Assessment of energy and angular dependence of LiF:Mg,Ti dosimeters irradiated in the quantity Hp(0.07)

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INTRODUCTION
Radiation metrology is the basis for achieving credibility metrology in dose measurements in several areas, it is also part of a structure to ensure radiological protection procedures, so as to avoid or minimize the possible biological effects induced by exposure to ionizing radiation [1]. The metrological reliability of the measurements performed in the dosimetry of patients or materials is achieved by establishing a metrological basis that must include: calibration and evaluation procedures using performance tests of experimental dosimetric systems [2].
For practices involving exposure to ionizing radiation, workers must use individual monitors capable of quantifying the effective dose received during the period of use [2].
Dosimeters are devices that have the purpose of quantifying the dose that the worker received in a certain region of the body during his period of activities involving ionizing radiation [1,3]. The use of the dosimeter is necessary to quantify the dose that the occupationally exposed individual received in a certain region of the body, using specific calculations for each region [1]. For the extremity, the dosimeter is used in the ring model, which has the purpose of quantifying the dose received in the region of the extremity [3].
A dosimeter must have, at least, one physical property that varies as a function of the measured dosimetric quantity [1,4], in addition to the physical properties, the dosimeter must pass the calibration tests; the energy dependence and angular dependence tests are essential tests for the dosimeter calibration process [1,2,4]. Dosimeter calibration is essential both in the medical area (technicians, technologists, nurses, and doctors) and in the research area (production of radiopharmaceuticals and performance in different stages of the fuel cycle). In Brazil, so far, there are no standards for extremity dosimeters, therefore, in this work the recommendations of the Evaluation Committee of Testing and Calibration Services (CASEC) for whole-body dosimeters adapted for extremity dosimeters were used [1,3,5]. The objective of this work is to evaluate the dependence of the dose evaluated in thermoluminescent dosimeters as a function of the energy and incidence angle of radiation.

MATERIALS AND METHODS
In this section, all the characteristics, definitions and relevant information of the materials, methods and equipment used in this work will be presented.
The equations used in this session are recommended by the CASEC standard for whole-body dosimetry, however, in this work, adapted for extremity dosimetry.
The irradiations were performed in the quantity Hp(0.07) using a phantom rod in (Figure 1

Energy Dependence
For the energy dependence test, 10 TLDs were selected. Initially, the batch started with the quantity of 43 dosimeters, which was reduced to 23 after a batch homogeneity test, the dosimeters were irradiated in different quantitys of energies in an X-ray irradiation system (48, 65, 83 and 118 keV) and a source of gamma radiation with a source of Cesium-137, used as a reference.
The dosimeters were irradiated at a focus-object distance of 2.5 meters in the X-ray tests and 1 meter in the tests with the Cs-137 source.
The evaluation of the dosimeters was performed 60 minutes after the irradiations. Irradiations were performed at room temperature, with doses of 10 mSv, and the irradiation and reading process was repeated 5 times with each dosimeter at each energy used. Group 5 (Cs-137).

Angular Dependence
For the angular dependence test, the same 10 TLDs of the energy dependence test were used.
The dosimeters were irradiated with X-ray voltage at 150 kVp and a dose of 10 mSv. The detector was positioned at different angles to the source (0°, 20°, 40° and 60°) with clockwise rotation.
Irradiations were performed with a focus-object distance of 2.5 meters with readings after 60 minutes of irradiation. The process was repeated 5 times. The variation of the TL response as a function of the radiation incidence angle of each group must not exceed the limits presented in equation (2), where C is the conventional true value, A is the evaluated value for each dosimeter, Āi is the average of the evaluated values, si is the standard deviation of the samples and li is the confidence interval of si.
Equation (2) Group 1: normal incidence (zero degree);  li is the confidence interval of the standard deviation. Figure 5 presents the mean of the results of the energy dependence test in graph format.

Condition
Accepted Accepted Accepted Accepted Accepted respectively. Table 12 presents the results of the TL response angular dependence test using the equation recommended by CASEC. Figure 6 presents the mean of the angular dependence test results in graph format.

CONCLUSION
The results obtained in the energy and angular dependence tests present a small range of variation +0,5% for energy dependence and +0,001% for angular dependence, according to the CASEC recommendations.
In the energy dependence test, the average of the energies evaluated using the reference The results indicate that the dosimetry system studied meets the calibration requirements in the quantity Hp(0.07), using a phantom rod recommended by ICRU in Report 47 [7] in gamma radiation field (Cs-137) and X radiation at energies of 48, 65, 83 and 118 keV recommended by CASEC and adapted to the equipment used.