Answer to Question #14600 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

Our group uses an x-ray machine for testing dosimetry and radiation detection equipment. The x-ray machine has a maximum output of 320 kilovolts (kV) and 12.5 milliamps (mA). We have heard of reference to "dark current" associated with x-ray machines, when a machine is off and not in use. What is dark current? And is it a legitimate radiation safety concern with our type of x-ray machine?

A

Dark current (also known as thermionic emission) is not a radiation safety concern with your x-ray machine. It is a phenomenon associated with photomultiplier tube-type radiation detectors. Essentially, it is background noise caused by the random emission of electrons by the cathode under the influence of an applied electric field in the absence of x rays impacting the detector.

A typical photomultiplier tube consists of scintillator that produces light when bombarded with x rays. This light impacts a photocathode that absorbs it and liberates electrons. These electrons are accelerated through a series of plates called dynodes across which is applied an increasing high voltage. This process results in multiplication of the original electrons by several orders of magnitude. This greatly increases the sensitivity of the detector. However, because the dynodes are maintained at a high voltage, some electrons may be released (emitted) off the dynodes even when there is no radiation impacting the scintillator. This results in a background signal that may slightly reduce the sensitivity of the detector.

In summary, dark current is a phenomenon associated with certain radiation detectors, not with x-ray machines, whether they are in use or switched off.

For further information, see Tsoulfanidis and Landsberger (2011).

Thomas L. Morgan, PhD, CHP

Reference
Tsoulfanidis N, Landsberger S. Measurement and detection of radiation, 3rd edition. Boca Raton, FL: CRC Press; 2011.

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 20 June 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.