On-line production of neutron rich isotopes from uranium carbide targets

Experiments done using on-line mass separation of neutron-rich nuclei produced by 1 GeV proton beam on natural U by spallation reactions are performed to study the characteristics of different UCx targets. Special attention is devoted to reliably extract isotopic yields in case of complex decay schemes and to the valuation of release efficiency of some isotopic chains. New gamma-branching measuremet for ⁹²Rb are obtained. In addition, a formalism is developed to disantangle the contribution of β-decay in the target from direct production by fission and to provide more consistent data for the interpretation of release curves. Some results are shown for the Cs and Rb isotopic chains.     

Fission fragment production from uranium carbide disc targets

A possible solution for a target system aimed at the production of exotic nuclei as a result of high-energy fissions in ²³⁸U compounds has been analyzed. The configuration proposed is constituted by a primary proton beam (40 MeV, 0.4 mA) directly impinging on a UC₂ multiple-disc target inserted within a cylindrical tungsten box. In order to extract the fission fragments, the tungsten box has to be kept at 2000 ^°C. This system has been conceived to obtain both a high number of fission fragments (about 2 ^. 10¹³ atoms/s) and a quite low power deposition in the target. The power release and the fragment distribution have been calculated by means of the Monte Carlo code MCNPX. The thermal analysis of the proposed configuration shows the capability of the thermal radiation to cool the discs with a reasonable margin below the material melting point. Moreover, the possibility of increasing such margin with simple modifications of the target design is shown by means of parametric analyses. The thermal analysis of the tungsten box, also cooled by radiation, points out the necessity to heat it and/or shield it thermally, in order to take it at the requested temperature. Preliminary calculations of the target-induced activity have also been performed.     

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.     

The SPES direct UCx target

A possible solution for a target system aimed at the production of exotic nuclei as a result of high energy fissions in ²³⁸U compounds has been analyzed. The proposed configuration is constituted by a primary proton beam (40 MeV, 0.2 mA) directly impinging on uranium carbide disks inserted within a cylindrical carbon box. This system has been conceived to obtain both a high number of neutron rich atoms (originated from about 10¹³ fissions/s) and a low power deposition in the target. In order to extract the fission fragments, the box has to be hold at 2000^○C. The thermal analysis shows the capability of the thermal radiation to cool the disks with a reasonable margin below the material melting point. Moreover, the analyses of the thermo-mechanical behaviour and of the effusion times confirm the promising features of this target configuration.     

Multifoil UCx target for the SPES project --An update

The target system is one of the key issues for the facilities aimed at the production of neutron-rich radioactive ion beams. In the framework of the SPES project (Study for the Production of Exotic Species), the possibility of using a target configuration with a proton beam (40MeV, 0.2mA) directly impinging on multiple uranium carbide disks is investigated. The ²³⁸U fission fragments constitute the source for the exotic beams and for this purpose the disks are placed inside a graphite box at 2000 ^°C. The target is split into several thin disks in order to allow the cooling of the system by thermal radiation. In this way about ∼ 10¹³ fissions s ^-1 are obtained with a relative simple system and with relative low costs. Further steps have been performed compared to previous publications and now all the main parameters of the system have been analysed by means of calculation codes: the fission rates and the fission fragment distribution; the power deposition and the thermal analysis; the thermo-mechanical behaviours of the disks; the effusive and diffusive extraction release properties of the target.     

Release time calculations for the SPES direct UCx target

A Direct Target for a mid-term RIB ISOL-type facility is being developed at LNL, in the framework of the R&D for the SPES project [1]. Using a 40 MeV proton beam impinging on a UCx thick target of 2.5 g/cm³ density, a production rate of 10¹³ in target fissions per second is expected [2]. The crucial point, when short-lived isotopes are produced in the target, is to build systems (target + ion source) with good release properties and high efficiency. Monte Carlo simulations were performed using the GEANT4 toolkit [3] and the RIBO code [4] in order to optimize our target geometry and to estimate the average release time.     

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     

Production of neutron-rich isotopes in fission of uranium induced by neutrons of 20 MeV average energy

In the context of a parameter study conducted by several laboratories for future European radioactive beam facilities based on fast-neutron induced fission, in particular for the SPIRAL-II project at GANIL, we have measured the yields of neutron-rich isotopes in the mass range of 88 to 144. These nuclei were obtained as fission products of natural uranium bombarded by neutrons of 20 MeV average energy emitted by a thick carbon target irradiated by 50 MeV deuterons. Yields have been measured using on-line mass separation with the ion-guide method. Compared with proton-induced fission at 25 MeV the magnitude of cross-sections, except for the symmetric region, is similar. Z-distributions of isobars have the same width, 0.7 charge units, but their maxima are shifted by about 0.8 charge units, favouring production of the neutron-richer isobars. Our data allow calculations of absolute cross-sections for fission of natural uranium induced by neutrons of about 20 MeV. Received: 10 July 2000 / Accepted: 27 October 2000     

A facility upgrade at Texas A&;M University for accelerated radioactive beams

The Cyclotron Institute at Texas A&M University is carrying out an upgrade project which will lead to accelerated radioactive ion beams at intermediate energies. The project involves recommissioning a K150 cyclotron for acceleration of stable beams which will be used to produce radioactive ions. Both light-ion and heavy-ion guides will be used to stop and transport the radioactive ions to a charge breeding electron cyclotron resonance ion source. Following charge breeding, highly-charged ions will be injected into the K500 cyclotron, accelerated and then transported to existing equipment to carry out experiments.     

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     

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.     

Extraction of thermalized projectile fragments from gas

The NSCL gas cell and quadrupole ion-guide system has been used to study the thermalization of fast nuclear reaction products in a buffer gas. The fraction of radioactive ions that can be extracted from the gas cell is dramatically suppressed by space charge created by the stopping ions. The results of a review of the ion yields from the NSCL and from other gas cells from the literature with different sizes and different incident particle energies shows an overall consistency with a dramatic decline in extraction efficiency at high ionization rates.     

Intense radioactive-ion beams produced with the ISOL method

For fifty years the isotope separation on-line (ISOL) technique has been used for the production of radioactive-ion beams (RIBs). Thick-target ISOL facilities can provide very intense RIBs for a wide range of applications. The important design parameters for an ISOL facility are efficiency, rapidity and selectivity of all steps of the separation process. To achieve the anticipated beam intensities with the next-generation RIB facilities, the production rate in the ISOL target has to be increased by orders of magnitude. This is only possible by adapting the projectile beam for optimum production cross-sections and simultaneously minimizing the target heating due to the electronic stopping power of charged-particle projectiles. ISOL beams of 75 different elements have been produced up to now and further beam development is under way to produce a still greater variety of isotopes and to improve existing beams in intensity and purity. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: Ulli.Koster@cern.ch     

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.     

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.     

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     

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of <Superscript>232</Superscript>Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of ²³²Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of ²³⁵U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.     

New experimental studies on the quaternary fission of <Superscript>233, 235</Superscript>U(n<Subscript>th</Subscript>, f ) and <Superscript>252</Superscript>Cf (sf )

Experiments have been performed for studying quaternary fission (QF) in spontaneous fission of ²⁵²Cf, on the one hand, and for the neutron-induced fission reactions ^{233, 235}U(n_th, f ), on the other hand. In this higher-multiplicity fission mode, by definition, four charged products appear in the final state. In other words, as a generalization of the ternary-fission process, not only one but two light charged particles (LCPs) are accompanying the splitting of an actinide nucleus into the customary pair of fission fragments. In the two sets of measurements, which have used quite different approaches, the yields of several QF reactions with α-particles and tritons as the LCPs have been determined and the corresponding kinetic-energy distributions of the α-particles measured. The QF process can appear in two basically different ways: i) the simultaneous creation of two LCPs in the act of fission (“true” QF) and ii) via a fast sequential decay of a single but particle-unstable LCP in common ternary fission (“pseudo” QF). Experimentally the two varieties of QF have been distinguished by exploiting the different patterns of angular correlations between the two outgoing LCPs. The experiments described in the present paper are the first to demonstrate that both types of reactions, true and pseudo QF, occur with quite comparable probabilities. As a new result also, the kinetic-energy distributions related to the two processes have been shown to be significantly different. For all QF reactions which could be explored, the yields for ²⁵²Cf(sf) were found to be roughly by an order of magnitude larger than the yields found in the ²³³U(n_th, f ) and ²³⁵U(n_th, f ) reactions. An interesting by-product has been the measurement of yields of excited LCPs which allows to deduce nuclear temperatures at scission by comparison to the respective yields in the ground state.     

Production of neutron-rich nuclei in fission induced by neutrons generated by the p + <Superscript>13</Superscript>C reaction at 55 MeV

Cross-sections for the production of neutron-rich nuclei obtained by neutron-induced fission of natural uranium have been measured. The neutrons were generated by bombarding a ¹³C target with 55 MeV protons. The results, position of the maximum in the (Z, A)-plane, width and magnitude, are very comparable with those where the neutrons are generated by bombardment of natural ¹²C graphite with 50 MeV deuterons. Depending on the geometry of the converter/target assembly the isotope yields, however, are a factor of 2-3 lower due to less efficient production of neutrons per primary projectile, especially at small forward angles. Received: 8 November 2002 / Accepted: 20 December 2002 / Published online: 15 April 2003