THE REX-ISOLDE PROJECT R. von Hahn, M. Grieser, H. Podlech, R. Repnow, D. Schwalm Max-Planck-Institut f¨ ur Kernphysik, Heidelberg, Germany D. Habs, O. Kester, T. Sieber LMU Munich, Garching, Germany A. Schempp University of Frankfurt, Frankfurt, Germany G. Bollen, F. Ames CERN, Geneva, Switzerland U. Ratzinger GSI Darmstadt, Darmstadt, Germany L. Liljeby, K.G. Rensfelt Manne Siegbahn Institute of Physics, Stockholm, Sweden P. van Duppen Instituut voor Kern- en Stralingsfysica, Leuven, Belgium and the REX-ISOLDE collaboration Abstract REX-ISOLDE is an experiment at ISOLDE/CERN with a twofold aim: i) to demonstrate a novel efficient scheme for the acceleration of radioactive ions from the online mass separator ISOLDE. ii) to perform first nuclear physics ex- periments by studying the structure of neutron rich Na,Mg and K,Ca nuclei in the vicinity of the closed neutron shells N=20 and N=28 by Coulomb excitation and neutron trans- fer reactions. The radioactive ions are first accumulated in a Penning trap, then charge breeded to a charge to mass ra- tio of ≥1/4.5 in an Electron Beam Ion Source (EBIS) and finally accelerated up to 2.2 MeV/u. The Linac consists of a Radio Frequency Quadrupole (RFQ) accelerator, an interdigital H-type-Structure (IH) and three seven-gap res- onators, which allow to vary the energy between 0.8 and 2.2 MeV/u. This paper gives an overview of the different components of the accelerator. 1 INTRODUCTION REX-ISOLDE (Radioactive Beam EXperiment at ISOLDE) is a pilot experiment with energetic radioactive ions at the new ISOLDE (CERN) [1]. It makes full use of the low energetic 1 + -beams delivered from the on-line mass separator ISOLDE. These ions will be accelerated on the basis of a radically new concept (see fig. 1) . By continuous injection into a Penning trap (REX-TRAP), the 1 + ions are accumulated for 20 ms and cooled by collisions with a buffer gas. Bunches of 10 μs length with improved emittance are then transferred to the EBIS. The dense electron beam of the EBIS, focussed by a strong magnetic field of a solenoid strips the ions up to the stripping limit determined by the electron energy. After a breeding time of 20 ms a 100 μs long bunch is finally extracted. As the intensity of the radioactive ions is about orders of magnitudes smaller than the intensity of ions from the residual gas, a mass separator between the EBIS and the front end of the LINAC is required. The mass separator consists of an electrostatic 90 0 cylinder deflector and a 90 0 magnetic bender build up in a vertical S-shape. This sys- tem has been calculated to third order, providing a q/A- resolution of 1400. The seperator is described more pre- cisely in ref. [2, 3]. The following Linac allows to vary the ion- energy between 0.8 and 2.2 MeV/u. The ions are finally bend onto the target, which might be surrounded by an efficient Ge-detector array (MINIBALL). 2 THE LINEAR ACCELERATOR 2.1 The 4-rod-RFQ The first acceleration stage of the REX-ISOLDE LINAC is a 4-rod-RFQ which is designed to accelerate radioactive ions with a charge-to-mass ratio larger than 1/4.5 from 5 keV/u to 300 keV/u. The rf quadrupole field provides trans- verse focusing for the low energy ions while a modulation of the four rods performs a smooth bunching and accelera- tion of the injected 100 μs bunch. The 4-rod-RFQ is a well tested structure [4], which is already used in the GSI HLI- LINAC [5] and the Heidelberg high-current injector [6]. As the lay-out of the REX-ISOLDE-RFQ is very conservative the maximum voltage which can be obtained should even provide acceleration of ions with charge-to-mass ratios as small as 1/6.5. With a shunt impedance of 100 kΩm the expected power consumption will be about 60 kW. Design calculations with PARMTEQ have been per- formed to minimize the divergence of the beam at the exit of the RFQ. It turned out that one can avoid a rapid increase of the radial beam size in the drift tube between the end of the rods and the first triplet of the matching section. Con- necting the RFQ to the IH-structure the matching section consists in total of two triplets and a rebuncher. 947 0-7803-4376-X/98/$10.00  1998 IEEE