Jean-Paul Fabre

Glasses as active and passive components for scintillating fiber detectors

Scintillating fibers are of growing interest in high energy physics for applications in calorimetry and in tracking detectors At present plastic scintillating fibers are mainly used in these applications because of their high light yield and their fast decay rates; however, in thin fibers, required for high spatial resolution, these suffer from low attenuation lengths. Moreover, cross-talk is still a severe problem. As alternatives we will discuss the following two concepts: (1) using Ce- and Tb-doped multicomponent glasses as active core material of glass fibers and (2) using liquid scintillator filled glass capillary arrays. The optical properties of the rare earth doped glasses are described and the scintillation efficiency of the fibers and fiber bundles utilizing these glasses as core material are presented. Broader applications appear to the possible with liquid scintillator filled capillary arrays. Suitable liquid scintillators with high refractive index solvents and locally emitting solutes with high yields, short decay times and large Stokes-shifts are available. Arrays can be produced with and without extra mural absorber in various sizes and shapes. Theoretical estimates show that reflection losses at the liquid/glass interface do not effect the overall transmission up to length/diameter ratios of 105. In addition recent results have shown that the system resists radiation doses in the 100 kGy range. Further experimental results obtained at CERN with these arrays will be discussed.

Optical microscope for nuclear emulsion readout: system design and results in application

Experiments such as CHORUS at CERN require the inspection of a large amount of nuclear emulsion plates exposed to particle beams. Rare events need to be found, measured and analyzed. Their features are stored as grains in microscopic dimensions in a 3D stack of plates. A new, fully automatic immersion microscope system was developed for this purpose. It features high resolution, small depth of focus, large working distance, large field of view and synchronization of illumination and detector. An additional requirement is given by variations in the refraction index and in the relative thickness of immersion oil and emulsion. The approach used here is an imaging system based on a various objective lens with extreme numerical aperture, large working distance and wide field, combined with a matched high-aperture Koehler illuminator. The light source is a mercury arc lamp, combined with a filter package for the g- line. It includes liquid crystal elements for synchronized shuttering and variable attenuation. The theoretical resolution is less than 1 micron in x, y, z within a volume of 0.5mm diameter times 1 mm scanning depth in all situations within a predefined index range. Three identical pieces of the system have been built. The identical pieces of the system have been built. The experimentally measured resolution confirms the expectations and is better than 1 micron in all three dimensions. This is the result of a complex process of system design and manufacturing, unifying optical, opto-mechanical and opto-electronical contributions. This process spans from the early stages of feasibility and manufacturing up to the test and adjustment procedures. The three prototypes are operational since the fall of 1998 in the frame of the CHORUS project. Practical experience and application results are presented.

Charged-particle tracking with high spatial and temporal resolution using capillary arrays filled with liquid scintillator

We abstract developed a new technique that allows the trajectories of ionizing particles to be imaged with very high spatial and temporal resolution. This technique, developed for future experiments in high-energy physics, may also be applied in other field. Central to the technique is a detector consisting of a bundle of thin, glass capillaries filled with a liquid scintillator of high refractive index. These liquid-core scintillating fibers act simultaneously as a detector of charged particles and as an image guide. Track images seen at the readout end of the capillary bundle are amplified by an optoelectronic chain consisting of a set of image- intensifier tubes and read by a photosensitive CCD camera. We report here on results obtained with detector prototypes. A spatial resolution of 6-14 micrometers , dependent on image magnification prior to readout, has been obtained with 16 micrometers capillaries. The high scintillation efficiency of the liquid scintillator used and a large light attenuation length-- approximately 3 m for 20 micrometers capillaries--result in hit densities along the track of a minimum-ionizing particle of 8.5 mm-1 and 3.5 mm-1 at distances from the readout window of approximately 2 cm and approximately 1 m respectively. The radiation resistance of the detector is an order of magnitude greater than that of other types of tracking device of comparable performance. To complement the detector we have been developing a new readout system based around a gateable vacuum image pipeline (VIP) and an electron- bombarded CCD camera. These increase the spatial and temporal resolution obtained with detector and render it particlarly attractive as a microvertex detector for the observation of short-lived particles in high-energy physics experiments performed with evelated interaction rates.

New tool for high-resolution multichannel readout: megapixel electron-bombarded CCD image zoom tube

A hybrid image intensifier zoon tube, based on a thinned backside electron-bombarded CCD (EBCCD) 1024 X 1024 pixels (13.1 X 13.1 micrometers 2), to be used for the readout of a high resolution fiber detector in a high energy physics experiment, has been designed, manufactured and tested. This tube has a photocathode diameter of 40 mm and allows to change the image magnification (M) from 0.6 to 1.3. Owing to the low energy threshold of the EBCCD (2.5 keV) and the high operational voltage (15 kV), a gain (electrons per photoelectron) of 4000 has been attained. A spatial resolution of about 40 lp/mm (15% MTF) with an illumination of 2 (DOT) 10(superscript -4 lux and has been achieved. The EBCCD tube is gateable by applying appropriate voltage pulses to the focusing electrode. The high gain and the excellent space resolution of this device make it very interesting for many applications in high energy physics, astrophysics, medical diagnostics and very low light imaging.

High-speed gateable image pipeline

We present the results obtained with a prototype of a high speed gateable Vacuum Image Pipeline (VIP) for selection of non-repetitive images from a continuous stream. It allows snapshots with a very short exposure time (of the order of 10 nanoseconds) to be accepted (or rejected) after a decision time of a few microseconds. The VIP is a vacuum tube equipped with a photocathode, a system of metallic grids and a phosphor screen. Photoelectrons produced by the images focused on the photocathode are guided by a uniform magnetic field parallel to the tube axis. By acting on the grid potentials, the drift time of the photoelectrons inside the tube can be adjusted between 0.3 and 2 microseconds. An image among many others can then be selected by an external trigger with a time resolution between 4 and 30 ns depending on the delay time. The selected photoelectrons are finally accelerated by a high potential (+15 kV) onto the phosphor screen where they reproduce the triggered image. A spatial resolution of 33 lp/mm at a magnetic field of 0.1 T has been measured. The VIP is a useful tool for high energy physics and astrophysics experiments as well as in high speed photography.

CHORUS scintillating fiber tracker and optoelectronics readout system

An essential component of the CERN WA95/CHORUS experiment is a scintillating fiber tracker system made up of more than one million scintillating fibers, for the precise track reconstruction of particles. The design and construction of the tracker system as well as its opto-electronics readout are discussed. Performances of the detector are presented.

Scintillating fiber tracker system of the CHORUS experiment

The CERN WA95/CHORUS Collaboration has been constructing a detector to search for neutrino oscillations. An essential component of the detector is a scintillating fiber tracker system for precise track reconstruction of particles. An overview of the tracker system design, its opto-electronics readout, data processing and test beam measurements is presented.