M. Rodríguez-Villafuerte

Theory of heavy charged particle response (efficiency and supralinearity) in TL materials

Abstract We describe the theory of heavy charged particle (HCP) response (efficiency and supralinearity) in thermoluminescent (TL) materials. The HCP TL relative efficiency is treated in the framework of modified track structure theory (MTST) using Monte Carlo (MC) calculations of radial dose distributions in condensed phase LiF. HCP TL fluence response is modelled in the framework of the extended track interaction model (ETIM) which treats both HCP fluence response supralinearity and saturation using trapping centre (TC) and luminescent centre (LC) radial occupation density profiles. These are based on the MC radial dose distributions and experimental measurements of optical absorption and sensitisation dose response. Analytical ETIM calculations (up to fourth-order nearest-neighbour track interactions) are used to model the TL fluence response of the components of composite peak 5 in LiF:Mg,Ti (TLD-100). Monte Carlo track interaction calculations (MCTIM) are also described which can model even higher-order nearest-neighbour track interactions appropriate to the high temperature peaks in LiF:Mg,Ti (TLD-100), and as well, model the HCP TL fluence response in both parallel and non-parallel HCP beam geometries.

The use of a reflective scanner to study radiochromic film response.

GafChromic MD-55-2 film response was studied using a flatbed, reflective Microtek ScanMaker E3 scanner, under different conditions to optimize its use. Irradiations were performed using 60Co gamma rays in a 0-300 Gy dose interval to produce a calibrated step wedge. A 24-bit colour-scale mode along with image splitting into its red, green and blue components is suggested as an improved dosimetry method over a 256 grey level (8-bit) mode, extending the dynamic range for this film. Diverse film orientation and positioning, a black or a white background, and individual step film scanning were evaluated. Unwanted normalization is overcome by adding reference black and white steps adjacent to the radiochromic film, ensuring reproducibility. The use of a red filter was found to be equivalent to the use of the red component of the image after image colour splitting. The useful range for MD-55-2 film is extended up to 300 Gy if colour components, other than red, are used to evaluate the response. Comparisons with optical density measurements show that inexpensive commercial scanners might be a good alternative to densitometers.

Radiation transmission, leakage and beam penumbra measurements of a micro-multileaf collimator using GafChromic EBT film

Micro‐multileaf collimator systems coupled to linear accelerators for radioneurosurgery treatments require a rigorous dosimetric characterization in order to be used in 3D conformal and intensity modulated stereotactic radiosurgery and radiotherapy applications. This characterization involves high precision measurements of leaf transmission, leakage and beam penumbra through the collimation system and requires the use of detectors with high spatial resolution, high sensitivity and practically no energy dependence. In this work the use of GafChromic EBT radiochromic film to measure the basic dosimetric properties of the m3‐mMLC (BrainLAB, Germany) micro‐multileaf collimator system integrated to a 6 MV linear accelerator, is reported. Results show that average values of transmission and leakage radiation are 0.93±0.05% and 1.08±0.08%, respectively. The 80–20% beam penumbra were found to be 2.26±0.11 mm along the leaf side (perpendicular to leaf motion) and 2.31±0.11 mm along the leaf end (parallel to leaf motion) using square field sizes ranging from 9.1 to 1.8 cm. These measurements are in agreement with values reported in the literature for the same type of mMLC using different radiation detectors. PACS number: 87.56.N‐

Thermoluminescent response and relative efficiency of TLD-100 exposed to low-energy x-rays.

The dose-response of LiF:Mg,Ti (TLD-100) exposed to 15 and 35 kVp (8.0 +/- 0.1 and 8.1 +/- 0.1 keV effective energy respectively) x-rays and 60Co gamma-rays has been measured in the dose interval from (1.2-5.4) x 10(3) Gy for x-rays, and from 0.14 to 850 Gy for gamma-rays. In both cases the total TL signal and glow curve peaks 3 to 9 show supralinearity. The supralinearity function f(D) is similar for both x-ray beams, except for peak 8, where a 30% difference is observed. The maxima of f(D) for the total TL signal and peaks 5 to 8 are 2.1, 1.7, 6.4, 3.3 and 7.5 respectively for 8.1 keV x-rays and 3.7, 3.1, 13.6, 9.9 and 11.0 for gamma-rays. The measured relative efficiencies for x-rays with respect to 60Co, for the total TL signal and peaks 5 and 7, were 1.04, 0.97 and 3.2 respectively.

Small photon beam measurements using radiochromic film and Monte Carlo simulations in a water phantom

This work reports the use of both GafChromic EBT film immersed in a water phantom and Monte Carlo (MC) simulations for small photon beam stereotactic radiosurgery dosimetry. Circularly collimated photon beams with diameters in the 4-20 mm range of a dedicated 6 MV linear accelerator (Novalis, BrainLAB, Germany) were used to perform off-axis ratios, tissue maximum ratios and total scatter factors measurements, and MC simulations. GafChromic EBT film data show an excellent agreement with MC results (<2.7%) for all measured quantities.

Study of the TL response of LiF:Mg,Ti to 3 and 7.5 MeV helium ions: Measurements and interpretation in terms of the track interaction model

Abstract We have measured the LiF:Mg,Ti (TLD-100) thermoluminescent response, efficiency and supralinearity, to 3 and 7.5 MeV helium ions in the fluence interval between 6×107 to 1.5×1011 cm−2. Ion irradiations were performed using a flux of elastically scattered ions from a thin gold target. The measured efficiencies with respect to 60Co γ-rays are equal to 0.074 and 0.174 at 3 and 7.5 MeV, respectively, for the response of peak 5. The supralinearity functions for peaks 4, 5, 6a, 6b, 7, 8, 9 and the total TL signal indicate that supralinearity is an increasing function of energy and that it increases with peak temperature. The data for peaks 5, 8 and 9 have been successfully described by Monte Carlo simulations of the track interaction model. The fitted parameters show that peak 5 displays a special behaviour which requires the use of a much larger effective track radius than the higher temperature peaks.

Response of radiochromic dye films to low energy heavy charged particles

Abstract We have studied the possible use of radiochromic dye films (RCF) as heavy charged particle dosemeters. We present the results of irradiating two commercial RCF (GafChromic HD-810 and MD-55-1) with 1.5, 2.9 and 4.4 MeV protons, 1.4, 2.8, 4.7, 5.9, 6.8 MeV 4He ions and 8.5 and 12.4 MeV 12C ions, at proton doses from about 1 Gy up to 3 kGy, helium ions doses from 3 Gy to 5 kGy and carbon ion doses from 30 Gy to 20 kGy. The films were scanned and digitized using commercial equipment. For a given particle, the response per unit dose at different energies indicates an energy dependence of the sensitivity, which is discussed. Comparison was made for the use of a standard spectrophotometer to obtain optical density readings versus a white light scanner.

Supralinear response and efficiency of LiF:Mg,Ti to 0.7, 1.5 and 3 MeV protons

Abstract The thermoluminescent (TL) response, supralinearity function, and relative efficiency with respect to 60 Co γ-rays, of TLD-100 exposed to 0.7, 1.5 and 3 MeV protons have been measured. The TL response of the glow curve peaks using two different deconvolutions, separating in either 7 (3–9) or 9 peaks (3–10 and peaks 6a and 6b), was investigated. Results for the supralinearity function of peaks 5 and 7–9 and the relative response of peaks 5 and 7 are insensitive to the chosen deconvolution scheme. At all energies we observe that peak 5 is slightly supralinear and that supralinearity increases as a function of the peak temperature. The relative efficiencies, measured for the total TL signal (area under the glow curve) and peaks 5 and 7, decrease with the proton energy having values of 0.33, 0.46 and 0.70 (total TL signal), 0.22, 0.32 and 0.53 (peak 5) and 1.4, 2.6 and 3.6 (peak 7) at 0.7, 1.5 and 3 MeV, respectively. These data agree with the systematics reported in the literature. The measurements of the supralinearity function are compared with Monte Carlo track interaction model calculations. The analysis suggests that the model ion track effective radius and the saturation radius are independent of the incident ion energy.

Two Flat-Backed Polydesmidan Millipedes from the Miocene Chiapas-Amber Lagerstätte, Mexico

Two species of fossil polydesmidan millipedes (Diplopoda: Polydesmida) embedded in amber are described from Miocene strata near Simojovel, in the Chiapas Highlands, Mexico. Maatidesmus paachtun gen. et sp. nov., placed into Chelodesmidae Cook, 1895, and Anbarrhacus adamantis gen. et sp. nov., assigned in the family Platyrhacidae Pocock, 1895. Morphological data from fossil specimens have been recovered using 3D X-ray micro-computed tomography and regular to infrared-reflected microscopy. Both fossil species are recognizable as new primarily but not exclusively, by collum margin modification and remarkable paranotal and metatergite dorsal sculpture.

Effect of correction methods of radiochromic EBT2 films on the accuracy of IMRT QA

Radiochromic films are dosimeters with more favorable characteristics than other two-dimensional (2D) radiation detectors. Transmission film scanners using a linear charge-coupled device (CCD) array have a drawback of variation in response along the detector array, which may result in a nonuniform transmission of signal over the scanned image. This study uses 2D gamma index analysis to compare two methods for correcting the nonuniform response of EBT2: the first method was based on the new red:blue method proposed by the manufacturer (to compensate for small nonuniformities in the film coating) and the second method, proposed by Menegotti et al. (2008), was based on dose-dependent matrix correction factors. The gamma index analysis shows that both the methods are comparably accurate for all the criteria values used for evaluation (1 mm/1%, 2 mm/2%, 3 mm/3%). Centers around the world use both the methods to correct EBT2 local heterogeneities, but it is important to note that the former method has several advantages such as less time consumption and easy implementation.