Charles Brecher

Scintillation Properties of SrHfO

In this paper we report on the scintillation properties of cerium doped strontium - and barium hafnate. Radioluminescence, pulse height, scintillation decay and timing spectra are presented. Radioluminescence spectra of SrHfO3:Ce3+ and BaHfO3:Ce3+ consist of a broad band due to Ce3+ emission peaking at 410 nm and 400 nm, respectively. The light yield of BaHfO3:Ce3+ and SrHfO3:Ce3+ is approximately 40 000 photons/MeV when compared to a crystal of BGO. The principal decay time constant for SrHfO3:Ce3+ and BaHfO3:Ce3+ is 42 and 25 ns, respectively. A timing resolution of 276 ps (FWHM) was obtained with transparent optical ceramic of SrHfO3:Ce3+.

_{3}

The authors report a new scintillator based on a transparent ceramic of Lu/sub 2/O/sub 3/:Eu. The material has an extremely high density of 9.4 g/cm/sup 3/ and a light output comparable to CsI:Tl. Its narrow-band emission at 610 nm perfectly matches the spectral response of charge coupled devices (CCDs). To enhance the spatial and contrast resolution, the authors have developed a special process to pixelate the scintillator and prevent the spread of light within the scintillator volume. The imaging performance of this pixelated device was evaluated using a thermoelectrically cooled CCD camera. The new scintillator is expected to play a major role in digital radiographic systems when readout technologies capable of taking advantage of the transparency are developed further.

:Ce

Despite the acknowledged advantages of CsI:Tl for most scintillator applications, its use for CT and other high-speed imaging has been hindered by a high degree of afterglow in its scintillation decay. We have found that a particularly effective way to suppress this afterglow is to codope the material with certain dipositive rare earth ions capable of trapping the vagrant carriers that give rise to it. We have extensively studied the manner in which one such ion, Eu2+, alters the spectroscopic and kinetic properties of the scintillation, and have developed a coherent mathematical model consistent with the experimental results. But the beneficial effect of Eu2+ appears to be restricted only to relatively short times (say les 200 ms ) after the end of the excitation pulse. To be effective at longer times, the codopant should also provide some nonradiative means to annihilate the trapped carriers before their escape can enhance the low-level long-term emission. And, as predicted by the model, this is exactly what Sm2+ does. In this paper we describe the experimental effort to characterize the behavior of the CsI:Tl, Sm material system. Spectroscopically, we find that the familiar broad Tl emission becomes distorted and progressively shifted to shorter wavelengths. Kinetically, we find that the afterglow of the emission is substantially reduced relative to conventional CsI:Tl (no codopant), and that this effect is always found, regardless of the conditions of excitation. And finally, we find that the material shows virtually no memory of its previous excitation history (the so-called hysteresis phenomenon), in stark contrast with both conventional CsI:Tl and the corresponding material codoped with Eu. Various aspects of these effects and their dependence on the concentrations of the dopants are also discussed.

^{3+}

Although LSO is one of the most successful scintillator developments for medical diagnostics in the last two decades, good single crystals are not commercially available in any quantity. Consequently, we explored the feasibility of developing a ceramic version of the material, which requires a considerably lower temperature to consolidate the material to essentially crystalline density. Consolidation of the ceramic was achieved by hot pressing at temperatures up to 1700degC and pressure of 8000 psi. Hot pressing causes a loss of oxygen resulting in strong coloration of the ceramic, which had to be removed by heating (ldquobleachingrdquo) in air at approximately 1100degC. The resultant specimens were colorless and highly translucent, but the anisotropic nature of the crystal structure precluded the achievement of full transparency. The scintillation performance of the resulting ceramics was characterized and compared with that of high light-output LSO single crystals. The scintillation efficiency as measured by energy spectra generally fell in the range of 50-60% of that of the crystals. While this would be adequate for PET applications, the limited transparency provides a barrier to such use. An alternative application would be in signal integrating techniques, such as CT, where it could provide an alternative to GOS but with higher speed. Here, the problem of afterglow assumes major importance. The afterglow is a function of many factors, including conditions of excitation. Further work on improving the LSO ceramics is considered.

and BaHfO

Phase-pure transparent LSO ceramic was obtained, with its scintillation properties rivaling those of single crystalline LSO. The transparent LSO ceramic was prepared by a nano-technology approach. The densities of the LSO ceramic increased with increasing sintering temperature. Further SEM examination confirmed the decrease of the porosity during densification of the ceramic. Pores were segregated at grain boundaries after sintering. Hot isostatic pressing (HIPing) was employed to further densify the ceramic and eliminate porosity after sintering. Transparent polycrystalline LSO ceramics were obtained after the final HIP. XRD examination confirms single monoclinic LSO phase. A light output as high as 30,100 ph/MeV was obtained using a 22Na excitation source. LSO ceramic showed an energy resolution of 15% (FWHM) at 662 keV (137Cs source) and a fast scintillation decay of 40 ns due to the 5d \ura 4f transition of Ce3+. The excellent scintillation and optical properties make LSO ceramic a promising candidate for future gamma-ray spectroscopy as well as medical imaging applications.

_{3}

Suppression of afterglow in co-doped CsI:Tl is found to be an order of magnitude more effective in CsI:Tl, Sm than in CsI:Tl, Eu. Rate equations predict that deep electron traps introduced by co-doping with samarium effectively scavenge electrons from shallow traps associated with thallium, thus suppressing afterglow in the time domain of tens of milliseconds. In addition, combined radioluminescence and thermoluminescence experiments suggest that electrons released by samarium recombine non-radiatively with trapped holes, thus providing a mechanism for suppression of hysteresis. Ab initio quantum chemistry calculations support the conclusion that non-radiative charge-transfer transitions in CsI:Tl, Sm are enabled by the presence of low-energy excited states of within the ground configuration.

:Ce

Lanthanide gallium/aluminum-based garnets have a great potential as host structures for scintillation materials for medical imaging. Particularly attractive features are their high density, chemical radiation stability and more importantly, their cubic structure and isotropic optical properties, which allow them to be fabricated into fully transparent, highperformance polycrystalline optical ceramics. Lutetium/gadolinium aluminum/gallium garnets (described by formulas ((Gd,Lu)3(Al,Ga)5O12:Ce, Gd3(Al,Ga)5O12:Ce and Lu3Al5O12:Pr)) feature high effective atomic number and good scintillation properties, which make them particularly attractive for Positron Emission Tomography (PET) and other γ- ray detection applications. The ceramic processing route offers an attractive alternative to single crystal growth for obtaining scintillator materials at relatively low temperatures and at a reasonable cost, with flexibility in dimension control as well as activator concentration adjustment. In this study, optically transparent polycrystalline ceramics mentioned above were prepared by the sintering-HIP approach, employing nano-sized starting powders. The properties and microstructures of the ceramics were controlled by varying the processing parameters during consolidation. Single-phase, high-density, transparent specimens were obtained after sintering followed by a pressure-assisted densification process, i.e. hot-isostatic-pressing. The transparent ceramics displayed high contact and distance transparency as well as high light yield as high as 60,000-65,000 ph/MeV under gamma-ray excitation, which is about 2 times that of a LSO:Ce single crystal. The excellent scintillation and optical properties make these materials promising candidates for medical imaging and γ-ray detection applications.

^{3+}

While polycrystalline ceramic of Ce+3 doped lutetium oxyorthosilicate (LSO) has demonstrated scintillation characteristics equivalent to those of single crystal material, it lacks in optical quality. It is projected that if their grain size could be reduced to the nanometer range they would be smaller than the wavelength of light thereby minimizing scattering and substantially improving optical quality. In this investigation ceramic LSO:Ce that is much more transparent than would be expected from a highly anisotropic material, has been successfully produced by hot pressing at 75 MPa in a graphite die and furnace. The conditions necessary for powder processing and densification were optimized so as to produce dense LSO:Ce ceramic discs with an average grain size of 700 nm. Appreciable improvement in optical properties was observed, with decay and emission levels comparable with LSO single crystals, the light output was some 20% below that of single crystal. The degradation of light output in the nanoceramic is attributed to the formation of quenching centers associated with the loss of oxygen during densification, to which such nanomaterials are highly susceptible.

Ceramics

Despite its obvious advantages, well known CsI:Tl scintillator has two characteristic properties that undermine its use in clinical and high speed imaging: the presence of an afterglow component in its scintillation decay, and a hysteresis effect that causes drift in the scintillation yield after exposure to high radiation doses. We have previously reported that the addition of a second dopant, Sm2+, to the CsI:Tl crystals, significantly suppresses both afterglow and hysteresis. Here we report on the fabrication and characterization of the Sm co-doped CsI:Tl microcolumnar films to examine if these properties are preserved in films as well. Our preliminary data suggests that the Sm co-doped CsI:Tl films significantly improve temporal response relative to their CsI:Tl counterpart, and that the newly developed films demonstrate excellent spatial resolution. Various aspects of these effects and their consequences for imaging performance are discussed in this paper.

A new X-ray scintillator for digital radiography

Microcolumnar CsI:Tl remains a highly desirable sensor for digital X-ray imaging due to its superior spatial resolution, bright emission, high absorption efficiency, and ready availability. Despite such obvious advantages, two characteristic properties of CsI:Tl undermine their use in clinical and high speed imaging: a persistent afterglow in its scintillation decay, and a hysteresis effect that distorts the scintillation yield after exposure to high radiation doses. In our earlier work we have discovered that the addition of 0.05 to 0.5 mol percent of Sm2+ to crystals of CsI:Tl suppresses their afterglow by a factor of up to 50, even when subjected to a very high exposure of 120 R. This additive also diminishes hysteresis by an order of magnitude, which is a major accomplishment. Consequently, our work is now focused on developing codoped micro- columnar CsI:Tl,Sm films that can potentially combine excellent properties of the current state-of-the-art CsI:Tl films with the reduced afterglow and hysteresis observed in codoped crystals. While our earlier attempts in CsI:Tl,Sm film fabrication, reported at the previous IEEE meeting, demonstrated obvious advantages of the approach, the recent work has succeeded in producing films that show improvement by at least a factor of 7 in afterglow and 150% in brightness compared to the standard CsI:Tl films. We report these important results in this paper, along with other recent advances in film growth and new imaging results.