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 pr omising results [1]. Because of its somewhat lower efficiency with res pect to other PET crystals at 511 keV the depth of the crystal is a cr ucial 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 ene rgy absorption events as a function of the interaction distance from t he 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 o f the square section sides with a 5 mu m reflective layer. The crystal s 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 config uration, we obtained a lower limit of 6.8 +/- 0.3 cm on the average 'b ulk' (not including the losses from reflection) light attenuation leng th. This lower limit is significantly higher than the attenuation leng th 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.