Laboratori Nazionali del Sud

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The  MEDEA – SOLE – MACISTE detection complex

MEDEA  is an array for gamma-rays and light charged particles, designed to operate in a large vacuum vessel. Its present configuration consists of 180 barium fluoride crystals, arranged in a ball shape of 22 cm inner radius. The closely packed BaF2 ball covers polar angles from 30° to 170° and the whole azimuthal range. The typical thickness of each module is 20 cm, which is about 10 radiation lengths, and corresponds to the range of  about 300 MeV protons and about 1 GeV alpha particles. The detection efficiency of each module is higher than 95% for γ-ray energies from 1 to 300 MeV.

Barium fluoride is a scintillating material  with an interesting property: its light emission has two distinct components, with different decay times (600 ps and 600 ns), whose relative amount is a function of the incident radiation type. The fast to slow component ratio is maximum for γ-rays and decreases for light charged particles with increasing Z. The discrimination of  g- rays and light charged particles is achieved using a pulse-shape analysis method: the separate integration of the total output signal and the fast component allows for the identification of  γ-rays, Z=1 isotopes and Z=2 particles from deposited energies as low as a few MeV.  Moreover, the presence of the fast component allows for a very good time resolution (less than 1 ns). Combining the time of flight information and the pulse shape analysis a good discrimination between γ-rays and neutrons is obtained. Recently, the design and electronics of some modules have been modified to optimize the neutron detection.

At very forward angles, MACISTE, 15 m far from the target, can identify the reaction products conveyed on it by SOLE, a superconducting solenoid  placed just at the exit of the MEDEA reaction chamber. SOLE is operated at liquid Helium temperature till a maximum magnetic field of 5 Tesla and presents an angular acceptance of about 6° and a momentum acceptance of about 20%.  MACISTE consists of  four telescopes with gas chambers, for energy loss and two-dimensional position information, followed by plastic scintillators for the residual energy measurement, arranged in a variable geometry resembling a photographic diaphragm. A central hole is left for beam transit.  The long flight path allows for  detection and identification of projectile-like fragments and heavy residues emitted at very forward angles.

If necessary, MEDEA can be complemented with auxiliary detectors. As an example, it has already been used with PPAC, telescopes for detection and identification of Z>2 ions and neutron detectors located outside the vacuum chamber.

MEDEA has been conceived for experiments with the heavy ion beams of the LNS Superconducting Cyclotron and it is particularly suited for the measurement of quantities related to the mechanisms of high energy  γ-ray and neutral pion production, to the dynamics of nucleus-nucleus collisions and to the properties of hot nuclei.

During about 25 years of operation, MEDEA has given important contributions to a series of open questions in the intriguing field of intermediate energy heavy ion physics. In particular, thanks to MEDEA experimental results, probes for the different reaction times have been identified and studied, limits of collective motion have been put in evidence, information on nuclear EOS and its symmetry term have been obtained.

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