Josilene C. Santos

Monte Carlo simulation of the response functions of CdTe detectors to be applied in x-ray spectroscopy.

In this work, the energy response functions of a CdTe detector were obtained by Monte Carlo (MC) simulation in the energy range from 5 to 160keV, using the PENELOPE code. In the response calculations the carrier transport features and the detector resolution were included. The computed energy response function was validated through comparison with experimental results obtained with (241)Am and (152)Eu sources. In order to investigate the influence of the correction by the detector response at diagnostic energy range, x-ray spectra were measured using a CdTe detector (model XR-100T, Amptek), and then corrected by the energy response of the detector using the stripping procedure. Results showed that the CdTe exhibits good energy response at low energies (below 40keV), showing only small distortions on the measured spectra. For energies below about 80keV, the contribution of the escape of Cd- and Te-K x-rays produce significant distortions on the measured x-ray spectra. For higher energies, the most important correction is the detector efficiency and the carrier trapping effects. The results showed that, after correction by the energy response, the measured spectra are in good agreement with those provided by a theoretical model of the literature. Finally, our results showed that the detailed knowledge of the response function and a proper correction procedure are fundamental for achieving more accurate spectra from which quality parameters (i.e., half-value layer and homogeneity coefficient) can be determined.

Evaluation of conversion coefficients relating air-kerma to H*(10) using primary and transmitted x-ray spectra in the diagnostic radiology energy range.

According to the International Commission on Radiation Units and Measurements (ICRU), the relationship between effective dose and incident air-kerma is complex and depends on the attenuation of x-rays in the body. Therefore, it is not practical to use this quantity for shielding design purposes. This correlation is adopted in practical situations by using conversion coefficients calculated using validated mathematical models by the ICRU. The ambient dose equivalent, H*(10), is a quantity adopted by the IAEA for monitoring external exposure. Dose constraint levels are established in terms of H*(10), while the radiation levels in radiometric surveys are calculated by means of the measurements of air-kerma with ion chambers. The resulting measurements are converted into ambient dose equivalents by conversion factors. In the present work, an experimental study of the relationship between the air-kerma and the operational quantity ambient dose equivalent was conducted using different experimental scenarios. This study was done by measuring the primary x-ray spectra and x-ray spectra transmitted through materials used in dedicated chest radiographic facilities, using a CdTe detector. The air-kerma to ambient dose equivalent conversion coefficients were calculated from these measured spectra. The resulting values of the quantity ambient dose equivalent using these conversion coefficients are more realistic than those available in the literature, because they consider the real energy distribution of primary and transmitted x-ray beams. The maximum difference between the obtained conversion coefficients and the constant value recommended in national and international radiation protection standards is 53.4%. The conclusion based on these results is that a constant coefficient may not be adequate for deriving the ambient dose equivalent.

Application of a semi-empirical model for the evaluation of transmission properties of barite mortar

The aim of this study was to estimate barite mortar attenuation curves using X-ray spectra weighted by a workload distribution. A semi-empirical model was used for the evaluation of transmission properties of this material. Since ambient dose equivalent, H(⁎)(10), is the radiation quantity adopted by IAEA for dose assessment, the variation of the H(⁎)(10) as a function of barite mortar thickness was calculated using primary experimental spectra. A CdTe detector was used for the measurement of these spectra. The resulting spectra were adopted for estimating the optimized thickness of protective barrier needed for shielding an area in an X-ray imaging facility.

Direct measurement of clinical mammographic x‐ray spectra using a CdTe spectrometer

Purpose To introduce and evaluate a method developed for the direct measurement of mammographic x‐ray spectra using a CdTe spectrometer. The assembly of a positioning system and the design of a simple and customized alignment device for this application is described. Methods A positioning system was developed to easily and accurately locate the CdTe detector in the x‐ray beam. Additionally, an alignment device to line up the detector with the central axis of the radiation beam was designed. Direct x‐ray spectra measurements were performed in two different clinical mammography units and the measured x‐ray spectra were compared with computer‐generated spectra. In addition, the spectrometer misalignment effect was evaluated by comparing the measured spectra when this device is aligned relatively to when it is misaligned. Results The positioning and alignment of the spectrometer have allowed the measurements of direct mammographic x‐ray spectra in agreement with computer‐generated spectra. The most accurate x‐ray spectral shape, related with the minimal HVL value, and high photon fluence for measured spectra was found with the spectrometer aligned according to the proposed method. The HVL values derived from both simulated and measured x‐ray spectra differ at most 1.3 and 4.5% for two mammography devices evaluated in this study. Conclusion The experimental method developed in this work allows simple positioning and alignment of a spectrometer for x‐ray spectra measurements given the geometrical constraints and maintenance of the original configurations of mammography machines.

Evaluation of mean conversion coefficients from air-kerma to H *(10) using secondary and transmitted x-ray spectra in the diagnostic radiology energy range

Ambient dose equivalent H *(10) is an operational quantity recommended by the IAEA to establish dose constraints in area monitoring for external radiation. The direct measurement of H *(10) is not common due to the complexity in the calibration procedures of radiation monitors involving the use of expanded and aligned radiation fields. Therefore, conversion coefficients are used to assess H *(10) from the physical quantity air-kerma. Conversion coefficients published by international commissions, ICRU and ICRP, present a correlation with the radiation beam quality. However, Brazilian regulation establishes 1.14 Sv Gy-1 as unique conversion coefficient to convert air-kerma into H *(10), disregarding its beam quality dependence. The present study computed mean conversion coefficients from secondary and transmitted x-ray beams in order to improve the current assessment of H *(10). The weighting of conversion coefficients corresponding to monoenergetic beams with the spectrum energy distribution in terms of air-kerma was used to compute the mean conversion coefficients. In order to represent dedicated chest radiographic facilities, an anthropomorphic phantom was used as scatter object of the primary beam. Secondary x-ray spectra were measured in the diagnostic energy range at scattering angles of 30°, 60°, 90° 120° and 150° degrees. Barite mortar plates were used as attenuator of the secondary beam to produce the corresponding transmitted x-ray spectra. Results show that the mean conversion coefficients are about 43% higher than the recommended value accepted by Brazilian regulation. For secondary radiation measured at 100 kV the mean coefficient should be 1.46 Sv Gy-1, which represent the higher value in the mean coefficient set corresponding to secondary beams. Moreover, for transmitted x-ray beams at 100 kV, the recommended mean conversion coefficient is 1.65 Sv Gy-1 for all barite mortar plate thickness and all scattering angles. An example of application shows the discrepancy in the evaluation of secondary shielding barriers in a controlled area when the shielding goals is evaluated. The conclusion based on these results is that a unique coefficient may not be adequate for deriving the H *(10).

Characterization of OSL dosimeters for use in dose assessment in Computed Tomography procedures

This study describes the characterization of an Al2O3:C OSLD (Landauers Luxel™ tape) for dose evaluation in Computed Tomography. The irradiations were conducted using both a constant potential X-ray equipment and a 64-slice clinical CT scanner, and the readouts were performed using a Risø TL/OSL reader. The following aspects were studied: batch homogeneity, energy response, linearity of dose response, reproducibility, reusability, and effect of uncertainties with the normalization of OSL signals per their response to beta radiation. A group of 330 dosimeters from the 452 irradiated with the same dose presented OSL signals within the interval of 4.7% from the average. The dosimeters presented energy-dependent response in good agreement with results found in the literature. The air kerma response of the OSL signal showed a linear trend for both the constant potential X-ray device and the clinical CT scanner, with differences in their slopes of approximately 10%. Reproducibility, reusability, and effect of beta normalization were analyzed by separating 72 dosimeters in 3 groups. The results obtained in this study together with those of previous works indicate that this type of dosimeter is adequate for dose evaluation in CT clinical applications.

Estimation of dose distributions in mammography into a tissue equivalent phan-tom

Mammography is considered the most suitable technique for the identification, tracking and staging of breast diseases, especially cancer. However, the use of ionizing radia-tion carries a risk of inducing cancer in this kind of breast imaging procedure. In this way, it is important to know the dose distribution in typical mammography techniques. The aim of this work was to estimate the dose distributions in dif-ferent positions parallel to the image plan and different depths of a tissue equivalent phantom using LiF thermoluminescent dosimeters (TLD). LiF TLDs pellets were positioned on the xy plan on a phantom BR12 at different depths. The set was irra-diated in a GE Senographe DS System for a maximum field size. The dosimeters were irradiated using a Mo/Mo an-ode/filter combination, 100 mAs, and different X-ray tube voltages (25, 28, 30 and 32 kVp). It was also used a fixed X-ray tube voltage (28 kVp), 100 mAs, and different anode/filter combinations (Mo/Mo, Mo/Rh and Rh/Rh). Dose variation in the y direction was more significant than in the x direction. It were observed variations up to 8% and 28.2% in x and y direc-tions respectively. For the tube voltage 28 kVp, the maximum dose in the phantom surface in the y-axis was 16(1) mGy for Mo/Mo, and 15.0 (9) mGy for Mo/Rh and 12.8(6) for Rh/Rh.