Search

The CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab utilizes six iron-free superconducting coils to provide an approximately toroidal magnetic field. The six sectors are instrumented individually to form six independent spectrometers. The forward region (8deg < (theta) < 45deg) of each sector is equipped with a lead-scintillator electromagnetic sampling calorimeter (EC), 16 radiation lengths thick, using a novel triangular geometry with stereo readout. With its good energy and position resolution, the EC is used to provide the primary electron trigger for CLAS. It is also used to reject pions, reconstruct pi-0 and eta decays and detect neutrons, This paper treats the design, construction and performance of the calorimeter.

Electromagnetic calorimeter modules are being constructed for the CEBAF Large Acceptance Spectrometer that will utilize plastic plate scintillators that can be over 4 meters in length. At shower maximum, it is expected that up to 100 Gray may be deposited over a ten year lifetime in the detector elements. Although it is believed that PVT-based scintillators are radiation-hard at that level, the large size of the plates motivated an ageing test of a 2 meter long sample so as to gain a realistic estimate of the loss in intrinsic light output and especially in attenuation performance of the chosen scintillator (Bicron BC412). A test setup was designed so that the sample was irradiated (by a /sup 60/Co line source) at a rate sufficiently low (0.5 Gray/hr) to allow complete diffusion of oxygen into the material during the irradiation. Additionally, in order to ensure complete diffusion and to gain a pessimistic estimate of damage, a pure oxygen atmosphere was circulated through the sample container during the irradiation, and the total dose was increased to a total of 160 Gray. Within the same setup, tests were also made of two additional materials: (1) a 1 meter long sample of an uniquely bright, fast green scintillator with superior attenuation properties, and (2) a customized acrylic scintillator (with a high doping level of fluors) read out by embedded (into smooth grooves cut into the surface of the scintillator) wavelength shifting fibers doped with the same green fluor as the aforementioned scintillator. The data indicates that the BC412 shows no loss in intrinsic light output, but does have an attenuation loss amounting to 10% at 1 meter and 16% at 2 meters. The fast green scintillator (and fibers) showed no damage. When read out by WLS fibers, the acrylic scintillator displayed a loss in intrinsic light output, but no change in attenuation. >

A low dose rate evaluation of several scintillators and wavelength shifters of interest to the CEBAF program was performed with a 60Co line source to a total dose of 160 Gray at a rate of 0.5 Gy/hr in a slowly flowing oxygen atmosphere. The samples included the following: (1) a 2 meter piece of Bicron BC412 scintillator being extensively used in the CEBAF Large Acceptance Spectrometer, (2) a 1 meter long piece of green scintillator doped with a uniquely fast green fluor (G2), (3) wavelength shifting fiber dopedalso with G2, and (4) grooved acrylic-based scintillator that is readout with the WLS fiber. The data indicates that the BC412 shows no loss in intrinsic light output, but does have an attenuation loss amounting to 10% at 1 meter and 16% at 2 meters. The fast green scintillator (and fibers) showed no damage. When read out by WLS fibers, the acrylic scintillator displayed a loss in intrinsic light output, but no change in attenuation.

Optical transmission, light output and time characteristics are given for long scintillator strips fabricated at the Yerevan Physics Institute using the extrusion method. It is shown that at 45% relative (to anthracene) light output, good transmission (2.5/2.9 m attenuation length with photomultiplier direct readout and 3/3.5 m attenuation length fiber readout) and time characteristics (average decay time 2.8 nsec) were obtained.

The angular dependences of proton photoproduction from the nuclei 12 C, 63 Cu and 208 Pb irradiated by bremsstrahlung γ-quanta with maximum energy 4.5 GeV, both in the cumulative region (i.e. in the kinematical region in which the production of protons in the collision of γ-quanta of the given energy with the quasi-free nuclear nucleon is forbidden) and in the non-cumulative region, are investigated. The experimental data obtained are compared with the results of theoretical calculations of cumulative proton photoproduction according to the following models: the “quasi-two-body” scaling model, the low-nucleon correlation model, the fluctuon model and the cluster model.