Answer to Question #1442 Submitted to "Ask the Experts"

1. The exposure rate measurement was made at a technique of 100 mA and 1.0 second.
    
2. The GM survey meter used for the measurement is energy compensated.
    
3. The response time of the GM survey meter used is short enough to allow for a reasonably accurate measurement of exposure in the time interval over which the exposure is made.
    
4. All of the x-ray exposures taken on this machine are at 70 kVp.
    
5. The occupancy factor where the measurement was made is 1, that is, you are present at this location during the entire eight-hour workday, and all x-ray exposures made by this machine are taken during this time period.
    
6. If this is a measurement of scatter radiation, no additional contribution from primary radiation in this area is assumed. If this is a measurement of primary radiation from this x-ray machine, there is no additional contribution from scatter radiation in this area, that is, the x-ray beam points in the same direction all of the time, and the use factor is 1.

For the calculation, set up with the given data and solve for the desired units with conversion factors as needed:

Note: the constants (1) and (1) in the numerator correspond to the occupancy factor and the use factor mentioned above. CONCLUSION: If the assumptions made to complete the calculation are all correct, the exposure is below the general public limit of 100 mrem/yr (1 mSv/yr) and well below the occupational limit of 5000 mrem/yr (50 mSv/yr). These are the limits in most states. It should be noted that for a typical x-ray room, the average kVp may be significantly higher, for example, 90 kVp. Exposure rates are proportional to the kVp squared, so increases in kVp will lead to greater increases in calculated exposures. If the mA used for the measurement was lower than 100 mA, the calculated exposure will also increase, but in a linear manner. Also, if the survey meter is not energy compensated, there is a good chance that the measurement is overestimated due to the properties of the detector. If you are not at this location all day, every day, and all x rays are not taken in during the times you are there, the above calculation will be an overestimation of the actual exposure. Obviously, the information given to calculate the dose is not perfect, but is acceptable for estimating the exposure. The user must realize what assumptions are made and what effect each parameter has on the calculation. RECOMMENDATIONS: In order to determine the actual dose received by an individual, the best method is to wear a personal dosimeter to verify that no dose limits are exceeded. Since the initial calculations indicate that there is likely not a concern, it would be acceptable to begin to track doses on a monthly basis using a dosimeter. If you are measuring the dose to a particular area, you can post an environmental dosimeter for a period of three to six months. Note that this is not necessarily indicative of an individual's dose, but may be acceptable to verify that public dose limits are not exceeded.

Ken "Duke" Lovins, MS, CHP