Γ-ray Multiplicity Distributions Research Articles

Neutron and gamma multiplicities calculated in the consistent framework of the Hauser-Feshbach Monte Carlo code FIFRELIN

Monte-Carlo simulations to calculate the number of prompt particles emitted during fission were performed using the Fifrelin code and compared to recent experimental data. We show that we are able to reproduce both the neutron and γ-ray multiplicity distributions as a function of the pre-neutron mass of the fission fragments using a single consistent set of parameters. This result was made possible by using an energy-dependent spin cut-off model, driving the initial total angular momentum of the fission fragments, together with microscopic level densities from the HFB plus combinatorial method. We also discuss, how the initial excitation-energy sharing shapes the TKE-dependent γ-ray multiplicity.

High resolution gamma-ray spectroscopy of fast-neutron induced fission

Experiments to perform precision spectroscopy of fast neutron induced fission were carried out during the ν-Ball experimental campaign at the ALTO facility of IJC Laboratory Orsay. Low energy fission of 232Th(n,f), 238U(n,f) and spontaneous fission of 252Cf were studied using this hybrid highresolution spectrometer and calorimeter. New observables such as γ-ray multiplicity distributions correlated with specific fission fragments are presented and discussed. A new method using fast-timing techniques to detect prompt fission neutrons in coincidence with prompt fission γ-rays is described.

Angular momentum distribution for the formation of evaporation residues in fusion of 19F with 184W near the Coulomb barrier

We present γ-ray multiplicity distributions for the formation of evaporation residues in the fusion reaction 19F + 184W → 203 83Bi 120 at beam energies in the range of 90–110 MeV. The measurements were carried out using a 14 element BGO detector array and the Heavy Ion Reaction Analyzer at the Inter University Accelerator Centre. The data have been unfolded to obtain angular momentum distributions with inputs from the statistical model calculation. Comparison with another neighboring system, viz. 19F + 175Lu → 194 80Hg 114 with nearly similar entrance-channel mass asymmetry, hints at the depletion of higher angular momenta after crossing of the Z = 82 shell in the compound nucleus.

Progress on the europium neutron capture study using DANCE

The accurate measurement of neutron capture cross sections of the Eu isotopes is important for many reasons including nuclear astrophysics and nuclear diagnostics. Neutron capture excitation functions of 151,153Eu targets were measured recently using a 4π γ-ray calorimeter array DANCE located at the Los Alamos Neutron Science Center for En=0.1–100keV. The progress on the data analysis efforts is given in the present paper. The γ-ray multiplicity distributions for the Eu targets and Be backing are significantly different. The γ-ray multiplicity distribution is found to be the same for different neutron energies for both 151Eu and 153Eu. The statistical simulation to model the γ-ray decay cascade is summarized.

Statistical γ-ray multiplicity distributions in Dy and Yb nuclei

The statistical γ-ray multiplicity distributions following the reactions 163 Dy( 3He ,αx n) 162−x Dy and 173 Yb( 3He ,αx n) 172−x Yb have been studied. The mean value 〈 M〉 and standard deviation σ M have been extracted as functions of excitation energy. The method is based on the probability distribution of k-fold events, where an α-particle is observed in coincidence with signals in k γ-ray detectors. Techniques for isolating statistical γ-rays and subtracting random background, cross-talk and neutron contributions are discussed.

Gamma-ray multiplicity distributions in 16O + 152Sm fusion near and below the Coulomb barrier

We have studied γ-ray multiplicity distributions for the system 16O + 152Sm at five beam energies in the range 60–80 MeV, near and below the Coulomb barrier. The data were obtained using a large array of γ-ray detectors and an efficient evaporation-residue detector system utilizing an electrostatic deflector. The data are in good agreement with predictions of angular momentum distributions calculated using a coupled-channels model incorporating the effects of deformation.

Multiplicity distributions for the decay of erbium produced by symmetric fusion

Measurements have recently been made of detailed γ-ray multiplicity distributions for the reaction 80Se ( 80Se, xηγ) 160−x Er at different bombarding energies. Here we show that the excitation of collective surface modes in the entrance channel accounts for the general features of the observed distributions at energies near the barrier.

Restoration of γ-ray multiplicity distributions from experiments with low efficiency multiplicity filters

The restoration of γ-ray multiplicity distributions from experimental p-fold coincidence distributions is discussed. It is shown that the restoration of the multiplicity from measurements with low total detection efficiency is an “incorrectly posed problem”. While in the literature the analysis of the experimental data has been attempted only in terms of the lowest central moments of the multiplicity distribution, in this paper an unfolding method based on the minimization of the directioned discrepancies in the probability space is used. The method is found to work very well even if the total efficiency Ω ≲ 0.1. Realistic tests and a comparison with the usual method of analysis are presented.

Bombarding energy dependence of angular momentum transfer in deep inelastic collisions

Measurements of first and second moments of γ-ray multiplicity distributions from deep inelastic collisions of 86Kr + 154Sm are reported. A global systematics of the angular momentum distributions from deep inelastic reactions with projectile masses ≳ 40 is presented. The average angular momentum is found to depend linearly on the incident channel average angular momentum, while no simple systematics for the second moment appears obvious. In order to illuminate the question whether the angular momentum transfer process reaches statistical equilibrium in deep inelastic collisions, numerical calculations have been performed on two models: a two-sphere classical model including the collective modes of twisting, bending, wriggling and tilting, and a statistical equilibrium Fermi-gas model. The two-sphere classical model is not able to account for the observed second moments, and neither does the Fermi-gas model give an explanation of the deep inelastic multiplicity data.

Gamma-ray multiplicity studies for 40Ar and 86Kr induced reactions leading to Er compound nuclei at limiting angular momenta

The moments of the γ-ray multiplicity distributions as a function of evaporation residue and of E γ have been determined from γ-multiplicity measurements for the 116, 120, 122, 124Sn( 40Ar, xn) reactions at 161 ≦ E(Ar) ≦ 236 MeV and for the 76Ge( 86Kr, xn) reaction at 314 ≦ E(Kr) ≦ 376 MeV. Even at the highest incident energies where l gr ≈ 120 units, it is not possible to populate nuclei with l > 65 units, the limiting l predicted for a rotating liquid drop. For the lowest incident energies these data are in agreement with the decay of a compound system. At the highest incident energy, however, significant deviations are observed from the expected decay of an equilibrated system. Such deviations are indicative of pre-equilibrium particle decay. Statistical calculations utilizing the code GROG12 are used to substantiate these conclusions. No evidence for a lower l-cut in the population of the compound system is observed; however, these data would not be sensitive to an l cut ≲ 10 units. The moments of inertia extracted from the edge of the “collective E2 bump” in the 〈 M γ 〉 versus E γ data are in agreement with those predicted for a rotating liquid drop. The average multiplicity of the noncompound processes is estimated to be 8–11 for l gr ≲ 100 units and increasing to 21 for l gr ≲ 120 units.

Gamma decay after heavy ion reactions studied by first and higher order multiplicity measurements

First and higher order multiple coincidence rates have been measured for the γ-rays from 16O induced reactions on targets of 148,150Nd at various bombarding energies. Average values and shape parameters for the γ-ray multiplicity distributions have been extracted from the data. The multiplicities are connected to angular momentum through complete fusion cross sections, and conclusions are drawn on spin distributions of entry states in the final nuclei as well as on the decay pattern from these states.