We review the principal characteristics driving the design of precision calorimeters composed of inorganic crystal scintillators now in operation (L3, CLEO II) or developed for the next generation of particle physics experiments. The unique discovery potential of these detectors (1.5 to 50 m3 of crystals; 104 to > 105 elements) is the result of their high electron and photon energy resolution over a wide energy range, uniform hermetic acceptance and fine granularity.

Experiments at CERN's multi-TeV Large Hadron Collider (LHC) will search for the Higgs particles thought to be responsible for mass, and for many other new physics processes. In order to exploit the intrinsically high resolution of crystal detectors, exceptionally high speed (1 to 30 ns decay time) and radiation resistance are required. BaF2 and CeF3 are currently the preferred choices, and higher density alternatives such as PbWO4 are under investigation.

Lower energy, high luminosity experiments that will measure rare particle decays, and explore the violation of the fundamental “CP” symmetry that may be related to the predominance of matter over antimatter in our universe, have chosen Cesium Iodide for its combination of high light output, speed, and radiation resistance.

Recent developments by Caltech include the use of photons generated by an H− beam from an RFQ accelerator to calibrate and provide sub-percent resolution in the L3 BGO calorimeter, and an in situ optical bleaching technique that renders large BaF2 crystals now mass produced in China radiation hard up to dose levels ≳l 10 MegaRads.

A large tracking detector consisting of scintillating plastic optical fibers has been chosen by the D0 collaboration as a part of a planned upgrade at the Fermilab Tevatron. The tracker will utilize a state of the art photodetector known as the Visible Light Photon Counter. The benefits of fiber tracking in high energy physics will be presented along with recent progress in several key areas, including: optimization of scintillating dyes and light yields, fiber construction, fiber ribbon manufacture and placement, optical transmission and photodetection. The current status of the D0 development effort will be outlined, including results from the characterization of 5000 channels of VLPC. Finally, results from simulations of expected detector performance will be shown and discussed.

Over the years a number of scintillator materials have been developed for a wide variety of nuclear detection applications in industry, high energy physics, and medical instrumentation. To expand the list of useful scintillators, we are pursuing the following systematic, comprehensive search: (1) select materials with good gamma-ray interaction properties from the 200,000 data set NIST crystal diffraction file, (2) synthesize samples (doped and undoped) in powdered or single crystal form, (3) test the samples using sub-nanosecond pulsed x-rays to measure important scintillation properties such as rise times, decay times, emission wavelengths, and light output, (4) prepare large, high quality crystals of the most promising candidates, and (5) test the crystals as gamma-ray detectors in representative configurations. An important parallel effort is the computation of electronic energy levels of activators and the band structure of intrinsic and host crystals to aid in the materials selection process.

A large effort has been undertaken since nearly 2 years by 14 research institutes (the “Crystal Clear” collaboration, CERN RD project 18), in order to develop and investigate new dense, radiation hard and fast inorganic scintillators suitable for very good resolution electromagnetic calorimeters to be built at the new high luminosity accelerators. Other fields of interest like nuclear physics, gamma ray astronomy, medical imaging (PET), safety systems and non destructive industrial applications are also rapidly growing. Progress on cerium fluoride will be reviewed with some comments about mass scale production and cost considerations. The pro's and con's of potentially cheaper solutions based on fluoride glasses or lead compounds will be discussed. Finally some attractive candidates will be mentioned for low energy gamma ray applications.

A summary of some experimental results and achievements on BaF2 scintillating crystals research and development done in Shanghai Institute of Ceramics (SIC).

It is now well established that in a number of wide gap ionic crystals a specific intrinsic luminescence can be observed due to radiative hole transitions between the upper core band and valence band. The progress in knowledge about core-valence (CV) transitions is considered in papers [1-3].

This report summarizes the radiation resistance of the current production BaF2 crystals. An approach to implement optical bleaching in situ is also presented. By using optical bleaching current production quality BaF2 crystals could serve as an excellent candidate to construct a precision electromagnetic calorimeter at future hadron colliders.

The very large volumes needed to build a crystal calorimeter for High Energy Physics experiments bring cost considerations at the front of the stage. A reasonable compromise between cost and performances must be reached. One possible solution is to look at very dense materials like PbWO4, which will reduce the detector volume, even if the relatively low light yield will impose some limitations to the energy resolution. A review of the different results obtained by the Crystal Clear collaboration will be given for this crystal, including spectroscopic data and radiation damage measurements.

Large single crystals of CeF3, 40 mm in diameter up to 140 mm in length, have been grown using the vertical Bridgman technique. Their optical and scintillation properties and their radiation hardness have been measured.

Cerium fluoride offers a reasonable compromise between parameters like the density, the light yield, the scintillation characteristics (particularly the decay time) and the radiation hardness, and is considered today as the best candidate for large electromagnetic calorimeters in future High Energy Physics experiments. Details on the performances of large crystals produced by different manufacturers all over the world and measured by the Crystal Clear collaboration will be shown and the usefulness of a good collaboration between the industry and the users will be highlighted by some examples on the light yield and radiation hardness improvement.

Cerium Fluoride, a new, dense, fast and radiation hard scintillator, is at present the favored candidate for high-resolution calorimetry at future colliders. This crystal has been extensively studied – in particular by the Crystal Clear Collaboration – and the conditions for production of large, high-quality crystals are now being optimized in seven firms all over the world. Recently the collaboration has received several CeF3 crystals, up to 17cm long, from three companies. A small assembly of such crystals with silicon photodiode readout was exposed to high energy electrons and muons in the H2-beam of the SPS at CERN. The results of these tests with longitudinally segmented towers are presented and discussed, in particular in terms of light yield and energy resolution.

Cerium-doped lutetium orthophosphate (LuPO4:Ce) is one of the most promising new scintillator materials. In this paper we report on the effects of total or partial replacement of Lu by Yb. When not doped with Ce, crystals of YbxLu1-xPO4 scintillate rather poorly, the emission being due to the Yb charge transfer transition. Activation with Ce reduces the light output from Yb even further, while correspondingly the addition of Yb to LuPO4:Ce also degrades that material's scintillation performance. We suggest that the nonradiative decay of the (Yb2+-Ce4+) charge transfer state is responsible for the mutual quenching of these ions. The differences in performance of LuPO4:Yb and LuPO4:Ce indicate that energy transfer mechanisms ordinarily associated with materials containing molecular anionic groups may not be quite sufficient to explain the results. Suggestions are presented that under γ-excitation the lattice-to-ion energy transfer in orthophosphates is accomplished by sequential capture of charge carriers rather than the more conventional mechanism of exciton hopping between molecular groups.

The journey of YAP from interesting laboratory curiosity to a material with commercial possibilities has taken two full decades. The reported YAP properties which motivated this effort are its rapid (~ 27 ns) dominant (~ 97%) decay which results in negligible afterglow; high density (~ 5.55 g/cm3); brightness (~ 50% of NaI:Tl); energy resolution (~ 6% at 455 keV); physical ruggedness; and chemical nonreactivity. Lone among its draw backs as a practical scintillator is its emission which is centered in the UV (~ 350 nm). Three firms now offer YAP crystals of sufficient size and quality that industrial applications are becoming possible.

To date, the physical properties of YAP have only been published on small laboratory samples. Here we report a comprehensive and comparative evaluation of physical properties of material from all known commercial sources. Further, we speculate on the physical origin of variations among samples. Finally, we attempt to predict what can ultimately be expected from YAP as further refinements in its processing techniques are perfected.

We have tested a single crystal of Ce-activated lanthanum beryllate BEL(Ce) as a potential gamma detector material. The density (6.1 g.cm−3) and decay time (50 ns) are competitive with other recently developed materials. The scintillation efficiency is 57- to -95% that of BGO. For an excitation wavelength of 340 nm, the emission spectrum is a broad peak centered at 450 nm. The H2 annealed sample is transparent for wavelengths greater than 400 nm. We are continuing a program to improve the scintillation efficiency by varying the crystal growth conditions.

The crystal was prepared by M. Long and E.W. O'Dell of AlliedSignal, Inc., using the Czochralski technique.

An anomaly of the temperature dependence of the unit cell parameter has been observed for β–PbF2 single crystals at 200 K that is interpreted as a phase transition to a pseudocubic lattice. Such a pseudocubic phase is observable at room temperature after uniaxial plastic deformation of the bulk single crystals. The structural aspects of the β→α transition have been established. The as-grown crystals of α–PbF2 phase are shown to undergo a phase transition at 100 K.

Spectral and kinetic parameters of PbX2 (X=F, Cl, Br, I) crystals emission are investigated under pulse photo- (∆t=l.0 ns) and X-ray (∆t=l.5 ns) excitation in the temperature region from 77 to 293 K. X-ray luminescence of the PbF2, PbC12 and PbBr2 crystals is considered as radiative decay of the self-trapped excitons at the defects, while subnanosecund and nanosecond emission components are regarded to the result of the thermal quenching. The nature of the fast X-ray luminescence for PbI2 crystals is discussed from the viewpoint of the possibility for observation of radiative core-valence transition.

The spectral and kinetic properties of photoluminescence of Pb2+ aggregated phase in CsC1 host, together with scintillation characteristics of CsCI:Pb crystals, are reported in the 10 - 300 K temperature range. Absorption, steady-state excitation and emission spectra of Pb2+ phase in CsCI host are similar to those of CsPbCl3 bulk crystal (emission peak at 419 nm at 10 K). The decay of the 421 nm luminescence of the Pb2+ phase in CsCl shows single exponential behavior with extremely short decay time of 40 ps at 421 nm and 10 K,which is considerably shorter than the decay times found in the decay of CsPbCl3 bulk emission (0.45, 2.8 and 12 ns at 418 nm and 10 K). In the scintillation decay of CsCI:Pb, two components with 0.95-1 ns and 2-3 ns decay times were found and no slower component is present at room temperature.

Several 2-(2'-hydroxyphenyl)benzothiazole, -benzoxazole, and -benzimidazole derivatives have been prepared. Transmittance, fluorescence, light yield, and decay time characteristics of these compounds have been studied in a polystyrene matrix and evaluated for use in plastic scintillation detectors. Radiation damage studies utilizing a 60Co source have also been performed.

The new generation of high energy accelerators - LHC, CEBAF, RHIC - have encouraged the development of more robust plastic scintillators. Monitors and detectors for these machines will require plastic scintillator with greater radiation tolerance. Although the major cause of radiation damage in plastic scintillator is the creation of color centers in the base plastic, the most successful approach to date has been to utilize fluors which circumvent, rather than solve, the radiation damage problem. Two techniques have been found to be useful. First, increase the concentration of fluors so that the optical density of the fluors for absorption of the scintillation photons remains much higher than the optical density of the radiation induced color centers. Second, use fluors which emit at longer wavelengths than traditional fluors, thus avoiding radiation induced color centers. The present work attempts to meet these requirements by modifying the structure of the 3-HF molecule.

We have produced fast wavelength shifters using mixtures of various Coumarin dyes with DCM in epoxy-polymers (DGEBA+HHPA) and measured the properties of these wavelength shifters. The particular mixtures were chosen because there is a substantial overlap between the emission spectrum of Coumarin and the absorption spectrum of DCM. The continuous wave and time-resolved fluorescence spectra have been studied as a function of component concentration to optimize the decay times, emission peaks and quantum yields. The mean decay times of these mixtures are in the range of 2.5-4.5 ns. The mean decay time increases with an increase in Coumarin concentration at a fixed DCM concentration or with a decrease in DCM concentration at a fixed Coumarin concentration. This indicates that the energy transfer is radiative at lower relative DCM concentrations and becomes non-radiative at higher DCM concentrations.