Introduction

As one of the most important inventions of the 19^th century, X-ray technology plays a major role in our daily life. Whether we require an X-ray examination for medical reasons or have our luggage inspected before getting on an airplane, X-rays allow us an "inside view". X-ray astronomy conducts scientific investigations of astrophysical systems using the X-radiation that they produce. X-rays are also used for industrial applications such as X-ray inspection of casting products for the automobile industry or for analytical applications.

X-rays, what are they?

X-rays are generally invisible and travel in straight lines at the speed of light. Although they cannot be deflected by means of a lens or prism, the beam can be diffracted or bent by a chrystalline grid. When passing through a material, X-rays are partly absorbed in the transmission direction. The relation between absorption and penetration depends first of all on the kind of material (its atomic number) and on the energy of the X-rays. The energy itself is a function of the acceleration of the emitted electrons (the high voltage connected between anode and cathode) and the electrical current passing the filament wire. Another important property of X-rays is that they can destroy or impair living cells.

Description of an X-ray set

The main component of an X-ray set is the X-ray tube. The traditional one consists of a glass bulb while the sophisticated one consists of a metal ceramic envelop under vacuum. Inside the evacuated area, a cathode comprises the filament wire and the anode, the target with a high melting point. An electrical current drives electrons through the low resistance filament wire. In that way, the filament will get hot and electrons are emitted. Due to the connected high voltage between cathode and anode, the emitting electrons are accelerated in the direction of the target. X-rays are produced when these electrons strike the target (which is normally made of tungsten due to its high melting point). To prevent damage of the target, it is cooled by fluid substances or air depending on the X-ray power. One of the main tasks for medical applications is to keep the exposure to radiation for the patient as low as possible. This is determined by the product of mA and time (mAs). To increase the output, tubes with a rotating anode of a diameter up to 200 mm for a better loss of heat are used. The anode is accelerated up to 9000 rpm within less than 1 second. Due to the extreme stress of the bearings, Philips developed a special wear resistant spiral groove bearing which can provide continuous operation.

For industrial applications, tubes with a fixed anode are generally used. Here the heat is carried away by a copper extension of the anode block. The copper block itself is cooled by liquid which is usually circulated by means of a pump.

To get radiation output from an X-ray tube, a sufficient high voltage has to be supplied to anode and cathode. Therefore, a special high voltage generator is necessary which converts the main voltage into a range of up to 225,000 V. The high voltage generator also incorporates the filament transformer to supply the required filament current which is normally in the range of 4.5 A. Depending on the application, most of the high voltage sets also include an electronic power supply with highly sophisticated power electronic devices to control and monitor the data.

For manual operation of an X-ray set, a separate external control unit to display and set the operating data is generally connected to the power supply. Integrated in automatic inspection systems, the control functions are usually carried out by means of the system computer interfaced to the X-ray set.

Different Technology

Especially in the security field, different attempts were made to increase the performance of explosives detection systems during the last 10 years. The most critical aspects which have to be taken into consideration are detection and false alarm rate without ignoring a sufficient throughput of the bag to be inspected. Actually there are more or less two different detection technologies which are used today. Although the use of a linear detector array (LDA) allows a high inspection speed, its output signal does not provide sufficiently detailed substance specific information. This technique is mainly used as a 1^st or 2^nd level machine in multilevel inspection systems.

The coherent X-ray scatter spectra analysis, however, enables a better detection and much lower false alarm rate (<1%).  It also allows a precise differentiation between harmless and dangerous substances (such as "black powder" and honey) but at a lower throughput. Therefore this technology is just right for use as a 2^nd and 3^rd level machine.

Dual energy

Dual energy explosives detection systems base their decision on an estimation of the density of objects in a bag. This is done by a combined evaluation of two different X-ray images generated at two different X-ray voltages (e.g. 150 kV and 75 kV). Additionally, dedicated image processing is used to separate different objects superimposed on one another in the projected image. The measured densities are compared with library values of the densities of known explosives.

Multi view

This technique uses two X-ray systems with two different views (bottom view, top view). The two images of these views are used for a combined evaluation called three-dimensional density reconstruction. The estimated material density values generated from the two projection image pixel data are compared with the typical density data of explosives for identifying image objects of potential threat.

CT systems

Computed X-ray tomography is a technique which has been in use in medical applications for more than 20 years. An X-ray tube and a linear detector array are rotated around the body to be examined producing projection data of thin slices from many different views (profiles). Using a mathematical process (back projection), the density distribution of the irradiated slices is reconstructed (cross-sectional image). This enables 3-dimensional density information to be gathered. These density values are compared against values for explosives. Compared to dual energy and multiview X-ray projection systems, CT systems do have a lower throughput at the moment but a superior detection capability.

The use of coherent X-ray scatter is a new approach to the inspection of airport baggage.

The X-ray beam of a high-power X-ray tube is collimated to a special shape by a primary beam collimator. The collimator contains diaphragms with a shape of a cone and separates the basic X-ray beam into different "radiation rings" with different diameters.

The beam irradiates a small volume of the bag and penetrates it. The X-radiation scattered into a small fixed angle in the forward direction is focused by a secondary scatter collimator which is coupled to a detector. After preprocessing, the detector signals are evaluated by the computer. With this procedure, 3-dimensional or tomographic information is gained. Each substance has its specific spectra fingerprint when exposed to radiation. For plastic explosives, most industrial explosives and even home-made explosives, the scatter spectra exhibit characteristic diffraction peaks corresponding to their polycrystalline structure.