Heavy Mass Region Research Articles

The gross theory model for neutrino-nucleus cross-section

The nuclear gross theory, originally formulated by Takahashi and Yamada (1969 Prog. Theor. Phys. 41 1470) for the β-decay, is applied to the electronic-neutrino nucleus reactions, employing a more realistic description of the energetics of the Gamow–Teller resonances. The model parameters are gauged from the most recent experimental data, both for β--decay and electron capture, separately for even–even, even–odd, odd–odd and odd–even nuclei. The numerical estimates for neutrino-nucleus cross-sections agree fairly well with previous evaluations done within the framework of microscopic models. The formalism presented here can be extended to the heavy nuclei mass region, where weak processes are quite relevant, which is of astrophysical interest because of its applications in supernova explosive nucleosynthesis.

SPIRAL2 at GANIL: A world leading ISOL facility at the dawn of the next decade

To pursue the investigation of a new territory of nuclei with extreme N / Z , called “terra incognita”, several projects, all aiming at the increase by several orders of magnitude of RIB intensities are now under discussion worldwide. In Europe, two major new projects have been approved recently: FAIR@GSI, using the so-called “in-flight” method, and SPIRAL2@GANIL, based on the ISOL method. The main goal of SPIRAL2 is clearly to extend our knowledge of the limit of existence and the structure of nuclei deeply in the medium and heavy mass region ( A = 60 –140), which is today an almost unexplored continent. SPIRAL2 is based on a high power, CW, superconducting driver LINAC, delivering 5 mA of deuteron beams at 40 MeV (200 kW) directed on a C converter+ Uranium target and producing therefore more than 10 13 fissions/s. The expected radioactive beam intensities for exotic species in the mass range from A = 60 to A = 140 , of the order of 10 6–10 10 pps will surpass by two order of magnitude any existing facility in the world. These unstable atoms will be available at energies between a few keV/n to 15 MeV/n. The same driver will accelerate high intensity (100 μA to 1 mA), heavier ions up to Ar at 14 MeV/n producing also proton rich exotic nuclei. In applied areas SPIRAL2 is considered as a powerful variable energy neutron source, a must for studying the impact of nuclear fission and fusion on materials. The intensities of these unstable species are excellent opportunities for new tracers and diagnostics either for solid state, material or for radiobiological science and medicine. The technical design has reached the point where SPIRAL2 is ready for construction. Project status and foreseen schedules will be presented. Scientific and technical R&D programs in collaboration with EU and International partners for the facility as well as for the associated innovative new instruments will be discussed.

The isospin admixture of the ground state and the properties of the isobar analog resonances in medium and heavy mass nuclei

Within the framework of quasiparticle random phase approximation (QRPA), Pyatov-Salamov method [23] for the self-consistent determination of the isovector effective interaction strength parameter, restoring a broken isotopic symmetry for the nuclear part of the Hamiltonian, is used. The isospin admixtures in the ground state of the parent nucleus, and the isospin structure of the isobar analog resonance (IAR) state were investigated with the inclusion of the pairing correlations between nucleons for the medium and heavy mass regions: 80 <A < 90, 102 <A < 124, and 204 <A < 214. It was determined that the influence of the pairing interaction between nucleons on the isospin admixtures in the ground state and the isospin structure of the IAR state is more pronounced for the light isotopes (N ≈ Z) of the investigated nuclei

Structure properties of even–even actinides at normal and super deformed shapes analysed using the Gogny force

Mean field and beyond mean field methods implemented with the D1S force are used in large scale calculations to study the structure properties of fifty five even–even actinides at normal and isomeric potential deformations. The mass region covered is spanned by the 226–236Th, 228–242U, 232–246Pu, 238–250Cm, 238–256Cf, 242–258Fm and 250–262No nuclides. Hartree–Fock–Bogolyubov (HFB) calculations performed with constraints placed on axial and triaxial quadrupole deformations as well as octupole deformations serve to build tensors of inertia and potential energy landscapes up to very large elongations. Long lived spin isomers are investigated through blocking calculations performed with and without breaking time reversal symmetry. Configuration mixing calculations are performed to make predictions for shape isomers and π = + vibrations in the isomeric potentials and to calculate moments of inertia at low spin. Moments of inertia are also investigated through cranking HFB calculations to high frequency so as to explore in this regime pairing properties of the D1S force in the heavy mass region. The WKB method is adopted in half-life calculations for the γ-back and fission decay modes of shape isomers. All these predictions are challenged through extensive comparisons with measurements for multipole moments, moments of inertia, spin and shape isomers as well as superdeformed phonons, inner and outer potential barrier heights, and shape isomer lifetimes. Most predictions match the experimental data. Finally we predict a shallow minimum on top of the triaxial inner barriers of N ≃ 154 nuclei. This topological property offers a possible explanation for the unexpected structures observed in fission transmissions measured a long time ago for 252Cf and neighboring nuclei.

Dynamical Effects in the Region of Heavy and Super-Heavy Nuclei

The transfer-induced fission channel has been studied in the collision of 340 MeV 2 8 Si on 2 3 2 Th as a function of the atomic number of the projectile-like fragments (PLF) by using a 4π detector array. The average excitation energy of the target-like fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. The measured ratio of transfer-fission yield to PLF singles, Y f , first increase up to a net charge transfer ΔZ = 4, then shows a plateau around the values Y f = 0.4 - 0.6. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfer AZ <6, suggesting a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of the deexcitation, such effects are important in the population of nuclei in the heavy and superheavy mass region by transfer reactions. Moreover pre- and post-scission multiplicities of neutrons and alpha particles have been simultaneously measured for the fission-like channel of the same reaction. Dynamical model calculations using HICOL code predict that 90% of the observed events are of quasi-fission type while the remaining 10% are from compound nucleus fission decay. From a comparison of PACE2 Statistical Model predictions with the measured pre-scission neutron multiplicity, the fission delay is estimated to be of 5 + 7 - 3 ×10 - 2 0 s, which overlaps with the average duration of fission-like process from the contact to the scission point (2×10 - 2 0 s) as determined from HICOL-based dynamical calculations.

Problems of Dynamical Calculation for Synthesis of Superheavy Elements

We discuss the fusion-fission process in super heavy mass region. In dynamical calculation, we list up the problems and discuss the sensitivity of the parameters. Using the fluctuation-dissipation model we investigate the fusion-fission process and estimate the fusion cross section precisely.

Ground state structure of some double-λ hypernuclei by a three-body model using a simple coordinate space approach

New experimental data on the binding energyB λλ ofλλ6He, reported very recently, come up with the valuesB λλ = 725 ±0.14 MeV and ΔBλλ = 101 ±0.2 MeV which are substantially lower than the old dataB λλ = 109 ±0.8 MeV and ΔBλλ = 4.7±10 MeV in use in literature since 1966. In view of the new data we decided to undertake a re-study of the λλ 6 He hypernucleus using the same three-body model (α-λ-λ) with a simple coordinate space variational approach which was employed earlier with the old data onλλ/6He. After fitting different λ-λ potentials to the new data of λλ 6 He we have applied our method to study some double-λ hypernuclei in light, medium and heavy mass regions and have determined the structural quantities like Bλλ, the r.m.s. values of core-λ (〈rcore-λ〉〉) and λ-λ (〈rλ-λ〉〉) distances theoretically. The core-λ interaction considered is of Woods-Saxon type. The strength and the range of the core-A potential have been adjusted to reproduce the λ-binding energy(B λ) . These are in good agreement with the relativistic mean field (RMF) results. Our study shows that the λ-λ bonding energy ΔBλλ decreases with increasing mass number from λλ 10 Be to λλ 210 Pb of a double-A hypernucleus

Fission of targetlike fragments populated in the multinucleon transfer reactions of 340 MeV28Sion232Th

The transfer-induced fission channel has been studied in the collision of 340 MeV ${}^{28}\mathrm{Si}$ on ${}^{232}\mathrm{Th}$ as a function of the atomic number of the projectilelike fragments (PLF's) by using a $4\ensuremath{\pi}$ detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer $\ensuremath{\Delta}Z$ from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with ${Z}_{\mathrm{TLF}}=90--96 (\ensuremath{\Delta}Z=0--6)$ have average excitation energies up to about 100 MeV and angular momenta up to about $40\ensuremath{\Elzxh}.$ The measured ratio of transfer-fission yield to PLF singles, ${Y}_{f},$ first increases with increasing net charge transfer up to $\ensuremath{\Delta}Z=4$ and then shows a plateau around the values ${Y}_{f}=0.4--0.6$ followed by a decrease for higher $\ensuremath{\Delta}Z$ transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to $\ensuremath{\Delta}Z&lt;~6,$ where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of deexcitation, as also indicated by the large fission times ${(t}_{f}=10--100 \mathrm{zs})$ derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.

Most probable charge of fission products in 24 MeV proton induced fission of238U

The charge distributions of fission products in 24 MeV proton-induced fission of {sup 238}U were measured by the use of an ion-guide isotope separator on line. The most probable charge (Z{sub p}) of the charge distribution was discussed in view of the charge polarization in the fission process. It was found that Z{sub p} mainly lies on the proton-rich side in the light mass region and on the proton-deficient side in the heavy mass region compared with the postulate of the unchanged charge distribution. The charge polarization was examined with respect to production Q values. {copyright} {ital 1998} {ital The American Physical Society}

The GDR at finite temperature in medium and very heavy mass regions

Experimental investigations of the giant dipole resonance at finite temperature provide new insight into different properties of hot rotating nuclei. Here we discuss the studies aiming at the understanding of the role of thermal and quantal fluctuations in the measured GDR width. We also present the first studies of the properties of super-heavy nuclei through the γ-decay of the GDR in the hot compound prior to fission.

Observation of Coulomb excitation of unstable nuclei utilization of a secondary beam

An experimental technique to detect γ-rays from secondary-beam Coulomb excitation was developed. A secondary beam of 76Kr produced by the inverse reaction 9Be( 70Ge, 3n) was Coulomb excited by a 208Pb target. Weak γ-rays from the secondary-beam Coulomb excitation were discriminated from those of a primary beam by measuring triple coincidences of primary γ-rays, scattered particles and emitted γ-rays from the second target. Coulomb excitation of the unstable nucleus was observed for the first time in the medium heavy mass region. The observed yield of the 2 + → 0 + transition in 76Kr is consistent with that which is estimated from the known B(E2: 0 + → 2 +) for this nucleus.

G Pair Included Fermion Dynamical Symmetry Model for Medium and Heavy Mass Nuclei

A new description of the hexadecapole degree of freedom in the medium and heavy mass region in the framework of the fermion dynamical symmetry model is proposed. In this letter, the k-active case is discussed in detail, in which the pairing and multipole operators generate Sp10 group. The energy spectra are derived for two limiting cases. The results show that these limits and those in the s.d.g IBM have one to one correspondence when N1 (=N boson number in the s.d.g IBM) ⩽ Ω1/5.

Nuclear data sheets for A = 194

Spectroscopic data for known nuclei of A=194 (Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At) have been evaluated and presented with adopted level properties and associated γ rays. No experimental data exist on 194Hf and 194W, but amongst other nuclides in heavy mass region these have been included and nuclear structure calculations by 85Ro21, 85He14, 84Sa16, and 83Ba47. Except for the isotopic identification, nothing much is known about the level structure of 194Bi, 194Po, and 194At. Since the 1997 evaluation of A=194 (77Ha46), the following are the main additions to the present revision: excited state data are now available for 194Os through a 2-neutron transfer reaction and for 194Tl from 194Pb ɛ decay and (Hl,xn γ) study. New high spin data for 194Au, 194Hg, 194Tl, and 194Pb from in—beam γ—ray studies reveal band structures. A new isomer is tentatively identified in 194Bi; its ɛ decay reveals a low lying 0+ state as the first excited state in 194Pb. From α decay studies, 194At isotope has been identified. Further experimental work is needed in the following causes: in a recent neutron capture work on 194Ir, only about 60 of the 900 reported γ rays can be placed in a level scheme, although a large number of levels are indicated from primary γ rays. The existence of isomerism in 194Bi remains to be confirmed through a more detailed study of 194Bi ɛ decay. The radioactive decays of 194Os (6.0 y), 194Ir (31.85 ms), and 194Tl (33.0 min and 32.8 min) need more definitive work. The newly discovered first excited state in 194Pb with J π=0+ has been interpreted as the starting member of a collective band; however, high—spin members of such a band have not yet been found in the (hl,xn γ) studies of 194Pb.

Binding energies of a hyperon in finite nuclei with Nijmegen interaction

Hyperon (Λ, )-nucleus potentials are derived on the basis of the Nijmegen interaction. Experimental values of B Λ in the p-shell region are nicely reproduced and a characteristic behavior is predicted in the heavier mass region. Discussion on B  and Λ  is also given.

Binding Energies of a Hyperon in Finite Nuclei with Nijmegen Interaction

Hyperon (A, Sand Ac)·nucleus potentials are derived on the basis of the Nijmegen interaction. Experimental values of BA in the p·shell region are nicely reproduced and a characteristic behavior is predicted in the heavier mass region. Bs and rs are investigated in relation to the choice of SN hard core radii, the result of which is consistent with the events in emulsion experiments. Discussion on BA, is also given.

Microscopic description of the (p, pi -) continuum at intermediate proton energy.

The continuum spectra of the (p,${\ensuremath{\pi}}^{\mathrm{\ensuremath{-}}}$) reaction on nuclei in the medium and heavy mass region at ${E}_{\mathrm{p}\mathrm{\ensuremath{\sim}}200}$ MeV are studied in terms of the standard pion-absorption-production model. The threshold energy and the linear rise of the pion cross section with the excitation energy are reproduced. By including the medium effect on the rescattering pion and rho meson fields we can also account for the absolute value of the cross section.

Half-life ofTi44

The half-life of $^{44}\mathrm{Ti}$ has been measured to be ${T}_{\frac{1}{2}}=54.2\ifmmode\pm\else\textpm\fi{}2.1$ yr, somewhat higher than previously published values of 46.4\ifmmode\pm\else\textpm\fi{}1.7 and 48.2\ifmmode\pm\else\textpm\fi{}0.9 yr. The present value was obtained from the specific activity and the radioisotope concentration of several Ti${\mathrm{O}}_{2}$ samples, each spiked with a $^{44}\mathrm{Ti}$ activity of about 1.2 \ensuremath{\mu}Ci. The specific activity was measured via the 1157 keV $\ensuremath{\gamma}$ line from the decay of the $^{44}\mathrm{Sc}$ daughter. $^{44}\mathrm{Ti}$/Ti concentrations were measured with the Argonne FN tandem accelerator in conjunction with an Enge split-pole magnetic spectrograph using the technique of accelerator mass spectrometry. An overall sensitivity of ${10}^{\ensuremath{-}10}$ was achieved which proves accelerator mass spectroscopy to be a powerful tool for radioisotope detection also in a region of rather heavy ion masses. A detailed description of the subtleties of accelerator mass spectroscopy when applied to measure absolute radioisotope concentrations in this heavy mass region is presented.RADIOACTIVITY Measured half-life of $^{44}\mathrm{Ti}$ via accelerator mass spectroscopy.

Pre-equilibrium effects in (n, 2n) reactions at 14.2 MeV+

With a view to study the pre-equilibrium effects in neutron-induced reactions, the activation cross-sections for (n, 2n) reactions at 14.2±0.2 MeV in the heavy mass region have been measured using the versatile mixed powder technique and high resolution Ge(Li) detection. The experimental cross-sections are found to be consistently smaller than the predictions based on the statistical theory and this is attributed to the effect of pre-equilibrium decay in these reactions. The cross-sections due to pre-equilibrium decay were estimated using exciton, hybrid and unified models. When this cross-section was included in comparing the experimental cross-sections with theory, we obtained better agreement, within the limitations of the present-day preequilibrium theories.

Some neutron activation cross sections in the heavy mass region at 14 MeV

Neutron activation cross sections were measured for the reactions, 158Gd( n, α) 155Sm, 176Yb( n, 2 n) 175Yb, 174Hf( n, 2 n) 173Hf, 181Ta( n, α) 178 m Lu and 208Pb( n, p) 208Tl, at 14.2 ± 0.2 MeV, using the mixed-powder technique and high-resolution Ge(Li) detection. The measured ( n, 2 n) cross sections were compared with the predictions based on statistical and preequilibrium theories. The ( n, α) and ( n, p) cross sections were compared with the recent empirical estimates.

Extended-Skyrme-force calculation: Theoretical description and application to Ca and Sn isotopes

The original Skyrme force is extended by a momentum dependent three-body term. A study is made on its influence on nuclear matter properties and properties of finite nuclei. The problem of spin-instability is treated in two nuclear models. A force parametrization is proposed, giving good estimates for all nuclear matter quantities (binding energy per nucleon, incompressibility, reduced mass, isospin symmetric energy, etc.), as well as properties of finite nuclei throughout the whole mass region (ground-state energy, nucleon distributions, rms radii, single-particle removal energies, etc.). The proposed parametrization satisfies the spin stability conditions. The force is able to describe satisfactorily the ground-state properties, even fine effects of two nuclear mass regions, one belonging to the light mass region ($A=40\ensuremath{-}48$) and one belonging to the medium heavy mass region ($A=104\ensuremath{-}132$).NUCLEAR STRUCTURE Skyrme force extended with momentum dependent three-body term, spin stability conditions, application to Ca and Sn isotopes.