In beam test of the new Time-of-Flight spectrometer for the 8πLP setup based on the CORSET concept

A. Brondi1, A. Di Nitto1, G. La Rana1, R. Moro1, M. Trotta1, E. Vardaci1, A.Ordine1, A. Boiano1, A. Chizhov2, G.N. Kniajeva2, N.A. Kondratiev2, E.M. Kozulin2, M. Cinausero3, E. Fioretto3, P. Mastinu3, G. Prete3, V. Rizzi3, N. Gelli4,

F. Lucarelli4

1 Dipartimento di Scienze Fisiche and INFN, Napoli,2 Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia, 3 INFN, Laboratori Nazionali di Legnaro, 4 Dipartimento di Fisica and INFN, Firenze

INTRODUCTION

The detection capability of fission fragments in the 8πLP apparatus has been improved, in the framework of the FIESTA collaboration, with the design of a new Time-of-Flight spectrometer based on the CORSET concept from the FLNR in Dubna [1]. The basic system consists of four arms mounted on opposite sides of the beam, each arm containing a START and a STOP detector. The STOP detectors are position sensitive. The information of the time of flight and of the position of the fragments in each respective arm along with the two-body kinematics allows the reconstruction of the masses and kinetic energies of the fragments. The START and STOP detectors use a system of microchannel plates and allow to achieve a time resolution of ~180ps measured with a triple alpha source. From our simulations, it is possible to achieve a mass resolution of ≈1 amu with a flight path of 30 cm.

THE 4-ARM TOF SPECTROMETER

In order to install the full spectrometer system in the 8πLP support frame we have designed and produced three types of pyramidal capsules out of carbon fibre. Each one has such a shape to fit inside the frame of the BALL. Each capsule contains a START and a STOP set of detectors (Fig.1). Standard NIM electronics (CFDs and TACs) is used to guarantee a time resolution of ~180ps.

FIG.1: the START and STOP detectors and the carbon fiber capsule

Four arms have been mounted inside the frame as shown in Fig.2. Mechanical work is in progress to

guarantee a more precise anchoring of the STOP detectors inside the capsules.

FIG.2: the TOF arms mounted inside the 8πLP frame

The full system of 4 arms has been tested with the

reaction 32S + 109Ag at Elab = 180 MeV. This reaction has been chosen because the M-TKE distribution of the fission fragments has been measured by the CORSET setup in a different experiment. In this report a brief account will be given of the preliminary data processing.

FIG.3: TOF-TOF matrix

Fig.3 shows the TOF-TOF matrix. Three loci are clearly

visible. From kinematical considerations, two of them correspond to the fragments coming from quasi-elastic reactions, and the one in the middle corresponds to the region of the fission fragments.

FIG.4: Mass-TKE distribution from the TOF-TOF matrix of Fig3

In order to obtain the Mass-TKE distribution from the

TOF-TOF correlation matrix it is necessary to use the conservation laws applied to the two body kinematics. When this is done the matrix in Fig.4 is obtained. The most intense areas correspond to the quasi-elastic processes. The mass and TKE measured for such reactions is consistent with the geometrical efficiency of the TOF spectrometer and with the behavior expected for the mass transfer of these type of reactions. Since the correction for the kinematics efficiency still needs to be applied to the matrix in Fig. 4, further processing may induce some slight variation of the pattern in Fig.4.

FIG.5: Mass-TKE distribution from the TOF-TOF matrix of Fig.3

By projecting the M-TKE matrix on the x-axis we obtain

the mass distribution. If only the symmetrical part is projected on the y-axis (region marked in Fig.4 as FF), we obtain the TKE distribution for the symmetric mass region. The average value is in full agreement with the Viola systematics. This gives us confidence on the correctness of the whole analysis procedure.

FIG.6: TKE distribution for the symmetric mass splitting

One other interesting observable is the folding angle distribution of the fragments in the center of mass reference frame. This gives an indication of the degree of the two-body nature of the reaction. The folding angle distribution for the symmetric mass region is given in Fig.7. The fact that this distribution is peaked around 180° is an indication that the two partners come from a two-body splitting, as expected for the reaction under exam.

FIG.7: Center of mass folding angle distribution for the symmetric mass splitting.

[1] N.A. Kondratiev et al., Proceedings of the 4th International Conference on Dynamical Aspects of Nuclear Fission, October 19-23, 1998, Casta-Papiernicka, Slovak Repubblic, Y. Oganessian, J. Kliman and S. Gmuca Eds., World Scientific Publishing Co., 2000, p.431-437.