A. C. Russo

A study for the use of the LNS crystal ball multidetector as a neutral pions spectrometer

Abstract The response of the large BaF2 multidetector Crystall Ball to high energy photons and neutral pions was studied by simulations carried out with the code GEANT3. The properties of the detector relevant to π0 detection, such as resolution, linearity and efficiency are discussed. The performance of the system can be improved by considering a cluster of 8 detectors around the illuminated one to follow the development of the electromagnetic shower. As a π0 spectrometer the Crystal Ball covers the whole angular range and permits the detection of π0s over 100 MeV kinetic energy. Estimated π0 efficiencies vary between 60% for pio emitted at rest to 35% at 100 MeV kinetic energy.

Measurement of the Dalitz decay of π0 produced in heavy ion collisions at intermediate energies

Abstract An experimental search for events with three electromagnetic showers originating from the Dalitz decay of the π 0 meson ( π 0 → γe + e − ) has been carried out using a BaF 2 multidetector array covering a large fraction of the overall solid angle in an experiment where the main decay mode π 0 → γγ was measured at the same time. Neutral pions were produced in the reaction 36 Ar + 27 Al at 3.4 GeV laboratory incident energy. Identification of the π 0 3-body decay was achieved by an invariant mass analysis after a selection of the events. A branching ratio Γ ( γ e + e − )/ Γ (2 γ ) = (1.32 ± 0.11) × 10 −2 was deduced.

Subthreshold pion production from nucleus-nucleus collisions around 100 MeV/nucleon

Several global variables were tested with the aim to determine the impact parameter in nucleus-nucleus collisions producing pions at incident energies around 100 MeV/nucleon. The experimental set-up includes the MEDEA multidetector, part of which is used as aπ0 spectrometer, and an additional hodoscope of plastic scintillators to cover very forward angles. A statistical model was used to generate bothinclusive and pion-triggered events. Selection ofwell measured events was made through the measured total parallel momentum. Among the different global variables which were tested, the average parallel velocity was seen to give the best correlation with the impact parameter.

Detection of η mesons in heavy-ion collisions at 100 MeV/nucleon

Abstract A study of the detection of η mesons in heavy-ion collisions at 100 MeV/nucleon by a large array of BaF 2 modules has been carried out using full GEANT3 simulations. The most probable decay modes of the η mesons have been considered. Invariant mass spectra from the detection of two or more electromagnetic showers are discussed. The different sources of contamination are evaluated. The detection limits in terms of production cross section are discussed. An analysis of experimental data taken at 95 MeV/nucleon with the MEDEA array showed a few events which are compatible with the η → γγ decay.

On the cosmic-ray induced background in neutral pion production measurements with a BaF2 multidetector

Abstract The problem of the cosmic-ray induced background in neutral pion production measurements with barium fluoride multidetectors is discussed. As a reference example, the response to cosmic rays of the MEDEA photon spectrometer is studied. The interaction of the cosmic radiation with the experimental filter has been treated by means of full Monte Carlo computer simulations with the GEANT3 code. The results of the simulations are compared with experimental data and general criteria to minimize the cosmic background are discussed.

Light charged particles associated with subthreshold neutral pion emission in the16O+27Al reaction

The production ofZ=1 andZ=2 particles associated with neutral pion emission in the16O+27Al reaction at 94 MeV/nucleon has been studied. Results are compared with previous findings obtained by charged pions in the same collision at the same bombarding energy and with the prediction of a dynamical model based on a numerical solution of the Boltzmann-Nordheim-Vlasov equation.

Study of the CLAMSUD magnetic spectrometer focal plane by a simulation code

Abstract A simulation of the performances of the CLAMSUD magnetic spectrometer as a pion detector was carried out by the code GEANT2. A survey of the geometry and the magnetic field of the CLAMSUD spectrometer is first presented. The investigation by means of the code GEANT2 of the spectrometer characteristics (such as solid angle and momentum resolution), pion discrimination by a ΔE-t analysis and muon contamination of pion spectra is reported.

Experimental Evidence of “In Medio” Effects in Heavy-Ion Collisions at Intermediate Energies

Heavyion collisions at bombarding energies ranging from about 100 MeV/nucleon up to a few GeV/nucleon represent a unique tool to study the excitation of non-nucleonic degrees of freedom like baryonic resonances in excited nuclear matter far from ground-state conditions, i. e. outside the usual domain of existing nuclear structure information. Indeed, in a recent paper1 we have demonstrated the existence of the elementary indirect process N N → N Δ → N Nπ0 in 36Ar+27Al collisions at around 100 MeV/nucleon and we have deduced from experimental data the relative cross section. Notwithstanding Δ → Nπ is by far the most favoured decay channel (B.R.~ 100% 2), it is not however the best-suited one to study the signals of excitation and propagation of Δ(1232)-resonance in nuclear matter because of the high distortion introduced by the final-state interactions of pions with the surrounding medium. In this context, the electromagnetic decay Δ →Nγ would be, on the contrary, much more appropriate due to the almost complete absence of interaction of photons with nuclear matter. The free branching ratio of that decay channel is, however, only 6 · 10−3 2, and the successful realization of an experiment aimed to the detection of γ’s coming from Δ decay has to reckon with the existence of several serious drawbacks: i) in order not to have contamination from other mechanisms (such as statistical photon emission and/or giant-resonance de-excitation) a lower energy cut-off of at least 25–30 MeV must be imposed on the data and this strongly reduces the yields, ii) it is well known that high-energy photons are mostly emitted in the elementary direct process N N →N Nγ so that one has to identify a reasonable ensemble of conditions on the available observables apt to disentangle the indirect mechanism from the direct one, iii) in order to reduce as much as possible the strong background due to photons coming from π0 decays, the bombarding energy should not be much larger than 100 MeV/nucleon and, at the same time, it should not be much smaller than that value because of the consequent reduction of the phase space available for the excitation of the Δ resonance.

The low energy kaon program at the celsius storage ring

The CLAMSUD spectrometer has been recently installed at the jet-target position of the CELSIUS ring at “THE SVEDBERG LABORATORY.” The physical purpose is the study of kaon production at energies below the N-N threshold. Due to the low cross-section and short lifetime of kaons we increased the solid angle by means of two quadrupoles positioned at the entrance of the dipole. The experimental quality of the measurements due both to the beam characteristics and to the CLAMSUD detector will be shown.

Evidence of Δ(1232)-Resonance Excitation in Subthreshold Pion Production at Intermediate Energies

After almost twenty years of intense research activity, it is nowadays rather well accepted that the main responsible of subthreshold pion production in heavy-ion collisions at intermediate energies (around 100 MeV/nucleon) has to be localized in the “in-medium” incoherent nucleon-nucleon collisions which take place in the overlap volume of the two reacting nuclei at the early stage of quite central collisions. In this framework two different channels are deemed possible: the “direct” NN →NNπ one and the “indirect” NN → NΔ → NNπ one. In the latter, the excitation and decay of the baryonic resonance acts as a doorway state which might be responsible of the large energy pooling needed in the subthreshold particle production. Up to now, all previous statements rely only on the results of indirect tests such as the agreement of the predictions of microscopic theoretical models, based on that picture, with both inclusive and semi-exclusive experimental data[1,2,3,4].