Answer to Question #143 Submitted to "Ask the Experts"

Q

Where might I find research results on the radiation output of electron beam welders and the potential health risks to those exposed to radiation from working with electron beam welders for many years?

A

Electron Beam welders are industrial devices used to make very clean, precise welds in various metals. The devices use an intense beam of electrons to heat and melt the base metals to be welded and any filler metal. The heat comes from the absorption of the electrons in the metal. Since electrons can be stopped by all matter, including air, the welding process is almost always conducted in a vacuum chamber. Otherwise, most of the electrons would never reach the parts to be welded. A technology related to electron beam welding is the heating and purification of metals using electron beams to heat, melt and vaporize the metal. Extremely high-purity rare metals can be produced by this method which must also be done under a high vacuum. Anytime a beam of electrons is accelerated in a vacuum and made to strike a material, it is possible to generate x rays. The higher the atomic number of the material struck by the electron beam, the greater the probability of producing x rays. Thus, an electron beam striking metal has a reasonably high probability of producing x rays.

A common material welded using the electron beam process is an alloy of copper, tin, nickel, and other metals. The maximum energy of the x rays produced will be determined by the maximum voltage used to accelerate the electrons, which in turn is dependent on the size of the weld, the metals involved, and the depth of penetration. The energy of the x rays is important because it will determine the ability of the x rays to penetrate the vacuum chamber walls.

In many industrial e-beam welding devices, the vacuum chamber walls are steel and have a thickness of approximately 2 centimeters (3/4 to 1 inch). There are minimal published data describing the intensity of the x-ray fields produced inside e-beam welding or melting devices. Using an approximation suggested by B. Lindell (Lindell 1968), a copper metal target (50 centimeters from the electron gun) being struck by electrons having a maximum energy of 150 keV and an average energy of 75 keV, a beam current of 50 mA (150 kVp and 50 mA are typical high machine settings), would produce a radiation field of around 59 R per second. Assuming the beam impinges on a 1 centimeter spot (very large for a welder), the scatter at 1 meter (at the chamber walls) is calculated to be 0.85 R per hour. For maximum welding efficiency, the electron beam should be highly focused. However, this tends to limit the actual number of electrons that are capable of acceleration and also severely reduces the scatter potential. Thus, a highly focused beam will not produce the maximum number of x rays. The attenuation of the x rays by the walls of the vacuum chamber will further reduce the x-ray field outside the welder enclosure. Assuming 1.5- and 2-centimeter-thick steel chamber walls, the calculated measurable exposure rate outside the device would be from 0.1 to 1 mR per hour. Actual measurements made around e-beam welders and melters do not usually exceed 0.05 to 0.1 mR per hour at the surface of the chamber, and are generally not detectable using instruments designed to measure very low energy x rays.

Additionally, viewing ports to look inside the chamber during welding are made of thick leaded glass that has the same attenuation capability as the steel walls. The calculated external exposure rates would mean that a worker who had his or her body pressed against the exterior wall of an e-beam welder for two hours each day would receive a total annual dose of around 200 microsieverts (20 millirem) in a year. We have not seen any detrimental health effects from doses in this range even when received for a full working lifetime. It is important to make sure all door and vacuum interlocks that prevent energization and acceleration of the electron beam are functioning properly, that all fittings and connections meet the manufacturer's specifications, and that the leaded glass in the viewing ports not be replaced with non-leaded glass. These precautions will prevent excessive levels of x-ray leakage. Additionally, some e-beam welders use a radiofrequency (RF) source to produce the electron beam. This carries two potential hazards: the potential of microwave radiation and the potential of causing x rays in the microwave-generating tube. The US Food and Drug Administration limits the emission of electronically produced x-ray radiation outside a piece of equipment intended for general industry use to 0.5 mR per hour. I have not found any e-beam equipment that uses RF electron production systems having any detectable radiation external to the RF tube.

In the case of the second concern, also associated with the RF generator, the manufacturers are required by the Federal Communications Commission to not interfere with conventional communications equipment, including radio and TV transmission, computers, portable radios, and portable telephones. RF fields measured around e-beam equipment have been well below the limits set by either government or consensus standard-setting organizations. Because x rays are electromagnetic radiation, like visible light, once the power is turned off to the electron gun, no more electrons are created, no more electrons are accelerated toward the metal being welded, and no more x rays are being created or present in the vacuum chamber. So when the vacuum chamber is opened up, there are no x rays trying to get out. However, depending on the metals being welded, there could be vaporization of the metal and deposition of the metal dust on the inside walls of the vacuum chamber.

You should ask your company industrial hygienist or safety engineer if the dust from the metals being welded present a potential hazard, since it is generally required that personnel at least partially enter the vacuum chamber to position and retrieve the materials being welded. The other potential concerns would be due to the residual heat present in the materials just welded and other general safety issues such as heavy moving doors, hydraulic systems and hoses, hoists, and other lifting equipment, and the high voltages associated with the electron gun.

Tony LaMastra, CHP

Reference:
Lindell B. Occupational hazards in using x-ray analytical work. Health Phys., 15:481-486; 1968.