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The energy dependence of the optical potential for the elastic scattering of 6 He on 209 Bi at near and subbarrier energies is studied. Elastic angular distributions and the reaction cross section were simultaneously fitted by performing some modifications in the ECIS code. A phenomenological optical model potential with the WoodsSaxon form was used. There are signatures that the so-called breakup threshold anomaly (BTA) is present in this system having a halo projectile 6 He, as it had been found earlier for systems involving stable weakly bound nuclei.One of the most widely used methods for studying the interplay between different reaction mechanisms at near barrier energies is the study of the energy dependence of the optical potential. This energy dependence is produced by polarization potentials originated from the coupling between the elastic scattering and different reaction mechanisms, such as inelastic excitation, transfer of nucleons or clusters of nucleons, breakup, fusion, fission, and quasifission. These mechanisms may produce polarizations of different signs, attractive or repulsive. The net effect on the energy dependence of the total optical potential depends on the importance and strength of the different specific polarization potentials. For systems containing only tightly bound nuclei, usually the most important couplings are inelastic excitations of target and projectile and the transfer of nucleons or clusters of nucleons between them. For these systems, an attractive polarization potential is produced, and, consequently, they usually show enhancement of the fusion cross section at energies near and below the Coulomb barrier, when compared with predictions from one-dimensional barrier penetration models (or no-coupling calculations). For these systems, the energy dependence of the optical potential shows the so-called threshold anomaly (TA), which manifests itself as a decrease in the strength of the imaginary potential as the incident energy decreases toward and below the nominal Coulomb barrier. As the real and imaginary parts of the nuclear potential are connected through the dispersion relation, due to the general principle of causality [1,2], this behavior of the imaginary potential is accompanied by a bell shape of the real part of the potential, at the same energy region.When weakly bound nuclei are involved in the interaction, the breakup channel may remain open at energies below the Coulomb barrier, with very large cross section. Then, the imaginary part of the potential does not vanish near or even below the barrier, since an important reaction channel does not close at this energy regime. Actually, for stable weakly bound nuclei, such as 6,7 Li and 9 Be, it has been observed [3-5] that the strength of the imaginary part of the optical potential increases * lubian@if.uff.br as the energy is reduced toward the nominal Coulomb barrier. From this characteristic of the imaginary potential and because of the dispersion relation, the strength of the real part of the potential decreases at...

Este trabalho aborda conceitos associadosà interação da radiação eletromagnética com a matéria, através de uma aplicação experimental de baixo custo, capaz de gerar imagens digitais comumente utilizadas emáreas de medicina nuclear, radiodiagnósticos, ciências ambientais, indústria, etc. Além de aproximar ao funcionamento básico de tecnologias inovadoras, esta prática também tem por objetivo permitir ao aluno compreender as leis físicas associadas, que normalmente são apresentadas nas disciplinas teóricas, mas que o aluno geralmente tem dificuldade de compreensão. O kité baseado na detecção da radiação gama, abordando fenômenos associados a interação de fótons com a matéria, leis do inverso do quadrado da distância e de atenuação de um feixe de fótons,ângulo sólido e resolução de imagens. Palavras-chave: interação de fótons com a matéria, radiação gama, contador Geiger-Müller, geração de imagens This work addresses concepts associated with the interaction of electromagnetic radiation with matter, through a low cost experimental application able to generate digital images commonly used in nuclear medicine, radiodiagnostics, environmental science, industry, etc. Besides approaching the basic concepts of innovative technologies, this practice also aims to enable students to understand the physical laws associated, which are usually presented in theoretical subjects, but students often have difficulty understanding. The kit is based on the gamma detection, addressing phenomena associated with the interaction of photons with matter, laws of the inverse square and attenuation of photons, solid angle and imaging resolution. Keywords: Interaction of photons with matter, gamma detection, Geiger-Müller counter, imaging IntroduçãoAspectos da Física das Radiações são indispensáveis para permitir que estudantes de diversasáreas adquiram uma compreensão mais abrangente de conceitos físicos necessários para o entendimento do avanço de pesquisas científicas e tecnologias inovadoras. O mais elementar dos casos talvez seja o uso de conceitos provenientes da interação da radiação com a matéria na construção de modelos de transferência de energia para o meio e na elaboração de processos * Endereço de correspondência: meigikos@if.uff.br.analíticos na indústria e na medicina. Os resultados dessas interações permitem descrever, por exemplo, forças resultantes que atuam em sistemas quânticos: a interação de fótons pela ação de campos eletromagnéticos, atuando sobre partículas carregadas do meio; partículas carregadas que têm sua ação em elétrons do meio aproximada por interações Coulombianas consecutivas; nêutrons que atuam sobre prótons e nêutrons de núcleos atômicos pela força nuclear forte [1]. Por outro lado, estes mesmos conceitos fazem parte da vida cotidiana dos alunos, sendo vastamente aplicados nasáreas de radiotera-

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