Answer to Question #8340 Submitted to "Ask the Experts"

Category: Radiation Basics

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

Q

I am using an old (1990) x-ray diffractometer at 10 mA, 30 kV operating condition. I am not exposed to the direct beam; however, I have to work unprotected near the beam while aligning samples (one to two minutes, three to four times a day). What is the level of risk I face? The apparatus is at chest level so any scattered beam will pass through the chest and abdomen.

A

The extent of radiation exposure from the x-ray diffraction unit you describe cannot be adequately estimated without more specific information about the instrument, although we can review some of the expected doses consistent with recommendations (second to last paragraph). If you are abiding by recommended procedures and the unit has been properly designed and/or modified, you should not have any dose problems. You should have received proper training, and appropriate dosimetric surveys of the diffraction unit and the room should have been conducted. In particular, the unit should have been surveyed at installation and should be routinely surveyed with appropriate instrumentation at least once a year. Also, anytime that changes in configuration, shielding, procedures, etc., are made that can affect exposure in the area, additional surveys should be made. Such surveys should consider effects of scatter and leakage; naturally, there should be no direct access to the primary beam or to the diffracted beam (the direct beam of diffraction units may deliver dose rates of tens of sieverts per second, and the diffracted beam can deliver tens of sieverts per hour. Because of the low energies involved, the x rays are strongly attenuated in superficial tissues and it is primarily these tissues that might be damaged by excessive exposure.

You do not say what kind of facility you work at, but if the company employs a radiation safety officer, and surveys have not been performed, you should contact him/her and request that surveys be taken under circumstances of use such as the alignment procedures that you use. If there is any possibility of significant finger/hand exposure you should wear a finger or wrist dosimeter to record any significant exposure. In such a case, make sure that the dosimeter is sensitive to the x-ray energies of interest. For your case, operating at 30 kV, energies of interest will range from about 5 keV to 30 keV, with the effective energy probably being less than 15 keV. If there is any significant scatter or leakage that might expose your body, you should also wear a whole-body dosimeter; again, confirm that the dosimeter is suitable for these lower energies.

A good reference to refer to with regard to measurement and protection considerations applicable to x-ray diffraction units is the 2001 HPS/ANSI Standard 43.2, Radiation Safety for X-ray Diffraction and Fluorescence Analysis Equipment. If you or someone at your facility is a member of the Health Physics Society, this standard is available through the members home page (go to the HPS site and click on Members Only in the left column. If your unit is operating in accordance with the standard recommendations, the dose rate to the operator's body should not exceed 0.0025 mSv per hour from scattered and leakage radiation and other possible stray radiation. Additionally, when an operator is carrying out any regular procedures with the machine, he/she should not receive more than 0.25 mSv  to the extremities (primarily fingers or hands for diffraction procedures) and not more than 0.025 mSv  to the whole body in one hour. These latter numbers apply to alignment procedures that might be carried out. For the exposure times (approximately 35 hours per year if you work 50 weeks per year, 40 hours per week) that you quote, these dose rates would translate to potential doses of 8.3 mSv extremity dose and 0.83 mSv whole-body dose. These values represent about 1.7 percent of the respective extremity and whole-body dose limits in the United States, clearly not a dose concern. If the machine is an open-beam unit, dose rates external to the tube housing when all shutters are closed should not exceed 0.025 mSv prt hour at 5 cm from the housing surface, and dose rates near the controls used for adjusting machine parameters/conditions should not exceed 0.25 mSv per hour to extremities and 0.025 mSv perhour to the whole body. For an enclosed beam unit, the dose rate within 5 cm for any surface should not exceed 0.0025 mSv per hour during normal operation. Many more recommendations are contained in the standard, and I strongly recommend that you review it if you are not familiar with it.

As a final note, if you are not able to obtain survey information relative to your diffraction unit, perhaps because no one at the company has the capability, and you are working in the United States, you might arrange through your employer to speak with someone in the radiation control program in your state. He/she may be able to assist you in getting appropriate measurements made or recommending someone who can perform them. You can find links to the various program directors though the Conference of Radiation Control Program Directors. Good luck.

George Chabot, PHD, CHP

 

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Answer posted on 9 June 2009. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.