Answer to Question #11875 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

Q

I have a Harshaw 6600 Plus thermoluminescent dosimeter (TLD) reader. How many dosimeters are needed to do the calibration to get the reader calibration factor (RCF) and element correction coefficients (ECCs)? For verification of the results, should I do the calibration at a secondary standards dosimetry laboratory (SSDL) or could it be done in a cobalt-60 (60Co) unit?

A

As you know, the RCF is the most critical factor used in conversion of a TLD readout to an appropriate dose. Therefore, you should attempt to determine the RCF with a high level of accuracy and confidence. This usually requires preselection of a group of dosimeters that show acceptably high precision. This is normally done by irradiating a relatively large group of TLDs (at least 20, but better close to 50) and selecting at least 10 dosimeters that all fall within the range of about 0.9 to 1.1 times the mean value. You can improve precision by reducing the width of this interval, depending on what you find for the response characteristics of the TLDs.

Once you have selected at least 10 dosimeters for the RCF determination, it is important that they be irradiated in an acceptable configuration and to a well-characterized gamma radiation field so that the determined RCF will be reliable in allowing conversion of a TLD reading to the dose quantity of interest. Depending on what dose quantity is of concern, acceptable phantoms may be required for the irradiations. The radionuclide most commonly used for these irradiations is cesium-137 (137Cs), which through decay of daughter barium-137m (137mBa), produces 662 kiloelectronvolt (keV) photons.  If your facility has a suitable source (you note that you have access to a 60Co source), and you have the equipment and ability to deliver known doses under requisite conditions, you can perform the irradiations yourself. Many facilities decide to use an SSDL because these facilities have demonstrated the necessary qualifications for performing such irradiations and are generally recognized by regulating groups/agencies as having acceptable means and expertise for carrying out such irradiations. If you perform the irradiations yourself, you should maintain all the appropriate documentation to demonstrate that the irradiations were performed in an acceptable manner and that all equipment and measurements were consistent with acceptable standards.

The ECCs are used to correct the readouts of respective dosimeters to yield the appropriate doses. The ECC for a particular dosimeter (element) represents the ratio of the mean reading for the entire group of dosimeters to be used divided by the reading of the respective dosimeter. This is desirable since the dosimeters selected for general use often show considerably more variability about the mean of the group than do the calibration dosimeters, and establishing response corrections for individual TLD elements provides means for yielding more accurate dose estimates. It is common for the group of dosimeters to be used for regular field use to have deviations as great as 25% to 30% of the mean value. How many dosimeters you require for ECC determinations depends on how many additional dosimeters (beyond the RCF dosimeters) you will be using. You may require dosimeters for other types of testing, including such things as energy dependence and angular dependence in addition to dosimeters you will be using on a more routine basis. The dosimetric requirements for determining ECC values are generally not as restrictive as those required for the reader calibration. It is necessary that you simply be able to provide the same dose to all of the dosimeters in a given group; performing the required irradiations at your own facility may be more easily done than would be the RCF dosimeter irradiations.

George Chabot, CHP, PhD

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