Molecular break up process under low pressure conditions: characterization of a gaseous stripper for the implementation of a low energy AMS

O objetivo desse trabalho foi estudar a viabilidade de adaptação do Implantador Iônico da Universidade de São Paulo para a técnica de Accelerator Mass Spectrometry (AMS), tendo em vista as baixas energias utilizadas por este acelerador de partículas. A técnica de AMS, amplamente utilizada para a análise de Carbono-14 no estudo de datação de fósseis, requer que algum componente do acelerador garanta a quebra de moléculas de massa 14, contaminantes no processo de medição dos átomos de carbono com massa 14. Com a utilização de energias da ordem de dezenas de keV no acelerador, o estudo do processo de quebra de moléculas para estas energias foi realizado através do projeto, construção e caracterização de um stripper gasoso. Para caracterizar o stripper implementado foram realizadas medidas de seção de choque para troca de carga do feixe e quebra molecular em função da pressão de gás injetado no stripper. Também investigou-se a influência do átomo utilizado como gás, através de três diferentes gases injetados no stripper: Hélio, Argônio e Xenônio. Alguns feixes posivos foram produzidos no Implantador para o estudo destas seções de choques: Ar+, Ar(2+), CO+, CO2(+) e O2(+). O projeto do stripper foi idealizado para minimizar a variação de pressão no interior do implantador, visando preservar as condições da fonte de íons. Curvas de perfil de pressão de gás dentro do tubo do stripper foram calculadas segundo as teorias da Tecnologia do Vácuo que permitiram tanto a estimativa desta variação quanto a quantidade de gás no interior do stripper. Também foram realizados estudos do perfil de pressão em stripper com outras geometrias (cônicas abertas e fechadas), buscando estimar a otimização da espessura do stripper em função da injeção de gás em sua base. Baseando-se nos resultados, foram apontadas mudanças necessárias no atual estágio de adaptação desse acelerador de partículas para que se torne possível a concepção de um sistema AMS em sua linha de pesquisa.The present work aimed on studying the feasibility of adapting the Ion Implanter of University of Sao Paulo to the Accelerator Mass Spectrometry technique, taking into account the low energies employed by this type of particle accelerator. The AMS technique, largely applied to the Carbon-14 analysis for fossil dating, requires the breaking of molecules with mass 14 by some component in the accelerator, since these lead to interference on the Carbon-14 counting process. By employing energies on the level of keV in the accelerator, the study of the breaking process of the molecules for this energy was accomplished by means desiring, building and characterization of a gaseous stripper. In order to characterize the installed stripper, measurements were taken of the charge state exchange and molecular break up process as a function of the pressure of the gas injection into the stripper. The influence of the atom employed as gas was also investigated. The experiments were realized with the following different gases: Helium, Argon and Xenon. Some positive beams were produced on the implanter: Ar+, \\Ar(2+), CO+, CO2(+) and O2(+). The designed stripper was idealized to minimize the pressure variation on the inside of the implanter in order to preserve the conditions of the ion source. Pressure profiles of the gas inside the stripper were calculated according to the Vacuum Technology theory, which allowed estimating the quantity of gas inside the stripper. Studies on the pressure profile for different stripper geometries (open and closed conic forms) were also carried out to estimate the optimization of the stripper thickness as a function of the gas injection on its base. Based on the results, specifications for further work and changes on the current system were listed to make it possible to implement the AMS system

Physics with Positron Beams at Jefferson Lab 12 GeV

Positron beams, both polarized and unpolarized, are identified as essential ingredients for the experimental program at the next generation of lepton accelerators. In the context of the Hadronic Physics program at the Jefferson Laboratory (JLab), positron beams are complementary, even essential, tools for a precise understanding of the electromagnetic structure of the nucleon, in both the elastic and the deep-inelastic regimes. For instance, elastic scattering of (un)polarized electrons and positrons off the nucleon allows for a model independent determination of the electromagnetic form factors of the nucleon. Also, the deeply virtual Compton scattering of (un)polarized electrons and positrons allows us to separate unambiguously the different contributions to the cross section of the lepto-production of photons, enabling an accurate determination of the nucleon Generalized Parton Distributions (GPDs), and providing an access to its Gravitational Form Factors. Furthermore, positron beams offer the possibility of alternative tests of the Standard Model through the search of a dark photon or the precise measurement of electroweak couplings. This letter proposes to develop an experimental positron program at JLab to perform unique high impact measurements with respect to the two-photon exchange problem, the determination of the proton and the neutron GPDs, and the search for the $A^{\prime}$ dark photon