Measurement of the (Gd,La) 2 O 2 S:Tb phosphor eciency for x-ray imaging applications D. Cavouras a, *, I. Kandarakis a , C.D. Nomicos b , A. Bakas b , G.S. Panayiotakis c a Department of Medical Instrumentation Technology, Technological Educational Institution of Athens, Agiou Spyridonos Street, Aigaleo, 122 10 Athens, Greece b Department of Electronics, Technological Educational Institution of Athens, Agiou Spyridonos Street, Aigaleo, 122 10 Athens, Greece c Department of Radiology, Technological Educational Institution of Athens, Agiou Spyridonos Street, Aigaleo, 122 10 Athens, Greece Received 10 October 1998; received in revised form 27 July 1999; accepted 28 July 1999 Abstract (Gd,La) 2 O 2 S:Tb phosphor performance was tested under x-ray excitation for medical imaging applications with tube voltages from 70 to 180 kVp and phosphor coating weights from 20 to 240 mg/cm 2 . The emission spectrum of (Gd,La) 2 O 2 S:Tb was also measured and it was centered at 549 nm. (Gd,La) 2 O 2 S:Tb was found ecient in a wide range of x-ray tube voltages and adequately compatible with photocathodes, photodiodes, and orthochromatic ®lms currently used in x-ray medical imaging. # 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Phosphor materials are employed as radiation to light converters in detectors of x-ray or gamma ray medical imaging systems (Arnold, 1979; Haque and Stanley, 1981; Zweig and Zweig, 1983; Yae and Rowlands, 1997). The light emitted by phosphors is usually captured by optical detectors (®lms, photo- cathodes or photodiodes) incorporated in the radiation detector system. Gd 2 O 2 S:Tb and La 2 O 2 S:Tb have been previously shown to be two of the most ecient phos- phors (Giakoumakis et al., 1989; Kandarakis et al., 1996). Gd 2 O 2 S:Tb is ecient at medium to high diag- nostic x-ray energies while La 2 O 2 S:Tb is more ecient at lower energies. Gd 2 O 2 S:Tb is widely used in radio- graphic intensifying screens and in detectors of digital radiographic units while La 2 O 2 S:Tb is incorporated in intensifying screens of some radiographic cassettes (Arnold, 1979). In the present study the luminescent properties of a phosphor mixture consisting of 50% Gd 2 O 2 S:Tb and 50% La 2 O 2 S:Tb were determined, in order to investi- gate if this mixture is adequately ecient in a wider range of x-ray energies than the energy range of each phosphor used alone. The luminescence evaluation of the mixture, denoted as (Gd,La) 2 O 2 S:Tb, was accom- plished by determining the following parameters: 1. The absolute eciency (AE) of the phosphor, which is de®ned as the ratio of the light energy ¯ux emitted by an irradiated phosphor over the incident x-ray or gamma ray exposure rate (Ludwig, 1971; Cavouras et al., 1996; Kandarakis et al., 1996, 1997). AE is very important if the phosphor per- formance is evaluated in relation to the patient radi- ation dose burden. 2. The eective eciency (EE), which is the absolute eciency with respect to the spectral compatibility Radiation Measurements 32 (2000) 5±13 1350-4487/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S1350-4487(99)00245-0 www.elsevier.com/locate/radmeas * Corresponding author. Tel./fax: +301-9594-558 (home); fax: +301-5910-975 (work). E-mail address: cavouras@medisp.bme.teiath.gr (D. Cavouras).