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Performance of the Huygens detectors at intermediate energies
NUCLEAR PHYSICS A H~:vlH~
Nuclear Physics A583 (1995) 457-460
Performance of the Huygens detectors at intermediate energies R.J.M. Snellings a, R.W. Ostendorf~, P.G. Kuijer a, T.M.V. Bootsma a, A. van den Brink a, A.P. de Haas ~, R. Kamermans ~, C.T.A.M. de Laat ~, G.J. van Nieuwenhuizen ~, C.J. Oskamp a, A. P6ghaire b, C.J.W. TwenhSfel~, J.M. Voerman a ~Universiteit Utrecht / NIKHEF Dept. of subatomic physics P.O. Box 80.000, 3508 TA Utrecht, The Netherlands bGANIL, BP 5027, 14021 Caen Cedex, France An experiment with the Huygens detectors has been performed at GANIL (France). These Huygens detectors comprise two detectors: a central detector, which consists of a time projection chamber (TPC) surrounded by a plastic scintillator barrel, and a CsI(T1) wall in the backward region. In addition, in the forward hemisphere a plastic hodoscope ('MUR') is used in conjunction with the forward plastics of the Huygens plastic scintillator barrel. In this paper first results will be presented. 1. I N T R O D U C T I O N The Huygens detectors, a central time-projection chamber surrounded by a plastic scintillator barrel and a CsI(T1)-backward wall, have been built for the study of flow and multi-fragment emission in heavy-ion collisions at intermediate energies (10-100 MeV per nucleon). These detectors in combination with a plastic scintillator hodoscope allow to study reactions at intermediate energies on an event by event basis, i.e., without introducing a priori constrains on the analysis. The detectors were placed in a large vacuum vessel, the 'Nautilus' (3.2 m in diameter, 3.6 m in length), at GANIL. The 3BAr + 4STi reaction was studied at incident energies of 45 MeV and 95 MeV per nucleon. 2. T H E D E T E C T O R
SETUP
In heavy-ion reactions at intermediate energies, the particle flux and the dynamical range in energy and charge of the emitted particles, have a strong angular dependence. Therefore each detector is designed to cope with the count rates and the dynamical range in Z and E expected in its angular region. An overview of the complete system is given in fig. 1. 2.1. T h e Central D e t e c t o r
The central detector [1,2] is a gas filled volume with microstrip read-out surrounded by 32 thick (10 cm) plastic scintillators at 30 ° < 8 < 900 (sideward) and 16 thin (1 cm) plastic scintillators at 10° < 8 < 30 ° (forward). The read-out of the central detector is 0375-9474/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved. SSDI 0375-9474(94)00704-7
458c
R.J.M. Snellings et al. I Nuclear Physics A583 (1995) 457-460
Z //,,~X
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Central
Trigger
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Figure 1. Overview of the detector setup: the Huygens detectors and the MUR
performed by dedicated electronics. Particle identification can be obtained by combining the A E from the gas, determined by the microstrips, with the corresponding E from the plastic scintillators. The polar scattering angle 8 is derived from the microstrips drift time signal. The azimuthal scattering angle ¢ is determined by combining the ¢ from the fired microstrips on a track, which results in a ¢ resolution of approximately 4 °. The ionization chamber was operated at a pressure of 60 mbar (isobuthane). The anode voltage was set at 260 V and the drift field at 100 V/cm. The pressure chosen is a compromise between particle identification and energy threshold. With a higher pressure, the number of primary electrons would increase, leading to a better resolution and a larger dynamical range in energy, it would, however, also lead to a higher energy threshold. 2.2. T h e M U R
The forward hodoscope 'MUR' [3] consist of 96 plastic scintillators covering the angular range 3° < 8 < 30 ° with respect to the beam. These 96 plastic counters are arranged in seven concentric rings. The inner two rings consist of eight counters each, while the five outer rings each contain sixteen counters. In our experiment the three inner rings were operated at a different gain than the other four, because they were not covered by the forward plastic scintillators of the Huygens central detector. The gain was chosen such that these first three rings cover 1 < Z < 8. For the other rings a coverage of 1 < Z < 2 was sufficient since particles with Z > 2 are already stopped in the forward plastics of the central detector. Fig. 2 demonstrates particle identification for the outer rings above E = 35 MeV per nucleon (particles with energies less then 35 MeV per nucleon are stopped in the forward plastics of the central detector). Particles identified in the outer four 'MUR' rings are used to calibrate the forward plastics of the central detector.
R.J.M. Snellings et al. /Nuclear Physics A583 (1995) 457-460
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150
200
250
300
350
400
Z distribution (A.U.) Figure 2. Z distribution for one MUR module in an outer ring.
3. RESULTS In this paper we present first preliminary results on the multiplicity distributions for aSAr -t- ~ T i at 45 and 95 MeV per nucleon. These multiplicities are not corrected for the detector response. In order to enhance the centrality of the selected events, the trigger was defined as a coincidence of at least two sideward plastics (30 ° < 8 _< 90°). The charged particle multiplicity distribution is normalized such that the total sum equals 1, see Fig. 3. In this figure we compare our results with results for the reaction 3SAr + SSNi at 40 and 95 MeV per nucleon measured by INDRA [4]. Qualitatively, these curves show a similar behaviour where it should be noted that our trigger tends to suppress the more peripheral events, hence the decrease at low multiplicity. Quantitatively, we can compare the maximum charged particle multiplicity divided by the total charge in the system, which would be a measure for the violence reached in the event and the efficiency of the detector setup. In the case of 95 MeV per nucleon these numbers are 36/46 = 0.78 for INDRA and 30/40 = 0.75 for our setup. These numbers nicely demonstrate the ability to measure events in which the system completely breaks up into fragments of Z = 1 and Z=2.
These results together with the good position resolution of fragments in the central drift chamber will ensure the feasability to address the questions on multi-fragment emission and flow with the Huygens detectors. 4. A C K N O W L E D G M E N T S We would like to thank the LPC (Caen, France) for making the MUR available. We further like to thank colleages from GANIL for valuable help in all phases of the experiment. This work was performed as part of a research program of the " Stichting voor Funds-
460c
R.J.M. Snellings et al. / Nuclear Physics A583 (1995) 457-460
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charged particle multiplicity Figure 3. The normalized multiplicity distributions of the complete detector setup for 45 and 95 MeV per nucleon. The open points are data from INDRA for 3SAt + SSNi at 40 and 95 MeV per nucleon. Both data sets are not corrected for the detector efficiency.
menteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (NWO).
REFERENCES 1. 2. 3. 4.
T.M.V. Bootsma et al., To be published in Nucl. Instr. and Meth. T.M.V. Bootsma, Ph.D. thesis Utrecht University (1993) G. Bizard et al., Nucl. Inst. and Meth. A244 (1986) 483. C.O. Bacri et al., Nuclear Physics GANIL 1992-1993 a compilation 164.
Nuclear Physics A583 (1995) 457-460
Performance of the Huygens detectors at intermediate energies R.J.M. Snellings a, R.W. Ostendorf~, P.G. Kuijer a, T.M.V. Bootsma a, A. van den Brink a, A.P. de Haas ~, R. Kamermans ~, C.T.A.M. de Laat ~, G.J. van Nieuwenhuizen ~, C.J. Oskamp a, A. P6ghaire b, C.J.W. TwenhSfel~, J.M. Voerman a ~Universiteit Utrecht / NIKHEF Dept. of subatomic physics P.O. Box 80.000, 3508 TA Utrecht, The Netherlands bGANIL, BP 5027, 14021 Caen Cedex, France An experiment with the Huygens detectors has been performed at GANIL (France). These Huygens detectors comprise two detectors: a central detector, which consists of a time projection chamber (TPC) surrounded by a plastic scintillator barrel, and a CsI(T1) wall in the backward region. In addition, in the forward hemisphere a plastic hodoscope ('MUR') is used in conjunction with the forward plastics of the Huygens plastic scintillator barrel. In this paper first results will be presented. 1. I N T R O D U C T I O N The Huygens detectors, a central time-projection chamber surrounded by a plastic scintillator barrel and a CsI(T1)-backward wall, have been built for the study of flow and multi-fragment emission in heavy-ion collisions at intermediate energies (10-100 MeV per nucleon). These detectors in combination with a plastic scintillator hodoscope allow to study reactions at intermediate energies on an event by event basis, i.e., without introducing a priori constrains on the analysis. The detectors were placed in a large vacuum vessel, the 'Nautilus' (3.2 m in diameter, 3.6 m in length), at GANIL. The 3BAr + 4STi reaction was studied at incident energies of 45 MeV and 95 MeV per nucleon. 2. T H E D E T E C T O R
SETUP
In heavy-ion reactions at intermediate energies, the particle flux and the dynamical range in energy and charge of the emitted particles, have a strong angular dependence. Therefore each detector is designed to cope with the count rates and the dynamical range in Z and E expected in its angular region. An overview of the complete system is given in fig. 1. 2.1. T h e Central D e t e c t o r
The central detector [1,2] is a gas filled volume with microstrip read-out surrounded by 32 thick (10 cm) plastic scintillators at 30 ° < 8 < 900 (sideward) and 16 thin (1 cm) plastic scintillators at 10° < 8 < 30 ° (forward). The read-out of the central detector is 0375-9474/95/$09.50 © 1995 ElsevierScienceB.V. All rights reserved. SSDI 0375-9474(94)00704-7
458c
R.J.M. Snellings et al. I Nuclear Physics A583 (1995) 457-460
Z //,,~X
(---(/~////
\\1~Jl Y I
/^ ~ , - , /
Central
Trigger
~ockwo~
Figure 1. Overview of the detector setup: the Huygens detectors and the MUR
performed by dedicated electronics. Particle identification can be obtained by combining the A E from the gas, determined by the microstrips, with the corresponding E from the plastic scintillators. The polar scattering angle 8 is derived from the microstrips drift time signal. The azimuthal scattering angle ¢ is determined by combining the ¢ from the fired microstrips on a track, which results in a ¢ resolution of approximately 4 °. The ionization chamber was operated at a pressure of 60 mbar (isobuthane). The anode voltage was set at 260 V and the drift field at 100 V/cm. The pressure chosen is a compromise between particle identification and energy threshold. With a higher pressure, the number of primary electrons would increase, leading to a better resolution and a larger dynamical range in energy, it would, however, also lead to a higher energy threshold. 2.2. T h e M U R
The forward hodoscope 'MUR' [3] consist of 96 plastic scintillators covering the angular range 3° < 8 < 30 ° with respect to the beam. These 96 plastic counters are arranged in seven concentric rings. The inner two rings consist of eight counters each, while the five outer rings each contain sixteen counters. In our experiment the three inner rings were operated at a different gain than the other four, because they were not covered by the forward plastic scintillators of the Huygens central detector. The gain was chosen such that these first three rings cover 1 < Z < 8. For the other rings a coverage of 1 < Z < 2 was sufficient since particles with Z > 2 are already stopped in the forward plastics of the central detector. Fig. 2 demonstrates particle identification for the outer rings above E = 35 MeV per nucleon (particles with energies less then 35 MeV per nucleon are stopped in the forward plastics of the central detector). Particles identified in the outer four 'MUR' rings are used to calibrate the forward plastics of the central detector.
R.J.M. Snellings et al. /Nuclear Physics A583 (1995) 457-460
=I
459c
J 50
I00
150
200
250
300
350
400
Z distribution (A.U.) Figure 2. Z distribution for one MUR module in an outer ring.
3. RESULTS In this paper we present first preliminary results on the multiplicity distributions for aSAr -t- ~ T i at 45 and 95 MeV per nucleon. These multiplicities are not corrected for the detector response. In order to enhance the centrality of the selected events, the trigger was defined as a coincidence of at least two sideward plastics (30 ° < 8 _< 90°). The charged particle multiplicity distribution is normalized such that the total sum equals 1, see Fig. 3. In this figure we compare our results with results for the reaction 3SAr + SSNi at 40 and 95 MeV per nucleon measured by INDRA [4]. Qualitatively, these curves show a similar behaviour where it should be noted that our trigger tends to suppress the more peripheral events, hence the decrease at low multiplicity. Quantitatively, we can compare the maximum charged particle multiplicity divided by the total charge in the system, which would be a measure for the violence reached in the event and the efficiency of the detector setup. In the case of 95 MeV per nucleon these numbers are 36/46 = 0.78 for INDRA and 30/40 = 0.75 for our setup. These numbers nicely demonstrate the ability to measure events in which the system completely breaks up into fragments of Z = 1 and Z=2.
These results together with the good position resolution of fragments in the central drift chamber will ensure the feasability to address the questions on multi-fragment emission and flow with the Huygens detectors. 4. A C K N O W L E D G M E N T S We would like to thank the LPC (Caen, France) for making the MUR available. We further like to thank colleages from GANIL for valuable help in all phases of the experiment. This work was performed as part of a research program of the " Stichting voor Funds-
460c
R.J.M. Snellings et al. / Nuclear Physics A583 (1995) 457-460
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10 -2
•
10
,
¢r
A •
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-3
•
10
•
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10
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A ~r * A
/r *
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A GM~t//. • 95MeV/u
•
,,
A j ~ l ~ j , , I
5
. . . .
10
I
. . . .
15
I
20
. . . .
~ . . . .
25
.
¢e
I , , ~ 1 ~
30
35
charged particle multiplicity Figure 3. The normalized multiplicity distributions of the complete detector setup for 45 and 95 MeV per nucleon. The open points are data from INDRA for 3SAt + SSNi at 40 and 95 MeV per nucleon. Both data sets are not corrected for the detector efficiency.
menteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (NWO).
REFERENCES 1. 2. 3. 4.
T.M.V. Bootsma et al., To be published in Nucl. Instr. and Meth. T.M.V. Bootsma, Ph.D. thesis Utrecht University (1993) G. Bizard et al., Nucl. Inst. and Meth. A244 (1986) 483. C.O. Bacri et al., Nuclear Physics GANIL 1992-1993 a compilation 164.