Cerium activated rare-earth tri- halides represent a well-known family of high performance inorganic rare-earth
scintillators - including the high-light-yield, high-energy-resolution scintillator, cerium-doped lanthanum tribromide.
These hygroscopic inorganic rare-earth halides are currently grown as single crystals from the melt - either by the
Bridgman or Czochralski techniques - slow and expensive processes that are frequently characterized by severe cracking
of the material due to anisotropic thermal stresses and cleavage effects. We have recently discovered a new family of
cerium-activated rare-earth metal organic scintillators consisting of tri-halide methanol adducts of cerium and lanthanum
- namely CeCl3(CH3OH)4 and LaBr3(CH3OH)4:Ce. These methanol-adduct scintillator materials can be grown near
room temperature from a methanol solution, and their high solubility is consistent with the application of the rapid
solution growth methods that are currently used to grow very large single crystals of potassium dihydrogen phosphate.
The structures of these new rare-earth metal-organic scintillating compounds were determined by single crystal x-ray
refinements, and their scintillation response to both gamma rays and neutrons, as presented here, was characterized using
different excitation sources. Tri-halide methanol-adduct crystals activated with trivalent cerium apparently represent the
initial example of a solution-grown rare-earth metal-organic molecular scintillator that is applicable to gamma ray, x-ray,
and fast neutron detection.
Ceramic materials show significant promise for the production of reasonably priced, large-size scintillators. Ceramics
have recently received a great deal of attention in the field of materials for laser applications, and the technology for
fabricating high-optical-quality polycrystalline ceramics of cubic materials has been well developed. The formation of
transparent ceramics of non-cubic materials is, however, much more difficult as a result of birefringence effects in
differently oriented grains. Here, we will describe the performance of a few new ceramics developed for the detection of
gamma- and x-ray radiation. Results are presented for ceramic analogs of three crystalline materials - cubic Lu<sub>2</sub>O<sub>3</sub>, and
non-cubic LaBr<sub>3</sub>, and Lu<sub>2</sub>SiO<sub>5</sub> or LSO (hexagonal, and monoclinic structures, respectively). The impact of various
sintering, hot-pressing and post-formation annealing procedures on the light yield, transparency, and other parameters,
will be discussed. The study of LaBr<sub>3</sub>:Ce shows that fairly translucent ceramics of rare-earth halides can be fabricated
and they can reach relatively high light yield values. Despite the fact that no evidence for texturing has been found in our
LSO:Ce ceramic microstructures, the material demonstrates a surprisingly high level of translucency or transparency.
While the scintillation of LSO:Ce ceramic reaches a light yield level of about 86 % of that of a good LSO:Ce single
crystal, its decay time is even faster, and the long term afterglow is lower than in LSO single crystals.
We have investigated the applicability of phosphate glasses as host systems for the formation of rare-earth-activated
gamma- and x-ray scintillators. Glass scintillators have generally suffered from low light yields, usually attributed to
inefficient energy transfer from the glass matrix to the luminescent center. Our research on these phosphate glasses has
shown that their structural properties can be readily varied and controlled by compositional alterations. The melting and
pouring temperature of ~1050°C for these phosphate glasses is significantly lower than the processing temperatures
generally associated with the formation of silicate glass scintillators. The calcium-sodium phosphate glasses will
tolerate relatively high cerium concentrations based on the initial melt compositions, and the light yield for gamma-ray
excitation at 662 keV was determined as a function of cerium concentration up to the saturation level. The rare-earth-activated
Ca-Na phosphate glass primary-component decay time was in the range of 32 to 42 nsec for various Ce
concentrations with the contribution of the light output of the primary component ranging from 80 to 90%. Studies of the
effects of co-doing with both Ce and Gd were also carried out in the case of the Ca-Na phosphate glass hosts. The
effects of post-synthesis thermochemical treatments in a variety of atmospheres and at various processing temperatures
were also investigated for the Ce-activated Ca-Na phosphate scintillators.
When activated with an appropriate rare-earth ion (e.g., Ce or Nd), rare-earth orthophosphates of the form REPO4 (where RE = a rare-earth cation) and alkali rare-earth double phosphates of the form A<sub>3</sub>RE(PO<sub>4</sub>)<sub>2</sub> (where A = K, Rb, or Cs) are characterized by light yields and decay times that make these materials of interest for radiation-detection applications. Crystals of the compound Rb<sub>3</sub>Lu(PO<sub>4</sub>)<sub>2</sub> when activated with ~0.1 mol % Ce exhibit a light yield that is ~250% that of BGO with a decay time on the order of ~40 nsec. The cerium-activated rare-earth orthophosphate LuPO<sub>4</sub>:Ce is also characterized by a high light yield and a relatively fast decay time of ~25 nsec. Additionally, the rare-earth orthophosphates are extremely chemically, physically, and thermally durable hosts that recover easily from radiation damage effects. The properties of the rare-earth orthophosphates and double phosphates that pertain to their use as X- and gamma-ray detectors are reviewed. This review includes information related to the use of Nd-doped LuPO<sub>4</sub> as a scintillator with a sufficiently energetic, short-wavelength output (λ=90 nm) so that it can be used in conjunction with appropriately activated proportional counters. Information is presented on the details of the synthesis, structure, and luminescence properties of lanthanide double phosphates that, when activated with cerium, are efficient scintillators with output wavelengths that are sufficiently long to be well matched to the response of silicon photodiode detectors.