G. di Caporiacco

Pulse height defect for Al27 and Mg24 ions in silicon surface barrier detectors

Abstract Silicon surface barrier detectors are shown to exhibit a “pulse height defect” for a low energy Al27 and Mg24 ions. The ions were produced inside the detector as the recoils of the Si28 (γ, p) and (γ, α) reactions by monochromatic γ-rays of 17.6 MeV. The pulse height defect ranges from 70 to 152 keV for Al27 ions of 91 keV and 225 keV respectively, while for Mg24 ions it is 138 keV and 145 keV for energies of 910 keV and 1105 keV. It is temperature independent (from −70°C to +20°C) and does not vary when the electric field is varied by a factor 2. The evidence suggests that the effect is due to a reduction of the efficiency in the electron-hole production when the ions are sufficiently slowed down.

Ersicson's fluctuations in the photodisintegration of Si28

Abstract The cross section of the reactions Si 28 (γ, p) Al 27 and Si 28 (γ, α) Mg 24 as been measured in an energy range ΔE = 117 keV around 17.62 MeV gamma ray energy. Six different transitions (γ, p 0 ), (γ, p 1 ), (γ, p 2 ), (γ, p 4 ), (γ, α 0 ) and (γ, α 1 ) have been separately analysed; they show strong, uncorrelated fluctuations, the amplitude of which agrees with Ericsons predictions (pure statistical reaction and purely E1 gamma absorption being assumed). No free parameter, as the effective number of channels, is required for the comparison. Some contribution of direct effect (up to, say, 50%) is not excluded. The relative yield of (γ, α) to (γ, p) transitions is, on the average, lower (by a factor about 10) than the statistical model would predict, on the basis of transmission coefficients. This is probably an example of the isobaric spin selection rules, as applied to photodisintegration by Gell-Mann and Telegdi.

Partial-wave analyses of the (3π)-system in the reaction π−p → π−π−π+p at 11.2 GeV/c

Abstract The results are presented of two partial-wave analyses of the (3π) − system in 30 000 events of the reaction π − p → π − π − π + p at 11.2 GeV/ c . Both techniques incorporate the assumptions of the isobar model and are (a) the University of Illinois program which fits in terms of the (3π) density matrix elements and (b) an amplitude parametrisaton including possible effects of both spin non-flip and spin flip at the baryon vertex. The results obtained with these independent programs are found to be very close. The proportions of contributing states, their t -dependence and the phases of off-diagonal density matrix elements are discussed for the mass range 0.9 M (3 π ) 1.9 GeV/ c 2 . The A 1 phenomenon is investigated in the context of a recent model; values of the mass, width and cross sections for the A 1 , A 2 , A 3 states are given, and an interesting variation in the 2 + P M =1 wave is observed at high masses.

Energy and angular distributions of α-particles in the 20Ne(n, α)17O reaction at 14 MeV☆

Abstract Energy and angular distributions for α-particles in the 20 Ne(n , α) 17 O reaction have been obtained at E = 14.2 MeV, by using a fast-cycle cloud chamber. Departures from the typical evaporation distribution are observed in the low and high energy parts of the α-particle spectrum; the low energy part, however, is not especially significant owing to the distortion introduced by the (n, nα) reaction. For the high energy α-particles the effect can be traced back to a direct interaction. The c.m. differential cross section peaks at backward angles, an effect which becomes more pronounced for the more energetic α-particles. This feature, which is analogous with the reported results for the 12 C(n , α) 9 Be and 16 O(n , α) 13 C reactions, suggests that heavy particles stripping plays an important role in the 20 Ne(n , α) 17 O reaction.

An experimental study of longitudinal and azimuthal correlations between pairs of mesons produced in π−p collisions at 11.2 GeV/c and their possible interdependence

SummaryAzimuthal and longitudinal correlations between charged mesons have been studied on a large sample of four- and six-prong events produced in π−p collisions at 11.2 GeV/c in the 2 m CERN HBC. The effect introduced by the ρ-meson has been investigated. The Goldhaber, Goldhaber, Lee and Pais (GGLP) effect is observed and compared with available data on π−p at 40 GeV/c and on pp in the momentum interval (100÷400) GeV/c. The ρ-meson accounts for the azymuthal asymmetry of π+π− pairs, but not for the like-charge pair asymmetry. The longitudinal correlations, however, cannot be justified by the presence of the ρ-meson. A short-range correlation survives for Δy≲1, due neither to the ρ-meson nor to Bose-Einstein statistics.RiassuntoSi studiano le correlazioni longitudinali e azimutali tra coppie di mesoni carichi prodotti nelle interazioni con 4 e 6 particelle cariche nello stato finale. Si utilizzano i dati di un esperimento di ampia statistica di π−p a 11.2 GeV/c nella camera a bolle di 2 m del CERN, riempita con idrogeno. Si indaga l’effetto indotto in tali correlazioni dalla presenza del mesone ρ. Si misura l’effetto di Goldhaber, Goldhaber, Lee e Pais e lo si confronta con quanto osservato in altri esperimenti. La presenza del mesone ρ rende conto dell’asimmetria azimutale delle coppie π+π−, ma non giustifica quella delle coppie di mesoni di ugual carica. Inoltre le correlazioni longitudinali non risultano dipendere dal mesone ρ; si costata la presenza di una correlazione a corto raggio (Δy≲1) non dovuta nè al mesone ρ nè alla statistica di Bose-Einstein.РеэюмеИэучаются аэимутальные и продольные корреляции между эаряжен-ными меэонами на больщой выборке четырех- и щестилучевых событий, обраэо-ванных при π−p соударениях при 11.2 ГзB/c в 2M водородной пуэырьковой камере в ЦЕРНе. Исследуется зффект, обусловленный ρ меэоном. Наблюдается зффект Голдхабера, Голдхабера, Ли и Пейса. Проводится сравнение с имеюшимися данными для π−p соударений при 40 ГзB/c и pp соударений в интервале импульсов от 100 до 400 ГзB/c. ρ меэон общясняет аэимутальную асимметрию π+π− пар, но не асим-метрию одноименно эаряженных пар. Однако продольные корреляции не могут быть общяснены наличием ? меэона. Короткодействуюшая корреляция не сохрана-ется для Δy/≲1, ни вследствие ? меэона, ни вследствие статистики Боэе-Эйщтейна.

Study of A2− → ηπ− in the reaction π−p → π−π−π+π0p at 11.2 GeV/c

SummaryThe analysis of A2−→ηπ− at 11.2 GeV/c confirms that A2− is essentially produced by natural-parity exchange. In the differential cross-section as a function oft′ there is evidence of the dip at smallt′ values. The branching ratio (A2→ηπ)/(A2→ϱπ) is 0.18±0.05.RiassuntoL’analisi del decadimento A2−→ηπ− nel sistema di Gottfried-Jackson conferma che il mesone A2− è essenzialmente prodotto mediante scambio di parità naturale. La sezione d’urto differenziale mostra un decremento per piccoli valori dit′. Il rapporto di decadimento (A2→ηπ)/(A2→ϱπ) risulta essere 0.18±0.05.РеэюмеАналиэ A2−→ηπ− при 11.2 ГзВ/c подтверждает, что A2− преимушественно рождается в реэультате обмена с естественной четностью в системе Готтфри-да-Джексона. В дифференциальном поперечном сечении, как функцииt′, имеется провал при малыхt′. Отнощение ветвей распадов (A2→ηπ)/(A2→ϱπ) составляет 0.18±0.05.

A fast and low cost pulsed data A–D acquisition system

A system is presented for data acquisition from a set-up of detectors submitted to high counting rates in pulsed regime during a short period of the cycle. The acquisition concerns analog charge signals that produce corresponding voltage levels on capacitive memories. In the time elapsing between two successive phases, the levels exceeding a prefixed bias are digitized and transferred to a computer. The structure we propose is modular, and allows expansion up to several hundreds of channels. The basic module includes 16 channels, each disposing of 32 capacitive memories. Of particular interest are the ease of construction and the low cost, mainly due to the use of recently introduced integrated analog memories.