Aumann, T.

Caesar, C.

Gernhäuser, R.

Heil, M.

Holl, M.

Johansen, J. G.

Kelic-Heil, A.

Kiselev, O.

Körper, D.

Litvinov, Y.

Movsesyan, A.

Paschalis, S.

Petri, M.

Plag, R.

Scheit, H.

Schrock, P.

Simon, H.

Syndikus, I.

Törnqvist, H.

Wamers, F.

GSI Repository

Heavy-ion tracking detectors for the R3B setup∗S. Paschalis1, T. Aumann1,2, C. Caesar1, R. Gernha¨user3, M. Heil2, M. Holl1, J.G. Johansen1,A. Kelic-Heil2, O. Kiselev2, D. Ko¨rper2, Y. Litvinov2, A. Movsesyan1, M. Petri1, R. Plag1, H. Scheit1,P. Schrock1, H. Simon2, I. Syndikus1, H. To¨rnqvist1, F. Wamers4,1,2, and the R3B collaboration1TU Darmstadt, Germany; 2GSI, Darmstadt, Germany; 3TU, Mu¨nchen, Germany; 4EMMI @ GSI, Darmstadt,GermanyOverviewThe high-intensity radioactive ion beams foreseen withinthe FAIR project will give us access to study the most ex-otic nuclear systems. In order to fully explore these rareshort-lived species the experimental apparatuses are under-going major upgrades. In this challenging environment theR3B collaboration is working towards the development ofa versatile system offering an unprecedented combinationof high-efficiency, high-resolution and high-rate-capablekinematically complete measurements.A pivot point in these developments is the installationof the new superconducting magnet GLAD. This magnet iscapable of bending effectively the high-rigidity beams andallows for projectile-like neutrons to be detected at zerodegrees and the bent beam to be momentum analysed. Aseries of charged-particle detectors are placed along the tra-jectory of the beam in order to measure precisely the posi-tion, charge and time of the incoming beam particles andthe outgoing beam-like fragments. Although the currenttechnologies of charged-particle detectors offer solutionsthat could meet our resolution requirements alone, the chal-lenge is to combine this with minimum material budget,high-rate capability, radiation hardness, large active area,large dynamic range, low cost and high flexibility.Heavy-ion tracking detectorsAlong these lines we are focusing our developments onthe following detectors: 1) Ultra-thin scintillating fiber de-tectors. 2) Ultra-thin resistive strip Si detectors. 3) Largearea time-of-flight wall. In this report we present the cur-rent status of these developments.Plastic-scintillator fiber detectorsSome of the advantages of using plastic-scintillator ma-terial is the low cost, fast response and high-rate capability.Five new fiber detectors are being developed for the needsof precision position measurement. Each detector planeprovides precise information in one dimension and, wheretwo-dimensional information is needed two detector planesare packed closely into one detector. All planes are builtusing a 200 μm square fiber and have varying dimensionsranging from small 5x5 cm2 active area up to 110x80 cm2.∗Work supported by BMBF (05P12RDFN8) and (05P12WOFNF), byGSI via the GSI-TU Darmstadt cooperation contract and by HIC for FAIR.The total number of fibers needed to cover these areas isalmost 3x104 and the total length exceeds 60 km of fiber.The light collection is performed by multi-pixel photoncounters (MPPC) and multi-anode photomultiplier tubes.The boards for powering the MPPCs and providing a firststage amplification are designed and built in-house at theelectronic workshop of the Institut fu¨r Kernphysik at theTechnical University of Darmstadt. The electronic readoutis performed using a combination of NXYTER, FEBEXand TAMEX depending on the physical location of the de-tector and the needs for a high multi-hit capability read-out. We have currently tested, for example, a fully inde-pendent single-fiber readout based on NXYTER [1]. Inmany cases, however, the large number of fibers makes itimpractical and costly to read out all fibers individually. Areadout scheme, in which the different fibers are bundledin a unique way in both ends, enables a way of determin-ing the hit fiber with a reduction of the readout channels to2√Nfiber per plane. This reduces the readout of the 3x10 4fibers to 944 channels, but at the same time compromisesthe multi-hit capability of the detector. A simple exampleof the bundling is shown in Fig. 1.Resistive strip Si detectorsThe Si detectors, in general, are one of the most pop-ular tracking devices, since they combine high-resolutionposition and energy-loss measurements. The Si micro-stripdetectors deliver excellent position resolution, but have alarge number of readout channels and the total energy-lossreconstruction for heavy-ions is not trivial. For these rea-sons, without excluding the use of micro-strip technologyfor certain experiments, we are also working on optimisingthe concept of resistive charge division. Detectors basedon this technology can deliver excellent position and en-ergy resolution when the deposited energy is sufficientlyhigh, as is the case for most heavy-ion experiments at R3B.In addition, the number of readout channels is significantlyreduced compared to the micro-strip Si detectors.However, these detectors suffer from slow charge col-lection as the charge is defused at the resistive side with atime-constant defined by the detector capacitance and theanode resistance. This limits the rate capability of the de-tector. Compared to the micro-strip detectors they alsohave higher noise levels due to the presence of the anode re-sistance and due to the generally higher capacitance. Thisnoise is, however, not the dominant factor in the energy-loss measurements as discussed in [2].GSI SCIENTIFIC REPORT 2013 NUSTAR-KR-10doi:10.15120/GR-2014-1-NUSTAR-KR-10 FAIR@GSI 147Figure 1: A fiber-bundling scheme that reduces the numberof readout channels to 2√Nfiber pr. plane.These limitations of the resistive charge division detec-tors can be reduced by e.g. segmenting the detectors andreducing the anode resistance, which however increases thenumber of readout channels and worsens the position res-olution, respectively. The aim of this work is to find theoptimum trade off between the number of channels, posi-tion resolution, thickness, and rate capability, and to takefull advantage of the varying pulse shapes in this type ofdetectors.We have performed a detailed characterisation and com-parison of two types of resistive charge division detectors:a continuous two-dimensional detector and a strip Si de-tector using an 241Am source. In addition, the read out ofthese detectors is performed using digital electronics andthe waveforms of the pulses are recorded for further of-fline processing. We have found that both the position res-olution and the pulse rise time are significantly improvedin the case of the strip detector compared to a continuoustwo-diminensional detector. Namely, the position resolu-tion obtained for an 241Am alpha source and a 3 KΩ an-ode resistance has improved threefold to be σ ≈ 150 μm(see Fig. 2 adopted from Ref. [3]) and the signal rise timefrom the central detector area has become ten times fastert ≈ 350 ns. We plan to verify these results in an in-beamtest where the limits of high-rate capability will also betested. The currently used detectors have a thickness be-tween 100 and 300 μm and typically a 5x5 cm2 active area.In the future we plan to test also larger detectors with area7x7 cm2 and 10x10 cm2.Figure 2: Position resolution obtained with a Si resistivestrip 140 μm thick detector and an 241Am source.Large area time-of-flight wall detectorA large-area plastic-scintillator wall is used in the R3Bsetup to obtain simultaneously excellent time-of-flight andenergy-loss measurement. An optimised wall, with an ac-tive area of 110x80 cm2 (see Fig. 3), is currently being builtand will be tested in-beam this year. The optimisation isdriven mainly by the requirement for a precise energy-lossmeasurement at high rate.Design-wise one can highlight the following differencesof this detector compared to older versions: The lightguides at the ends of the paddles have been removed andthe paddle width and photomultiplier diameter have beenchosen to fit well with minimal loss of light collection.The paddles are oriented vertically with respect to the dis-persive axis of the dipole magnet. In this way the pad-dles are mainly hit in their central area minimising thenon-uniformity and the “smiley effects” in the total energyreconstruction. Vertically oriented paddles also share thebeam rate more evenly. Four such planes with vertical pad-dles are placed behind each other and are slightly shiftedwith respect to each other so that there are effectively nodead areas between the paddles.The paddles are produced with a maximum variation ofa few percent in their thickness. The energy-loss mea-surement is, however, anticipated to be much better thanthis variation. In order to correct for non-uniformities inthe thickness of the paddle and the position dependent re-sponse of the scintillator, this detector is packed closely onthe same frame with the aforementioned large area fiberdetectors that cover the same active area.On the electronics readout side there have also been ma-jor advances. It is found that the energy-loss measurement,when performed with a standard QDC type of electronics,suffers greatly from rate dependencies. The new readout isbased on a time-over-threshold technique which effectivelyNUSTAR-KR-10 GSI SCIENTIFIC REPORT 2013148 doi:10.15120/GR-2014-1-NUSTAR-KR-10cancels any baseline variation and at the same time gives amuch higher dynamic range. These electronics are a com-bination of the in-house TAMEX board and a QTC add-ondelivering simultaneously good time and energy measure-ments.Figure 3: A CAD drawing of the plastic-scintillator wall.In front of the paddles, the tilted frames foreseen to supportthe large-area fiber planes are also shown.References[1] P. Schrock et al., “Development of a Thin Large-Area FiberDetector for Radioactive-Beam Experiments” GSI-SR2012-PHN-ENNA-EXP-69, GSI Report 2013-1[2] S. Paschalis et al., “Position-Sensitive Si Detectors forIn-Beam Tracking at the R3B Setup” GSI-SR2012-PHN-ENNA-EXP-68, GSI Report 2013-1[3] I. Syndikus master thesis, “Testing of Position Sensitive Sili-con Detectors for the R3B-Setup” February 2014.GSI SCIENTIFIC REPORT 2013 NUSTAR-KR-10doi:10.15120/GR-2014-1-NUSTAR-KR-10 FAIR@GSI 149

Heavy-ion tracking detectors for the $R^{3}B$ setup

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Heavy-ion tracking detectors for the $R^{3}B$ setup

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Heavy-ion tracking detectors for the R3BR^{3}BR3B setup

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Heavy-ion tracking detectors for the $R^{3}B$ setup