Answer to Question #14461 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment and Shielding — Equipment

The following question was answered by an expert in the appropriate field:

Q

I have read that an x-ray machine does not contain any radioactive elements. However, I came across an article that said thorium is added to the tungsten target which is radioactive.

My questions are:

  1. Will tungsten (with thorium added or not) inside the x-ray tube leak outside the tube if the x-ray machine wears out and will it cause any harmful effect to people who accidentally touch or ingest it in that case?
  2. Will x ray make the tungsten target (with thorium added or not) radioactive after years of use?
  3. If thorium is added to tungsten, can x ray generate induced radioactivity?

I see there is conflicting information online so I would like to ask for clarifications here.

A

To summarize, the tungsten and thorium in x-ray tubes are not a hazard to people working with the tubes or patients being imaged by the equipment it contains.

In an x-ray tube, x rays are produced when a tungsten filament wire is heated which releases electrons. These negatively charged electrons are accelerated by an electric field and impact on a target, usually made of tungsten. X rays are produced when these electrons pass near the positively charged nuclei of the tungsten atoms. In addition to x rays, this process also produces substantial heat in the target. 

Tungsten is a very dense metal and has a high melting point (3422oC). This makes it ideal for producing the electrons and for production of the x rays. Electrons are produced when the filament wire is heated above 2200oC. Adding small amounts of thorium to the tungsten in the filament wire reduces this temperature substantially, to about 1700oC. This increases the efficiency of electron production and increases the life of the filament wire.

The filament wire, target, and associated components are contained in a thick glass enclosure which in turn is contained within a steel housing. This double layer of protection prevents release or leakage of the thorium or tungsten. Unless this housing and the glass containment vessel are breached, no leakage will occur, and no one will be exposed.

At the end of the service life, because x-ray tubes and other parts of the imaging equipment contain electric and electronic components that contain heavy metals and other hazardous materials, federal and state environmental protection regulations require removal and disposal by a qualified vendor.

Tungsten, as used in x-ray tubes, is a solid metal that is not radioactive. It is not considered a toxic metal by the US Environmental Protection Agency. Thorium is radioactive, but only very small amounts are used in x-ray tubes. 

X rays, at the energies typically used in diagnostic medical imaging equipment, cannot induce radioactivity in either the filament wire which contains the thorium or in the tungsten target.

The tungsten wire filament is very slightly radioactive because of the presence of small amounts of thorium. The amount of radioactivity will not increase because of the x rays.

For your information, up until about 1990, the Coleman Company, Inc., manufactured and sold white gas-powered lanterns. These lanterns were very popular camping accessories. The light-producing element (the "mantle") was made of a meshed nylon fabric that was dipped in thorium nitrate. The heat from the burning gas caused the thorium to emit an incandescent glow.

Please be assured that x-ray tubes are not hazardous to handle or operate because of the tungsten or thorium content.

Thomas L. Morgan, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 29 March 2022. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.