A large acceptance MAGNetic spectrometer for EXcyt beams

Laboratori Nazionali del Sud -Catania (Italy), IPN -Orsay (France)

Stable and Radioactive heavy ion Tandem beams (E=0.5 - 8 MeV/u)

International collaboration:

Total number of researchers: 25

Number of Italian researchers: 15 (12.7 FTE)

Scientific project of the MAGNEX exper iment

The advent of new facilities for the production of Radioactive Ion Beams (RIB) like EXCYT, presently under construction at LNS-Catania, will dramatically increase our knowledge of basic nuclear structures and reaction mechanisms. New experiments involving weakly bound nuclei will show the fundamental influence of the isospin degree of freedom, providing crucial information on nuclear observables of astrophysical interest. The MAGNEX spectrometer has been conceived in order to explore a consistent part of such nuclear physics, taking into account RIB low intensities, wide energy and mass ranges and the excellent properties of Tandem beams.

Spokesperson : Prof. Angelo Cunsolo Angelo.Cunsolo@lns.infn.it

- INFN - Laboratori Nazionali del Sud, Catania , Italy - INFN Sezione di Catania, Catania, Italy - Univ. Degli Studi di Catania, Dip. Di Fisica , Catania, Italy - Univ. Degli Studi di Messina, Dip. di Fisica , Messina , Italy - INFN Sezione di Pisa, Pisa, Italy - Flerov Laboratory of JINR, Dubna, Russia - GANIL, Caen, France - Institut de Physique Nucléaire, Orsay, France

The MAGNEX set-up

The MAGNEX spectrometer is a general-purpose instrument to be used both for inclusive measurements and in coincidence with other detectors. The second and higher-order optical aberrations are partially compensated for by the shape of the dipole entrance and exit field boundaries (hardware correction), and partially by means of trajectory reconstruction (software correction). A set of surface coils mounted on the dipole tips provides tuneable quadrupolar and sextupolar field used for compensation of second order kinematic effect.

Maximum magnetic rigidity 1.8 T m

Solid angle 51 msr

Momentum acceptance ±10%

Deflection angle 55°

Focal plane angle 61°

Focal plane length 92 cm

Maximum focal plane height 32 cm

1st order momentum resolution for 1 mm2 target 5400

Path length of the central trajectory 5.76 m

The MAGNEX functioning principle is here summarised:

the entrance position-sensitive start detector(PSD), provides the start signals for the TOF;

the quadrupole magnet focuses the particles in the vertical direction;

the huge dipole or bending magnet disperses particle trajectories on the focal plane depending on their magnetic rigidity;

the focal plane detector (FPD), measures the impact of particles on the focal plane and serves to identify the deflected ions. It measures ion direction, energy, charge and mass.

The PSD detector, based on the microchannel plate technology

The prototype of FPD detector.

FPD

Dipole Quadrupole

PSD

Target

As shown in the table hereby, the predicted energy and mass resolution coupled to the large solid angle and momentum acceptance are well beyond present limits. In addition the accepted magnetic rigidities will cover all the possibilities offered by a Tandem accelerator together with some particularreaction product obtained by Cyclotron beam.

Last results about spectroscopy of light exotic nuclei

In the 15C case the ground and the first excited state are clearly seen together with a structure at 8.5 MeV (Γ∼0.3 MeV). The results on 15C indicate behaviour similar to that of 11Be nucleus, for which a narrow state far from the neutron emission threshold and a broad bump, in the region where the SDR mode could be expected, are observed. The observation of narrow resonances well beyond the neutron emission threshold can be interpreted as due to the polarization of the core. A microscopic approach to 15C strength distribution has been set up based on QRPA theory and Dynamical Core Polarization model (DCP). The 15C excitation energy spectrum up to 16 MeV has been obtained. Both the s1/2 and d5/2 strengths distributions are found strongly fragmented, being concentrated between 9 and 15 MeV excitation energy range of 15C.

The magnets and power supplies are almost completed and will be delivered to the LNS for the beginning of May 2002. The rotating platform is going to be commissioned (the administrative procedure is at advanced step) and the good will be built and delivered in 3 months from the date of contract signature. The PSD has been built and successfully tested. The FPD has been designed and a prototype has been successfully tested under beam at the LNS. The construction was started and the detector will be delivered for the end of 2002. The silicon calorimeter is almost completed needing a few months for the arrival of the detectors at the LNS. The vacuum and gas handling system has been studied and is partly built, being completed within the half part of 2002. The acquisition system has been designed and the software is being developed. The detailed project of the control system and scattering chamber will be ready for June 2002. The CEX program is going on with new experiments foreseen at the IPN-Orsay in the spring of 2002 and development of the CEX-QRPA theory with the Giessen collaborators.

The optics of the large acceptance magnetic spectrometer MAGNEX has been extensively studied. The use of 3D finite elements calculations allowed a realistic description of the shape of the magnetic elements. Different solutions have been tested in order to achieve the required field uniformity. The main results are stored in two NIMA articles (presently in press).

Main results of MAGNEX on ion Optics

Status of the art

The 15N(7Li,7Be)15C and 14C(7Li,7Be)14B reactions have been studied at 57 MeV incident energy. Experimental results have been strongly limited by the impossibility to use the Split-Pole spectrometer to detect 7Be ejectiles. For 14B all the presently known states have been observed.