Fragment mass dependence of angular anisotropy in 15 MeV proton-induced fission of 244Pu

Angular distributions of fission fragments with mass number A=97-159 have been measured by the radiochemical recoil-catcher method in the proton-induced fission of ²⁴⁴Pu with the incident energy of 15 MeV. Angular anisotropies of extreme asymmetric mass division products even up to the fragment mass ratio of A _H /A _L ∼ 1.85 are found not any different from those of the typical asymmetric mass division products with A∼ 138, which indicates that no clear evidence is observed for the existence of an additional saddle point configuration in the extreme asymmetric mass division. The correlation between the saddle point state evaluated from the angular anisotropy and the mass division mode is discussed.     

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     

Modeling a neutron-rich nuclei source

The deuteron break-up process in a suitable converter gives rise to intense neutron beams. A source of neutron-rich nuclei based on the neutron-induced fission can be realised using these beams. A theoretical optimization of such a facility as a function of the incident deuteron energy is reported. The model used to determine the fission products takes into account the excitation energy of the target nucleus and the evaporation of prompt neutrons. Results are presented in connection with a converter-target specific geometry. Received: 1 December 2000 / Accepted: 12 May 2001     

Fragment dependence of high energy γ-ray emission in the spontaneous fission of 252Cf

The high energy γ-ray emission accompanying the spontaneous fission of ²⁵²Cf has been measured in coincidence with individual fission fragments selected by discrete γ-ray transitions. The enhancement of the γ-ray emission probability in the energy range E_γ= 3–8 MeV has been observed for the fission fragments in the region of nearly symmetric mass splitting, confirming results reported in previous investigations. The γ-γ coincidence technique employed in the present work clearly demonstrate that the major contribution to this enhancement is caused by the fission channels where one fragment is near to the N= 82 or Z= 50 shell closures. The high energy γ-ray emission probability does not show any significant dependence on the number of neutrons emitted in the fission process, supporting the hypothesis that high energy γ-rays are mainly emitted from the fragments after the neutron evaporation. Received: 22 December 1998     

Semi-microscopical description of the scission configuration in the neutronless fission of 252Cf

The neutronless fission of ²⁵²Cf is studied in the frame of a molecular model in which the scission configuration is described by two aligned fragments interacting by means of Coulomb (+ nuclear) forces. The study is carried out for different distances between the fragments tips and excitation energies. For a given deformation, the fragment's total energy is computed via the constrained Hartree-Fock + BCS formalism. The total excitation energy present in the fragments is supposed to contribute only to the fragments deformation and the asymptotic value of the kinetic energy is equated to the inter-fragment potential at scission. These two constraints are yielding a few fission channels for a fixed tip distance and excitation energy. Discarding those fission channels corresponding to a disequilibrium in the sharing of the excitation energy between the two fragments, we establish the most likely scission configurations for a specified excitation energy. Received: 24 September 1999     

Yields of products from fast neutron-induced fission of 233U measured by means of an isotope separator on-line (ISOL) system

A comprehensive study has been carried out of the yield pattern of fission products formed in fast neutron-induced fission of ²³³U. The isotope separator on-line facility at Studsvik to the R2-0 nuclear reactor was used for rapid separation of the fission products. At a target temperature of 2250 ^°C fission products of the elements from zinc (Z = 30) to barium (Z = 56) are released, with the exception of yttrium, zirconium, niobium, molybdenum, technetium, ruthenium and rhodium. The individual isotopes are then available for study, implying that an almost complete mapping of the yield distribution can be made. In the analysis, the delay between production and measurement and the overall separator efficiency for three consecutive elements (the one under study and its parent and grand parent) are taken into account. Independent and/or cumulative yields have been obtained for 203 nuclear species, among them 59 isomeric states. Received: 19 April 2000 / Accepted: 19 June 2000     

Single-particle spin effect on fission fragment angular momentum

Independent isomeric yield ratios (IYR) of ¹²⁸Sb, ¹³⁰Sb, ¹³²Sb, ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe, and ¹³⁸Cs have been determined in the fast neutron-induced fission of ²⁴³Am using the radiochemical and γ-ray spectrometric technique. From the IYR, fragment angular momenta (J _rms) have been deduced using the spin-dependent statistical model analysis. From the J _rms-values and experimental kinetic energy data deformation parameters (β) have been deduced using the pre-scission bending mode oscillation model and the statistical model. The J _rms- and β-values of fission fragments from the present and earlier work in the odd-Z fissioning systems ( ²³⁸Np ^* , ²⁴²Am ^* and ²⁴⁴Am ^* ) are compared with the literature data in the even-Z fissioning systems ( ^{230, 233}Th ^* , ^{233, 234, 236, 239}U ^* , ^{239, 240, 241, 242}Pu ^* , ²⁴⁴Cm(SF), ^{245, 246}Cm ^* , ²⁵⁰Cf ^* and ²⁵²Cf(SF)) to examine the role of single-particle (proton) spin effect. It was observed that i) in all the fissioning systems J _rms- and β-values of the fragments with spherical 82n shell and even-Z products are lower than the fragments away from the spherical neutron shell and odd-Z products, which indicate the effect of nuclear structure. ii) For both even-Z and odd-Z fission products J _rms-values increase with Z _F ²/A _F due to increase in Coulomb torque. iii) The J _rms- and β-values of even-Z fission products are comparable in all the fissioning systems. However, for odd-Z fission products they are slightly higher in the odd-Z fissioning systems compared to their adjacent even-Z fissioning systems. This is possible due to the contribution of the extra single-particle (proton) spin of the odd-Z fissioning systems to their odd-Z fragments. iv) The yield-weighted fragment angular momentum and elemental yields profile shows an anti-correlation in even-Z fissioning systems but not in the odd-Z fissioning systems.     

High-spin study of odd- A 49In isotopes beyond the neutron mid-shell

The ^{115, 117, 119, 121}In nuclei have been produced as fission fragments in three reactions induced by heavy ions: ¹²C + ²³⁸U at 90MeV bombarding energy, ¹⁸O + ²⁰⁸Pb reaction at 85MeV, and ³¹P + ¹⁷⁶Yb at 152MeV. Their level schemes have been built from gamma-rays detected using the EUROBALL III and IV arrays. High-spin states of ^{117, 119, 121}In nuclei have been identified for the first time. Moreover, isomeric states lying around 2.5MeV have been established in ^{119, 121}In from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. Most of the observed states can be described by coupling a proton g _9/2 hole to a Sn core, while the intruder band based on an orbital from the π[g _7/2/d _5/2] sub-shells behaves as the ground-state band of neighbouring Cd isotopes. Received: 24 May 2002 / Accepted: 11 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CEA-DIF, DAM/DCRE/SDE/LDN, 91680 Bruyères-le-Chatel, France. RID="c" ID="c"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="d" ID="d"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm     

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     

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.     

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by ⁴⁸Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas. Received: 1 May 2001 / Accepted: 4 December 2001     

Fast fission mechanism and duality of the diffusion process

The reaction of ²³⁸U with ¹²C was studied radiochemically with the purpose of elucidating fast fission characteristics. From the difference in the mass distribution below and above the critical energy where fast fission is predicted to set in, fast fission component was extracted in far-asymmetric mass region and interpreted as the mass diffusion following the Fokker-Planck equation. Anomalous charge dispersion widths in the corresponding mass region and a sudden increase of the whole mass distribution width at the critical energy were also observed to support the above result. The reaction time of fast fission deduced from the width and position of the mass distribution was 5×10⁻²¹s as well by taking into account the effect of neutron emission during the diffusion process, which turned out to be more than one order of magnitude longer than the corresponding life time of typical deep inelastic scattering but substantially short compared to ordinary fusion-fission life time. Evaluation of the driving potential for mass drift required dinuclear configuration be of an elongated or deformed form for fast fission in contrast to a more compact form for the deep-inelastic process. Received: 11 November 1997     

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     

On the nature of the 17 μs isomer of the <Superscript>133</Superscript>Sb valence nucleus

The decay of the 17 μs isomer of ¹³³Sb was re-investigated experimentally. It was produced by thermal neutron induced fission of ²⁴¹Pu. Its detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble), and the γ-rays, and conversion electrons from the isomer. The interpretation of the level scheme is based on shell model calculations, where empirical two-body matrix elements were employed. The good agreement between theory and experiment suggests that the isomer is the 21/2⁺ member of the 2p-1h πg _7/2ν(f _7/2 h _11/2 ⁻¹) configuration. Received: 20 January 2000 / Revised version: 28 February 2000     

New high-spin microsecond isomers in 131Sb

In this work we have identified and studied the decay of two new microsecond isomers 19/2^- and 23/2⁺ in ¹³¹Sb. This neutron-rich nucleus was produced by thermal neutron-induced fission of ²⁴¹Pu. The detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble) and the γ-rays or conversion electrons from isomers. These new data on high-spin states complete the level scheme previously obtained from β-decay. A large fraction of the members of the πg _7/2ν(h _11/2 ^-1 d _3/2 ^-1) and πg _7/2νh _11/2 ^-2 multiplets are now known and were compared with a multi-particle shell model calculation. Received: 7 June 2000 / Accepted: 19 September 2000     

Fission of highly excited fragments from collisions of 750 A.MeV 238U-ions on 208Pb

Fragments of relativistic 750 A.MeV U-projectiles were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and ΔE-ToF techniques. For elements between Ge and Sb, measurements of isotopic yield distributions and velocities revealed three processes: fragmentation, low-energy fission, and high-energy fission. The last of these regimes is presently reported. First and second moments of distributions of mass numbers, atomic numbers and velocities of the corresponding fragments allowed us to identify ¹⁰¹ ₄₃Tc₅₆ as the most probable fragment of a high energy symmetric fission reaction. Moreover, we could deduce a hypothetical mean fissioning fragmentation product ²⁰⁸Rn and its highly excited pre-fragmentation parent ²²⁷Ra produced in a primary abrasion reaction at an excitation energy of about 290 MeV. Received: 26 January 1998 / Revised version: 16 March 1998     

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.     

First identification of rotational bands in 103Tc: Evolution of intrinsic proton states of the 97-105 43Tc isotopes

High-spin states of ¹⁰³ ₄₃Tc have been identified for the first time, this nucleus being produced as fission fragment following the fusion reaction ³⁷Cl + ¹⁷⁶Yb at 170 MeV bombarding energy. The high-spin level scheme has been built from the prompt gamma rays detected using the Euroball III array. It exhibits similarities with those of the neighbouring isotopes and isotones. All the band head configurations observed in the ^97-105Tc isotopes are identified from the behaviour of the rotational bands built on them. The single-proton states located around the Fermi level are discussed as a function of deformation of these nuclei. Received: 19 January 2001 / Accepted: 23 February 2001     

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