High-Accuracy Mass Determination of Unstable Cesium and Barium
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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN-EP/99-12 15 February 1998 High-Accuracy Mass Determination of Unstable Cesium and Barium Isotop es a b c;1 d;2 F. Ames , G. Audi , D. Beck , G. Bollen , b a;3 c c M. de Saint Simon , R. Jertz , H.-J. Kluge , A. Kohl , c;4 b e f a;d M. Konig , D. Lunney , I. Martel , R.B. Mo ore , T. Otto , g d;5 f ;d Z. Patyk , H. Raimbault-Hartmann , G. Rouleau , a;6 a;7 c a G. Savard , E. Schark , S. Schwarz , L. Schweikhard , a;8 h;9 H. Stolzenb erg , J. Szeryp o , and d the ISOLDE Collab oration a Institut fur Physik, Johannes Gutenberg-Universitat, D-55099 Mainz, Germany b CSNSM-IN2P3-CNRS, F-91405 Orsay-Campus, France c GSI, Postfach 110552, D-64220 Darmstadt, Germany d CERN, CH-1211 Geneva 23, Switzerland e Instituto de Estructura de la Materia, CSIC, Madrid, Spain f Foster Radiation Laboratory, McGil l University, Montreal, H3A 2B1, Canada g Soltan Institute for Nuclear Studies, Hoza _ 69, PL-00- 681 Warsaw, Poland h Institute of Experimental Physics, Warsaw University, PL-00-681 Warsaw, Poland PACS numb er: 21.10.Dr, 27.60.+j, 32.10.Bi, 07.75.+h Abstract Direct mass measurements of short-lived Cs and Ba isotop es have b een p erformed with the tandem Penning trap mass sp ectrometer ISOLTRAP installed at the on- line isotop e separator ISOLDE at CERN. Typically, a mass resolving power of 600 000 and an accuracy of m 13 keV have b een obtained. The masses of 123;124;126 122m Ba and Cs were measured for the rst time. A least-squares adjustment has b een p erformed and the exp erimental masses are compared with theoretical ones, particularly in the frame of a macroscopic-microscopic mo del. 117142 123128;131;138144 Key words: ATOMIC MASSES Cs, Ba; Measured masses. On-line mass sp ectrometry.Penning trap. Least-squares adjustment of data. Macroscopic-microscopic mass mo del calculations. (submitted to Nuclear Physics A) 1 Intro duction The binding energy of the atomic nucleus is one of the most fundamental prop erties of such a many-body system. Accurate mass data serve as testing grounds for nuclear mo dels and stimulate their further improvement. Further- more, systematic investigation of the binding energy as a function of proton and neutron number allows the direct observation of nuclear prop erties like pairing, shell and subshell closures, as well as deformation e ects, and leads to a deep er understanding of nuclear structure. Large e orts are presently de- voted, at several nuclear physics lab oratories around the world, to apply new mass sp ectrometric techniques, such as time-of- ight, Smith-RF or Schottky mass sp ectrometry, for the accurate mass determination of short-lived isotop es far from the valley of b eta stability [1]. A particularly successful approach has proven to b e the use of a Penning trap as a mass sp ectrometer. Here we rep ort on results obtained by ISOLTRAP which is a Penning trap mass sp ectrometer installed at the on-line mass sep- arator facility ISOLDE at CERN in Geneva. During its rst decade of op era- tion ISOLTRAP has b een steadily improved and presently o ers an accuracy 7 in mass determinations of 10 or b etter. It is applicable to all ion b eams available at ISOLDE of isotop es with half-lives down to ab out one hundred milliseconds. Highly accurate mass measurements far from stabilityhave to date b een car- ried out on isotopic chains of alkali and alkali earth isotop es [2,3], on rare earth isotop es [4] and most recently on neutron de cient mercury isotop es 208 and isotop es of various elements in the vicinityof Pb [5,6]. In this pap er we rep ort on mass measurements on radioactive cesium and 117142 123128;131;138144 barium isotop es for Cs and Ba. Typically, the masses were determined with a resolving power of R 600 000 and an accuracy of 1 present address: Instituut voor Kern- en Stralingsfysica, Celestijnenlaan 200 D, B-3001 Leuven, Belgium 2 corresp onding author: georg.b [email protected], fax/phone: +41 22 767 8990/5825 3 present address: Bruker Franzen Analytik GmbH, Fahrenheitstr. 4, D-28359 Bre- men, Germany 4 present address: Data Care AG, Eisenstrae 58, D-65428 Rsselsheim 5 present address: Dresdner Bank AG, D-60301 Frankfurt, Germany 6 Argonne National Lab oratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA 7 present address: Arthur Andersen, Mergenthalerallee 10-12, D-65760 Eschb orn 8 present address: Baub o den Systemhaus GmbH, Munchfeldstr.1-5, D-55122 Mainz, Germany 9 present address: Lab oratoire de Physique Corpuskulaire de Caen, F-14050 Caen Cedex, France 2 122 m 123;124;126 m 13 keV. The masses of Cs and Ba have b een measured for the rst time. The measurements have b een carried out in several b eam times spread over a p erio d of several years. In the present pap er the complete set of results will b e presented and discussed, including the results of the very rst 123;125;127;131 series of measurements already published [2] and the data on Ba published recently [4]. Except for the latter barium isotop es all data were included in the latest up date of the mass tables of 1995 [7]. The adjustment pro cedure will be discussed as well as the large impact of the ISOLTRAP measurements on the general evaluation of atomic masses. The mass data will b e compared with the results of nuclear mass mo dels. Work will b e presented in which the ISOLTRAP data have b een used for a lo cal adjustment of a macroscopic-microscopic mass formula. 2 The Principle of Penning Trap Mass Determination The ion storage metho d has b ecome a widely applied technique. The theory of the Penning trap and its applications are describ ed in detail in a number of publications [8{13]. Therefore only a brief description will be given here, concentrating on the principle of Penning trap mass sp ectrometry as applied in the case of ISOLTRAP. Charged particles are stored in a Penning trap by a sup erp osition of a homo- geneous magnetic eld and an axial electrostatic quadrup ole eld. The motion of an ion con ned in such an ideal trap can be describ ed as a sup erp osition of three indep endent harmonic oscillations: an axial oscillation with frequency along the magnetic eld lines and two circular motions p erp endicular to z the magnetic eld with frequencies (reduced cyclotron motion) and + (magnetron motion). All frequencies dep end on the trapping voltage but in a uniform magnetic eld and a pure quadrup olar electrostatic eld the sum of the two radial frequencies equals the cyclotron frequency given by c q + = = B : (1) + c 2m It has b een shown that by using a radio-frequency (RF) azimuthal quadrup ole it is p ossible to excite the ion motion directly at the sum frequency + + [12,13]. If the ions are initially prepared in a pure magnetron orbit the application of such an RF eld at this frequency with well chosen amplitude c and interaction time T will convert the ion motion in the radial plane into a RF pure cyclotron motion. This is accompanied by a large increase in the kinetic energy in the radial plane, which is used to detect the resonance by a time-of- ight technique [14]: The ions are ejected from the trap after excitation and are allowed to drift through the fringe eld of the magnet to a channel plate 3 detector where the time of ight is measured. During their adiabatic passage through the inhomogeneous part of the magnetic eld the radial energy gained by the RF excitation is converted into axial energy. This leads in resonance to a reduction in the time of ight from the trap to the detector. The resolving p ower of this exp eriment is determined by the interaction time T with the RF eld. In the measurements rep orted here the interaction time RF was generally chosen to be T = 0.9 s. This corresp onds to a Fourier-limit RF of ab out 1.2 Hz for the half-width of the cyclotron resonances and a resolving power of R = = (FWHM) 600 000 for nuclides in the mass region c c of interest here. In o -line measurements a resolving power of R 8 000 000 was obtained for stable Cs by using a longer interaction time (T = 12 s) RF and a corresp ondingly lower amplitude for the RF excitation. The resolving powers given here corresp ond to mass resolutions of m(FWHM) 200 keV and m(FWHM) 13 keV, resp ectively. 3 Exp erimental Setup and Pro cedures The tandem Penning trap mass sp ectrometer ISOLTRAP was rst installed at the on-line mass separator ISOLDE-I I at CERN. After the move of ISOLTRAP to the new PSB-ISOLDE facility [15], a numb er of mass measurements in the cesium and barium isotopic chains have b een rep eated and new determinations have b een p erformed. Figure 1 is a schematic of the setup as it was used for most of the measurements rep orted here. It consisted of two main parts, each adapted to a well de ned task. The rst part was a Penning trap in an electromagnet.