332 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 1, FEBRUARY 2014 Sample-to-Sample Variation in Single Crystal YAP:Ce Non-Proportionality Samuel B. Donnald, Student Member, IEEE, Mohit Tyagi, Harold E. Rothfuss, Jason P. Hayward, Member, IEEE, Merry Koschan, Member, IEEE, Mariya Zhuravleva, Fang Meng, and Charles L. Melcher, Senior Member, IEEE Abstract—In this paper, nine different samples of YAIO Ce have been collected and analyzed. The light yield non-proportion- ality of each sample was measured and used to classify each sample as proportional or non-proportional. A variety of scintillation and optical measurements were conducted on each sample, and the proportional samples were generally found to have a higher light output and better energy resolution. In addition, a strong linear correlation was found between scintillation decay time and the degree of non-proportionality. Based on absorption mea- surements as well as radioluminescence data, it was determined that the non-proportional samples all shared a range of increased absorption near the cerium 5d absorption edge between about 325 and 400 nm. The increased absorption has been reported in literature, and it is believed to be the result of a material defect introduced during growth. Thermoluminescence glow curves were measured for two representative YAIO Ce samples, one from each proportionality grouping, and it was determined that there was an observable change in defect structure, but there were no additional traps visible in the glow curves of either the propor- tional or non-proportional samples. However, the intensity of the 105 K thermoluminescence peak was found to be approximately a factor of two greater in the non-proportional samples. Since the lifetime of this peak is known to be between 25 and 81 ns, it was determined to be the likely cause of the slower decay in the non-proportional samples. Index Terms—Luminescence, non-proportionality, scintillation detector, YAP:Ce. I. INTRODUCTION I N the past three decades, a great deal of time and effort has gone into growing and characterizing YAIO Ce YAP:Ce . Based on the resulting literature, it is evident that YAP:Ce is an excellent scintillation material because of both its scintillation performance and its mechanical properties. The material is non-hygroscopic and is one of Manuscript received May 18, 2013; revised July 29, 2013; accepted August 04, 2013. Date of current version February 06, 2014. This work was supported in part by the U.S. Department of Energy under Grant DE-NA0000473. S. B. Donnald, M. Tyagi, M. Koschan, F. Meng, M. Zhuravleva, and C. L. Melcher are with the Scintillation Materials Research Center at the University of Tennessee Knoxville, Knoxville, TN 37920 USA (e-mail: sdonnald@utk.edu; mtyagi@utk.edu; mkoschan@utk.edu; fmeng@utk.edu; mzhuravl@utk.edu; cmelcher@utk.edu). J. P. Hayward is with the Department of Nuclear Engineering, University of Tennessee Knoxville, Knoxville, TN 37920 USA, and also with Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA (e-mail: jhayward@utk.edu) H. E. Rothfuss is with the Siemens Molecular Imaging, Knoxville, TN 37932 USA (e-mail: harold.rothfuss@siemens.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2013.2277860 Fig. 1. Picture of the nine samples used in the study. All of the samples are optically transparent, are of various shapes and sizes, and differ in surface fin- ishing. the hardest scintillation materials available [1]. It has been measured to have a light output 3.8 times BGO and has been measured to have an energy resolution as good as 4.38% at 662 keV [2]. The excellent energy resolution of this sample stems in part from its high light yield, but can be primarily attributed to its nearly ideal light yield non-proportionality [3]. The excellent proportionality leads to an excellent intrinsic resolution, which has been measured to be as good as 1.3% [2]. However, not all samples of YAP:Ce display an ideal light yield non-proportionality; many samples have been found to be substantially non-proportional [4]. Although a small sample-to-sample variation in light yield non-proportionality is expected in all scintillation materials, it is very rare to see a scintillation material where different samples can exhibit either a proportional or non-proportional response without the addition of impurities into the crystal lattice. II. EXPERIMENTAL DETAILS A. Overview of Samples For this study, we have collected 9 individual samples of YAP:Ce, shown in Fig. 1. Of these samples, two were grown in-house, and the other seven were either borrowed or purchased. All nine of the YAP:Ce samples were optically transparent and of good quality. The samples were classified as either proportional or non-proportional based on a simple figure of merit which will be discussed in II.B. The samples that were found to be proportional were labeled as P01 through P04. Likewise, the samples that were found to be non-propor- tional were labeled as NP01-NP05. Table I gives an outline of all known growth conditions, the cerium concentration (in the melt), and the sample origin. For the most part, growth information is lacking; this is because of either the sample’s old age or the sample not being obtained directly from the actual crystal grower. U.S. Government work not protected by U.S. copyright.