ANKE, a new facility for medium energy hadron physics at COSY-Jülich

Nuclear Instruments and Methods in Physics Research A 462 (2001) 364–381

ANKE, a new facility for medium energy hadron physics at COSY-Ju¨lich S. Barsova, U. Bechstedtb, W. Bothec, N. Bongersb, G. Borchertb, W. Borgsb, W. Bra¨utigamb, M. Bu¨scher*,b, W. Cassingd, V. Chernysheve, B. Chiladzef, J. Dietrichb, M. Drochnerg, S. Dymovb,h, W. Erveng, R. Esserb,i,1, A. Franzenb, Ye. Golubevaj, D. Gottab, T. Grandeb, D. Grzonkab, A. Hardt2,b, M. Hartmannb, V. Hejnyb, L.v. Hornb, L. Jarczykk, H. Junghansb, A. Kacharavaf,h, B. Kamysk, A. Khoukazl, T. Kirchnerm, F. Klehrn, W. Kleino, H.R. Kochb, V.I. Komarovh, L. Kondratyuke, V. Kopteva, S. Kopytob, R. Krauseb, P. Kravtsova, V. Kruglovh, P. Kulessab,k, A. Kulikovh,p, N. Langl, N. Langenhagenm, A. Lepgesb, J. Leyi, R. Maierb, S. Martinb, G. Macharashvilif,h, S. Merzliakovh,p, K. Meyerb, S. Mikirtychiantsa, H. Mu¨llerm, P. Munhofenb, A. Mussgillerq, M. Nekipelova,b, V. Nelyubina, M. Nioradzef, H. Ohmb, A. Petrush, D. Prasuhnb, B. Prietzschkm, H.J. Probstb, K. Pyszr, F. Rathmanns, B. Rimarzigm, Z. Rudyk, R. Santol, H. Paetz gen. Schiecki, R. Schleichertb, A. Schneiderb, Chr. Schneiderm, H. Schneiderb, U. Schwarzt, H. Seyfarthb, A. Sibirtsevd, U. Sielingg, K. Sistemichb, A. Selikovh, H. Stechemessern, H.J. Steinb, A. Strzalkowskik, K.-H. Watzlawikb, P. Wu¨stnerg, S. Yashenkoh, B. Zalikhanovh, N. Zhuravlevh, K. Zwollg, I. Zychoru, O.W. B. Schultb, H. Stro¨herb a

High Energy Physics Department, Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia b Institut fu¨r Kernphysik, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany c DESY, 22603 Hamburg, Germany d Institut fu¨r Theoret. Physik, Justus-Liebig Universita¨t Giessen, Heinrich-Buff Ring 16, 35392 Giessen, Germany e Institute for Theoretical and Experimental Physics, Cheremushkinskaya 25, 117259 Moscow, Russia f High Energy Physics Institute, Tbilisi State University, University Street 9, 380086 Tbilisi, Georgia g Zentrallabor fu¨r Elektronik, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany h Laboratory of Nuclear Problems, Joint Institute for Nuclear Research, Dubna, 141980 Dubna, Russia i Institut fu¨r Kernphysik, Universita¨t zu Ko¨ln, Zu¨lpicher Str. 77, 50937 Ko¨ln, Germany j Institute for Nuclear Research, Russian Academy of Sciences, 117312 Moscow, Russia k Institute of Physics, Jagellonian University, Reymonta 4, 30059 Cracow, Poland l Institut fu¨r Kernphysik, Universita¨t Mu¨nster, W.-Klemm-Street 9, 48149 Mu¨nster, Germany m Institut fu¨r Hadronen- und Kernphysik, Forschungszentrum Rossendorf, 01474 Dresden, Germany

*Corresponding author. E-mail address: [email protected] (M. Bu¨scher). 1 Now at BICRON, Vertriebs GmbH, 42929 Wermelskirchen, Germany. 2 Now at FH Aachen, Abt. Ju¨lich, Germany. 0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 1 4 7 - 5

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n

Zentralabteilung Technologie, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany Institut fu¨r Schicht- und Ionentechnik, Forschungszentrum Ju¨lich, 52425 Ju¨lich, Germany p Dubna Branch, Moscow State University, 141980 Dubna, Russia q Fachhochschule Mu¨nchen, Fachbereich Physikalische Technik, 80335 Mu¨nchen, Germany r Institute of Nuclear Physics, Radzikowskiego 152, 31342 Cracow, Poland s Physikalisches Institut II, Universita¨t Erlangen, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany (working at IKP II, Forschunsgzentrum Ju¨lich) t Universita¨t GH Paderborn, Abt. Soest, FB Elektr. Energietechnik, Steingraben 21, 59494 Soest, Germany u The Andrzej Soltan Institute for Nuclear Studies, 05400 Swierk, Poland o

Received 4 July 2000; received in revised form 5 October 2000; accepted 5 October 2000

Abstract ANKE is a new experimental facility for the spectroscopy of products from proton-induced reactions on internal targets. It has recently been implemented in the accelerator ring of the cooler synchrotron COSY of the Forschungszentrum Ju¨lich (FZ-Ju¨lich), Germany. The device consists of three dipole magnets, various target installations and dedicated detection systems. It will enable a variety of hadron-physics experiments like meson production in elementary proton–nucleon processes and studies of medium modifications in proton–nucleus interactions. # 2001 Elsevier Science B.V. All rights reserved. PACS: 29.30.
h; 29.40.
n Keywords: Internal targets; Magnetic spectrometer; K+-meson detection

1. Introduction The cooler synchrotron and storage ring COSY at the Forschungszentrum Ju¨lich is a unique facility for medium-energy hadron physics [1]. It provides both unpolarized and polarized proton beams in the momentum range between 294 and 3450 MeV/c, which may be electron cooled up to 615 MeV/c and stochastically cooled above 1500 MeV/c in order to achieve the highest phase space densities and compensate beam deteriorations due to beam–target interactions. In the future also deuterium beams will become available. The beams are used by a large international community at internal installations (COSY-11 [2], COSY-13 [3], and EDDA [4]) and for external experiments (GEM [5], MOMO [6], NESSI [7] and TOF [8]). New additional experiments are being set up (JESSICA [9]) or planned (PISA [10]), TETHYS [11]). The apparatus for Studies of Nucleon and Kaon Ejectiles (ANKE) is a new internal experiment in one of the straight sections of COSY (see Figs. 1 and 2). It was planned and has been built by an

international collaboration between 1994 and 1997 and was installed, commissioned and first used for physics experiments in 1998. It will be exploited to investigate proton–proton, proton–neutron (via deuteron targets) and proton–nucleus collisions. The physics program includes meson production in elementary collisions and modifications due to the nuclear medium as well as hadron–hadron interactions via their final-state interactions (FSI). Thus, ANKE will provide precision data for a more detailed understanding of hadrons in the non-perturbative regime of QCD. An overview of the Physics with ANKE can be found in [12]. With ANKE, the advantages of internal target experiments in storage ring accelerators can be exploited [13]. Windowless targets like gas jets or pellets will be used so that background reactions are minimized. High luminosities can, nevertheless, be achieved since the beam particles repeatedly pass the target (up to 106 times per second). With such targets, the degradation of the energy information for the projectile protons and the reaction products is largely avoided. Deterioration of the beam quality can be compensated by

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Fig. 1. Floor plan of COSY with ANKE. The central magnet D2, the adjacent detector support A and the platform with the electronics B (outside the concrete shielding) are rigidly connected and can be moved on rails.

stochastic cooling of the circulating COSY beam. The energy selection is easy and excitation functions can be studied during acceleration (or deceleration). At COSY, the possibility to switch energy between subsequent acceleration cycles has been implemented (‘‘Superycycle mode’’), which allows background studies (for example at energies below a corresponding production threshold) simultaneously. For polarized targets, an easy and fast change of polarization is possible, which reduces systematic errors in single- or doublepolarization experiments. The present paper describes in more detail the ANKE facility with its detectors and the options for possible experiments. A separate technical paper is in preparation which presents the capabilities of ANKE for K+ identification [14].

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Details about the properties of the magnets are given in [15]. (b) The target, which can be: *

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2. The ANKE facility ANKE (Fig. 2) consists of: (a) A magnetic system, comprising:

a dipole magnet (D1) which deflects the circulating COSY beam by an angle a off its straight path onto a target, a large spectrometer dipole magnet (D2) to momentum analyze, in forward direction including 08, the reaction products, which originate from collisions of the beam with a target in front of the magnet (beam deflection –2a), and a third dipole magnet (D3, identical to D1) to deflect the beam by a back into the nominal orbit.

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a strip target (carbon, polyethylene or any other solid-state target), onto which the beam is moved after injection and acceleration, a cluster-jet target [16], which produces a beam of hydrogen or deuterium clusters that cross the COSY beam, a frozen pellet target [17] for high-luminosity measurements (in preparation),

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Fig. 2. Layout of ANKE consisting of the magnets D1–D3, a target chamber and vacuum chambers. Detector systems for positively and negatively charged ejectiles are placed at the side exits of D2 as well as in forward direction (FD) and in backward direction (BD) near D1. Typical trajectories of ejectiles are indicated with emission angles of 08 or  108. B=bellows, BPM=beam position monitors. The coordinate system has its origin in the center of the pole pieces of D2.

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a polarized storage-cell gas target [18], fed by a Stern–Gerlach-type atomic beam source (ABS) for single- and double-polarization experiments (also in preparation).

Studies on effective neutron targets will be performed using deuterium targets and tagging the appropriate events by detecting the proton spectators [19].

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(c) Detection systems both for positively and negatively charged particles: * *

A large side detector arrangement consisting of plastic scintillator counters for TOF measurements, multi-wire proportional chambers

(MWPC) for tracking and range telescopes for particle identification (see Section 3.2), the forward detector (FD in Fig. 2) comprising scintillator hodoscopes, Cherenkov counters and fast proportional chambers [20] to measure the high-momentum particles close to the circulating COSY beam, a backward detector (BD) made of hodoscopes and multi-wire drift chambers, where magnet D1 is used as a spectrometer of backward emitted particles, a combination of scintillation and Cherenkov counters together with wire chambers to identify negatively charged pions and kaons (under construction), and

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silicon strip counters close to the target for vertex reconstruction (being designed) and detection of low-energy spectator protons (under construction).

R&D studies are performed for a compact, large solid angle photon-detection system [21]. 2.1. Magnets The magnetic spectrometer consisting of three dipole magnets is the main part of the ANKE setup (see Fig. 2). Its principal aim is to separate the ejectiles from the circulating COSY beam in order to identify and momentum analyze them. In the design of the magnets the following requirements were taken into account: *

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the possibility to study ejectiles emitted at angles around 08 from the target, the possibility to analyze both positively and negatively charged reaction products, simultaneously, a large gap height of the main spectrometer magnet D2 for maximum angular acceptance, focusing properties of D2 in a wide-momentum range, variation of the range of detected ejectile momenta independent of the beam energy, short distance between target and detectors to minimize decay-in-flight losses of kaons, maximum space for setup of targets and detector systems, ramping of the magnets during COSY-acceleration cycles, and high transverse field homogeneity to allow the circulation of the COSY beam.

The C-shaped magnets of ANKE have been manufactured by ANSALDO, Italy. They have been built in a very compact fashion by glueing together sheets of 1.5 mm magnet iron. The coils are narrow and the necessary space for their support has been minimized. Stainless-steel supports near the D2 coils have been installed in order to reduce the influence of the magnetic forces on the gap distance to  0.7 mm (see Fig. 2). D2 has a gap height of 200 mm. The dimensions of the yoke are: 1400 mm (width), 2820 mm (depth) and

2660 mm (height); its total weight is about 65 t. For more technical details concerning this magnet we refer to [15]. The large opening in the return yoke enables the installation of detectors for negatively charged particles. D2 is focusing in the horizontal direction for ejectiles leaving the dipole to either side, while defocusing vertically. The focusing property is exploited for particle identification of positive ejectiles (see Section 3.2). D1 is horizontally focusing for particles which leave the target at angles close to 1808 (see Fig. 2). The magnets D1 and D3 have gap heights of 90 mm like the COSY dipoles. The field strengths of the magnets can be chosen independently of the COSY bending magnets since separate power supplies are used for D1/D3 and D2. The maximum field strength of D2 is 1.57 T. The magnetic properties of all three magnets have been studied in the layout phase with the two-dimensional code POISSON [22] and the 3Dcodes PROFI [23] and MAFIA [24]. Field-map and floating-wire measurements for D2 have shown a very good agreement with expectations from the calculations [15]. 2.2. Vacuum system As an internal experiment ANKE is part of the COSY accelerator and such is its ultrahighvacuum system. This system consists of special chambers with thin exit windows in magnets D1 and D2 for particles produced in the beam–target interactions, a target chamber in front of D2, as well as installations for beam-position monitors, pumps and valves. In magnet D3 a standard COSY vacuum tube is used. Beam-line bellows both upstream and downstream of D2 enable horizontal movement (in x-direction, see Fig. 2) of the magnet and target station perpendicular to the direction of the undisturbed beam, in order to optimize the spectrometer momentum acceptance for individual experiments (see Section 2.4). The vacuum chambers of D1 and, in particular, of D2 are large. Perturbing eddy currents during ramping the magnets are almost completely avoided by use of thin 2 mm stainless-steel sheets for the vacuum chamber top and bottom plates [25]. As a further benefit, this construction also

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allows to make optimum use of the gap height of D2. Fig. 3 shows the layout of the chamber and its fixing at D2 which makes use of the suspension bridge technique. The vacuum chamber of D1 has one exit window for backward going positively charged particles of 595 mm length and 100 mm height. The D2 chamber has windows of dimensions 1190 mm (length) and 248 mm (height) both to the positive and negative particle side. In addition, there are smaller windows downstream for high-momentum ejectiles. Presently, aluminium foils of 0.5 mm thickness are used as window material. In order to further reduce the Coulomb scattering in future experiments aiming at optimum momentum resolution (see Section 2.4) thinner aluminium foils or windows based on carbon-fibre compounds are considered and being investigated by the Zentralabteilung Allgemeine Technologie (ZAT) of the FZJu¨lich.

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2.3. Target systems Different targets are available or are in preparation for use at ANKE: strip targets, a cluster-jet target, a frozen pellet target and a polarized target including a storage cell. *

For commissioning and phase-1 experiments with ANKE thin strips of solid target material have been placed at the target position 300 mm in front of D2 (see Fig. 2). The strips typically have triangular shapes and dimensions of 2 mm (width at base), 20 mm (length), 40 mg/cm2– 1.5 mg/cm2 thickness and are suspended from a movable target holder inside the vacuum pipe (see Fig. 4). Luminosities in the range up to (2–3)  1032/(cm2 s) are achieved. Different targets can be inserted simultaneously into the target chamber so that comparative measurements can be performed without breaking the COSY vacuum.

Fig. 3. Drawing of the D2 vacuum chamber with thin top and bottom plates and its stabilisation through the fixing to the magnet yoke. Thin windows at both sides (and in forward direction) are used for the exit of the ejectiles. The positions of the start detectors of the K+ identification system and of the chamber MWPC1 are also shown.

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Fig. 4. Three strip targets can be inserted into the ANKE target chamber, e.g. for studies of the A dependence of the subthreshold K+ production. In the figure, the beam is prepared below the target and steered after acceleration onto the target in use.

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A cluster-jet target has been constructed at the Universita¨t Mu¨nster, Germany, for studies of proton–proton and proton–deuteron interactions [16]. It is similar to the one built by the same group for the COSY-11 experiment [2]. Such a target provides area thicknesses of up to 1014 atoms/cm2 (corresponding to luminosities of up to a few 1030/(cm2 s)) with typical dimensions of less than 10 mm perpendicular to the beam direction [16]. This target will enable the use of stochastically cooled beams at ANKE with lifetimes of hours, as the experience at COSY-11 has shown, since the beam heating can be compensated for. The target chamber has been built in such a way that a fast switching between strip targets and the cluster-beam target is possible. A pellet target for use at ANKE is being built by the Institute for Theoretical and Experimental Physics (ITEP), Moscow, Russia, and the Moscow Energy Institute (MEI), Russia, in a collaboration with FZ-Ju¨lich [17]. A similar target is in use at the CELSIUS accelerator in Uppsala, Sweden [26]. It will provide luminos-

ities which are expected to be about two orders of magnitude larger than those of the cluster target thus allowing high statistics and/or low cross-section measurements. A storage-cell target with an atomic beam source (ABS) for polarized protons and deuterons is also under development [18]. A collaboration of the universities of Erlangen and Ko¨ln, Germany, the Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia, and the FZJu¨lich is building the ABS and the storage cell, similar to the one at IUCF [27] and HERMES [28]. The polarized atoms of the ABS will be fed into a tube of about 400 mm length and appropriate cross section (300 mm2), collinear with the circulating COSY beam. By this the effective thickness will be increased by about 1–2 orders of magnitude as compared to a crossed-beam target. A value of almost 1014 atoms/cm2 is envisaged.

The position of the target in the vacuum system along the beam direction is – within certain limits – a free parameter at ANKE. The distance from D2 affects the geometrical acceptance of the spectrometer, and it influences the distance of the focal surface from the exit window of D2 [29,30]. The closer the target to the entrance of D2, the further away is the focus for ejectiles. Thus, the loss through decay-in-flight for short-lived particles increases, since the range telescopes must be located along the focal surface. A distance of 300 mm from the magnet iron of D2 has been chosen as the target position for the strip and cluster target (this position is depicted in Fig. 2). For the pellet and storage-cell target, a larger distance will be necessary. 2.4. Operation modes of ANKE For each measurement the field strength of D2 has to be adapted to the experimental requirements. Given the beam momentum, the deflection angle a of the COSY beam in D1 is determined by a ¼ 12B D2 leff =ðBrÞ with B D2 the field strength in the center of D2, leff the effective field length in D2 along the COSY-

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beam trajectory and (Br) the magnetic rigidity of the beam. (Br) is related to the beam momentum by p ¼ ðBrÞ=3:3356 for p in GeV/c and (Br) in Tm. Since D1/D3 and D2 have separate power supplies, a variety of experimental conditions can be realized. Three typical cases are illustrated in Fig. 5: *

In Fig. 5a the case for the subthreshold reaction pA ! pK þ X is illustrated for an incident proton energy of 1.0 GeV (corresponding to a momentum of 1.7 GeV/c). B D2 has a value of 1.30 T while the field strength of the COSY bending magnets is 0.81 T. The appertaining value of a is 10.18 with this setting, the ‘‘K+ mode’’. Ejectiles, emitted at 08 with momenta between 110 and 510 MeV/c exit through the

Fig. 5. Three typical operation modes of ANKE. See text for details.

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D2 side window towards the focal-surface detectors. Fig. 5b demonstrates the situation for a high COSY-beam energy (Tp2.6 GeV, p3.4 GeV/c), e.g. for the investigation of the reaction pp ! daþ 0 . In this case, D2 is at its maximum field strengths of 1.57 T. The deflection angle a is 6.28. In order to optimize the acceptance for high-momentum deuterons, the target (and simultaneously the orbit of the circulating beam) is shifted in the –x direction, in the frame of the coordinate system defined in Fig. 2. It can be seen that the distance between beam and vacuum tube at the exit of D2 is as small as 15 mm. Ejectiles with momenta up to 80% of the beam momentum can be analyzed. Fig. 5c finally shows an example for ejectiles with very small momenta, originating from the reaction pC !pCZ for a projectile energy of Tp=1.58 GeV ( p=2.34 GeV/c). For B D2 =0.60 T momenta of produced particles down to 300 MeV/c and with a charge of 4 can be analyzed. a is equal to 3.48. Simultaneously emitted protons will be detected behind the windows at the far end of the D2 vacuum chamber.

For each of the settings the magnet D2 needs to be displaced horizontally perpendicular to the nominal beam direction. For this purpose, D2 is installed on rails. ( The magnets D1 and D3 have fixed positions at an angle of 3.68, which minimizes edge focusing.) Bellows allow the vacuum chambers to follow this movement (see Fig. 2). Aluminium bridges borne by D1 and D2 as well as by D2 and D3 guide the movement, carry the weight of the vacuum chambers and provide the forces for the necessary deformation of the bellows. The target position with respect to D2 can be adjusted perpendicular to the COSY beam. In the case of the K+ mode, shown in Fig. 5a, this position as well as B D2 are kept constant while the projectile energy is varied in the measurement of protoninduced K+ production on nuclear targets (Tp=1.0–2.3 GeV). In fact the target is placed at x=
80 mm. In this mode, ejectiles with the same momentum hit the same telescopes in the focal surface as elucidated in Fig. 6 (see also Section 3.2).

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Fig. 6. The settings of ANKE for the investigation of the subthreshold K+ production at projectile energies of Tp=1.0 (right-hand side) and 2.3 GeV. In this K+ mode, the field strength of D2 and the relative position of target and D2 are kept constant. Particles with given momenta imping on the same detector, independent of Tp.

a changes with (Br) of the beam between 10.18 and 5.58. Magnet D2 together with the detector platform are displaced correspondingly. 2.5. Angular and momentum acceptance of ANKE Simulation calculations have shown that the horizontal acceptance of ANKE is approximately  (128–158) for positively charged particles which are emitted in the forward direction and hit the K+-detection system (see Section 3.2). The vertical acceptance depends on the exit position at D2 [29], i.e. on the momenta of the ejectiles and the operation mode of ANKE; typically it ranges between  38 and  58. As an example, the acceptance for a typical setting of ANKE is shown in Fig. 7a. The momentum acceptance depends on the operation mode and is large as the examples of Fig. 5 show. For the different experiments the acceptances are determined by measurement of well-studied reactions, for example pp ! dp+, as well as by simulations. Fig. 7(bottom) shows the horizontal acceptance for the same setting as in Fig. 7(top) (K+-mode) 3. Detector systems of ANKE 3.1. General considerations In proton-induced interactions with elementary or nuclear targets, ejectiles of all charge states are

Fig. 7. (top) Vertical angular acceptance for positively charged particles calculated for a beam momentum of 2.73 GeV/c and a D2-field strength of 1.30 T. The target – D2 distance is 300 mm. (bottom) Horizontal acceptance of D2. The shaded areas are the acceptances in the horizontal plane for vertical emission angles y=08. The gap between the two acceptance ranges is caused by the corner structures of the vacuum chamber.

produced either directly or via the decay of shortlived reaction products (for example f ! K+K
). Positively charged particles ( p, p+, K+) are deflected to the right-hand side of the circulating COSY beam (see Figs. 1, 2), while negatively charged secondaries (p
and K
) are bent to the left-hand side. Neutrals not decaying close to the target can in principle be detected in detectors

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behind D2. For this purpose a window in the vacuum system of D2 has been implemented. The first-generation detector setup has been designed to optimize identification and measurement of K+ mesons, produced in proton–nucleus collisions, in particular at energies below the free NN-threshold at 1.58 GeV (‘‘subthreshold production’’). Operating ANKE with B=1.30 T, kaons with momenta between 110 and 525 MeV/c will leave D2 through the side exit window and are horizontally focused. The experimental difficulty is to detect the kaons, which are produced with small probability in a huge background of protons and positively charged pions. For example at an incident beam energy of 1.0 GeV the ratio K+: p+: p is 1: 106: 107 in the forward cone y 5108. The detection system, which has been optimized for kaon identification is described in Section 3.2 below. For the study of correlated emission of K+ and particles like protons and deuterons, detectors in the forward direction are needed. Together with the backward detection system at D1 they will also be used for the study of correlated particles from proton-induced deuteron breakup. The description of the corresponding setup can be found in Section 3.3. The presently realized negative particle detector system is also shown in Fig. 2. 3.2. K+ detection system Guided by GEANT simulations [30] a system of segmented detector telescopes in the focal plane of D2 has been developed (see Fig. 2). All particles hitting one of the 15 telescopes fall into a momentum bite of about 10%, provided they originate from the target. The simulations have shown that this momentum spread is acceptable in view of the necessary highly effective particle identification. Thus, protons, pions and kaons can be discriminated on the basis of their time-offlight (TOF), energy loss (DE) and range. The time-of-flight is measured between the 23 thin start detectors close to the exit window of D2 and stop detectors, which are part of the telescopes (see inset of Fig. 2) at the focal plane. Start and stop detectors consist of vertically oriented plastic

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scintillators (Pilot-U, Nuclear Enterprises, and BC408, Bicron, respectively). Starting from the low-momentum side, the start scintillators 1 and 2 have a thickness of 0.5 mm, whereas it is 1.0 mm for detectors 3–5 and 2.0 mm for the rest. All detectors are 50 mm wide and 270 mm high. The thickness of the start scintillators is a compromise between sufficient light output and small angular spread by multiple scattering. The stop detectors are 10 mm thick and 100 mm wide; their heights vary between 520 and 1000 mm adapted to the vertical acceptance of D2. For all scintillators the light is detected at both ends via photomultipliers (Philips). For the start detectors 1 inch phototubes (XP2972) are used and 2 inches tubes (XP2020) for the stop detectors. The phototubes are shielded against the stray field of D2 by a combination of mmetal and iron cylinders. Additional shielding with iron plates of up to 5 mm thickness is used for the tubes closest to the magnet. A precise time information (DtTOF400 ps (FWHM) for the stop-DE, and 600 ps (FWHM) for the start–stop TOF, respectively) is achieved by constant fraction discriminators and mean timers, both especially developed for this purpose [31]. In addition, spatial information is obtained from the time difference of both phototubes. The resolution obtained for the stop detectors is better than 30 mm (FWHM). The energy loss of the ejectiles is also used for particle identification. In order to increase the difference in the energy losses of pions and kaons, tapered copper degraders of variable thicknesses are employed (see Deg. I in Fig. 2). Particles lose energy traversing the degraders and thus the energy loss of kaons in the DE detectors is towards the end of their range. Simulations [32] have shown that the best discrimination between p+ and K+ is achieved when kaons just penetrate the DE counters and are stopped in Deg. II. The momentum variation across the width of the telescopes is taken into account by the trapezoidal shape of degrader Deg. I. The veto counters can be effectively used for discrimination against pions and fast scattered protons. The protons from the target are already stopped in the first degrader, while kaons at most make it to the second one. The veto counters

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register, however, the delayed decay products of the stopped kaons. This is used for K+ identification at subthreshold energies [14]. The telecopes for higher ejectile momenta each have lucite Cherenkov detectors between the stop scintillator and the first degrader (see Fig. 2). While pions have sufficiently high velocities to produce a Cherenkov signal, the light output for kaons is weak or absent. The start scintillators and the stop telescopes have been built at the FZ-Ju¨lich with contributions from partners of the international collaboration. Test measurements with one start counter and one stop telescope have been performed with protons and pions at the synchrocyclotron of the PNPI and with proton, pion and kaon beams at the synchrotron of ITEP in order to experimentally confirm the expectations from simulations [33]. Besides the start scintillators and stop telescopes, the K+-detection system contains two multiwire-proportional chambers (MWPC 1 and 2) [34] for particle tracking in order to determine their emission angle and momentum. Both chambers have been built at the Forschungszentrum Rossendorf, Germany. The sensitive areas are 350  1300 and 600  1960 mm2. Both consist of three planes of anode wires (vertical, +308,
308). The tungsten wires have a thickness of 20 and 25 mm in the smaller and larger chamber, respectively. Their spacing is 2.54 mm. The cathodes are C- or Al-covered Mylar foils with a thickness of about 20 mm. The distance between the cathode and anode planes is 5 mm. Special care has been taken to avoid any massive structure for the frames and to keep, nevertheless, the wires and cathode foils sufficiently tensioned. The forces are compensated by pre-formed Stesalith frames, onto which the wires or foils are fixed. The chambers are operated with a gas mixture of 70% argon and 30% CO2 with some alcohol admixture. All chambers have been tested with radioactive sources and cosmic rays. In addition, the behavior of the smaller chamber was tested at the BIG KARL facility of COSY [5]) in order to determine the response to protons: a spatial resolution of s=0.7 mm horizontally and 1.5 mm vertically was achieved [34,35]) for angles of incidence as large as 508 with respect to the normal of the MWPC.

Fig. 2 shows the whole arrangement of detectors at the exit window of D2. In order to obtain as long a flight path as possible for TOF measurements, the start detectors are placed immediately behind the exit window (see Fig. 3) – in fact the support frame is fixed to the magnet. The whole detector system is positioned on a movable platform, which is rigidly connected to D2 such that the system follows the movements of D2 (see Section 2.4). It has been commissioned at its final measurement position with the horizontal component of cosmic radiation. 3.3. Other detection systems The forward detection system of ANKE consists of three multiwire proportional chambers for track reconstruction (MWPC 3–5 in Fig. 2) and two scintillator hodoscopes with two planes each. One of these hodoscopes (‘‘forward hodoscope’’) is placed behind the forward MWPCs, while the second one is located near the high-momentum edge of the K+ detection system (‘‘side wall’’). Cherenkov detectors are placed behind both hodoscopes [36]. The MWPC’s have two wire planes each, with horizontally and vertically arranged wires at a distance of 1 mm as well as two strip planes, which are inclined by  188 with respect to the wires. The chambers are optimized for operation at high event rates, which are expected to be as high as 107 s
1 per cm of detector width. The forward hodoscope consists of two vertical sets of 8 and 9 scintillators (polystyrene, produced at JINR) in the first and second layer, respectively. Six counters in each plane at the low momentum side have a width of 80 mm and a thickness of 20 mm. The others are 15 mm thick and have a smaller and varying width (40 and 60 mm in the first plane, 40, 50, and 60 mm in the second plane), because the count rate strongly increases at the high-momentum region close to the accelerator beam tube. The height of all scintillators is 360 mm. They are read out at both sides via lucite light guides and phototubes (Philips XP4222B, XP2972). The 16 Cherenkov detectors behind the forward hodoscope are made of lucite (80 mm wide, 50 mm

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thick, 300 mm long). By selecting a proper inclination angle b with respect to the vertical axis, see Fig. 8, discrimination of deuterons against protons of the same momentum is possible [36] if these counters are used as veto counters: The opening of the light cone of the protons is larger than that of the deuterons. Hence, total reflection can be achieved for the protons whereas the deuterons do not produce signals at the photomultipliers. The angle b can be adjusted for optimal particle discrimination depending on their momenta. The counters have been tested and used under beam conditions during an experiment on o-meson production in the reaction pd ! dopsp. These measurements require the detection of slow spectator protons psp in coincidence with fast (2.0 GeV/c) deuterons. The latter have to be discriminated against a high background of protons. The measurements showed that at an inclination angle of b=108 a proton-suppression factor of Fp=25 and, simultaneously, a deuterondetection efficieny (i.e. signals below a certain discrimination threshold) of ed=75% can be

Fig. 8. Geometry of the Cherenkov counters behind the forward hodoscope for proton–deuteron discrimination.

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achieved. By varying the discrimination threshold, different values of Fp and ed can be chosen (e.g. Fp=15, ed=90%). The side wall consists of two layers of 5 and 6 scintillators (polystyrene), which have a width of 100 mm, a height of 1000 mm, and a thickness of 10 mm. They are also connected to phototubes (Philips XP2020) at both ends via lucite lightguides. In this case, straight Cherenkov detectors will be utilized. The wire chambers and the corresponding scintillator hodoscope have been produced at the Joint Institute of Nuclear Research (JINR), Dubna, Russia, while the scintillators of the side wall were built by PNPI. The Cherenkov detectors have been developed and constructed at the High Energy Physics Institute, Tbilisi State University (HEPITU), Georgia and JINR. For monitoring three different procedures are used. A telescope of three scintillation detectors (50  50  5 mm3, photomultipliers XP2020) outside the target chamber essentially counts pions from the target and allows relative monitoring. This device has been built at JINR, in cooperation with Moscow State University (MSU), Russia. Relative monitoring is achieved as well from the telescopes at the low momentum side which view the target directly. Absolute monitoring is achieved with silicon-strip detectors, which are used to detect proton–proton and proton– deuteron elastic scattering from the cluster target. These counters may also allow the study of reactions on quasi-neutron targets via the detection of spectator protons from pd reactions [19]. For future measurements it is planned to observe fast elastically scattered protons in the forward detectors for luminosity monitoring. The backward detection system (built at JINR) consists of three multiwire drift chambers and a two-layer scintillator hodoscope. The MWDCs contain an x and y plane each; they have a cell width of 40 mm and a cathode wire spacing of 2 mm. The cathodes are 9 mm apart. Each layer of the scintillator hodoscope contains 8 scintillation counters (dimensions 280  62  5 mm3 in the first and 280  62  20 mm3 in the second plane, polystyrene). The signals are read out with XP2020 PMs.

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3.4. Electronics and data acquisition Digitization of the signals from the scintillation and Cherenkov detectors is performed with CAMAC (FERA) and FASTBUS QDC and TDC modules. Readout is initiated by a trigger, based on information on TOF and energy loss of the ejectiles. With one specially built VME-module for each stop counter it is possible to set a common TOF gate (length variable between 3 and 23 ns) in coincidence with up to 16 individually adjustable start–stop combinations [31]. Sixteen of these modules are needed for the ANKE TOF trigger, which can select pions, kaons or protons within about 70 ns. For the necessary decision time, the analog signals are delayed by 60 m long cables (300 ns). The system has been developed by FZJu¨lich and RWTH Aachen. All scintillator detectors are equipped with LEDs (built at FZRossendorf), which allow to monitor the pulse heights of the counters and to check the triggerand data acquisition system. For the MWPCs a highly integrated readout system, based on chips RAL 111 and RAL 118 of the Rutherford Appleton Laboratory, Oxford, Great Britain, is used [37]. The electronics boards are placed directly on the chambers. This readout system was developed at the FZ-Rossendorf, Germany, and the Zentrallabor fu¨r Elektronik (ZEL) of the FZ-Ju¨lich [37]. The multi-crate data-acquisition system supports the standard readout systems CAMAC, FASTBUS and VME, designed by the ZEL, FZJu¨lich [38], to meet the requirements of ANKE, e.g. recording of high trigger rates for mediumsized events (55 kbyte). For a total trigger rate of 10 kHz, approximately 50% of the events are written on tape. The readout time per event is about 100 ms. The crates are read out in parallel using powerful and cost efficient INTEL compatible PCs (single board), running under UNIX (NetBSD). The data are transmitted in clusters of sub-events via a Fast-Ethernet connection to the event builder and are written on tape using a fast DLT tape drive. In order to ensure the correctness of every event, each readout system employs a synchronization module developed for this purpose. These modules are interfaced by a ring-like bus system. The parallel readout system is scalable

over a wide range and is supported by an extensive body of software. The software enables interactive communication with the individual subsystems, which is particularly useful during commissioning and for general diagnostic purposes. 4. Commissioning and detector tests 4.1. Commissioning Commissioning of ANKE with the dipoles in the COSY ring started with developing appropriate acceleration procedures. ANKE may affect the operation of COSY by *

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a mismatch of the currents of D1/D3 and D2 causing closed orbit distortions and, therefore, lower acceptance and lower beam intensity at injection, eddy currents in the vacuum chambers and the magnet iron disturbing the balance of the threemagnet chicane, edge focusing of the dipoles disturbing the symmetry of the COSY lattice, inherent sextupole components and those induced by vacuum chamber eddy currents, which affect the dynamic acceptance of COSY, and an increased RF impedance due to the structure of the vacuum chambers leading to intensity limitations at higher beam momenta.

The perturbation of the ion optics by ANKE was minimized by an appropriate setting of the quadrupoles in the telescopic straight sections of COSY. At the target position b-function values in both planes of 3–4 m have been achieved. The ramping currents of the two power supplies for D1/D3 and for D2 are controlled such that they follow the COSY-beam momentum. The nonlinear I(B) dependence has been deduced from field-map measurements and forms the basis for the computer-controlled ramping procedure. These steadystate currents were also confirmed by a floating wire measurement over the whole setup of all three ANKE magnets [15]. Dynamic effects due to eddy currents and different magnetic inertia of the dipoles are corrected by backleg windings in D1 and D3. Inherent and induced sextupole components as well as the change of the impedance

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turned out to be non-critical. Acceleration up to the maximum COSY energy at an angle a=6.28 or – restricted by the maximum D2 field of about 1.6 T–up to 1.2 GeV (1.9 GeV/c) at the maximum deflection angle of a=10.68 is routinely achieved with intensities of (1–4)  1010 protons in the ring. The next commissioning step was the control of the beam-target overlap. In the case of the cluster target the procedure is straightforward. The cluster beam is switched on after the COSY beam is prepared at the requested energy. Only little horizontal steering may be necessary for a good overlap of cluster and accelerator beam. The procedure for the strip target is more complicated. Due to the 105 and more times higher density only a small part of the beam is allowed to hit the target. The target is mounted in a fixed position with its tip close to the beam axis (see Fig. 4). At injection and acceleration, the beam circulates below and sidewards of the target. During the experiment cycle, the beam is slowly moved upwards onto the tip of the target and continously used up such that the count rates are acceptable for the detectors and for the data-acquisition system. A feedback loop controlled by an appropriate detector signal enables constant count rates (see Fig. 9), thus minimizing deadtime losses. Typical cycle times for the strip target experiments are in the range of 20–60 s. In addition to the vertical steering, a careful horizontal steering has to be applied in order to make full use of the beam. Recent developments aimed at the investigation of a pure horizontal steering in order to avoid acceptance losses due to the large amplitude of the two-steerer bump which is used for the vertical beam movement. At injection and acceleration the beam can be passed beside the targets by controlling the amplitude of the D1–D2–D3 chicane. During the experimental cycle, the beam is horizontally shifted by a foursteerer bump which, in contrast to the vertical twosteerer bump, allows to precisely control both beam position and angle at the target. 4.2. Tests and first measurements The goals of the studies in the running-in phase of ANKE were checks of the performance of all

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Fig. 9. Oscilloscope picture of a typical COSY cycle. During data acquisition the rates at the ANKE detectors stay approximately constant.

the detectors, in particular of those for the K+ identification (see Fig. 2), as well as of the electronics and data acquisition. The detectors showed the properties which were anticipated from design studies with simulation calculations and from laboratory tests. The thicknesses of the degraders in the stop telescopes have been tuned during the first experimental runs for optimum discrimination of kaons from the background (Deg. I) as well as for stopping the kaons in front of the veto counters (Deg. II). Also the electronics and, in particular, the delays of the fast trigger system needed adjustment. This trigger – based on flight times and energy losses of the ejectiles (see Section 3.4), together with the selection of delayed signals from K+ decay products in the veto counters (see Section 3.2) – provides an online suppression factor for background of about 105. The two-body reaction pp ! dp+ with nearly mono-energetic pions from a CH2-target (polyethylene) as well as from an H2 cluster target has been used for momentum calibration and for the determination of the momentum resolution of the spectrometer. The result of the calibration is consistent with simulation calculations within Dp/p=1% [15]. The momentum resolution is in the range of one to a few percent, depending on

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the momenta of the particles and the setting of ANKE [39]. It is limited by multiple scattering of the ejectiles in the materials between the target and MWPC 2. A typical result of the calibration measurements is shown in Fig. 10. The projectile energy was Tp=495 MeV kinematically corresponding to pp=315 MeV/c. The momentum resolution amounts to 2.2%. The measured momentum resolution as a function of the pion momentum is compared with the results of the calculated values in Fig. 10c. There are plans to improve the resolution for pions to better than 1% by installing thinner exit windows. The horizontal angular acceptance of ANKE with good focusing at the positive particle side is given by the geometrical range in which the ejectiles pass through a well-defined magnetic field region. It has been checked with pion and proton

ejectiles from proton–carbon reactions that this is the case at least for  128. Fig. 11 shows the timeof-flight spectra for different start counters and the stop telescope 11. Peaks with a resolution of 670 ps (FWHM) for pions and 800 ps (FWHM) for protons are observed for the start detectors from 8 through 21 corresponding to emission angles W=
128 to +158. Due to aberation of D2, ejectiles with larger emission angles are not horizontally focused onto the telescopes, thus causing an enhanced width of the TOF peaks. As the first physics experiment at ANKE, the K+ production in proton–nucleus interactions has been investigated at different projectile energies, in particular at energies far below the free nucleon– nucleon threshold of 1.58 GeV. Momentum spectra of the kaons have been measured for carbon (diamond) targets at Tp=1.0, 1.2, 1.5, 1.8, 2.0, 2.2

Fig. 10. Momentum spectra of pions from the two-body reaction pp ! dp+. (a) singles spectrum (b) exclusive spectrum where the deuterons have been detected in the forward detector (FD in Fig. 2). The coincidence condition eliminates the background from threebody reactions. (c) Dependence of the resolution on the pion momentum.

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Fig. 11. Time-of-flight spectra for three different start counters and the stop detector in telescope 11 of the K+-detector system. Narrow peaks are observed for protons and pions with the start detectors 8 and 21, defining the acceptance range for this particular experiment. While the TOF-resolution is about 0.8 ns for protons in the case of start 8 and 21, it is 1.2 ns for the combination start 5– stop 11. The corresponding smearing of ejectile momenta does not allow a separation of kaons from background particles for start detector 5.

and 2.3 GeV. At energies of 1.0 and 2.3 GeV the mass dependence of these momentum spectra has been determined for carbon, copper and gold targets. Fig. 12 shows a TOF spectrum between start counters 8–18 and the stop counter of telescope 11. As in Fig. 11 two distinct peaks for pions and protons originating from the target are observed. In addition, there is a broad background distribution of ejectiles produced in secondary reactions on the pole shoes of D2 or in structure elements of the vacuum chamber. These background particles can be largely suppressed with the help of the vertical MWPC information. As shown in the upper part of the figure, particles from the target in general have different vertical angles when passing through the chambers at a certain height. The dashed histogram shows the events remaining after selecting the ejectiles coming from the target. The scattered background is suppressed by more than one order of magnitude and in the range where kaons are expected, a small peak becomes visible. The time resolution for pions (and kaons) is 650 ps FWHM. Thus, the TOF difference between pions and kaons is 8s which allows a very good discrimination between target pions and kaons. The proton distribution is significantly broader (1000 ps FWHM) due to the following reasons: (i) The flight paths for the various start counters have different lengths. In case of the pion distribution this effect has been corrected for by shifting the individual peaks from Fig. 11 onto

Fig. 12. TOF spectrum for telescope 11 summed over start counters 8–18 which cover horizontal emission angles of jyj5128. The data were taken at Tp=2.3 GeV and B=1.57 T in D2 using a carbon target. The solid histogram is the raw spectrum. The dashed histogram shows the remaining events after selecting particles from the target (see text and illustration on top). The shaded histogram are pion and kaon events which are inside gates on the energy losses in the stop- and DEcounters.

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each other. (ii) Telescope 11 covers a momentum byte of 8%. This causes a variation of the proton velocities in contrast to the pions which have velocities close to the speed of light. The intrinsic counter resolution for protons is 550 ps due to the larger light output as compared to pions and kaons. The shaded histogram in Fig. 12 has been obtained by setting cuts on kaons in the energyloss spectra of the stop and DE counters of telescope 11. Only two peaks remain which can be attributed to pions and kaons [40–42]. The kaon peak of the dashed histogram (only MWPC cut) is shifted relative to the ‘clean’ peak due to non-symmetric background [14]. As in the case of the target protons, the kaon TOF has not been corrected for the different flight paths which causes a slight widening of the distribution. The pions which are seen are the ones in the high-energy tail of the Landau distributions ‘leaking’ into kaon gates. The data shown in Fig. 12 have been obtained at Tp=2.3 GeV where the kaon-to-pion ratio is reasonably large. At this high energy, the MWPC and energy-loss cuts are sufficient to identify kaon events. At the lowest energy, Tp=1.0 GeV, at which K+-production has been measured with ANKE, the kaon-to-pion ratio is 104 times smaller. In this case, it is necessary to use additional criteria for background suppression, in particular, to set gates on delayed events in the veto counter as it was discussed in Section 3.2. A more detailed description of these data-analysis procedures has been presented in [40–42] and a dedicated publication is in preparation [14] where it is shown that kaon production in proton– nucleus collisions can be studied with ANKE even at energies far below threshold down to Tp=1.0 GeV.

Acknowledgements We would like to acknowledge the support we received during design, construction, and installation of ANKE. The infrastructure sections of all our collaborating institutions were of indispensable help. In particular, we would like to mention G. Krol and Th. Sagefka (Accelerator Division

IKP, FZJ), U. Rindfleisch (Construction IKP, FZJ), M. Karnadi (Data Acquisition Division IKP, FZJ), D. Protic and J. Pfeiffer (Detector Lab IKP, FZJ), J. Bojowald, N. Dolfus, H. Labus, G. Lu¨rken, and R. Nellen, (Electronics IKP, FZJ), H. Hadamek, (Mechanics Workshop IKP, FZJ), W. Bertram and H. Maselter (ZAT, FZJ), M. Freitag, M. Sobiella, and J. Hutsch (FZ-Rossendorf) as well as our IKP-II technicians G. D’Orsaneo, W. Ermer, and P. Wieder. Our thanks also go to P. Bachmann (Philips Research Laboratory, Aachen, Germany) and to P. Koidl (Fraunhofer Institut Angewandte Festko¨rperphysik; Freiburg, Germany) for the preparation of diamond-foil targets. The COSY accelerator crew did an excellent job not only during the preparation phase but also in commissioning ANKE in the accelerator ring. Finally, we would like to thank the national funding agencies of Georgia (Department of Science and Technology), Germany (BMBF, DFG, FZJ and State of North-Rhine Westfalia), Poland (Polish State Committee for Scientific Research, Grant #2 P03B 101 19) and Russia (Russian Ministry of Science, Russian Academy of Sciences, Grant #99-02-04034, 99-02-18179a), as well as the European Community for their financial support.

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