3124 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 52, NO. 6, DECEMBER 2005 CdWO Crystal in Gamma-Ray Spectrometry M. Moszyn ´ski, Fellow, IEEE, M. Balcerzyk, M. Kapusta, Member, IEEE, A. Syntfeld, Member, IEEE, D. Wolski, G. Pausch, J. Stein, and P. Schotanus Abstract—The properties of CdWO (CWO) crystals in gamma spectrometry were studied. Several small samples of 10 10 3 mm size, typically used in CT X-ray detectors, were tested and then compared to the performance of a larger crystal of 20 mm in diameter and 20 mm in height. The light output, energy resolution, and nonproportionality of the CWO response versus gamma-ray energy, were measured and compared with those of a small BGO to discuss further the origin of the intrinsic resolution of pure undoped scintillating crystals. A high light output of phe/MeV and a good energy resolution of % for 662 keV gamma rays from a Cs source were measured for the small samples coupled to an XP3212 photo- multiplier. Common nonproportionality curves and consequently common intrinsic resolutions of small CWO and BGO suggest that they represent fundamental characteristics of the heavy oxide scintillating material themselves. Index Terms—CdWO , energy resolution, non-proportionality, scintillators. I. INTRODUCTION N EW high-Z scintillator detectors are needed to signifi- cantly improve current border monitoring instrumentation utilizing plastic detectors, as well as, for the development of highly efficient handheld instrumentation. One of the possible candidates is an undoped CdWO (CWO) crystal [1]–[8]. A high density of 7.9 g/cm and a high Z value of tungsten (74) assure comparable detection efficiency to that of BGO. The emission maximum is situated at 475–480 nm as measured in [2] with 2 cm thick sample. The relative light output corresponds to 38% of that of NaI(Tl), as measured with a bial- kali photocathode photomultiplier (PMT) [2]. A good energy resolution of 6.8% was reported for a 2.5 cm in diameter and 1.2 cm thick CWO crystal in [2]. A comparable energy resolution of 6.8% was reported in [3] for a larger, 3 cm 3 cm, CWO crystal. An application of CWO in the gamma spectrometry is limited because of a long main decay time constant of about 15 s of the light pulse [2], [4], which does not allow measure- ments at high counting rates. However, in the border monitoring equipment, one is looking for traces of gamma rays emitted by Manuscript received October 12, 2004; revised July 26, 2005. This work was supported in part by the International Atomic Energy Agency, Research Con- tract No. 12596/Nuclear Security Multi-donors Fund and by the Polish Com- mittee for Scientific Research Grant No. 3 T10C 010 26 and SPUB no. 621/E- 78/SPB/IAEA/0-13/DWM21/2004-2006. M. Moszyn ´ski is with the Soltan Institute for Nuclear Studies, PL 05–400 Swierk-Otwock, Poland (e-mail: marek@ipj.gov.pl). M. Balcerzyk, M. Kapusta, A. Syntfeld, and D. Wolski are with the Soltan Institute for Nuclear Studies, PL 05-400 S ´ wierk-Otwock, Poland. G. Pausch and J. Stein are with Target Systemelectronic GmbH, D-42651 Solingen, Germany (e-mail: G.Pausch@target-systems-gmbh.de). P. Schotanus is with SCIONIX Holland B.V., 3980 CC Bunnik, The Nether- lands (e-mail: scionix@wxs.nl). Digital Object Identifier 10.1109/TNS.2005.855704 well-shielded radioactive sources. Thus, high rate capabilities of the detectors are less important. Still the main goal is the con- struction of detectors with high efficiency to rule out innocent alarms on one side and being able to detect a heavily shielded nuclear material on the other side. In this respect, the good tem- perature stability of the light output of CWO must be pointed out [1], [5]. In [3], the CWO crystal was proposed as the compact gamma ray spectrometer for planetary lander missions. In [4] the pulse shape discrimination capability of CWO was studied in order to detect charged particles in a gamma ray background. Finally, the CWO crystal was also proposed for a detection of thermal neutrons [6], [7]. Since the afterglow of CWO upon an X-ray irradiation is very low [5], [8], the crystal is widely applied for X-ray detection in CT scanners. The aim of this work was to study the properties of CWO in gamma spectrometry. Four small samples of 10 10 3 mm size, typical for use in CT X-ray detectors, were tested. Two of them were characterized by a low afterglow, according to the CT detector requirements. Two others showed about a 5 times higher afterglow. Finally, the performance of a larger crystal with 20 mm diameter and 20 mm height, applicable in the border monitoring, was studied. The light output, energy resolution and nonproportionality of the CWO response were measured versus gamma-ray energy for all studied crystals. The nonproportionality and energy res- olution characteristics were compared to those of mm BGO [9] to discuss further the origin of the intrinsic resolu- tion of pure undoped scintillating crystals. A study [9] suggested that both curves represent the fundamental characteristics of the BGO crystal material itself. A collection of new data in this re- spect was postulated at the Non-proportionality Workshop held in Portland in 2003 during IEEE NSS-MIC conference [10]. II. EXPERIMENTAL DETAILS All the CWO crystals were delivered by Scionix. Four crys- tals of 10 10 3 mm size had the exit face polished and the others grounded. Two crystals, marked by L, showed very low afterglow, according to the CT detector requirements. The other two, marked by H, exhibited about 5 times higher afterglow ac- cording to the manufacturer data. The large crystal of 20 mm in diameter and 20 mm in height was grounded at the side surface, while the exit and top faces were polished. Unfortunately, the large crystal showed a slight yellow color suggesting a selfab- sorption of the light [1]. In all measurements the crystals coated with Teflon tape were coupled by silicone grease to a Photonis XP3212 photomulti- plier having a high blue sensitivity of 12.1 A/lm blue. A signal 0018-9499/$20.00 © 2005 IEEE