CAVITY`S THEORY OF BRAGG-GRAY APPLYED TO IONIZATION FREE- AIR CHAMBER.

Transcription

1 CAVITY`S THEORY OF BRAGG-GRAY APPLYED TO IONIZATION FREE- AIR CHAMBER. Silva J. L. D. F., Cardoso R. S. and Peixoto J. G. P. Instituto de Radioproteção e Dosimetria (IRD / CNEN - RJ) Av. Salvador Allende S/N Rio de Janeiro, RJ joseluiz@ird.gov.br ricardo@ird.gov.br guilherm@ird.gov.br ABSTRACT Ionization s chambers are kinds of dosimeters more useful to measurements such as required in radiotherapy. Are detectors of non pulse type (mean effect) and gaseous, calibrate in Kerma, unit Gray (Gy).The objective of this work is to relate the physical theory existing and the construction shape of the primary standard, that is the ionization of free-air chamber with variable volume, manufactured by Victoreen Instruments, used to measure magnitude air Kerma performed by the Laboratório Nacional de Metrologia das Radiações Ionizantes from Instituto de Radioproteção e Dosimetria - LNMRI-IRD. For in such a way, will be done an evaluation of the chamber capability, and the sensibility of radiations of low energy, range from 20kV to 80kV, in the qualities of mammography not attenuate, mammography attenuate, diagnostic, therapy e protection through the construction s acquaintance, dimensions of pieces components and operational tests. Concerning the chamber s construction, the mechanical set consists of two cylinders of aluminium concentrically that moves axially between itself, objectifying the variation of internal volume (variable volume). The practical application of the Bragg-Gray cavity s theory can be applied to the chamber, where the reasons of the absorbed dose in different means to the same fluency is equal to the reason of stopping powers of two means. From the definition of Kerma we can obtain the adsorbed dose and existing particles in (CPE), the absorbed dose is equal to the Kerma s collision. 1. INTRODUCTION Ionization s chambers are the most common dosimeter useful to accuracy measurements, as that required in radiotherapy (ATTIX, 1986). The first works about the free air ionization chamber are in period between 1895 and 1897, when PERRIN reported his project (PEIXOTO, 2002). The model of free air ionization chamber considered as conventional described by KEMP (PEIXOTO, 1991) is of parallel plates. Considered as an instrument of high precision by DAY (1948) e KEMP (1954), this chamber has originally settled volume and offer the advantage in measurements of therapeutic X rays. The construction of a free air ionization chamber of parallel plates consists basically in association among a lead shielded box, and a assemblage of three coplanar plates, located at the opposite side of the high tension plate, parallel to it, and rounded by guard wires. The incident beam is equidistant and parallel to all plates. The distance of the plates from the beam is designed to put them beyond the range of substantially all the secondary electrons originating in the beam. The electric field distortion effects are minimize by a system of wires guard, G, responsible to keep the uniformity in the region of ions gathering, as showed Figure 1. 1

2 Figure 1: Diagrammatic plane of the free air ionization chamber of parallel plates (ATTIX, 1986). V Volume effective; V Volume; P Point used for reference; e 1,2,3 electrons; G Guard ring. 2. IONIZATION S CHAMBERS OF VARIABLE VOLUME Attix in 1957 propose the ionization chamber of variable volume, which mechanical and electrical construction consists of two cylinders of aluminium concentrically that moves axially between itself, objectifying the variation of internal volume. The set is also compound of an external shield and a collecting rod. The shield is to avoid that the scattering radiation s penetrates in a sensible part of itself. Just the function of the collecting rod is as an electrode anode (+), filament shape to capture ions (-). Ionization s chambers are kinds of dosimeters more useful to measurements such as required in radiotherapy. Are detectors of non pulse type (mean effect) and gaseous, calibrate in Kerma, unit Gray (Gy) The chamber VICTOREEM In the year of 1966 was manufactured by Victoreen Instruments the ionization chamber of variable volume model 481, that is now being useful in Laboratório Nacional de Metrologia das Radiações Ionizantes do Instituto de Radioproteção e Dosimetria LNMRI-IRD Figure 2. Figure 2: Ionization chamber of variable volume manufactured by Victoreen Instruments, model

3 2.2. Objective of this work The objective of this work is to relate the physical theory existing and the construction shape of the primary standard, that is the ionization of free-air chamber with variable volume, manufactured by Victoreen Instruments, used to measure magnitude air Kerma performed by the Laboratório Nacional de Metrologia das Radiações Ionizantes from Instituto de Radioproteção e Dosimetria LNMRI-IRD. For in such a way will be done an evaluation of the chamber capability, and the sensibility of radiations of low energy, range from 20kV to 80kV in the qualities of mammography not attenuate, mammography attenuate, diagnostic, therapy e protection, through the construction s acquaintance, dimensions of pieces components and operational tests Implantation of the chamber The use of the chambers of variable volume is restricting to some laboratories of standardization. Nevertheless, preliminary informations about work of the ionization free air chamber of variable as a primary standard, to obtain magnitude air Kerma, were get by the Italian laboratory ENEA (LAITANO, 1984). As in this laboratory was useful the model 480 from Victoreen in the process, the LNMRI chose the same model to lead the implantation process of Kerma magnitude, started in 1991 (PEIXOTO, 1991). 3. CONSTRUCTION SHAPE Concerning the chamber s construction, the mechanical set consists of two cylinders of aluminium concentrically that moves axially between itself, objectifying the variation of internal volume (variable volume). The set is also compound of an external shield and a collecting rod. The shield is to avoid that the scattering radiation s penetrates in a sensible part of itself. Just the function of the collecting rod is as an electrode anode (+), filament shape to capture ions (-) Figure 3. Figure 3: Diagrammatic plane of the free air ionization chamber of parallel plates (ATTIX, 1986) (a) in condition collapsed; (b) in condition extended. W Window of maylar; E Output for the electrometer; A; A ; B; B e V - Volumes. 3

4 4. BRAGG-GRAY CAVITY S THEORY 4.1. The theory Reporting the Bragg-Gray cavity s theory to understand the function of the chamber we get a practical application of this theory, where the reason of the absorbed dose in different means to the same fluency is equal to the reason of stopping powers of two means Gathering of ions The X-rays penetration in the chamber by the collimator do inside the chamber a conical trunk (region of interest), where the air is ionizing and the formed ions-pairs, are collected through an applied (ddp) to the components: rod (anode) and cylinders (cathode), forming an electrical field between then. These ions are entered by the associate electronic (electrometer) by the system. How much bigger the ddp applied to the chamber, minor the ionic recombination and more the amplitude of pulse. In the work region of the chamber, the response obtained is proportional to the numbers of the ions-pairs produced. When the chamber reaches the region of saturation an increase of ddp do not causes an increase of current (not proportional region) Results From de definition of Kerma, that is the expectation value of the energy transferred to charged particles per unit mass at a point of interest, we can obtain the absorbed dose, and existing charged particles equilibrium (CPE), the absorbed dose is equal to the Kerma`s collision, as follows: de K = tr dm [Gy] (1) D d = Ε dm [Gy] (2) if existing charged particles equilibrium (CPE) K = D With the difference between the average of ionization current lectures, with the chamber extended and collapsed, then get (Q), alike the Equation 3. Q = Q E Q C (3) Applying the lecture of the ionization s current (Q), in the relation showed by the Equation 3, available in the comparisons to establish the magnitude Kerma rate (BURNS, 2001), get the magnitude to the primary standard.. K = Q ρar V W ar e 1 1 g ar where, is the air density to the relating conditions; W ar is the average energy spent by the electron with charge e to produce a ion pair; g ar is the initial ion fraction energy lost from the electron from the production of bremsstrahlung in the air; and Π k i is the product of the correction factors of air attenuation and saturation. i K i (4) 4

5 5. CONCLUSIONS According with the proposal of the present work shall be obtained and conferred all qualities tabled for the qualities of mammography not attenuate, mammography attenuate, diagnostic, therapy and protection. REFERENCES 1.PEIXOTO, J.G.P., Implantação de um padrão primário da grandeza exposição para feixes de raios-x, Tese de M.Sc., COPPE/UFRJ, Rio de Janeiro, RJ, Brasil (1991) 2.PEIXOTO, J.G.P., Rastreabilidade e controle de qualidade em mamografia; Medidas e modelos, Tese de D.Sc., UERJ, Rio de Janeiro, RJ, Brasil (2002). 3.ATTIX, F.H., Introduction to Radiological Physics and Radiation Dosimetry, 1ª ed., Wiley & Sons Inc, New York, USA (1986). BURNS, D.T., Degrees of equivalence for the key comparison BIPM.RI(I)-K2 between national primary standards for low-energy X-rays, Summary Report for BIPM.RI(I)- K , BIPM, França (2003). 5