Answer to Question #9915 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I am a board-certified therapeutic medical physicist and I work in a proton radiotherapy facility. I have a TLD (thermoluminescent dosimeter) reader, and I want to use various types of TLD chips to try to estimate neutron doses in a solid material outside the region of the proton dose, but nearby. I plan to use a combination of different TLD types (TLD-100, TLD-600, TLD-700) for this, but I'm a novice when it comes to doing TLD measurements. I see vendors who are selling many other different TLD types such as TLD-1000/6000/7000 or TLD-200/400/500/800. I can't seem to find any reference or site that has a listing of data on different common TLD types, what they're used for, and some details on how to use them. For instance, I'd like to know for different TLD types what the preferred temperature profile is for obtaining light curves with the reader, what the recommendations are for annealing, and perhaps some basic information on energy dependence, especially for neutron-sensitive TLDs, since the neutrons I deal with are higher energy than those one sees in a reactor. I'm not asking you to provide this information. My questions are: Is there a place I can go to get information and specifications on many different TLD types? Is there a good reference you can recommend that talks about how to do TLD dosimetry in mixed radiation fields with neutrons?
Thermoluminescence dosimetry is a broad area that has received much attention in the literature. In the way of summary regarding the use of TLDs for neutron dosimetry, I will say that most TLD systems for such applications are albedo type dosimeters, meaning that they rely on the reflection of neutrons incident on the body back into the dosimeter, which is normally worn on the chest area and maintained in proximity to the body surface.
The most common TLDs for neutron dosimetry use a phosphor enriched in an isotope that exhibits a high absorption cross section for thermal neutrons. The body acts as a moderator of fast neutrons and some of the moderated neutrons find their ways back to the active dosimeter element. Often this active element is a 6Li enriched phosphor such as 6LiF(Mg, Ti), also known as TLD-600; the 6Li undergoes an (n,a) reaction, and the alpha particle and the triton that are produced deposit their kinetic energies readily in the TLD element. Such dosimeters almost always include a neutron insensitive element intended to measure the gamma dose that accompanies the neutron dose. A typical such gamma-sensitive element might be enriched in the isotope 7Li, such as 7LiF(Mg.Ti), commonly referred to as TLD-700. This phosphor is quite insensitive to thermal neutrons and if it is paired with a similar geometry and mass 6LiF(Mg,Ti) element, the net neutron response is obtained by subtracting the gamma response of the 7Li element from the gamma plus neutron response of the 6Li element. The material TLD-100 is a LiF(Mg,Ti) that contains natural lithium with both 6Li and 7Li, not especially useful for neutron dosimetry.
The TLD-1000, 6000, and 7000 are common designations for alternative LiF phosphors containing a different dopant mixture from the LiF noted above, namely LiF(Mg,Cu,P), which has a considerably higher sensitivity than the LiF(Mg,Ti); The 1000, 6000, and 7000 designations have similar meanings to the 100, 600, and 700 descriptions. The LiF phosphors have the particular personnel dosimetry advantage that they are very nearly soft tissue equivalent in terms of effective atomic number. Another Li-based phosphor that is tissue-equivalent is lithium tetraborate, Li2B4O7 sometimes referred to as TLD-800 when doped with Mn. It is possible to enrich this element in 6Li and/or 10B, both of which have large thermal neutron cross sections for the (n,a) reaction.
Other commonly denoted phosphors are TLD-200, 300, and 400, CaF2(Dy), CaF2(Tm), and CaF2(Mn) respectively, all relatively high sensitivity phosphors but not used for typical neutron dosimetry. Another high sensitivity phosphor is TLD-900, CaSO4(Dy), also not for typical neutron dose assessment. The material designated as TLD-500 is Al2O3(C), which is a high sensitivity material with good thermal stability. It has gained special prominence for use as a dosimeter in optically stimulated luminescence (OSL), primarily for photon and some beta dosimetry. There are hundreds, perhaps thousands, of materials that exhibit luminescence upon heating after exposure to ionizing radiation, but relatively few have gained much popularity in radiation dosimetry.
You can find a lot more information on the Internet and in many journal publications. You might want to review some of the questions that have been asked on the HPS Ask the Experts website at this link. Type in “TLD” or “thermoluminescent dosimeter” in the search box, and related questions and answers should appear. You can find reasonable summary discussions in Glenn Knoll’s well-known book, Radiation Detection and Measurement, Wiley & Sons, 4th ed., 2010. There are numerous other textbooks that have varying levels of detail about many topics. The book by McKeever et al., Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing, 1995. is a good one, although it doesn’t contain information on some of the latest phosphors. You can find other books at various book supplier sites, including Amazon.com.
Good luck in your neutron dose measurements.
George Chabot, PhD