Going to the Source: X-ray Tubes

X-ray fluorescence (XRF) technology is a powerful technique for elemental analysis. The XRF process starts with irradiating a solid or liquid sample with high energy x-rays from a controlled X-ray source. In handheld and portable Energy-dispersive X-ray Fluorescence (EDXRF) instruments, the X-ray source is typically an X-ray tube, which has several advantages over radioisotopes including less stringent licensing issues and the fact that X-ray emissions from tubes can be easily modified for specific analytical applications. In this blog, we’ll explain how an X-ray tube works. The major X-ray tube components are the cathode, anode, the tube envelope, the tube housing, and the window.

Cathode

The cathode serves to expel the electrons from the circuit and focus them in a beam on the focal spot of the anode. It is a controlled source of electrons for the generation of X-ray beams. The electrons are produced by heating the filament, i.e., a coil of wire made from tungsten, placed within a highly polished nickel focusing cup providing electrostatic focusing of the beam on the anode. Heat is used to expel the electrons from the cathode.

Anode

The anode represents the component in which the x-rays are produced. It is a piece of metal, shaped in the form of a beveled disk, connected to the positive side of electrical circuit. The anode converts the energy of the electrons into X-rays and dissipates the heat, considered the byproduct.

Envelope

An airtight enclosure that houses the cathode and anode. It is often made from metal and ceramic because these materials are able to withstand the tremendous amount of heat generated during X-ray production, but they can also be made of glass.

Housing

Provides protection and absorbs excess radiation.

Window

The X-ray tube window typically is made from beryllium because it allows X-rays to pass through but has sufficient strength to hold the vacuum required for the X-ray tube to operate. When an electrical current is passed through the cathode, the electrons generated by the cathode are accelerated by high voltage towards a metal target, or anode. X-rays known as Bremsstrahlung (“braking radiation”) are produced when the electrons are suddenly decelerated upon collision with the anode. When an atom in the sample is struck with an X-ray of sufficient energy (greater than the atom’s K or L shell binding energy), an electron from one of the atom’s inner orbital shells is dislodged. The atom regains stability, filling the vacancy left in the inner orbital shell with an electron from one of the atom’s higher energy orbital shells. The electron drops to the lower energy state by releasing a fluorescent X-ray. The energy of this X-ray is equal to the specific difference in energy between two quantum states of the electron. These X-rays all have sufficient energy to pass through the X-ray tube window and reach the sample. The measurement of this energy is the basis of XRF analysis.

Requirements for Handheld X-ray Tube Based XRF Analyzers In the United States devices with X-ray tubes as the signal source are regulated by state, often the same state agency that controls radioactive material when the state is an agreement state. In almost all states, a registration, but not a license, is required. Usually, the registration form provides the state with information about the device, its purpose, and your company. Many states charge an annual registration fee. In a few states, there are more strict requirements such as an on-site device inspection and certification by a local radiation safety consultant as a prerequisite to obtaining the registration and authorization to use the device.

Contact your state regulator for further information, particularly prior to transporting or operating an XRF analyzer in another state.

To learn more about XRF technology, handheld and portable XRF analyzers, and their many applications in the mining industry, download the free eBook, XRF in the Field: Technology for the Non-Scientist.