Answer to Question #2320 Submitted to "Ask the Experts"

Q

What is the best type of survey meter to check for leakage from an x-ray diffraction unit?

A

In asking this question, you acknowledge the fact that you are aware x-ray diffraction units can present serious radiation safety concerns for operating personnel. This is especially true for custom-built or older "open beams" systems, which may lack modern enclosures and beam/component interlocks. Several accidents have been well documented through the 1960s and 1970s and one where a university researcher here in Pennsylvania was not aware of an extremity overexposure until they noticed acute radiation burns on his fingers. This prompted an investigation by our state Radiation Control Program staff and the U.S. Food & Drug Administration (FDA), who have regulatory authority over the use and manufacture of such analytical x-ray equipment respectively. FDA has produced publications on radiation safety with x-ray diffraction units in the late 1960s and 1970s, and they developed a training video The Double Edged Sword on the subject in the late 1970s. This training video may not be readily available however. Nonetheless, I would note a more recent reference I helped work on and one that is available online to HPS members. Specifically, the American National Standard ANSI/HPS N43.2-2001, Radiation Safety for X-ray Diffraction and Fluorescent Analysis Equipment, should be on hand for all x-ray diffraction equipment users. Equally important, your specific state radiation control regulations should be consulted.

Regarding the best survey meter to check for leakage, as one might expect, it depends on what you're trying to achieve. The first consideration should be to detect any "stray radiation" from the diffraction unit. This stray radiation can be scatter or leakage of the primary beam through the tube housing, scatter from x-ray beams impinging on components, streaming from poorly installed shutters or other devices, scatter from a sample in the beam, fluorescent x-rays, etc. Thus, energies can be up to the maximum kilovoltage applied (typically 100 keV), and beam areas may be quite small. If you just want to detect the stray x rays, I'd say a thin NaI crystal probe with a thin window would have the highest intrinsic efficiency. This is due to the fact that the detector is a solid medium and of a material with a high interaction cross section for low-energy x rays. Also with the expected small beam areas, you may want to mask the detector face with a sheet of lead and focus in on any leakage locations. If you don't have a thin NaI probe, a mica "end window" Geiger-Mueller (G-M) probe would work too. You may not see a very low-level x-ray leakage, but if you've got a problem, you'll no doubt detect it with an end window G-M probe. An end window G-M is preferred to a "pancake" type G-M tube, because x-ray photon interactions with counting gas increase with the longer path length in the tube. Again, one could mask the window to focus in on a leakage point. Both the thin NaI crystal and end window G-M tube should see x rays from 10 to 100 keV.

Once stray radiation is found you should try to quantify it in terms to dose to air in microgray per hour (uGy/hr) near the surface for any extremity exposure potential. Similarly, measurements from 30 to 100 cm may be useful for whole-body exposure scenario assessments. This is where an understanding of beam area and energy and detector response is a must. Ideally with a wide-area leakage beam of sufficient x-ray flux, one could use an ionization chamber with a mylar or other thin window. Most ion chambers have a flat-energy response. However, more likely you'll have a low dose rate and/or small cross sectional area, so you will need to use the thin NaI probe or end window G-M tube. Most manufactures have energy response curves for their probes. Nonetheless, Figure 10-20 in Radiation Detection and Measurement by Glenn F. Knoll (publisher, John Wiley & Sons) provides efficiency curves for various thickness NaI crystals, relative to a standard ¹³⁷Cs/^137mBa 662 keV calibration point. For a 2 mm crystal, it may over respond by a factor of between 5 to 20 below 100 keV. Similarly, for the end window G-M tube (which I've personally evaluated), it may over respond by a factor of 2 to 5.5 when calibrated to 662 keV gamma rays (see the proceedings of the HPS 1987 Midyear meeting, page 85, Figure 8). An additional consideration with both the NaI and G-M probe is, at higher dose rates (that is, > 1×10³ µGy/hr), their response will decrease due to dead time losses. Exposure of thermoluminescent dosimeters (TLDs) at critical measurement points is recommended for dose verification. Lastly, and as important, you'll need to do a probe area correction if the beam area is less than the probe area, which was uniformly irradiated during calibration. This is discussed in the above noted ANSI Standard, but the simple ratio of areas provides the needed scaling factor.

David J. Allard, CHP