Answer to Question #12025 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

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

Q

I am a university radiation safety officer. An art expert wants to examine a university painting in our library with a large-animal x-ray machine. My job is to ensure that no member of the public receives more than 0.02 millisievert per hour (mSv h-1). I am concerned about stray and scattered x rays. All I have is a Geiger-Mueller (GM) detector and a gamma-ray energy-response curve.

I will look into obtaining the assistance of our hospital's medical physicist, but perhaps you can help me in the meantime.

  • How do I calculate the dose (in mSv h-1) from 50 kilovolt potential (kVp), 4 milliamp (mA) x rays for one minute per picture? I don't know who is going to be operating the x-ray machine or what model they are using.
  • Will my GM detector (a Ludlum Model 3 with a 44-9 probe) be of any use in measuring x rays?
  • We have a limited scope state health department license for radionuclides only. Should I get the department of health involved? Are we putting our license at risk?
  • There is likely to be an audience for this procedure, as we are building our art analysis body of work here at the university. Is it acceptable to use shielding, distance, kVp, mA, time duration, number of pictures, and filter calculations to keep everyone safe?
  • Any advice on creating a standard operation procedure (SOP) for this event?
A

This is an interesting question, and there are several considerations.

First, 50 kVp sounds like a very high voltage for radiographing a painting. Usually much lower voltages are used—on the order of 25–30 kVp or less. Sometimes radiographers use voltages as low as 15–20 kVp or Grenz rays from superficial radiotherapy (i.e., very low-energy x rays that do not penetrate more than a few millimeters of skin). Ionization chambers for use at these low energies usually have a metalized polyester front window. You may want to explore why 50 kVp is being used; it may be that the x-ray unit cannot produce a lower kVp.

This project should have no impact on your radionuclide license as the project is using x rays.

It would be preferable to have a diagnostic medical physicist assist you as opposed to a radiotherapy physicist. The therapy physicist may not have instrumentation that can accurately measure such low-energy x rays.

For 25–30 kVp (mammography range) x rays, a chamber with an extremely thin front window is required. In general, thin-window GM detectors of the type you cite are not well suited to making dose-rate measurements at the x-ray energies of concern here. If the instrument is not equipped with a response-flattening filter, it will most likely yield a considerable overresponse, and unless you have x-ray spectral information, it may be difficult to determine the appropriate energy-response correction factor. Some manufacturers offer snap-on filters that will flatten the dose response, although these can sometimes suppress the lower-energy photon response.

Most of the scattered radiation will come from the wall behind the artwork. The artwork itself will absorb only a small amount of radiation and only a small amount will be scattered. Depending on where you will be making measurements, intensities may be quite low, and having an instrument with integration capability may be desirable to obtain reasonably accurate measurements.

Because of likely uncertainties associated with theoretical determinations of both the intensity and the x-ray energy spectral quality at possible locations of scattered or direct radiation, measurement results with an appropriate detector would most likely be more reliable than calculations. X-ray intensities vary theoretically as the square of the applied voltage and directly with tube current, so if voltage and/or current change, it is possible to estimate expected dose rates or air kerma rates at fixed locations, based on measurements made earlier under known conditions at the same respective locations. This assumes that machine orientation and other influencing factors, such as shielding, have not changed. Such estimates are also subject to uncertainty because of the effect of inherent and added filtration in the machine, often increasing the effective x-ray energy and producing a greater-than-voltage-squared dependence of intensity on voltage. Some additional measurements are desirable to identify any such influences.

I would not recommend getting the state department of health involved. However, you may want to check to see what their requirements are for setting up and operating an x-ray unit in a public space. Some states require a trained operator and some require special permission for temporary use of an x-ray unit.

Joel Gray, PhD, FAAPM

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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