H.L. Ravn

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.

Chemically selective laser ion-source for the CERN-ISOLDE on-line mass separator facility

Abstract Radioactive atoms produced in proton-induced nuclear reactions and released from thick targets have been ionized resonantly by laser radiation in a hot tube connected to the target container. Pulsed tuneable lasers with a repetition rate as high as 10 kHz have been applied for stepwise resonant excitation and photoionization in the last step. In this way the efficiency and selectivity of the target and ion source system which serves as an injector to the on-line isotope separators at CERN-ISOLDE could be improved. In a series of off-line and on-line studies the ionization of Sn ( E i = 7.3 eV), Tm ( E i = 6.2 eV), Yb ( E i = 6.2 eV) and Li ( E i = 5.4 eV) was investigated. An ionization efficiency of up to 15% was obtained for Yb. The ratio of the laser-ionized and surface-ionized ion currents was measured as a function of temperature for different ionization cavity materials (W, Ta, Nb and TaC). It was shown that this ratio, i.e. the selectivity, rises for Tm from 10 to 10000 with falling temperature and is strongly dependent on the material. Since the lasers are pulsed the ion beam becomes bunched with a pulse width of about 10–50 μs. This width is strongly dependent on the potential drop along the tube (caused by the electric current used for heating the tube) and on the alignment of the laser beams with respect to the tube axis. The selectivity could be further improved by a factor of 10 using gated detection of the bunched ion beam.

Ion source with combined cathode and transfer line heating

Abstract The plasma-discharge ion-source concept used in connection with the high temperature thick targets at the CERN-ISOLDE facility is described. Derived from the construction of the tubular surface ionizer, the same current which heats the transfer line between the target and the ion source is also used to heat the disc-shaped cathode. This construction has the advantage that the less volatile nuclear reaction products are transferred to the center of the source by diffusion along a rising temperature gradient, so that adsorption losses on the walls are minimized. The design, which exists in several versions dependent on the element to be ionized and the coupling between the target and ion source, is discussed and examples of the measured efficiencies are given.

The ISOLDE laser ion source for exotic nuclei

At the ISOLDE on line mass separator a system of copper vapor lasers and dye lasers serves for resonant ionization of atoms inside a hot cavity attached to the target. Radioactive ion beams of Yb, Ag, Mn, Ni, Zn, Be, Cu, Cd and Sn were produced with the Resonance Ionization Laser Ion Source (RILIS). Two and three step excitation schemes are used, providing an ionization efficiency of about 10%. Thanks to the use of the RILIS it became possible to ionize beryllium efficiently at ISOLDE, and all particle stable Be isotopes could be separated for the first time. Separation of Ag and Cu nuclear isomers was achieved in the ion source by appropriate tuning of the laser wavelength. New isotopes of Ag, Mn, Zn, Cd and Sn were found, including the r process “waiting point” nucleus 129Ag.

New techniques at ISOLDE-2

Abstract Experiments at the reconstructed on-line isotope separator facility ISOLDE-2 at the CERN 600 MeV synchro-cyclotron are now in progress. Over the past year the extracted proton beam has been raised to 1.3 >A, and the design value of 5 >A is now within reach. The target and ion-source technology developed before the reconstruction has been further improved so as to permit the utilization of the high proton-beam intensities and to increase the range of elements available for study to 30. With a proton-beam intensity of 0.7 >A the radioactive Cs+ beams after mass separation have increased by a factor of 300 and, in the most favoured cases, are exceeding 6 nA. This permits the visualization of the separated radioactive ion beams by means of the usual beam scanners. A new target concept, a finely divided metal powder at ultra-high temperatures, features a short delay component which enhances the yield of very short-lived products. In the case of a niobium powder target at 2200°C for the production of Rb, the new N = Z nuclide 74 Rb (T 1 2 = 59 ms ) could be identified with a collection rate of 450 atoms per second. The new beam-optical system supplies four measuring stations simultaneously and allows up to ten groups to take data during an ISOLDE run (typically 100 h). The over-all performance and the stability of this system are perhaps best illustrated with the on-line spin measurements: about 20 new spins in isotopes of Rb, Cs, and Au have been measured by means of an atomic beam magnetic resonance (ABMR) apparatus. This Gothenburg-Uppsala experiment requires the ion beam to be directed onto its oven and kept steady within an area of 0.5 mm2. The joint set-up (ISOLDE+ABMR) acts as an instrument that gives a beam of nuclei selected in Z, A, and spin value.

Target Techniques for the isolde on-line isotope separator

Abstract New target and ion-source systems have been developed for an isotope separator on-line (ISOLDE-2) with the 600 MeV CERN synchro-cyclotron. The system constitutes a further development of the molten-metal target for production of alkali elements by means of surface ionization. The design has been modified to permit the use of much larger proton-beam intensities than before, and the method has been extended to new regions of elements. The paper lists 27 elements, which either have already been separated or which are believed to be readily available with the techniques described here. Two different types of surface-ionization sources are used to produce alkali, alkaline-earth and rare-earth elements from molten-metal targets. A new series of target systems for noble-gas production is considered, where the molten metals are connected with a plasma ion source. An improved version of the molten Pb target has been developed, where higher yields of the short-lived Hg isotopes have been obtained by electromagnetic stirring of the target melt. Finally, we have begun to consider the use of targets involving irradiation and/or transfer of chemical compounds. Preliminary results from on-line separation of Po and At from molten ThF 4 ·LiF are reported.

Short-lived isotopes of alkali and alkaline-earth elements studied by on-line isotope separator techniques

Abstract Ion beams of many isotopes of Rb, Cs, Ba, Fr, and Ra have been produced at the ISOLDE facility, CERN, by bombarding heavy element targets with 600 MeV protons, followed by on-line surface ionization and electromagnetic mass separation. Overall production yields are reported, and the prospects for measuring formation cross-sections of short-lived nuclear species are discussed, taking the production of Cs from La as an example. The following half-lives of new nucleides have been determined: 75 Rb (21±3) sec, 76 Rb (36·8±1·5) sec, 116 Cs (3·6±0·2) sec, 136m Cs (19±2) sec, 227 Fr (2·4±0·2) min, 228 Fr (39±1) sec, 229 Fr(50±20) sec, 229Ra (4·0±0·2) min. The systematics of average beta ( β + , EC) strength functions has been updated.

Intense Beams of Radioactive Halogens Produced by Means of Surface Ionization

Abstract A negative surface-ionization source has been developed for on-line separator use in order to make intense ion beams of nuclear-reaction produced halogens. It consists of a planar LaB 6 surface onto which the mixed volatile nuclear reaction products are allowed to impinge. A transverse permanent magnetic field and an intermediate electron catcher electrode, inserted before the separator extraction gap, is used to separate the ion and electron fractions. The efficiency of the source is shown to be up to 50% for the halogens, Br and I. i.e. very similar to that obtained by the same process for positive ionization of alkalis on a tungsten surface. During periods of several weeks the source has been used for a number of on-line experiments and has allowed the identification of a series of new nuclei. Yields and halflives of 42,43 Cl and 70,71,72m,76m,93,94 Br are reported.

Decay of Neutron-Rich Mn Nuclides and Deformation of Heavy Fe Isotopes

The use of chemically selective laser ionization combined with {beta} -delayed neutron counting at CERN/ISOLDE has permitted identification and half-life measurements for 623-ms {sup 61}Mn up through 14-ms {sup 69}Mn . The measured half-lives are found to be significantly longer near N=40 than the values calculated with a quasiparticle random-phase-approximation shell model. Gamma-ray singles and coincidence spectroscopy has been performed for {sup 64,66}Mn decays to levels of {sup 64,66}Fe , revealing a significant drop in the energy of the first 2{sup +} state in these nuclides that suggests an unanticipated increase in collectivity near N=40 . {copyright} {ital 1999} {ital The American Physical Society }

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.