Acquadro, J.C.

Added, N.

Anjos, R.M.

Carlin, N.

Gomes, P.R.S.

Liguori Neto, R.

Medina, N.H.

Rizzuto, M.A.

Santos, Guaciara

English

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleA Bragg Detector Built for AMS PurposesPermalinkhttps://escholarship.org/uc/item/3q10s6dvJournalRevista de Física Aplicada e Instrumentação, 13(3)AuthorsSantos, GuaciaraAcquadro, J.C.Anjos, R.M.et al.Publication Date1998-09-03 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaRevista de Fsica Aplicada e Instrumentac~ao, vol. 13, no. 3, Setembro, 1998 39A Bragg Detector Built for AMS PurposesG.M. Santos1, J.C. Acquadro2, R.M. Anjos1;, N. Added2;, R. Liguori Neto2;,P.R.S. Gomes1;y, M.A. Rizzuto, N.H. Medina2;, N. Carlin1;1 Instituto de Fsica,Universidade Federal Fluminense,Av. Litoranea s/n, Gragoata, Niteroi, R.J., 24210-340, Brazil2Instituto de Fsica, Universidade de S~ao Paulo,C.P. 20516, S. Paulo, 01498-970, BrazilRecebido 22 de Abril, 1998A Bragg detector has been designed and built. In this paper, results of tests, performed withdierent scattered beams, are reported. The energy resolution of 1.7% and charge resolutions of1.3% 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 seustestes, realizados com diferentes feixes incidentes. Resoluc~oes em energia de 1.7% e em numeroatômico de 1.3% mostram que o detetor e apropriado para ser usado no Programa Brasileiro deAMS.I IntroductionThe Accelerator Mass Spectrometry (AMS) techniqueis, at the present, the most powerful method for massspectrometry[1]. It is particularly attractive since itallows the determination of concentrations with a sen-sitivity down to 1 atom of isotope in 1015 atoms, usingfew 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 identifyunambiguously each ion.In the last few years, eorts have been devoted tothe implementation of this technique in Brazil. Die-rent institutions are involved with it. The acceleratorused is the Pelletron 8UD Tandem at USP. The initialmajor emphases are on projects based on the determi-nation of 36Cl, 26Al, 14C and 10Be concentrations, withmain 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 AMStechnique is the presence of isobaric contamination onnatural samples, like 10B in 10Be, 26Mg in 26Al and36S in 36Cl. In all these examples, the chemical sepa-ration of the contaminants, in the levels required forAMS purposes, is very dicult. Therefore, one needs adetection system that is able to separate elements thatdier by one unit of charge in the mass energy regionof interest. Complex systems, as multi-anode ionizationchambers, magnetic spectrometers, electrostatic detec-tors, gas-lled magnets and velocity lters may also beused in the application of the AMS technique.Preliminarly tests of the detection of unstable be-ams were performed in the laboratory with a positionsensitive ionization chamber, coupled with a silicon bar-rier position sensitive detector [2,3]. The specic energyloss of the incident particle in the gas is proportionalto its Z2 and it is dependent on its energy. The gas de-tector measures the energy loss E and the solid statedetector measures its residual energy. The electric -eld between anode and cathode is perpendicular to theparticle incident direction. The analysis of the energyand the biparametric E E spectra for the 26Al and36Cl measurements have shown that, although the peakpositions of the isobars could be separated, the back-ground tails of 36S and 26Mg on the 36Cl and 26Al spec-tra were too high to be used for AMS purposes. Theenergy loss resolution of this detector is of the orderFellow of the Conselho Nacional de Desenvolvimento Cientco e Tecnologico - CNPqyAutor para contato: Paulo R. S. Gomes. Fone e fax:(021) 620 6735, E-mail:paulogom@if.u.br40 G.M. Santos et al.of 4%. Therefore, we decided to change the detectionsystem.A new detector has been designed and built at UFFand USP: a Bragg detector (BD)[4,5]. This kind of de-tector has been used before[6,7] for AMS purposes. Itis expected to have good energy and charge resolutionover a wide range of atomic number and energy. In thispaper we report on the performance of this detector.Section 2 describes the general characteristics of a BD.In section 3 the design of our BD is described. Resultsof tests are shown in section 4. In section 5 a summaryis presented.II Bragg detector general cha-racteristicsA Bragg detector has an applied electric eld parallel tothe incident particle direction, and enough gas pressureto stop it[4,5]. This last property allows the determina-tion of the range of particle in the detection medium,and its total energy. The nuclear charge of the parti-cle can be obtained by determining the portion of theenergy that the particle has lost over a given path len-gth and the spatial distribution of electrons producedalong the path of the particles in the gas (the so-calledBragg curve). The range of the particle is proportionalto its A=Z2 ratio. The time dependence of the ioniza-tion current collected at the anode is related with theBragg curve and, therefore, the range of the particles isdetermined by the time length of the signal. Identicalparticles at dierent energies dier in their range buthave roughly the same specic ionization in their Braggmaxima. Projectiles with dierent nuclear charges havedierent specic ionization at their Bragg maxima. Forisobars, the ratio of their range is equal to their Z2ratio. Therefore, the nuclear charge of the particle isobtained by integration of the specic ionization aroundthe Bragg peak, while the total energy is obtained, asusual, from the total ionization current.The discrimination between the ionization near theBragg maximum (charge identication signal) and thetotal ionization (total energy signal) is obtained by theuse of two main ampliers, with dierent shaping ti-mes, after the split of the output signal from the pre-amplier. For the total ionization, the shaping time islonger than the total electron collection time (> 6s),while the other one operates with shaping time of theorder of the collection time of the electrons from theBragg maximum (from 0.1 to 0.5 s).III Detector DesignFigure 1 shows a schematic layout of our Bragg de-tector. It has a cylindrical geometry and contains 12equipotential guard rings separated by ceramic rings.The detector assembly is a spare part of the S. Paulo's8UD NEC Pelletron accelerator tube, made of stainlesssteel. Figures 2a and 2b are pictures of the detector.The diameter and length of the active detector volumeare 5.3 cm 15.5 cm, respectively. The entrance window,made of aluminized mylar foils, works as cathode. Theinner diameter of the window is 2.7 cm and its thicknessis 1.7 m (230 /cm2).Figure 1. Schematic layout of the Bragg detector.Figure 2a. Picture of the Bragg detector parts.Revista de Fsica Aplicada e Instrumentac~ao, vol. 13, no. 3, Setembro, 1998 41Figure 2b. Picture of the Bragg detector.The electrons produced by the particle in gas ioni-zation process drift to the anode and start inducing asignal when they cross the Frisch grid. The grid con-sists of parallel 20 m tungsten wires coated with ashof Au, separated by 1 mm. A constant drift velocity isobtained applying a voltage to the rings, which producea homogeneous electric eld along the drift direction.The rings are 12 mm apart from each other. The Frischgrid eciency is 98.3% and it is located at 10 mm fromthe anode and 155 mm from the cathode (active depth).A chain of 11 resistors of 100 K is used as a voltagedivider between the Frisch grid and the cathode. Theanode consists of a Cu round plate with 65 mm diame-ter. In order to avoid charge collection on the Frischgrid, the electric eld on the region between the gridand the anode must be higher than in the region bet-ween the cathode and the grid.The gas used is the usual P 10 (90% Ar, 10% CH4).For the beam energies of interest, 8  Z  17, 45MeV  ELab  72 MeV, calculations performed by theTRIM[8] code have shown that the gas pressure requi-red to stop the detected particles are within the rangefrom 120 to 200 Torr.Table 1 summarizes the design parameters of ourBragg detector.cTable 1. Design parameters of the Bragg detectorcathode to grid distance 155 mmgrid to anode distance 10 mmgrid wire diameter 20mgrid wire spacing 1 mmcathode window diameter 27 mmcathode window thickness 1:7m (230 m/cm2)grid eciency 98.3%geometrical transmission 97.5%gas P 10 (10% CH4, 90% Ar)dIV Detector PerformanceThe rst tests of the BD were performed with beamsof 28Si, 35;37Cl and 32S (Z = 14, 16, 17). The voltageterminal was kept xed, and therefore, due to their dif-ferent charge states, the beams had dierent energieson a 197Au target. The beam energies used were in therange from 45 MeV to 72 MeV.During the rst tests, the BD was xed at the ex-tremity of a large multipurpose scattering chamber, at15 with the beam direction. The gas pressure was setat 165 Torr. During the tests we have noticed that ithad changed within 10% of this value, causing some de-terioration in the detector performance. A new controlof the gas pressure is being built. Further tests, with35Cl, 37Cl and 36S beams, were performed with the de-tector placed at zero degree with the beam direction, asshown in gure 3. A much better gas pressure stabilitywas achieved using a higher gas ux.Tests have been performed varying the detector,grid and anode voltages. The optimum conditions wereachieved for a detector bias of 1000 V; Frisch grid andanode voltages at 100 V and 200 V, respectively. Thesevalues lead to a cathode-Frisch grid eld of 73.3 V/cm42 G.M. Santos et al.and a grid - anode eld of 100 V/cm. For the gas pres-sure of 165 Torr, this corresponds to reduced elds of0.61 V/cm Torr and 0.44 V/ cm Torr, respectively. Theratio grid-anode eld / cathode-grid eld, for these con-ditions, is 1.37.Figure 3. The detector placed at zero degree with the Pe-lletron beam direction.Best values for amplier shaping time were alsochecked. For the total energy signal, the values of 3 and6 s leads to similar resolutions, and the longer one wasthen used. For the charge signals, a short shaping timeis required in order to the signals be independent of theparticle energies. That was achieved for the shortestshaping time of the Mechtronics 510 amplier, 0.2 s.Figure 4a shows the biparametric spectra of theBragg Peak versus Total Energy for the 28Si, 37Cl and32S scattered beams, and the detector placed at 15.Actually this gure shows the superposition of 3 die-rent spectra, one for each beam. The beam energies arein the range from 64 MeV to 68 MeV. One can noticethat the Z signals are independent of the energy foreach beam and that unique Z separation is obtained,with very good separation of the elements. The projec-tion of the spectra on the Bragg peak axis is shown ingure 4b.Figure 5c shows the superposition of biparametricspectra of 35Cl, 37Cl and 36S beams, taken with thedetector at zero degree and energies of 64.0 MeV, 60.5MeV and 62.4 MeV, respectively. Figures 5a and 5bare the projection energy and energy loss spectra, res-pectively. One can notice that the 36S counts are wellseparated from the 35;37Cl, as it should be from the36Cl.The detector energy resolution E(FWHM) / Eachieved was 1.7% for these beams. The charge re-solution resolution Z(FWHM) / Z was derived to be1.3%. The separation between the positions of the pe-aks of two adjacent elements( (C17 - C16) / C17) is8.6%.Figure 4a. Biparametric spectra of the Bragg Peak versus Total Energy for the 28Si, 37Cl and 32S scattered beams, andthe detector placed at 15. The beam energies are in the range 64 - 68 MeV. An auxiliary pulser response is present inthe spectra. Figure 4b. The projection of the spectra of gure 4a on the Bragg peak axis, with energy calibration of 0.30MeV/channel.Revista de Fsica Aplicada e Instrumentac~ao, vol. 13, no. 3, Setembro, 1998 43Figure 5. Superposition of spectra of 35Cl, 37Cl and 36S beams, taken with the detector at zero degree and energies of 64.0MeV, 60.5 MeV and 62.4 MeV, respectively. (5a)- the projection energy- with energy calibration of 0.34 MeV/channel (5b)-the projection energy loss spectra- with energy calibration of 0.41 MeV/channel (5c)- the biparametric spectra.V ConclusionsWe have designed and built a Bragg detector. Re-sults obtained in the tests show that it has a very goodperformance, better than the previously used positionsensitive ionization chamber, and comparable with re-ported Bragg detector performances in the literature[4,5,6]. It is ready to be used for Nuclear Reactionsand high precision AMS measurements, since it canseparate neighbor charges and isobars. We intend toimprove the gas pressure control unit and to test theoptimum thickness of the window. Finally, tests withdierent beam energies will be performed, in order tostudy the dependence of the resolution with the rangeof the particles.References1 Proceedings of the Intern. Conf. on AMS; Nucl. Instr.Meth. in Phys. Res. B 52 (1990); B 92 (1994); B 123(1997).2 C. Tenreiro, R.M. Anjos, J.C. Acquadro, G. Kremer,G. Ramirez; Nucl. Instr. Meth. in Phys. Res. B 92,89 (1994).3 G.M. Santos, J.C.Acquadro, R. M. Anjos, P.R.S. Go-mes; C. Tenreiro, R. Liguori Neto, M. M. Coimbra,C.R. Apolloni, A.M.M. Maciel, N.H. Medina, M.A. Ri-zutto, N. Carlin Filho; Nucl. Instr. Meth. in Phys. Res.B 123, 34 (1997).4 C.R. Gruhn, M. Binimi, R. Legrain, R. Loveman, W.Pang, M. Roach, D.K. Scott, A. Shotter, T.J. Symons,J. Wouters, M. Zisman, R. Devries, Y.C. Peng, W.Sondheim; Nucl. Instr. Meth. 196, 33 (1982).5 C. Schessl, W. Wagner, K. Hartel, P. Kienle, H.J. Ko-ner, W. Mayer, K.E. Rehm; Nucl. Instr. Meth 192, 291(1982).6 M.F. Vineyard, B.D. Wilkins, D.J. Henderson, D. G.Kovar, C. Beck, C.N.Davids, J.J. Kolata; Nucl. Instr.Meth. in Phys. Res. A 255, 507 (1987).7 H.R.Andrews, G.C. Ball, R.M. Brown, R.J.J. Cor-nett, W.G. Davies, B.F. Greiner, Y. Irnahori, V.T. Ko-slowsky, J.McKay, G.M. Milton, J.C.D. Milton, K.W.Allen; Nucl. Instr. Meth. in Phys. Res. B 52, 243(1990).8 J.F. Ziegler, J.P. Biersack and U. Littmark; The Stop-ping and Range of Ions in Solids Pergamon Press, NewYork (1985).

A Bragg Detector Built for AMS Purposes

https://escholarship.org/content/qt3q10s6dv/qt3q10s6dv.pdf?t=sgps7r

A Bragg Detector Built for AMS Purposes

Abstract

Similar works

Full text

Available Versions

eScholarship - University of California