0000000000049581
AUTHOR
Kari Perajarvi
showing 48 related works from this author
Beta Decay of 111Tc to 111Ru
1998
The β-decay of the very neutron-rich 111Tc nucleus has been observed. A partial decay scheme of 111Ru is presented, complementing the high-spin levels earlier reported in spontaneous fission work.
First on-line laser spectroscopy of radioisotopes of a refractory element
1999
The first fully on-line isotope shift measurement of a radioactive refractory element is reported. Collinear laser-induced fluorescence measurements were made on the radioactive isotopes ${}^{170,172,173,174}\mathrm{Hf}$ produced with a flux of $2--3\ifmmode\times\else\texttimes\fi{}{10}^{3}$ ions per second from an ion-guide fed isotope separator. The method may be applied to all elements and isomers with lifetimes as short as 1 ms. The systematics of the new charge radii measurements are well reproduced by theory, with the maximum deformation in the chain occurring significantly below the midshell.
First decay scheme of 113Tc and identification of 113Ru m
1998
Very neutron–rich fission products of the mass chain A=113 obtained from the IGISOL on–line mass separator have been investigated by γγ coincidence techniques and γ-spectra multiscaling. Gamma–rays following β–decay of 113Tc have been observed for the first time and a new 0.5 s isomeric state has been found in 113Ru.
Status report of the Jyvaskyla ion guide isotope separator on-line facility
1997
The ion guide isotope separator facility IGISOL of the University of Jyvaskyla has been moved to the new K-130 heavy ion cyclotron laboratory. The totally reconstructed facility is described in detail. The primary beams and targets, helium pumping, separator beam line construction and separator beam diagnostics are discussed. The spectroscopy stations are introduced with illustrative examples from the research program, including beta-delayed proton and neutron spectroscopy, gamma-ray spectroscopy with and without arrays, conversion electron spectroscopy, collinear laser spectroscopy and nuclear level lifetime spectroscopy.
Production of refractory elements close to the Z=N line using the ion-guide technique
1998
Production of neutron-deficient isotopes of refractory elements in the A = 80-88 region was studied using the IGISOL technique and the 165 MeV Si-32 + Ni-nat reaction. Radioactive isotopes of Y through Mo could be produced up to the M-T = + 1 line. New information on the decay of the A = 82 and 85 nuclei, including a more detailed decay scheme and more accurate half-life for Y-82, was obtained. (C) 1998 Elsevier Science B.V. All rights reserved.
First observation of β decay ofNb108toMo108
1996
The \ensuremath{\beta}-delayed \ensuremath{\gamma} decay of $^{108}\mathrm{Nb}$ has been studied using the on-line mass-separator technique. The measured \ensuremath{\beta}-decay half-life 0.20\ifmmode\pm\else\textpm\fi{}0.03 s agrees with the previous measurement based on \ensuremath{\beta}-delayed neutron detection. The lifetime of the first ${2}^{+}$ state in $^{108}\mathrm{Mo}$ was measured to be 0.5\ifmmode\pm\else\textpm\fi{}0.3 ns, which is consistent with a deformation \ensuremath{\beta}\ensuremath{\approxeq}0.4. Spin and parity of ${2}^{+}$ are suggested for the ground state of $^{108}\mathrm{Nb}$. \textcopyright{} 1996 The American Physical Society.
STUDIES OF 12C USING β-DECAYS
2008
The nuclear structure of states in 12 C have been a subject of interest for both theory and experiment since the early days of nuclear physics. Many open questions remain, especially concerning the existence and properties of 0+ and 2+ states in the triple alpha continuum. A series of experiments have been performed using β-decay of 12 N and 12 B to probe these states. The latest experiment was performed at KVI using an implantation method, measuring the sum energy of the three α-particles directly. Preliminary results from this experiment will be presented.
Production of neutron deficient rare isotope beams at IGISOL; on-line and off-line studies
2004
This article reports on recent on-line yield measurements employing the light-ion and heavy-ion reaction-based ion guide systems and new results on a-recoil ion transport properties in ion guides with and without electric fields. In addition, the presently used ion guide designs for fusion evaporation reactions are introduced. The present study investigated different schemes for ion extraction from the gas cell. The addition of an extra ring electrode between the traditional skimmer electrode and the exit hole led to transmission independent of the primary beam intensity as opposed to strong intensity dependence observed earlier with the plain skimmer only. Furthermore, the mass resolving p…
New ion-guide for the production of beams of neutron-rich nuclei between Z=20–28
2005
It has been shown for the first time that quasi- and deep-inelastic reactions can be successfully incorporated into the conventional Ion-Guide Isotope Separator On-Line (IGISOL) technique. This is of particular interest for characterizing the decay properties of refractory elements and is applied to neutron rich nuclei between Z = 20-28. As a first step of this project, the kinematics of quasi- and deep-inelastic reactions, such as {sup 197}Au({sup 65}Cu,X)Y, were studied. Based on these studies, a specialized IGISOL target chamber was designed and built. This chamber was tested in on- and off-line conditions at the Jyvaskyla IGISOL facility. Yields of radioactive, projectile-like species s…
Cryogenic helium as stopping medium for high-energy ions
2008
We have investigated the survival and transport efficiency of Ra-219 ions emitted by a Ra-223 source in high-density cryogenic helium gas, with ionisation of the gas induced by a proton beam. The combined efficiency of ion survival and transport by an applied electric field was measured as a function of ionisation rate density for electric fields up to 160 V/cm and for three temperature and density combinations: 77 K, 0.18 mg/cm(3), 10 K, 0.18 mg/cm(3) and 10 K, 0.54 mg/cm(3). At low beam intensity or high electric field, an efficiency of 30%, is obtained, confirming earlier results. A sharp drop in efficiency is observed at a "threshold" ionisation rate density which increases with the squ…
On-line laser spectroscopy of refractory radioisotopes at the JYFL IGISOL facility
1998
A major objective of the laser-IGISOL program has been realized with the first ever on-line observation of collinear laser induced fluorescence from an ion of a refractory element. The measurements demonstrate that the IGISOL can be operated in a mode that produces ion beams of good emittance with reasonable extraction efficiency. The technique has been used to study the neutron-deficient Hf isotopes.
A new isomer in 125La
1999
Levels in 125La have been studied via β+/EC decay of on-line mass-separated 125Ce using the HIGISOL technique. A new (390 ± 40) ms isomer is definitely attributed to 125La by conversion electron measurements of the 107 keV E3 isomeric transition.
β-delayed neutron decay of 104Y, 112Tc, 113Tc and 114Tc: test of half-life predictions for neutron-rich isotopes of refractory elements
1999
Abstract Beta-decay gross properties of neutron-rich isotopes 104Y and 112,113,114Tc produced in 25 MeV proton-induced fission of 238U have been measured. Decays of 104Y with a half-life of 180±60 ms and of 114Tc with a half-life of 150±30 ms are reported for the first time. Beta-delayed neutron emission probabilities of 112,113,114Tc are determined as (1.5±0.2)%, (2.1±0.3)% and (1.3±0.4)%, respectively. Comparison of the observed Pn values with a recent theoretical calculation based on deformed quasiparticle random phase approximation (QRPA) shows good overall agreement for Tc isotopes. However, comparison of beta-decay half-lives of neutron-rich isotopes of Y to Rh with the QRPA model and…
The $\beta$-delayed one- and two-proton emission of $^{27}$S
2001
In an experiment performed at the GANIL LISE3 facility, radioactive 27S isotopes have been produced by projectile fragmentation of a 95 AMeV 36Ar primary beam. After selection by means of the LISE3 separator, the isotope of interest was implanted in a silicon-detector telescope where its half-life ( T 1/2 = 15.5(15) ms) and its main decay branches were measured.
The 1+→0+ Gamow–Teller strength of the 58Cug.s.→58Nig.s. transition
2001
The ground-state branch in the beta decay of Cu-58 has been remeasured by using gamma-ray detection combined with ion-guide-based on-line mass separation. The measured value 80.8(7)% is three times more precise and in agreement with the earlier reported value. The deduced Gamow-Teller strength of this transition, to be used for the calibration of the charge-exchange reactions, is 0.0821(7). (C) 2001 Elsevier Science B.V. All rights reserved.
Upgrade and yields of the IGISOL facility
2008
The front end of the Jyvaskyla IGISOL facility was upgraded in 2003 by increasing its pumping capacity and by improving the radiation shielding. In late 2005, the skimmer electrode of the mass separator was replaced by a sextupole ion guide, which improved the mass separator efficiency up to an order of magnitude. The current design of the facility is described. The updated yield data, achieved with and without the additional JYFLTRAP purification, using both fusion evaporation reactions and particle induced fission is presented to give an overview of the capability of the facility. These data have been determined either by radioactivity measurements or by direct ion counting after the Penn…
Decay of the neutron-rich isotope 113Ru to 113Rh
2002
The decay of neutron-rich isotope 113Ru obtained as on-line mass separated product of proton-induced fission has been investigated by γγ coincidence and spectrum multiscaling measurements. Decay schemes for both low- and high-spin isomers of 113Ru have been constructed. The level scheme of 113Rh is considerably extended. Systematics of the lowest-lying rhodium levels is smooth. The picture of shape coexistence established for neutron-rich Rh isotopes near-neutron midshell is confirmed with the observation of a K = 1/2 deformed band, with its 3/2+ state at 600 keV being the lowest-lying level and of probable 7/2+ and 5/2+ band members. A large fraction of β feeding is found to populate high-…
A new 400 ms isomer in125Ce→125La
1998
By using the IGISOL technique, adapted for the study of products of heavy ion induced fusion-evaporation reactions, it has been possible to identify a new isomer in the mass chainA=125.
Investigation of a gas-catcher/ion guide system using alpha-decay recoil products
2002
Abstract 219Rn recoils from the alpha decay of 223Ra have been used to study the efficiency and delay time distributions of a gas-catcher/ion guide system. Ions with charge states up to +4 were coming out of the gas cell. Combining efficiency and delay time measurements, ion survival times in plasma free conditions can be deduced.
A study of on-line gas cell processes at IGISOL
2010
Abstract The laser ion source at the IGISOL facility, Jyvaskyla, has been used to study the effects of the passage of a primary beam through an ion guide via the dynamic time profiles of yttrium and related molecular compounds. The accessibility of a neutral fraction for laser ionization is shown to be restricted to the nozzle region in the presence of a weak plasma. The survival of this neutral fraction cannot be explained by recombination of the buffer gas alone and perhaps indicates a suppression of the transport of ions due to a plasma-generated electric field. The concept of a competition between the processes that create and destroy the ion of interest is used to explain the different…
Studying exotic nuclides close to the N = Z line at the HIGISOL facility
2003
The ion guide [1, 2] for heavy-ion fusion-evaporation reactions (HIGISOL) which was developed by Beraud et al. [3] has been implemented at the IGISOL facility in Jyvaskyla [4]. This system was modified over the past 5 years. Figure 1 shows the present set-up. The HIGISOL takes advantage of the different angular distributions of primary beam and reaction products: the primary beam is stopped in front of the stopping chamber and the reaction products enter the stopping chamber through a thin foil passing the beam stop. This so called “shadow” method removes the plasma effect since the primary beam is not ionising the stopping gas. In order to improve ion optical properties, mainly to reduce t…
R-matrix analysis of theβdecays ofN12andB12
2010
The β decays of 12N and 12B have been studied at KVI and JYFL to resolve the composition of the broad and interfering 0+ and 2+ strengths in the triple-α continuum. For the first time a complete treatment of 3α decay is presented including all major breakup channels. A multilevel, many-channel R-matrix formalism has been developed for the complete description of the breakup in combination with the recently published separate analysis of angular correlations. We find that, in addition to the Hoyle state at 7.65 MeV, more than one 0+ and 2+ state is needed to reproduce the spectra. Broad 03+ and 22+ states are found between 10.5 and 12 MeV in this work. The presence of β strength up to the 12…
Beta-decay half-lives of $^{70}$Kr and $^{74}$Rb
2000
Abstract Beta-decay half-lives of two nuclei close to N = Z line, 70 Kr and 74 Rb, have been measured at the ISOLDE mass-separator facility at CERN. Importance of these half-lives on two ingredients explaining existence and development of the Universe, the astrophysical nucleosynthesis and the Standard Model, are discussed.
Medium-spin structure of neutron-rich Pd and Cd isotopes
2003
Cadmium isotopes are traditionally considered as good examples of nearly spherical, vibrational nuclei, which can be described by quadrupole vibrators in the collective model, as well as the U(5) dynamical symmetry in the interacting boson model (IBM). However, the shape coexistence of more deformed intruder states originating from proton 2p–4h excitation across the Z = 50 major shell is another interesting structural aspect, whose presence in Pd-isotopes has also been demonstrated [1,2].
Beta decay of neutron-rich 118Rh and the lowest excited states in 118Pd
2000
Beta decay of a refractory isotope 118Rh produced in symmetric fission and mass separated by the ion guide technique has been applied for the study of low-lying excited states of 118Pd. The yrast band in 118Pd has been observed up to a 6+ state and the lowest states of the asymmetric γ-band have been identified. The measured half-life of 118Rh is (300±60)ms. The systematics of the excited states in neutron-rich Pd-isotopes implies the saturation towards an O(6) symmetry at N = 70.
Off-line studies of the laser ionization of yttrium at the IGISOL facility
2007
A laser ion source is under development at the IGISOL facility, Jyvaskyla, in order to address deficiencies in the ion guide technique. The key elements of interest are those of a refractory nature, whose isotopes and isomers are widely studied using both laser spectroscopic and high precision mass measurement techniques. Yttrium has been the first element of choice for the new laser ion source. In this work we present a new coupled dye-Ti:Sapphire laser scheme and give a detailed discussion of the results obtained from laser ionization of yttrium atoms produced in an ion guide via joule heating of a filament. The importance of not only gas purity, but indeed the baseline vacuum pressure in…
Measurement of the IAS resonance strength in 23Mg
2000
Abstract Beta decay of 23 Al to excited states in 23 Mg has been studied using low-energy proton and high-energy gamma-ray detection combined with ion-guide-based on-line mass separation. For the first time, a T =3/2 isobaric analogue state, at 7801(2) keV, was observed to decay by both proton and gamma emission, with a proton branching of 0.17(8)%. The deduced resonance strength ωγ =2.2(10) meV is in agreement with upper limits reported from 22 Na(p, γ ) reaction studies. Shell-model calculations are incorporated.
Transport of ions in ion guides under flow and diffusion
2000
Abstract A model for ion transport simulations in the ion guide stopping chamber is introduced. Computed transport time distributions are compared to the experimental data obtained in on- and off-line conditions. It is shown that millisecond transport times are feasible and that diffusion of ions is an important mechanism in transport.
Level structure of 99Nb
1998
The β decay of 97Sr to 97Y has been investigated using ion-guide on-line mass separation and a 10 Ge-detector array to record γ−γ coincidences to a detection limit well below that of former studies. Similarities are found in the β-decay patterns of 99Zr and of its isotone 97Sr and also in the γ-ray decay rates and branchings of the corresponding levels in their respective daughters 99Nb and 97Y. This indicates a persisting influence of the d5/2 neutron shell closure for 99Nb. The level structure of 99Nb and the β-feeding pattern are discussed in the frame of the interacting boson-fermion plus broken pair model and the microscopic quasiparticle phonon model.
βdecay of neutron-rich118Agand120Agisotopes
2003
$\ensuremath{\beta}$ decays of on-line mass-separated neutron-rich ${}^{118}\mathrm{Ag}$ and ${}^{120}\mathrm{Ag}$ isotopes have been studied by using $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ and $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence spectroscopy. Extended decay schemes to the ${}^{118,120}\mathrm{Cd}$ daughter nuclei have been constructed. The three-phonon quintuplet in ${}^{118}\mathrm{Cd}$ is completed by including a new level at 2023.0 keV, which is tentatively assigned the spin and parity of ${2}_{4}^{+}.$ The intruder band in ${}^{118}\mathrm{Cd}$ is proposed up to the ${4}^{+}$ level at 2322.4 keV. The measured $\ensuremath{\beta}$-decay half…
The β-decay approach for studying 12C
2008
6 pags., 3 figs. -- 9th International Conference on Clustering Aspects of Nuclear Structure and Dynamics (CLUSTERS'07) 3–7 September 2007, Stratford upon Avon, UK
Production of neutron-rich nuclei in fission induced by neutrons generated by the p + 13C reaction at 55 MeV
2003
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.
New interpretation of shape coexistence in99Zr
1997
Levels in ${}^{99}\mathrm{Zr}$ populated by $\ensuremath{\beta}$ decay of on-line mass separated ${}^{99}$Y have been studied by $\ensuremath{\gamma}$-ray spectroscopic methods, including $\ensuremath{\gamma}$-$\ensuremath{\gamma}$ coincidences recorded with an array of ten Ge detectors and level-lifetime measurements. The formerly reported strongly collective character of the 53-keV transition turns out to be questionable. This implies a revision of the experimental evidence for shape coexistence in ${}^{99}\mathrm{Zr}.$ Transition rates and $\ensuremath{\gamma}$-ray branching ratios make a new level at 679 keV with ${t}_{1/2}=9$ ns another candidate for a deformed state.
High-resolution studies of beta-delayed proton emitters at IGISOL facility
1998
Beta-decays of23Al and41Ti have been studied by applying ion guide techniques, gamma detection and a gas-Si telescope for charged-particle detection. The experimental beta-decay strength of41Ti was found to be quenched by a factor ofq2 = 0.64 compared to our shell model calculations below 8 MeV excitation energy in41Sc. This result is in line with the generally accepted value ofq = 0.77. The isobaric analogue state of the41Ti ground state was measured to contain 10(8)% isospin impurity. This relatively high value could be well reproduced by the shell model calculation in thesdfp space. Gamma-decay of theT = 3/2 isobaric analogue state in23Mg was observed together with its decay by proton em…
Precise branching ratios to unbound 12C states from 12N and 12B β-decays
2009
6 pages, 2 tables, 4 figures.--PACS nrs.: 21.45.-v; 23.40.-s; 27.20.+n; 21.60.De.--Printed version published Aug 3, 2009
Independent yields of neutron-rich nuclei in charged-particle induced fission
1997
Abstract Yields of fission products have been studied at the recently upgraded IGISOL facility in Jyvaskyla using 50 MeV H2+ beam (Ep = 25 MeV) and thin natTh and natU targets. The independent yields of fission products in the mass regions A = 99−112 (elements Y, Nb and Tc) and A = 127−134 (elements Sn, Sb and Te) have been obtained. Comparisons of the experimental isotopic distributions with theoretical ones are presented. Enhancement of the production rate of the doubly magic 132Sn is observed in the experimental yield curve.
Status of HIGISOL a new version equipped with SPIG and electric field guidance
2001
A new HIGISOL chamber devoted to the study of short-lived products from heavy-ion-induced fusion-evaporation reactions is proposed. It enables, via the extraction of ions by means of a SPIG (SextuPole rf Ion Guide), to improve the mass resolving power by a factor 2.5 compared to the previous system using a skimmer-ring assembly. The gas cell was also equiped with an electric field for faster transportation of recoiling ions to the nozzle where they are ejected with the gas jet. The first results obtained both with a radioactive α-source and cyclotron beam will be reported.
New levels in 118Pd observed in the beta-decay of very neutron-rich 118Rh isotope
2006
We investigate the β decay of very neutron-rich 118Rh isotope using on-line mass-separated sources which are produced by applying 25 MeV proton induced symmetric fission of natural uranium at the IGISOL facility. The β–γ and γ–γ coincidence spectroscopy is employed to establish the level scheme of daughter nucleus 118Pd. Five low-lying new levels are identified for the first time with tentative spin and parity assignments based on the even-mass Pd systematics.
High-resolution study of the beta decay of 41Ti
1997
Abstract An improved high-resolution study of the beta decay of 41 Ti, produced in the 40 Ca( 3 He, 2n) reaction at 40 MeV, has been performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility. The beta-delayed radiation was detected by a low-energy charged-particle detector and a large Ge detector for gamma rays. The experimental beta-decay strength and its distribution, extracted from delayed-proton data, are compared with results of shell-model calculations in the sdfp space. The lowest J π = 3 2 + , T = 3 2 state in 41 Sc, the isobaric analogue state of the 41 Ti ground state, is estimated to contain 10% isospin impurity.
Nuclear moments and charge radii of the171Hf ground state and isomer
2000
The magnetic moment, quadrupole moment and mean-square charge radial changes, relative to 172Hf, have been measured for the 171Hf ground state and the ½-[521] isomeric state, using on-line laser spectroscopy. The magnetic moments of the isomer and ground states are found to be + 0.526(16)µn and -0.674(12)µn, respectively. The spectroscopic quadrupole moment of the ground state is found to be +3.463(27) b. No change in mean-square charge radius is observed between the ground state and isomeric state. The observation of an inversion in the odd-even staggering at 171Hf is consistent with the suggested deformation trends.
Improved ion guide for heavy-ion fusion-evaporation reactions
1998
The ion guide for heavy-ion-induced reactions developed originally for the SARA facility in Grenoble has been implemented at the Jyvaskyla IGISOL facility. For the Cd-116(Ar-40, 6n)Dy-150 reaction an efficiency of 0.5% relative to the number of reaction products entering the stopping chamber was obtained. This is 3.5 times higher than previously obtained at SARA and corresponds to a yield of about 100 ions/(p mu C mb). Mass-separated yields for the Ar-36 + Mo-92 and Ar-36 + Mo-94 reactions were measured. (C) 1998 Elsevier Science B.V. All rights reserved.
Isomeric state of 80Y and its role in the rp-process
2003
The HIGISOL facility has been used to investigate carefully the isomeric transition 228.5 keV in 80Y. We have measured the electron internal conversion coefficient for this transition αK = 0.50 ± 0.07 which gives the value for half-life of “bare” isomeric state T 1/2 = 6.8 ± 0.5 s. The isomeric state should play an important role in the rp-process calculations.
Branching ratios in theβdecays ofN12andB12
2009
Absolute branching ratios to unbound states in C-12 populated in the beta decays of N-12 and B-12 are reported. Clean sources of N-12 and B-12 were obtained using the isotope separation on-line (ISOL) method. The relative branching ratios to the different populated states were extracted using single-alpha as well as complete kinematics triple-alpha spectra. These two largely independent methods give consistent results. Absolute normalization is achieved via the precisely known absolute branching ratio to the bound 4.44 MeV state in C-12. The extracted branching ratios to the unbound states are a factor of three more precise than previous measurements. Branching ratios in the decay of Na-20 …
Ground-state spin of 59Mn
2001
Beta decay of $^{59}$Mn has been studied at PSB-ISOLDE, CERN. The intense and pure Mn beam was produced using the Resonance Ionization Laser Ion Source (RILIS). Based on the measured $\beta$-decay rates the ground-state spin and parity are proposed to be $J^{\pi}$ = 5/2$^{-}$. This result is consistent with the systematic trend of the odd-A Mn nuclei and extends the systematics one step further towards the neutron drip line.
Superasymmetric fission at intermediate energy and production of neutron-rich nuclei with A < 80
1997
Abstract The yields of neutron-rich Ni, Cu, Zn, Ga and Ge-isotopes were measured in 25 MeV proton induced fission of 238 U using the ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus and show the potential of this reaction for the production of neutron-rich exotic nuclei around Z = 28.
Non-analog β decay of 74Rb
2001
The magnitude of the Coulomb mixing parameter δ 1 has been experimentally deduced, for the first time, for the β decay of 74 Rb. The estimated magnitude is derived from the feeding of the non-analog first excited 0 + state in 74 Kr. The inferred upper limit of 0.07% is small compared to theoretical predictions. The half-life was measured to be 64.90(9) ms. 2001 Published by Elsevier Science B.V.
Isomeric state of $^{80}$Y and its role in the astrophysical rp-process
2001
5 pages, 7 figures.-- PACS nrs: 21.10.Tg; 23.20.Nx; 27.50.+e.
Production of neutron-rich isotopes in fission of uranium induced by neutrons of 20 MeV average energy
2000
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 is…