Estimation of Service Life of X-Ray Tube Anodes

Abstract

Progress in modern X ray techniques to a large extent is due to improvements in the performance char acteristics of X ray tubes. In addition to better X ray focusing, it is also very important to increase the service life of the X ray tube under conditions of increased ther momechanical load.Anode rotation leads to intensive electron bombard ment of its focal tracks. The heat generated at the focal spot is expanded all over the anode and emitted from its surface. As the focal spot is briefly exposed to the electron beam, its temperature rapidly increases, and then gradu ally decreases. Such temperature dynamics continues during the entire exposure session, whereas during the pause between the exposure sessions the mean tempera ture of the anode becomes lower. Usually, after 5 10 cycles the dynamics is leveled.Thus, the character of the first temperature cycle is due to the anode rotation and depends on the rotation period, while the second temperature cycle is determined by the operating mode of the X ray tube (namely, the exposure time and the pause duration).Typical operating modes of the 20 40BD40 125 X ray tube (Svetlana Roentgen, Ltd., St. Petersburg) are given in Table 1. This X ray tube anode has the following characteristics: anode diameter, 70 mm; substrate thick ness, 12 mm; target thickness, 1 mm; working surface slope angle, 17°. The mean radius of the working track is 27.5 mm. The anode rotation rate is 50 rpm; effective focal spot size, 2 × 2 mm. The substrate and target are made of molybdenum and tungsten, respectively. The service life of the X ray tube is 20,000 exposure sessions.The X ray tube service life was calculated based on the temperature fields and the number of thermal cycles before appearance of cracks in anodes made of monocrystalline and polycrystalline Mo and W. Calculation of Temperature Fields and Stress States of Rotating X Ray Tube AnodesThe anode temperature is increased above mean level near the focal spot. The anode size is not taken into account in calculation of the temperature variation amplitude. This is valid if the anode rotation rate is n > 0.17·а·d·(r 0 h 2 ) [1], where а is the temperature conductiv The X ray tube anode life (time interval before cracks in substrate or coating appear) was estimated for differ ent operating conditions. The calculations were based on the Manson equation and experimental results on the plastic properties of polycrystalline and monocrystalline materials. The limiting number of exposure sessions before appearance of cracks in the substrate or the monocrystalline coating was 80,000 250,000, which is 30 100 times greater than in the polycrystalline coating or 4 5 times greater than in the polycrystalline W-Re coat ing, respectively. It was demonstrated in this work that the exposure dose power for a monocrystalline anode was virtually invariable for ~5000 exposure sessions, whereas in the case of a polycrystalline anode ~1000 exposure sessions caused a decrease in the dose power. This can be regarded as evidence of larger damageability of p...