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Due to their high sensitivity and fast decay time, Rapidex screens have been successfully used in back-scatter imaging systems, where the photons which are Compton scattered from the target object, rather than those attenuated by it, are used to generate the image. In recent years backscatter detectors have found growing use in security applications, as this technique allows detection of organic materials, such as explosives or human tissue. It also enables the positioning of the detector side by side compactly with the x-ray source with clear operational advantages for the scanning of vehicles and other moving targets. In this application high sensitivity is essential, as backscatter detectors operate at relatively low energies, when the Compton effect is the predominant method of interaction between photons and free electrons and the probability of scattering back an incident photon towards the origin is higher. In addition, the energy of a scattered photon is always lower than the energy of the incident photon, hence the need of phosphor screens with high sensitivity such as the Rapidex range. Fast decay time is also important, as backscatter system often operate according to a flying spot technique, which was developed by American Science & Engineering (AS&E) in the 1970s.
A selection of materials for optimum performance in each application. Scintacor offers three types of Gadox scintillator screens (sometimes referred to as luminescent screens or intensifying screens):
Luminex (made of terbium-doped gadolinium oxysulphide (Gadox:Tb). This phosphor, also known as P43
Rapidex (made of praseodymium-doped gadolinium oxysulphide (Gadox:Pr) for hard X-ray applications (50-300 KeV)
MeVex (is a very high coating weight screen made of terbium-doped gadolinium oxysulphide (Gadox:Tb) for use at high-energy energy levels (>450 KeV – 25 MeV).
The properties of these screens are listed in Table 1.
Light Output (1)
MTF % @ 2lp/mm (2)
MTF % @ 5lp/mm (2)
Attenuation % (2)
Decay to 10% μs
Afterglow @ 20ms %
(1)Relative to Lanex Regular benchmark screen. X-ray source: 150kV unfiltered
(2)X-ray source 70kV
Size & geometry.
Unlike other commercially available products, Scintacor X-ray screens can be ordered in single units and custom-cut to any shape and size, to match the specific requirements of each application.
Large sheets are available with linear dimensions up to 1,000×1,750mm. On request, the screens can also be supplied in precision-cut strips as thin as 3-4mm for coupling to linear diode arrays. They can be mounted on a variety of substrates for easier handling and deployment.
Levy and West, the founders of the renowned laboratories which later became Applied Scintillation Technologies and then Scintacor, were among the pioneers who explored the use of zinc sulphide phosphors for fluoroscopic X-ray imaging. Since then, many additional phosphor systems have been developed and today we find gadolinium oxysulphide (Gd2O2S or simply Gadox / GOS) phosphor is one of the most widely adopted for several reasons. The material morphology is well known and well controlled; the luminescent efficiency is relatively high (approximately 15%) and the emitted light matches well with the spectral response of silicon light detection devices such as charge-coupled detectors (CCD) and complementary metal oxide semiconductors (CMOS).
Gadox scintillator screens were originally developed and commercialized by companies which supplied photographic film and were often described as ‘fine’, ‘regular’ or ‘fast’ depending upon the film speed and spatial resolution of the film with which they were paired. These screens are still widely available up to a maximum size of 14’’x 17’’ but they do not always provide the optimal solution for digital X-ray imaging. Conversely, Scintacor phosphor screens have been specifically designed around digital X-ray imaging systems to provide differentiated performance over standard scintillator screens, in both lens-coupled systems, and direct-coupling to solid-state light detectors, such as CMOS and CCD.