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Proposal for upgrading CHIMERA 4π detector A. Pagano1, M. Alderighi2, F. Amorini3, A. Anzalone3, N. Arena1, L. Auditore4, R.Barnà4, R.Bassini5, J.Blicharska6, C.Boiano5, C.Calì3, V.Campagna3, G. Cardella1, S. Cavallaro3, M. B. Chatterjee7, M. D’Andrea1, E. De Filippo1, D. De Pasquale4, A. Di Stefano3, F. Fichera1, E. Geraci8, N. Giudice1, F. Giustolisi3, A. Grzeszczuk6, N. Guardone1, A. Grimaldi1, P. Guazzoni5, E. La Guidara3, G. Lanzanò1, G. Lanzalone3, C. Maiolino3, D. Nicotra1, M. Papa1, S. Pirrone1, G. Politi1, F. Porto3, C. Rapicavoli1, E. Rosato9, F. Rizzo3, G. Rizza1, S. Russo5, P. Russotto1, S. Salamone3, G. Saccà1, M. Sassi5, W. U. Schröder10, G. Sechi2, A. Trifirò4, J. T!ke10, M. Trimarchi4, S. Urso1, M. Vigilante9, W. Zipper8, and L. Zetta5

1 INFN Catania and Dipartimento di Fisica e Astronomia, Università di Ca-tania, Italy 2 INFN Milano and Istituto di Fisica Cosmica, CNR, Milano, Italy 3 INFN-LNS and Dipartimento di Fisica e Astronomia, Università di Cata-nia, Italy 4 INFN and Dipartimento di Fisica, Università di Messina, Italy 5 INFN Milano and Dipartimento di Fisica, Università di Milano, Italy 6 Institute of Physics, University of Silesia, Katowice, Poland 7 Saha Institute Of Nuclear Physics, Kolkata, India 8 INFN Bologna and Dipartimento di Fisica, Università di Bologna, Italy 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 4π charged-particle detector array CHIMERA in its full configuration has successfully been operated at the Catania Laboratori Na-zionali 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 identifi-cation. Successful tests of specific pulse shape discrimination methods for CHIMERA silicon elements suggest that a corresponding upgrade of this de-tector array is feasible.

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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. Fragmenta-tion 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 re-action 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 4 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 Re-solving 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 system-atic 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 tele-scopes covering polar angles between 1° and 176° with full azimuth symmetry about the beam axis. The CHIMERA detector telescopes each consist of a 300-µm 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 an-gles). 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) identification. These techniques also allow for isotopic (Z, A) identification for light ions (Z<10) stopped in the second (CsI(Tl)) element of a telescope in a large angular range. 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 ener-

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gies. 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 4π configuration motivated extended experimental campaigns during the peri-ods January-July 2003 and January-July 2004 [9], in which several different ex-periments 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<10, over the entire solid angle. An illustration of the detector capability is given in Fig. 1, where the charges of fragments from the reaction 124Sn+64Ni at 35 MeV/nucleon are plotted vs. their velocity compo-nents VPAR parallel to the beam axis. A class of events is represented in Fig. 1, for which the sum of the charges of the three heaviest detected fragments is close to the total charge of the colliding projectile/target system.

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

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For orientation, approximate domains populated mainly by fragments identified as projectile-like (PLF), target-like (TLF), or those of intermediate mass (IMF), re-spectively, have been outlined in the figure.

In the corresponding experiments, parallel velocities ranging from those of relatively slow TLFs up to those of the fastest PLFs were measured with thresh-olds as low as 0.2 cm/ns. This was possible with a time-of-flight (TOF) technique, even though the 300-µm thick, planar silicon detectors of CHIMERA have a high capacitance (∼1 nF). The solid curve in the charge-velocity space of Fig. 1 demar-cates the region where the particles are fully stopped (left) from that where they punch through (right) the silicon detectors. The ∆E-E method provides charge identification with resolving power as good as ∆Z/<Z> ≈ 1 %.

In addition to a separation of p, d, t, and He isotopes, isotopic identification was also possible for light IMFs. In Fig. 2, angle-and energy-integrated average

N/Z ratios are plotted vs. atomic numbers Z, measured for products emitted from the reactions 124Sn + 27Al, from the neutron rich reaction system 124Sn + 64Ni, and from the relatively neutron poor system 112Sn + 58Ni. The latter two reactions em-ploy magic projectile and target nuclei. All reactions have been investigated at a bombarding energy of 35 MeV/nucleon. For each atomic number, the <N/Z> value was normalized to that of the most abundant natural isotope [10], corre-sponding to the bottom of the beta-stable valley.

The experimental data presented in Fig. 2 already show a number of interest-ing features. Notice, for example, the similarities in the shapes of the three sets of data, contrasted by systematically larger <N/Z> values for the two neutron rich

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

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systems relative to the neutron poorer system124Sn + 27Al. This effect appears to present evidence for a memory of the entrance isospin asymmetry and/or for in-creased isospin transfer in the reaction involving the most asymmetric system. Remarkable in the data displayed in Fig. 2 are also the different magnitudes and odd-even effects in the populations of LCPs and light IMFs. Detailed analysis of these experimental results is underway.

In spite of very interesting experimental results obtained in the REVERSE campaigns, it is of concern that isotopic identification with CHIMERA is pres-ently possible only with relatively high thresholds on the particle kinetic energy. This has been one of the reasons for choosing an inverse-kinematics situation for these measurements. In order to render CHIMERA an equally superb experimental tool also for experiments conducted in normal kinematics and to extend possible isospin physics experiments to less intense secondary beams, it is highly desirable to lower the current energy thresholds for IMF isotope identification. In the fol-lowing, results of tests are described, where a simple pulse shape method has been applied successfully to CHIMERA silicon detectors. The method promises to ex-tend the isotope identification capability of CHIMERA down to particle kinetic energies of a few MeV/nucleon. 3. APPLICATION OF A PULSE SHAPE ANALYSIS FOR PARTIC LE-ID WITH CHIMERA

The shape of signals produced in the collection of electrons and holes from a silicon detector is a rather complex phenomenon arising from various processes occurring along the path of ionizing radiation in the detector material. They result in contributions to the signal rise and delay times, due mainly to the formation time of a plasma column associated with the high ionization produced by an in-coming heavy ion and to the time delay due to the erosion (decay) of this plasma column. As a consequence, the signal rise time is closely related to the length of the ionization track and its ionization density. The shape of the electronic signal depends strongly on the side of the detector from which the particle enters, i.e., whether it is injected from the front, where the electric field is highest, or from the rear [11]. The former is the situation preferred for TOF applications. There are good prospects that a detailed analysis of the shape of the current pulse may ren-der full information regarding mass and charge of a range of particles produced in heavy-ion collisions. In particular, particle identification (PSD) methods based on rise time measurements have been suggested already in the past [12]. Recent suc-cessful applications of this method have been reported by Mutterer et al. [13]. In these studies, employing a neutron transmutation silicon (n-TD) detector technol-ogy, an energy threshold of less than 2.5 MeV/nucleon was achieved for heavy ions with charge up to Z ∼ 12. The small-area (5cm2), ∼250-µm thick, homogene-ous n-TD silicon detector employed in the above measurement was irradiated from the rear.

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Application of PSD methods to CHIMERA silicon detectors faces specific prob-lems. These detectors were made (by Eurisys Mesures, France, or by Micron, UK) from silicon produced in a float-zone technique. In the CHIMERA array, the sili-con detectors are configured for particle front injection, which is the preferred configuration for TOF measurements, because it guarantees the shortest possible rise times and therefore the best time resolution. Geometries with rear particle in-jection into the detectors provide significantly poorer timing resolution, making them less desirable for TOF measurements. This unfortunate circumstance pre-vents a straightforward combination of rise-time PSD method and TOF technique for CHIMERA.

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

Improvements in the implementation of PSD techniques in the CHIMERA setup have to comply with the following important constraints:

i) the large capacitance (~1nF) of the 25-cm2/300-# n-type planar silicon detectors;

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

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ii) a front injection configuration of the Si detectors (optimizing TOF resolution);

iii) a high silicon bulk resistivity (ρ > 5kΩ cm); iv) compatibility with the existing front end.

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

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2E MV= 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 parti-cle. 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 pa-rameterized 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 func-tion contains three fit parameters, P0, P1, and P2, and is given by the formula

( ) ( ) ( ) ( )' / 20 , , 0 , , 0 / 0 1 1 A Pi ii i

C A Z E C S R A Z E P A E E P e− = + − × − − − (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 off-set 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 varia-tion 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 sig-nals. 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 par-ticle energy, a series of dedicated experiments were undertaken using the tan-dem/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 ex-tended by a second CFD chain. In practice, the negative timing pulse from the fast main timing amplifier (T) is split using a passive splitter.

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The start signal for the rise time measurement is derived from one of the two sig-nals, 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 meas-urements [15]. The average electronic noise obtained for this preamplifier is plot-ted in Fig. 5 vs. the input (= detector) capacitance.

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

Fig. 4: Electronics used for rise time and energy meas-urement.

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4. TEST RESULTS AND DISCUSSION

In order to evaluate sensitivity and large-scale feasibility of particle identifi-

cation 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, that will presented in a separated publication. In the following, after a brief summary 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-µm thick planar silicon detector typical of those used for CHIMERA experiments.

Results of tests performed at the tandem accelerator can be summarized as fol-lows:

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 lit-tle. - 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) 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 Cyclo-tron 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 ex-periments, a combination of TOF and PSD methods is employed for particle ID. The silicon detector is configured for front particle injection, which is the configu-ration 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,

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the signal rise time is found to be nearly independent of particle charge and en-ergy. Consequently, no PSD information is available for efficient charge identifi-cation 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).

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

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

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Data shown in Fig. 7 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 reac-tion, 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 sili-con detector, information provided by the TOF measurement.

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 require-ment that the particles are stopped in the silicon detector.

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Consequently, for nuclear fragments with intermediate atomic numbers (Z ≤ 12), it is possible to achieve complete isotopic identification from simultaneous meas-urement 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 num-bers. 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. Conse-quently, and in anticipation of much increased future demand for improvements of this detector’s capabilities by the international LNS users’ community, a pro-posal to upgrade the CHIMERA detection system appears feasible, justified, and timely.

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