Answer to Question #9333 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

My department is currently using a TLD (thermoluminescent dosimeter) badge, and for the past eight years I have never had any exposure detection recorded. My only explanation for this is the combination of our very large x-ray suites (distance factor) and the lack of any fluoroscopy devices (only general x ray/CT). Should I be satisfied with my above reasoning, or do I need to switch our dosimeter type/manufacturer for a more accurate/sensitive reading?

A

You do not specify the particular TLD design or vendor that you are using, and this can have some influence on the energy and dose sensitivity of the system, but most standard TLDs used for personnel monitoring in the diagnostic radiology area are effective for photon energies ranging from about 20 keV to more than 1,000 keV. This should be adequate for most all diagnostic procedures, although some of the lower-energy photons that accompany some techniques may be missed. The usual vendor service has a minimum reportable dose of about 0.1 mSv above background.


If your dosimetry badges are being processed monthly and if you are doing only routine radiographic and CT procedures and are abiding by all the recommended safety procedures to minimize your dose, it is entirely possible that you would have no recorded dose above the minimum (usually reported as "M" by the dosimetry vendor). This does not mean that your monthly dose from x-ray procedures that you have carried out is zero, but only that it is less than 0.1 mSv. If you were using a more sensitive monitoring technique, such as the optically stimulated luminescence (OSL) technique that currently is quite common, you might well see some ascribable monthly doses between 0.01 and 0.1 mSv. The typical OSL system can measure net doses to about 0.01 mSv, and the energy sensitivity also often extends well below 20 keV.

In theory, then, using a standard TLD system, it is possible, but unlikely, that you could be receiving an annual dose from your x-ray work as high as 1.2 mSv and have no positive reading reported from your dosimetry vendor. Such a dose represents about 40 percent of the typical annual background radiation dose received by an individual in the United States. It also represents about one quarter of the ALARA goal of 5 mSv generally recommended for many occupationally exposed medical workers and about 2½ percent of the annual legal limit of 50 mSv for occupationally exposed individuals.

In my opinion, the amount of possibly "missed" dose that could exist when using typical TLD systems for personnel monitoring is not a significant safety concern. However, if you are personally concerned and have the influence to change the type of system you are using, you might investigate the use of a reliable OSL system. You can learn more about such a system by typing "OSL dosimetry" into Google or some other search engine. Good luck.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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