Answer to Question #12299 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
I can estimate x-ray radiation from a generator operating at a constant potential in kilovolts (kV) and current in milliamps (mA), for a given filter (e.g., 1 millimeter [mm] beryllium + 1 mm aluminum), using a standard graph of exposure per mA per minute at 10 centimeters (cm) against kV. However, what I would like to do is estimate x-ray radiation produced by an electron microscope operating at, say, 45 kV. Can I use the same graph for this purpose, with some corrections?
The construction and operation of an x-ray tube designed to produce x rays and an electron microscope designed to probe materials but which produces adventitious x rays is not quite the same. Is there an approximate formula I can use to estimate the leakage x rays produced by an electron microscope?
The bottom-line answer to your question is that no, there is no available formula for estimating leakage intensity from an electron microscope. I shall, however, provide some additional discussion in the way of explanation.
X-ray production in electron microscopes occurs partly from characteristic x rays produced when the electron beam interacts in the target material and possibly other material, causing ejection of electrons from atoms and the subsequent emission of the characteristic x rays of discrete energies. Usually, however, the x rays of greater concern would be bremsstrahlung radiation (as from an x-ray generator) produced somewhat by direct electron-beam interactions in the sample and possible interactions in machine components and by back-scattered electrons that may interact with machine components. These interactions are electromagnetic in character, typically occurring as electrons accelerate in the electric fields of nuclei, with the resultant emission of bremsstrahlung x rays, which can have energies as high as the energies of the incident electrons.
While it is possible to predict the theoretical bremsstrahlung yield and intensity when the voltage, current, and characteristics of the medium that intercepts the electrons are known, these quantities are generally not fully known, especially for the stray and scattered electrons that may account for most of the x rays of possible concern from electron microscopes. Additionally, the leakage pathways of x rays produced in the unit may not be very obvious, and there is no acceptable "recipe" for predicting leakage intensities from electron microscopes.
As a consequence, it is generally not useful to attempt calculations that will be helpful in predicting x-ray concerns. Fortunately, newer electron microscopes that are properly installed and maintained typically have zero or near zero emission of x rays external to the microscope casing. Possible leakage points may be locations, especially in older instruments, where the metal casing is reduced—for example at locations where sealing gaskets are used, as in some breaks defining separate column sections.
The best and most recommended way to determine x-ray emission intensity and possible associated personnel exposure problems is through radiation survey measurements with appropriate instrumentation. Measurements should be made with the instrument operating at maximum useful voltage and current. Any new electron microscope installation should be surveyed to ensure that no significant x rays are present in the work environment. Additionally, any physical changes to a microscope that could affect the leakage of x radiation from the instrument should be followed up by an appropriate radiation survey. Alterations such as changes in shielding materials and changes in viewing ports would be examples of changes that could affect measurable radiation levels.
The U.S. Food and Drug Administration (FDA), which has developed and promulgated performance standards for many electronic products that emit ionizing radiation (10 CFR 21.1010), has not recommended a specific standard for electron microscopes that would limit dose rates from these devices. In a brief search of the internet, I have noticed that a number of facilities have adopted the same performance standard for electron microscopes that the FDA has recommended for television receivers and x-ray cabinets; namely, 5 microsieverts per hour (μSv h-1) at 5 cm from any accessible surface of the device. Naturally, the usual requirements that apply to general radiation safety would also apply to facilities using electron microscopes.
George Chabot, PhD