Performance of upstream interaction region detectors for the FIRST experiment at GSI

2012 JINST 7 P02006 PUBLISHED BY IOP PUBLISHING FOR SISSA RECEIVED:November 25, 2011 REVISED:January 10, 2012 ACCEPTED:January 20, 2012 PUBLISHED:February 13, 2012 Performance of upstream interaction region detectors for the FIRST experiment at GSI Z. Abou-Haidar,mC. Agodi,dM.A.G. Alvarez,mM. Anelli,aT. Aumann,vG. Battistoni,a A. Bocci,mT.T. B¨ ohlen,r,sA. Boudard,qA. Brunetti,b,kM. Carpinelli,b,k G.A.P. Cirrone,dM.A. Cortes-Giraldo,xG. Cuttone,dM. De Napoli,dM. Durante,v J.P. Fern´ andez-Garc´ıa,xC. Finck,pM. I. Gallardo,xB. Golosio,b,kE. Iarocci,c,h F. Iazzi,f,iG. Ickert,vR. Introzzi,fD. Juliani,pJ. Krimmer,yN. Kurz,vM. Labalme,o Y. Leifels,vA. Le F` evre,vS. Leray,qF. Marchetto,fV. Monaco,f,wM.C. Morone,g,z P. Oliva,b,kA. Paoloni,cV. Patera,a,bL. Piersanti,c,hR. Pleskac,vJ.M. Quesada,x N. Randazzo,jF. Romano,d,uD. Rossi,vV. Rosso,l,nM. Rousseau,pR. Sacchi,f,w P. Sala,aA. Sarti,c,h,1C. Schuy,vA. Sciubba,c,hC. Sfienti,d,sH. Simon,vV. Sipala,j,k E. Spiriti,eL. Stuttge,pS. Tropeadand H. Younisf,i aIstituto Nazionale di Fisica Nucleare - Sezione di Milano, Milano, Italy bIstituto Nazionale di Fisica Nucleare - Sezione di Cagliari, Cagliari, Italy cIstituto Nazionale di Fisica Nucleare - Laboratori Nazionali di Frascati, Frascati, Italy dIstituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud, Catania, Italy eIstituto Nazionale di Fisica Nucleare - Sezione di Roma 3, Roma, Italy fIstituto Nazionale di Fisica Nucleare - Sezione di Torino, Torino, Italy gIstituto Nazionale di Fisica Nucleare - Sezione di Roma Tor Vergata, Roma, Italy hDipartimento di Scienze di Base e Applicate per l’Ingegneria, “La Sapienza” Universit` a di Roma, Roma, Italy iDipartimento di Fisica, Politecnico di Torino, Torino, Italy jIstituto Nazionale di Fisica Nucleare - Sezione di Catania, Catania, Italy kUniversit` a di Sassari, Sassari, Italy lIstituto Nazionale di Fisica Nucleare - Sezione di Pisa, Pisa, Italy mCNA, Sevilla, Spain nUniversit` a di Pisa, Pisa, Italy oLPC-Caen, ENSICAEN, Universite de Caen, CNRS/IN2P3, Caen, France pInstitut Pluridisciplinaire Hubert Curien, Strasbourg, France qCEA-Saclay, IRFU/SPhN, Gif sur Yvette Cedex, France 1Corresponding author. c 2012 IOP Publishing Ltd and SISSA doi:10.1088/1748-0221/7/02/P02006 2012 JINST 7 P02006 rEuropean Organization for Nuclear Research CERN, Geneva, Switzerland sMedical Radiation Physics, Karolinska Institutet and Stockholm University, Stockholm, Sweden tUniversitat Mainz Johann-Joachim-Becher, Mainz, Germany uCentro Studi e Ricerche e Museo Storico della Fisica “Enrico Fermi”, Roma, Italy vGSI Helmholtzzentrum fur Schwerionenforschung, Darmstadt, Germany wDipartimento di Fisica, Universit` a di Torino, Torino, Italy xDepartamento de Fisica Atomica, Molecular y Nuclear, University of Sevilla, 41080-Sevilla, Spain yIPN-Lyon, Universite de Lyon, Universite Lyon 1, CNRS/IN2P3, Villeurbanne, France zDipartimento di Biopatologia e Diagnostica per Immagini, Universit` a di Roma Tor Vergata, Roma, Italy E-mail: [email protected] ABSTRACT: The FIRST (Fragmentation of Ions Relevant for Space and Therapy) experiment at GSI has been designed to study carbon fragmentation, measuring 12C double differential cross sections (∂2s/∂q∂E) for different beam energies between 100 and 1000 MeV/u. The experimental setup integrates newly designed detectors in the, so called, Interaction Region around the graphite target. The Interaction Region upstream detectors are a 250 µm thick scintillator and a drift chamber optimized for a precise measurement of the ions interaction time and position on the target. In this article we review the design of the upstream detectors along with the preliminary results of the data taking performed on August 2011 with 400 MeV/u fully stripped carbon ion beam at GSI. Detectors performances will be reviewed and compared to those obtained during preliminary tests, performed with 500 MeV electrons (at the BTF facility in the INFN Frascati Laboratories) and 80 MeV/u protons and carbon ions (at the INFN LNS Laboratories in Catania). KEYWORDS: Wire chambers(MWPC, Thin-gap chambers, drift chambers, drift tubes, proportional chambers etc); Gaseous detectors; Scintillators, scintillation and light emission processes (solid, gas and liquid scintillators); Particle tracking detectors (Gaseous detectors) –1– 2012 JINST 7 P02006 Contents 1 Introduction 1 2 The experimental setup 2 2.1 The Beam Test Facility (BTF) 2 2.2 The LNS zero degree test setup 2 2.3 DAQ system 2 3 The Start Counter 3 3.1 Start Counter performances: efficiency and time resolution 4 4 The Beam Monitor drift chamber 6 4.1 Track reconstruction 7 4.2 Hit detection efficiency 8 4.3 Spatial resolution 9 5 Detector performance in the FIRST experiment at GSI 11 5.1 Start Counter preliminary results 11 5.2 Beam Monitor preliminary results 12 6 Conclusions 14 1 Introduction The FIRST (Fragmentation of Ions Relevant for Space and Therapy) experiment [1] is performed at the GSI heavy ion synchrotron (SIS) in Darmstadt. Its aim is to measure carbon fragmentation on nuclei at different energies between 100 and 1000 MeV/u. The study of fragmentation processes is relevant to different fields of physics, including both basic research and applications. In particular, different reaction mechanisms can be better understood by means of the analysis of double differential cross sections as a function of the energy and of the angle with respect to the beam. The FIRST experimental setup consists of two main regions: the Magnet Region and the Interaction Region. While the former integrates an already existing setup composed by a dipole magnet (ALADiN, [2]), a TPC (Music IV, [3]), a neutron detector (LAND, [4]) and a time of flight scintillator wall, the latter is made up of newly designed detectors installed around the target. The FIRST setup aims for a suitable particle identification capability providing a sM M10%, (where M is the fragment mass). A 10% relative uncertainty on the fragment mass is mandatory in order to have a clear separation of all the ions and isotopes under study. The required precision on fragment mass directly translates into requirements on the time and momentum resolution for all the detectors used in the FIRST experiment. The mass measured in the spectrometer is given –1– 2012 JINST 7 P02006 by M=k|R|f(b)where k=0.3Z m0and f(b)=p1b2 b=1 bg. The relative uncertainty on M is hence related to the time and momentum resolutions by the relation ⇣sM M⌘2=✓sp p◆2 +⇣gst t⌘2 (1.1) where sp p=sR Rhas been used. The Interaction Region target upstream detectors are a thin scintillation counter (Start Counter), designed for triggering and timing purposes, a drift chamber (Beam Monitor) dedicated to a precise ion impinging point measurement and a pre-target fragmentation detection, while the downstream ones are a silicon Vertex Detector and a thick scintillator (Proton Tagger) used to detect light fragments produced at large angles. In this paper we review the design of the upstream detectors as well as test beam results obtained with 500 MeV electrons (at the Beam Test Facility of the INFN LNF Laboratories in Frascati) and with 80 MeV/u protons and carbon ions (at the Zero Degree Line of the INFN LNS Laboratories in Catania). Section 2describes the experimental setup, the test beam facilities and the DAQ for the two tests that have been performed. The Start Counter is described in section 3, the Beam Monitor in section 4. The preliminary results of the FIRST experiment obtained at GSI are shown in section 5. 2 The experimental setup 2.1 The Beam Test Facility (BTF) The drift chamber has been tested at the BTF experimental facility (figure 1), with 500 MeV electrons extracted in 10 ns spills at a frequency of 25 Hz. In each spill the number of electrons followed a Poisson distribution with Ne=1. A lead glass calorimeter (Beam Dump) was used to select single electron events. An example of the calorimeter charge distribution measured during the spills is shown in figure 2. The RF signal from the accelerator has been used to trigger the acquisition. 2.2 The LNS zero degree test setup The experimental setup at the LNS zero degree line is shown in figure 3. 80 MeV/u protons and fully stripped 12C ions were delivered at ⇠1 MHz, in order to reproduce GSI-like conditions. A 4⇥4⇥8 cm3plastic scintillator (BC-404) was used as beam dump as well as a trigger. The event rate was ⇠1 kHz, due to the electronics dead time. 2.3 DAQ system Both tests on beam shared the same NIM and VME electronics. While the former is used for the trigger logic and handles the dead time of the DAQ, the latter comprises a 64 channels multihitmultievent CAEN V1190B TDC (with 100 ps time resolution and 1 µs window width) and a 16 channels CAEN V792N QDC (with 0.1 pC resolution over a charge conversion window ranging from 0 to 400 pC). Data acquisition is performed by means of a Motorola MVME5100 CPU. The low voltage for the electronics of the Start Counter and the Beam Monitor is supplied by a low ripple low noise quad independent DC channel GW INSTEK GPS4303, while a CAEN N1470 –2– 2012 JINST 7 P02006 Figure 1. Experimental setup at the BTF facility; the position of the various detectors is shown, as well as the other elements of the setup. Charge (pC) 0 50 100 150 200 250 300 350 400 Entries / 1 pC 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 Figure 2. Typical BTF beam dump calorimeter charge histogram. The various peaks shown in the histogram are related to 0, 1, ... , n detected particles. four channels programmable HV power supply provides Beam Monitor high voltage. Every signal coming from the Start Counter has been split in order to measure both time and charge. Signals coming from the Beam Monitor are discriminated and then read by a TDC. Trigger time signal is also acquired in order to fix a time reference. During tests, temperature and pressure have been recorded by a barometric weather station. 3 The Start Counter The Start Counter has been designed for triggering and timing purposes. It has been optimized through a careful balancing of detector time resolution (time of flight measurements will be used to discriminate the different fragments) and thickness minimization. The design goal was to have a pre-target particle interaction probability of less than 1% with respect to the on-target one, reducing to a negligible amount the Start Counter contribution to the cross section measurement systematics. –3– 2012 JINST 7 P02006 Figure 3. Radioactive ion beam facility at INFN LNS. The experimental setup at the Zero Degree Line is shown as well as the other elements of the setup. In FIRST a 5-8 mm thick graphite target was foreseen, so the final Start Counter layout, taking into account the different material density and interaction cross sections, was designed as a 250 µm thick disc, 52 mm diameter, of plastic scintillator (EJ-228). Light is collected by means of 160, 1 mm diameter, step-index plastic optical fibers (reference standard: IEC 60793-2-40) radially glued and grouped in four bundles, and finally connected to four fast Hamamatsu UBA H10721201 photomultipliers (40% quantum efficiency). The fibers have a low attenuation (<200 dB/km) for the scintillator output wavelength (400 nm) and their layout has been chosen in order to maximize the light collection from the scintillator. A picture of the Start Counter is shown in figure 4. A temporal resolution of the order of 250 ps (standard deviation) must be achieved, in order to fulfill the precision on fragment time of flight measurement required by FIRST. The signals coming from the four photomultipliers are amplified by a factor of twenty by means of custom electronics boards, designed by the LNF electronics workshop and embedded in the detector. During LNS test beam the Start Counter has been tested, scanning different thresholds and different PMT gain values, in order to find the most suitable working point. 3.1 Start Counter performances: efficiency and time resolution Two main parameters have been chosen to benchmark the Start Counter: efficiency and time resolution. The efficiency has been defined as the fraction of passing particles that fire at least 3 PMTs out of 4. –4– 2012 JINST 7 P02006 Figure 4. Start Counter assembly. PMT time and charge information have been acquired in order to carry out time slewing correction on carbon ions, so as to refine and adjust the final results. The charge-time correlation distributions that have been used to compute the correction are shown in figure 5, choosing as fitting function: T=p0+p2 (Qp1)2.(3.1) No time slewing correction has been applied on proton data, since their measured charge variation was not appreciable. To estimate the time resolution, after equalization of the delays of the four TDC channels, we performed a Gaussian fit to the time difference distribution of two Start Counter readout photomultipliers, as shown in figure 6. All the possible combinations of photomultipliers were used to measure the time resolutions, giving results consistent within 10 ps. Using the data sample taken at LNS, both efficiency and time resolution have been evaluated as functions of the discrimination threshold with protons (⇠1 M events) and with carbon ions (⇠1.5 M events). The results are shown in figure 7. Since the energy release of 12C in the FIRST experiment is comprised between that of protons (lower limit) and of 80 MeV/u carbons (upper limit) available at LNS, the results obtained with p and 12C must be interpreted as worst and best-case scenarios, respectively, for assessing detector performances. For this reason the efficiency evaluated with carbon ions is very promising, showing a flat distribution consistent with 100% in the full threshold range, while the results obtained with protons are useful to constrain detector limitations. –5– 2012 JINST 7 P02006 Charge (pC) 100 150 200 250 300 350 400 Time (ns) -22.2 -22 -21.8 -21.6 -21.4 -21.2 -21 / ndf 2 χ 110.591 / 65 p0 0.003± -22.041 p1 3.238± -51.058 p2 501.902± 11077.324 Figure 5. Charge-time correlation distribution of a Start Counter PMT. t (ns)Δ -23 -22.8 -22.6 -22.4 -22.2 -22 -21.8 -21.6 -21.4 -21.2 -21 Entries / 0.05 ns 0 1000 2000 3000 4000 5000 6000 Constant 44.7866± 6358.8979 Mean 0.0006± -22.1058 Sigma 0.0004± 0.1007 Figure 6. Time difference (Dt) distribution of two Start Counter readout photomultipliers, obtained on LNS data (Carbon ions). A fit with a Gaussian function is superimposed. 4 The Beam Monitor drift chamber The Beam Monitor, shown in figure 8, is a drift chamber designed for charged particles trajectory reconstruction. This detector provides the ion impinging point on the target, together with valuable information on possible projectile pre-target fragmentation. The requirements for fragmentation measurements in FIRST are a single cell spatial resolution <200µm with an high particle detection efficiency. The detector is made of alternated horizontal and vertical wire layers (or planes). Each layer is composed of three rectangular cells, 16 mm ⇥10 mm along the beam direction, for a total of 36 sense wires (see figure 9). The geometrical layout has been optimized in order to minimize ions interactions with the wires still maintaining the required cell resolution. The twelve planes (six on each “view”) provide tracking redundancy and ensure a high tracking efficiency and an excellent spatial resolution. In order to minimize tracking ambiguities, the consecutive layers of each view –6– 2012 JINST 7 P02006 Threshold (mV) 20 40 60 80 100 120 140 160 180 200 Efficiency 0.5 0.6 0.7 0.8 0.9 1 Protons Carbon Ions Threshold (mV) 20 40 60 80 100 120 140 160 180 200 Time Resolution (ps) 50 100 150 200 250 300 350 400 Protons Carbon Ions Carbon Ions (after TS corr.) Figure 7. Start Counter performances measured at LNS: the top picture shows the efficiency versus threshold for protons (full triangle) and carbon ions (full cross); the bottom picture shows the time resolution versus threshold for protons (full triangle), carbon ions with (full cross) and without (full circle) time slewing correction. Error bars (statistical uncertainty) are smaller than marker size. are staggered by half a cell. Custom front-end electronics boards, designed by the LNF electronics workshop, are embedded in the detector and provide wire signal amplification by a factor of 10. Different gas mixtures, as well as operating high voltages, have been tested on electron, proton and carbon ion beams in order to choose the most suitable operating point. Hydrocarbons free mixtures are preferred because of safety issues. 4.1 Track reconstruction The hits recorded by the Beam Monitor are used to reconstruct the trajectories of charged particles in the detector active volume; furthermore, such information is fundamental to discriminate –7– 2012 JINST 7 P02006 x (cm) 0.2 0.4 0.6 0.8 11.2 y (cm) -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 0 1000 2000 3000 4000 5000 6000 7000 Figure 18. GSI beam spot as reconstructed by the drift chamber. x BS position (cm) -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 y BS position (cm) -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 Figure 19. Mean beam spot position (BS), in the x,y plane, recostructed using the Beam Monitor from the full FIRST data sample. Each point is related to a given run. Finally, after all the tracks have been reconstructed, we could measure the GSI beam spot as it was seen by the drift chamber just before the target, shown in figure 18. The mean beam spot position, as reconstructed in the various runs taken from FIRST experiment is shown in figure 19, showing that variations of few millimiters occurred during the data taking. 6 Conclusions We have developed the upstream Interaction Region detectors for the FIRST experiment: the Start Counter, a fast scintillator (250 µm thick) used for trigger and timing purposes and the Beam Monitor, a drift chamber used for particle trajectories tracking. – 14 – 2012 JINST 7 P02006 A functional test of FIRST upstream Interaction Region detectors has been carried out at the LNF Beam Test Facility and at the LNS Zero Degree Line. The aim of the test was to verify the Beam Monitor and Start Counter performances as well as to find the most suitable working point in GSI-like conditions. Both detectors have been successfully operated, fulfilling FIRST requirements: a Start Counter time resolution less than 250 ps and a Beam Monitor spatial resolution of the order of 100 µm have been measured in a wide range of configurations. The FIRST experiment took data on August 2011, performing 400 MeV/u 12C on 8 mm graphite collisions (⇡18 M events) and 12C on Au (⇡2 M events). Both the detectors ran smoothly, showing a very stable behavior for the entire duration of the experiment. The detectors have completely matched the expectations: the measured time and spatial resolution are respectively st⇡150 ps and sx⇡140 µm. Acknowledgments We would like to thank B. Buonomo L. Foggetta, A. Mengucci and F. Romano for their generous help in the preparation and pursuance of the BTF and LNS test. We would like to acknowledge M. Arba, L. La Delfa and M. Tuveri (INFN Sez. Cagliari), M. Anelli, S. Cerioni, G. Corradi, D. Riondino and R. Rosellini (INFN, LNF) , M. Magi (Dipartimento di Scienze di Base e Applicate per l’Ingegneria, Universit` a di Roma “La Sapienza”), M. Capponi and A. Iaciofano (INFN, Sez. Roma3) for the technical design and mechanical work on the Interagion Region, and Filippo Bosi (INFN Sez. Pisa) for his help and suggestions. We would like also to acknowledge the support given to the FIRST experiment from Prof. C. Scheidenberger and the GSI accelerator staff. This work has been supported by the European Community FP7 - Capacities, contract ENSAR n 262010. This work was also supported by Junta de Andalucia and the Spanish Ministerio de Ciencia e Innovacion Contracts P07-FQM-02894, FIS2008-04189 and FPA2008-04972-C03. Finally some of the authors would like to thank CNRS-In2p3 for the support. The research leading to these results has received the financial support of the Belgian company Ion Beam Applications (IBA). References [1] V. Patera, Nuclear physics experiment for hadrontherapy application,Nuovo Cim. C 34 (2011) 179. [2] J. Hubele et al., Fragmentation of gold projectiles: From evaporation to total disassembly,Z. Phys. A 340 (1991) 263. [3] G. Bauer et al., A multiple sampling time projection ionization chamber for nuclear fragment tracking and charge measurement,Nucl. Instrum. Meth. A 386 (1997) 249. [4] LAND collaboration, T. Blaich et al., A Large area detector for high-energy neutrons,Nucl. Instrum. Meth. A 314 (1992) 136. [5] P. Avery, Applied Fitting Theory I, General Least Squares Theory, (1991), http://phys.ufi.edu/avery/fitting/fitting1.pdf. – 15 –