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Caracterização de um Espectrômetro Compton para Medição de Espectros de Feixes de Fótons de Energia Alta
Abstract
O método de espectrometria Compton apresenta-se como uma técnica eficiente para medição de espectros de feixes de fótons de energia alta que são produzidos em aceleradores lineares clínicos e unidades de radioterapia de Co-60, usadas em tratamento de pacientes com câncer. Neste trabalho foi feita a caracterização de um espectrômetro Compton com o objetivo de medir o espectro de altura de pulsos (PHS, do inglês Pulse Height Spectra) para uma unidade de Co-60 de uso clinico, com a finalidade de obter o espectro de fluência de fótons para posteriormente aplicar este método na medição e reconstrução do espectro de bremsstrahlung de um acelerador linear clínico. Previamente foi feita a caracterização de um detector de NaI(Tl), obtendo as funções de resposta em energia para o espectrômetro Compton. Aqui apresentamos a medição dos espectros de altura de pulsos em configuração Compton para energias de raios X de 70 a 100 kV com ângulo Compton de 90o, as quais foram feitas com o fim de obter a resposta do espectrômetro a estas energias, para depois disso medir o espectro de altura de pulsos para a unidade de Co-60 em diferentes ângulos Compton.
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Multiple Compton scattering results in an incorrect evaluation of Compton profile, which usually examines the case when the photon has undergone only one Compton collision in the sample. The probability of a photon being scattered several times may be significant for a target of finite dimensions both in depth and lateral dimensions. The present measurements are carried out to study the energy and intensity distributions of 0.279, 0.662, and 1.12 MeV {gamma} rays multiply scattered from the copper target of various thicknesses at a scattering angle of 60 degree sign . The scattered photons are detected by a NaI(Tl) scintillation detector whose detector response unfolding, converting the pulse-height distribution to a photon energy spectrum, is obtained with the help of an inverse response matrix. We observe that with an increase in target thickness, the number of multiply scattered photons saturates at a particular value of the target thickness (saturation depth). The multiply scattered fraction starts saturating above a particular energy window around the centroid of inelastic scattered peak. The signal-to-noise ratio is found to be decreasing with an increase in target thickness. Monte Carlo calculations based upon the package developed by Bauer and Pattison [Comptom Scattering Experiments at the HMI (1981), HMI-B 364, pp. 1-106] support the present experimental results.
The response function, converting the observed pulse-height distribution of a NaI(Tl) detector to a true photon spectrum, is obtained experimentally with the help of an inverse matrix approach. The energy of gamma-ray photons continuously decreases as the number of scatterings increases in a sample having finite dimensions when one deals with the depth of the sample. The present experiments are undertaken to study the effect of target thickness on intensity distribution of gamma photons multiply backscattered from an aluminium target. A NaI(Tl) gamma-ray detector detects the photons backscattered from the aluminium target. The subtraction of analytically estimated singly scattered distribution from the observed intensity distribution (originating from interactions of primary gamma-ray photons with the target) results in multiply backscattered events. We observe that for each incident gamma photon energy, the number of multiply backscattered photons increases with increase in target thickness and then saturates at a particular target thickness called the saturation thickness (depth). Saturation thickness for multiply backscattering of gamma photons is found to decrease with increase in energy of incident gamma-ray photons.
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A straightforward presentation of the broad concepts underlying radiological physics and radiation dosimetry for the graduate-level student. Covers photon and neutron attenuation, radiation and charged particle equilibrium, interactions of photons and charged particles with matter, radiotherapy dosimetry, as well as photographic, calorimetric, chemical, and thermoluminescence dosimetry. Includes many new derivations, such as Kramers X-ray spectrum, as well as topics that have not been thoroughly analyzed in other texts, such as broad-beam attenuation and geometrics, and the reciprocity theorem. Subjects are layed out in a logical sequence, making the topics easier for students to follow. Supplemented with numerous diagrams and tables.
The photon flux spectra from two 6 MeV Linear Accelerators and a 2 MeV Van de Graaff Generator which are in normal use for radiotherapy, have been measured using a 75 × 75 cm NaI (Tl) crystal spectrometer. The X-ray beam was scattered by aluminium sheets to reduce the intensity and energy range, and the primary spectra calculated from the measured scattered spectra using the Klein-Nishina formula. The crystal spectrometer was calibrated from theoretical Monte Carlo calculations which were compared with experimentally obtained isotope spectra. The measured pulse height spectra were converted to photon flux spectra by a digital computer. Photon flux spectra were obtained for different field sizes and under different operating conditions, and the variation in the spectra over the X-ray field was investigated. The results were compared with theoretically calculated thick target spectra using published formulae for the bremsstrahlung cross-sections.
The bremsstrahlung spectrum from an 8-MeV linear accelerator has been measured using a NaI(T1) spectrometer system. The spectrum shows a low-energy cutoff at 0.4 MeV and the maximum photon energy to be approximately 6% greater than the nominal energy. The maximum emission of energy fluence was 1.6 and 1.8 MeV for measured and calculated values, respectively. The fast neutron dose in the photon beam was approximately 0.09% of the x-ray dose. The weighted mean energy was 2.3 MeV, measured value, and 2.4 MeV, calculated value.
A Compton spectrometer with a high-efficiency Ge detector was used to measure photon spectra from clinical accelerators. The response-function matrix for the detector system was determined using ten radioactive sources that emitted gamma rays of single energy ranging from 279-1525 keV. The system was tested by measuring the spectrum from a 60Co teletherapy machine. The energy distribution of tungsten target bremsstrahlung produced by 2.2, 2.5, 15, 20, and 25 MeV electrons in two clinical accelerators were determined. Theoretical thin-target calculations overestimate the number of photons in the high-energy regions of the spectra, as was expected. However, theoretical thick-target calculations underestimate the numbers of high-energy photons.
Bremsstrahlung spectra from thick targets of Al and Pb have been measured absolutely (photons per incident electron) along the beam axis for electrons of 10-, 15-, 20-, 25-, and 30-MeV incident energy. The spectra have a 220-keV low-energy cutoff. The targets were cylinders with nominal thicknesses of 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm diameter by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and the collimator effect. Spectra were calculated using the EGS4 Monte Carlo system for simulating the radiation transport. The simulation model included the small amount of material upstream of the target. This material contributed about 40% of the spectrum, but its presence or absence had little effect on the calculated bremsstrahlung yield. The shapes of the measured and calculated spectra were in excellent agreement. The ratio of the total number of photons in each measured spectrum to those in the corresponding calculated spectrum varied from 0.97 +/- 0.06 to 1.12 +/- 0.06, depending largely on the atomic number of the target. Absolute spectral measurements in the literature agreed with our calculations of spectral shape but showed a range of +/- 30% in the number of photons per incident electron relative to the calculated values, which is contrary to our result.
The bremsstrahlung spectra from a 25 MeV linear accelerator and a 19 MeV betatron were measured using a NaI(T1) spectrometer system. The spectra show a low energy cut off at 0.6 and 0.4 MeV, respectively, and the maximum photon energies were 26.8 and 19 MeV, respectively. Cross sections, thin and thick target photon spectra and total electron energy losses (collision and radiative) were computed using a digital computer for tungsten (Z=74) and platinum (Z=78) targets. The measured spectra were compared to the calculated spectra for thin and thick targets using electron kinetic energies of 26.75 and 19 MeV. The photon spectra from the 2 therapy units are different; however, depth dose data (10X10 cm, 100 cm TSD) are approximately the same. In addition, narrow beam attenuation coefficients in lead for the 2 machines were measured. The effective energy derived from the first HVL was 8.1 MeV for the linear accelerator and 7.8 MeV for the betatron.
An experiment to determine the peak of the energy spectrum of the photon beam from a Toshiba LMR-15 medical linear accelerator is described. It is found that the flattening filters removed much of the bremsstrahlung spectrum below approximately 1 MeV, resulting in a photon spectrum which peaks around 1.2 MeV.
Thick-target bremsstrahlung spectra produced by electron bombardment of a 0.2 radiation-length gold-tungsten target were measured for electron energies in the range of 5.3 to 20.9 MeV. A large NaI spectrometer coupled to a pulse pile-up rejection system measured pulse-height distributions for bremsstrahlung emitted in the forward direction, and a response matrix developed for the spectrometer was used to convert the pulse-height distributions into photon energy spectra. The shapes of the measured spectra show good agreement with those obtained by others for the same target configuration. Apparatus and experimental methods are described and thick-target bremsstrahlung spectra are presented for electron energies of 5.3, 10.0, 14.8, 16.4, 18.22, and 20.9 MeV.