Answer to Question #9806 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
My plant has a fixed nuclear level gauge with 3.7 x 109 Bq of 137Cs. How I can conduct the survey for the following by survey meter or get an accurate reading:
- Dose rate at source surface
- Dose rate at detector surface
- Dose rate at 1 meter far from source
The dose rate at 1 meter from the source is clear but the dose rate at source surface and detector surface are not clear for me. How I can get these readings?
There are a number of factors that will influence whether you can make the appropriate measurements and how accurate such measurements will be.
To begin with, you should have available a survey meter that has been properly calibrated and is known to provide acceptably accurate measurements when irradiated uniformly with 137Cs (137mBa) gamma radiation. How accessible the source holder is and how much access you have to the system detector will affect whether or how well you are able to make the measurements. There are many different types of radioactive level gauges applied to a wide variety of level or thickness measurements, and some are more difficult to access than others.
Assuming that you can expose the source and place your survey instrument at a meter from the source (distance measured from the center of the source to the center of the detector), you should be able to make an adequate measurement at the 1-meter distance, as you have inferred.
Making a measurement at a location corresponding to the surface of the system detector may require moving the detector out of its normal location and placing the survey meter so that the center of the detector is located at the same distance from the source that the system detector surface on which the gamma rays are first incident is normally located. In some systems this may not be possible and you may have to settle for locating the survey meter surface at the same distance as the system detector surface normally lies.
The source is normally contained in a source holder, and the best you can do is to measure the dose rate at the surface of the holder where the beam emerges or at the closest accessible surface. Again, such a measurement may not be possible, depending on how the system is configured and how much access you have. In any event, if you attempt such a measurement, keep in mind that most of the level measuring systems emit a partially collimated beam of radiation, some beams being much narrower than others, depending on the specific application. This can be a problem for measurements with a survey meter because often the survey meter may have volume dimensions larger than the width of the beam in at least one dimension. If this is the case, the survey-meter detector volume will not be irradiated uniformly, and the readings will be lower than the actual dose rates because the survey meter is calibrated in a uniform field. Field nonuniformity is also often a problem, even in a broad field, as one attempts to make measurements close to the source because the gamma field intensity changes noticeably with small changes in distance from the source, and a survey meter volume may be irradiated nonuniformly along its depth. Problems with nonuniform irradiation of the survey-meter volume can be reduced by using a small-volume detector, preferably one whose active volume dimensions are smaller than any beam width measurement at the survey-meter detector location.
Also keep in mind that the dose rate from a small-dimension source goes up rapidly with decreasing distance from the source. Thus, while the expected soft-tissue dose rate at 1 meter from a small 3.7 x 109 Bq 137Cs source would be approximately 0.086 μGy s-1, the dose rate at a close distance, say 5 cm, would be about 33 μGy s-1. The dose rates are not so high that they preclude reasonable measurements, but you should avoid unnecessary exposure as you make your measurements. For measurements, especially those close to the source, it is desirable to set up your survey instrument at the desired location while the source is in its shielded configuration. You can then expose the source and read the meter, often without having to have any body part in the radiation beam.
If the configuration of your system is such that you are not able to make useful measurements close to the source, you may be able to make a reasonable calculation of the expected dose rate at the source holder surface or at some other specified distance. This is especially true if the active source dimensions are relatively small (a few cm or less), in which case you may be able to make a measurement of the dose rate at one meter and then extrapolate to the dose rate at a closer distance, using the inverse square law, which implies that the dose rate decreases with the inverse square of the distance from the source. Thus, as we showed above, a measured dose rate of 0.086 μGy s-1 at one meter would translate to an expected dose rate of (0.086 μGy s-1)(100 cm/5 cm)2 = 33 μGy s-1 at 5 cm. The calculated dose rates at close distances will tend to overestimate the respective actual dose rates because the source has finite dimensions and the dose rate will not follow a true inverse square relationship when the distances of interest are not large compared to source dimensions.
The details of what measurements you make and how you make them will depend on the specific characteristics of the level measuring gauge system that you are using. I wish you success in your measurements.
George Chabot, PhD, CHP