Measurement of absolute light yield and determination of a lower limit for the light attenuation length for YAP:Ce crystal

In small animal PET applications YAP:Ce scintillator has given some promising results [1]. Because of its somewhat lower efficiency with respect to other PET crystals at 511 keV the depth of the crystal is a crucial parameter Past measurements of the light attenuation length for YAP:Ce scintillator have given rather different results with values as low as 3 cm [2]. We have Studied the pulse height spectra of full energy absorption events as a function of the interaction distance from the photomultiplier window for five match like 2 x 2 x 30 mm(3) YAP:Ce crystals. Each crystal was covered on the four long sides and on one of the square section sides with a 5 mu m reflective layer. The crystals were the middle row of a bundle of 5 by 5 such crystals. By using a transport model for-light within the crystals, for this crystal configuration, we obtained a lower limit of 6.8 +/- 0.3 cm on the average 'bulk' (not including the losses from reflection) light attenuation length. This lower limit is significantly higher than the attenuation length measured by other authors. From these measurements and calculations we have also determined the average absolute light yield for the five crystals to be 24000 +/- 1100 photons/MeV.