Answer to Question #9960 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

In my new work in a dosimetry section, I need to know if there is any way to recognize different kinds of TLD's (thermoluminescent dosimeters) like LiF:Mg,Tl 100, 200, 600, 700, and 900.

Unfortunately, I know of no reliable ways to distinguish among the various TL (thermoluminescent) materials through simple visual inspection. An obvious exception to this would be if it is known that a particular dosimeter element was used in a characteristic geometry or configuration that would identify it. There are less direct ways that may be used for identification if you have irradiation and readout capabilities available.

The TLD-100, 600, and 700 materials are all LiF(Mg,Ti); assuming all are in the same geometry, all yield about the same TL output per unit dose of gamma radiation so that irradiating them to the same doses of gamma radiation will not yield outputs that distinguish them from each other. They do, however, all contain differing amounts of the 6Li isotope. The TLD-100 contains natural lithium with about 7.5 percent of the lithium ions being 6Li and the remainder 7Li; the TLD-700 is enriched in 7Li and contains a very small amount of 6Li, and the TLD-600 is enriched in 6Li to the extent of about 96 percent. The isotope 6Li exhibits a large thermal neutron absorption cross section. If a group of dosimeters (all of the same mass and geometry, containing TLD-100, 600, and 700 is irradiated uniformly with thermal neutrons there will be clear and easily interpreted differences among the responses of the three dosimeter types, with the 600 yielding a light output from the thermal neutron dose that is more than ten times that of the 100. The TLD 700 would yield virtually no response to the neutrons. One slight complication may arise from gamma radiation that might be emitted by the neutron source. If this is very significant, one might have to use some known TLD-700 elements to evaluate the gamma contribution and subtract this from the gross readings.

The TLD-200 that you refer to is not a designation for LiF(Mg,TI). It generally refers to a different phosphor, CaF2(Dy). This phosphor is roughly 30 times more sensitive than LiF(Mg,Ti) so it is easy to distinguish by the difference in gamma response that would be observed. There is a LiF(Mg,Cu,P) phosphor that has a 200 designation but usually it is referred to as TLD-200A or GR-200. It is also much more sensitive than LiF(Mg,Ti). The TLD 900 that you mention normally refers to another high sensitivity phosphor, CaSO4(Dy), which has a usual sensitivity about 20 times that of LiF(Mg,TI). Thus, gamma irradiation and subsequent readout might be a possible means to differentiate it from the standard LiF(Mg,Ti).

Lastly, if you have the capability to do glow curve analysis, you could use the shapes and structures of the glow curves following gamma irradiation to distinguish among the LiF, CaF2(Dy) and CaSO4(Dy), since all exhibit different light output vs. temperature characteristics. There are other chemical/physical measurement processes that could be used to identify and/or quantify the constituents of a TLD, but such methods are beyond our discussion here and probably beyond what you would wish to implement. I hope you enjoy your new position in the dosimetry section.

George Chabot, PhD

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