Spin Distribution Of Compound Nucleus Research Articles

Microscopic description of target spin distribution after inelastic scattering to the continuum

Microscopic modeling of inelastic scattering to the continuum is applied to neutron induced reaction on spherical and axially deformed even-even targets. The spin distributions of the residual compound nucleus formed after the fast inelastic process are calculated with two microscopic models and compared to the prescription usually associated to the semi-classical exciton model. As the semi-classical exciton model does not account for angular momentum conservation, it is often assumed that it is the same as the compound nucleus spin distribution, but this forgets the dynamics of the reaction. It is found that microscopic approaches drastically reduce the average spin value in comparison to what was previously assumed. This strongly impacts (n,n’ γ) as well as isomer production cross sections when high spin levels are involved. New spin cut-off parameters are deduced from the microscopic calculations that can be used as an alternative to previous prescriptions which neglect the reaction dynamics when they are applied in the context of pre-equilibrium emission process.

Investigation of Weisskopf-Ewing approximation for the determination of ( n,p ) cross sections using the surrogate reaction technique

The present study explores the limitations of the surrogate reaction method for determining the $(n,p)$ cross sections for the target nuclei in the mass region $A\ensuremath{\approx}50$ for the neutron energies 1--20 MeV. In the past few years there have been several experimental attempts for determining the $(n,p)$ and $(n,xp)$ cross sections using the surrogate reaction method. But this method has not been benchmarked with the experimentally well-known $(n,p)$ cross sections. The surrogate reaction method with Weisskopf-Ewing approximation may help in providing good constraints for the cross-section data, but this approximation has not been validated yet for $(n,p)$ reactions. In this paper, we have examined the validity of the Weisskopf-Ewing approximation for the $(n,p)$ reactions and also check the sensitivity of the surrogate reaction results with respect to the compound nucleus spin distribution. We have simulated the cross sections obtained through the surrogate reaction method for $n+^{48}\mathrm{Ti}, n+^{53}\mathrm{Cr}, n+^{56}\mathrm{Fe}$, and $n+^{59}\mathrm{Co}$ reactions for the different schematic spin distributions of the compound nucleus and studied the effect of assuming the validity of the Weisskopf-Ewing approximation. It has been observed that the proton decay probabilities of the compound nucleus for the $(n,p)$ channel are strongly spin dependent, therefore the Weisskopf-Ewing approximation is violated. We have also observed that the cross sections obtained using the Weisskopf-Ewing approximation show clear dependence on the spin distribution of the compound nucleus. It has also been observed that, due to the large pre-equilibrium contributions in the $(n,p)$ reactions at higher neutron energies, the use of the surrogate reaction method for the neutron energies greater than $\ensuremath{\approx}15$ MeV may not be suitable. It is concluded that the surrogate reaction method relying solely on the Weisskopf-Ewing approximation is not sufficient for determining the $(n,p)$ cross sections for the target nuclei in mass range $A\ensuremath{\approx}50$ and further development and exploration of the surrogate technique is required.

First simultaneous measurement of fission and gamma probabilities of237U and239Np via surrogate reactions

Fission and gamma decay probabilities of 237 U and 239 Np have been mea- sured, for the first time simultaneously in dedicated experiments, via the surrogate re- actions 238 U( 3 He, 4 He) and 238 U( 3 He,d), respectively. While a good agreement between our data and neutron-induced data is found for fission probabilities, gamma decay prob- abilities are several times higher than the corresponding neutron-induced data for each studied nucleus. We study the role of the different spin distributions populated in the surrogate and neutron-induced reactions. The compound nucleus spin distribution popu- lated in the surrogate reaction is extracted from the measured gamma-decay probabilities, and used as input parameter in the statistical model to predict fission probabilities to be compared to our data. A strong disagreement between our data and the prediction is ob- tained. Preliminary results from an additional dedicated experiment confirm the observed discrepancies, indicating the need of a better understanding of the formation and decay processes of the compound nucleus.

Calculation of spin distribution for several fission reaction systems induced by nucleons and heavy ions

Compound nucleus spin distribution has been calculated for several fission reaction systems induced by nucleons and heavy ions. Determination of the spin distribution for these systems is based upon the comparison between the experimental data of the fission fragment angular distributions as well as the prediction of the standard saddle-point statistical model (SSPSM). For the systems, the two cases, namely with and without neutron emission corrections were considered. This method is used for the first time to determine compound nucleus spin distribution. Afterwards, our theoretical results have been compared with the data obtained from the coupled-channel technique as well as the Wong model, and satisfactory agreements were found. Furthermore, we have introduced a semiclassical approximation relation for the compound nucleus spin distribution of these systems.

Feeding of the11/2−isomers in stable Ir and Au isotopes

Excited states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$ were studied and absolute partial $\ensuremath{\gamma}$-ray cross sections were measured using the ($n,{n}^{'}\ensuremath{\gamma}$) reaction. A Compton-suppressed germanium-detector array (GEANIE) for $\ensuremath{\gamma}$-ray spectroscopy was used for the measurement and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided energetic neutrons. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute partial $\ensuremath{\gamma}$-ray cross sections were measured up to incident neutron energy of 20 MeV for several transitions feeding directly the $11/{2}^{\ensuremath{-}}$ isomers and ground states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$. The feeding of the $11/{2}^{\ensuremath{-}}$ isomers, which originate from the odd proton occupying the ${h}_{11/2}$ orbital, was found for the three targets to be very similar and increasing relative to the feeding of the corresponding ground state with increasing neutron energy up to ${E}_{n}~10$ MeV. Above this neutron energy the opening of the $(n,2n)$ reaction channel strongly affects the population of the isomers and leads to a decrease of their relative population compared to the population of the ground states. The experimental results are compared with theoretical predictions from the GNASH reaction model calculation implementing a version of the spin distribution for the pre-equilibrium reaction piece with either a compound nucleus spin distribution (CN-GNASH) or a Feshbach-Kerman-Koonin (FKK-GNASH) quantum mechanical spin distribution. The effects of the spin cutoff parameter values on the population of states are examined. Evidence is presented that FKK-GNASH provides a description of the experimental data that mitigates the need for adjustment of the level density parameter to fit the data.

Capture barrier distributions: Some insights and details

The ``experimental barrier distribution'' provides a parameter-free representation of experimental heavy-ion capture cross sections that highlights the effects of entrance-channel couplings. Its relation to the $s$-wave transmission is discussed, and in particular it is shown how the full capture cross section can be generated from an $l=0$ coupled-channels calculation. Furthermore, it is shown how this transmission can be simply exploited in calculations of quasifission and evaporation-residue cross sections. The system $^{48}\mathrm{Ca}+^{154}\mathrm{Sm}$ is studied in detail. A calculation of the compound-nucleus spin distribution reveals a possible energy dependence of barrier weights due to polarization arising from target and projectile quadrupole phonon states; this effect also gives rise to an entrance-channel ``extra-push.''

Reaction dynamics at the barrier for heavy compound systems

To investigate basic properties of the fusion reaction dynamics for heavy compound systems, the partial wave distribution σl extracted from measured γ multiplicities can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems, and a possible effect of the Z=82 shell on the enhancement of evaporation residue production. The measured spin distribution can provide information on the single partial wave cross sections, which is hidden in the integral fusion cross section. Moreover, it can reveal signatures in the high-spin region, which could be an indication of a stabilization due to an increase in the potential hole by shell correction energies Eshell in the vicinity of a closed shell. The systematic investigation and understanding of the fusion-fission reaction dynamics, together with the understanding of the structure of transfermium nuclei, which are stabilized only via shell effects, are essential for a successful program aiming at the synthesis of new elements at GSI, Dubna, or elsewhere. We started a series of experiments to measure those properties for the reactions at the Laboratori Nazionali di Legnaro, Italy. In order to extract the compound nucleus spin distribution, γ multiplicities are measured using the γ-detector array GASP and its inner ball in the multiplicity filter mode.

Isomeric yield ratio for the 95Mo (p,n) [formula omitted] reaction

Excitation functions and isomeric yield ratios for the 95Mo (p,n) 95m,g Tc reactions up to E p = 28 MeV are analyzed with a statistical model. In the vicinity of the threshold energy E p ⩽ 12 MeV, the experimental isomeric yield ratios are well reproduced by the calculation, while the calculation overestimates the experimental value beyond that energy. This suggests that the contribution of a pre-equilibrium process occurs above E p ∼ 12 MeV. The relationship between the isomeric yield ratio and compound nucleus spin distribution is discussed.

A new polarization potential for heavy ion fusion spin distribution

The real part of the polarization potential which depends on both energy and angular momentum is calculated in a simple way using dispersion relation. A barrier penetration model (BPM) has been used to explain the fusion cross-section and compound nucleus spin distribution for32S+64Ni system in the energy range 50–75 MeV. It is also shown that the polarization potential which only depends on energy, is not adequate to give rise to correct spin distribution even after including any radial dependence. The proposed polarization potential with implicitE andl dependences is able to explain both fusion cross-section and average spin values.

Fission-fragment angular distributions for the 19F + 208Pb near- and sub-barrier fusion-fission reaction

Fission cross sections and angular distributions have been measured for the 19F + 208Pb reaction at bombarding energies from 83 to 105 MeV. The fission excitation function is well reproduced on the basis of the coupled-channels theory. The fission-fragment angular distributions are calculated in terms of the transition-state theory, with the transmission coefficients extracted from the excitation function calculation. It is found that a discrepancy between the observations and the predictions in angular anisotropy of fission fragments exists at near- and sub-barrier energies, except for lower and higher energy regions where the discrepancy tends to disappear. Moreover, the anisotropies as a function of the center-of-mass energy show a shoulder around 82 MeV. Our results clearly indicate the considerable effects of the coupling on the sub-barrier fusion cross section and on the near-barrier compound-nucleus spin distribution, and confirm the prediction of an approximately constant value for the mean square spin of a compound nucleus produced in a far sub-barrier fusion reaction.

Heavy-ion induced fusion-fission systematics and the effect of the compound-nucleus spin distribution on fission-barrier determination

Fission and evaporation residue excitation functions have been measured for reactions using 18O and 19F projectiles to produce 158Dy, 168Yb, 178W, 188Pt and 210Po compound nuclei. The results of the measurements were compared with statistical model calculations performed using the Hauser-Feshbach code ALERT1. The initial calculations used fixed parameters and showed excellent agreement with the data for all but the two lightest systems studied, 158Dy and 168Yb. Further calculations with different parameter sets gave better agreement for the light systems. It is concluded that the calculations are best able to describe the data in those situations for which the angular momentum distribution following fusion produces significant population in or above the region where the fission barrier is equal to the neutron binding energy. This is not the case for the light systems. The fission barriers for the heavy systems are consistent with those predicted by the rotating finite range model.

Spin distribution of compound nucleus formed by near-barrier fusion.

Spin distributions of compound nuclei formed by heavy-ion fusion reactions near the barrier are calculated, based on a direct reaction description we recently developed. The mean values of the distributions thus calculated are found to fit the observed data very well.