Detection of organic materials by spectrometric radiography method

Abstract —In this paper we report a spectrometric approach to dual-energy digital radiography that has been developed and applied to identify specific organic substances and discern small differences in their effective atomic number. An experimental setup has been designed, and a theoretical description proposed based on the experimental results obtained. The proposed method is based on application of special reference samples made of materials with different effective atomic number and thickness, parameters known to affect X-ray attenuation in the low-energy range. The results obtained can be used in the development of a new generation of multi-energy customs or medical X-ray scanners.

Index Terms — multi-energy radiography, effective atomic number, spectrometry, scintillation detectors, ZnSe-materials.

I. INTRODUCTION Presently, most dual-energy X-ray security systems for luggage inspection as well as for medical diagnostics use multi-detector systems operating in the current mode. Such X- ray instruments rely on two types of detectors – one a “thin” scintillator with low effective atomic number which therefore transmits the high energy portion of the incident radiation, i.e., a low energy detector (LED); and the second a “thick” scintillator with high effective atomic number seeking near total absorption of the incident radiation, i.e., a high energy detector (HED) (see, for example, [1-5]). Such combined detector systems can receive and analyze two type of signals: one from the LED detector, with its response to both the low- energy and the high-energy components; and the other from the total absorption (HED) detector, which records predominantly the high-energy component. The dual-energy approach allows substantial improvement of the image quality of X-ray inspection systems [6-13] as compared with systems based on conventional broad spectrum

This work was supported in part by NATO SfP-982823 and STCU 4115 projects. *Corresponding author: S.V. Naydenov is with Concern “Institute for Single Crystals”, Kharkov, Ukraine. Phone: +38-057-341-03-51, fax: +38- 057-719-59-97; e-mail: sergei.naydenov@gmail.com V. D. Ryzhikov and G. M. Onyshchenko are with the Institute for Scintillation Materials of the National Academy of Sciences of Ukraine.
P. Lecoq is with European Research Centre, European Organization for Nuclear Research (CERN), Geneva, Switzerland. E-mail: paul.lecoq@cern.ch C.F. Smith is with Lawrence Livermore National Laboratory (LLNL), Livermore, CA, USA. E-mail: cfsmith@nps.edu

X-ray radiography. However, in this method there is an unavoidable source of error due to the presence in the LED detector of a large fraction of high-energy quanta which interfere with the detection of the low-energy quanta. This ultimately affects the quality of the obtained image, especially in the identification of organic materials with small values of eff 5 10 Z ∝ − . This drawback can be largely avoided by fabricating the LED with specially designed scintillators based on doped zinc selenide compounds such as ZnSe(Te), which have a low eff 33 Z ≈ and a low density 2 5.4 g cm ρ = . With such scintillators and rather small detector thickness (e.g., hundreds of microns), absorption in the high energy range (80-90 keV) does not exceed 10-15%, i.e., the detector shows a roughly spectrometric behavior. A unique combination of useful physical characteristics, accompanied by high light output (up to 120-140% in comparison to CsI(Tl)), allow its application in inspection equipment as the most efficient scintillator (among the known variety of scintillation materials) for low energy detectors. Its use in customs inspection systems enable high-speed automatic sorting of loads with potentially dangerous enclosures [1]. In applications of this scintillator for medical diagnostics (i.e.,
the study of biological objects in 3D tomographic mode), it is possible not only to observe a clear difference between muscular and osseous tissues, but to reliably detect small deviations (2-3%) of calcium content in the bone tissue [3]. It has been confirmed [4] that the use of ZnSe(Te) in the LED component of a dual energy system ensures detection with high probability of illegal and dangerous substances in loads and luggage. At the same time, the engineering solution proposed in that patent and the specified design of the dual- energy detector array do not allow detection of all potentially dangerous substances with sufficiently low error probability. However, a theoretical approach developed to exploit the potential of multi-monochromatic radiation in several narrow and mutually separated spectral ranges [5] predicts very high (90-95%) accuracy in determination of eff Z and other important param

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