This solution describes the basics of an x-ray tube for medical and basic-science applications. Through multiple choice questions, the principle of the acceleration of electrons onto a high-Z target for x-ray production is discussed. Some details about the underlying physics for generating Bremstrahlung and line-emission x-rays as well as the nature of Compton scattering and photo-electrical effect are discussed. Finally, this discussion is a summary of two solutions. The first solution briefly introduces the concept of an x-ray tube, while the second solution talks in detail about the inner workings of an x-ray tube through the multiple choice questions.
In a typical x-ray tube, electrons are accelerated onto a target, made of for instance copper, and when these high-energy electrons interact with the copper atoms, x-rays are produced. This x-ray production process is extremely inefficient. About 99% of the electrons (or rather the electron energy) is converted into heat, while only 1% of the electrons are converted to x-rays. The high voltage applied to the x-ray tube is typically in the range of 30 to 150 kV.
To prevent getting exposed to x-rays while working with an x-tube, it is typically enclosed by lead. The main motivation for using lead is that it is a high-Z material that is an excellent absorber of x-rays (higher Z mtrl -> better absorber). In addition, using thicker lead is more effective than using thinner lead, simply because more x-rays are absorbed by the thicker one than a thinner one.
1. What charge is needed to focus the electrons that are aimed at the target?
Answer: b. Positive. The target (anode) is usually operated with a positive bias relative to the cathode to effectively guide the electrons onto the target.
2. What type of anode is used so that the focused electron beam always strikes a different place?
a. Stationary anode
b. Rotating anode
c. Static anode
d. Target anode
Answer: b. Rotating anode. The sensitive area of the anode is typically shaped as an annular ring that is rotated to make sure the electron beam strikes different portion of the anode.
3. What problems can arise from using small focal spots?
a. Increased danger from radiation
b. Burning out the filament
c. Heating up the target too much
d. All of the above
Answer: c. Heating up the target too much. With too small focal spots on the anode, the local energy density is too high, risking the anode material to melt.
4. To use an anode cooling chart to prevent overheating the x-ray tube, calculate the heat units in which way?
a. kVp x exposure seconds
b. kVp + mA x exposure seconds
c. kVp x mA x exposure seconds
d. kVp/mA x exposure seconds
Answer: c. kVp x mA x exposure seconds. The x-ray thermal energy is measured in heat units, which is equivalent to kVp x mA x exposure seconds.
5. What happens when high energy electrons strike the atoms of the anode?
a. X-rays are produced
b. Direct current is produced
c. X-rays and gamma rays are produced
d. Any of the above may occur
Answer: a. X-rays are produced. In the anode, about 1% of the electron energy is converted to x-rays, while the rest of the electron energy (~99%) is converted to heat.
6. For an x-ray photon to be produced ...
The principle and underlying physics governing the use of an x-ray tube for medical and basic-science applications are discussed. The atomic-physics processes relevant to an x-ray tube, such as such as Bremstrahlung and line-emission x-rays, Compton scattering and photo-electrical effect are also elaborated upon. A brief discussion of the optimization for specific applications is also being made.