A. Del Zoppo

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.

BaF2 scintillator: A stand-alone detector for γ-rays and light charged particles

Abstract The performances of thick (≥ 10 cm) BaF 2 crystals as detectors for γ-rays and light charged particles have been tested. The capability of BaF 2 of identifying Z = 1 particles by itself alone is shown. A new method to perform γ-particle and particle mass discrimination to be used in multiple coincidence experiments with highly segmented detection system is presented.

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 Trojan Horse Method in nuclear astrophysics

Abstract The basic features of the Trojan Horse Method are discussed together with a review of recent applications, aimed to extract the bare astrophysical S(E)-factor for several two-body processes. In this framework information on electron screening potential U e was obtained from the comparison with direct experiments.

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.

Measurement of neutron yield by 62 MeV proton beam on a thick Beryllium target

In the framework of research on IVth generation reactors and high intensity neutron sources a low-power prototype neutron amplifier was recently proposed by INFN. It is based on a low-energy, high current proton cyclotron, whose beam, impinging on a thick Beryllium converter, produces a fast neutron spectrum. The world database on the neutron yield from thick Beryllium target in the 70 MeV proton energy domain is rather scarce. The new measurement was performed at LNS, covering a wide angular range from 0 to 150 degrees and an almost complete neutron energy interval. In this contribution the preliminary data are discussed together with the proposed ADS facility.

Prompt dipole radiation in fusion reactions

Abstract The prompt γ-ray emission is investigated in the 16 A MeV energy region by means of the 36,40Ar + 96,92Zr fusion reactions leading to a compound nucleus in the vicinity of 132Ce. The dynamical nature of this radiation is confirmed. We show that the prompt γ radiation has an angular distribution pattern consistent with a dipole oscillation along the symmetry axis of the dinuclear system. The data are compared with calculations based on a collective bremsstrahlung analysis of the reaction dynamics.

Different isotopic composition as a signature for different processes leading to fragment production in midperipheral 58Ni+58Ni collisions at 30 MeV/nucleon

We report on the results of experiments performed to investigate the 58 Ni + 58 Ni 30 MeV/nucleon reaction. In midperipheral collisions a massive fragment (4 Z 12) production has been observed. The emission patterns exhibit features consistent with dynamical fragmentation of a neck zone between the interacting nuclei, while in addition and at the same time, the projectile-like and targetlike residues are subject to statistical decay. The nature of the fragments produced via the two different mechanisms differ both for what concerns charge distribution and isotopic composition. In particular, neutron rich fragments can be produced in dynamical processes, even if the starting nuclear matter presents a N/Z ratio close to the unity.  2001 Elsevier Science B.V. All rights reserved.

Performance of a symmetric BGO-NaI anti-compton shield

Abstract A compact symmetric BGO-NaI shield for accurate discrete γ-ray spectroscopy with germanium detectors is described. Tests of the shield performed with radioactive sources are presented. The characteristics are expressed in terms of the Compton suppression factor, of the average detection efficiency and of the peak to total ratio deduced from the analysis of the response function of a Ge detector inserted into the shield.