ISOL beams of hafnium isotopes and isomers

The production of ISOL beams of hafnium is described. Radioactive Hf isotopes were produced at ISOLDE by 1.4 GeV proton-induced spallation in Ta and W foils. Chemical evaporation in form of HfF₄ and mass separation in the molecular sideband HfF₃⁺ after electron impact ionization provided intense and pure beams. Beams of ^158-185Hf and short-lived isomers down to 1.1 s ^177mHf were observed, but the method could be extended to reach even more exotic isotopes: down to about ¹⁵⁴Hf (N=82) on the neutron-deficient side and up to neutron-rich ¹⁸⁸Hf.     

Target-ion source unit ionization efficiency measurement by a method of stable ion beam implantation

At the IRIS facility a rather precise method of the target-ion source unit ionization efficiency measurement has been developed. The method exploits an off-line mass-separator for the implantation of the ion beams of selected stable isotopes into a tantalum foil placed inside a Faraday cup in the focal plane of the mass-separator. After the implantation of the required amount of the investigated species, tantalum foil has been inserted into the volume of the target-ion source unit prepared for the on-line utilization at the IRIS on-line separator. The first tests have ensured the ionization efficiency values (90±10)% for Rb and (85±10)% for Cs in the empty combined target-ion source unit, which was used as a reference one. For the target-ion source unit with UC target material inside the measured value of the ionization efficiency of Rb was (52±20)%.     

Integrated target-ion source unit for on-line production of radioactive short-lived isotopes

A new version of integrated target-ion source unit (ionising target) has been developed for the on-line production of radioactive single-charged ions. The target is able to withstand temperatures up to 2500 ^°C and acts also as an ion source of the surface and laser ionisation. Off-line and on-line experiments with the ionising target, housing tantalum foils as a target material, have been carried out at the IRIS (Investigation of Radioactive Isotopes on Synchrocyclotron) facility. The off-line surface ionisation efficiency measured for stable atoms of Li, Rb and Cs was correspondingly 6% , 40% and 55% at the target temperature of 2000 ^°C and 3-10% for atoms of rare-earth elements Sm, Eu, Tm and Yb at a temperature of 2200 ^°C. The off-line measured values of the ionisation efficiency for stable Gd and Eu atoms by the laser beam ionisation inside the target were 1% and 7%, respectively. The radioactive beam intensities of neutron-deficient rare-earth nuclides from Eu to Lu produced by the integrated target-ion source unit have been measured at a temperature of 2500 ^°C. The results of the integrated target-ion source unit use for on-line laser resonance ionisation spectroscopy study of neutron-deficient Gd isotopes have been also presented.     

Recent developments and on-line tests of uranium carbide targets for production of nuclides far from stability

The capacity of uranium carbide target materials of different structure and density for production of neutron-rich and heavy neutron-deficient nuclides have been investigated. The yields of Cs and Fr produced by a 1 GeV proton beam of the PNPI synchrocyclotron and release properties of different targets have been measured. Yields and release efficiencies of Cs and Fr produced from a high density UC target material and from low density UCx prepared by the ISOLDE method at IRIS in the collaboration with PARRNe group from Orsay are compared. The yields from ISOLDE original target are presented for comparison as well.     

Parallel trends observed in the dynamics and kinetics of the family of reactions O(3P)+XY→OX+Y (X,Y, halogen atoms) using the quasi-classical trajectory method

Summary We calculate several features in the dynamical behaviour of the reactions O(³P)+XY→OX+Y (XY=Br₂, BrCl) using the quasi-classical trajectory method. The results are compared to analogous results for the reactions O(³P)+Cl₂, I₂. The similarities observed in this homologous series of reactions indicate that the dynamics are influenced to a significant degree by the long-range repulsive forces between the reactants.     

Production of low- and high-energy radioactive-ion beams by fragmentation

The physics opportunities made possible by beams of rare isotopes are among the richest available in nuclear science. The rare-isotope accelerator (RIA) now under development is an innovative accelerator that will define the state of the art for all such facilities. A novel aspect of the RIA project is the conversion of the most intense high-energy heavy-ion beams into both fast and reaccelerated exotic beams. Along with target fragmentation in next-generation high-power ISOL targets, RIA will use projectile fragmentation in a high-energy separator/gas-filled ion collector system to provide an extensive range of thermalized ions for reacceleration. In addition, a second high-energy separator will provide the same or larger range of ions for high-energy experiments. A brief overview of the RIA accelerator concept, the layout of the facility, and production techniques will be given along with information on the present R&D efforts in gaseous-ion collection. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: morrissey@nscl.msu.edu     

Reacting polymers with highly correlated initial conditions

We propose and theoretically study an experiment designed to measure short-time polymer reaction kinetics in melts or dilute solutions. The photolysis of groups centrally located along chain backbones, one group per chain, creates pairs of spatially highly correlated macroradicals. We calculate time-dependent rate coefficients κ(t) governing their first-order recombination kinetics, which are novel on account of the far-from-equilibrium initial conditions. In dilute solutions (good solvents) reaction kinetics are intrinsically weak, despite the highly reactive radical groups involved. This leads to a generalised mean-field kinetics in which the rate of radical density decay - ∼S(t), where S(t) ∼t ^{- (1 + g/3)} is the equilibrium return probability for 2 reactive groups, given initial contact. Here g≈ 0.27 is the correlation hole exponent for self-avoiding chain ends. For times beyond the longest coil relaxation time τ, - ∼S(t) remains true, but center of gravity coil diffusion takes over with rms displacement of reactive groups x(t) ∼t ^1/2 and S(t) ∼ 1/x ³(t). At the shortest times ( t 10^-6s), recombination is inhibited due to spin selection rules and we find ∼tS(t). In melts, kinetics are intrinsically diffusion-controlled, leading to entirely different rate laws. During the regime limited by spin selection rules, the density of radicals decays linearly, n(0) - n(t) ∼t. At longer times the standard result - ∼d ³(t)/d (for randomly distributed ends) is replaced by ∼d2x ³(t)/d ² for these correlated initial conditions. The long-time behavior, t > τ, has the same scaling form in time as for dilute solutions. Received 18 May 2000     

First results from laser spectroscopy on bunched radioactive beams from the JYFL ion-beam cooler

A new RFQ ion-beam cooler and buncher, installed after the mass-separating magnet of the ion guide isotope separator, IGISOL, JYFL, has dramatically increased the scope of on-line laser spectroscopy at this facility. The device, operated in a bunching mode, has permitted new measurements on short-lived radionuclei in the Ti, Zr and Hf chains at a sensitivity two orders of magnitude greater than that previously achieved. The device has also opened new prospects for laser-based nuclear spectroscopy at the facility, particularly collinear resonance ionisation spectroscopy. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: pc@mags.ph.man.ac.uk     

Photofission for the production of radioactive beams: Experimental data from an on-line measurement

A PARRNe 1 experiment (Production d'Atomes Radioactifs Riches en Neutrons) aimed at the production of neutron-rich radioactive noble gases produced by photofission has been performed at CERN. The LEP Pre-Injector (LPI) has been used to deliver a 50 MeV electron beam. The results obtained show clearly that the use of an electron beam to produce neutron-rich fission fragments for futur RNB facilities is an option that should not be neglected. Received: 20 July 2001 / Accepted: 5 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: ibrahim@ipno.in2p3.fr Communicated by D. Guerreau     

High-precision measurement of the 14O half-life using a mass-separated radioactive beam

A high-precision measurement of the ¹⁴O half-life has been performed using a mass-separated radioactive beam in combination with a germanium detector set-up. This is the first ¹⁴O half-life measurement with a contamination-free source. The final result of 70.560 ± 0.049 seconds is in agreement with the generally adopted mean value. Received: 5 April 2001 / Accepted: 6 August 2001     

Feasibility of high power refractory metal-foil targets for EURISOL

EURISOL foil-targets have to withstand a primary proton beam of 1 GeV kinetic energy and up to 100 μA beam current. These foil targets will be based on previous high-power target concepts, i.e. the RIST target [J.R.J. Bennett et al., Nucl. Instrum. Meth. Phys. Res. B 126, 117 (1997)] or high power targets used at TRIUMF [P. Bricault et al., Nucl. Instrum. Meth. Phys. Res. B 204, 319 (2003), M. Dombsky et al., Nucl. Instrum. Meth. Phys. Res. B 204, 191 (2003)]. A single target unit is capable of dissipating up to 25 kW, hence, several target units can be merged together by individual transfer lines to one common ion source. The single target units will be irradiated by a proton beam in a time sharing mode to distribute the primary proton beam current to the individual target units. In this feasibility study the necessary properties of high-power foil targets are discussed and the requirements to design a foil target according to the proton beam parameters [CITE] for the future EURISOL facility are given.     

Neutron-rich rare isotope production below the Fermi energy and its application to the Texas A&;M RIB upgrade

Recent efforts to produce and separate neutron-rich rare isotopes in peripheral collisions below the Fermi energy are presented. The experiments have been performed at the Cyclotron Institute of Texas A&M University employing beams from the K500 Superconducting Cyclotron. Two magnetic separators were used: the MARS recoil separator and the Superconducting Solenoid Line (BigSol Line). An enhanced production of neutron-rich nuclides in comparison with high-energy fragmentation mechanisms has been observed and attributed to the role of the N/Z and the nuclear periphery of the target. From a practical viewpoint, these reactions below or around the Fermi energy offer a novel way to access very neutron-rich rare isotopes. The experience obtained in the production of rare isotope beams (RIB) below the Fermi energy will be exploited in the ongoing RIB upgrade of the Cyclotron Institute that will involve production, stopping and reacceleration of rare isotopes.     

Radioactive beam facilities: European perspectives

During the last ten years, the cooperation in Nuclear Physics between the different European countries has significantly increased, in particular through extensive discussions initiated by NuPECC, the Nuclear Physics European Collaboration Committee. In this context, perspectives in physics with radioactive beams have been reviewed and a high priority has been given to the developments of performing facilities in Europe including both in-flight and ISOL facilities. In this presentation, the main challenges are recalled and the various existing national and transnational projects are reviewed as well as the accompanying research and development programs. A special attention is given to the EURISOL project, a European program aiming at a preliminary design study of a second-generation European ISOL radioactive beam facility. Received: 1 May 2001 / Accepted: 4 December 2001     

Physical preseparation: A powerful new method for transactinide chemists

In recent years, the concept of physical preseparation of single atoms was introduced into the field of transactinide chemistry. In this approach, the transactinide element of interest is isolated in a physical recoil separator and then extracted from this machine. The beam as well as the unwanted reaction products are strongly suppressed, allowing for the investigation of new chemical systems that were not accessible before. The most important aspects of the technique are discussed and the advantages for chemistry experiments with transactinides are presented, using some of the chemical studies that were performed with preseparated isotopes as examples.     

Target ion source and charge exchange cell development for the EXCYT facility

The EXCYT facility at the INFN-LNS is based on a K-800 superconducting cyclotron delivering stable ion beams on a Target Ion Source (TIS) assembly to produce the required nuclear species, and on a 15 MV Tandem for post-accelerating the radioactive beams. For some ion beams such as for Li, the extraction efficiency from the TIS is higher when obtained by positive ionisation, while the injection into the Tandem is possible only after a charge exchange to obtain negative ions. In this work we present the procedures together with the results of the production of ^6,7,8,9Li beams extracted at EXCYT during the last year. The production of the radioactive elements was performed by sending a ¹³C⁴⁺ primary beam of 45 MeV/u on a graphite target. The ionisation of the production species was achieved by a tungsten positive surface ioniser. The Li⁺ has been extracted from TIS at different energies to cross-check the transmission and the charge exchange efficiency. To perform the conversion from positive to negative ions we employed a Charge Exchange Cell (CEC) containing Cs vapours. The Li beam interacts with the latter in a two-step reaction, thus converting its charge from +1 to –1. The CEC was already characterised during off-line tests; the results obtained at EXCYT confirmed both the isotopic shift effect and the efficiency values at several given extraction energies. Future improvements of the TIS and the CEC are discussed.     

Hyperthermal surface-collisions of water cluster cations

Size-selected, protonated water cluster cations (H₂O)_nH⁺, 4 n 32, are scattered at normal incidence from the surface of a diamond-coated silicon wafer at collision energies 0 E _coll 500 eV. The size distribution of collision-induced fragment-ions and the ion yield of scattered particles are analyzed, using a secondary time-of-flight mass spectrometer, as a function of the cluster size, n, and the collision energy, E _coll. Even at low impact energies only very small fragment-ions can be detected, with a maximum fragment size of 35% of the colliding parent cluster ions. For clusters consisting of more than 10 molecules, the protonated water dimer (H₂O)₂H⁺ becomes the predominant fragment-ion. The total charge survival yield obeys a nonlinear increase with cluster size; for the largest clusters investigated, more than 35% of the impacting ions survive the surface collision in the cationic charge state.     

Prospects for exotic beam facilities in North America

There are several nuclear physics laboratories in North America that have on-going research using energetic and stopped radioactive beams. These include the large ISOL-type programs ISAC at TRIUMF and HRIBF at Oak Ridge and the in-flight fragmentation program at the NSCL of Michigan State University. There are also smaller, more specialized, programs using a variety of techniques at the 88-inch cyclotron of Berkeley, ATLAS at Argonne, the Cyclotron Institute of Texas A&M University, the Nuclear Structure Laboratory at Notre Dame University, and the Nuclear Structure Laboratory at SUNY/Stony Brook. There are also three projects on the horizon in North America for new capabilities in both the near term and more distant future. The intensities of the in-flight fragment beams at the NSCL will be increased dramatically very soon as the Coupled Cyclotron Project will be completed and commissioned for research by mid-2001. A new project, ISAC-II, has been approved in Canada. For the longer term, the United States is considering construction of a major new facility, the Rare Isotope Accelerator (RIA), which would have a very high-intensity heavy-ion driver linac. The RIA facility is proposed to utilize both ISOL and in-flight production mechanisms. Received: 1 May 2001 / Accepted: 4 December 2001     

Yields of neutron-rich isotopes around Z = 28 produced in 30 MeV proton-induced fission of 238U

Heavy ^65-70Co, ^68-74Ni, ^70-76Cu and ^74-81Ga isotopes were produced at the LISOL facility by means of 30 MeV proton-induced fission of ²³⁸U. Production rates were deduced and compared to two types of cross-section calculations: the empirical model (V. Rubchenya, private communication) and the PROFI code. Comparison with experimental data favors the latter model. Yields using different beam-target combinations and different energies are calculated and discussed. Received: 11 February 2002 / Accepted: 26 April 2002     

New low-energy RIB separator CRIB for nuclear astrophysics

An in-flight RIB separator at low energies, which is the first extensive separator at low energies, called CRIB, is just under construction at CNS. This consists of a double-achromatic magnetic separator, a window-less gas target, and a Wien filter. Some characteristics of the CRIB are described. Possible experimental plans are also discussed in our nuclear astrophysics project for the study of the explosive hydrogen burning process, especially on the onset mechanism. Received: 1 May 2001 / Accepted: 4 December 2001     

Collision-induced fragmentation and neutralization of methanol cluster cations

This contribution addresses the inelastic interaction of positively charged molecular cluster ions with a solid surface at kinetic energies up to 30 eV/molecule. We report experimental results on the scattering of mass-selected, protonated methanol cluster cations (CH₃OH)_nH⁺, n = 4-32, off a diamond-coated silicon surface. In particular we provide fragment size distributions of methanol cluster ions following their impact on the target, as well as surface-induced neutralization probabilities of methanol cluster ions as a function of the size and the kinetic energy of the parent clusters. Received 30 November 2000