LiF:Mg,Ti TLD response as a function of photon energy for moderately filtered x‐ray spectra in the range of 20–250 kVp relative to

Abstract

The response of LiF:Mg,Ti thermoluminescent dosimeters (TLDs) as a function of photon energy was determined using irradiations with moderately filtered x-ray beams in the energy range of 20-250 kVp relative to the response to irradiations with 60Co photons. To determine if the relative light output from LiF:Mg,Ti TLDs per unit air kerma as a function of photon energy can be predicted using calculations such as Monte Carlo (MC) simulations, measurements from the x-ray beam irradiations were compared with MC calculated results, similar to the methodology used by Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)]. TLDs were irradiated in photon beams with well-known air kerma rates using the National Institute of Standards and Technology traceable M-series x-ray beams in the range of 20-250 kVp. For each x-ray beam, several sets of TLDs were irradiated for times corresponding to different air kerma levels to take into account any dose nonlinearity. TLD light output was then compared to that from several sets of TLDs irradiated at similar corresponding air kerma levels using a 60Co irradiator. The MC code MCNP5 was used to account for photon scatter and attenuation in the holder and TLDs and was used to calculate the predicted relative TLD light output per unit air kerma for irradiations with each of the experimentally used photon beams. The measured relative TLD response as a function of photon energy differed by up to 13% from the MC calculations. We conclude that MC calculations do not accurately predict the relative response of TLDs as a function of photon energy, consistent with the conclusions of Davis et al. [Radiat. Prot. Dosim. 106, 33-43 (2003)]. This is likely due to complications in the solid state physics of the thermoluminescence process that are not incorporated into the simulation.

Supplement 2 for the 2004 update of theAAPMTask Group No. 43 Report: Joint recommendations by theAAPMandGEC‐ESTRO

“…Davis et al 41 reported a TLD over-response under 200 keV relative to 60 Co (k bq decrease) up to 10% (0.6% Type A uncertainty, k = 1) for LiF:Mg,Ti TLD chips using low-energy x-ray beams. Corresponding results of Nunn et al 42 and Carlsson Tedgren et al 43 were 13% (3.5% combined standard uncertainty, k = 1) and 7% (1.9% combined standard uncertaintiy, k = 1), respectively. Carlsson Tedgren et al 43 discussed differences between their results and those of previous studies in view of TLD handling (annealing and read out parameters) and formulation, and expressed concern regarding the applicability of determinations obtained using x-ray beams to other photon fields.…”

“…X-ray transitions are listed after the elemental symbol for the daughter nuclide (i.e., Xe). Subgroups are enumerated for the K a and K b transitions, while the mean energies and total intensities are given for the other transitions, e.g., While these results are well within the range of corresponding findings of studies [41][42][43] employing x-ray beams, they are significantly higher than corresponding findings of Reed et al 45 On average, the measured Λ values (as revised by Rodriquez and Rogers 44 to include their f rel , P phant , and k rel bq corrections) were 4.3% and 5.9% lower than the original values for 103 Pd and 125 I seeds, respectively. As expected, a closer agreement was observed between these revised Λ values and their corresponding MC calculations.…”

“…38 Since the publication of the 2004 TG-43U1 report 2 and its 2007 supplement, 4 a terminology has been introduced to rigorously describe dosimeter characteristics, 39,40 and important work has been reported on the complexity of using TLD dosimetry for low-energy sources. [41][42][43][44][45] In this terminology, 39,44 D med (Q) is the absorbed dose to the medium med at the point of measurement (normally the detector midpoint) in the absence of the detector for a given radiation quality Q. This is related to the detector measurement for the same radiation quality M(Q) and the absorbed dose calibration coefficient for the detector N D,med (Q) according to:…”

“…37 The hypothesis of k rel bq equal to unity prevailed in TLD dosimetry studies of low-energy sources until the work of Davis et al and subsequent high-quality determinations of k bq reported in the literature. [42][43][44][45] See reference 37 for a review. Davis et al 41 reported a TLD over-response under 200 keV relative to 60 Co (k bq decrease) up to 10% (0.6% Type A uncertainty, k = 1) for LiF:Mg,Ti TLD chips using low-energy x-ray beams.…”

“…71 Kennedy et al 53 reported dosimetry results of TLD measurements and MC estimations of the model 9011 125 I source, with comparisons also made to the model 6711 125 I source. Measurements performed using TLD-100 1 mm 3 cubes calibrated with 60 Co photons based on the methods of Nunn et al 42 Irradiations were performed in two high-purity 11 9 30 9 30 cm 3 polymethyl methacrylate (PMMA) phantoms having independent composition analyses. This material was chosen over solid water mixtures due to the reduced uncertainties when correcting measured results to yield absorbed dose to water.…”