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EMRIC: A new setup for very small angle correlation measurements
Nuclear Instruments and Methods in Physics Research A275 (1989) 133-141 North-Holland, Amsterdam
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EMRIC: A NEW SETUP FOR VERY SMALL ANGLE CORRELATION MEASUREMENTS F. MERCHEZ, S. KOX, C. PERRIN, J. MISTRETTA, J.C . GONDRAND and L.N . IMOUK Institut des Sciences Nucléaires, 53 avenue des Martyrs, 38026 Grenoble Cedex, France
P. GRETILLAT and E. SCHWARZ
Institut de Physique de L'Université, 2000 Neuchâtel, Switzerland
Received 30 March 1988 and in revised form 22 August 1988 A new detection device for light determination of correlation functions precisely determined from information correlation function of light charged MeV/nucleon .
charged particle correlation measurements has been designed . This device permits the at very small relative momenta . In addition the angular part of the relative momenta is delivered by a multiwire chamber. This setup opens new possibilities for the study of the particles as illustrated by preliminary results for the reaction system z° Ne+ Z'A] at 30
1 . Introduction When light particles are detected m close proximity in space and time, their wave functions of relative motion are modified by final state interactions [1] and quantum statistical symmetries [2]. By measuring twoparticle correlation functions at small relative momenta (q) it should be possible to obtain information about the space-time characteristics of the subset which has emitted these particles. The detection of pairs of light particles produced by the decay of particle unbound states also provides insight into source properties . Indeed the relative populations of these states may indicate the excitation energy or temperature of the emitting system [3]. In addition, perturbations of the correlation function can yield information on the proximity of the source due to the long range Coulomb force [3] . The properties of the correlation function in the region of very small relative linear momentum are of great interest ; in addition a knowledge of the angular dependence of the relative momenta may give extended power to the measurements [1]. To better constrain the theory our objectives are threefold: (a) to extend the relative momentum range as low as possible (well below 10 MeV/c), (b) to achieve good angular resolution allowing a complete determination of q (modulus and direction), (c) to cover a large solid angle, in order to obtain good statistics in a reasonable amount of time . These conditions must be combined with a wide dynanuc range in energy and good particle discrimination . In the intermediate energy range, accurate and complete measurements in the region of small q can only be achieved by detecting the particles at very small relative angles . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Up to now the experimental devices in use were not well adapted for the determination of small relative momenta. In thus article we describe a new experimental device (EMRIC) which has been specifically designed to perform small angle correlation measurements between light charged particles in the intermediate energy range. In section 2 we describe the different parts of our setup and section 3 is devoted to the discussion of its performance. In section 4 some experimental results are discussed to show the new possibilities opened up by this setup. 2. The experimental setup The device EMRIC is composed of an array of 16 Csl(TI) scintillators (see fig. 1 (top)) used in conjunction with a multiwire proportional chamber (MWPC) . A schematic drawing of the device is also shown in fig. 1 (bottom) . Each CsI module is independent and the array can approximately fit in a spherical geometry with respect to the center of the target. The multiwire chamber allowed a precision of 0.5 X 0.5 mm2 in the determination of the impact point for particles detected in the CsI crystals over a surface of 16 X 16 cm2. A large solid angle (= 40 msr at 80 cm) is then obtained with a large number of detectors and consequently of combinations for the coincident particles. Thus this setup allows the collection of good statistics needed in this kind of experiment. The thickness of our detectors (10 cm) permits the detection of protons up to 200 MeV. The length of the dead area between two adjacent CsI detectors has been reduced to less than 3 mm . This corresponds to a relative detection angle of 0.2° at 80
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F. Merchez et al. / EMRIC: setup for angular correlation measurements
EMRIC
Fig. 1 . Top : photograph of the EMRIC setup. Bottom: schematic view of the EMRIC setup . Particles are detected in an array of 16 Csl(TI) (see top) used in conjunction with a multiwire chamber which delivers the X and Y information of the impact point .
cm which allows the measurement of relative momenta of the order of 1 MeV/c . EMRIC was placed outside of the reaction chamber behind a stainless steel window of 38 ~L m . The large size of the window (20 X 60 cmz ) enabled us to measure the correlation function over a wide angular domain . It was also possible to move the position of the target in the reaction chamber (either downstream or upstream) . Additional detectors were placed in the reaction chamber to detect events producing particles at large relative q. The beam was stopped in a movable Faraday cup allowing the detection of light particles through a thin window at the end of the tube . Careful shielding was employed in order to protect the highly sensitive MWPC from secondary particles produced in this Faraday cup . 2 .1 . The CsI scintillators The properties of CsI(TI) scintillators for particle discrimination have been known for a long time [4,5] but the interest in this type of detector has been re-
newed recently [6] . Many such crystals are now currently used for light charged particle detection in the intermediate energy range [7-9] . The choice of this type of detector has been made considering three characteristics . First this material of low hygroscopy permits its use in air and without windows thus lowering the detection threshold . Secondly it produces two components of very different decay time constants (= 400 ns and 7 [is) m its scintillation process allowing light particle discrimination by pulse shape analysis [8] without additional (AE) detectors . Finally, its price is quite low as compared to other inorganic scintillators and its mechanical resistance is good . However, CsI has a lower scintillation efficiency than Nal and the decay time constant of the slow component does not allow counting rates larger than several thousands particles per second . In addition the wavelength of the radiation produced in the CsI(TI) scintillator is larger ( = 560 rim) than that for the common range of photocathode sensitivity [10] . Our scintillators are optically coupled with a silicon rubber to a 10-stage XP2012 phototube of RTC . The diameter of this photomultiplier is close to that of the CsI scintillator allowing for a compact setup ; its gain is also well adapted for our dynamic range of detection . We have chosen not to use a light guide, this function being assumed by the CsI crystal itself. The crystal was wrapped on its side in three layers of 10 ~im white nylon to provide a diffuse reflection . An input window composed of a 10 [Lm mylar foil with a carbon deposit (= 10 11 in) was used to shield the detector from external light . No major dependence has been found on the impact point of the particles on the CsI surface . Some crystals with bad intrinsic resolution were rejected . An optical fiber was used to send light produced by a diode to the photocathode of the photubes . The diode output was regulated and used to check the stability of the gain of the photomultiplier [ll] . Particle identification is based on a pulse shape analysis of the two components (fast and slow) produced by the particles in the crystal [8] . In this technique the pulse is charge integrated during two different time gates (see in fig . 2) . The first one (GI) is set to integrate the fast component of the signal and the second one (G2) is delayed and set on the slow component . The slow component has been shown to depend on the type (mass and charge) of the detected particle thus allowing particle discrimination [8] . The charge integration of the pulse over these two gates with the specific values indicated in fig. 2 gave the best results . The information delivered by QDCs (charge to digital converters) allows the mass and charge identification for particles ranging from Z = 1 up to Z = 3 [8] . In the following we will name Sl (respectively S2) the information for the fast (respectively slow) component of the pulse .
F. Merchez et al / EMRIC: setup for angular correlation measurements
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2.2 .2 . Readout system
G1
G2
Fig. 2 . Pulse shape discrimination technique . The signal delivered by the Csl(TI) crystal is integrated m charge during two time gates GI and G2 . The timing of these gates is adjusted to separate the fast and slow components of the pulse . Two signals (Sl and S2) are delivered by QDC units (charge to digital converters) . Mass and charge discrimination is then achieved in the two dimensional map Sl-S2 .
The digital readout system is based on a fast encoder which generates the address of the center of gravity of each track . It has been described in detail elsewhere [12] and we will only recall its main features. The pulses coming from the wires are amplified and stored in fast registers if strobed by a trigger of a minimum width of 10 ns . The encoder then reads the registers, codes the address of the fired wires and determines the coordinates of the center of gravity of the cluster. Before sending it to the computer, the encoder module checks whether three conditions are satisfied ; (a) the number of clusters per plane should be less than a threshold given by the user, (b) the cluster should be limited in size, (c) the events with an empty plane can be rejected . No events are treated during the dead time of the computer . The X and Y information from the MWPC are contained m one 16-bit word and two 16-bit words per detected particle. All these words are sent to the computer for recording on magnetic tape.
2 .3 . Electronics and acquisition The thickness of our CsI crystals (10 cm) allowed the detection of protons up to 200 MeV . This range is typical of the particles produced in nuclear reactions in the intermediate energy range. Hence our setup can be used for various accelerators that deliver beams in the range 15-100 MeV/nucleon .
2.2. The multiwire chamber Good position resolution for the particles is achieved by a multiwire proportional chamber associated with a fast digital readout system .
2.2.1 . Chamber characteristics
The chamber is constituted of two 20 x 20 cmz planes giving X and Y information for the impact point of the particles . Each plane is composed of 192 gold plated tungsten wires (10 gm), separated by 1 mm (placed at 80 cm of the target, this corresponds to an angular resolution better than 0.1') . The gap between the wires and the anode plane is 5 mm. The gas mixture used is a standard argon-isobutane-freon magic gas. The total areal thickness of this chamber has been minimized to 17 mg/cmz (equivalent 12 C) which introduces an energy threshold of about 2 MeV for protons . This chamber was initially designed for the detection of charged pions at minimum ionization [12] and is also very efficient for the detection of light charged particles (even for the most energetic ones) . The counting rate supported by this chamber is high compared to the sum of those for the 16 CsI modules placed behind .
2.3 .1 . The electronics
The associated electronics, schematized in fig. 3, are composed of two distinct functions (logic and analog) . The task of the logic part is to start the acquisition whenever a given number of CsI detectors are fired . The fast signal associated with a CsI detector is obtained by inverting the pulse delivered by the last dynode of the phototube. The time pickoff is made by a constant fraction discriminator (CFD) . The resulting signals for each CsI are sent to a multiplicity unit with analog output proportional to the number of inputs (during a gate of 80 ns) . Multiplicities larger than a given value are selected by the output of a discriminator (by setting its threshold) . This module, which starts all the functions related to the acquisition, is vetoed by the general dead time corresponding to the recording of the previously selected events . The output signal is then used to start the acquisition and the encoding of the center of gravity of the wire response of the MWPC . It also starts the fast decision module which records only the parameters of detectors which have been fired [12] . As the pulses delivered by Of crystals are quite long (= 10 Ws) a special device has been designed which detects pileup during the charge to digital conversion time. This module delivers a signal which is recorded in a pattern unit for further analysis . The fast electronic units associated with the logic and readout system of the MWPC are located close to the detectors. Level translation modules are then used (NIM/ECL and ECL/NIM) to send signals to the control room on an ECL standard. These modules furnish (in the control room) the signals for the scalers . They also generate the fast and slow component
136
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F. Merchez et al / EMRIC. setup for angular correlation measurements gates for charge integration of the pulse delivered by the CsI detectors. The analog part of EMRIC's electronics is simply composed of a passive division and a variable attenuation of the signals delivered by the anode of the ghototube . The two resulting signals are sent to the QDC (charge to digital converters) . These QDC units were designed at ISN [13] and allowed 12-bit coding . This 8-channel module allows individual gates and a chose of two sensitivities for integration matched to our need for the integration of the fast and slow components of the CsI signals. 2.3.2. The acquisition system This acquisition is based on a VME system . It uses CAMAC standard for the encoders, fast decision modules, scalers and a pattern unit . Four microprocessors 68000 work in parallel, along with a microcomputer (MacIntosh from Apple) to control the experiment online. Using the MacPLINTH card developed at CERN [14] it was possible to treat a sample of the information from the VME bus via a MacVEE module . Using a Mac-CC module to control a CAMAC crate, the MacIntosh also piloted the 32-channel high-voltage LeCroy module . Information was stored on magnetic tape . Such an acquisition system is transportable and our setup can thus be used at various accelerators . 2 .4 . Energy calibration In this type of measurement we need careful energy calibrations for all the detected particles to obtain good precision for the small relative momenta. The response of CsI crystals is not linear with the energy of the detected particles (especially in the low energy domain) . In addition we must also check the stability of the calibration for a large number of detectors . A fast calibration technique is thus required for all the particles and for the whole dynamic range. Three different techniques have been checked (energy loss in a 0 E silicon detector, secondary beams, and time of flight measurement) but we will limit ourselves to the description of the last one, which gave the best results . This technique was based on a time of flight measurement of secondary light particles . A beam stop was placed well before the reaction chamber and the time of flight (TOF) of the light particles produced was measured with two thin (200 pm) plastic scintillators separated by a flight path of 5 m . These particles were then detected in the CsI detectors placed on the beam axis after the Faraday cup which was removed . Each event was characterized by three parameters - Sl, S2 (fast and slow components of the signal delivered by the CsI) and TOF - from which we can extract the type of particle and its energy . One should notice that all kinds of particles at all energies are produced in the beam
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Fig. 4 . Display of the time of flight (TOF) vs Sl map obtained with the time of flight technique (see text). The different isotopes detected in EMRIC are clearly separated and their energy (E) is calculated from the TOF information. We have only displayed the 1 .2,3H and alpha particles for the sake of clan ty
stop and that the complete calibration can be achieved in a single measurement. Typical results obtained with this technique (corrected for energy losses in the last scintillator and in the front windows of the CsI cells) are shown in figs . 4 and 5 . The map of Sl versus TOF (fig. 4) provides particle identification and consequently the determination of energy (E) . In fig. 5 one observes a nearly linear relation between S2 and E for light particles with a slight change of slope in the vicinity of 30 MeV . These features were also observed for the fast component Sl . This technique appeared as a rapid (about 15 min for one cell) and precise (time of flight resolution was about 2% for protons of 100 MeV) method of calibration.
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Fig . 5 . Energy calibration (E) vs S2 for different isotopes detected in EMRIC.
138
F. Merchez et al. / EMRIC : setup for angular correlation measurements
3. Performance 3.1 . Particle identification With the pulse shape analysis technique particles of Z < 4 have been generally identified by mass and charge in EMRIC . In fig . 6 we have displayed the map of the information corresponding to the charge integration of the fast (Sl) and slow (S2) components of the pulse delivered by the CsI(TI) crystals . It appears that particle discrimination was achieved for almost all the dynamic range in the map of SI-S2 as illustrated by fig . 6 . Typically one wanted to detect protons having up to 100 MeV . This led to the detection of 300 MeV alphas full range due to the nonlinear response of the CsI with the mass of the detected particles. The addition of a very thin sheet of plastic scintillator in front of the CsI was shown [81 to extend the charge discrimination up to Z=20 . 3 .2 . Energy resolution
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The resolution in energy for our CsI detectors has been estimated by using proton and alpha beams of well defined energies produced by the tandem of the CRN at Strasbourg (ranging between 16 and 40 MeV) and by the synchrocyclotron of Orsay (201 MeV protons and 218 MeV alphas) . At Orsay we have placed our detector in the focal plane of the spectrometer "Montpellier" . Using this spectrometer, the elastically scattered beam particles were selected with high precision . The use of various
targets ('97Au, ' z C and ' H) and angles permitted us to cover a wide energy range . The results obtained were twofold . First, for the fast component signal (SI), an energy resolution of about 1 .3% and a very linear relation with the energy were observed for protons in the range 50-170 MeV (see fig . 7) . Similar resolutions were measured for deuterons, tritons and alpha particles . This resolution slightly worsened (1 .5%) for 198 MeV protons which were stopped only at the very end of the CsI crystal . Secondly, the slow component signal (S2) showed a resolution about twice that of the fast component . This may be due to statistics (a lower number of photoelectrons produced in the phototube). We also performed a test of efficiency in coincidence with a plastic scintillator which indicated a loss of 10% for the 200 MeV protons (mainly due to geometrical problems) . For the Strasbourg run we used particles produced by Rutherford scattering of the beam on a gold target . The detectors were placed well below the grazing angle out of the reaction chamber . The measured energy resolutions are shown to figs . 8a and b for proton and alpha beams in the energy range 16-40 MeV . One can notice an improvement of the resolution as the energy increases . In this energy domain FWHM resolutions of about 600 keV were measured . One may notice that during our experiments the energy straggling in the different windows slightly worsens these resolutions for low energy particles .
Fig . 6. Identification plane of Csl(TI) crystals of EMRIC obtained with the pulse shape discrimination technique schematized m fig . 2 . The map of the fast (Sl) and slow (S2) component of the pulse clearly allows the separation of isotopes of mass < 7 .
3.3. The MWPC Each particle detected in a CsI cell is also detected in the MWPC, which allows the X and Y localization of the impact point . Each wire being independent, multiple
F. Merchez et al. / EMRIC. setupfor angular correlation measurements Protons
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24
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26
139
The detection efficiency for light charged particles in the MWPC is very close of 100% even for the most energetic ones. This was checked by selecting events having a signal in CsI and then looking for the MWPC response . Fig. 9 displays the MWPC plane (X- Y) in coincidence with the 16 CsI of EMRIC . One can clearly see the dead area between detectors which corresponds to a width of about 3 wires in X and Y coordinates . 3 .4 . Pileup rejection
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34
35
36
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Fig . 8 . Energy resolution of Csl(TI) in the energy range 16-40 MeV, (a) protons, (b) alphas . particle events were detected in this MWPC . The very small dead length between Csl detectors and the fine angular resolution of the multiwire chamber allow detection of particles with relative angle less than 0 .2 ° at 80 cm . This corresponds to relative momenta q smaller than 1 MeV/c for two protons of identical linear momentum of 200 MeV/c . The Csl detectors being placed close to the MWPC, the angular straggling m the MWPC was negligible . Events with two particles having exactly the same X or Y values in the MWPC were rejected as they could not be resolved . 200-
Y(mm)
The large solid angle and decay time constant of our CsI detectors could lead to the recording of events with two particles hitting the same detector . The control and the rejection of these erroneous events was performed off-line . First the information delivered by the MWPC (number of detected particles and their X and Y coordinates) permits to determine if a Csl detector has been fired by two particles in very close proximity of time (same beam burst) . Secondly a pattern unit word informs us of the pileup which occurred during the encoding time (10 Ws) of the signals of the Csl detectors by QDCs (see section 2 .3) . 4. Experimental results We now investigate some new representations provided by the fine angular determination obtained with our setup . For this purpose we use results obtained in an experiment performed at the SARA facility in Grenoble. The light charged particles produced in the reaction 2° Ne+ 27AI at 30 MeV/nucleon were detected in EMRIC positioned at a distance of 82 cm from the target, at an average laboratory angle of 22 ° . The resulting experimental threshold, including air and MWPC, was about 4 MeV for protons and 19 MeV for alphas . The accumulation of the data presented in figs . 10-12 was obtained in a beam period of about 10 hours (for a target thickness of 1 mg/cm2 ). 4.1 . Correlations at very small relative angles
100
X(mm) Fig . 9 . Display of the X and Y information delivered by the MWPC. The events were recorded whenever a particle fired one of the 16 Csl detectors of EMRIC . The dead area between contiguous detectors (3 mm) is clearly identified .
EMRIC has been specifically designed to obtain good detection efficiency for the events at very low relative momenta . This characteristic is very important for the study of the correlation function R (q) but also for a detailed analysis of the relative production of intermediate mass fragments excited to unbound states which subsequently decay by particle emission . In fig . 10 we show the usual correlation function constructed from the modulus of the difference of the two momenta and defined by :
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140
F Merchez et al. / EMRIC: setupfor angular correlation measurements
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where p t and p2 are the laboratory momenta of particles 1 and 2, and q is their momentum of relative motion . The normalization constant C 2 is adjusted to give R (q) = 0 for large relative momenta where correlations due to final state interactions should vanish . The a-a coincidences detected in EMRIC clearly exhibit three components. The maxima at 20, 50 and 100 MeV/c correspond respectively to the decay of 8 Be(g .s .), the ternary decay of 9Be* (2 .43 MeV) and the decay of 8 Be* (3 .04 MeV) . For the latter the use of detectors placed at higher relative angles will allow a better detection efficiency. It is also possible to display the location of the a-a coincidences detected in EMRIC. For each pair, we have reported from the center of fig. 11 the vector r = rt - r2 where r1 and r2 represent the detection coordinates of particle 1 and 2 in the plane of the MWPC. In fig. 11 we have selected the events with 5 < q < 30 MeV/c (decay of 8Be(g .s .)) to illustrate the need of a fine grained detection system. Indeed most of the a-a pairs with such low relatively momenta are detected near the contiguous borders of the cells (the modulus of r being smaller than 40 mm). 4.2. Two dimensional correlation function
The angular resolution of our detection system also allows a complete determination of the relative momentum vector of the detected particles (modulus and direction) . As an example we have constructed in fig. 12 the vector q for the deuteron-alpha coincidences . The events are displayed in the map qrqt with qt
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Fig. 11 . Spatial display of the correlation function calculated from eq . (1) for the a- a coincidences detected m EMRIC. The events are plotted as a function of their relative position in the plane of the MWPC (see text) for a relative momenta range of 5-30 MeV/c (8Be(g .s .)).
being parallel to the sum of the impulsion of the detected particles (p = p1 +p2) and qr belonging to the plane formed by the beam axis and the vector p (and orthogonal to qt). One can clearly distinguish the particles associated with the decay of the first excited state of 6Li * (2 .186 MeV) . This kind of display is of great interest to study mean field effects of the emitter on the subsequent decay of the 6 Li* [3] and can be used to study the lifetime of the source emitting light charged particles [1].
80 40 0 -40 -80 -80 -40 0 40 80 Fig. 12 . Two dimensional display of the relative momentum vector for a-deuteron coincidences . The location of the events corresponds to the decay of the first excited state of 6Li* (see text)
F. Merchez et al / EMRIC:: setupfor angular correlation measurements
5. Summary and conclusion In this contribution we have presented the main characteristics of EMRIC, a new detection system well adapted for measurements of very small relative momenta for coincident light particles. Some preliminary results have been obtained which have been used to illustrate the sensitivity of our setup and its new features . EMRIC allows the construction of the complete vector of the relative momenta of the two detected particles. In addition the distributions can be measured with high statistics . This type of detector will thus certainly be very useful in systematic and detailed studies of the emitting sources produced in heavy ion collisions in the intermediate energy range. Acknowledgements We would like to thank G. Bosson, R. Foglio, A. Maurice, L. Pitrot, J. Pouxe for their technical support. We would also like to thank G. Chesneau and J. Guillot for their help during the measurement at the synchrocyclotron of Orsay.
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EMRIC: A NEW SETUP FOR VERY SMALL ANGLE CORRELATION MEASUREMENTS F. MERCHEZ, S. KOX, C. PERRIN, J. MISTRETTA, J.C . GONDRAND and L.N . IMOUK Institut des Sciences Nucléaires, 53 avenue des Martyrs, 38026 Grenoble Cedex, France
P. GRETILLAT and E. SCHWARZ
Institut de Physique de L'Université, 2000 Neuchâtel, Switzerland
Received 30 March 1988 and in revised form 22 August 1988 A new detection device for light determination of correlation functions precisely determined from information correlation function of light charged MeV/nucleon .
charged particle correlation measurements has been designed . This device permits the at very small relative momenta . In addition the angular part of the relative momenta is delivered by a multiwire chamber. This setup opens new possibilities for the study of the particles as illustrated by preliminary results for the reaction system z° Ne+ Z'A] at 30
1 . Introduction When light particles are detected m close proximity in space and time, their wave functions of relative motion are modified by final state interactions [1] and quantum statistical symmetries [2]. By measuring twoparticle correlation functions at small relative momenta (q) it should be possible to obtain information about the space-time characteristics of the subset which has emitted these particles. The detection of pairs of light particles produced by the decay of particle unbound states also provides insight into source properties . Indeed the relative populations of these states may indicate the excitation energy or temperature of the emitting system [3]. In addition, perturbations of the correlation function can yield information on the proximity of the source due to the long range Coulomb force [3] . The properties of the correlation function in the region of very small relative linear momentum are of great interest ; in addition a knowledge of the angular dependence of the relative momenta may give extended power to the measurements [1]. To better constrain the theory our objectives are threefold: (a) to extend the relative momentum range as low as possible (well below 10 MeV/c), (b) to achieve good angular resolution allowing a complete determination of q (modulus and direction), (c) to cover a large solid angle, in order to obtain good statistics in a reasonable amount of time . These conditions must be combined with a wide dynanuc range in energy and good particle discrimination . In the intermediate energy range, accurate and complete measurements in the region of small q can only be achieved by detecting the particles at very small relative angles . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Up to now the experimental devices in use were not well adapted for the determination of small relative momenta. In thus article we describe a new experimental device (EMRIC) which has been specifically designed to perform small angle correlation measurements between light charged particles in the intermediate energy range. In section 2 we describe the different parts of our setup and section 3 is devoted to the discussion of its performance. In section 4 some experimental results are discussed to show the new possibilities opened up by this setup. 2. The experimental setup The device EMRIC is composed of an array of 16 Csl(TI) scintillators (see fig. 1 (top)) used in conjunction with a multiwire proportional chamber (MWPC) . A schematic drawing of the device is also shown in fig. 1 (bottom) . Each CsI module is independent and the array can approximately fit in a spherical geometry with respect to the center of the target. The multiwire chamber allowed a precision of 0.5 X 0.5 mm2 in the determination of the impact point for particles detected in the CsI crystals over a surface of 16 X 16 cm2. A large solid angle (= 40 msr at 80 cm) is then obtained with a large number of detectors and consequently of combinations for the coincident particles. Thus this setup allows the collection of good statistics needed in this kind of experiment. The thickness of our detectors (10 cm) permits the detection of protons up to 200 MeV. The length of the dead area between two adjacent CsI detectors has been reduced to less than 3 mm . This corresponds to a relative detection angle of 0.2° at 80
134
F. Merchez et al. / EMRIC: setup for angular correlation measurements
EMRIC
Fig. 1 . Top : photograph of the EMRIC setup. Bottom: schematic view of the EMRIC setup . Particles are detected in an array of 16 Csl(TI) (see top) used in conjunction with a multiwire chamber which delivers the X and Y information of the impact point .
cm which allows the measurement of relative momenta of the order of 1 MeV/c . EMRIC was placed outside of the reaction chamber behind a stainless steel window of 38 ~L m . The large size of the window (20 X 60 cmz ) enabled us to measure the correlation function over a wide angular domain . It was also possible to move the position of the target in the reaction chamber (either downstream or upstream) . Additional detectors were placed in the reaction chamber to detect events producing particles at large relative q. The beam was stopped in a movable Faraday cup allowing the detection of light particles through a thin window at the end of the tube . Careful shielding was employed in order to protect the highly sensitive MWPC from secondary particles produced in this Faraday cup . 2 .1 . The CsI scintillators The properties of CsI(TI) scintillators for particle discrimination have been known for a long time [4,5] but the interest in this type of detector has been re-
newed recently [6] . Many such crystals are now currently used for light charged particle detection in the intermediate energy range [7-9] . The choice of this type of detector has been made considering three characteristics . First this material of low hygroscopy permits its use in air and without windows thus lowering the detection threshold . Secondly it produces two components of very different decay time constants (= 400 ns and 7 [is) m its scintillation process allowing light particle discrimination by pulse shape analysis [8] without additional (AE) detectors . Finally, its price is quite low as compared to other inorganic scintillators and its mechanical resistance is good . However, CsI has a lower scintillation efficiency than Nal and the decay time constant of the slow component does not allow counting rates larger than several thousands particles per second . In addition the wavelength of the radiation produced in the CsI(TI) scintillator is larger ( = 560 rim) than that for the common range of photocathode sensitivity [10] . Our scintillators are optically coupled with a silicon rubber to a 10-stage XP2012 phototube of RTC . The diameter of this photomultiplier is close to that of the CsI scintillator allowing for a compact setup ; its gain is also well adapted for our dynamic range of detection . We have chosen not to use a light guide, this function being assumed by the CsI crystal itself. The crystal was wrapped on its side in three layers of 10 ~im white nylon to provide a diffuse reflection . An input window composed of a 10 [Lm mylar foil with a carbon deposit (= 10 11 in) was used to shield the detector from external light . No major dependence has been found on the impact point of the particles on the CsI surface . Some crystals with bad intrinsic resolution were rejected . An optical fiber was used to send light produced by a diode to the photocathode of the photubes . The diode output was regulated and used to check the stability of the gain of the photomultiplier [ll] . Particle identification is based on a pulse shape analysis of the two components (fast and slow) produced by the particles in the crystal [8] . In this technique the pulse is charge integrated during two different time gates (see in fig . 2) . The first one (GI) is set to integrate the fast component of the signal and the second one (G2) is delayed and set on the slow component . The slow component has been shown to depend on the type (mass and charge) of the detected particle thus allowing particle discrimination [8] . The charge integration of the pulse over these two gates with the specific values indicated in fig. 2 gave the best results . The information delivered by QDCs (charge to digital converters) allows the mass and charge identification for particles ranging from Z = 1 up to Z = 3 [8] . In the following we will name Sl (respectively S2) the information for the fast (respectively slow) component of the pulse .
F. Merchez et al / EMRIC: setup for angular correlation measurements
135
2.2 .2 . Readout system
G1
G2
Fig. 2 . Pulse shape discrimination technique . The signal delivered by the Csl(TI) crystal is integrated m charge during two time gates GI and G2 . The timing of these gates is adjusted to separate the fast and slow components of the pulse . Two signals (Sl and S2) are delivered by QDC units (charge to digital converters) . Mass and charge discrimination is then achieved in the two dimensional map Sl-S2 .
The digital readout system is based on a fast encoder which generates the address of the center of gravity of each track . It has been described in detail elsewhere [12] and we will only recall its main features. The pulses coming from the wires are amplified and stored in fast registers if strobed by a trigger of a minimum width of 10 ns . The encoder then reads the registers, codes the address of the fired wires and determines the coordinates of the center of gravity of the cluster. Before sending it to the computer, the encoder module checks whether three conditions are satisfied ; (a) the number of clusters per plane should be less than a threshold given by the user, (b) the cluster should be limited in size, (c) the events with an empty plane can be rejected . No events are treated during the dead time of the computer . The X and Y information from the MWPC are contained m one 16-bit word and two 16-bit words per detected particle. All these words are sent to the computer for recording on magnetic tape.
2 .3 . Electronics and acquisition The thickness of our CsI crystals (10 cm) allowed the detection of protons up to 200 MeV . This range is typical of the particles produced in nuclear reactions in the intermediate energy range. Hence our setup can be used for various accelerators that deliver beams in the range 15-100 MeV/nucleon .
2.2. The multiwire chamber Good position resolution for the particles is achieved by a multiwire proportional chamber associated with a fast digital readout system .
2.2.1 . Chamber characteristics
The chamber is constituted of two 20 x 20 cmz planes giving X and Y information for the impact point of the particles . Each plane is composed of 192 gold plated tungsten wires (10 gm), separated by 1 mm (placed at 80 cm of the target, this corresponds to an angular resolution better than 0.1') . The gap between the wires and the anode plane is 5 mm. The gas mixture used is a standard argon-isobutane-freon magic gas. The total areal thickness of this chamber has been minimized to 17 mg/cmz (equivalent 12 C) which introduces an energy threshold of about 2 MeV for protons . This chamber was initially designed for the detection of charged pions at minimum ionization [12] and is also very efficient for the detection of light charged particles (even for the most energetic ones) . The counting rate supported by this chamber is high compared to the sum of those for the 16 CsI modules placed behind .
2.3 .1 . The electronics
The associated electronics, schematized in fig. 3, are composed of two distinct functions (logic and analog) . The task of the logic part is to start the acquisition whenever a given number of CsI detectors are fired . The fast signal associated with a CsI detector is obtained by inverting the pulse delivered by the last dynode of the phototube. The time pickoff is made by a constant fraction discriminator (CFD) . The resulting signals for each CsI are sent to a multiplicity unit with analog output proportional to the number of inputs (during a gate of 80 ns) . Multiplicities larger than a given value are selected by the output of a discriminator (by setting its threshold) . This module, which starts all the functions related to the acquisition, is vetoed by the general dead time corresponding to the recording of the previously selected events . The output signal is then used to start the acquisition and the encoding of the center of gravity of the wire response of the MWPC . It also starts the fast decision module which records only the parameters of detectors which have been fired [12] . As the pulses delivered by Of crystals are quite long (= 10 Ws) a special device has been designed which detects pileup during the charge to digital conversion time. This module delivers a signal which is recorded in a pattern unit for further analysis . The fast electronic units associated with the logic and readout system of the MWPC are located close to the detectors. Level translation modules are then used (NIM/ECL and ECL/NIM) to send signals to the control room on an ECL standard. These modules furnish (in the control room) the signals for the scalers . They also generate the fast and slow component
136
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F. Merchez et al / EMRIC. setup for angular correlation measurements gates for charge integration of the pulse delivered by the CsI detectors. The analog part of EMRIC's electronics is simply composed of a passive division and a variable attenuation of the signals delivered by the anode of the ghototube . The two resulting signals are sent to the QDC (charge to digital converters) . These QDC units were designed at ISN [13] and allowed 12-bit coding . This 8-channel module allows individual gates and a chose of two sensitivities for integration matched to our need for the integration of the fast and slow components of the CsI signals. 2.3.2. The acquisition system This acquisition is based on a VME system . It uses CAMAC standard for the encoders, fast decision modules, scalers and a pattern unit . Four microprocessors 68000 work in parallel, along with a microcomputer (MacIntosh from Apple) to control the experiment online. Using the MacPLINTH card developed at CERN [14] it was possible to treat a sample of the information from the VME bus via a MacVEE module . Using a Mac-CC module to control a CAMAC crate, the MacIntosh also piloted the 32-channel high-voltage LeCroy module . Information was stored on magnetic tape . Such an acquisition system is transportable and our setup can thus be used at various accelerators . 2 .4 . Energy calibration In this type of measurement we need careful energy calibrations for all the detected particles to obtain good precision for the small relative momenta. The response of CsI crystals is not linear with the energy of the detected particles (especially in the low energy domain) . In addition we must also check the stability of the calibration for a large number of detectors . A fast calibration technique is thus required for all the particles and for the whole dynamic range. Three different techniques have been checked (energy loss in a 0 E silicon detector, secondary beams, and time of flight measurement) but we will limit ourselves to the description of the last one, which gave the best results . This technique was based on a time of flight measurement of secondary light particles . A beam stop was placed well before the reaction chamber and the time of flight (TOF) of the light particles produced was measured with two thin (200 pm) plastic scintillators separated by a flight path of 5 m . These particles were then detected in the CsI detectors placed on the beam axis after the Faraday cup which was removed . Each event was characterized by three parameters - Sl, S2 (fast and slow components of the signal delivered by the CsI) and TOF - from which we can extract the type of particle and its energy . One should notice that all kinds of particles at all energies are produced in the beam
4000~Si(a .u .) 4He
2000-a
0
To. F
100
(ns)
200
Fig. 4 . Display of the time of flight (TOF) vs Sl map obtained with the time of flight technique (see text). The different isotopes detected in EMRIC are clearly separated and their energy (E) is calculated from the TOF information. We have only displayed the 1 .2,3H and alpha particles for the sake of clan ty
stop and that the complete calibration can be achieved in a single measurement. Typical results obtained with this technique (corrected for energy losses in the last scintillator and in the front windows of the CsI cells) are shown in figs . 4 and 5 . The map of Sl versus TOF (fig. 4) provides particle identification and consequently the determination of energy (E) . In fig. 5 one observes a nearly linear relation between S2 and E for light particles with a slight change of slope in the vicinity of 30 MeV . These features were also observed for the fast component Sl . This technique appeared as a rapid (about 15 min for one cell) and precise (time of flight resolution was about 2% for protons of 100 MeV) method of calibration.
Energy (MeV)
160
120 3H. .
s ' (Q .u 0~
800
1600
2400
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Fig . 5 . Energy calibration (E) vs S2 for different isotopes detected in EMRIC.
138
F. Merchez et al. / EMRIC : setup for angular correlation measurements
3. Performance 3.1 . Particle identification With the pulse shape analysis technique particles of Z < 4 have been generally identified by mass and charge in EMRIC . In fig . 6 we have displayed the map of the information corresponding to the charge integration of the fast (Sl) and slow (S2) components of the pulse delivered by the CsI(TI) crystals . It appears that particle discrimination was achieved for almost all the dynamic range in the map of SI-S2 as illustrated by fig . 6 . Typically one wanted to detect protons having up to 100 MeV . This led to the detection of 300 MeV alphas full range due to the nonlinear response of the CsI with the mass of the detected particles. The addition of a very thin sheet of plastic scintillator in front of the CsI was shown [81 to extend the charge discrimination up to Z=20 . 3 .2 . Energy resolution
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Fig. 7 . Energy resolution of Csl(TI) for protons m the energy range 50-200 MeV .
The resolution in energy for our CsI detectors has been estimated by using proton and alpha beams of well defined energies produced by the tandem of the CRN at Strasbourg (ranging between 16 and 40 MeV) and by the synchrocyclotron of Orsay (201 MeV protons and 218 MeV alphas) . At Orsay we have placed our detector in the focal plane of the spectrometer "Montpellier" . Using this spectrometer, the elastically scattered beam particles were selected with high precision . The use of various
targets ('97Au, ' z C and ' H) and angles permitted us to cover a wide energy range . The results obtained were twofold . First, for the fast component signal (SI), an energy resolution of about 1 .3% and a very linear relation with the energy were observed for protons in the range 50-170 MeV (see fig . 7) . Similar resolutions were measured for deuterons, tritons and alpha particles . This resolution slightly worsened (1 .5%) for 198 MeV protons which were stopped only at the very end of the CsI crystal . Secondly, the slow component signal (S2) showed a resolution about twice that of the fast component . This may be due to statistics (a lower number of photoelectrons produced in the phototube). We also performed a test of efficiency in coincidence with a plastic scintillator which indicated a loss of 10% for the 200 MeV protons (mainly due to geometrical problems) . For the Strasbourg run we used particles produced by Rutherford scattering of the beam on a gold target . The detectors were placed well below the grazing angle out of the reaction chamber . The measured energy resolutions are shown to figs . 8a and b for proton and alpha beams in the energy range 16-40 MeV . One can notice an improvement of the resolution as the energy increases . In this energy domain FWHM resolutions of about 600 keV were measured . One may notice that during our experiments the energy straggling in the different windows slightly worsens these resolutions for low energy particles .
Fig . 6. Identification plane of Csl(TI) crystals of EMRIC obtained with the pulse shape discrimination technique schematized m fig . 2 . The map of the fast (Sl) and slow (S2) component of the pulse clearly allows the separation of isotopes of mass < 7 .
3.3. The MWPC Each particle detected in a CsI cell is also detected in the MWPC, which allows the X and Y localization of the impact point . Each wire being independent, multiple
F. Merchez et al. / EMRIC. setupfor angular correlation measurements Protons
0
24
25
26
139
The detection efficiency for light charged particles in the MWPC is very close of 100% even for the most energetic ones. This was checked by selecting events having a signal in CsI and then looking for the MWPC response . Fig. 9 displays the MWPC plane (X- Y) in coincidence with the 16 CsI of EMRIC . One can clearly see the dead area between detectors which corresponds to a width of about 3 wires in X and Y coordinates . 3 .4 . Pileup rejection
N
;Y (b)
34
35
36
Energy (MeV)
oi
39
40
Fig . 8 . Energy resolution of Csl(TI) in the energy range 16-40 MeV, (a) protons, (b) alphas . particle events were detected in this MWPC . The very small dead length between Csl detectors and the fine angular resolution of the multiwire chamber allow detection of particles with relative angle less than 0 .2 ° at 80 cm . This corresponds to relative momenta q smaller than 1 MeV/c for two protons of identical linear momentum of 200 MeV/c . The Csl detectors being placed close to the MWPC, the angular straggling m the MWPC was negligible . Events with two particles having exactly the same X or Y values in the MWPC were rejected as they could not be resolved . 200-
Y(mm)
The large solid angle and decay time constant of our CsI detectors could lead to the recording of events with two particles hitting the same detector . The control and the rejection of these erroneous events was performed off-line . First the information delivered by the MWPC (number of detected particles and their X and Y coordinates) permits to determine if a Csl detector has been fired by two particles in very close proximity of time (same beam burst) . Secondly a pattern unit word informs us of the pileup which occurred during the encoding time (10 Ws) of the signals of the Csl detectors by QDCs (see section 2 .3) . 4. Experimental results We now investigate some new representations provided by the fine angular determination obtained with our setup . For this purpose we use results obtained in an experiment performed at the SARA facility in Grenoble. The light charged particles produced in the reaction 2° Ne+ 27AI at 30 MeV/nucleon were detected in EMRIC positioned at a distance of 82 cm from the target, at an average laboratory angle of 22 ° . The resulting experimental threshold, including air and MWPC, was about 4 MeV for protons and 19 MeV for alphas . The accumulation of the data presented in figs . 10-12 was obtained in a beam period of about 10 hours (for a target thickness of 1 mg/cm2 ). 4.1 . Correlations at very small relative angles
100
X(mm) Fig . 9 . Display of the X and Y information delivered by the MWPC. The events were recorded whenever a particle fired one of the 16 Csl detectors of EMRIC . The dead area between contiguous detectors (3 mm) is clearly identified .
EMRIC has been specifically designed to obtain good detection efficiency for the events at very low relative momenta . This characteristic is very important for the study of the correlation function R (q) but also for a detailed analysis of the relative production of intermediate mass fragments excited to unbound states which subsequently decay by particle emission . In fig . 10 we show the usual correlation function constructed from the modulus of the difference of the two momenta and defined by :
EY12(Pl, P2)°C12( 1 +R(q»EYI(Pl)Y2(P2),
(1)
140
F Merchez et al. / EMRIC: setupfor angular correlation measurements
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8
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20
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100
120
140
27 AI ( 20 Ne, act) X E/A = 30 MeV eav = 22'
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,
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where p t and p2 are the laboratory momenta of particles 1 and 2, and q is their momentum of relative motion . The normalization constant C 2 is adjusted to give R (q) = 0 for large relative momenta where correlations due to final state interactions should vanish . The a-a coincidences detected in EMRIC clearly exhibit three components. The maxima at 20, 50 and 100 MeV/c correspond respectively to the decay of 8 Be(g .s .), the ternary decay of 9Be* (2 .43 MeV) and the decay of 8 Be* (3 .04 MeV) . For the latter the use of detectors placed at higher relative angles will allow a better detection efficiency. It is also possible to display the location of the a-a coincidences detected in EMRIC. For each pair, we have reported from the center of fig. 11 the vector r = rt - r2 where r1 and r2 represent the detection coordinates of particle 1 and 2 in the plane of the MWPC. In fig. 11 we have selected the events with 5 < q < 30 MeV/c (decay of 8Be(g .s .)) to illustrate the need of a fine grained detection system. Indeed most of the a-a pairs with such low relatively momenta are detected near the contiguous borders of the cells (the modulus of r being smaller than 40 mm). 4.2. Two dimensional correlation function
The angular resolution of our detection system also allows a complete determination of the relative momentum vector of the detected particles (modulus and direction) . As an example we have constructed in fig. 12 the vector q for the deuteron-alpha coincidences . The events are displayed in the map qrqt with qt
AX(mm)
Fig. 11 . Spatial display of the correlation function calculated from eq . (1) for the a- a coincidences detected m EMRIC. The events are plotted as a function of their relative position in the plane of the MWPC (see text) for a relative momenta range of 5-30 MeV/c (8Be(g .s .)).
being parallel to the sum of the impulsion of the detected particles (p = p1 +p2) and qr belonging to the plane formed by the beam axis and the vector p (and orthogonal to qt). One can clearly distinguish the particles associated with the decay of the first excited state of 6Li * (2 .186 MeV) . This kind of display is of great interest to study mean field effects of the emitter on the subsequent decay of the 6 Li* [3] and can be used to study the lifetime of the source emitting light charged particles [1].
80 40 0 -40 -80 -80 -40 0 40 80 Fig. 12 . Two dimensional display of the relative momentum vector for a-deuteron coincidences . The location of the events corresponds to the decay of the first excited state of 6Li* (see text)
F. Merchez et al / EMRIC:: setupfor angular correlation measurements
5. Summary and conclusion In this contribution we have presented the main characteristics of EMRIC, a new detection system well adapted for measurements of very small relative momenta for coincident light particles. Some preliminary results have been obtained which have been used to illustrate the sensitivity of our setup and its new features . EMRIC allows the construction of the complete vector of the relative momenta of the two detected particles. In addition the distributions can be measured with high statistics . This type of detector will thus certainly be very useful in systematic and detailed studies of the emitting sources produced in heavy ion collisions in the intermediate energy range. Acknowledgements We would like to thank G. Bosson, R. Foglio, A. Maurice, L. Pitrot, J. Pouxe for their technical support. We would also like to thank G. Chesneau and J. Guillot for their help during the measurement at the synchrocyclotron of Orsay.
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