Evaporation residues from heavy (Z=90–93) compound nuclei at excitation energies of Ex ≃ (75–110) MeV

Evaporation residue cross sections have been measured in reactions⁴⁰Ar +¹⁴⁰Hf,¹⁸¹Ta,¹⁸²W,¹⁸⁴W,^natRe at bombarding energiesE/A=(5.5–6.3) MeV/u. The cross sections do not decrease with increasing nuclear temperature, as expected from statistical model calculations. Rather they are found to be roughly constant. This behavior is predominantly affected by an enhanced emission probability of charged particles. Influence of a ‘fission delay’, which is expected atE ^x>60 MeV from measurements of pre- and post-scission neutrons, on thexn- cross sections has been observed at excitation energiesE ^x>90 MeV. The possibility to produce transfermium isotopes in ‘hot fusion’ reactions has been investigated in an irradiation of²³²Th with⁴⁰Ar. Upper limits of 10 nb for evaporation residues were obtained.     

Dynamical decay of nuclei at high temperature: Competition between particle emission and fission decay

A generalized diffusion equation is propounded to follow the time evolution of an excited nucleus towards fission including along the particle decay. This theoretical model is built in order to try to analyse the anomalous behaviour of particle emission observed in many experimental data for heavy-ion induced reactions. Some calculations for the systems¹⁹⁴Hg,¹⁷⁰Yb and²⁴⁸Cf are presented. A possible extension of this generalized formalism is suggested to deal more consistently with the experimental data.     

Stochastic model of angular distributions of fragments originating from the fission of excited compound nuclei

The anisotropy of angular distributions of fission fragments and the average multiplicity of prescission neutrons were calculated within a stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations. This approach was combined with a Monte Carlo algorithm for the degree of freedom K (projection of the total angular momentum I onto the fission axis). The relaxation time τ _K in the coordinate K was considered as a free parameter of the model; it was estimated on the basis of a fit to experimental data on the anisotropy of angular distributions. Specifically, the relaxation time τ _K was estimated at 2 × 10^?21 s for the compound nuclei ²²⁴Th and ²²⁵Pa and at 4 × 10^?21 s for the heavier nuclei ²⁴⁸Cf, ²⁵⁴Fm, and ²⁶⁴Rf. The potential energy was calculated on the basis of the liquid-drop model with allowance for finiteness of the range of nuclear forces and for the diffuseness of the nuclear surface. A modified one-body viscosity mechanism featuring a coefficient k _s that takes into account the reduction of the contribution from the wall formula was used to describe collective-energy dissipation. The coefficient k _s was also treated as a free parameter and was estimated at 0.5 on the basis of a fit to experimental data on the average prescission multiplicity of neutrons.     

On the decay of compound nuclei following alpha-particle and12C induced reactions

Multiple coincidence rates have been measured using a detector system consisting of a Ge(Li) spectrometer and eight NaI(Tl) or eight liquid scintillators. Reactions induced byα-particles with energies of 51–55 MeV and 118 MeV¹²C ions are studied. The data are analysed to give the first and second central moments of the distribution of the number ofγ-rays feeding individual levels in the final nuclei. When these numbers are compared to spin distributions calculated with the statistical model code GROGI the relative importance of dipole and quadrupole deexcitation modes can be ascertained. In particular, in the¹²²Te(α, 4n)¹²²Xe reaction theγ-decay prior to the entry into the ground band is well described as a statistical process proceeding to 50% by dipole and 50% by quadrupole radiation. In the¹⁶⁶Er(α,4n)¹⁶⁶Yb and¹⁹²Os(α,4n)¹⁹²Pt reactions the relative amount of quadrupole radiation is larger and it seems that the dipole and quadrupole decay takes place via separate cascades. In the¹⁶⁴Dy(¹²C, 7-8n) reactions the average multiplicity is independent of spin, suggesting that the nucleus forgets the spin of the entry state before the process enters into the ground band. In the¹⁷⁶Yb(¹²C, 8n)¹⁸⁰Os reaction, finally, the nucleus definitely retains memory of the entry state during the decay. In this last case the multiplicity measurement is combined with aγ-ray singles measurement to give an average excitation energy prior to theα-decay and the average moment of inertia characterising the decay of the high-spin states.     

Comparison between experimental decay of the compound nuclei150Gd and evaporation calculations

The deexcitation of the compound nucleus¹⁵⁰Gd formed by¹⁵⁶O+¹³⁴Ba and⁴⁰Ar+¹¹⁰Pd have been studied. Excitation functions were determined for various exit channels: (HI, × n) withx=3, 4, 5 (HI, pxn) withx=2, 3, 4 (HI, axn) withx=3 and 5. A systematic comparison has been made between these experimental data and the results of evaporation calculation using the ALI CE code. The very good agreement obtained for the total residue cross section disappears completely when a more detailed analysis of the different contributions is performed. Absolute and relative cross sections are not reproduced; excitation functions of each individual channel (HI, 3n), (HI, 4n) etc. ... are not found at the right place in energy. A very preliminary explanation for such a shift towards higher energies is suggested.     

Interaction of gold nuclei with photoemulsion nuclei at energies in the range 100–1200 MeV per nucleon and cascade-evaporation model

The interaction of gold nuclei with photoemulsion nuclei at energies in the range 100–1200 MeV per nucleon was studied experimentally. A consistent comparison of the experimental data obtained in this way with the results of the calculations based on the cascade-evaporation model is performed.     

Analysis of the 2νββ decay and muon-capture reactions for the mass A=46 and A=48 nuclei using the nuclear shell model

Physics of Atomic Nuclei - We discuss the two-neutrino double-beta decays of 46Ca and 48Ca by using the nuclear shell model with well-tested two-body interactions. We also discuss the ordinary...     

The fission processes 236U(α, α′f) at Eα = 50 MeV and 235U(d,pf) at Ed = 23 MeV and the total kinetic energy release in fission of excited nuclei

The fissioning nucleus ²³⁶U was investigated by two different reactions. Total kinetic energies (TKE) as function of the excitation energy above the highest fission threshold are shown to have different behaviour for E_X below the two quasiparticle threshold 2Δ and above. A strong dependence on the mass splitting is observed.     

Mass yield distributions for the reactions Ca+Ca,Nb+Nb at EA = 400 MeV and Ca+Ca at EA = 800 MeV in the molecular-dynamical model

Mass yield distributions obtained on the basis of the molecular-dynamical model are presented for the reactions Ca+Ca, Nb+Nb at $\text{E}\text{A}\text{= 400}\text{MeV}$ and Ca+Ca at $\text{E}\text{A}\text{= 800}\text{MeV}$. For the fragments with masses up to a quarter of the mass of the initial nucleus the model predicts a power law for mass spectra with almost the same value of the exponent. Such behaviour is roughly a result of the superposition of the fireball break-up and the disintegration of spectator regions.     

High-resolution study of four-nucleon pickup via the (d, 6Li) reaction at Ed = 55 MeV on sd-shell nuclei: (II). odd-A nuclei

The ²⁵Mg(d, ⁶Li)²¹Ne and ²⁷Al(d, ⁶Li)²³Na reactions were studied with a broad-range magnetic spectrograph at E_d = 55 MeV with an energy resolution of ~ 40 keV. Angular distributions were obtained from θ_lab = 10°–37° for some eighty transitions to states up to E_x ? 9.5 MeV. The data were analyzed with zero-range and finite-range DWBA. Experimental spectroscopic factors are compared with those predicted by the SU(3) model and by microscopic shell-model calculations made with the Chung-Wildenthal interaction in the full (1sd)ⁿ space.     

Dynamic and statistical simulation of the fission of highly excited atomic nuclei with allowance for the relaxation stage

Statistical and combined dynamic-statistical approaches to simulating the fission of excited nuclei are compared. It is shown that in the statistical approach, the complete suppression of the fission pro cess during time τ _s does not result in double consideration of the emission of light particles. The importance of dynamic simulation of fission at the stage of relaxation of fission rate R _f (t) to its quasistationary value is demonstrated.     

Direct and compound nuclear contributions to the 13C(6Li,t) reaction at Elab = 25 MeV

The reaction ¹³C(⁶Li, t)¹⁶O has been studied in the incident energy range 24–26 MeV. Complete angular distributions have been measured at E_⁶Li, = 25 MeV in the angular range θ_lab = 8°–172°, with the reaction ⁶Li(¹³C, t)⁶O being used for the backward angle measurements. Cross sections for evaporation residues from the fusion of the ⁶Li + ¹³C system have been measured in the incident ⁶Li energy range 9.2–35.1 MeV. Compound nuclear contributions to the transfer cross sections have been calculated using the Hauser-Feshbach statistical theory with the assumption that the compound-nucleus formation cross section is equal to the measured fusion cross section. By comparison of the compound nuclear calculations with backward angle data it is found that the sharp cutoff approximation commonly used to represent the initial angular momentum distribution of the compound nucleus is not adequate for the ¹³C(⁶Li, t)¹⁶O reaction. Good fits to the backward angle data can be obtained by using a smooth cutoff approximation. The forward angle cross sections have been compared with exact finite-range distorted-wave Born approximation calculations to extract transferred angular momenta and spectroscopic strengths. The present results differ from those of an earlier study. These differences are due to the inclusion of forward angle data in the present study.     

Giant dipole resonance in the hot and thermalized 132Ce nucleus: damping of collective modes at finite temperature

The gamma decay of the giant dipole resonance (GDR) in the 132Ce compound nucleus with temperature up to approximately 4 MeV has been measured, using the reaction 64Ni + 68Zn at E(beam) = 300, 400, and 500 MeV. The gamma and charged particles measured in coincidence with recoils are consistent with a fully equilibrated compound nucleus emission. The GDR width, obtained with the statistical model analysis, is found to increase almost linearly with temperature. This increase is rather well reproduced within a model including thermal shape fluctuations and the lifetime of the compound nucleus.     

Angular distributions of fission and deep inelastic products of the system Ar+Au at 285 MeV

The angular distribution of the heavy products from the interaction of 285 MeV ions⁴⁰Ar on an Au target has been analysed through a large angular range (4°–165° lab angles). The results exhibit two components: the fission component corresponding to relatively symmetric masses, and a deep inelastic one which peaks in the angular range 100°–140° (c.m angles). The fission component exhibit the usual 1/sinθ pattern, proving that, in spite of a zero fission barrier, the life time of the system exceeds the rotation period. The relative cross sections for the fission and deep inelastic processes are discussed.