About 120 baryons and baryon resonances are known, from the abundant nucleon with $u$ and $d$ light-quark constituents up to the recently discovered $\Omega_b^-=bss$, and the $\Xi_b^-=bsd$ which contains one quark of each generation In spite of this impressively large number of states, the underlying mechanisms leading to the excitation spectrum are not yet understood Heavy-quark baryons suffer from a lack of known spin-parities In the light-quark sector, quark-model calculations have met with considerable success in explaining the low-mass excitations spectrum but some important aspects like the mass degeneracy of positive-parity and negative-parity baryon excitations are not yet satisfactorily understood At high masses, above 18 GeV, quark models predict a very high density of resonances per mass interval which is not observed In this review, issues are identified discriminating between different views of the resonance spectrum; prospects are discussed how open questions in baryon spectroscopy may find answers from photo- and electro-production experiments which are presently carried out in various laboratories

Baryon spectroscopy

In this paper we summarize the results of a research programme on lead-tungstate (PWO) crystals performed by the CMS Collaboration at CERN, as well as by other groups who promoted the progress of the PWO scintillation crystal technology. Crystal properties, mass production technology, scintillation mechanism, origin of colouring, defects in crystal and radiation induced phenomena, light yield improvement and results of beam tests are described.

Lead tungstate scintillation material

The detector system TAPS (Two-Three-Arm Photon Spectrometer) has been designed and installed to study high-energy photons as well as neutral mesons produced in relativistic heavy-ion reactions. The spectrometer will consist of up to 384 individual BaF/sub 2/ modules packed in arrays of 64 scintillators. Each spectrometer arm, which carries two detector blocks, can be moved around the target location independently. The design concept, the specifications of the individual scintillator, and results from test on subarrays performed with monochromatic photons and charged particles are presented. Initial experiments have been performed exploiting heavy-ion beams up to a projectile energy of 1 GeV/u. The commissioning of the apparatus and the first experiments have shown that the envisaged performance has been fully achieved. >

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The BaF/sub 2/ photon spectrometer TAPS

https://hadron.physics.fsu.edu/~skpark/document/arXiv_nucl-ex_0306023.pdf

Study of Non-Strange Baryon Resonances with Meson Photoproduction

New results on the Roper resonance and the P-11 partial wave

The Two Arm Photon Spectrometer TAPS - comprising 384 plastic-BaF2 scintillator telescopes - was tested with monochromatic photons in the energy range between 45 and 790 MeV. The energy resolution for a collimated photon beam hitting the central detector module was determined to sigma/E = 0.59% X E(gamma)-1/2 + 1.9% (E, given in GeV). For the the fast scintillation component alone sigma/E = 0.79% X E(gamma)-1/2 + 1.8% has been measured. The position resolution of the point of impact amounts to DELTAx = 2 cm (FWHM) at the highest energies which corresponds to 30% of the diameter of an individual module. Monte Carlo simulations using the code GEANT3 are in good agreement with the experimental results.

/pdf/response-of-taps-to-monochromatic-photons-with-energies-j08cjpz5ug.pdf

Response of TAPS to monochromatic photons with energies between 45 and 790 MeV

PbWO/sub 4/ has been selected as the most appropriate scintillator material for future calorimeter projects at LHC to detect high energy photons in the multiple GeV range. In spite of the low yield of scintillation light, the applicability of the scintillator to detect photons and particles at energies well below 1 GeV has been studied using monoenergetic electrons between 180 and 855 MeV. The individual crystals (20/spl times/20/spl times/150 mm/sup 3/) are characterized by the optical quality and the response to low energy /spl gamma/-sources using hybrid photo diode tubes. The energy resolution of the EM-shower deposited by electrons in a 3/spl times/3 crystal matrix amounts to /spl sigma//E=2.39%//spl radic/E+0.20% (E given in GeV). The obtained time resolution of /spl sigma//spl les/130 ps allows photon/particle discrimination via time-of-flight measurement.

Response of a PbWO/sub 4/-scintillator array to electrons in the energy regime below 1 GeV

The study of the performance and application of PbWO/sub 4/ in electromagnetic calorimetry at energies far below 1 GeV has been continued. The significantly improved optical and scintillation properties of 15 cm long Nb/La-doped crystals, optimized for the ECAL/CMS calorimeter, are documented. The lineshape, energy and time response of a 5/spl times/5 matrix are tested with monoenergetic photons up to 790 MeV energy and compared to previous measurements. First attempts have been made to enhance the scintillation yield by suitable dopants (Mo,Tb) for applications at very low photon energies. As a first large scale project at medium energies, the proposed concept for a compact photon spectrometer to be implemented into the ANKE magnetic spectrometer at COSY (KFA Julich) is illustrated.

Electromagnetic calorimetry with PbWO/sub 4/ in the energy regime below 1 GeV

For the first time, full size lead tungstate crystals of different suppliers, quality and dopant concentration have been irradiated with gamma-rays at low temperatures down to -25degC at IHEP Protvino. In contrast to the behavior at room temperature, increased damage and extremely slow recovery processes have been observed. These first results are discussed in the light of several very different interpretations. Further more sensitive measurements are under preparation. The outcome will have a strong impact on the presently assembled ALICE-PHOS detector and the design of the EM calorimeter of PANDA at the future FAIR facility.

Radiation Hardness and Recovery Processes of PWO Crystals at $-25\, ^{\circ}$ C

For the first time, the response function to high-energy photons of a 3 times 3 matrix comprising large volume LYSO crystals was measured using energy marked photons provided by the tagged photon facility of MAMI. The crystal quality was determined based on the optical transparency, the intrinsic radioactivity and the luminescence yield. Energy and time resolutions for photons up to 490 MeV photon energy have been deduced from the reconstruction of the electromagnetic shower deposited into the crystal array and the data delivers very promising results.

W. Doring

Papers

Response of TAPS to monochromatic photons with energies between 45 and 790 MeV

Response of a PbWO/sub 4/-scintillator array to electrons in the energy regime below 1 GeV

Electromagnetic calorimetry with PbWO/sub 4/ in the energy regime below 1 GeV

Radiation Hardness and Recovery Processes of PWO Crystals at $-25\, ^{\circ}$ C

High-Energy Photon Detection With LYSO Crystals