R. Chipaux

EXTENSIVE STUDIES ON CEF3 CRYSTALS, A GOOD CANDIDATE FOR ELECTROMAGNETIC CALORIMETRY AT FUTURE ACCELERATORS

Abstract In the framework of its search for new heavy, fast and radiation hard scintillators for calorimetry at future colliders, the Crystal Clear Collaboration performed a systematic investigation of the properties and of the scintillation and radiation damage mechanisms of CeF 3 monocrystals. Many samples of various dimensions up to 3 × 3 × 28 cm 3 were produced by industry and characterised in the laboratories by different methods such as: optical transmission, light yield and decay time measurements, excitation and emission spectra, gamma and neutron irradiations. The results of these measurements are discussed. The measured light yield is compared to the theoretical expectations. Tests in high energy electron beams on a crystal matrix were also performed. The suitability of CeF 3 for calorimetry at high rate machines is confirmed. Production and economical considerations are discussed.

Further results on cerium fluoride crystals

Abstract A systematic investigation of the properties of cerium fluoride monocrystals has been performed by the “Crystal Clear” collaboration in view of a p

Charge-transfer luminescence and spectroscopic properties of Yb3+ in aluminium and gallium garnets

Luminescence of Yb 3 - from the charge-transfer state with broad emission bands and short radiative lifetimes (few to tens of nanoseconds depending on the host lattice and the temperature) is attractive for the development of fast scintillators capable of discriminating very short events. The most important currently considered application is that in solar neutrino (v e ) real-time spectroscopy, since the v e capture by 1 7 6 Yb is followed by a specific emission signature which can accordingly excite the Yb 3 + fluorescence. Studies on scintillation and luminescence in aluminium garnets containing Yb 3 + have shown that these materials meet some of the required properties for such scintillators. In defining our priorities, the best compromise between host crystal, Yb 3 + concentration, production method, post-growth treatment and performance is to be considered based on the studies of charge-transfer luminescence and quenching mechanisms. The experiments have been extended to a large number of compounds: YAG:Yb-YbAG, YGG:Yb-YbGG, YAP:Yb-YbAP, LaYbO 3 in the form of single crystals and/or powders. In garnets, the temperature-dependent fluorescence intensity and decay time under X-ray and VUV excitations decrease at low temperatures (T<100 K) and demonstrate the important role played by the traps. The thermoluminescence peaks show a strong dependence on the crystal history, composition and impurities introduced intentionally. The fluorescence intensity and decay time are also dependent on Yb 3 + concentration and the presence of Yb 2 + . The results trace the major directions to optimised scintillators in terms of their efficiency and lifetime.

Ytterbium-based scintillators, a new class of inorganic scintillators for solar neutrino spectroscopy

Abstract The observed deficit of the solar neutrino flux is now well established. This puzzling problem of todays particle physics could be resolved soon. The most likely explanation would be the vacuum neutrino oscillation phenomenon, indirectly proving the non-zero mass of these fleeting particles. Following the proposition of Raghavan of using 176 Yb as a target for low-energy solar neutrino spectroscopy, an intense R&D work has started a few years ago to define a suitable scintillator incorporating a large amount of ytterbium. Recently, the observation of UV scintillation in mixed yttrium/ytterbium aluminium garnets opened the field of investigation to a new class of scintillating crystals with interesting luminescence properties, very attractive not only for neutrino physics but also for radiation detection, in general. Their luminescence properties present some peculiarities that make them interesting by themselves.

Beam tests of lead tungstate crystal matrices and a silicon strip preshower detector for the CMS electromagnetic calorimeter

Tests of lead tungstate crystal matrices carried out in high-energy electron beams in 1996, using new crystals, new APDs and an improved test set-up, confirm that an energy resolution of better than 0.6% at 100 GeV can be obtained when the longitudinal uniformity of the struck crystal is adequate. Light loss measurements under low dose irradiation are reported. It is shown that there is no loss of energy resolution after irradiation and that the calibration change due to light loss can be tracked with a precision monitoring system. Finally, successful tests with a preshower device, equipped with silicon strip detector readout, are described.

Crystal structure of lead tungstate at 1.4 and 300 K

Abstract The crystal structure of lead tungstate, PbWO 4 , is tetragonal, scheelite type, space group I 4 1 / a . This compound is frequently subjected to lead deficiency, due to the difference in the vapour pressure of the two oxides, WO 3 and PbO, used in the crystal growth. One group has reported that lead vacancies can order in a crystal structure derived from the scheelite type, but of lower symmetry and described by the space group P4 / nnc or P 4 . We report here on neutron diffraction measurements performed on three different powdered samples. Our measurements do not show any indication of structural transition between 1.4 and 300 K. The structure is undoubtedly the tetragonal one. The existence of a lead deficient structure remains unconfirmed.

Studies of Lead Tungstate Crystal Matrices in High-Energy Beams for the CMS Electromagnetic Calorimeter at the LHC

Using matrices of lead tungstate crystals, energy resolutions better than 0.6% at 100 GeV have been achieved in the test beam in 1995. It has been demonstrated that a lead tungstate electromagnetic calorimeter read out by avalanche photodiodes can consistently achieve the excellent energy resolutions necessary to justify its construction in the CMS detector. The performance achieved has been understood in terms of the properties of the crystals and photodetectors.

PERFORMANCE OF A CERIUM FLUORIDE CRYSTAL MATRIX MEASURED IN HIGH-ENERGY PARTICLE BEAMS

Abstract A cerium fluoride matrix composed of nine longitudinally segmented towers, approximately 25 X 0 long, has been tested in electron, muon and pion beams of momenta ranging from 10 to 150 GeV/ c . The results are discussed in terms of light yield, electronic noise, energy and position resolution. In spite of serious imperfections in geometry and quality of some of the crystals, an electron energy resolution of ∼0.5% has been obtained with a silicon photodiode readout, for energies above 50 GeV. The performance of cerium fluoride in a beam, its high density, high light yield and fast response, radiation resistance and ruggedness make it a very good candidate for high-resolution calorimetry at future colliders. The best conditions for production of large high-quality crystals are being studied in several firms over the world. Many CeF 3 crystals, 2 × 2 cm 2 or 3 × 3 cm 2 in cross section and up to 28 cm long, were received in 1994 from four companies, some of them with excellent light yield and radiation hardness.

Experimental study of an impact-parameter optical discriminator

Abstract An optical discriminator, sensitive to the impact parameter has been tested in a particle beam. The device is based on the detection of Cherenkov light produced and trapped in a thin spherical crystal shell viewed by a fast photodetector. The detector is only sensitive to charged particles with a nonzero impact parameter relative to the centre of the crystal sphere. A prototype using a LiF crystal has been tested and the experimental results are in agreement with expectations. We outline the foreseen developments in order to use such a detector for a fast selection of B events in fixed-target experiments at proton accelerators. We also discuss the use of the detector as presently conceived for the fast selection of strange particles.

Simulation of light collection in the CMS lead tungstate crystals with the program Litrani: coating and surface effects

The accuracy of the lead tungstate electromagnetic calorimeter of the CMS experiment under construction at CERN relies, among other things, on the correction of the calibration parameter from variations due to crystal ageing. This ageing will be measured by a so-called monitoring system, but the relation between monitoring and calibration parameter variations is not so trivial, and depends much on the overall optical characteristics of crystal and photodetector. We present here simulations done with the program Litrani based on real ageing data for a realistic CMS crystal with a defined surface quality (optically polished, with or without one lateral face slightly depolished), covered by coatings of various characteristics, from totally absorbing to nominal aluminum or diffusing medium. The correlation coefficient between monitoring and scintillation signals depends greatly on these characteristics, and varies between about 1.3 and more than 10 (the optimum being one).