H. Podlech

Safe Coulomb Excitation of 30Mg

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the island of inversion.

Accelerated radioactive beams from REX-ISOLDE

In 2001 the linear accelerator of the Radioactive beam EXperiment (REX-ISOLDE) delivered for the first time accelerated radioactive ion beams, at a beam energy of 2 MeV/u. REX-ISOLDE uses the method of charge-state breeding, in order to enhance the charge state of the ions before injection into the LINAC. Radioactive singly-charged ions from the on-line mass separator ISOLDE are first accumulated in a Penning trap, then charge bred to an A/q < 4.5 in an electron beam ion source (EBIS) and finally accelerated in a LINAC from 5 keV/u to energies between 0.8 and 2.2 MeV/u. Dedicated measurements with REXTRAP, the transfer line and the EBIS have been carried out in conjunction with the first commissioning of the accelerator. Thus the properties of the different elements could be determined for further optimization of the system. In two test beam times in 2001 stable and radioactive Na isotopes (Na-23-Na-26) have been accelerated and transmitted to a preliminary target station. There Ni-58- and Be-9- and H-2-targets have been used to study exited states via Coulomb excitation and neutron transfer reactions. One MINIBALL triple cluster detector was used together with a double sided silicon strip detector to detect scattered particles in coincidence with gamma-rays. The aim was to study the operation of the detector under realistic conditions with gamma-background from the beta-decay of the radioactive ions and from the cavities. Recently for efficient detection eight tripple Ge-detectors of MINIBALL and a double sided silicon strip detector have been installed. We will present the first results obtained in the commissioning experiments and will give an overview of realistic beam parameters for future experiments to be started in the spring 2002.

'Safe' Coulomb excitation of Mg-30

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the island of inversion.

The 7-gap-resonator–accelerator for the REX-ISOLDE-experiment at CERN

Abstract The Radioactive Beam Experiment at ISOLDE (REX-ISOLDE-Experiment) which presently is being developed and under construction at CERN serves to investigate exotic, very neutron rich, radioactive nuclei [1] (Radioactive beam EXperiment at ISOLDE: Coloumb Excitation and Neutron Transfer Reactions of Exotic Nuclei, Proposal to the ISOLDE commitee, CERNSIC94-25). A linear accelerator delivers radioactive ions which are produced by the isotope separator ISOLDE with energies between 0.85 and 2.2 MeV/u. The Linac will consist of a RFQ-accelerator, an interdigital H-Structure (IH) and three 7-gap-resonators with variable energy. While the LMU Munich is responsible for the frontpart of the accelerator, the backpart is being built by the MPI [2] (H. Podlech, Master Thesis, MPI-H-V21-1997, Heidelburg, 1997). After estimation of the voltage of one resonator to 1.75 MV at 90 kW, the design velocities were fixed to 5.4%, 6.0% and 6.6% of the velocity of light. Three downscaled models (1:2.5) were built in order to optimize the shuntimpedance and the field-distribution at the operation frequency of the amplifiers of 101.28 MHz. The optimization of all low power resonators is now successfully finished. Extensive beam dynamic calculations were made in order to optimize the transmission of the beam up to the target. It turned out that final energies between 0.85 and 2.2 MeV/u with nearly 100% transmission can be achieved. The acceptance in the x-plane is 1.2π mm mrad (norm.) and in the y-plane 3.0π mm mrad (norm.). The bunchlength of the fully accelerated beam (2.2 MeV/u) is 2.4 ns at the target. The development of the resonators was accompanied by extensive MAFIA calculations. It could be demonstrated that spiral-resonators like 7-gap-resonators can be calculated with MAFIA with an accuracy of 1% in comparison with experimental results. Presently, the tanks and the half shells of the power type resonators are manufactured in the workshops of the MPI.

The 7-gap-resonator accelerator for the REX-ISOLDE project at CERN

Abstract The REX-ISOLDE experiment which is presently under construction at CERN is intended to investigate exotic, very neutron-rich, radioactive nuclei. Axa0linear accelerator will deliver radioactive beams which are produced by the isotope separator ISOLDE, with energies between 0.85 and 2.2xa0MeV/u. The linac will consist of an RFQ accelerator, an interdigital H-structure (IH) and three 7-gap-resonators for variable final energy. The 7-gap-resonators are being operated at a frequency of f =101.28xa0MHz with a pulsed power of P =90xa0kW (duty cycle 1:10). After the fabrication of the power type resonators, high-power tests were performed in order to prove the design voltage of 1.74xa0MV at 90xa0kW rf power. These tests were carried out with a 32 S beam and showed that the 7-gap-resonators exceed their design voltage; voltages between 1.77 and 1.88xa0MV were reached. In this paper the status of the 7-gap-resonator accelerator and the results of the high-power test are reported.

The RFQ-accelerator for the Heidelberg high current injector

Abstract At the MPI fur Kernphysik, a new type of accelerator is under construction which consists of a CHORDIS ion source [1] , two RFQ and eight 7-gap resonators [2] . The High Current Injector will deliver 1–3 orders of magnitude higher intensities in comparison to the existing tandem-postaccelerator combination. The RFQ is designed to accelerate ions with a minimum charge to mass ratio of q / m =1:9 up to an energy of 0.478 MeV/u. Both resonators operate at a frequency of 108.48 MHz with a pulsed power of 90 kW (25% duty factor (DF)). By means of an optimizing algorithm, it was possible to lower the electrode voltage of the RFQ-accelerator from 71 kV to 60 kV maintaining a slightly reduced final energy and a particle transmission of about 80% with ion currents of 10 mA [3] . A new electrode profile was developed in order to combine sufficient cooling with good mechanical stability. Moreover the fabrication process of the RFQ-stems and the electrodes were simplified and optimized. Both resonators are tuned and tested with low level measurements and under high power with a H + 2 -ion beam.

Status of the REX-ISOLDE project

The radioactive beam experiment REX-ISOLDE, a pilot experiment testing a new concept of post acceleration of radioactive ions at ISOLDE/CERN is in progress. Singly charged radioactive ions delivered by the online mass separator ISOLDE are accumulated in a Penning trap (REX trap), charge bred in an electron beam ion source (EBIS), separated from the residual gas in a mass separator and then accelerated in a linac with output energies between 0.8 and 2.2 MeV/u. The REX trap is in operation, a first test beam was already injected. The design phase of the EBIS is finished and the construction has been started. The superconducting magnet is delivered. The linac consists of a radiofrequency quadrupole (RFQ) accelerator, an interdigital IH-structure and 3 seven gap resonators to vary the final energy.

RES-ISOLDE - Post-accelerated radioactive beams at CERN-ISOLDE

The ISOLDE RIB-facility at CERN has today been producing a vast range of radioactive beams since more than 30 years. The low-energy beams of ISOLDE will be complemented by a post-accelerator, REX-ISOLDE, currently being assembled. In order to convert the pseudo-DC, singly-charged beam from the ISOLDE mass separators into a cooled and bunched beam at higher charge states, a novel scheme of trapping, cooling, and charge-state breeding has been devised, using a linear Penning trap and an Electron Beam Ion Source (EBIS). This allows for subsequent acceleration by a short, cost-effective LINAC consisting of an RFQ, an IH-structure and three seven-gap resonators, reaching 0.8–2.2 MeV/u. The installation of REX-ISOLDE is well underway and the first post-accelerated radioactive beams are expected to be obtained during late 2000.

The REX-ISOLDE project

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 experiments 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 transfer reactions. The radioactive ions are first accumulated in a Penning trap, then charge breeded to a charge to mass ratio of /spl ges/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 resonators, 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.