No abstract

▪ Abstract  Scintillators are the primary radiation sensor in many applications such as medical diagnostics, medical radiographs, and industrial component inspection. Some of the limitations in the properties of single-crystal scintillators are discussed for imaging applications, and the advantages of a new class of polycrystalline ceramic scintillators are described in detail. After the important scintillator properties of transparency, X-ray stopping power, light output, primary speed, luminescent afterglow, and radiation damage are described, the processing and performance of ceramic scintillators (Y,Gd)2O3:Eu,Pr; Gd2O2S:Pr,Ce,F; and Gd3Ga5O12:Cr,Ce are discussed. Ceramic scintillator uses and trends are presented in light of issues related to their uses in advanced medical and industrial X-ray detectors for CT imaging applications. Finally, some of the challenges are given for successfully developing a polycrystalline ceramic scintillator for use in photon-counting applications.

No abstract

Phase transitions in the group-IV transition metal zirconium were studied by the energy-dispersive xray-diffraction technique with a synchrotron source to a pressure of 32 GPa. A first-order phase transition between an co phase and a bcc phase (isostructural with group-V transition elements) was observed during compression and decompression in a pressure range of 30 ± 2 GPa, which is in qualitative agreement with the recent first-principles theoretical predictions. The observation of the bcc phase in Zr (a group-IV element) under pressure at room temperature signifies a pressure-induced electronic configuration similar to that of a group-V element. PACS numbers: 64.70.Kb, 61.10.-i, 62.50.-hp, 64.30.+tCrystal structures of elemental metals tend to have certain sequences when viewed as functions of atomic number or hydrostatic pressure. The most prominent example of this phenomenon is the d transition metals, where all three transition series, excluding the four magnetic 3d metals, show the canonical hep-" bcc-• hep -fee sequence as their atomic numbers increase, or as their d bands become progressively filled. 1 " 4 It is believed that the d electrons play a critical role in determining the crystal structures of these transition metals. Similar transition-metal structure sequences are expected to occur in individual transition metals with increasing pressure since compression leads to an increase in delectron population by transfer of electrons from the s band. 1,2 This pressure-induced phase-transformation sequence has received extensive experimental as well as theoretical attention. Calculations of the crystal structures using one-electron theory, based on electron transfer from the s to the d band under compression, are given in reviews by McMahan 3 and Skriver. 4 The phenomenon of electron transfer from the s to the d band under pressure is well known for transition metals. 5 Physically, this phenomenon can be attributed to the greater fraction of the atomic volume occupied by the sp core in transition metals. The bottom of the s band (B s ) rises faster than that of the d band (Bj) because the s-band electrons have a greater increase in kinetic energy than the d electrons since they are repelled by the orthogonality effect as they are more confined to the core region under compression. This larger increase in B s reduces the spacing between the s and d bands, and causes the increase in occupancy of the d band. It is this increase in ^/-band occupancy with pressure of the transition metals with a partially filled d band like Ti, Zr, and Hf that makes the pressure effects equivalent to alloying with d-rich metals. Some model calculations 6,7 show that the pressure-induced electron transfer from the s band to the d band is greater for the first several columns of transition element series. Therefore, the transition metals in the first several columns are expected to show phase transitions at modest pressures, while ultrahigh pressures greater than 200 GPa will likely be required for the phase transitions of the ...

No abstract