. Il valore del rapporto di decadimento èstato trovato essere pari a (8.4±1.5)%. Si confronta questo risultato con diverse previsioni teoriche.РезюмеПредставлены результаты эксперимента по измерению отношения ветвей распадаR=ω→нейтральные/щрполный. Массы частиц, роЗденных в реакции π− + p → n + (недостающая масса) при 2.12 ГэВ/с, измерялись с неопределенностью ±15 МэВ, используя счетную технику для измерения времени пролета и угла испускания нейтронов. Запрещение отсчетов вокруг водородной мишени отбирало нейтральные моды распада. Пик для величин ω/фон составлял 0.4 для нейтральной моды и 0.55 для полного распада. Обсуздаются поправки к экспериментальной величине. Гыло найдено, что экспериментальная величина отношения ветвей распада равна (8.4±1.5)%.

Further evidence for the decay mode φ → ηγ and measurement of the branching ratio Γ(φ→ηγ)Γ(φ→total)

The reaction π−p → ηφ has been studied at 1.8 GeV/c incident pion momentum using the Bologna-CERN NBC set-up, in order to investigate the electromagnetic decay mode φ → ηγ. We observed (27 ± 6) events, yielding a branching ratio Γ(φ → ηγ)Γ(φ → total) = (7.3 ± 1.9)%. The theoretical implications of this result are discussed.

Measurement of the branching ratio Λ(X0→γγ)/Λ(X0→ total)

Abstract (68 ± 12) events of the type X 0 → γγ yield the following branching ratio: Λ (X 0 → γγ )/ Λ (X 0 → total = (1.71 ± 0.33)%.

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)%.

A study of the fractional charge content of the cosmic radiation

SummaryAn analysis of the fractional charge content of the cosmic radiation at 450 m above sea level is reported. The following upper limits are established: number of fractional charges (±1/3)e<4.5·10−10 cm−2 sr−1 s−1 with 90% confidence level, number of fractional charges (±2/3)e≤≤(1.7±1.7)·10−10 cm−2 sr−1 s−1, number of fractional charges (±4/3)e≤≤(1.6±0.8)·10−7 cm−2 sr−1 s−1.RiassuntoSi riportano i risultali di una analisi del contenuto di cariche frazionaric nella radiazione cosmica. I limiti superiori ottenuti sono: numero di cariche frazionarie ±1/3e≤4.5·10−10 cm−2 sr−1 s−1 con un livello di fiducia del 90%, numero di cariche frazionarre ±2/3e≤(1.7±1.7)·10−10 cm−2 sr−1 s−1, numero di cariche frazionarie ±4/3e≤(1.6±0.8)·10−7 cm−2 sr−1 s−1.