Hyperfine Interact (2012) 211:49–52DOI 10.1007/s10751-011-0556-5

SiPM-based veto detector for the pion beam at FOPI

Gamal Ahmed · Pual Bühler · Olaf Hartmann · Johann Marton ·Ken Suzuki · Johann Zmeskal

Published online: 10 January 2012© Springer Science+Business Media B.V. 2012

Abstract Recently the FOPI collaboration carried out an experiment to study thein medium properties of the K+K− system by using the pion beam interactions at1.7 GeV/c. The experiment with a pion beam poses specific requirements to thedetectors and therefore the original FOPI setup needed modifications. The newhardware developments for this experiment include the replacement of the vetodetector with another more compact design. Within this report we describe thedesign and results of a test measurement of the new FOPI veto detector system withthe pion beam.

Keywords SiPM · Veto detector · FOPI · Pion beam

1 Introduction

The FOPI detector is installed at the GSI, Darmstadt, Germany [1]. It allows identify-ing (event by event) light charged particles and intermediate mass fragments. Neutralhadrons (like K0

s , �) can be reconstructed from their decay into charged particles.Since its assembly by an international collaboration in 1990, the experimental setuphas been changed and extended several times [2]. The FOPI collaboration carried outa measurement to study the in medium properties of the K+K− system by using theπ− (

1.7 GeV/

c) + A → K+K− + X reaction [3]. The original FOPI setup was mainly

G. Ahmed (B) · P. Bühler · O. Hartmann · J. Marton · K. Suzuki · J. ZmeskalStefan Meyer Institute for Subatomic Physics of the Austrian Academy of Sciences,Vienna, Austriae-mail: gsmoawad@hotmail.com

G. AhmedFaculty of Science, Physics Department, Al-Azhar University, Cairo, Egypt

50 G. Ahmed et al.

Fig. 1 The veto detector scintillator parts, where LU, LD, RD and RU refer to the positions of theveto detector scintillator parts with respect to the pion beam direction. The dark purple areas showoverlapping of scintillators parts. The red rectangles define the SiPMs positions and the light violetrectangle shows the target area and position. The veto detector system (Scintillator + SiPMs) washoused inside an aluminum vacuum tube. The drawing shows the setup of the veto detector systeminside the inner hole of the GEM-TPC

designed for heavy ion collision experiments. For experiments with a pion beam,the original FOPI setup needs modifications. In the present case a Time ProjectionChambers with Gas Electron Multipliers (GEM-TPC) has been used. The GEM-TPC sits in the inner hole of the Central Drift Chamber (CDC). Around the beamaxis as consequence there is no sufficient space available for the previously usedphotomultiplier (PMT) based veto detector and therefore the veto detector has tobe exchanged by another more compact design (see Fig. 1). The main function of theveto detector is to discriminate when the pion beam hits the target. When chargedparticles penetrate off beam-axis it gives a veto to the data acquisition. Furthermorethe veto detector is located close to the target, where a strong magnetic field isproduced by the solenoid (0.6 Tesla) [4].

2 Detector design

The new veto detector prototype (Fig. 1) consists of 4 scintillator parts, designedto form together a octagon disk-like structure with an external diameter of ∼8 cm.Internally it confines the cross-section of the fiducial target volume to a circle with adiameter of 4 cm. For the readout SiPMs (MPPCs, S10931–100P) have been used[5]. SiPMs have numerous advantages as compared to other photodetectors. Thelow operation voltage, fast timing, compactness, and moreover the insensitivity tomagnetic fields make them excellent candidates for different applications [6–8] andmade MPPCs the preferred choice to fit the new veto detector design requirements.Measurements of dark current, dark count rate and cross-talk and time resolution ofthis device have been presented in Ref. [9, 10].

SiPM-based veto detector for the pion beam at FOPI 51

Fig. 2 Counts as a function of the pion spills for the veto detector (Left) and the Halo1 detector(Right)

3 Beam test and results

The FOPI detector comprises several sub-detectors. The Halo1 detector, sitting∼2.15 m upstream from the solenoid, is composed out of four scintillator bars readout by PMTs and arranged in a cross shape around the beam line. Thus a rectangulararea of adjustable size is defined. The purpose of the Halo1 detector is to monitor thealignment and the focusing of the beam. The Start counter is placed ∼2 m upstreamthe solenoid in the beam line. This detector is used to count the incoming pions andto define the reference time (start time) for all other detectors.

For testing the new veto detector, the pion beam with a momentum of 1.7 GeV/chas been used and the average beam intensity was ∼8 × 103 pions/spill of 2 s duration.

Figure 2 shows the test result for the response and a comparison betweenHalo1 and the veto detector. LU, LD, RD and RU refer to the positions of theveto detector scintillator parts in respect to the pion beam as described in Fig. 1.MPPCs photosensors were operated in a high temperature environment withoutany temperature stabilization and even at such operating conditions the achievedefficiency was >97%. The veto detector efficiency is estimated as the probabilitythat one or both photosensors attached to one scintillator to give a signal at the sametime. Efficiency has been calculated for each scintillator part, by using the followingequation:

Vef f = POR =[

1 −[

1 −(

2R1 + R

)]2]

, R = PAND

POR, (1)

where PAND is the number of cases in which both photosensor (A, B) attached to onescintillator gave a signal at the same time. POR is the number of cases in which onlyone photosensor gave a signal.

4 Conclusion

A new veto detector for the FOPI pion beam experiments has been built. Theexperimental test of the veto detector with the pion beam showed that, the vetodetector has succeeded to monitor the pion beam with very good efficiency (∼97%),

52 G. Ahmed et al.

and therefore it has been successfully used during the experiment to study the inmedium properties of the K+K− system.

Acknowledgements This work is partly supported by Hadronphysics2 (proj 227431) and FWFAustrian Science Fund (P 21457-N16).

References

1. FOPI Collaboration: Technical Proposal GSI Report, 88–03 (1988)2. Münzer, R., et al.: Nucl. Instr. and Meth. A 617, 300 (2010)3. Study of Pion-induced In-medium Production and Propagation of Strangeness., FOPI Proposal,

March 2007.4. Bühler, P., et al.: GSI scientific report, 2009.5. http://sales.hamamatsu.com/assets/pdf/parts_S/s10362–33series_kapd1023e05.pdf6. Suzuki, K., et al.: Nucl. Instr. and Meth. A 610, 75 (2009)7. Ahmed, G.S.M., et al.: Nucl. Instr. and Meth. A 628, 393 (2011)8. Ahmed, G., et al.: Nucl. Instr. and Meth. A 639, 107 (2011)9. Ahmed, G.S.M., et al.: J. Instrum. 4, P09004 (2009)

10. Ahmed, G., et al.: Nucl. Instr. and Meth. A 652, 528 (2011)