Alkaline-earth scintillators such as strontium iodide and other alkaline-earth halides activated with divalent europium represent some of the most efficient and highest energy resolution scintillators for use as gamma-ray detectors in a wide range of applications. These applications include the areas of nuclear nonproliferation, homeland security, the detection of undeclared nuclear material, nuclear physics and materials science, medical diagnostics, space physics, high energy physics, and radiation monitoring systems for first responders, police, and fire/rescue personnel. Recent advances in the growth of large single crystals of these scintillator materials hold the promise of higher crystal yields and significantly lower detector production costs. In the present work, we describe new processing protocols that, when combined with our molten salt filtration methods, have led to advances in achieving a significant reduction of cracking effects during the growth of single crystals of SrI2:Eu2+. In particular, we have found that extended pumping on the molten crystalgrowth charge under vacuum for time periods extending up to 48 hours is generally beneficial in compensating for variations in the alkaline-earth halide purity and stoichiometry of the materials as initially supplied by commercial sources. These melt-pumping and processing techniques are now being applied to the purification of CaI2:Eu2+ and some mixed-anion europium-doped alkaline-earth halides prior to single-crystal growth by means of the vertical Bridgman technique. The results of initial studies of the effects of aliovalent doping of SrI2:Eu2+ on the scintillation characteristics of this material are also described.
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 Lu2O3, and
non-cubic LaBr3, and Lu2SiO5 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 LaBr3: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.
Previous measurements of the scintillation properties of members of the single-alkali, rare-earth double-phosphate family have demonstrated high light output and fast decay times when exposed to ionizing radiation. Cerium-doped K3Lu(PO4)2 and Rb3Lu(PO4)2 scintillators have exhibited light outputs of 32,500 and 28,200 photons/MeV respectively and decay times of 37 and 34 nanoseconds respectively. Because of the ease with which the alkali constituents (Li, Na, K, Rb, Cs) of the crystal matrix may be interchanged (e.g. K2CsLu(PO4)2 and CsLi2Lu(PO4)2), the rare-earth double-phosphate family of scintillators provides an ideal system for the study of matrix effects on scintillation efficiency and kinetics. In order to better understand and to ultimately optimize the scintillation properties of these scintillators, new members of the rare-earth double-phosphate family have been synthesized by high temperature flux growth. These new samples, represented by the general formula (A,B)3Lu(PO4)2:Ce where A and B are alkali elements, incorporate mixed alkali rather than single alkali components and varying levels of Ce doping. Light output, scintillation decay times, and photoluminescence measurements for the most promising of the samples to date are reported. In this paper, we identify promising samples and results that clearly demonstrate outstanding light output, up to 270% of BGO, fast decay times, 29-39 nanoseconds, and peak emission wavelengths of ~ 400 nm for many of the samples.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.