Answer to Question #9016 Submitted to "Ask the Experts"
Category: Radiation Basics — Photons
The following question was answered by an expert in the appropriate field:
I am inquiring about a term that I have seen used many times, "soft x rays." In particular, I would like to know if it would be appropriate to use this term to describe bremsstrahlung-type x rays that are generated from a high-voltage radiofrequency amplifier electron tube with an anode potential of 21,750 volts DC. These x rays have been measured by OSL (optically stimulated luminescence) dosimeter badges at levels of 65.9 nSv s-1 deep dose equivalent and 62.6 nSv s-1 shallow dose equivalent at a distance of 1.092 meters from the tube. Another example is a distance of 71.1 cm from the electron tube anode area and passing through a glass window 6.35 mm thick; the OSL badges gave readings as high as 48.4 nSv s-1 deep dose equivalent and shallow dose equivalent doses as high as 92.5 nSv s-1. This term seems to carry with it the implication that the x rays are soft and would not have any harmful effects on any individual being exposed to them. As many people may be influenced by the incorrect usage of this term, I would greatly appreciate the information as requested.
With an anode potential of 21.75 kV, the average x-ray energy generated in the tube is probably less than 10 keV, but because of the filtration effects associated with the tube envelope and other possible constituents, it is very likely that the effective energy of x rays escaping from the tube is greater than 10 keV and may well exceed 15 keV, with the upper limit being defined by the anode voltage. The added 6.35 mm of glass-window filtration would serve to increase the effective energy.
The energy definition of "soft" x rays is somewhat arbitrary, but such x rays are commonly identified as being about 10 keV or less in energy. The fact that energies may be in the 10 keV region (i.e., "soft") does not mean, however, that they are harmless. At sufficiently high intensities they certainly represent a potential hazard to live skin tissue, the critical skin depth being generally less than 0.01 cm (where the basal layer of cells is affected). The annual dose limit generally applied to the live skin is 0.5 Sv. Additionally, the lens of the eye, for which the annual dose limit is generally taken as 0.15 Sv, lies at an approximate depth of 0.3 cm below the outer surface of the cornea. At 10 keV, 20% to 25% of x rays incident on the cornea would be available to produce dose to the lens after passing through the overlying tissue. At 15 keV, the available percentage would increase to about 65%. At 20 keV, the approximate maximum x-ray energy, about 80% of the incident x rays, would penetrate to the lens. Naturally, some dose would also be delivered to deeper tissues, but attenuation increases exponentially with depth.
Your reported OSL results of 48.4 nSv s-1 deep dose (interpreted at 1 cm depth) and shallow dose of 92.5 nSv s-1 (interpreted at depth of 0.007 cm) can be used to estimate an effective energy of the x rays for that exposure. Considering only the primary photons and soft tissue as the dose medium, this effective energy turns out to be 22 keV, which is essentially the maximum energy dictated by the anode voltage; the added 6.35 mm window may be partly responsible for hardening the x rays. While this energy is not very high, it is sufficient that many professionals would place these x rays in the "hard" x-ray energy category.
The data you quote for OSL measurements made at 1.092 m from the tube are not consistent with the described operating conditions. The quoted deep and shallow doses are almost the same, differing by only 5%, implying an x-ray energy much greater than theoretically possible—i.e., the effective energy interpreted for the measurements would be close to 2 MeV, about 100 times greater than the theoretical maximum energy. I suspect that either the reported values were in error or experimental conditions were not appropriate for the measurements; for example, the OSL badge could have been oriented improperly in the radiation field such that the radiation causing the response did not pass through the proper badge filtration. Improper reported readings could result from a misreading, but OSL dosimeters can be reread and this could have been done at the time if the user had requested such. Another possibility is that the OSL badges used for these measurements were unknowingly exposed to elevated radiation levels (compared to the control badge used for background subtraction) prior to or following their use for the test irradiations.
If we assume that the OSL measurements made through the glass window were correct, we may also estimate the unattenuated dose rate that would apply at the same 71.1 cm location if the x rays had not passed through the window. The transmission of 20 keV x rays through 6.35 mm glass at 20 keV is about 5%, implying that the measured dose rates would have increased by about a factor of 20 had the glass window not been present. Applying this factor leads to an unattenuated deep dose rate of about 972 nSv s-1 and a shallow dose rate of 1,850 nSv s-1 at the 71.1 cm distance. You do not cite any measurements of dose rates as high as these, but I do not know whether that is because such measurements have not been made or possibly because the measurements that have been made were in error. Such dose rates exceed the levels that define a high radiation area and are a concern if they are real and if personnel have access to such areas while the generator is operating.
The emission of bremsstrahlung x rays from high-voltage vacuum tubes can vary dramatically with direction relative to the tube orientation so that measured dose rates may show wide variations as well. Surveys are important to define expected dose rates at various locations, especially in areas that are accessible to personnel. If sufficient measurements have not yet been made to define the dose profiles in areas of possible concern, I would encourage you to attempt to have such measurements made so that proper radiation protection measures may be implemented as necessary. Good luck.
George Chabot, PhD, CHP