Answer to Question #10336 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Dosimeters were placed around cabinet x-ray systems and later analyzed at a laboratory. It was said that for two hours the emission rate was 4 μSv h-1. Later it was learned that the system was in operation for 11 hours, not two hours, and therefore the emission rate was calculated to be 0.8 μSv h-1 (based on simple division of the dosimeter reading divided by the number of hours the system is in operation). How can this be when a survey meter placed 5 cm from the surface of the system consistently measures 4 μSv h-1. What is wrong with this reasoning?
If you can trust the readings on the dosimeters and on the instrument to be accurate respresentations of the actual exposure rates at a particular point of interest, then we would have to conclude that someone made a mistake in recording results or operating times. It is probably more likely, however, that the dosimeters or the survey instrument, or both, was (were) not providing accurate estimates of the quantity that you are reporting (equivalent dose rate). While I cannot draw a definitive conclusion based on the information you have provided, there are a number of possible reasons why the dosimeter readings might not agree with the survey instrument readings. I will list several of these with a bit of elaboration.
- The dosimeters may have been at a different distance from the x-ray source(s) compared to the detector distance. If the effective center of the detector was placed at 5 cm and the dosimeters at about 11 cm, such a difference could account for your observations. You do not state what types of dosimeters and what type of survey instrument you used but, frequently, dosimeters, such as thermoluminescent dosimeters (TLDs), are small in dimensions compared to the active volumes of common survey meters, and when the detector is placed at a location, the normal procedure is to place the effective center of the active volume at the distance of interest. When the distance of interest is relatively small (which means when the detector dimensions are not a lot smaller than this distance), the detector volume will often be irradiated nonuniformly, and this can lead to misinterpretations of the actual dose rate at the distance of interest. The small dimension dosimeter is typically not subject to this same problem of nonuniformity of exposure since its dimensions are often small compared to the distance from the source.
- The dosimeters may have been placed at a different angular location from the x-ray source(s) compared to the orientation of the survey instrument. X-ray sources, depending on machine voltages and internal and external shielding often emit x rays at different intensities at different angles measured from a convenient reference direction.
- The dosimeters may have been calibrated to read a different quantity from that measured by the survey instrument. For example, the dosimeter could have been calibrated to read dose at 1 cm tissue depth, and the survey instrument could be reading dose rate at a different depth. When dealing with relatively low-energy x rays such differences can make a substantial difference in the readings, partly because of marked attenuation effects at low energies. For example, if the dosimeters had been calibrated to read personal dose equivalent at a depth of 1 cm, and the survey meter had been calibrated to read equivalent dose rate at the surface, the expected ratios of the personal dose equivalent rate to the exposure rate at photon energies of 10 keV, 20 keV, 30 keV, 50keV, and 100 keV would be about 0.0085,0.537, 0.968, 1.550, and 1.587, respectively.
- The dosimeters may have a different energy response from that of the survey meter. It is important that both the dosimeters and instrument have been shown to provide the same dose or exposure response to the x-ray energies being emitted. Depending on the kind of dosimeters and the kind of detectors being used, photon energy responses can vary significantly, and you must know the energy response characteristics of both to account for these variations.
- The dosimeters and/or survey instrument may not have been appropriate or not been suitably prepared for the intended measurements. For example, if a radiation detector that used materials of construction that had relatively high atomic numbers compared to air or soft tissue had been calibrated at relatively high photon energy, such as 662 keV from a 137Cs source, and then used to measure notably lower energy photons from an x-ray unit the instrument may well provide an over-response to the lower energy photons. This type of behavior has been observed commonly among common Geiger Mueller detectors that have not been equipped with an energy compensating shield. It is also a common effect observed when inorganic scintillation detectors, such as NaI(Tl) are used to attempt dose rate measurements in photon fields.
In summary, there are a variety of possible causes that could account for what you have observed. I hope the above information is helpful to you in trying to better define the specific cause of the difference you have observed in readings between the dosimeters and the survey instrument.
George Chabot, PhD