The Crystal Ball was built at SLAC and used in J/Y measurements at SPEAR and b–quark physics at DESY. It was moved to BNL at the end of 1995 for a program in baryon spectroscopy and h physics. The Crystal Ball consists of a highly-segmented sphere made of 672 optically isolated NaI(Tl) crystals, 15.7 radiation lengths thick (see Fig. 4). The sphere has an entrance and exit tunnel for the beam and a central spherical cavity. The solid angle of the CB is 93% of 4p sr. Electromagnetic showers in the spectrometer are measured with an energy resolution s_E/E ~ 1:7%/(E (GeV))^0.4; the angular resolution for photon showers at energies of 0.05– 0.5 GeV is s_q = 2^o–3^o in the polar angle and s_q = 2^o/sinq in the azimuthal angle. For the experiments at AGS a cylinder of scintillation counters functioned as a veto.

TAPS will be implemented as a forward wall at a distance of 1.8 m after the CB target. It covers the downstream tunnel area of the Crystal Ball (20^o with respect to beam direction). In its present configuration, TAPS consists of 528 individual BaF₂ hexagonally shaped detectors with an inner diameter of 59 mm and a length of 250 mm (corresponding to 12 radiation lengths). The thickness of the detector is sufficient to stop 180-MeV p± , 280-MeV K± , and 360-MeV protons. Each detector has a 5-mm plastic scintillator (NE102A) that serves as a charged-particle veto detector.

BaF₂ has two scintillation-light components with very different decay constants t_s = 0.76 ns and t_l = 620 ns. The relative light yield, which depends on the ionization density of the particle, enables a pulse-shape analysis to discriminate between particle species. The excellent time resolution of the TAPS detector of FWHM=0.5 ns and the long distance to the target allow an efficient TOF measurement for further particle identification. The relative energy calibration (matching of the individual detectors) is performed by measuring minimum-ionizing cosmic-ray muons, while the absolute calibration is done using the invariant mass of the p^o ® 2g and h ® 2g decays in an iterative method for each crystal. The experimentally obtained invariant-mass resolution for p^o mesons is 19 MeV FWHM and for the h meson (h ® 2g) the FWHM is 45 MeV. The detector constitutes a good system for the measurement of multiphoton events as well as protons or charged pions. Extensive Monte-Carlo simulations assure us that the combined detection system will handle the anticipated beam-related background rate.

Included in this request are funds to add a particle-identification detector and charged-particle tracker in the central region of the Crystal Ball. As shown by beam studies and simulations, a particle-identification counter and tracker will reduce our background and allow us to perform measurements that include final states with both neutral and charged particles [Bec02, Wat02]. Design and construction of these items are well within our expertise – in addition to construction of scintillation-counter detectors for experiments at LAMPF, LNS, PNPI, TRIUMF and JLab, one of us (WJB) has also participated in the construction of the same type of cylindrical MWPC as is proposed here while on sabbatical in 1987-88 at CEN-Saclay. Component construction and testing will take place at GW, Glasgow, and Pavia, with final assembly at MAMI.

A high-resolution, high-efficiency central-tracker detector is a vital part of the proposed experimental setup at MAMI. Two coaxial cylindrical multiwire proportional chambers (MWPC) similar to those developed at Saclay for the DAPHNE large-acceptance tracking detector [Aud91] will be used for this purpose. Because of time constraints we will borrow a set of DAPHNE chambers for Phase I, but chambers specifically designed for the Crystal Ball will be built for Phases II and III. The chambers cover the entire azimuthal angular range. The lengths of the cylinders are arranged such as to subtend a range of angles in the polar direction q from 21^o to 159^o, which corresponds to 94% of 4p sr for a source at rest.

The inner and outer cylindrical walls of the Phase-I MWPC (Fig. 5) are made of 1-mm Rohacel covered with 25-mm Kapton film. The interior surfaces are laminated with aluminum strips (0.1 mm thick, 4 mm wide and separated by 0.5 mm) that form the cathode. The anode surface consists of arrays of 20-mm diameter tungsten wires stretched parallel to the cylindrical axis at 2-mm intervals around the circumference. The inner chamber has a length of 360 mm; the internal diameter is 60 mm, and the external diameter is 68 mm. For the outer chamber, those dimensions are 560 mm, 92 mm, and 100 mm, respectively. The inner and outer cathode strips are wound helically in opposite directions at an angle of ± 45% with respect to the anode wires. The anode-to-cathode gap is 4 mm, and a mixture of Ar (74.5%), ethane (25%) and freon (0.5%) is used as a filling gas. The chamber provides a track-reconstruction efficiency of ~85% for p± and ~90% for protons. The resolution for the polar and the azimuthal angle (which is a function of the polar angle) is expected to be Dq_FWHM £ 2^o and Df_FWHM £ 4^o. The cylindrical chamber, as well as other prototypes, was tested at MAMI with a photon beam in order to estimate the event rate through the central tracker device [Wat02, Sta02, Bec02]. The results of these tests as well as simulations are being used in the Phase-II design, which will be similar to that of the Phase-I MWPC but smaller in diameter and make use of improvements in materials and electronics. A cost estimate of the Phase-II MWPC is $13K for materials (capacitors, copper coated vetronite, connectors, etc.) and about $47K for associated electronics (ADCs and TDCs). We request the costs for the electronics. Details will be finalized in Fall 2003 after our first full-beam running.

A cylindrical PID detector made of scintillation counters will be built for our program at MAMI. To cover the full f acceptance of the Crystal Ball we will need 32 1-cm wide detectors with tubes at each end. The characteristics of subminiature PM tubes (Hamamatsu R7400U) connected to thin, 20-cm long scintillators which fit between the outer wall of the LH₂ target and the inner radius of the supports for the MWPC have been tested at Glasgow. This is the simplest solution for placement of the PID counters for Phase 1 – we will be using the larger DAPHNE MWPC. The scintillators are 20 cm long by 1 cm wide and samples having thicknesses of 1 mm and 2 mm were tested. The Glasglow group is looking at the Landau distributions from cosmic rays at various distances along the scintillators. The results of these tests will be compared with simulations made by Polaski [Pol02]. Cost estimates include PMTs and bases ($32.6K), scintillator and light guides including machining and assembly ($16.3K), mechanical construction, assembly, frame, connectors, and cables ($6.0K), electronics and amplifiers ($17.0K), and readout system ($17.7K), for a total of $89.6K with our share of the costs being set at $40K. Final design decisions for the Phase-II PID will be made in Fall 2003 after our first full-beam running.

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