Binary collisions at intermediate energies revisited by ...



Isospin physics: Proposal for upgrading CHIMERA

A. Pagano1, M. Alderighi2, F. Amorini3, A. Anzalone3, N. Arena1, L. Auditore4, R. Bassini5, C. Boiano5, G. Cardella1, S. Cavallaro3, M. B. Chatterjee6, M. D’Andrea1, E. De Filippo1, F. Fichera1, E. Geraci7, N. Giudice1, F. Giustolisi3, A. Grzeszczuk8, N. Guardone1, A. Grimaldi1, P. Guazzoni5, E. La Guidara3, G.  Lanzanò1, G. Lanzalone1,3, D. Nicotra1, M. Papa1, S. Pirrone1, G. Politi1, F. Porto3, C. Rapicavoli1, E. Rosato9, F. Rizzo3, G. Rizza1, S. Russo5, P. Russotto1, G. Saccà1, M. Sassi5, W. U. Schröder10, G. Sechi2, A. Trifirò4, J. Tōke10, M. Trimarchi4, S. Urso1, M. Vigilante9, W. Zipper8, and L. Zetta5

1 INFN Catania and Dipartimento di Fisica e Astronomia, Università di Catania, Italy

2 INFN Milano and Istituto di Fisica Cosmica, CNR, Milano, Italy

3 INFN-LNS and Dipartimento di Fisica e Astronomia, Università di Catania, Italy

4 INFN and Dipartimento di Fisica, Università di Messina, Italy

5 INFN Milano and Dipartimento di Fisica, Università di Milano, Italy

6 Saha Institute Of Nuclear Physics, Kolkata, India

7 INFN Bologna and Dipartimento di Fisica, Università di Bologna, Italy

8 Institute of Physics, University of Silesia, Katowice, Poland

9 INFN and Dipartimento di Fisica, Università di Napoli, Italy

10 Department of Chemistry, University of Rochester, Rochester, N.Y. 14627, USA

ABSTRACT

Since January 2003, the 4( charged-particle detector array CHIMERA in its full configuration has successfully been operated at the Catania Laboratori Nazionali del Sud (LNS) accelerator facility. Physics experiments studying heavy-ion reactions at Fermi bombarding energies have used a variety of beams delivered by the LNS Superconducting Cyclotron. A brief discussion of the status of some of these experiments is presented, along with an outline of recent technical developments aimed at improving the CHIMERA response. Future experiments in the field of isospin physics in this energy domain, planned at LNS for both primary and less intense secondary particle beams, could benefit significantly from improved experimental (A-Z) particle identification. Successful tests of specific pulse shape discrimination methods for CHIMERA silicon elements suggest that a corresponding upgrade of this detector array is feasible.

1. INTRODUCTION

Heavy ion collisions at medium energies (10 MeV/nucleon < E/A < 100 MeV/nucleon) offer a unique opportunity to study the evolution of dissipative processes in a transitional domain where strong competition between one-body and two-body dissipation is expected [1]. The last two decades have witnessed an impressive sophistication in experimental technology, with the introduction of high-resolution detector arrays covering almost the entire solid angle. Fragmentation of the transient nuclear system into several massive clusters of intermediate mass (IMF) and light particles (LP) has attracted much attention, because the phenomenon appears to be closely related to the nuclear equation of state (EOS) [2, 3]. In this energy domain, it is advantageous to study the relaxation of the reaction system along isospin degrees of freedom, predicted to play a crucial role in the reaction dynamics [3]. Evidently, the role of isospin degrees of freedom could be strongly enhanced in nuclear reactions with secondary beams of projectiles far from the beta-stable valley [4]. This perspective motivates attempts to improve the available identification methods, in order to provide simultaneous mass and charge identification of IMFs in 4л detector systems existing or planned for the future [5].

2. THE CHIMERA MULTI-DETECTOR ARRAY

2.1) General performance

Since January 2003, CHIMERA (Charged Heavy Ion Mass and Energy Resolving Array), a new multi-element detector array for charged particles (LCP) and fragments (IMF), has been fully operational at the LNS-Catania Tandem and Cyclotron facility. The characteristic capabilities of the detector rely on a systematic time-of-flight (TOF) measurement, providing excellent velocity and mass resolution, and its high granularity [6] minimizing event mixing and pileup due to multi-firing events. Basically, CHIMERA is a set of 1192 individual detector telescopes covering polar angles between 1° and 176° with full azimuth symmetry about the beam axis. The CHIMERA detector telescopes each consist of a 300-(m thick planar silicon detector of large capacitance (( 700 pF), followed by a CsI(Tl) crystal of thickness ranging from 2 cm (backward angles) to 12 cm (forward angles). CHIMERA is designed to allow mass measurement of sufficient resolution to be made for fragments stopped already in the first stage of a telescope using a TOF technique. Timing resolutions of about 800 ps (FWHM) are typically achieved for elastically scattered projectiles of intermediate masses. Simultaneous (E-E and (E-TOF techniques are utilized to provide charge (Z) and isotopic (A) identification for particles stopped only in the second (CsI(Tl)) element of a telescope. Finally, pulse shape analysis of CsI(Tl) detector signals allows one to identify the light charged particles (p, d, t, 3He, 4He, 6Li,….) in a wide range of kinetic energies. Experimental methods adopted for CHIMERA are described in further detail in recent publications [7].

The forward, cone-shaped part of the CHIMERA has been running efficiently already since early 2000. This configuration was utilized in the “REVERSE” campaign, where the relatively light targets of 27Al and 58,64Ni were bombarded with 35-MeV/nucleon beams of 112Sn and 124Sn projectiles, in order to investigate nuclear multi-fragmentation [8]. The recent commissioning of the apparatus in its full 4( configuration motivated extended experimental campaigns during the periods January-July 2003 [8] and January-July 2004 [9], in which several different experiments were completed. In the following, a more detailed description of IMF identification with CHIMERA will be given and some experimental perspectives will be outlined.

2.2 ) Intermediate-mass fragment detection with CHIMERA

Employing various methods, the individual detector telescopes of the 4π array CHIMERA provide good charge (Z) identification for reaction products ranging from protons to heavy ions and good isotopic resolution for light charged particles (LCP) as well as for IMFs with Z 5k( cm);

iv) compatibility with the existing front end.

A typical example of a TOF-Energy (E) discrimination matrix obtained with CHIMERA is illustrated in Fig. 3.

[pic]

The three identification lines for the stopped particles highlighted in the plot correspond to mass numbers A = 7, A = 11, and A = 15, respectively. For particles “punching through,” i.e., those not stopped in the detector, identification is still possible with respect to charge (Z). In the following, the classical relation [pic] between kinetic energy (E) and velocity (V) is adopted, where M is the mass of the particle. Assuming for each silicon detector an ideal, linear charge-energy response function, one can determine the dependence of the time-zero phase parameter (C0’) on the particle energy, using a fit procedure. Evidently, such a procedure is necessary for calibrating the measured TOF-E distribution in terms of particle mass. The time-zero parameter was evaluated for each detector and it was found to be dependent on the mass and the energy of the detected particle. The desirable behavior of the time-zero parameter, independence of particle mass and energy, as expected in the ideal case (C0), was obtained only for particle energies above ( 12 MeV/nucleon. The value of C0’, for each detector, was parameterized using a semi-empirical universal function depending on the mass number A, on the atomic number Z, and on the energy E of the particle. This function contains three fit parameters, P0, P1, and P2, and is given by the formula

[pic] (1)

In the data analysis, the function is found to be similar for all silicon detectors. In Equ. 1, the subscript i represents the detector number, C0i (ideal case) is the time- zero phase independent on the energy and particle mass; and E0i is an energy offset value taking account of the charge-to-digital converter bias (“QDC pedestal”). Finally, the quantity Si represents the detector thickness, while R(A,Z,E) stands for the range of the particle in silicon, calculated with an iterative method. The variation of the phase C0’ with energy is simply interpreted in terms of variations in the charge collection times influencing the shapes of the detector timing response signals. These effects are expected to be of the order of a few ns, in the case of a front injection configuration [11]. Observed in the measurement was a maximum variation in C0’ of about 3ns.

In order to measure in detail the rise time of the charge signal as a function of particle energy, a series of dedicated experiments were undertaken using the tandem/cyclotron facilities of the LNS Catania. The simple logical scheme employed for the rise time measurements, is sketched in Fig. 4. Briefly, the standard CHIMERA electronic chain for silicon detectors was extended by a second CFD chain. In practice, the negative timing pulse from the fast main timing amplifier (T) is split using a passive splitter. The start signal for the rise time measurement is derived from one of the two signals, which is fed into a standard CHIMERA CFD using a 30% fraction and a 20-ns internal delay [10, 14]. The duplicate signal is fed into a modified CHIMERA 90%-fraction CFD, also with a 20-ns internal delay. To first order, the signal rise time is taken to be proportional to the time difference between the signals from the 90%-fraction and the 30%-fraction CFDs. In the series of tests reported here (see Sect. 5), silicon detectors were coupled to a new version of CHIMERA charge-sensitive preamplifiers (PAC) developed in order to optimize rise time PSD measurements [15]. The average electronic noise obtained for this preamplifier is plotted in Fig. 5 vs. the input (= detector) capacitance.

[pic]

4. TEST RESULTS AND DISCUSSION

In order to evaluate sensitivity and large-scale feasibility of particle identification schemes based on a pulse shape analysis, different technical studies were carried out both at the tandem and at the cyclotron accelerators of the heavy ion facility at the Laboratori Nazionali del Sud in Catania [16]. In addition, a series of off-line electronic simulations were performed in order to help optimizing the PSD identification technique [17]. In the following, after a brief summery of the low-energy experiment performed at the tandem, selected preliminary data taken at intermediate bombarding energy are presented and discussed. In all tests reported here, the test object was a two-pads, large-area ( ( 20 cm2 ), 300-(m thick planar silicon detector typical of those used for CHIMERA experiments.

Results of tests performed at the tandem accelerator can be summarized as follows:

1) Generally, a pulse-shape based particle ID method is feasible even with the large-area (~ 20-25 cm2) planar implanted n-type silicon detectors of the CHIMERA array.

2) Specifically, charge identification test show that:

- An energy threshold of ~ 4.0 MeV/nucleon (in front injection) is easily achieved

- dependence of the charge identification pattern on bulk temperature (around 0°c) is weak and affects the identification threshold only very little.

-a strong influence of the detector bias is clearly seen. Increasing the bias to values above the total-depletion voltage (after compensation for reverse bias current flowing through the load resistor) as expected, had the tendency to shorten the rise time and consequently to reduce the sensitivity of the particle ID method.

An example of charge identification at tandem energy is shown in Fig. 6, for the reaction 19F + 12C at 95 MeV. The detection angle of the particles was θ = 6° with respect to the beam direction. As obvious from this figure, charge resolution from hydrogen to fluorine is clearly obtained. Most of the identified particles are projectile-like fragments with velocities close to that of the beam.

Very promising results of PSD based particle identification at higher Cyclotron incident energy were obtained in a December-2003 test run using the reaction 40Ar + 12C at an incident energy of Elab(40Ar ) = 20 MeV/nucleon. In these test experiments, a combination of TOF and PSD methods is employed for particle ID. The silicon detector is configured for front particle injection, which is the configuration preferred for TOF measurements.

An example of a kinetic-energy/rise-time matrix for charge identification, obtained in the above cyclotron experiment, is illustrated in Fig. 7. For stopped particles, good charge resolution is seen up to the heaviest fragments produced in the reaction, i.e., the highest observed atomic numbers. On the other hand, for particles punching through the silicon detector, the signal rise time is found to be nearly independent of particle charge and energy. Consequently, no PSD information is available for efficient charge identification of “punch-through” particles. Notice that the observed dependence of the rise time on particle energy and charge is in fair agreement with the PSD effects already described by the formula in Equ. (1). Data shown in Fig. 6 have been obtained at a detection angle close to the grazing angle for that reaction. At such angles the cross section is dominated by peripheral collisions leading to the emission of PLFs. Consequently, the statistics for IMF emission and detection are very poor. Good statistics for IMFs were collected at larger detection angles.

In Fig. 8, PSD identification is shown for the same reaction, but for a detection angle of θ = 5°. The detector bias used in both cases was approximately 30% in excess of the full-depletion values, as is standard for CHIMERA operations.

Evidently, the identification procedure can take advantage of information carried by the time of flight signal provided by the silicon detector (see Fig. 2). As an example, in Fig. 9 the same identification matrix of Fig. 8 is shown but with the additional condition that particles be fully stopped in the silicon detector, information provided by the TOF measurement. Consequently, for nuclear fragments with intermediate atomic numbers (Z ( 12), it is possible to achieve complete isotopic identification from simultaneous measurement of both TOF and signal pulse shape.

In conclusion, heavy-ion studies with stable and exotic beams require the identification of nuclear fragments in a broad range of masses and atomic numbers. The recent experimental data presented above have clearly demonstrated that charge identification of fragments with intermediate charges (Z-values) is possible with low energy thresholds, for the planar n-type silicon detectors of large capacitance ( ( 1ns) employed in the CHIMERA multi-detector array. Of particular potential is a combination of rise time analysis with time-of-flight measurements, which could be employed for all CHIMERA detectors. Consequently, and in anticipation of much increased future demand for improvements of this detector’s capabilities by the international LNS users’ community, a proposal to upgrade the CHIMERA detection system appears feasible, justified, and timely.

5.-REFERENCES

[1] A.BONASERA, M. DI TORO AND CH. GREGOIRE NUCL. PHYS. A 463 (1987) 653; G. PEILERT, H. STÖCKER AND W. GREINER; REP. PROG. PHYS., 57, (1994) 533.

[2] M.F. Rivet, Inv. Talk , proc. Of IWM2001, Catania 28 November , 2001, pag.11; J.Richter and P.Wagner, Phys. Rep. 350, 1 (2001).

; A.Bonasera et al., Rivista del Nuovo Cimento, 23 , (2002) 1.

[3] Isospin Physics in Heavy Ion Collisions at Intermediate Energies, Edited by Bao An Li and W. Udo Schröder, ISBN 1-56072-888-4, Nova Science Publ. Inc., New York (2001)

[4] P.Chomaz, C.R. Physique 4(2003)

[5] B.Borderie, ‘’Summary of the round table on future developments for new calibration 4( detectors’’, proc. Of IWM2001, Catania 28 November , 2001, pag.173;

in the proc. of IWM2003,

[6] S.Aiello et al., Nucl. Physics A583 (1995) 461c.

[7] S. Aiello et al., Nucl. Inst. and Meth. A 369 (1996)50-54; S.Aiello, IEE Trans. On Nucl. Sci. Vol. 45, n° 4, 1877; S.Aiello et al., IEEE Trans. On Nucl. Sci. 47: (2), (2000) 114 ; N. Le Neindre et al., Nucl. Inst. and Meth. A 490 (2002) 251 ; M. Alderighi et al., Nucl. Inst. and Meth. A 489 ( 2002) 257;

[8] A.Pagano et al. Nucl. Phys. A 681 (2001) 331-338c

[9] A.Pagano et al., Proc. NN2003 Int. Conf. Moscow 17-22 June NPA(2004) in press

[10] A.Pagano et al., ‘’ The CHIMERA detector at LNS CATANIA Methodology-Experimental Results-Future development ‘’, proc. of 6th World Multi – Conf. on Systemics, Cybernetics and Informatics, Orlando Florida, July 14-18, 2002., pag.71

[11] C. A. J. Ammerlaan et al. , Nucl. Instr. & Meth. 22 (1963) 189; J. B. A. England, G. M. Field and T. R. Ophel, Nucl. Instr. & Meth. A280 (1989) 291. and reference therein ; G. Pausch, W. Bohne and D. Hilscher, Nucl. Instr. & Meth. A 337 (1994) 573 and reference therein.

[12] W. –D. Emmerich et al., Nucl. Instr. & Meth. 83 (1970) 131.

[13] M. Mutterer et al.,, IEEE Trans. on Nucl. Sci. 47 (2000) 756.

[14] Pagano for CHIMERA, ‘’Comments on Pulse Shape Discrimination on Silicon and CsI(Tl) detectors’’, selected oral contribution : SPES Meeting Legnaro-May-9th (2002)

[15] R.Bassini et al., Proceedings of NSS2003 Conference, Portland October 20-25, 2003, Contribution N36-45

[16] M.Alderighi et al., poster contribution to Int. NN2003Conf. Moscow 17-22 June NPA(2004) in press

[17] M.Alderighi et al. work in preparation, to be published

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Fig. 1- Charge vs. parallel velocity for products emitted in the reaction 112Sn+ 58Ni at 35 MeV/nucleon (see text).

Fig.- 7 Kinetic-energy/rise-time identification matrix, obtained by the double-CFD method (see text) for the detection angle of 2.5°.

Fig. 3- Plot of energy vs. TOF, as measured in one of the CHIMERA silicon detectors.

T

[pic]

Fig. 6 - Charge identification matrix for LCPs and light IMFs from 19F-induced reactions on 12C at 95 MeV, near the grazing angle.

Fig. 8 – Kinetic-energy/rise-time identification matrix obtained by the double-CFD method (see text) for a detection angle of 5°.

Fig. 9 - Zoom of the kinetic-energy/rise-time identification matrix of Fig. 8 (1500x2500 channels) obtained with additional requirement that the particle are stopped in the silicon detector.

Fig. 2 -Isotopic analysis in the reaction 112Sn+ 58Ni at 35 MeV/nucleon

E

QDC

CFD30% Start TAC

Si PA Amp Split

Rise

Time

CFD90% Stop TAC

Fig. 4: Electronics used for rise time and energy measurement.

Fig. 5 – Dependence of electronic noise on input capacitance, for the new CHIMERA charge-sensitive preamplifier (see [16])

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