Cerium-Doped Yttrium Aluminum Perovskite (Yap): Properties of Commercial Crystals
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ABSTRACT 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/cm'); 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.
INTRODUCTION Weber [1] reported the scintillating properties of cerium-doped yttrium aluminum pervoskite, YAlO 3 :Ce, subsequently vulgarized to YAP. A succession of workers in the former Eastern Block groups [2-5] have developed YAP using two growth methods, Czochralski vertical crystallization and horizontal oriented crystallization. Currently YAP is available from three commercial sources [6-8]. In our analysis we have used samples ranging in size from ,-, 35 pim grains from all three suppliers to a variety of optically finished single crystal which include 20 x 60 x 60 mm3 slabs [7]. Most of our measurements have been with Russian samples. During this work we identified several applications now using sodium iodide-based detectors which if YAP scintillators were substituted would realize a marked increase in performance. These include, but are not limited to, dosimetery (especially in field deployed survey meters), x-ray and low-energy -7-ray detection (especially in high-rate burst mode operations), and medical imaging (especially tomographic gamma cameras.) The decisive advantage of YAP for dosimetry is its mechanical ruggedness, its low Kedge x-ray energy, and its speed; for x-rays it is its speed, and for medical imaging it is its ability to be precisely fashioned into various small shapes and its speed. Although the decay speed of YAP benefits all these applications but currently can not be used in spectrometric measurements since no commercial electronics allow precise pulse height measurement at the - 50 MHz operating speeds of which YAP is capable.
131 Mat. Res. Soc. Symp. Proc. Vol. 348. @1994 Materials Research Society
RESULTS Figure 1 shows a calculation of the attenuation length of -y-rays by conversion process in YAP and sodium iodide for the energy region 20 to 3,000 keV (9]. 102
101
YA1O3:Ce
-.
•,(YAP)
E
10' 0 0
100
Total
Comptoon
Nal:TI
"I',
101 Total T,
Compton
100 C--
1
._
10'~ CD
.f-
Photoab
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