E. Kugler

The new CERN-ISOLDE on-line mass-separator facility at the PS-Booster

The ISOLDE on-line isotope separators have been operated since 1967 at the CERN-SC. This 600 MeV proton synchro-cyclotron had to be shut down in December 1990 after 33 years of service and it was decided to move ISOLDE to a new experimental area. The new on-line mass-separator facility is now under construction at the CERN PS-Booster. This accelerator provides an average current of about 2-mu-A of 1 GeV protons in very short high intensity pulses at low repetition rate. The beam can hit either one of the two target stations, the general purpose separator (GPS), a reconstructed ISOLDE-2 type machine (which can deliver beams simultaneously into three beam lines), and the high resolution separator (HRS), which is essentially the slightly modified ISOLDE-3 separator. The central GPS beam line and the HRS feed a common beam transport system to which most of the experiments will be connected. The new facility will be taken into operation in spring 1992.

Pulse shape of the ISOLDE radioactive ion beams

Abstract The 2.4 μs proton pulses from the PS Booster are delivered to the ISOLDE targets at a low repetition rate (typically 0.4 Hz). However, the synchronously produced radioisotopes have partially lost this time structure due to the delay incurred by the mass transfer processes used in their conversion into an ion beam. Since the pulse shape of the ion bunches is a vital information for the target development and the experiments, new techniques have been developed to measure it. These methods as well as the mathematics used for calculating the production yield will be described in detail. The limits of the techniques are discussed and a set of examples are presented.

CHEMICALLY SELECTIVE LASER ION SOURCE OF MANGANESE

Abstract Ion beams produced by on-line isotope separators are often contaminated by abundantly produced isobars. This significantly complicates, and in a number of cases renders impossible, the realization of physical experiments with such beams. In many cases the purity of the ion beams may be improved on the ionization stage of the isotope separation using a laser resonance ion source. The method of laser resonance ionization of atoms in a hot cavity has been applied for chemically selective production of Mn ion beams at the CERN-ISOLDE facility. Radiation of pulsed dye lasers and copper vapor lasers provided three-step excitation and ionization of Mn isotopes in the ordinary W surface ionizer, connected to the target container. High values of efficiency and selectivity obtained with the laser ion source made it possible to suppress substantially an admixture of 57 Fe in the 57 Mn ion beam and to perform Mossbauer experiments on implanted impurities in solids.

Methods for Production of Intense Beams of Unstable Nuclei: New Developments at ISOLDE

An overview of progress in the target and ion source techniques at the ISOLDE on-line mass separator is given. The production of high intensity beams of mass-separated radioactive nuclei by bombardment of targets with 600 MeV protons and 910 MeV 3He from the CERN synchro-cyclotron is discussed. Off-line tests performed in order to clarify the release properties of different target materials are described. The targets are metal powders or foils, alloys, carbides, oxides, intermetallic compounds or molten metals. The influence of reactive gases on the release rates and progress in ion-source techniques are also discussed. Recent on-line tests are described in details, and systems are suggested for the production of elements which are not yet available as primary products in on-line mass separators.

Recent developments of the ISOLDE laser ion source

The ISOLDE laser ion source (LIS), in which stepwise resonant laser ionization is performed inside a high-temperature cavity, has been extended to include the elements Be, Zn, Cu and Cd. The ionization efficiencies obtained are discussed with respect to the level of saturation of each step achieved with the available laser beam intensities. Because of a high ionization potential of about 9 eV the first resonant transition of Be, Zn and Cd is in the far ultraviolet (UV) region. The UV laser beam needed for the first resonant step of Be, Zn and Cd is achieved in a new laser setup via frequency tripling of the dye laser light. The ions created during the 30 ns laser pulse are extracted from the hot cavity of the ion source by the electrical field resulting from the ohmic heating of the cavity. The time distribution of the laser-ionized ion bunches was measured for various cavities and temperatures. The time structure of the ion bunch is discussed in order to gain insight into the development of LIS cavities.

New production systems at ISOLDE

Abstract New target systems for the ISOLDE on-line mass separator facility are presented. Targets of carbides, metal/graphite mixtures, foils of refractory metals, molten metals and oxides have been tested. Beams of high intensity of neutron-rich isotopes of a large number of elements are obtained from a uranium carbide target with a hot plasma-discharge ion source. A target of ZrO 2 has been shown to provide high intensity beams of neutron-deficient isotopes of Mn, Cu, Zn, Ga, Ge, As, Se, Br, Kr and Rb, while a SiC target with a hot plasma ion source gives intense beams of radioactive isotopes of a number of light elements. All these systems are rather chemically unselective. Chemically selective performance has been obtained for several systems, i.e.: the production of neutron-deficient Au from ( 3 He, pχn) reactions on a Pt/graphite target with a hot plasma ion source; the production of neutron-deficient Lu and LuF + and Hf and HfF 3 + from a Ta-foil target with a hot plasma ion source under CF 4 addition; the production of neutron-deficient Sr as SrF + and Y as YF 2 + form a Nb-foil target with a W surface ionizer under CF 4 addition; the production of neutron-deficient Se as COSe + from a ZrO 2 target with a hot plasma ion source under O 2 addition; and the production of radioactive F from a SiC target with a hot plasma ion source operating in Al vapour.

Use of refractory oxides, carbides and borides as targets for on-line mass separation

Abstract Ion beams of a number of elements have been produced at the CERN-ISOLDE facility by 600-MeV proton bombardment of thick targets consisting of a variety of metal oxides, carbides and borides. The nuclear reaction products are released from the target material at high temperature and continuously transferred to the ion source of the ISOLDE on-line mass separator. The materials studied are CaB6, ScC2, LaC2, TaC, ThC2, UC2, MgO, CaO, BaO, and ThO2. The obtained beam intensities are of the elements He, C, N, Ne, Mg, Al, Cl, Ar, Ca, Br, Kr, I, Xe, Ba, At, Rn. The intensities are high enough to facilitate the study of hitherto unknown nuclei.

Release from ISOLDE molten metal targets under pulsed proton beam conditions

Abstract By moving the ISOLDE mass separators from the 600 MeV Synchrocyclotron (SC) to the 1 GeV Proton-Synchrotron-Booster (PS) [1] the instantaneous energy density of the proton beam went up by 3 orders of magnitude. The developments of the molten metal target units and the optimization of the PS proton beam to cope with the effects of the thermal shocks induced by the proton beam are described. The energy density of the PS proton beam was reduced by spatial defocusing and time staggered extraction of the four PS-accelerators. The release from lanthanum, lead and tin targets is discussed for different settings of the proton beam and compared to the release observed at ISOLDE-SC. The yields of Hg isotopes are presented.

Release and yields from thorium and uranium targets irradiated with a pulsed proton beam

Abstract Radioactive ion-beams produced from various uranium and thorium targets at the ISOLDE mass separator facility are presented. The targets are bombarded with 1 GeV protons delivered by the PS-Booster at CERN. The target materials are uranium carbide and thorium carbide, both prepared from the oxides or diphtalocyanines of uranium and thorium. Due to the pulsed structure of the PS-Booster, release can be measured as a function of time for a number of elements. Beam intensities and delay time distributions of the alkali-metals, the earth-alkalis, the boron group and the noble gases measured from the described targets are discussed. Comparisons are made between the target materials and with ISOLDE beam intensities produced by 600 MeV protons from the CERN synchrocyclotron.

Recent development of high-temperature metal targets for ISOLDE

Abstract The improved target and ion-source system to be used at the SC-ISOLDE on-line mass separators is presented. Tests of a number of refractory metals with respect to their use as target materials are described. The materials discussed are pure metals and mixtures of graphite and platinum-like metals. Rapid release of a number of nuclear reaction products from powders and foils of titatium and thorium is shown. The release properties of rhenium and iridium are described. The enhanced release of refractory elements obtained from these metals by means of reaction with CF4 are related to the importance of in-grain diffusion versus surface desorption. The difference in performance between powder and foil targets is discussed. On-line tests of some of these materials combined with new high-temperature plasma-discharge type ion sources are described and the potential use of yet untested systems is discussed.