Results of a Bragg Detector Built for the AMS Program

Revista de F sica Aplicada e Instrumentac a ~ o, vol. 13, no. 3, Setembro, 1998 A Bragg Detector Built for AMS Purposes G.M. Santos 1 , J.C. Acquadro 2 , R.M. Anjos 1 , N. Added 2 , R. Liguori Neto 2 , P.R.S. Gomes 1 y , M.A. Rizzuto, N.H. Medina 2 , N. Carlin 1 Instituto de F sica, Universidade Federal Fluminense, Av. Litoranea s n, Gragoata, Niteroi, R.J., 24210-340, Brazil Instituto de F sica, Universidade de S~ao Paulo, C.P. 20516, S. Paulo, 01498-970, Brazil Recebido 22 de Abril, 1998 A Bragg detector has been designed and built. In this paper, results of tests, performed with di erent scattered beams, are reported. The energy resolution of 1.7 and charge resolutions of 1.3 show that it is suitable to be used in the Brazilian AMS program. Um detetor de Bragg foi projetado e construido. Neste artigo s~ao mostrados os resultados de seus testes, realizados com diferentes feixes incidentes. Resoluc~oes em energia de 1.7 e em numero at^omico de 1.3 mostram que o detetor e apropriado para ser usado no Programa Brasileiro de AMS. I Introduction The Accelerator Mass Spectrometry AMS technique is, at the present, the most powerful method for mass spectrometry 1 . It is particularly attractive since it allows the determination of concentrations with a sen- sitivity down to 1 atom of isotope in 10 15 atoms, using few milligrams samples. The determination of these ex- tremely low concentrations of rare isotopes is accomplis- hed using a Tandem accelerator as a magnetic and elec- trostatic analyzer and detection good enough to identify unambiguously each ion. In the last few years, e orts have been devoted to the implementation of this technique in Brazil. Di e- rent institutions are involved with it. The accelerator used is the Pelletron 8UD Tandem at USP. The initial major emphases are on projects based on the determi- nation of 36 Cl, 26 Al, 14 C and 10 Be concentrations, with main applications on geological dating archaeology, cli- matology, environmental and biological studies. One may say that one of the main technical pro- blems when dealing with the ultra high sensitive AMS technique is the presence of isobaric contamination on natural samples, like 10 B in 10 Be, 26 Mg in 26 Al and 36 S in 36 Cl. In all these examples, the chemical sepa- ration of the contaminants, in the levels required for AMS purposes, is very di cult. Therefore, one needs a detection system that is able to separate elements that di er by one unit of charge in the mass energy region of interest. Complex systems, as multi-anode ionization chambers, magnetic spectrometers, electrostatic detec- tors, gas- lled magnets and velocity lters may also be used in the application of the AMS technique. Preliminarly tests of the detection of unstable be- ams were performed in the laboratory with a position sensitive ionization chamber, coupled with a silicon bar- rier position sensitive detector 2,3 . The speci c energy loss of the incident particle in the gas is proportional to its Z 2 and it is dependent on its energy. The gas de- tector measures the energy loss E and the solid state detector measures its residual energy. The electric - eld between anode and cathode is perpendicular to the particle incident direction. The analysis of the energy and the biparametric E E spectra for the 26 Al and 36 Cl measurements have shown that, although the peak positions of the isobars could be separated, the back- ground tails of 36 S and 26 Mg on the 36 Cl and 26 Al spec- tra were too high to be used for AMS purposes. The energy loss resolution of this detector is of the order Fellow of the Conselho Nacional de Desenvolvimento Cient co e Tecnol ogico - CNPq y Autor para contato: Paulo R. S. Gomes. Fone e fax:021 620 6735, E-mail:paulogom@if.u .br