Nuclear Instruments and Methods in Physics Research A 732 (2013) 437–441

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Nuclear Instruments and Methods inPhysics Research A

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n CorrE-m

journal homepage: www.elsevier.com/locate/nima

Particle tracking at 4 K: The Fast Annihilation Cryogenic Tracking(FACT) detector for the AEgIS antimatter gravity experiment

J. Storey a,n, C. Canali k, S. Aghion b,c, O. Ahlén d, C. Amsler a, A. Ariga a, T. Ariga a, A.S. Belov e,G. Bonomi f,g, P. Bräunig h, J. Bremer d, R.S. Brusa i, G. Burghart d, L. Cabaret j, M. Carante g,R. Caravita l, F. Castelli l, G. Cerchiari l, S. Cialdi l, D. Comparat j, G. Consolati b,c, L. Dassa f,S. Di Domiziom, L. Di Noto i, M. Doser d, A. Dudarev d, A. Ereditato a, R. Ferragut b,c,A. Fontana g, P. Genova g, M. Giammarchi c, A. Gligorova n, S.N. Gninenko e, S. Haider d,S.D. Hogan o, T. Huse p, E. Jordan q, L.V. Jørgensen d, T. Kaltenbacher d, J. Kawada a,A. Kellerbauer q, M. Kimura a, A. Knecht d, D. Krasnický r, V. Lagomarsino r, A. Magnani g,S. Mariazzi i, V.A. Matveev e,s, F. Merkt t, F. Moia b,c, G. Nebbia u, P. Nédélec v,M.K. Oberthaler h, N. Pacifico n, V. Petráčekw, C. Pistillo a, F. Prelz c, M. Prevedelli x,C. Regenfus k, C. Riccardi y,g, O. Røhne p, A. Rotondi y,g, H. Sandaker n, P. Scampoli a,z,M.A. Subieta Vasquez f,g, M. Špačekw, G. Testeram, D. Trezzi c, R. Vaccaronem, S. Zavatarelli m

a Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, University of Bern, 3012 Bern, Switzerlandb Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italyc Istituto Nazionale di Fisica Nucleare, Sez. di Milano, Via Celoria 16, 20133 Milano, Italyd European Organisation for Nuclear Research, Physics Department, 1211 Geneva 23, Switzerlande Institute for Nuclear Research of the Russian Academy of Sciences, Moscow 117312, Russiaf University of Brescia, Department of Mechanical and Industrial Engineering, Via Branze 38, 25133 Brescia, Italyg Istituto Nazionale di Fisica Nucleare, Sez. di Pavia, Via Agostino Bassi 6, 27100 Pavia, Italyh University of Heidelberg, Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg, Germanyi Dipartimento di Fisica, Università di Trento and INFN, Gruppo Collegato di Trento, Via Sommarive 14, 38050 Povo, Trento, Italyj Laboratoire Aimé Cotton, CNRS, Université Paris Sud, ENS Cachan, Bâtiment 505, Campus d'Orsay, 91405 Orsay Cedex, Francek University of Zurich, Physics Institute, Winterthurerstrasse 190, 8057 Zurich, Switzerlandl University of Milano, Department of Physics, Via Celoria 16, 20133 Milano, Italym Istituto Nazionale di Fisica Nucleare, Sez. di Genova, Via Dodecaneso 33, 16146 Genova, Italyn University of Bergen, Institute of Physics and Technology, Alleegaten 55, 5007 Bergen, Norwayo University College London, Department of Physics and Astronomy, Gower Street, London WC1E 6BT, UKp University of Oslo, Department of Physics, Sem Sælands vei 24, 0371 Oslo, Norwayq Max Planck Institute for Nuclear Physics, Saupfercheckweg 1, 69117 Heidelberg, Germanyr University of Genoa, Department of Physics, Via Dodecaneso 33, 16146 Genova, Italys Joint Institute for Nuclear Research, 141980 Dubna, Russiat ETH Zurich, Laboratory for Physical Chemistry, 8093 Zurich, Switzerlandu Istituto Nazionale di Fisica Nucleare, Sez. di Padova, Via Marzolo 8, 35131 Padova, Italyv Claude Bernard University Lyon 1, Institut de Physique Nucléaire de Lyon, 4 Rue Enrico Fermi, 69622 Villeurbanne, Francew Czech Technical University in Prague, FNSPE, Břehová 7, 11519 Praha 1, Czech Republicx University of Bologna, Department of Physics, Via Irnerio 46, 40126 Bologna, Italyy University of Pavia, Department of Nuclear and Theoretical Physics, Via Bassi 6, 27100 Pavia, Italyz University of Napoli Federico II, Department of Physics, Via Cinthia, 80126 Napoli, Italy

a r t i c l e i n f o

Available online 30 May 2013

Keywords:AntihydrogenGravityScintillatorsTracking

02/$ - see front matter & 2013 CERN. Publishex.doi.org/10.1016/j.nima.2013.05.130

esponding author. Tel.: +41 764872546.ail address: james.storey@cern.ch (J. Storey).

a b s t r a c t

The AEgIS experiment is an international collaboration with the main goal of performing the first directmeasurement of the Earth's gravitational acceleration on antimatter. Critical to the success of AEgIS is theproduction of cold antihydrogen (H) atoms. The FACT detector is used to measure the production andtemperature of the H atoms and for establishing the formation of a H beam. The operating requirementsfor this detector are very challenging: it must be able to identify each of the thousand or so annihilationsin the 1 ms period of pulsed H production, operate at 4 K inside a 1 T solenoidal field and not producemore than 10 W of heat. The FACT detector consists of two concentric cylindrical layers of 400 scintillator

d by Elsevier B.V. All rights reserved.

Fig. 1. Overview of the Fast Annihilation Cryogenic Tdetector. The scintillating fibres are arranged in loopbeam axis. There are two scintillating fibres layers at r98 mm from the beam axis. Each layer consists of 400by 0.6 mm, with alternate fibres displaced radially bytransported from the cryogenic region to the room temby means of 2 m long clear fibres. Charged pionsannihilation are reconstructed using the scintillating fi

back to the H formation region to identify the annihila(For interpretation of the references to color in thisreferred to the web version of this article.)

J. Storey et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 437–441438

fibres with a 1 mm diameter and a 0.6 mm pitch. The scintillating fibres are coupled to clear fibres whichtransport the scintillation light to 800 silicon photomultipliers. Each silicon photomultiplier signal isconnected to a linear amplifier and a fast discriminator, the outputs of which are sampled continuouslyby Field Programmable Gate Arrays (FPGAs).

In the course of the developments for the FACT detector we have established the performance ofscintillating fibres at 4 K by means of a cosmic-ray tracker operating in a liquid helium cryostat. The FACTdetector was installed in the AEgIS apparatus in December 2012 and will be used to study the Hformation when the low energy antiproton physics programs resume at CERN in the Summer of 2014.This paper presents the design requirements and construction methods of the FACT detector andprovides the first results of the detector commissioning.

& 2013 CERN. Published by Elsevier B.V. All rights reserved.

1. The AEgIS experiment

The principal of equivalence between gravitational mass andinertial mass is a foundation of general relativity. The universalityof free-fall (UFF), the experimental evidence on which the weakequivalence principle is based, has been tested to be valid to a veryhigh precision (1 part in 10 trillion) by many experiments using avariety of techniques [1]. General relativity is a classical theory,which makes no distinction between matter and antimatterparticles. However, there has never been a direct verification ofthe weak equivalence principal with antimatter. The AEgIS experi-ment proposes to perform, for the first time, a test of the UFF withantimatter, by measuring the deflection of a beam of H atoms inthe Earth's gravitational field [3].

The principal of the experiment is as follows: cold H atoms(100 mK) are synthesised in a Penning–Malmberg trap and accel-erated toward a Moiré deflectometer (the classical counterpart toan atom interferometer) and annihilate on a position sensitivedetector. The acceleration of the anti-atoms in the Earth's gravita-tional field (g) can be measured from the shift in the shadowimage formed by the Moiré grating on the position sensitivedetector [4]. In the first phase of the experiment g will bemeasured with a 1% precision. However, the ultimate goal is toperform an atom interferometry experiment for which a relativeuncertainty for g of 10−10 has been obtained for a matter–matter

racking (FACT) antihydrogens aligned orthogonally to theadial distances of 70 mm andscintillating fibres separated0.8 mm. Scintillating light isperature readout electronics(orange lines) from the Hbre tracker and extrapolatedtion vertex (r, z-coordinates).figure legend, the reader is

experiment [2], but this will require the development of techni-ques to cool H to sub-millikelvin temperatures.

2. Antihydrogen detector

Critical to the success of the AEgIS experiment is the production ofultra-cold (100 mK) H atoms. An antihydrogen detector is required tomeasure the production and temperature of H atoms and for estab-lishing the formation of the H beam. The operating requirements arevery challenging: it must be fast enough to identify the hundred or soannihilations in the 1 ms period of pulsed H production, operate at 4 Kinside a 1 T solenoidal magnetic field and not produce more than10W of heat. The detector must operate in an isolation vacuum(10−6 mbar), inside a cylindrical volume with an inner radius of68mm and an outer radius of 103 mm.

In light of these requirements a design based on scintillatingfibres with silicon photomultiplier readout was chosen. The FastAnnihilation Cryogenic Tracking (FACT) detector consists of fourlayers of 1 mm diameter Kuraray SCSF-78 M multi-clad scintillat-ing fibres coupled to clear fibres of the same diameter whichtransport the optical signal from the cryogenic region onto arraysof 1 mm diameter Hamamatsu Multi-Pixel-Photon-Counters(MPPC) S10362-11-100C [5,6]. A schematic of the detector designis shown in Fig. 1. The detector design is optimised to reconstructthe position of the annihilation vertex along the beam axis,knowledge of which will enable measurement of antihydrogenproduction, temperature and beam creation. From Geant4 simula-tions an annihilation vertex resolution of s¼ 2:1 mm is expected,which is sufficient for the AEgIS requirements.

2.1. Design and construction

The main mechanical components of the FACT detector are thesupport for the scintillating fibres, the connector to join thescintillating and clear fibres, and the isolation vacuum vessel toseparate the detector from the ultra-high vacuum region in whichthe H beam is produced. The engineering design for the supportstructure and fibre connector is shown in Fig. 2.

The scintillating and clear fibres are mechanically coupled with apair of 7075 aluminium alloy plates in which 1.05 mm holes aredrilled to hold the fibres. The fibres are glued into the plate with a lowviscosity epoxy (Stycast 2850-FT) and then polished with a diamondtool to a planarity of better than 10 μm. Guide pins along the lengthof the connector ensure the alignment of the scintillating and clearfibres. The production of the connector is illustrated in Fig. 3.

Once polished the connectors holding the scintillating fibresare mounted onto the two cylindrical support structures. This isthe only point where the scintillating fibres are fixed to thesupport structure, thereby minimising the likelihood of damageto the fibres caused by thermal contraction of the plastic fibres atcryogenic temperatures. Round stainless steel bars across the

J. Storey et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 437–441 439

length of the support are used to constrain the fibres tothe support structure, but are positioned to allow movement ofthe fibre around the cylinder when the fibres contract at cryogenictemperatures. The scintillating fibres are shown in Fig. 4 beforeand after they are mounted on the support structure.

The clear fibres are routed parallel to the beam axis (Fig. 4) towardthe readout electronics which are located 2 m downstream of thedetector. Particular attention is given to the routing of the clear fibres

Fig. 3. Left: mounting scintillating fibres into one of the connector pairs inpreparation for glueing. Right: connector holding the scintillating fibres afterdiamond polishing.

Fig. 4. Scintillating fibres before (left) and after (right) being mounted on the support struplate containing 32 rectangular slots.

Fig. 2. Section view of the design for the scintillating support structure and fibreconnector. The beam axis is indicated by the arrow and the green cone illustratesthe H beam region. Each of the two scintillating fibre layers are mounted on240 mm long cylindrical support structures made from 7075 aluminium alloy(orange and pink cylinders). The fibres are located in U shaped grooves which arecut into the support with a precision of less than 10 μm using a ComputerNumerical Control (CNC) milling machine. The blue and green bars are theconnectors to couple the scintillating and clear fibres. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

to ensure that the minimum bending radius always exceeds 50 mm,since light losses become significant at smaller bending radii.

2.2. Readout electronics

The readout electronics is designed to detect continuously thelight from the scintillating fibres for the duration of the 100 msperiod during which H is produced. The readout scheme for asingle scintillating fibre channel is shown in Fig. 5. Scintillationlight is guided by means of a clear fibre onto a Hamamatsu Multi-Pixel Photon Counter (MPPC) consisting of 100 Geiger modeAvalanche Photo Diodes (APDs) operating in parallel with a totalphotosensitive area of 1�1 mm. The MPPC signal is connected toa fixed gain linear amplifier which produces a pulse with anamplitude of 10 mV for a single photoelectron and a duration of10 ns. The signal is connected to a fast discriminator with a TTLoutput that can be read directly from an FPGA. The threshold ofthe comparator is independent for each MPPC channel and can bevaried by means of a digital-to-analog converter (DAC). The gain ofeach MPPC can be controlled by adjusting the bias voltage with asecond DAC channel.

In order to minimise the heat load to the cryogenic region theMPPCs are the only component of the readout electronics placedinside the vacuum vessel. A 2-part plastic connector developed bythe T2K collaboration is used to couple the clear fibres to the MPPC[7]. Groups of 48 MPPCs are soldered onto a PCB holder whichconnects to a 50-pin DSub vacuum feedthrough on a ISO-K 160flange. Each clear fibre is glued into the plastic connector with lowviscosity epoxy (Stycast 2850-FT) and polished with a diamondtool. The identity of every clear fibre is labelled with five colouredglass beads which are threaded onto each clear fibre. Photos of anassembled vacuum feedthrough are shown in Fig. 6.

The vacuum and air-side electronics are shown in Fig. 7. Theair-side of the 50-pin DSub vacuum feedthrough is connected to abackplane PCB into which 4 analog boards are connected, eachwith 12 MPPC amplifier and comparator channels. A XilinxSpartan-6 FPGA development board is used to record the 48 TTLoutputs of the 4 analog boards with a sampling frequency of

cture. The clear fibres are organised into groups of 25 fibres and fixed into a circular

Fig. 5. Readout scheme for a single scintillating fibre channel.

Fig. 6. Left: two groups of 48 MPPCs, each mounted in a plastic connector and soldered to the PCB holder. Each PCB holder is connected to one of two 50-pin DSubconnectors. An additional 15-pin DSub connector is used to readout a PT-1000 thermistor mounted on each PCB. Right: clear fibres connected to the MPPCs. Each fibre isidentified by five beads of four different colors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Air and vacuum side electronics for the FACT detector, separated by an ISO-K160 vacuum feedthrough. The signals from the 96 MPPCs on the vacuum side arerouted via the backplane PCB to one of four analog boards. The design has beenoptimised in order to minimise the path length for the raw MPPC signals. Thereadout is supervised by a Xilinx Spartan-6 FPGA development board (bottom leftcorner). The complete readout system consists of 9 vacuum feedthroughs, 18backplane PCBs, 67 analog boards and 16 Spartan-6 development boards.

0.20

0.15

0.10

0.05

0.00

Trig

ger

Rat

e (H

z)

300250200150100500Temperature (kelvin)

Fig. 8. Trigger rate in scintillating fibre due to passage of cosmic rays as a functionof the fibre temperature.

J. Storey et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 437–441440

200 MHz. The time (number of 200 MHz clocks from external startreadout trigger) and the state of the 48 comparators is recordedinto the FPGA memory if one or more of the comparators is in alogic high state. Data acquisition and readout to an external PC isdirected by a state machine implemented in the FPGA with VHDLcode. The FPGA is also used to set a dual channel potentiometerwhich controls the bias voltage and comparator threshold for eachMPPC channel. The FPGA state machine is controlled by LabViewvia a USB UART interface.

3. Results

3.1. Test of scintillating fibres at 4 K

Tests of the plastic scintillating fibres at 4 K have been per-formed in order to study the effect of cryogenic temperatures onthe light yield, decay time and lifetime of the fibres. The apparatusto perform this measurement (Fig. 8) consists of three layers of1 mm diameter Kuraray SCSF-78 M multi-clad scintillating fibresarranged in loops at the bottom of a liquid helium cryostat. Thelight from each of the scintillating fibres is detected by threeHamamatsu MPPCs (S10362-11-100C), the output of which isamplified and then digitised by a LeCroy Wavepro 7100 10 GS/soscilloscope. The performance of the scintillating fibres is mon-itored using cosmic rays. The oscilloscope is triggered when the

signal of two of the three fibres exceeds four photoelectrons(30 mV) and an event is defined when a signal is observed inthe third fibre, corresponding to the passage of a cosmic raythrough all three scintillating fibre layers. The cryostat is comple-tely filled with liquid helium, which immerses the scintillatingfibres for 4 h, and the performance of the fibres is monitoredduring this period and during the warm-up of the cryostat toambient temperature. The rate of events as a function of tempera-ture is shown in Fig. 8, demonstrating ≈10% decrease in theperformance of the scintillating fibre as a function of temperature.Examination of the fibres under a microscope after many thermalcycles to 4 K reveals no sign of mechanical damage to the fibre.

3.2. Commissioning of the FACT detector

The FACT detector was installed in the AEgIS apparatus inDecember 2012 and is currently undergoing commissioning withcosmic-rays. The readout electronics have been used successfullyto set the bias voltage and comparator threshold for all MPPCsconcurrently. An example bias voltage and threshold scan areshown in Fig. 9.

Cosmic-ray tracks have been reconstructed in the detector.Current developments in progress include a feedback system tomaintain a constant MPPC gain; a Peltier based system to lowerthe MPPC dark count rate and exploitation of the external FPGAmemory in order to record events continuously for longer periods.

Fig. 9. Left: MPPC bias voltage as a function of the dark count (DC) rate. Right: comparator counts per second as a function of comparator threshold.

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References

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Section B 266 (2008) 351.

[4] C. Amsler, et al., Journal of Instrumentation 8 (2013) P02015.[5] Kuraray, ⟨www.kuraray.co.jp/en⟩.[6] Hamamatsu Photonics, ⟨www.hamamatsu.com⟩.[7] H. Kawamuko, et al., PoS PD07, 2007, p. 043.