Answer to Question #4037 Submitted to "Ask the Experts"

Category: Micro/Radio Waves, Radar & Powerlines — Microwaves and Radiofrequency

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

Q

What is the minimum voltage required to produce collateral ionizing radiation from a thyratron, amplitron, or klystron used in long-range radar systems? I believe the target in the vacuum tubes is tungsten. I am interested in x rays that would make it outside the tube and would have a potential for human exposure. In searching the Internet I have run across the term filtered effect. I am not sure if this term is referring to the glass tube's ability to stop soft x rays. Thank you for your help. I have spent many hours researching this and have not found an acceptable answer.

A

Whenever high-energy electrons bombard a target, the production of ionizing x rays, referred to as bremsstrahlug radiation, is one consequence that may require attention from a personnel exposure point of view. While the phenomenon can occur at relatively low-electron energies, such as a few thousand electron volts (eV), it is generally not of practical importance when dealing with devices such as you mention until energies exceed 10,000 eV and approach about 15,000 eV (15 keV) because the x rays produced at lower energies generally do not have sufficient penetrating power to penetrate the tube envelopes in sufficient numbers to produce significant doses. Electron energies of 15 keV imply that the accelerating voltage would exceed about 15 kV. It is important to note that new tube designs, especially if they incorporate changes in envelope and/or housing characteristics, should be evaluated to determine x-ray emission characteristics even if voltages do not exceed 15 kV. It has also been observed that as some high-voltage tubes age, the x-ray fields outside the tubes increase somewhat. This may be associated with pitting effects of the electrons on the anode or beam collector that can result in localized increased voltages, leading to higher electron energies and more penetrating bremsstrahlung (noted in the National Council on Radiation Protection and Measurements report NCRP Report No. 95, "Radiation of the U.S. Population from Consumer Products and Miscellaneous Sources," NCRP, Bethesda, 1987). The following discussion provides some additional information that may be useful to you.

When high-speed electrons strike a target (in this discussion we shall assume that the material is thick compared to the penetrating range of the electrons, as would be expected in the cases of interest) they generate x rays, called bremsstrahlung radiation, as a consequence of some electrons being accelerated in the Coulomb attractive fields of the target nuclei. The energy distribution of bremsstrahlung, produced by electrons of a single energy, is continuous and ranges from zero to the energy of the incident electrons; the average photon energy is about one-third of the electron energy. The total bremsstrahlung yield varies approximately linearly with the atomic number, Z, of the target material, so a material like tungsten (Z = 74) would produce about 2.5 times as much bremsstrahlung as would a material such as copper (Z = 29), all other factors being equal. The yield also varies linearly with the electron beam current and exhibits an even stronger dependence on the incident electron energy, showing an energy squared dependence. The electron energy, in turn, depends directly on the accelerating voltage; thus, doubling the voltage would increase total x-ray yield by a factor of four.

In the case of a high-voltage switching tube, such as the thyratron that you mention, the voltage is usually cited as the anode voltage, while for a microwave-generating device such as the klystron tube, the voltage may be referred to as the beam voltage. In any event, the voltage is very important since it determines the energies of the bremsstrahlung photons and strongly affects x-ray yield. Because of the shielding effect of the tube envelope and other structures around the tube, the total virgin energy distribution of bremsstrahlung radiation is not observed outside the tube. Lower-energy photons are attenuated and the higher-energy photons penetrate the tube housing more readily; this filtering effect (this is likely the process you refer to in your question) can sometimes result in radiation dose rates outside the tube that show a voltage dependence even greater than the theoretical voltage squared effect. Some large radar systems of the types used, for example, in air traffic control systems may operate at very high-peak power outputs, often exceeding several megawatts, and the combination of high current and high voltage may produce very significant x-ray fields that require appreciable shielding of the microwave generator tubes.

I hope this response is helpful to you.

George Chabot, 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.
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