Machine Sources of Radiation 1 Xray Tube The

Machine Sources of Radiation

1 -X-ray Tube • The first of the machine sources of radiation was the high-vacuum diode, called a Crooke’s tube. In this tube, electrons were accelerated across about 25, 000 V to a high velocity and then were abruptly stopped when they struck the anode. In accordance with Maxwell’s theory of electromagnetic radiation, some of the kinetic energy of the electrons was converted into electromagnetic energy in the form of X-rays due to the abrupt deceleration of the electrons (the following Figure). This method of generating X-rays is the forerunner of the modern X-ray tube used in diagnostic radiology, dentistry, and in industrial radiography.

The same type of X-ray generators are used in X-ray spectrometers and diffractometers in analytical chemistry and crystallography, and in inspection and control devices. Most of the X-rays from this type of generator are emitted at right angles to the path of the accelerated electron. In this type of X-ray generator, the full accelerating voltage must be applied across the electrodes of the tube. This limits the maximum kinetic energy of the electrons to several hundred thousand electron volts. A conventional X-ray generator, in which electrons that are emitted by a heated cathode are accelerated by the strong electric field and emit X-rays after being abruptly stopped by striking the target. The useful X-ray beam emerges through an opening in the tube housing (shield).

2 -Linear Accelerator • The linear accelerator (shown in the figure in the next slide), in principle, consists of a series of tubular electrodes, called drift tubes, with an electron source at one end a target at the other end for stopping the highenergy electrons. The electrodes are connected to a source of high-frequency alternating voltage whose frequency is such that the polarity of the electrode changes as the electron exits from one drift tube and thus is attracted to the next drift tube. Each successive drift tube is at a higher voltage than the preceding one. The electron’s gain in kinetic energy in e. V is thus equal to the voltage difference between successive drift tubes.

Illustration of the operating principle of a linear accelerator. (From Brobeck WM. Particle Accelerator Safety Manual, MORP 68 -12. Rockville, MD: National Center for Radiological Health; 1968. )

These gains in kinetic energy are cumulative. Thus, if we had a series of 30 electrode gaps with 100 k. V differences between them, and if the electron were injected into the system with a kinetic energy of 100 ke. V, the electron would emerge at the other end with a kinetic energy of Ek = 100 + (30 × 100) = 3100 ke. V = 3. 1 Me. V. • The energy gradient in linear accelerators typically is 2 – 4 Me. V/ft. At these high energies, the speed of an electron is almost that of the speed of light. The increase in kinetic energy goes to increased mass rather than to higher speed, and all the high-energy electrons travel at about the same speed

• In addition to generating X-ray by causing the electron beam to strike an internal target, a linear accelerator can be designed to be an electron irradiator, that is, to bring the electron beam out of the machine and to deliver a radiation dose with the electron beam. Linear accelerators are widely used to treat cancers, as well as in research and industrial applications.

3 -Cyclotrons are circular devices (shown in the next slide) in which charged particles such as protons and alpha particles are accelerated in a spiral path within a vacuum. The power supply provides a rapidly alternating voltage across the dees (the two halves of the circle). This produces a rapidly alternating electric field between the dees that accelerate the particles, which quickly acquire high kinetic energies. They

Illustration of the operating principle of a cyclotron. (From Brobeck WM. Particle Accelerator Safety Manual, MORP 68 -12. Rockville, MD: National Center for Radiological Health; 1968. )

spiral outward under the influence of the magnetic field until they have sufficient velocity and are deflected into a target. A deflector is used to direct the particles out through a window of the cyclotron into a target. Some of the particles are incorporated into the nuclei of the atoms of the target. These energized (excited) nuclei are unstable. Because of this upper energy limit, cyclotrons are not commonly used in high-energy physics research. • The principal use of cyclotrons is as radiation sources, especially for the manufacture of short-lived radionuclides, such as 110 -minute which is widely used in PET (positron emission tomography) scans for medical diagnostic studies.

• Indium-111 is produced in a cyclotron. The accelerated (bombarding) particles are protons. The target atoms are cadmium-111. When a proton enters the nucleus of a atom, the is transformed into by discharging a neutron. This reaction can be written as: . Other examples of cyclotron reactions include , , and.