R. Coniglione

MEDEA: a multi element detector array for gamma ray and light charged particle detection at the LNS-Catania

Abstract A 4 π highly granular Mutli Element DEtector Array (MEDEA) for γ-rays and light charged particles is described. Its basic configuration consists of 180 barium fluoride scintillator crystals, arranged in the shape of a ball, plus a forward angle wall of 120 phoswich detectors. The inner radius of the ball (22 cm) and the distance of the wall from the target (55 cm) allow the placement of other detectors.in the inner volume. The whole detection system operates under vacuum inside a large scattering chamber. Dedicated electronics has been designed and realized. It includes a powerful hardware second level trigger and preanalysis system, which allows on-line event selection, and a modular VME-bus based data acquisition system. In-beam performances of the system are also described.

Strong impact parameter dependence of hard photon production in intermediate energy heavy ion collisions

Abstract The dependence of the high energy photon production on the impact parameter has been investigated in the reaction 129 Xe+ 197 Au at 44 MeV/u using the multidetector array MEDEA. A strong dependence of the high energy photon production probability on the impact parameter has been observed, while the slope parameter of the photon spectrum is almost constant. The data support the interpretation of the hard photon production in terms of first chance n-p collisions.

The HADES time-of-flight wall

In the framework of the High Acceptance DiElectron Spectrometer collaboration a Time-of-Flight wall subdetector has been developed. In order to efficiently select rare events with dilepton pairs produced in the nuclear matter, in a high hadron multiplicity environment, such a wall must be able to resolve electrons from pions up to 0.5 GeV/c and from protons up to 2 GeV/c. The short flight path of about 2 m requires a time resolution better than 150 ps, while impact position and multiplicity measurements need adequate granularity. These requirements have been fulfilled by employing scintillator rods read-out at both ends by photomultipliers.

On the origin of fast proton emission in intermediate energy heavy ion collisions

Abstract Exclusive measurements of multiplicity distributions of fast protons emitted in 40Ar and 132Xe induced reactions at 44 MeV/u bombarding energy have been performed using MEDEA 4π detection sytem. The data reflect the impact parameter dependence of the overlap region size and scale from system to system as the number of participant protons. The hypothesis of pre-equilibrium emission following first order p-N collisions is consistent with the data.

Response of the BaF2 scintillator to light charged particles

The response of a large BaF2 scintillator crystal in terms of analog output and energy resolution dependence on the light charged particles kinetic energy in the 4–150 MeV/u range has been investigated. The occurrence of loss mechanisms in the kinetic energy to light yield conversion is observed. A model calculation, based on the electron-hole recombination to trapping competition and the occurrence of ionization quenching, gives results in very good agreement with the experimental data. The detector energy resolution is found to be consistent with the photoelectron statistics as the main contribution. A procedure for reproducible particle energy calibration, based on the cosmic muon energy deposition in the BaF2 as reference is established.

Response of MEDEA BaF2 detectors to 20–280 MeV photons

The response function of MEDEA BaF2 crystals to high energy photons, up to 280 MeV, has been studied using monochromatic γ-rays from the in flight annihilation of positron beams. The experimental response functions are compared to the results of Monte Carlo simulations based on the EGS3 code and parametrized over the whole investigated energy range.

Impact parameter dependence of linear momentum transfer and the role of two-body dissipation mechanisms in heavy ion collisions around the Fermi energy

Abstract High energy γ -rays ( E γ >35 MeV) have been measured in coincidence with heavy residues emitted in reactions induced by a 37 MeV/u 36 Ar beam on a 98 Mo target. The γ -ray yield increases strongly with increasing linear momentum transfer indicating the importance of two-body collisions in the transfer mechanism. The high energy γ -ray multiplicity has been used to correlate the linear momentum transfer to the impact parameter. This correlation is compared to dynamical BNV simulations to show the essential role of two body nucleon-nucleon collisions at these bombarding energies.

Rejecting escape events in large volume Ge detectors by a pulse shape selection procedure

Abstract The dependence of the response to γ-rays of a large volume Ge detector on the interval width of a selected initial rise pulse slope is investigated. The number of escape events associated with a small pulse slope is found to be greater than the corresponding number of full energy events. An escape event rejection procedure based on the observed correlation between energy deposition and pulse shape is discussed. Such a procedure seems particularly suited for the design of highly granular large volume Ge detector arrays.

The time of flight wall for the HADES spectrometer

In the framework of the HADES (High Acceptance DiElectron Spectrometer) collaboration we are developing the Time Of Flight (TOF) wall subdetector. In order to efficiently select events with e/sup +/e/sup -/ pairs produced in the nuclear matter, the TOF wall must be able to resolve electrons from pions up to 0.5 GeV/c and from protons up to 2 GeV/c. Because the short flight path of about 2 m requires a 100-150 ps time resolution, we are building a granular system made of scintillator rods readout on both ends by means of photomultipliers, capable of such performance.

MACISTE: the forward spectrometer for heavy ions at LNS-Catania

The compact heavy ion spectrometer MACISTE is going to be realized at the Laboratorio Nazionale del Sud-Catania (LNS). It should allow one to resolve the single mass unit up to A/spl ap/70 and the single charge up to Z/spl ap/45 when operating on ions around 50 MeV/amu. Coupling this device with other multidetectors already installed at LNS will give access to a large series of experiments concerning the projectile-like and the quasi-fusion physics at intermediate energy.