IL NUOVO CIMENTO

V OL . 111 A, N. 8-9

Agosto-Settembre 1998

The 8 LP project at LNL( )

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G. P RETE 1 , E. F IORETTO 1 , M. C INAUSERO 1 , M. G IACCHINI 1 , M. L OLLO D. FABRIS 2 , M. LUNARDON 2 , G. N EBBIA 2 , G. V IESTI 2 , M. C ALDOGNO 2 A. B RONDI 3 , G. L A R ANA 3 , R. M ORO 3 , E. VARDACI 3 , A. O RDINE 3 A. Z AGHI 3 , A. B OIANO 3 , P. B LASID 4 , N. G ELLI 4 and F. LUCARELLI 4

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INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy INFN and Dipartimento di Fisica dell’Universit´ a di Padova, Padova, Italy INFN and Dipartimento di Scienze Fisiche dell’Universit´ a di Napoli, Napoli, Italy INFN and Dipartimento di Fisica dell’Universit` a di Firenze, Firenze, Italy

(ricevuto il 15 Giugno 1998; approvato il 28 Luglio 1998)

Summary. — A 4 detection system sensitive to light charged particles is being developed at the Laboratori Nazionali di Legnaro (LNL) for the study of the reaction mechanisms produced in heavy-ion collisions at energies up to 20 A MeV. The 8 LP apparatus is a telescope assembly characterized by a large solid angle (90% of  ) and high granularity (262 modules). Particle identification at low energy is obtained by combining E -E , TOF and PSD techniques. Thresholds for particle identification range from 1 MeV for protons and 3 MeV for alpha-particles to about 2–3 A MeV for C ions. The system is fully operational for experiments.

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PACS 29.30.Aj – Charged-particle spectrometers: electric and magnetic. PACS 29.30.Ep – Charged-particle spectroscopy. PACS 01.30.Cc – Conference proceedings.

1. – Introduction The operation of the superconductive linac at LNL allows to study reaction mechanisms at energies of 20 A MeV. In this energy range important tests of the theoretical models on the de-excitation of hot nuclei and on the behaviour of nuclear matter can be performed with exclusive experiments. To investigate the decay of hot nuclei it is of crucial importance the detection of all the emitted particles to determine the excitation energy of the compound nucleus and to identify the reaction mechanism [1]. The measurement of light particles is a powerful tool for the investigation of many aspects related to the

( ) Paper presented at the XVI Nuclear Physics Divisional Conference Structure of Nuclei Under

Extreme Conditions, SNEC 98, Padova, March 31 - April 4, 1998. G

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Fig. 1. – The 8 LP apparatus.

formation, properties and de-excitation of hot rotating nuclei [1, 2]. Some of the current subjects of investigation are the competition between different de-excitation mechanisms and the dynamics of the fission process. The statistical model, as implemented in the available codes, is one of the main tools of interpretation. The limits of validity and its physical ingredients are still the subject of open questions. Very stringent tests of the statistical model require the measurement of energy spectra and multiplicity of light particles with high-statistics and low-energy thresholds. In addition, exclusive measurements involving the detection of particles in coincidence with evaporation residues, fission fragments, intermediate-mass fragments, gamma-rays, etc. require a large solid angle array of detectors. Often the channels of interest correspond to very low cross-sections. To this end a new light charged particle detection array with an angular coverage close to 4 , low identification energy thresholds and high granularity has been developed. 2. – Design of the apparatus The apparatus was designed to cover a solid angle as close as possible to 4 , with room enough to add heavy-ion trigger detectors in the scattering chamber and -array and neutron detectors outside. The light particle hodoscope is a set-up of 262 E -E telescopes organized as shown in fig. 1. The telescopes are made by 300 m Si detector followed by CsI(Tl) crystals 15 mm or 5 mm thick which allow the detection of light charged particles with energies up to 64 A MeV and 34 A MeV, respectively.  telescopes, missing the four at the corners and the The Wall is a matrix of central one for the exit hole of the beam. The detectors are all identical in shape. Each telescope has an active area of 25 cm2 , an angular coverage of 4 and subtends a solid angle of about 7 msr. Each module is independently fixed to an aluminium support structure. The Ring telescopes are mounted onto four mechanically independent supports which realize an extension of the Wall edges. The Ball has the same geometry of the “Indiana silicon sphere” [3] and consists of 7 rings coaxial around the beam axis. The detectors have 4 different trapezoidal shapes with an active area ranging from 7.2 cm2 to 17.8 cm2 , subtending a solid angle from 32 msr to 79 msr. The mechanical support is a honeycomb brass structure in which the telescopes are housed.




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The scattering chamber has a spherical shape with removable lids made of 3mm thick aluminum. This configuration gives an easy access to all detectors and connections and allows the use of external and neutron detectors. 3. – Identification techniques







The E -E technique implies an identification threshold due to the thickness of the E detector (about 6 A MeV for light particles in our configuration). To lower this threshold it is necessary to couple other identification methods to the E -E one in order to discriminate protons and alpha-particles stopped in the first element of the telescope. To this end we use the time-of-flight for the Wall and Ring detectors which are 60 cm and 40 cm apart from the target; the start reference signal comes from the accelerator RF or high-efficiency detectors. TOF method is no more effective for the Ball detectors due to the small distance from the target (15 cm) and the quite poor timing resolution obtained with large-area Si detectors at low energy. To overcome this problem the Pulse Shape Discrimination method was chosen. The particle discrimination by PSD analysis has been recently applied to silicon detectors [4, 5]. The technique is based on the difference of rise time for particles having different stopping power. In fact, the total charge collection time reflects the rise time of the output signals. The discrimination is enhanced if particles are impinging on the ohmic side (generally named “rear side”). This is due to the lower electric field in the entrance region and the lower velocity of the holes which are the main responsible for the signal formation. To perform PSD the Ball Si detectors are mounted with the rear side facing the target. From the electronic point of view the PSD was obtained using the same electronics as the TOF, but changing the fraction and the internal delay of the Constant Fraction Discriminator to have an output timing signal sensitive to the rise time of the input signal.




4. – System performances Several in-beam runs have been performed at the Tandem-Alpi facility of the Laboratori Nazionali di Legnaro to test the particle identification capability of the 8 LP basic modules. We have irradiated typical modules of the Wall, Ring and Ball [6] with light particles and intermediate-mass fragments produced in the reactions 12 C + 27 Al (98 MeV), 12 C + 58 Ni (98 MeV) and 58 Ni + 58 Ni (345 MeV). The Si energy calibrations were obtained by means of an alpha source and a calibrated pulser. The CsI energy calibrations were deduced from the measured energy loss in the transmission silicon detectors using their thickness and a stopping power code. A calibration point for high-energy protons was obtained using the elastically scattered protons produced by a 98 MeV 12 C beam on a polypropylene target [7]. In all these runs the time measurements were carried out using as start the prompt -rays detected in a cluster of 19 BaF2 (Dt = 1 ns) placed at 25 cm from the target. The stop timing signal was provided by the first element of the telescope ( E ). Good mass and charge identification of light particles (p, d, t and -particles) has been obtained with all the identification methods. Figures 2 and 3 show typical scatter plots for TOF and PSD, respectively. Time resolutions of 2 ns with 10 MeV -particles and an energy threshold of 1 MeV for protons, 3 MeV for -particles and 2–3 A MeV for intermediate-mass fragments have been measured. The total energy spectra for protons and alpha-particles are shown in figs. 4a) and b).




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Fig. 2. – Discrimination by the time-of-flight technique: protons and alphas produced in the reaction  . The figure shows a scatter plot of the TOF vs. the 12 C +27 Al at 98 MeV and detected at lab energy deposition E measured with the first stage (Si detector) of a Wall module.




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Fig. 3. – Discrimination by the PSD method: Light particle and intermediate-mass fragments pro duced in the reaction 12 C+58 Ni at 98 MeV and detected at lab . The figure shows a scatter plot of the PSD vs. the energy deposition DE measured with the first stage (Si detector) of a Ball module. The vertical band from 0 to 18 MeV is due to accidental coincidences.

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Fig. 4. – Total energy spectra for protons (a) and alpha-particles (b) produced in the reaction 12 C +  . The results of the PACE statistical 27 Al at 98 MeV and detected in the Wall module at lab code are also shown for comparison.

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They are compared with the results of statistical model calculations performed with the PACE code. The spectral shapes are reasonably well reproduced by the calculations. The PSD technique has been tested in several conditions of bias voltage to study the effect of the ballistic deficit on the shape of the spectra. No significant differences between the shape of the spectra for protons and alphaparticles have been observed for bias voltages equal or greater than the full depletion voltage. 5. – Evaporation residues trigger detector The 8 LP system was designed to host trigger detectors with the aim to select different reaction channels. In the forward direction Evaporation Residues (ER), Fission Fragments (FF), quasi-elastic and elastic products are present and one of the main requests is to develop a trigger detector able to select ER among other products. Due to the high rate produced by the elastic scattering, gas detectors are the best choice to overcome the radiation damage problem. A detector made of a Parallel Plate Avalance Counter (PPAC) followed by a Bragg Chamber (BC) was designed and time of flight vs. range measurement was used to discriminate the detected particles. Both PPAC and BC were operated in the same gas volume and I-Butane was used as stopping medium. The detector has an active area of 16 cm2 and the active length of the BC was 7 cm. To have a compact design, G10 sheets glued together were used for the detector body; the BC guard rings were obtained with standard photoengraving procedure on the body itself. A test run was performed at the XTU Tandem using the reaction 196 MeV 32 S + 100 Mo. The detector was lodged in the 8 LP system at 60 cm from the target substituting a WALL element at 5o. The start signal for the time-of-flight was supplied by the BaF2 cluster and the stop signal came from the PPAC. With a current of 1 nA and a target thickness of 200 mg/cm2 the counting rate of the trigger detector was 250 Kcps, mainly due to the elastic scattering of the beam. At this high rate the slow energy signals of the BC suffer from a heavy pile-up and are not useful for discrimination. For the range mea-

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Fig. 5. – PPAC time of flight vs. BC range for the reaction 32 S + 100 Mo at lab =5 . ER are in region 1 and FF are in region 2. The vertical band on the left is due to uncorrelated elastic scattering ions.

surement the time difference between the PPAC signal and the BC anode signal, analysed by a timing filter amplifier and a constant fraction discriminator, was used. In this way a signal proportional to the quantity L , R is obtained, where L is the BC active length and R is the range of the particle. Operating the detector at 120 torr the ER are completely stopped in the active volume of the BC and are discriminated from the other products in the scatter plot time-of-flight vs. L , R . A typical on-line matrix is shown in fig. 5 where ER (region 1) are clearly identified from FF (region 2) and uncorrelated elastic scattered ions (vertical band on the left of the matrix).

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6. – Electronics and acquisition system Low-cost, high-density, remote-controlled electronics was used to process the signals. The preamplifier signals are shaped and amplified in a 16-channel Silena 761F amplifier. These are NIM modules which provide slow linear and fast timing outputs and are remotely controlled by a RS-485 serial line. The amplifiers also house an analogue multiplexer for diagnostic purpose. The linear outputs are sent to a 32-channels Silena ADC. The fast timing outputs of the amplifiers are sent to a 16-channels CAMAC constant fraction discriminator (Caen C208). The timing signals are converted by 32-channels Silena TDC. The front-end is based on the FAIR bus system developed by INFN, Sezione di Napoli [8, 9]. FAIR (FAst Intercrate Readout) is a new trigger and readout oriented bus system. It is based on a hardware level protocol which allows both event building and synchronous data transfer without the need of CPUs. The data transfer rate, which is programmable, can be as fast as 25 ns/longword. The FAIR bus system is a natural highly extendible and scalable multi-crate system and can manage a large number of pa-

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rameters. It can be used either as a stand-alone bus or in the user-defined part of the VME V430 bus as a plug-in backplane. The 32-channels Silena ADCs and TDCs have full interface to FAIR system as well as VME bus. 7. – Control system The complexity of the apparatus demands an automated control system that allows the user to set, optimize, and monitor all the parameters. In particular, it is also required that the control system has to be able to manage different protocols of communication like RS-485, GPIB and CAENET. To this end we developed a control system in the LabView (National Instruments, Version 4.0.1) environment running on Macintosh and PC. We have designed the control system in the framework of a Master/Remote architecture and of the TCP/IP protocol. Using a GPIB-ENET (National Instruments) interface we control directly the GPIB Camac Crate Controllers (Kinetic Systems 3988). The Master is the computer physically connected to the hardware via GPIB and RS485 and it runs the programs that perform system configuration/control and database storage. Remote computers are connect via Ethernet with the Master, and run a user frendly interface to manage the controls. Some of the main tasks of the control program are: – setting and monitoring the high voltages; – setting and monitoring the telescope electronics; – monitoring the fault state of the power supplies of the NIM-BIN and CAMAC crates, and of the high voltage mainframes; – the detector database that is dynamically modified to reflect the changes in the physical apparatus. These control sub-systems are embedded in the 8 LP main control panel, as many other auxiliary instruments. They are accessible through pull-down menus or buttons. 8. – Conclusions We developed a new 4 light-particle detection system with low identification thresholds. This apparatus will allow to perform exclusive measurements dedicated to the study of reaction mechanisms at energies available with the new ALPI Linac at LNL. The detector is now operational and the first experiments will run in the first half of 1998.

REFERENCES H ILSCHER D. and R OSSNER H., Ann. Phys. (Paris), 17 (1987) 471. PARKER W. E. et al., Phys. Rev. C, 44 (1991) 774. KWIATKOWSKI K. et al., Nucl. Instrum. Methods A, 360 (1995) 571. PAUSCH G. et al., Nucl. Instrum. Methods A, 349 (1994) 281. PAUSCH G. et al., Nucl. Instrum. Methods A, 365 (1995) 176. F IORETTO E. et al., LNL Annual Report (1993), p. 178. F IORETTO E. et al., LNL Annual Report (1994), p. 196. B OIANO A., O RDINE E. and VARDACI E., Proceedings of the Fifth International Conference on Electronics for Particle Physics, 10 - 11 May 1995 (Lecroy Corporation, Chestnut Ridge, New York), p. 51. [9] F IORETTO E. et al., IEEE Trans. Nucl. Sci., 44-3 (1997) 1017.

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