Two-dimensional visualization of cluster beamsby microchannel plates

A. Khoukaz n, D. Bonaventura, S. Grieser, A.-K. Hergemöller, E. Köhler, A. TäschnerInstitut für Kernphysik, Universität Münster, D-48149 Münster, Germany

a r t i c l e i n f o

Article history:Received 23 July 2013Received in revised form26 August 2013Accepted 29 August 2013Available online 5 September 2013

Keywords:Cluster beamInternal targetClusterMicrochannel plateBeam visualization

a b s t r a c t

An advanced technique for a two-dimensional real time visualization of cluster beams in a vacuum aswell as of the overlap volume of cluster beams with particle accelerator beams is presented. Thedetection system consists of an array of microchannel plates (MCPs) in combination with a phosphorscreen which is read out by a CCD camera. This setup together with the ionization of a cluster beam by anelectron or ion beam allows for spatial resolved investigations of the cluster beam position, size, andintensity. Moreover, since electrically uncharged clusters remain undetected, the operation in an internalbeam experiment opens the way to monitor the overlap region and thus the position and size of anaccelerator beam crossing an originally electrically neutral cluster jet. The observed intensity distributionof the recorded image is directly proportional to the convolution of the spatial ion beam and clusterbeam intensities and is by this a direct measure of the two-dimensional luminosity distribution. Thisinformation can directly be used for the reconstruction of vertex positions as well as for an input fornumerical simulations of the reaction zone. The spatial resolution of the images is dominated by thegranularity of the complete MCP device and was found to be in the order of s� 100 μm.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Cluster beams can be produced and prepared from originallysolid, liquid or gaseous materials and are widely used in modernphysics experiments. Prominent examples for this are cluster jetbeams as internal targets for storage ring experiments [1–4] orinteraction studies of high-intense laser beams with streams ofclusters [5]. Of special interest are cluster beams produced viaexpansion of cooled gases or liquids in Laval nozzles by eithercondensation of the gas or by breaking up the liquid into a sprayof droplets. Depending on the production parameters these clusterstypically consist of 103–106 molecules [6,7]. Target streams forexperiments can be provided with freely adjustable target thicknessover several orders of magnitude up to about ρtarget ¼ 1015

atoms=cm2 in a distance of more than 2 m behind the nozzle [4].Due to the high mass of the individual clusters compared to theresidual gas background they can travel over several meters throughan ultra high vacuum chamber with a constant angular divergencedefined by the orifices used for the cluster beam preparation. Thisfact is of high interest if the experimental setup, e.g. of a clustertarget at an accelerator facility, requires large distances between thecluster generator and both the scattering chamber and the cluster

beam dump. Especially in this case with large distances a carefulcluster target beam alignment is mandatory and suitable devices forthis are of interest.

A further important quantity in scattering experiments usingcluster beams as targets, e.g. for accelerated ion beams, is thespatial distribution of the interaction region as well as its variationwith time. Here a precise knowledge of this information might beneeded, e.g. for the reconstruction of the tracks of the ejectilesproduced in the interaction volume or for realistic Monte-Carlocomputer simulations for the reactions of interest. One possibilityto gain such information about the vertex volume might be theinvestigation of fluorescence light produced by excitation ofatoms/molecules of the target beams by the passing ions. How-ever, this method commonly requires an optical access to theinteraction point which might be difficult or even not be possiblefor modern compact 4π�detectors such as the planned PANDAdetector at FAIR in Darmstadt [8]. Alternatively the required vertexinformation might be reconstructed by separate investigations onthe accelerator beam profile, e.g. using fluorescence light mea-surements [9,10] of (residual) gas close to the interaction region, incombination with a measurement on the cluster target beamprofile. In order to quantify the geometrical size and/or theintensity of a cluster beam at the interaction point of an experi-ment, different approaches can be followed. One commonlyapplied technique for cluster beams from originally gaseousmaterials is the use of thin scanning rods in one of the

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Nuclear Instruments and Methods inPhysics Research A

0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nima.2013.08.085

n Corresponding author. Tel.: þ49 2518334934; fax: þ49 2518334962.E-mail address: khoukaz@uni-muenster.de (A. Khoukaz).

Nuclear Instruments and Methods in Physics Research A 735 (2014) 12–18

differentially pumped vacuum stages. The principle of this methodis shown in Fig. 1.

A movable rod scans the cluster beam in the vacuum chamberand as soon as the rod interacts with the cluster stream theclusters of the overlap region are stopped and converted into anadditional gas background. The resulting gas load to the vacuumchamber can easily be measured by vacuum gauges and is directlyproportional to the cluster beam density. Furthermore, with theknown rod dimensions and the recorded pressure rise as afunction of the rod position, information about the cluster beamdiameter and the local target thickness can be extracted [4]. Withthis method only one-dimensional scans are possible which resultin data averaged over the rod axis. In addition, depending onthe used vacuum gauge and its possible read out speed, onesingle cluster profile measurement might take, e.g. 1 min, if asufficient spatial resolution is required. To overcome the previouslydescribed limitations, a method based on a microchannel platedevice (MCP) has been implemented. It allows for a precise clustertarget beam position measurement and adjustment, to measure inreal time two-dimensional cluster beam intensity distributions aswell as to monitor the spatial distribution of the vertex point if thecluster target is used in combination with an accelerator beam.

The principle of operation and the properties of this device aredemonstrated here by data obtained with hydrogen cluster beams.While electrically neutral clusters hitting a MCP device remainundetected, ionized clusters, e.g. produced by electron impact, canbe registered by this system. Therefore, the complete cluster beamcross-section can be investigated. If the MCP signal is read out by aphosphor screen and a CCD camera, the time needed to measure onecluster beam image is typically only limited by the required exposuretime of the CCD camera in order to collect a sufficient amount ofphotons. However, this time might be reduced, e.g. to a few seconds, ifthe electron current for cluster ionization is adjusted correspondingly.The presented system is routinely in operation as a diagnostic systemat a hydrogen cluster beam installation at the University of Münster.Furthermore, with this device it was possible to visualize and monitorthe interaction region of a hydrogen cluster target beam and a protonbeam in a storage ring experiment. In this case the ionization of theoriginally electrically neutral cluster beam resulted from the energyloss processes of the ion beam in the cluster beam.

2. Experimental setup

A hydrogen cluster beam is produced via adiabatic expansionof pre-cooled and compressed hydrogen in a Laval nozzle.

Details about the cluster jet generator, the experimental setup aswell as the operational parameters are presented in Ref. [4].Shortly behind the cluster generator the cluster beam is ionizedby an electron gun which can be operated in a continuous modeor, if a timing information is needed, in a pulsed mode. At thecrossing point of cluster and electron beams, i.e. 76 cm behind thenozzle, the diameter of the electron beam amounts to approxi-mately 7 mm and is larger than the cluster beam, i.e. 4 mm, at thatposition. By this a complete coverage of the hydrogen beam isguaranteed. The current of the 150 eV electron beam is adjusted tothe experimental needs, e.g. to the required intensity of ionizedclusters, and is typically in the order of a few microamperes. At theinteraction point clusters with both negative and positive chargesare produced from which in the following only the positivelycharged ones are further considered. After a drift path of approxi-mately 4.2 m, measured from the position of the electron beam,the ionized beam hits the MCP based detection system. A sche-matic view of this device as well as the electrical circuit is shownin Fig. 2.

The cluster beam, indicated by a vertical cone, passes anelectrically grounded grid with a spacing of 2.7 mm and hits a

Fig. 1. Determination of the size and intensity of a cluster beam by the scanningrod method.

Fig. 2. Schematic view of the MCP based setup for the detection of ionizedcluster beams.

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microchannel plate in Chevron assembly. While depending on theoperational parameters of the cluster jet generator the individualclusters have originally velocities in the order of 200–1000 m/s[4,11,12] before entering the detector, the positively electricallycharged clusters are accelerated after passing the grid due to thepotential of up to �3.8 kV at the entrance surface of the MCP. Bythis the kinetic energy of the clusters is high enough to produceprimary electrons which can be multiplied within the MCP device.The cluster beam itself is stopped at the surface of the MCP,converted into residual gas and is pumped away by a turbomole-cular pump. Directly behind the MCP a phosphor screen at apotential of þ3.0 kV is placed which is hit by the electrons fromthe MCP. The resulting image can be observed by a CCD camerabehind an UHV window outside of the vacuum.

In Fig. 3 a photography of the MCP device with the entrance grid isshown. The MCP (dark area below the grid) with a gain of 44� 106

has an active diameter of 40 mm and a pore diameter of ð12:070:5Þ μm. The adapted phosphor screen is of the type P43 whichproduces photons of green color. The entrance grid was produced by acommercial etching procedure with a bar width of 0.2 mm.

In Fig. 4 a schematic view of the complete setup is displayed,showing the final vacuum stage with the MCP detection system aswell as the CCD camera.

3. Visualization of cluster beams

To investigate the possibility to measure two-dimensional clusterbeam density profiles, cluster beams with different geometries have

been prepared by using special shaped collimators, placed 121 mmbehind the nozzle [13]. In Fig. 5 (top) microscopic views of twodifferent collimators are shown which have been fabricated by lasercutting of stainless steel cones. The constant angular divergences ofthe cluster beams passing these orifices are determined by the size ofthe collimator openings as well as the distance between the nozzleand the collimator. This in turn determines the sizes of the clusterbeams at the following vacuum chambers as well as at the position ofthe MCP assembly.

The resulting images recorded by the MCP system are pre-sented by the lower pictures of Fig. 5 [14]. The clearly visible gridspacing allows for a direct measurement of the cluster beam sizeat that distance from the nozzle, i.e. after approximately 5 m.While the round shaped cluster beam, prepared by a collimatorwith a diameter of 500 μm, is fully visible by the MCP system, therectangular shaped beam is not fully detected in one direction dueto the use of a collimator with a width of 190� 770 μm2. By this a

Fig. 3. Photography of the MCP device with the entrance grid. The ionized clusterbeam enters from the top and hits the MCP array.

Fig. 4. Schematic view of the complete MCP detection system with the CCDcamera.

Fig. 5. Microscopic view of two collimators (top) with round ð∅� 0:5 mmÞ and slitð0:77� 0:19 mm2Þ shaped orifices and resulting MCP images (bottom) of thecluster jet beam at a distance of approximately 5 m behind the nozzle.

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part of the large cluster beam is cut away by an orifice inside of thebeam dump stage which is located directly in front of the MCPsystem. The shape of this internal orifice is clearly visible. Thus thiskind of detector also allows for an online monitoring of the targetbeam adjustment and interferences, e.g. with vacuum compo-nents. The observed boundary smearing of the cluster beam is infull agreement with earlier findings using the scanning methodwith moveable rods (see Fig. 1).

For a more quantitative analysis of the recorded figures thetwo-dimensional intensity distributions have been analyzed. Fig. 6shows exemplarily the result for the circular cluster beam shownin Fig. 5. Here the x- and y-axes correspond to the directionsperpendicular to the spread direction of the cluster beam. By usinga homogeneously distributed electron beam for the cluster ioniza-tion, the intensity distribution observed with the MCP device is adirect measure for the local cluster beam intensity. A closerinvestigation of the intensity distribution exposes a slight asym-metry which might be caused by the not yet perfectly adjustedrelative position of the two collimators in the cluster source.Therefore, the MCP device presented here is highly suitable foronline collimator adjustment purposes during the preparation ofcluster beams, e.g. for hadron physics experiments.

To gain further information about the quality of the spatialresolution of the MCP system, one of the scanning rods of themonitor system with a diameter of drod ¼ 1 mm located at thescattering chamber, i.e. lscat ¼ 2:1 m behind the nozzle, was placedat a fixed position within the passing cluster beam. Fig. 7 (top) showsthe extracted MCP image obtained at a distance of lMCP ¼ 5:0 mbehind the nozzle. In addition to the already described clusterbeam image a sharp-edged shadow of the rod is clearly visible. Toextract information about the spatial resolution of the MCP device,a projection of this two-dimensional intensity image was determinedand is displayed in Fig. 7 (bottom).

The recorded data points are given by the crosses and the shapeof the envelope is in good agreement with the expected shape of aprofile of a circular cluster beam known from the scanning rodmethod (see Fig. 1). The narrow intensity drops are a result of theregular grid structure and by this an absolute spatial calibration ofthe MCP monitor was possible.

In Fig. 8 the measured position in units of the CCD camerapixels of the intensity drops is shown. As expected for an image ofa grid with constant bar distance, the data can be described wellby a first order polynomial function and expose no signal for a

non-linearity. With the pixel information of the CCD camera andthe known grid bar distance of dgrid ¼ 2:7 mm (center-to-center)the calibration factor was determined to be cMCP ¼ð44:3270:01Þ pixel=mm. Moreover, since the shape of the narrowintensity drops is given by a convolution of the known grid barwidth of dbar ¼ 200 μm and the intrinsic resolution of the MCPFig. 6. Two-dimensional intensity distribution obtained by the MCP device.

Fig. 7. MCP image (top) and projection (bottom) of the intensity distributionobtained with one scanning rod crossing the cluster beam flight path.

Fig. 8. Measured position in units of the CCD camera pixels of the intensity dropscaused by a shadowing effect of the grid bars.

A. Khoukaz et al. / Nuclear Instruments and Methods in Physics Research A 735 (2014) 12–18 15

detector itself, the latter information can be extracted from a fit tothe displayed data. An excellent description of the intensity dropswas achieved by a pure Gaussian distribution with a width ofsMCP ¼ ð10371Þ μm (RMS), representing an upper limit for theintrinsic resolution of the MCP detector. At this point it should bestressed that in general the extracted intensity distributions aregiven by a convolution of the local cluster beam density distribu-tion and the one of the electron beam. However, the spatialvariation of the electron beam intensity was found to be negligiblein the vicinity of the analyzed individual intensity drops and couldbe ignored here. Furthermore, the broad minimum visible in Fig. 7(bottom) is a result of the scanning rod placed inside of the clusterbeam. While the total width allows for a reconstruction of therod size, an even more interesting information is given bythe shape of the steep intensity drops from the rod itself. Againa good description of the data was possible by a convolution of arectangular rod geometry and a Gaussian detection resolution sres.Here it must be noted that the latter resolution combines con-tributions from the already determined intrinsic resolution of theMCP detector, i.e. sMCP, and additional effects such as possiblestray fields influencing the trajectories of the ionized cluster beamor minor divergences of the cluster beam itself. The best descrip-tion of the data is shown by the solid line and considers adetection resolution of sres ¼ ð13573Þ μm (RMS) in addition tothe known numbers for dgrid, dbar, and sMCP. Thus, the resolutionfor the detection of structures in the cluster beam or of a possibledisplacement at the position of the scattering chamber, i.e. 2.1 mbehind the nozzle, can be estimated to be better than

sscatrsres � lscatlMCP¼ ð5771Þ μm: ð1Þ

Therefore, the MCP monitor device presented here is highlysuitable for the observation and the quantitative investigationsof cluster beams used as target beams, e.g. in hadron physicsexperiments at storage rings or at high power laser facilities.

4. Visualization of the beam-target vertex regionin accelerator experiments

The capability of the previously described MCP monitor deviceto detect spatially resolved ionized clusters opens further possibi-lities for its use in combination with cluster beams as targets, e.g.in storage ring experiments. One valuable real time informationwhich can be extracted quantitatively is the size of the vertexregion, i.e. the interaction volume of the accelerated ion beamwith the cluster beam. A sketch of the operation principle is givenin Fig. 9.

A cluster beam, produced in a Laval nozzle and prepared by aset of one skimmer and one collimator, enters the scatteringchamber of the particle accelerator and is hit by the ion beam.Due to energy loss processes, e.g. based on Coulomb effects, acertain fraction of the cluster beam of the overlap volume isionized by the ion beam while the part of the cluster beam not hitby the ions remains in an electrically neutral state. The partiallyionized cluster beam enters behind the scattering chamber thebeam dump stage where the previously described MCP monitorcan be placed. Since only clusters from the interaction volume areionized, solely clusters from this region are expected to give acontribution to the resulting image.

A feasibility study of this method has been performed [15] during afew hours of an out of turn beam time at the ANKE experimentalfacility [16] which is installed at an internal target position of thecooler synchrotron COSY [17] at the Forschungszentrum Jülich. AtANKE a cluster jet target is installed which provides hydrogen clustertarget thicknesses of up to ρtarget ¼ 1015 atoms=cm2 in a distance of

70 cm behind the nozzle in combination with a target diameter ofapproximately 10 mm. For the studies presented here the finalturbomolecular pump of the beam dump stage was replaced by thedescribed MCP monitoring device and a thin cluster jet beam wasproduced with ρtargetr1014 atoms=cm2. The COSY accelerator wasoperated in a mode where low energetic protons, injected with amomentum of pinj ¼ 300 MeV=c, were stored and accelerated up to abeam momentum of pacc ¼ 2100 MeV=c. After a cycle length of 30 sthe proton beam was dumped and new cycles with the same timestructure and with newly injected protons followed. In Fig. 10 (top) animage recorded with the MCP device is shown which was obtainedduring the acceleration of the COSY beam. The exposure time of theCCD camera was set to 5 s. A bright area is visible which is caused byclusters ionized by the accelerator beam. The vertical axis(“z-position”) corresponds to the ion beam direction while thehorizontal axis (“x-position”) corresponds to the one transverse tothe beam direction. Note that the COSY beam has a comparably broaddiameter during the acceleration due to the low proton momentumshortly after the injection. In the second figure (Fig. 10, center) animage is shownwhich was recorded when the final COSY momentumwas reached, i.e. pacc ¼ 2100 MeV=c. Due to the adiabatic cooling ofthe accelerator beam the transverse diameter of the ion beam shrunkwhich can be seen directly. In a next step one steering magnet in frontof the ANKE scattering chamber was switched on in order to allow fora shift of the COSY beam transverse to its flight direction. The resultingimage is shown in Fig. 10 (bottom) where a clear displacement isvisible. The lower intensity is only a result of a lower proton beamintensity during this measurement. Therefore, this series of measure-ments proves that a direct observation of the interaction vertex ispossible by such a MCP device. For a more quantitative investigation a

Fig. 9. Sketch of the partial ionization of a cluster target beam by an acceleratorbeam.

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series of 145 images recorded at pacc ¼ 2100 MeV=c has been analyzedand combined in one averaged image. The resulting image and aprojection on the transverse axis is shown in Fig. 11. Note that for themeasurements presented here a grid was used where four joining gridbars in the center of this electrode were removed (see Fig. 11). Inaddition, each of the presented MCP pictures shown in Figs. 10 and 11

exposes a small intensity maximum on the lower right border whichshows up in Fig.11 (bottom) as a small maximum. This effect is only anartifact originating from spatially limited discharges of the detectorcaused by a small detector defect located at the respective detectoredge in combination with the increased high voltage settings duringthe discussed measurements. However, these facts are of no relevancefor the further discussion.

Due to time limitations of the measurement the MCP devicecould not be aligned perfectly relative to the cluster beam axis,which resulted in the fact that the signal is not in the center of theMCP screen. Nevertheless, solid quantitative information about thevertex region can be gained with good accuracy. The obtained dataindicated by crosses can be described well by the assumptionof a Gaussian intensity distribution with a width of sCOSY;MCP ¼ð3:1570:01Þmm (RMS). Similar to the situation where the clusterbeam was ionized by an electron beam here again the obtainedintensity distribution is given by a convolution of the local clusterbeam intensity distribution and the one of the COSY beam.However, in this special case the used cluster beam has a muchlarger diameter than the accelerator beam. Furthermore, thecluster beam can be assumed to have an almost homogeneousvolume density distribution at the interaction volume which issupported by measurements similar to the one shown in Fig. 1.Therefore, in this case sCOSY;MCP is, in very good approximation, adirect measure of the width of the COSY beam. Here again the wellunderstood small intensity drops are caused by the grid structure,which can be treated as described above. Since the cluster beam

Fig. 10. Visualization of the beam–target vertex region during the acceleration ofthe COSY beam (top) and at a flat top momentum of 2.1 GeV/c (center). The thirdfigure shows the vertex region also at 2.1 GeV/c but with a switched on dipolesteering magnet in front of the scattering chamber.

Fig. 11. Image and projection of the vertex region obtained by averaging 145individual images with an exposure time of 5 s each.

A. Khoukaz et al. / Nuclear Instruments and Methods in Physics Research A 735 (2014) 12–18 17

has a constant angular divergence given by the size of thecollimator of the cluster source and its distance from the nozzle,the width of the COSY beam can be reconstructed by simplegeometrical considerations. With the knowledge of the distancebetween the nozzle and the vertex point in the scattering chamberof lvertex ¼ 700 mm and the distance between the nozzle and theMCP of lMCP ¼ 1829 mm, the width of the COSY beam in thescattering chamber sCOSY;vertex can be written as

sCOSY;vertex ¼ sCOSY;MCP �lvertexlMCP

: ð2Þ

Thus the width of the COSY beam at the vertex point could beestimated to be approximately sCOSY;vertex ¼ 1:2 mm (RMS) whichis in good agreement with the expected accelerator beam size [18]as well as with earlier results on the COSY beam size at that targetplace [19]. Due to the non-ideal experimental conditions nouncertainties are given here. However, based on the achievedresults a precision for the determination of the vertex size isconservatively expected to be in the order of 150 μm (RMS).

5. Extended applications

In addition to the use of the MCP device presented here as amonitor for the time resolved, two-dimensional investigation ofboth the cluster beam thickness and the vertex size distributions,further applications are possible. If an adjustable positive retarda-tion potential Uret is applied to the entrance grid instead of thegrounding, this device works as an energy filter for the impingingionized clusters with positive charge. Depending on the stagnationparameters of the fluid before entering the Laval nozzle, theproduced hydrogen clusters typically have a mean speed ofvcluster ¼ 200–1000 m=s. The width of the velocity distributionΔvcluster also depends on the stagnation parameters and wasobserved to be in the range of Δvcluster=vcluster ¼ 0:5–8%. For detailssee Refs. [4,11,12]. By investigating the intensity variation recordedby the CCD camera as a function of the retardation potential Uret,information about the masses of the ionized clustersmcluster can begained. In detail clusters of charge q with a mass below

mcluster ¼2qUret

v2clusterð3Þ

can be reflected. This opens the possibility to investigate the massdistribution of cluster target beams, e.g. as a function of thestagnation parameters or of the nozzle geometry, in parallel tothe operation as target in experiments. Assuming, for example, aretardation voltage of Uret ¼ 1000 V and a cluster velocity ofvcluster ¼ 600 m=s, clusters of masses up to mcluster � 500;000 amucan be investigated. Moreover, by comparing the obtained dis-tributions with and without an accelerator beam passing thecluster beam, information about a possible fragmentation ofclusters caused by the ion beam can be gained. First pilot studieson cluster mass measurements with this device and using thedescribed cluster target have been performed successfully and arefollowed by extensive systematic investigations [20].

6. Summary

In summary we have presented an advanced technique whichenables, for the first time, the two-dimensional real time visuali-zation of cluster target beams as well as of vertex regions atinternal beam experiments at particle accelerators. The system is

based on a microchannel plate array in combination with aphosphor screen which is read out by a CCD camera. Full, timeresolved, two-dimensional information about the cluster beamthickness distribution are accessible if a dedicated electron gun isswitched on. A spatial resolution in the order of s� 100 μm hasbeen reached which allows for quantitative investigations on thesize, shape, and thickness distribution of the cluster beams itself.In addition it has been demonstrated that this device allows for adetailed two-dimensional investigation of the vertex region atstorage ring experiments using internal cluster jet targets. Herethe ionization of the clusters proceeds solely by the passing ionbeam if the previously used electron beam is switched off. With thisdevice a resolution of approximately s� 150 μm was achieved. Themethod presented here can principally also be used for furtherapplications, e.g. for laser–cluster interactions, where electricallycharged clusters are produced, or for the cluster mass investigation.

Acknowledgments

The authors would like to thank the COSY crew and the ANKEcollaboration for the outstanding support and for the possibility toperform the presented test measurements. We acknowledge theexcellent work done by our mechanical and electronic workshop.The research project was supported by BMBF (06MS253I,06MS9149I/05P09MMFP8, 06MS7190I/05P12PMFP5), GSI F&Eprogram (MSKHOU1012), EU/FP6 HADRONPHYSICS (506078), EU/FP7 HADRONPHYSICS2 (227431) and EU/FP7 HADRONPHYSICS3(283286).

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