arXiv subjects

Vladimir A. Protopopescu

Publications and source records attributed to Vladimir A. Protopopescu.

4 recordsLinked to original sources

arXiv2022

Directional-dependence of the event-by-event neutron-$γ$ multiplicity correlations in $^{252}$Cf(sf)

We differentiate the event-by-event n-$γ$ multiplicity data from \ce{^{252}Cf}(sf) with respect to the energies of the emitted particles as well as their relative angles of emission. We determine that neutron emission enhances $γ$-ray emission around $0.7$ and $1.2$ MeV, but the only directional alignment was observed for $E_γ\leq 0.7$ MeV and tended to be parallel and antiparallel to neutrons emitted in the same event. The emission of $γ$ rays at other energies was determined to be nearly isotropic. The presence of the emission and alignment enhancements is explained by positive correlations between neutron emission and quadrupole $γ$-ray emission along rotational bands in the de-exciting fragments. This observation corroborates the hypothesis of positive correlations between the angular momentum of a fragment and its intrinsic excitation energy. The results of this work are especially relevant in view of the recent theoretical and experimental interest in the generation of angular momentum in fission. Specifically, we have determined an alignment of the fragments angular momenta in a direction perpendicular to the direction of motion. We interpret the lack of $n$-$γ$ angular correlations for fission fragments near closed shells as a weakening of the alignment process for spherical nuclei. Lastly, we have observed that statistical $γ$ rays are emitted isotropically, indicating that the average angular momentum removed by this radiation is small. These results, and the analysis tools presented in this work, represent a stepping stone for future analysis of $n$-$γ$ emission correlations and their connection to angular momentum properties.

nucl-ex↗

arXiv2021

Structure in the Event-by-Event Energy-Dependent Neutron-Gamma Multiplicity Correlations in $^{252}\text{Cf}$(sf)

The emission of neutrons and gamma rays by fission fragments reveal important information about the properties of fragments immediately following scission. The initial fragment properties, correlations between fragments, and emission competition give rise to correlations in neutron-gamma emission. Neutron-gamma correlations are important in nonproliferation applications because the characterization of fissionable samples relies on the identification of signatures in the measured radiation. Furthermore, recent theoretical and experimental advances have proposed to explain the mechanism of angular momentum generation in fission. In this paper, we present a novel analysis method of neutrons and gamma rays emitted by fission fragments that allows us to discern structure in the observed correlations. We have analyzed data collected on \ce{^{252}Cf}(sf) at the Chi-Nu array at the Los Alamos Neutron Science Center. Through our analysis of the energy-differential neutron-gamma multiplicity covariance, we have observed enhanced neutron-gamma correlations, corresponding to rotational band gamma-ray transitions, at gamma-ray energies of $0.7$ and $1.2$ MeV. To shed light on the origin of this structure, we compare the experimental data with the predictions of three model calculations. The origin of the observed correlation structure is understood in terms of a positive spin-energy correlation in the generation of angular momentum in fission.

nucl-ex↗

arXiv2019

Event-by-Event Multiplicity Correlations in $^{252}$Cf(sf)

Excited nuclear fragments are emitted during nuclear fission. The de-excitation of these fission fragments takes place as sequential emission of neutrons followed by photons. A correlation between neutron and photon multiplicities accompanying fission is thus expected. Fission event generators based on established statistical nuclear physics models predict a negative event-by-event correlation in neutron-photon multiplicity. A survey of published experimental results of an event-by-event covariance between the neutron and photon multiplicities emitted following the spontaneous fission of $^{252}$Cf is presented. Analytic unfolding expressions are developed in this work to determine the bias introduced by background sources, particle misclassification, pulse pileup, and inelastic photon production. The published experimental data are re-analyzed using these unfolding techniques and are found to be in qualitative agreement with the predictions of model-based calculations. In particular, we have concluded that there exists a significant event-by-event neutron-photon emission competition following the spontaneous fission of $^{252}$Cf.

nucl-ex↗

arXiv2013

Reversible Simulations of Elastic Collisions

Consider a system of N identical hard spherical particles moving in a d-dimensional box and undergoing elastic, possibly multi-particle, collisions. We develop a new algorithm that recovers the pre-collision state from the post-collision state of the system, across a series of consecutive collisions, with essentially no memory overhead. The challenge in achieving reversibility for an n-particle collision (where, n << N) arises from the presence of nd-d-1 degrees of freedom during each collision, and from the complex geometrical constraints placed on the colliding particles. To reverse the collisions in a traditional simulation setting, all of the particular realizations of these degrees of freedom during the forward simulation must be saved. This limitation is addressed here by first performing a pseudo-randomization of angles, ensuring determinism in the reverse path for any values of n and d. To address the more difficult problem of geometrical and dynamic constraints, a new approach is developed which correctly samples the constrained phase space. Upon combining the pseudo-randomization with correct phase space sampling, perfect reversibility of collisions is achieved, as illustrated for n <= 3, d=2, and n=2, d=3 (and, in principle, generalizable to larger n). This result enables for the first time reversible simulations of elastic collisions with essentially zero memory accumulation. The reverse computation methodology uncovers important issues of irreversibility in conventional models, and the difficulties encountered in arriving at a reversible model for one of the most basic physical system processes, namely, elastic collisions for hard spheres. Insights and solution methodologies, with regard to accurate phase space coverage with reversible random sampling proposed in this context, can help serve as models and/or starting points for other reversible simulations.

physics.comp-ph↗