Organik malzemelerin çift enerjili X-ışını güvenlik sistemlerinde görüntülenmesi ve patlayıcı malzemelerin tespiti
Imaging organic materials on dual energy X-ray security systems and detection of explosive material
Yazar: OZAN YALÇIN
Danışman: PROF. DR. İSKENDER ATİLLA REYHANCAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Nükleer Araştırmalar Ana Bilim Dalı / Radyasyon Bilim ve Teknoloji Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Fizik ve Fizik Mühendisliği = Physics and Physics Engineering ; Radyoloji ve Nükleer Tıp = Radiology and Nuclear Medicine
Dizin: Elektromanyetik radyasyon = Electromagnetic radiation

Özet:
X-ışını güvenlik tarama cihazları, güvenlik sektöründe özellikle havaalanı, liman, gümrük, askeri tesis, kamu kurum ve kuruluşları, avm giriş çıkışlarında kritik tesis geçiş kontrol sistemi olarak giderek artan bir eğilimle kullanılmaktadır. Geçiş kontrol sırasında paket ve çantaların taranıp görüntülenebilmesi hem kontrol için zaman tasarrufu sağlamakta hem de tesis ve kişi güvenliğinin sağlanmasına destek olmaktadır.
Halihazırda kullanılan cihazların performansları, kullandıkları X-ışını enerjisi ve dedektör kabiliyetleri ile orantılı olarak değişmektedir. Donanım çözümleri her ne kadar performans çıktıları olan penetrasyon değeri ve tel çözünürlüğü değeri için önemli olsa da, X-ışını görüntülerinin işlenmesi ve analiz edilmesini sağlayan yazılım ve algoritmalar olmadan elde edilen gri-seviye görüntüler son kullanıcı için çok anlamlı değildir. Son kullanıcının farkındalığı arttırmak için taranan paketlerde yer alan nesne ve maddelerin sınıflandırılması, tespit edilmesi, renklendirilmesi ve tehdit unsuru olabilecek maddelerle ilgili işitsel/görsel uyarıların verilmesi yüksek öneme sahiptir. 
Bu çalışmaya konu olan, X-ışını görüntülemede organik maddelerin ayırt edilebilmesi ve patlayıcı tespiti konusu literatürde de oldukça fazla araştırma ve geliştirme çalışmalarına konu olmuştur. Birçok farklı teori ve yaklaşım bulunmasına rağmen bu tezde etkin atom numarası (Zet) hesaplamalarına dayalı bir yöntem tercih edilmiştir. Kullanılan X-ışını jeneratörü 160kV anot voltajına ve 0,5mA akım değerine ayarlanmıştır. Çift enerji düzeyli dedektörler kullanılarak, yapılan deneyler ile farklı maddelere ait 16bit çözünürlüklü yüksek enerji (YE) ve düşük enerji (DE) görüntüler elde edilmiştir. Görüntü çiftleri piksel bazlı analiz edilerek kütle zayıflatma katsayıları (µYE - µDE) hesaplanmıştır. Tüm görüntü analiz ve hesaplamaları için MATLAB'dan faydalanılmıştır. Kütle zayıflatma katsayıları taranan maddenin kalınlık (X) bilgisini de içerdiği için, genel çözüme ulaşmak için engel teşkil etmektedir. Maddenin kalınlık (X) bilgisinin yok edilebilmesi için kütle zayıflatma katsayıları oranlanarak R değeri bulunmuştur. R oranına bağlı olarak maddelerin organik yada inorganik olarak sınıflandırılması sağlanmıştır. Bir sonraki adım olarak, R oranı ile maddelerin etkin atom numarası (Zet) arasında bir bağıntı oluşturarak, patlayıcı maddelerin tespit edilebilmesi için bu yaklaşımdan faydalanılmıştır.
Gerçekleştirilen testler ve elde edilen sonuçlar değerlendirildiğinde, R oranına bağlı olarak materyallerin organik veya inorganik olarak sınıflandırılmasının sağlanabildiği görülmüştür. Ek olarak, patlayıcı tespiti için yapılan R – Zet yaklaşımının belirli bir oranda yüzde fark ile doğrulama yapabildiği görülmüştür.
Geliştirilen bu tip algoritmalar, cihazların performans çıktılarını arttırmaktadır ve erken uyarı ve tespit ile olası eylem ve saldırıların önüne geçilebilir. Böylelikle, cihazların kullanıldığı noktaların güvenlik düzeylerinin yükseltilmesi sağlanabilmektedir.

Abstract:
X-ray security devices are increasingly used as critical facility access control systems at airports, customs, shopping centres and many other places. Packages, bags, suitcases and parcels are passed through the devices can be scanned with X-ray and their contents can be viewed. Furthermore, the detection of dangerous materials like explosives, drugs can be achieved in a short time. Screening of packages without manual control saves time, supports facility and personal security. 
To create the X-ray images, unlike devices in others, the scanned object moves in X-ray security devices. Due to the movement of objects, the exposure time and the image acquisition time are limited. The objects are placed on the conveyor belt and the drum motor that provides the movement is started. The conveyor belt moves at a speed of 0.2 m/s towards. The imaging process is performed while continuing to move at this speed. After the object enters the armoured tunnel, it is detected by the infrared sensors and the generator is made to produce X-rays. If there is no object in the tunnel, there will not be produced X-rays; thus, radiation safety is ensured. By using the line detectors and signal processor, the images are taken line by line and X-ray images are obtained.
The performance of the X-ray security devices varies in proportion to the voltage (kV) and the anode current (mA) of the X-ray source, and the capabilities like the number pixel, pixel size and pixel resolution of the X-ray detectors. The higher the generator voltage, the greater the penetration amount into the steel surface; furthermore, the higher the detector resolution, the higher the image quality obtained. Therefore, detectors and generators are very important to ensure high performance.
Though hardware solutions are significant for the performance outputs like steel penetration and wire resolution, grey-level X-ray images are not very meaningful for the end-user without image analyzing and processing algorithms and software. Thus the X-ray devices should also have advanced functions such as shape analysis, object detection, object identification and material identification. To increase the awareness of the end-user, classification and detection of material, visual colouring, and giving visual/auditory warnings about the possible threats have great importance. Thanks to the researches, studies and advancing technology, many methods have been developed for analysis and image processing in X-ray devices. In addition to the objective description and classification methods such as "machine learning", there are many models such as the "Effective Atomic Number", which has a material description and classification methodology, independent of the shape of the object.
Nowadays, single energy level X-ray devices have been replaced by dual-energy level devices. Dual-energy level devices create two images for a package at the same time. Dual-energy level images contribute to both the classification of the materials in the package and the detection of hazardous materials by analyzing them in detail. Therefore, much more meaningful information about the objects in the scanned package is obtained. In dual-energy devices, images are provided by the double layer of detectors and the filter placed between the two layers.
The distinction of the organic materials in dual-energy X-ray imaging and the detection of explosive materials has been the subject of a great many research and development studies in the literature. Although there are a lot of different theories and approaches, a method based on the effective atomic number (Zeff) calculation has been preferred in this thesis. The effective atomic number can be described as the effective nuclear charge of an atom or the average atomic number of a chemical compound.
In this study, the X-ray source set to 160kV and  0,5mA. High energy (HE) and low energy (LE) images of different materials have been obtained in the experiment by using dual-energy level detectors. High energy and low energy images have 16bit resolution. By analyzing image pairs (HE, LE) the mass attenuation coefficients (µHE, µLE) were calculated. The mass attenuation coefficient for a material refers to how easily it can be penetrated by light, sound, particle, or electromagnetic radiation such as X-rays. All image analysis and calculations were performed using MATLAB. Using the code created in Matlab, an area of 441 pixels was scanned for ten low energy images of material, and the average pixel value was obtained. This process was repeated for ten high energy images of the same material. Mass attenuation coefficients (µHE, µLE) were calculated with the obtained average pixel density values of materials. Since the mass attenuation coefficient includes the thickness (X) information of the material, it is an obstacle to obtain a general solution. In order to eliminate the thickness value of the substance, the R (µHE/ µLE) value was calculated by proportioning the mass attenuation coefficients.
The organic substances focused in this study are compounds containing carbon, hydrogen, oxygen and nitrogen, and it is defined as the region where the effective atomic number is between 0 and 8 for general use in X-ray systems. However, the elements and compounds determined as samples are distributed in organic and inorganic classes. After the general characteristics of the prepared experimental setup were determined, the substances were classified as organic and inorganic depending on the ratio R (µHE/ µLE). Then, the organic and the inorganic classes were analyzed separately for the system, and the responses to the organic class were characterized especially. The relation between the ratio R and the effective atomic number (Zeff) was used to detect and identify explosives and threats like Ammonium Nitrate, PETN and C-4. 
It is clearly seen that it is possible to classify the materials as organic and inorganic depending on the ratio R when the results obtained from the tests were evaluated. Furthermore, it has been observed that the ratio R-Zeff approach for the detection and identification of explosive threats can verify with a certain margin of error. However, this study is only an approach and it should be known that it does not give the exact results. 
In future studies, the system characteristics will be obtained more clearly by increasing the number and type of organic samples used. Thus, higher sensitivity can be achieved in determining threatening materials. Not only with the organic class, but similar studies will also be carried out for the inorganic class, and the identification of inorganic threats will be provided. By using different methodologies together, the rate of correct detection will be increased. To give an example, by making effective mass density (ρeff) calculations instead of R-Zeff, ρeff - Zeff relations will be obtained in this way and materials such as sugar and grape vinegar with similar Zeff values could be distinguished. Besides, considering that separating only organic and inorganic classes is also insufficient, more awareness will be provided to the end-user by dividing these classes into subclasses within themselves. 
The algorithms to be developed will increase the performance output of the X-ray security devices. Possible actions and attacks could be prevented thanks to early warning and detection. Thus, it would be possible to increase the security levels of the points where the devices are used.